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SAND 79-7056 Unlimited Release UC-63a Distribution
The Design of a PhotoYoltaic System for a Southwest All-Electric Residence
E. M. Mehalick, G. O'Brien, G. F. Tully, J. Johnson; J. Parker General Electric Space Division
Prepared for Sandia Laboratories under Contract No. 13-8779
Printed February 1980
~,~================================================================~ SF 2900-Q(3-80)
Issued by Sandia Laboratories, operated for the United States Department of Energy by Sandia Corporation:
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SAND79-7056 Unlimited Release
Printed April 1980 Distribution Category UC-63a
THE DESIGN OF A PHOTOVOLTAIC SYSTEM FOR A SOUTHWEST ALL-ELECTRIC RESIDENCE
E. M. Mehalick, G. O'Brien,
G. F. Tully, J. Johnson, J. Parker
General Electric Space Division Valley Forge Space Center
P. O. Box 8555 Philadelphia, PA 19101
prepared for Sandia National Laboratories under Contract No. 13-8779
FOREWORD
This report presents the first of five detailed designs on the Detailed
Residential Photovoltaic System Preferred Designs Study. The program is
being conducted by the Advanced Energy Programs of the General Electric
Company, Space Division with Mr. E. Mehalick as Program Manager. The program
is being performed under Sandia Laboratories Contract 13-8779 under the direction
of Dr. G. Jones.
The design described in this report represents only one of five residential
system designs which will be developed under this program. The choice of
hardware for this design was made primarily based on available data within
the time constraints of this effort and should not be considered an endorsement
of this hardware. Future designs will consider the implementation of other
hardware e.g. ARCO-Solar high density Block IV modules, flat PV panels, selected
roof mounted concentrator arrays and various power conditioning equipment. The
complete set of system designs will, therefore, provide detailed design infor
mation on several manufacturers hardware options. In addition, Section 6 of
the report discusses several system alternatives which can impact the design
described and they will be treated in detail in subsequent designs.
The intent of the detailed design was to provide sufficient detail for obtaining
a system installation cost estimate. A design package for this installation can
be formulated using the three appendices (Appendix A, Power Conditioning Subsystem
Specification; Appendix B, Array Installation Instruction Manual; and Appendix C,
Electrical Installation Specificationsl along with a drawings package (architectural
drawings Al-A6 and electrical drawings El-E6). These drawings are included
i
as foldouts within the report text and a full size set will be delivered to
Sandia.
The system design development and preparation of this report was supported by
contributions of the following individuals: Mr. G. Tully of Massdesign
Architects and Planners, Inc. for the residential house design and photovoltaic
array installation details; Mr. G.W. Johnson of Johnson and Stover, Inc. for the
electrical system installation specification; General Electric employees includ
ing Mr. G.P. O'Brien for overall system design; Mr. J. Parker for system per
formance analysis; and Mr. R. Felice for electrical system design support.
Valuable suggestions and technical data support were also obtained from Mr.
P. Sutton of JPL and Dr. E. Kern of MIT-Lincoln Laboratories.
i i
SECTION
1
2
3
4
5
6
7
CONTENTS
TITLE
BACKGROUND
SUMt·1ARY
Application Description Design Criteria Project Team Report Format
SYSTEM DESCRIPTION
Functional Description System Design Requirements Performance Characteristics Design Tradeoffs
HOUSE DESIGN CHARACTERISTICS
Des i gn Fea tu res House Plans
SUBSYSTEM SPECIFICATIONS
Sol ar Array Power Conversion Subsystem PV Array Electrical Interface to Conventional House System
Miscellaneous Considerations
SYSTEM DESIGN ALTERNATIVES
PAGE
1-1
1-1 1-1 1-5 1-7
2-1
3-1
3-,1 3-7 3-10 3-13
4-1
4-1 4-3
5-1
5-1 5-12 5-29
5-36
6-1
Introduction 6-1 System Performance for the Southeast and 6-1 Northeast
5 kW System Design Details 6-10 Solar Thermal Hot Water System 6-20 HVAC Heat Recovery Options 6-22 Ground Source Heat Pumps 6-25
REFERENCES 7-1
iii
-2-
APPENDIX TITLE PAGE
A POWER CONVERSION SYSTEM SPECIFICATION A-1
B PHOTOVOLTAIC SHINGLE INSTALLATION INSTRUCTIONS B-1
C RESIDENTIAL PHOTOVOLTAIC ELECTRICAL INSTALLATION C-1 SPECIFICATIONS
0 PERFORMANCE SIMULATION MODEL AND INPUT DATA 0-1
E ECONOMIC MODEL AND ASSUMPTIONS E-1
F PV SHINGLE MODULE CONSTRUCTION DETAILS F-1
G DESIGN TRADEOFF DETAILS G-1
iv
SECTION 1
BACKGROUND
Application Description
The photovoltaic system design described in this report is for a residential
single story house located in the Southwest region of the country typified by the
Phoenix/Albuquerque weather environment.
The house is considered to be new construction in 1986 with a living floor
area of 149m2 (1600 ft2) and a rectangular south facing roof area of 104 m2 .
(1120 ft2). A perspective of the house is shown in Figure 1-1. The house design
includes passive solar and energy conservation features projected. for
1986. The system design includes an advanced performance 3 ton heat pump for heat-
ing and cooling and an electric hot water heater, thus no fossil fuels are required
Figure 1-2 shows the general site layout for the house. The garage is separate
from the house, and is shown on the front of the lot in Figure 1-2. A slightly
oversized equipment room is provided at the end of the house to simplify the
service entry for both grid and photovoltaic power systems and can also be utilized
for additional storage area. The PV array is planned for only the house roof and
not the garage roof. The lot size could vary between 1/6 and 1/4 acre.
The electrical energy derived from the PV system will be utilized for the
normal household electrical requirements including general appliances, lighting,
cooking, hot water heating and heat pump operation. When PV generated energy
exceeds the house requirements excess energy is directed back to the utility grid.
Design Criteria
The intent of the detailed design effort is to develop designs of residential
photovoltaic systems which can be used as reference designs in regard to the
1-1
; : ; .
GENERAL _ ELECTRIC .~"c. PI"'.IO"
Detailed Residential P. V. System Preferred Designs Sandia Lab. Contract Doc.#13-8779
Johnson & Stover, Inc. Electrical Engineers 127 Taunton St. Middleborough, Mass. 02346 617-947-8464
P.V.tmY
" rnalSst Si!gfl ArctlitKtI Md pt.,,... lne.
138 MI. AUburn 51./ Cambridge, M.u. 02138 1 617-491.096 1
. -_. - - i
__ fi~L __ .L
Southwest All-Electric House With P. V. Only Job No. 9515 Revised
I '.-
~IIE PlJ\N
Scale reer MM •• W'
Figure 1-2. Site Plan
Sheet No.
j\ill
1-3/1-4
installation details, typical equipment requirements and system performance
estimates. The level of design detail is sufficient to allow independent cost
estimates for installation of the system. Specifications for equipment are
based on currently available components or similar currently available equip
ment if the component is not available at present.
In general, the design considers the homeowner as the system user and therefor
operation and maintenance requirements typical of conventional HVAC systems were
developed. The design does not include any instrumentation, since it is assumed
a mature system design.
To evaluate the performance of the system design, typical hourly electrical
load profiles and space conditioning demands for the hypothetical house are used.
These load profiles were developed in the GE Regional Residential Study, Ref
erence 1 and updated in the Residential Load Center Program, Reference 2. All
the system performance analyses are completed based on these hourly loads and
using Typical Meteorological Year (TMY) weather data as developed by Sandia
Laboratories. Life cycle cost analyses based on system performance results and
system cost estimates were used for system sizing and tradeoffs.
Project Team
The project team for the design effort consisted of the participants listed
in Figure 1-3. General Electric Company, Advanced Energy Department, was the
prime contractor with responsibility for system design and integration and pro
ject management. Massdesign Architects and Planners provided the details of the
house design and analysis support related to the solar array installation.
Johnson and Stover, Inc., provided the installation drawings for the electrical
equipment associated with the photovoltaic system. Support
1 ·5
I O"l
JOHNSON & STOVER, INC.
ELECTRICAL ENGINEERS
-EQUIPMENT SPECIFICATION
-ELECTRICAL INSTALLATION DRAWINGS
I--
-PROJECT MANAGEMENT
GENERAL ELECTRIC COMPANY
ADVANCED ENERGY PROGRAMS
SPACE DIVISION
-SOLAR SYSTEM DEFINITION
-PERFORMANCE ESTIMATES
-MAJOR EQUIPMENT SPECIFICATIONS
MASS DESIGN ARCHITECTS AND PLANNERS
CAMBRIDGE, MASSACHUSETTS
-ARCHITECTURAL DRAWINGS
-THERMAL INSTALLATION DRAWINGS
-DOCUMENTATION
GENERAL ELECTRIC COMPANY
CORPORATE RESEARCH AND DEVELOPMENT
-INVERTER DESIGN REQUIREMENTS
FIGURE 1-3. PROJECT TEAM
in the area of inverter integration was provided by the GE Corporate Research
and Development Laboratory.
Report Format
The report is organized to present the design details in the main section of
the report and all background data and material in the Appendix. Section 2 is a -
concise summary section presenting the design in bullet form and discussing key
lessons learned from the design. All of the design details are presented in
basically the next three sections: Section 3 presents the System Description;
Section 4 presents the residential house design; and Section 5, the Subsystem
Specifications. The latter section includes most of the electrical installation
drawings and details and discusses the two primary sUbsystems; the array and
the power conversion subsystem. The last main section of the report presents
system design alternatives which can impact the photovoltaic system design. These
options include extending the Southwest design to the Northeast and Southeast
regions of the country; reviewing modifications to the system design for smaller
sized systems; and design alternatives which can change the system load such as solar
thermal hot water systems, hot water recovery units and ground source heat pumps.
Finally, the Appendix includes design details such as the PCS specification,
array installation manual, and electrical system installation specification. Back
ground material is also included on the Shingle module construction, simulation and
economic models, cost assumptions and input data utilized in the design tradeoffs
and several backup design tradeoffs and parametric variations.
1-7/8
SECTION 2
SUMMARY
This section presents a brief overview of the key system design elements and
a synopsis of key design tradeoffs. The details of each of the topic areas are
discussed more thoroughly in subsequent sections of the report.
A simple, utility feedback, photovoltaic system was designed for a single
story all electric house in the Southwest. The two major system elements are the
photovoltaic array and the power conversion subsystem. The system was sized
nominally at 8 kWp based on minimum life cycle costs but key parameters were evalua
ted parametrically and changes in some of the assumptions were also evaluated.
For example, sellback rates were varied parametrically indicating at higher sell
back rates (greater than 50%) all of the available roof area should be utilized for
the array, resulting in the 8 kW sized system. As the sell back rates decrease to
30% or less, a 5 kW system size becomes more appropriate. Assumptions of fixed
and variable system capital costs also affect system sizing, implying smaller system
sizes as fixed costs are lowered. Since the system design is applicable to all
regions, its performance was evaluated in Boston and Miami. The effects of vary
ing electrical load level on the system economics resulted in cost-to-benefit
ratio insensitivity to load level at sellback rates above 50%. Therefore, system
size would not be affected by slight variations in load levels. Profile changes
and dramatic changes in the load would affect system sizing.
In addition, concerns of the appropriateness of all electric residences were
also addressed from the standpoint of various options for supplying hot water
energy requirements. Solar thermal systems and heat recovery systems for supplying
2-1
hot water requirements ultimately affect the house electrical requirements. These
options will be evaluated in detail in subsequent designs.
Design Considerations
The design effort has identified several key considerations for residential
photovoltaic systems. The first consideration is the constraints on system de
sign imposed by the physical size of the roof. If modules are connected in series
up the slant height of the roof, the system operating voltage is limited to, at
a minimum, multiples of the slant height. This implies that, for different roof
sizes, each system would result in a different operating voltage and a transformer
would be required to match house loads. The addition of a transformer to match
the loads appears more practical and less costly than redesigning the inverter
package for each voltage input. Similar concerns result if the modules are con
nected in series along the roof length.
The second design consideration evaluated was the voltage range input to the
PCS. The voltage range and the shape of the integral distribution curve input to
the inverter is very dependent on the specific site due to the different module
operating temperatures at each site. Curves of integral max power point voltage
versus annual energy curves are included in this report for Albuquerque, Phoenix,
Boston and Miami. In addition, the amount of energy generated at or below the
NQCT rated voltage max power point, represents a small percentage of the total
energy generated. In fact, in Boston, no system output is generated at the NOCT
rated voltage. This implies that the NOCT voltage does not necessarily represent
nominal system operating voltage. Actual system peak power values over the course of
the year were also noted as higher than the NOCT rated value.
2-.2
The third key result identified is that the voltage range input to the in
verter can be reduced without significant losses of annual energy performance.
This result allows a narrow input range to the inverter and thus, reduces inverter
losses. Limiting the voltage input range to the inverter can be accomplished
by driving the array off the max power point beyond the set limits.
Summary System Design
The following pages summarize the system design in a concise bUllet format.
2-3
HOUSE DESCRIPTION
•
•
•
The house design is for a SINGLE-STORY residence of NEW CONSTRUCTION for the Southwest ~egion of the country .
The design includes PASSIVE SOLAR FEATURES and ENERGY CONSERVATION FEATURES projected for 1986 .
There is 149 m2 (1600 ft2) living area with 104 m2 tl120 ft2) south facing roof area .
• The house is ALL ELECTRIC with a 3- ton heat pump and electric hot water heater.
• The site layout has a detached garage with a lot area between 1/6 and 1/4 acre .
26' r J--------------~- I : ' I - - - --- - - ----- ----r- ] I MASTER I BEDROOM :
24'
I FAMIL V I I Romi I II
I I
I -- I
i :~:,"~,~ "-~L_jr - I I DINING I I I I BEDRODf: #2 ;I~~~-~£';1------=""'====-=-~---------- i
r I I ~:~~NATE D WOOD RAlSE Il". CE ILING :
I I I I I I L ____ _________ _ _____ _ __ , I
I , , I
~ - - - ------------- __ _ __ _ 1
2-4
0> N
SYSTEM DESCRIPTION
• The system is grid connected with an 8 kW NOCTarray rating using aGE BLOCK IV SHIN~LE MODULE ARRAY. The array consists of a total of 475 MODULES COVERING 93 m in a redundant parallel-series network.
• The power conversion subsystem uses_a 10 kVA LINE COMMUTATED MAX POWER TRACKING INVERTER to convert dc generated power to ac . A 15 kVA SINGLE PHASE ISOLATION TRANSFORMER is used to match ac supply voltage to the load.
• The system operation is PARALLEL AND SYNCHRONIZED WITH THE UTILITY.
• Excess generated power is FEDBACK to the utility.
• The system represents the SIMPLIEST PHOTOVOLTAIC DESIGN with a minimum of components and controls.
PV ARRAY
- ROOF MOUNTED SHINGLE 93m2
2- 5
UTILITY BACKUP
DC/AC INVERTER
MAX POWER TRACK
GENERAL LOADS
HEAT PUMP
HOT WATER
SYSTEr1 OPERATION
I The system has automatic startup and shutdown control.
I The system automatically shuts down with loss of the utility,
I System operation is summarized by the sequence below.
1. At sunrise in the automati c lIonll mode, the ac and dc contactors wi 11 close when the array bus voltage reaches a threshold of 180 Vdc.
2. During the daylight period the inverter will continue to operate as long as there is a net power output.
3. The inverter will track the maximum power operating point within ~l percent over the range 180 to 220 Vdc.
4. The interruption of utility-supplied power will cause the dc contactor to open and remain open until line voltage is restored.
5. At sunset, the inverter ac and dc contactors will open when the net power output falls to zero. These contactors will remain open throughout the night to eliminate the majority of the inverter parasitic losses.
UTILITY IERVICE-3WIRE. SINGLE PHASE. 240/120 VAC
ROOF ARRAY
258 x 19P SHINGLE
WALL
~-POWER CONVERstON SYSTEM S[P.VICE PJIIIEl
2-6
PHOTOVOLTAIC ARRAY
I The array consists of shingle PV modules connected in a 25 series by 19 parallel network covering 93 m2 of roof area.
I The array is oriented due south with a roof pitch of 260. The overall cell
packing efficiency is 76.3% over the 93 m2,',"V'
I The modul es are di rect mounted on top of the roofing felt and plywood roof sheathing. They form a weather tight roof.
• The modules are installed by an overlapping procedure similar to conventional shingles. Four electrical interconnections are made with flathead machine screws per module and two roofing nails are used per module for attachment to the roof.
ROOF EDGE
FINISHING ROW OF DUMMY EDGE
SHINGLES
POSITIVE TERMINATION SHINGLES
TWENTY FIFTH COURSE "SCM" SHINGLES TWENTY FOURTH COURSE "SCM" SHINGLES
THIRD COURSE "SCW SHINGLES SECOND COURSE "SCW SHINGLES
TWENTY THIRD COURSE "SCM" SHINGLES
STARTER COURSE
2-7
TYPICAL EDGE SHINGLE
PHOTOVOLTAIC MODULES
• The module is currently under development by General Electric Company as part of the JPL Block IV procurement.
• The module uses 19 ARCO-SOLAR 100 mm cells with an unencapsulated efficiency of 12.3% connected in a series circuit.
• For a NOCT of 640 C, the maximum power output is 17.14 Watts and 7.3 Volts at SOC conditions (1 kW/m2. 20°C ambient. 1 m/s wind speed).
• A summary of module characteristics include:
• •
Module weight: 3.85 kg
Total cell area: .1492 m2
'2 • :Exposed modul e a rea: 0. 1955 m
• Module packing factor: 0.763
2-8
POWER CONVERSION SUBSYSTEM
• The PCS provides the interface between the PV array and the normal residential utility service and loads.
• The subsystem consists of three main components: the inverter, the dc filter and transformer along with the associated control circuitry.
• The subsystem is sized for 10 kW of power output with a 15 kVA transformer sized to accommodate the out 0 f phase ac vo ltage and current.
• The subsystem characteristics are summarized in the Table below.
• The subsystem can be obtained from the Gemini Corporation, marketed by Windworks, Inc.
KEY INVERTER DESIGN CHARACTERISTICS
OUTPUT POWER RATING
OUTPUT VOLTAGE:
IN PUT VOLTAGE:
FULL LOAD POWER FACTOR:
FULL LOAD EFFICIENCY:
FULL LOAD HARMONIC DISTORTION
OPERATING TEMPERATURE
10 kW CONTINUOUS
240 VAC Utility Residential Service
200 + 20 Vdc
60% Minimum
92% r1inimum
30% Maximum
00 to 400 C
POWER CONVERSION SYSTEM
2-9
PV INTERFACE WITH UTILITY AND HOUSE SERVICE
• Interface arrangements employ conventional wlrlng runs and equipment as much as possible to facilitate acceptance by local regulatory authorities.
• The PV array source treated as a conventional utility service entrance to residence with the raceway parallel to utility line.
• An external disconnect switch provides an external break. By strict code interpretation, this switch may be eliminated, especially as PV installations increase.
• An equipment room of 66 ft2 floor area is located on the west end of the house. The equipment rooms will also house the heat pump and electric hot water heater and can be used for extra storage.
• All PV related equipment is wall mounted except for the transformer with electrical connection to the household service panel.
POSITIVE BUS BAR PENETRATION
,------STANDARD WEATHERHEAD :r-----~AIN LOOP
I I I
I STANDARD ENTRANCE t----1 FOR UTILITY : SERVICE I I I I I I I
~ :
/ DISCONNECT fol
~'~ L-_SW_I_TC_H ____ ~'~'--~ I I
: 'METER SOCKET : BY UTILITY CO •
•
SOLAR COLLECTING ARRAY
--WJ~~~-:---1 NEGATIVE BUS BAR PEN ETRA TI ON
i'
-------------------------~
2-10
ARRAY SIZING
•
• • •
Energy sell back price the utility is willing to pay the homeowner affects system sizing.
Sellback rates greater than 50% imply full roof arrays (93m2)
Lower sell back rates (-- 30%) imply roof arrays of 50m2
At higher sellback rates, the economics are insensitive to load level"
o I-0( ~
IiL w z w OJ
o l-IIII o U
ALBUQUERQUE
SELL BACK ~ PS/PE
r-DESIGN POINT
I
0 i-
~ !:: u. w z w OJ
0 l-I-III 0 U
PHOENIX
1.6
1.4 SELL BACK ~ PS/PE
0.7
~DESIGN I POINT
o O~~2~O~~40~~6~0--~80~~
o O·~~2~0~4~~r-~~~o
COLLECTOR AREA, M2 COLLECTOR AREA, M2
2.0
1.8
1.6
!2 I- I.I! < II::
l- 1.2 u:: w
1.0 z W III
0 0.8 t-t-In 0.6 0 U
0.11
0.2
0
0
PHOENIX
2 Array Area = 92.9 m
SELL BACK
_____ M!.!..Q PS/PE
====;;;;;;;~~:: 0.3 0.5
0.7
"0.5 1.0 1.5 ACTUAL LOAD/NOMINAL LOAD
2-11
DESIGN PERFORMANCE
• Net system output for both Albuquerque and Phoenix is greater than the total electrical load.
• Phoenix shows better load matching characteristics
• Overall system efficiency based on incident insolation for qross array area is withi n the 8% to 8.4% range for tile Southwes t.
2.0
1.5
:z:: 3: 2 >-C!1 a: 1.0 w z w
0.5
o
PHOENIX
UTILITY MAKEUp· 8.7 MWH UTILITY FEEDBACK -10.3 MWH % OF LOAD SUPPLIED
DIRECTLY -45%
JFMAMJJASOND
2-12
2.0
1.5
0.5
ALBUQUERQUE
NET FROM UTILITY
PV SYSTEM OUTPUT 18.3 MWH
LOAD 14.8 MWH
UTI L1TY MAKEUP -9 MWH UTILITY FEEDBACK -12.6 MWH % OF LOAD SUPPLIED
DIRECTL Y -39%
J FMAMJJASOND
SECTION 3
SYSTEM DESCRIPTION
Functional Description
System
The grid connected residential photovoltaic system for the Southwest is
designed to meet both space conditioning requirements and all conventional
electrical load requirements for an all electric residence. The system is
comprised of two major subsystems, the solar array and the power conditioning
subsystem (PCS). Figure 3-1 shows a system block diagram. An 8kW peak photo-
voltaic array has been designed for the house. 2 The 93 m solar array uses a
shingle solar cell module, being developed by General Electric as part of the
JPL Block IV procurement, in a highly redundant series/parallel matrix. The
photovoltaic generated power is supplied to a 10 kVA power conversion subsystem
which is controlled to track the solar array maxi~u~ rower operating point and
feed the 240 Vac output power directly to the house loads or back to the utility
when excess power is generated. The PV power is isolated from the utility by a
15 kVA transformer.
The overall system is connected in parallel with the utility service to supply
the residential load. Power generated by the array in excess of residential
loads will be fedback into the utility grid. Residential loads in excess of the
array power source will draw power from the utility grid. With this arrangement,
all house load demands are met, no electrical storage is required, and all net
energy output of the photovoltaic system is utilized. The utility is expected
to supply the meters to measure the utility energy requirements and to measure
the feedback energy to the utility since they already do normally supply resi-
3-1
W I
N
PVARRAY
- ROOF MOUNTED SHINGLE 93m2
UTILITY BACKUP
DC/AC INVERTER
MAX POWER TRACK
GENERAL LOAD~
HEAT PUMP
HOT WATER
dential metering. Protection from lightning-induced voltage transients is
provided by varistors connected between the negative dc line and ground and be
tween the ac lines and ground at the utility service entry. The remainder of
the electrical equipment consists of switches and circuit breakers to isolate
the inverter on both the ac and the dc sides and an exterior disconnect switch
to meet interpreted code requirements. System operation is described below:
1. At sunrise in the automatic "on" mode. the ac and dc contactors will close
when the array bus voltage reaches a threshold of 180 Vdc.
2. During the daylight period the inverter will continue to operate as long
as there is a net power output.
3. The inverter will track the maximum power operating point within +1 percent
over the range 180 to 220 Vdc.
4. The interruption of utility-supplied power will cause the dc contractor to
open and remain open until line voltage is restored.
5. At sunset, the inverter ac and dc contactors will open when the net power
output falls to zero. These contactors will remain open throughout the
night to eliminate the majority of the inverter parasitic losses.
The system represents the simplest photovoltaic design with a minimum of
components and controls. A key to the implementation of this design is the
acceptance of the feedback energy by the utility and the price the utility
will pay to the homeowner for this energy.
Due to economic uncertainties, both in system costs and the price the
utility is willing to pay for feedback energy, a system smaller than the 8kW
nominal design may be more economical. Therefore, design modifications for a
3-3
reduced size system were also developed. Functionally, the system operation
remains the same as described above, however, the roof array is reduced to 48.9 m2
with a 4.3 kW NOCT array peak power. The power conversion subsystem specifica
tion would also be reduced to 5 kVA, if available. Standard sizing of inverters
may preclude the availability of specifically sized systems and an additional
evaluation would be required in a design effort, to select the standard size.
Components
A one ~ine diagram of the system components is shown in Figure 3-2. The
major functional elements of the system are briefly described below with further
details contained in Section 5.
Solar Array-- The solar array consists of 475 shingle modules connected electrical
ly in a highly redundant series/parallel circuit arrangement with 25 modules in
series and 19 parallel circuits. The module electrical circuit terminates in
positive and negative busbars which are connected to cabling, run in conduit
to the equipment room. The negative busbar is grounded. The array output is
8 kW under NOCT conditions.
Inverter -- The inverter is a Silicon Control Rectifier (SCR) thyristor bridge
circuit providing unidirectional current flow on the input dc side and alternat
ing current flow on the output ac side. It is sized at 10 kVA and is line commu
tated.
Transformer -- The 15 kVA transformer provides both isolation of the ac and dc
circuits and matching of the normal ac line voltage to the output dc voltage of
the array.
3-4
(.0 I ~,
UTILITY SERVICE-3 WIRE, SINGLE PHASE, 240/120 VAC
ROOF ARRAY
25S x 19P SHINGLE
~~~~~Y WiL > Or-~~t------
WALL
SWl
FIGURE 3-2.
POWER CONVERSION SYSTEM SEP.VICE pfl.'in
- ?) ;:~: .. I -I C1 o~-.---.:=--.
1 m<~ ) BRANCH J CIRCUIT
Residential Photovoltaic One Line Diagram
dc Filter This filter smooths the dc current flow which is subject to high
harmonics as a result of the switching action of the thyristor valves.
Inverter Controls -- These controls provide the timing signals for firing of the
thyristor valves and, in turn, control the level and direction of power flow
through the power conversion system.
Maximum Power Control -- This control circuit modifies the inverter timing control
circuit in a manner to operate the array at its dynamic maximum power point.
RFI Filter -- This filter attenuates high frequency output harmonics to minimize
radio and TV interference.
Input dc Contactor-- This contactor closes only when the ac Line Contactor is
closed.
Output ac Contactor -- This contactor closes only when the array available output
power is greater than the Power Conversion System no-load losses.
Other Electrical Interconnection Items
Exterior Array Fused Switch -- This switch provides a visible exterior discon
nect, and is fused only if required by a strict code interpretation, since array
short circuit current is only slightly over full load current.
Varistors -- Induced lightning transient protection for the inverter and house
hold equipment is provided by the varistors on the dc and ac residence input
lines.
3-5
Interior Disconnects--Isolation of the Power Conversion System for installa
tion and service is provided by the interior dc switch and the circuit breaker
in the service panel. The circuit breaker also protects the main service from
short circuits in the inverter array system.
System Design Requirements
A review of the environmental conditions applicable to the Southwest region
of the country as encompassed by the Albuquerque/Phoenix climates, the goals for
the implementation of residential PV systems in 1986, and the specific constraints
associated with residential house designs have lead to the broad system design
requirements listed in Table 3-1. The environmental conditions listed are
typical of the Southwest region. The module design temperatures are dictated
by the JPL defined thermal cycle test requirement of -40 to +900 C. The lower
limit is obviously not applicable for the Southwest but from the standpoint of
widest market penetration across the U.S., it is retained.
The UL997 standard IIWind Resistance of Prepared Roof Covering Materials ll
requires shingles to survive a 26.8 m/s wind speed. A review of the wind data
over a 23 year period for Phoenix and Albuquerque shows the recorded extremes
were 33 m/s for Phoenix and 40 m/s for Albuquerque which is higher than the UL
requirement. The occurrences of wind gusts in the 26.8 m/s range, however,
is minimal. As a matter of fact, the lowest measurable range of wind gusts
occurrences for Phoenix (11-13.9 m/s range) occurred less than 1% of the time,
while in Albuquerque, the winds fall into this range 2% of the time. Therefore,
a design requirement consistent with the UL997 appears acceptable, particularly
when larger market requirements are considered.
3-7
TABLE 3-1
System Design Requirements
Environmental Conditions Phoenix ·Albuguergue
Ambient temperature: -23 to 470 C -27 to 40.60 C Module Thermal Cycle Test: - - -40 to 900 C - - -Wind:
Extremes % of Time Between (11-13.9 m/s)
Moderate Lightning Area: Isokeraunic Level
Ha i 1 : Annual Horizontal InsolatiJn:
Application Reguirements
• Electrical Load Summary:
33 m/s <1%
26
Low 2156 kWh/m2
Average Daily Base Electrical Load 19.1 kWh/day
- Average Daily Hot Water Load 10.6 kWh/day
- Average Daily Heat Pump Load 13.9 kWh/day Annual Electrical Load 15937 kWh
• Load: Single Phase 240/120 Vac
• Life: 20 Year Design Life • Meet Local Building and Electrical Codes • ~1eet Local Fire District Regulations • Site Constraints (General Considerations)
Roof Area Available Roof Slope
House Orientation
- Sun Rights
• System Operational Constraints Disconnect Photovoltaic System if Utility is Interrupted
Operate in Parallel and Synchronized with the Utility
3-8
40 m/s
2%
47
- -2119 kWh/m2
19.1 kWh/day
12.6 kWh/day
8.9 kWh/day 14766 kWh
The lightning environment is deemed moderate for the region of interest and
imposes only nominal requirements on the design. The risk of hailstone damage in
the Southwest is also relatively moderate. The tempered glass cover of the shingle
module has successfully passed initial hailstone tests conducted by JPL and there-
fore, the array design should meet the nominal requirements.
The table also lists the average daily load requirements for the two regions.
The heat pump average load requirements are based on the annual electrical input
to the heat pump required to satisfy both the space heating and cooling requirements.
Since no specific site is considered for the installation,no specific local building,
fire or electrical codes are imposed. Only the general Model Code considerations
appropriate for the Southwest were used in the design. The overall electrical
design tries to assure safety in the normal residential application.
The house is a new construction, and therefore, the site constraints also did
not impose any significant design requirements. The house orientation was selected
at due south; the roof slope was selected at a 260 (2 to 1 pitch) consistent with
standard framing member sizes; and the roof area was specified consistent with
the aesthic features of the house design at 104 m2. No landscape shadowing or
surrounding building shadowing was imposed on the design.
System operating constraints require utility grid connection with the PV
system operating in parallel and synchronized with the utility. If utility
interruption occurs, the PV system will disconnect.
3-3
Performance Characteristics
The residential photovoltaic system design has a calculated system performance
for Phoenix and Albuquerque as summarized in Table 3-2. The solar array consists
of 475 shingle modules in a 25 series by 19 parallel matrix. Each module has 10
100mm diameter cells for a total nominal solar cell area of 70.9 m2. The exposed 2 glass coverplate area of shingles accounts for 92.9 m of module area which results
in a 0.763 packing efficiency. The gross roof area, which is 16.0 m wide by 6.5 m
high including all shingle edge members, amounts to 104.3 m2 and yields an overall
roof packing efficiency of 68%. The rated output of the solar array at Standard
Operating Conditions (including an NOCT of 64 0 C) is calculated to be 8 kW. On an
annual basis, the ac energy output for Phoenix is 17455 kWh for a total insolation
of 2347.6 kWh/m2 on the 26 0 sloped roof. Thus, the overall system conversion ef-
ficiency is 8%. For Albuquerque, the corresponding system efficiency is 8.4% with
18336 kWh ac energy output with an insolation level of 2350 kWh/m2. For both loca-
tions the solar array conversion efficiency runs between 9.1% in Phoenix to 9.5%
in Albuquerque with less than 1% annual energy loss due to the series resistance
losses in the shingle module bus strips, the termination bus bars, and the cabling
between the bus bars and the inverter. The annual Power Conversion Subsystem ef-
ficiency amounts to approximately 88% (88.3% in Phoenix and 88.7% in Albuquerque)
resulting in the overall system efficiencies noted.
The I-V characteristics of the array at reference conditions are summarized on
Figure 3-3 .. The NOCT array output is 8 kWp at 183 volts and 43.7 amperes.
3-10
TABLE 3-2
SUMMARY OF SYSTEM PERFORMANCE
PARAMETER
NUMBER OF MODULES (25 SERIES X 19 PARALLEL)
TOTAL SOLAR CELL AREA (M2)
TOTAL EXPOSED r10DULE AREA CM2)
TOTAL GROSS ROOF AREA CM2)
ARRAY OUTPUT AT SOC NOCT = 64°C (kW PEAK)
~NNUAL DC ENERGY INPUT TO INVERTER (kWh)
ANNUAL AC ELECTRICAL ENERGY OUTPUT (kWh)
ANNUAL INSOLATION ON ARRAY SURFACE (kWh/M2)
SYSTEr1 OUTPUT OVERALL SYSTEM EFFICIENCY INSOLATION X ARRAY AREA
3-11
VALUE
475
70.9
92.9
104.3
8.03
PHOENIX
19763
17455
2347.6
ALBUQUERQUE
20682
18336
2350
8% 8.4%
w CI) I 0:. ........
~ N « ..... ~ 2 w a: CC :J U
50 ~ => ...........
40
30
20
I
VMP =183 10
I I o 1. ______ .. _____ I I . _ ..... .1 .!
o 50 100 150 200
VOLT/,G£:, VOLTS
250
NOTES
1. ARRAY CONFIGURATION 25S BY 19P
2. VALUES REFLECT BUS. BAR AND CABLING LOSSES
3. INSOLATION = 100 mW/CM2
300
FIGURE 3-3 SOLAR ARRAY. I-V CHARACTERISTICS AT REFERENCE CONDITIONS
Design Tradeoffs
System performance analyses were conducted to provide an economic basis
for establishing collector array size for both the Phoenix and Albuquerque
houses. In addition, studies were conducted for the Phoenix house to determine
the sensitivity of collector tilt (roof slope) and load variations on performance.
The models and input data used for all of these analyses are discussed in Ap
pendi x D.
Array Tilt Sensitivity
The sensitivity of collector tilt or roof slope angle to net system output
was investigated with the results as shown in Figure 3-4. For this analysis, the
array consisted of 25 series collectors by 19 parallel circuits for a collector
area of 92.9 m2. The results indicate that the system output is maximized with
a roof slope of 26 0 which tends to confirm the trend established in earlier stud
ies, that the optimum tilt angle for PV arrays serving residential electric loads
would be about 10 degrees less than the latitude for the Southwest. The actual
design slope was selected as 26.60, which conforms to the use of standard framing
techniques (8 11 vertical 16 11 horizontal). It is also evident that only small
differences in net system output occurs over the range of 20 to 300 tilt.
Collector Array Sizing
Array sizing was evaluated for both the Phoenix and Albuquerque houses. Since
the PV system output is proportional to the array size, and the size of the array
impacts the system cost, the effect of array size is best evaluated in terms of
economic factors, such as levelized annual cost-to-benefit ratio. The array size
3-13
PHOENIX ARRAY AREA = 92.9 m2
17.5
~ s: 17.0 :i!:. r-' :::> 0-r-:::> 0 :i!: 16.5 ~ DESIGN w
POINT r-ei)
26° >-eI)
r-w z
16.0
o o 20 25 30 35 40
ROOF SLOPE, DEG.
FIGURE 3-4. Array Tilt Angle Sensitivity
was varied by maintaining 25 series modules along the roof yielding the same
system operating voltage but varying the number of parallel circuits. The
resulting area variation was from approximately 20 m2 to 93 m2 which is the
maximum array area available. Using the economic assumptions discussed in
Appendix E, the cost-to-benefit ratio was calculated for each system at three
different utility sellback energy prices. The results are shown in Figure 3-5.
The trends are similar for both Phoenix and Albuquerque. With the range of
this evaluation, the optimum array size varies as the sell back to buy price
ratio.
For sellback to buy energy price ratios of 50% and above, the optimum array
2 area is the maximum allowable for the roof of 92.9 m , since the cost to benefit
ratio continues to decrease. If the sellback ratio is only 30% of the buy rate,
3-14
ALBUQUERQUE PHOENIX
SELL BACK RATIO PS/PE 1.6
o 1.4 0 -
1.2~ SELL l- I 0.3 < ...
\ BACK a: 1.2 < RATIO PS/PE w CI: I l- .. ....... I-Ul iL 1.0 -W lL
o:at I w z z "1 0• 3 ~ 0.8 w
Dl 0.5 0 r-0ESIGN 0 I- 0.6 POINT I- 0.6 0.7 l- I l-ll) 0 I
lI)
u 0 u·'1 I-.-OESIGN u I POINT
o I I u·""1 I I I I lAo
0 0 20 40 60 80 0 20 40 60-SlJ-------nf0
COLLECTOR AREA, M2 COLLECTOR AREA, M2
FIGURE 3-5. Cost-To-Benefit Ratio as a Function of Collector Area
o the optimum array area is reduced to approximately 50 m~. As the
array area is decreased, the amount of energy sold back to the utility decreases
to zero and thus, all the curves tend to single cost-to-benefit ratio.
It is apparent that the energy sell back price the utility is willing to pay
the homeowner effects the optimum system size. Ratios of 50% appear feasible so . 2
the design area was selected at 92.9 m. This array area also presents the most
challenging roof layout. To complete the design effort, however, design modifi
cations were also developed for a reduced array area of 48.9 m2, corresponding to
system size for utility feedback rates below 50% of the buy rate. Appendix G,
Design Tradeoffs, discusses additional system cost assumption variations which
also can effect the system size selection. Designing the system with a full roof
array and considering the design modifications for a reduced array size cover the
spectrum of system sizes which may result due to economic un~ertainties. The
design modification , for a reduced system size is discussed in Section 6
System Design Alternatives.
Load Sensitivity
The system performance evaluations were based on average residential loads
as discussed in Appendix D. Variations in electrical loads from these average
values (exclusive of space conditioning loads), reflecting a range of energy
conservation practice within the home, were examined and their effect on the cost
to benefit ratio evaluated. The space conditioning loads were not varied since
the design of the houses represented tight, well insulated construction with
thermostats set at relatively low settings. It is important to note that only
~-16
the electrical load levels were varied while the load profile remained unchanged.
The results of this variation are shown in Figure 3-6. The trend shown in the
figure implies that energy conservation is penalized with higher cost to benefit
ratio, particularly at low sell back rates. This is a direct result of the dis
tribution of the PV energy used directly in the house and that portion sold
back to the utility since the net system output is the same for all the cases.
As the load increases, more PV energy can be directed to the load and less is
sold back. The energy used directly provides a greater benefit than being sold
to the utility at a reduced rate. As the load is reduced, the reverse is true,
less PV energy is used directly in the house and more is sold back. It is also
noted in the figure that as the sellback price increases the effect of load level
on the cost-to-benefit ratio is negligible. At 70% sellback rates, the economic
performance is insensitive to the load. This implies that at those sellback rates
it makes no difference if the generated load is utilized directly in the house
or sold back to the utility. Even at 50% sellback, only a small change in cost-tc
benefit ratio over the load variation range results. Additional general discussio
on load matching and system sizing are presented in Reference 12.
For the present analysis, with the nominal design load profile, 55% of the
system output exceeds the immediate load demand and is sold back to the utility.
When the actual load is reduced to 50% of nominal, 69% of the system output is
sold back; whereas, when the load is increased to 150% of nominal, only 46% is
sold back to the utility. Closer matching of PV power generation and load pro
files would result in more of the load being supplied directly by the PV system
and less energy being supplied by the utility.
3-17
2.0
1.8
1.6 o !< 1.4 0::
I- 1.2 u. LU Z LU al
o 0.8 l-I-~ 0.6 u
0.2
o o
PHOENIX
Array Area 2 = 92.9 m
SELL BACK RATIO PS/PE -------=========-=::. -;;-;;-~::_ 0.3 0.5
0.7
0.5 1.0 ACTUAL LOAD/NOMINAL LOAD
1.5
FIGURE 3- 6. Cost- To-Benefi t Ratio as a Function of El ectri c Load
Design Performance
The monthly performance of the nominal design PV system is shown in Figure
3-7 for Phoenix and Albuquerque for the design collector area of 92.9 m2. Both
the monthly load and PV system output is shown. When the total PV system output
is greater than the load for that month, there is a net flow of energy to the
utility. Actually, there is a flow to and from the utility each day but the net
over the month is shown on the figure. Similarily, the net from the utility is
shown when the load is greater than the PV output. The percent of the total load
supplied directly by the system is also shown on the figure. The net system
output for both locations is greater than the total load. Notice, also, that
despite the higher load and smaller system output in Phoenix (see Figure 3-n
3-18
J: ~ :E
the cost to benefit ratio from Figure 3-5 for the 92.9 m2 array area is more
attractive for Phoenix than for Albuquerque for the same sellback ratio. This
is due to the higher cost of electricity in Phoenix and because of the shape
of the load and system output curves of Figure 3- 7. The curves reflect closer
matching of system output and load, for Phoenix, thereby resulting in a greater
portion of the load supplied directly by the system.
PHOENIX
2.0
1.5 J: ~ :E
2.0
1.5
ALBUQUERQUE
NET FROM UTILITY
>' >'
" a: " a: UJ
1.0 z w
0.5
o
NETFROM J UTILITY
~------------------~ UTILITY MAKEUP· 8.7 MWH UTIliTY FEEDBACK ·10.3 MWH % OF LOAD SUPPLIED
DIRECTLY 45%
JFMAMJJASOND
~ 1.0 w
0.5
PV SYSTEM OUTPUT 18.3 MWH
J F M A M J
"'-- LOAD 14.8 MWH
UTILITY MAKEUP·9 MWH UTILITY FEEDBACK ·12.6 MWH % OF LOAD SUPPLIED
DIRECTLY ·39%
FIGURE 3-7. PV System Monthly Performance Summary
3-19/20
SECTION 4
HOUSE DESIGN CHARACTERISTICS
Design Features
To complete a detailed design of a photovoltaic power system, a hypothetical
house design was developed. Primary guidelines followed in the design were to
utilize energy conservation features and passive solar features. The house is
a single-family detached house on a separate lot, providing three bedrooms, two
baths, a kitchen, a living/dining room, a family room on one floor, totalling
1600 square foot in area planned for the southwest. By 1986 such a house will
be semi-luxury item; it is therefore, carefully designed and has a number of
features which would not be found in a more moderately priced house.
Given the relative reluctance of the building industry to make major changes
in the way it does business, it will be fortunate if by 1986, the standard energy
conserving house is as good as a moderately advanced house in 1979. On this as
sumption, six inch studs with R-19 insulation, R-25 insulation at the cathedral
ceiling, and R-38 insulation at the attic was used. The edge of the slab is
insulated and all east, west, and north windows are triple-glazed, with the
south windows double-glazed. Other energy conserving features consist of in
sulation outside the floor joists; insulation between the window headers; in
;ulation underneath the slab; and great pains taken to minimize infiltration.
"igure 4-1 shows several of the energy conservation features.
4-1
R-25 (8") F.G.
COMPOSITION SHINGLES--------~/
1/2" PLYWD--SHTHG _~
'<, ,i \-
GUTTER~~l :~~~ WOOD EXTER. ;1' . FINISH OVER . .t:::' :::::"":::>Ll-__ _
1 / 2" PLY 4---n1r-=--.t~--
2" URETHANE INSULATION-~~il
OF
~~~~~,~' 2 LAYERS R-19 : I: \ liJNiFIBERGLASS INSULAnm; in'\\.) '.)1 --;..--_. ,~;-
I :"....;.-!..L.~ LOWER CHORD OF p(n )( h tr0o~ TRUSS
ATTIC CONDITION
OUTSIDE JOISTS --~-2x10 JOISTS
~::r;:=======~
2" URETHAN·~~~=.l-
INSULATION BETWEEN HEADERS ---:ld-W~
WOOD SASH W/ DOUBLE OR ----Ii TRIPLE GL
INSULATED ~ METAL EXTER. -DOORS
2x6 STUDS ---+t+.~-J W/R-19 F.G. INSULATION
R-IO INSUL. OUTSIDE FOUNDATIONS (PROTECTED)
1/2" GYPSUM BD.
4" FLOOR SLAB
R-7 INSULATION BELOW SLAB
FI GURE 4-1 Typi ca 1 Wood Frame Construct; on Ener9Y r:ons~rvati on Features
4-2
Since this house is intended to go in the southwest, where heat requirements
vary dramatically between low and high elevations, the amount of south glazing
can be varied significantly between a house in Phoenix and a ~ouse in Albuquerque,
for instance. In Phoenix, a moderate amount of south ~lilzing. with generous
patio doors leading out onto the terrace would be an attractive design
feature; but a very wide overhang is required to protect the house from
solar gain in the summer. In Albuquerque, the glazing would be more wide-
spread, with sliding glass doors in the bedrooms. Since all but one of the
major rooms in the house communicates directly with the south, the house
would have excellent passive gain characteristics. Only the floor slab is.
designed as passive storage. Here again, there is a distinction between climates
like Phoenix and those like Albuquerque; in the latter case, additional mass
within the dwelling might prove cost-effective.
A major design requirement of the building is a continuous rectangular roof
area facing south at a tilt of latitute minus 100
, totally unshaded. to aCCClrll0date
the photovoltaic array of about 95 square meters (1000 square feet). Because of
this very large and potentially monotonous design feature, the house was built
on two levels, with an offset in plan between the living areas and the bedroom
areas. This creates a clerestory at the top of the living room, whic~ also func
tions to release hot air during the summer time periods. The offset also provides
an attractive entrance.
House Plans
Detailed plans for the house were developed for locating the PV system equip
ment and to detail cabling runs from the roof top power system. Figure 1-2 showed
4-3
the general site layout for the ~ouse.
The floor plan shown in Figure 4-2 has been worked out carefully to insure
that each bedroom can receive either a double bed or two single beds, along
with space for a large dresser, ample closet space, and plenty of room to
move around. The master bedroom, in addition, has a dressing room connecting
the bedroom to its private bath. The equipment room is located on an exterior
wall and since it is located close to the bedrooms, it features careful
sound-proofing, with ventilation provided through an acoustically baffled louvre.
In the living area, a small recirculating fireplace occurs in the middle of
the plan, with a chimney rising on the ~orth side of the roof. A large beam
runs down the middle of the living room to carry the very long roof joists
supporting the photovoltaic array. The dining alcove is set at the northeast
corner of the living room, while a separate family room near the entrance could
also be used as a dining space. An eating counter/bar separates the kitchen
from the family room, allowing the two to be formed into one large country
kitchen. It is very important in a small house to have at least two
acoustically isolated living spaces, to allow two different age groups within
a family to enjoy separate occupations at the same time.
The elevations shown in Figures 4-3 and 4-4 are straightforward, but feature
large overhangs, heavy rakeboards. and relatively heavy window trim, all of which
give a strong character to the house. A band of trim is provided at window level
on Figure 4-3 to separate the lower plywood sheets from the upper ones, and tie
the house together visually. The recessed and protected entry way on the north
elevation of Figure 4-4 has a skylight to provide plenty of light. A small
4-4
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4-7/4-8
retaining wall takes maximum advantage of the difference in elevation between
the two sections of the house. On the south roof, the projecting living room
provides an opportunity to break up the large photovoltaic array by the ad
dition of a small additional roof area with ordinary asphalt shingles as shown
on the south elevation of Figure 4-4.
In section, the living area and the bedroom areas are completely different as
shown in Figure 4-5. In the living area, a cathedral ceiling rises "to a high
ridge below which is a clerestory; while over t~E kitchen there is a hung
ceiling. In the bedroom wing, standard roof trusses are used to help hold
down the cost and simplify the termination of a large number of partitions
found in that area. Thermally, there will be a louvre connecting the upper
part of the living area with the attic space over the bedrooms, both of
which will be ventilated with a fan located on the west wall. In addition,
an attic vent helps to cool the uninsulated area over the kitchen. The
details relating to the photovoltaic system are relatively straightforward.
Figure 4-6 shows several details from the sectional views of Figure 4-5. At
the ridge and along the two ends of the roof aluminum flashing is used to cover
the edges of the photovoltaic array. Otherwise, the construction of the house
is not affected in any way by the existence of the photovoltaic cells (except
for the lack of a finished roof over the #15 building paper which is laid
over the plywood roof deck) .
Besides the northeastern windows being triple-glazed, there is a preference
system for the location of windows. North windows are absolutely minimized;
east and west windows are minimized, except at the expense of north windows;
for instance, in the master bedroom the major windows are on the west rather
4-9
than on the north. Finally, as many windows as possible have been moved to
the south wall, and the absolute amount of south glass is controlled to
respond to the particular climate conditions.
4-10
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4-15/4-16
Module
SECTION 5
SUBSYSTEM SPECIFICATIONS
Solar Array
The residential array design for this wouthwest application uses the GE
Block IV Shingle modules in a 25 series by 19 parallel circuit. The array covers
2 the 16.1 m by 6.5 m roof for a total active array area of 92.9 m. The array
IV characteristics was shown previously in Figure 3-3.
The solar array consists of 475 Shingle modules. Figure 5-1 shows the module
which has nineteen 100 mm diameter cells connected in a series. Table 5-1 presents
a summary of the design details and performance parameters for the module. The
shingle solar cell modules are designed to meet the requirements of JPLDocument
No. 5101-83 entitled, "Block IV Solar Cell Module Design and Test Specification for
Residential Applications". This specification established design and qualifica
tion test requirements which are summarized in Table 5-2. The electrical output
rating of these modules is specified under conditions which are called Standard
Operating Conditions (SOC) as defined by the combination of parameters listed in
Table 5-3. The solar cell temperature under these conditions is called The Nominal
Operation Cell Temperature (NOCT). The direct roof mounting of the solar cell
modules in this design will result in a NOCT of approximately 640 C. This value
is extrapolated from an NOCT measurement of 57 0 made by JPL on the shingle modules
tested as part of the PRDA-38, Phase I effort. The difference of 70 C between these
two values accounts for the change in definition of the insolation level for the 2 2 NOCT measurement from 80 mW/cm to 100 mW/cm .
While not specifically stated as a module design requirement, the intended
service life of the photovoltajc module installation should be in excess of 20
3-1
TABLE 5-1
Summary of Design Characteristics
fo~ the Block IV Shing1 ~ Modu1 es
Th1rdGeneration
PARAMETER (JPL 955401)
Solar Cell Diameter (mm) 100
Solar Cell Supplier Areo-Solar
Number of Solar Cells 19
Total Solar Cell Area (m2) 0.1492
Exposed Module Area (m2) 0.1955
Module Packing Factor 0.763
ftocr (~) M
Maximum Power Output 17.14 at SOC (watts)
Areal SP2cific Output 87.7 (W/m module area)
Module Weight (kg) 3.85
Areal Spe~ifie Weight 19.7 (kg/m module area)
Power-to-Weight Ratio 4.45 (W/kg) .
NOCT at 100 mW/cm2 insolation
years with a minimum of periodic maintenance. The construction details of the
module are included in Appendix F.
5-2
U1 I
W
32.20 _I ----- --- 26.40 ..
~... 10.20 ... J.4- 6.00
3.00
- 26.35 ---r
9.353
(-) t 3.50 T-
19.353
-1 _____ _ -I
MODULE WEIGHT 3.85 kg
TOTAL CELL AREA .1492 m2
EXPOSED MODULE AREA .1955 m2
MODULE PACKING FACTOR .763 DIMENSIONS IN INCHES
Fi gure 5-1.
ARCO-SOLAR 100mm CELLS 12.3%
Outline Dimension of the Third-Generation Block IV Shingle Module
TABLE 5-2
Summary of Block IV Residential Module Design
and Qualification Test Requirements
• THERMAL-CYCLING, 50 CYCLES, _400 TO 90°C • HUMIDITY CYCLING, 5 CYCLES, 23 TO 40.50 C, 90 TO 95% RH • MECHANICAL CYCLING, 10,000 CYCLES, ~ 1.7 kPa
(OR WIND RESISTANCE TEST PER UL 997)
• TWISTED MOUNTING SURFACE AT 20 mm/m • HAIL H1PACT 20 mm DIAMETER ICEBALLS AT 20.1 m/sec. TERMINAL VELOCITY • AT LEAST 1500 Vdc WITHSTANDING VOLTAGE BETWEEN CIRCUIT TERMINALS
AND EXPOSED CONDUCTIVE SURFACES • GROUNDING TERMINAL FOR EXPOSED CONDUCTIVE SURFACES
.--. , ... --.-.---------~---------.:.
TABLE 5-3
Definition of Standard Operating Conditions (SOC)
• INSOLATION = 100 mW/cm 2
• AIR TEMPERATURE = 20°C
• WIND SPEED = 1 m/s • MODULE OPEN-CIRCUITED • MOUNTING TYPICAL OF INSTALLATION • ORIENTATION NORMAL TO DIRECT BEAM RADIATION AT SOLAR NOON
----- -_ ...... _---_ ... _-_ ..... -._ ... __ ._ ..... -
Solar Array Installation
The General Electric Photovoltaic Shingle Module has been designed to serve
as a weather-tight element while acting to collect solar energy in electrical
5-4
form. As such, a PV array constructed with these shingles as building blocks
actually is the roof of the residence and displaces conventional asphalt shingles.
The shingle modules are installed by an overlapping procedure similar to conven
tional shingle installation. Appendix B provides step-by-step installation
instructions for the current design. The four electrical terminals of each module
are interconnected as described in Appendix F using a flat-head machine
screw to apply the contact force. The shingles are attached to the roof sheathing
by nailing through the substrate at two marked places per shingle with ordinary
roofing nails. The module-to-module interconnectors between overlapping layers
form a series/parallel matrix of interconnected modules, as shown in Figure 5-2.
with the current increasing as modules are added in the parallel direction across
the length of the roof from gable-to-gable, and voltage increasing as modules are
added in the series direction along the slant height of the roof from eave-to-ridge.
The negative terminations at the eave and the positive terminations at the ridge
for each solar cell circuit are attached to busbars running the length of the roof.
The details of the solar array installation on the southern residential roof
surface are shown in Figure 5-3, which is GE Drawing 47E254983. The 475 shingle
solar cell modules are electrically interconnected by integral conductors to produce
a single matrix of 25 series modules (or 400 series cells) by 19 parallel modules.
Four types of special dummy shingles (some dummy shingle types have different
dimensions) shown in Figure 5-4 are used at the roof edges to maintain the weather
tight roof surface and to provide the method for electrical termination at the
array negative and positive buses. Copper foil strips, which are contained within
these dummy termination shingles, are soldered to a copper busbar for both the
5-5
• -'
•
•
_--"-..... ---.---..--9--...... - ..... --+------POSITIVE BUS BAR
• • •
• • • • • • • • •
• • • .. • • • • •
• • •
• • •
MODULE-TO-MODULE INTERCONNECTOR
_--41_.-.-...... -__.------ NEGATIVE BUS BAR
Figure 5-2. Module Interconnection Electrical Schematic
positive (see Detail B of Figure 5-3) and the negative (see Detail C of Figure 5-3)
array terminals. These busbars, which are bonded to the plywood sheathing as shown
with Pliobond 20, conduct the current from the entire array to AWG #4/0 cables
which are run in conduit to the equipment room. Pliobond 20 is an adhesive product
of Goodyear Chemicals which has good bonding properties and insulating properties.
The busbsrs are bent and inserted through the roof rake as shown in Detail A of
Figure 5-3 and the connections made to the cable also as shown in Figure 5-3.
These connections are then covered with heat shrinkable insulation. The east and
west edges of the roof, as well as the top edge, are covered with an a 1 umi num
structural angle which is snapped into anchor clips which are nailed into the
5-6
17
LAST FOIL CONNECTION TO POSITIVE e,US BAR
----- 52'g" '-------1
'ID TYPICAL POSITIVE TERMINATION
DETAIL"!:>' '
5CALE'I/IO
180' TWIST ONL Y
BUS BAR ~T~Y~PTf~n-~--'--'--n~-----r-----n 12L'00~i= ,TO POSITIVE 'L 1"2.0
.38 ~ .250 DIA +------r-----'~~---.L..--___r_----..:=..,r__-------l '-41--.31 DETAil ''A'
SCALE:I/2
MAKE FROM a542N 110 PI2'-,------" (lOFT LTHS) ,
COVER WITH FLEXSEAL '" I WHITE HYPAlON 5UPPORTED'~ WITH G'XG"POLYE'STER SCRIM
TERMINATION OF BUS BAR.s
SCALE: 1/1
NEGATIVE BUS bAR
a
INVERTED ITEM 5 (REF)
E5TAIOU5HED ,,===*,.
BUS BAR MAKE FROM PI4 MA,'L:COPPER ALY~I2.2
.11.S1HK x.625 WIDE (12. FT lEN"THS)
PLAN VIEW SCALE, 1/10
RIDGE PLATE e Cli PS NOT SHOWN
NEGATIVE CONNECTION
'TERMINAL SCREW. TORQUE 10-15 IN/LS
I. 52',," ARRAY .1 PARAMETERS FOR S'OLAR CELL ARRAY
SCALE: NONE (SEE PLAN ,,,ew)'
10 REF
POSITIVE CONNECTION
, SOLAR ARRAY MATRIX INTERCONNECTION
BASE LI NE_~ _ Y--J;"'=i=i:'e:' =r=: -- SOLDER FOIL TO N.EGATiVE BUS BAR
VIEW A-k· SCALE72[i
INTERLOCKING OF SOLAR CELL MODULES ROOF OUTLINE 7 TYPICAL NEGATIVE
. TERMINATION,
NEGATIVE BUS BAR
FIRST cour,SE OF ITEM Z (REF)
NOTES: I.DRAWINC, TERMS ANDTOl PI:R
tiE .sPEC 5.3000:0. Z,BOND ITEM 13 TO ITEM 12 U51N~
ITEM 2.9. 3.COVER ITEM 14 TERMINATIONS
WITH ITEM 28 AS SHOWN: 4BOND ITEMI4 TO ROOF USINl;
ITEM 22 AFTER CADWELDINC; AND SOLDERINt; ARE OVER.
DE TAl L ·C.' ,SCALE: 1/10 Figure 5-3. Solar Array Installation
5-7/5-8
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GENERAL __ ELECT'RIC .PAC. D'V'.'ON
Detaled ResidentIal P.V. System PrefarI8d Daalgns SanIe Lab. COntract Doc.#13-Sn9
'Johnson & Stover, Inc. Electrical Engineers 127 Taunton St. Middleborough, Mass. 02346 617-947-8464
massC-esian ~""~II'IC.
138 Mt. Auburn St. I CambridQl. MIa. 02138/817-491-0981
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Sheet No.
Figure 5-4. Summary of Shingle Types
5-9/5-10
plywood sheathing. This prevents the entrance of water from the sides or top and
provides the tie-down needed to survive extreme wind lifting forces. Silicon sealant
is run between the top flange of the angle and the shingles to form a water-tight
joint.
Figure 5-5 summarizes the erection procedure for the array installation. A com
plete single-row, working from left to right or right to left, is completed before
moving onto the next row. The installation starts at the roof eave with the place
ment of dummy shingles, the negative busbar and then the attachment of the negative
termination shingles to the busbar. The initial chalk line alignment of the nega
tive termination shingles is critical to the roof installation. A simple aluminum
alignment bar with four pins that fit into the interconnection holes is used to
space the shingles at 32.4 inches from center to center of the interconnection holes
along the chalk line. After this row of shingles is nailed in place, the SCM shingle
positioning is automatically determined and they are electrically connected as
discussed in Appendix F and nailed in place. It is important to nail the shingles
in place at the points marked on the shingles especially for the bottom and top rows
to eliminate striking the busbars. This procedure continues to the roof ridge where
the positive busbar is placed and the positive termination shingle leads are soldered
to it. The positive terminal shingles have two copper foil leads and both are con
nected to the busbar, except for the last terminal shingle installed. The two leads
provided enable one to be cut off for the roof edge module connection since busbars
are terminated 12 inches from the roof edge to eliminate possibilities of striking
them with nails and to limit the accessibility to them from the roof edge (See Detail
B of Figure 5-3).
The installation is completed with the edge and ridge aluminum angle placement.
5-11
Figure 5-6 shows the completed roof with the modules connected in parallel and
series circuits identified by the numbering system indicated. (e.g., 10/1 refers
to the tenth series module in the first parallel circuit and 8/4 refers to the
eighth series module in the fourth parallel circuit). The figure also gives the
number of the different shingle types required for the installation.
Appendix B provides an installation instruction manual developed by Massdesign
for the array with a step-by-step description and sketches. Appendix B also pre
sents several important safety considerations for the installation procedure.
Power Conversion Subsystem (PCS)
The power conversion subsystem, with the associated cabling and switchgear,
provides the interface between the residential roof photovoltaic array and the normal
residential utility service and loads. Functionally, it must convert the available
array dc power to a usable and acceptable ac form. A line commutated inverter manu
factured by the Gemini Corporation and marketed by Windworks, Inc. of Mukwonago,
Wisconsin is available in the 10 kVA range for this application. It can be supplied
according to the specification included as Appendix A to meet the preferred control
options. The specification details the complete PCS, as shown in Figure 3-2, which
is packaged in three units, the inverter, dc filter and transformer along with the
control circuitry. Standard controls, filters and transformer options are provided
by Gemini to encompass the requirements specified for the residential design. Pro
curement of the basic inverter bridge, desired controls, filter, and trans~ormer
from a single source is recommended to assure compatible matching of pes components.
Subsystem Requirements
General-- The dc to ac conversion must be accomplished in a manner consistent with
residential electric practice. The conversion technique should accommodate the
5-12
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GENERALe ELECTRIC .".c. PI"'.IO" Detaled Residential P.V. System Prefanad Designs &nIa Lab. COntract Doc.#13-Sn9
Johnson & Stover, Inc. Electrical Engineers 127 Taunton St. MiddJeborough. Mass. 02346 617-947-8464
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Figure 5-6. Overall PV Shingle Array Ins tallati on
5-15/5-16
large range of solar array electrical parameter variation, residential load varia
tion, and utility line voltage variations. The interface must also provide safe
interconnections under all operational and failure modes, and provide feedback of
any excess array power to the utility. Equipment selection should use proven
technology to the extent available.
The requirements for converting the variable dc power and voltage of a photovol
taic array to the utility residential fixed ac voltage low impedance interface can be
reliably and economically met by solid state electronic inverter device, acting as a
current source to transfer power. Either a self commutated or line commutated inver
ter can meet the requirements, but the latter type, selected for this application,
is commercially available at more attractive prices. The line commutated inverter,
in its simplest single phase form required for residential service, has the disadvan
tages of poor power factor and high harmonics. It has a large technological applica
tion base, however, and has been tested and applied in photovoltaic applications by
both the NASA Lewis Research Center and the MIT Lincoln Laboratories.
These summary requirements have been analyzed for the southwest residential
design, and detailed in a specification, Appendix A, for the PCS. Key parameters for
the PCS are summarized in Table 5-4.
Array Input to the PCS--Array IV characteristics were shown in Figure 3-3 at SOC
conditions. The characteristics indicate a maximum power rating of 8 kW and rated
voltage of 183 Vdc. The array maximum power point and associated voltage, however,
exhibits a dynamic variation with insolation, temperature and wind variations.
The array maximum power point and voltage was examined on an hourly basis for
5-17
TABLE 5-4
Inverter Characteristics
RATINGS
POWER RATING OUTPUT VOLTAGE
NOMINAL INPUT VOLTAGE INPUT VOLTAGE LIMITS
MAXIMUM POWER VOLTAGE RANGE
FULL LOAD EFFICIENCY
FULL LOAD POWER FACTOR
FULL LOAD HARMONIC DISTORTION
FULL LOAD AUDIBLE NOISE
RFI
ENVIRONMENT
AMBIENT OPERATING TEMPERATURE
RELATIVE HUMIDITY
PHYS I CAL SIZE
DC FILTER
INVERTER
TRANS FORt~ER
HEIGHT INCHES
15
30
20
WIDTH INCHES
12
24
17
5-18
10 kW CONTINUOUS 240 ac UTILITY LINE CONTROL
200 Vdc LOW 180 Vdc HIGH 220 Vdc
180-220 Vdc
92% MINIMUM
60% MINH~UM
30% MAXIMUM
60 dB @ 1 METER
200V MAXIMUM FROM 5 KHZ TO 3~1HZ
UP TO 95%
DEPTH INCHES
12
9
15
WEIGHT LBS.
120,
60
300
Albuquerque and Phoenix based on TMY weather data. The Albuquerque weather
data provided the extremes of variation and, therefore, was chosen for the
power conversion system design requirements. Figures 5-7 and 5-8 illustrate
the distribution of the maximum power point and maximum power voltage, res
pectively as a function of the array annual energy for Albuquerque. Similar
curves for Phoenix are shown in Appendix G.
Subsystem Sizing
The annual distribution curves for maximum power and voltage, Figure 5-7
and 5-8, can be used for system sizing specification and operating voltage
specification. The power distribution curve of Figure 5-7 indicates 20.1 MWh
of annual energy at an 8kW array rated level and an actual peak of 11.2 kW
with 20.7 MWh of annual energy, -or 3% additional energy. To capture the bulk
of this additional energy and use a standard rating for the pes a 10 kw size
was selected. At a 10 kw limit the annual energy collection will be greater
than 99.7% of the potential energy available with an 11.2 kw rating.
Examination of the variation of voltage at the maximum power point versus
the annual energy collection as shown in Figure 5-8 shows an extermely wide
swing from 160 to 246 Vdc. In this case, only a small portion of the annual
energy is collected at a limit of the array rated voltage (183 Vdc) or less.
The voltage can be expressed around the mid-point of the range as 203 ~ 21%
Volts. An inverter designed to accommodate this wide input voltage will have
poorer effi ci ency, power factor and hi gher harmon; cs than woul d be true with
a smaller input range. Since the curve is "s" shaped with relatively flat ends,
the hourly simulation analyses were made with excursion limits placed upon the
5-19
I::l Do
22
20 ----
~ 18 o ~ J: 16 a::~ w ... Z:::' W • 14 ..JDo <{v ::l..J 12 Zw Z> <{W >..J 10 <{a:: a:: w a::~ 8 <{~ a:: I-S<{ 6 g
4
2
ARRAV CONFIGURATION 20.1 MWH
- - 25SBV 19P - -- -- --. BLOCK IV SHINGLE 8.0 KW
O~ ____ ~ ____ -L ______ L-____ ~ ____ -L ____ ~ ______ ~ ____ ~ ____ ~ ____ ~
o
22~ 20 :c ~!Z :::loo; 18 n.!::. t-- 0. :::l o .E
16 >.> "v a: UJ
\..IJ L'J 14 ~ "-1 w~
-' --'0 12 g> za: 2~ 10 <{>
>~ <{~ 8 L a::::l a:~ <{:"; 0:)( 6 <{<{ -'2 0t- 4 CI)<{
2
1 2 3 4 5 6 7 8
S'OLAR ARRAV MAXIMUM POWER OUTPUT ...... P (KWI
Fi gure 5-7. Integral Distribution of Solar Array
Maximum Power Point Power for Albuquerque
NOTES
1. ARRAV CONFIGURATION 25 S BY 19 P
BLOCK IV SHINGLE
1.9 MWIi
----------
SOLAR ARRAV MAXIMUM POWER VOLTAGE",Vmp (VOL TSI
Figure 5-8. Integral Distribution of Solar Array Maximum Power Potnt Voltage
for'Albuquerque
5-20
9 10
11.2 KW I I
20.7 MWH
245.5 V I I
20.7 MWH
maximum power voltage of 200~ 10% Volts. The results of this analysis are shown
in Figure 5-9. The annual energy collection with the limits is 99.5% of the un
restricted case. The power duration curve shown in Figure 5-10 for the maximum
power voltage limit case illustrates the small triangular area (energy) above
10 kw. Thus, the selected power limit of 10 kW and a voltage limit of 200 ±
10% Vdc input to the inverter will still result in collection of greater than
99% of the annual energy available if there were no limits on the inverter input.
Inverter Specification
The interconnection diagram for the pes is shown in Figure 5-11. The inverter
package contains the basic dc to ac conversion circuitry and the operational control
circuitry.
Conversion Circuit-- The basic conversion circuit consists of an SCR Thyristor
bridge circuit with SCR phase angle firing control electronics and initial adjust
ments for voltage and current limit settings. Commutation of the SCR's to the off
state is accomplished by the reversal of ac line voltage as seen by the bridge
through the transformer, hence, line commutated. The phase angle firing control
)oint is dynamically adjusted within limits to transfer the maximum available array
lower. The initial adjustments provide for three set points: dc turn-on voltage,
V load line slope, and current limit. Suitable fusing is also included on the dc
nput to the bridge and the ac output from the bridge.
:artup/Shutdown--The control circuitry for daily operation is shown in Figure 5-12.
,ne inverter will operate in its normal mode when the main on/off switch, Sl, is
closed. The operation is illustrated by following the sequence of circled numbers
around the diagram: In the morning when the sun rises the array dc voltage rises
5-21
... ::l a. I-::l 0 .... )0:1: ~
O~ o:~ UI z ' UI .J .J> <V ::lUI ZO z< <I-)o..J <0 0:> 0:1-« 0:
~ 0 In
=-~ A
0..
~
~ I.J.J
6 0..
22
20
18
16
14
12
10
8
6
4
2
0 0
ALBUQUERQUE SINGLE FAMILY RESIDENCE
NOTES
1. ARRAY CONFIGURATION 25S BY 19P BLOCK IV SHINGLE
2. THE SYSTEM OPERATES AT THE MAXIMUM POWER POINT BETWEEN 180 AND 220 VDC
150 160. 170 180 190 200 --SOLAR ARRAY VOLTAGE -V. VOLTS
210 220 230
FigllY'p. 5-9. IntegY'''l nistriolJtion of Solar Array Voltaqe with 12- Restrictions for Albuquerque
11
8
7
6 .
5
4
3
2
o o 500 1000 1500 2000 2500 3000 3500 4000 4500
DURATION, HOURS AT POWER> P
Figure 5-10. POVlt'r Duration Plot for Albuquerque Residence with
Voltage Restriction (180-220 Vdc)
5-22
240
U"l I N W
UTIL AC
{ 'i i .........--OUTSIDE WALL
1Tl' r----, ~ . ,,,, UTILITY r-
METfR 240/12.0 VAC, "Ol-l~. S'N~LE PHASE, 3 WIRE N ---r~Q --~-~.--------------------.
L2.
- Lr SURGE --ro PROT[C - I - - 1
'--___ ...... 1'- I I SERV'"CE 'r-)
I .. PANEL V ..£.
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\ SURG.E I ;> I ZOOA ( . ( " PROTECTOR > I I('~ ~
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I . I . I I ('OA 1 I DC o-r--f<D-~H-D+-+--6-0A~--t---jt--to Jr-
""' .J"\ I '" I -'"' ". - ~ ".r.,.,. . J\. J\. I ;::---Oi--+----' ........ I " - I
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ISOLATION : FILTER INVERTER I DISCONHECT I I
L _______ ..;. ____________ , POWER CONVERSION SYSTeM '--------, w~
[I 1~~6~-+~~~~~MJ
SERVICE GROUND ~
It PIP! --+ '
ROUND ROO
t.n I N .+::>
TO OC F1L TER
CDt
J:= !
I
AU,OMAilC OPERATION FOR YOc.., MINiMUM AUTOMATIC SHUTDOWN ON AC LINE LOSS NO AC LOAD IN OFF CONDITION
- - , r-'-- ·-VDe )MIN I I
S 1 (oNI OFF CONTROL) I
LO<4IC OR ---,''" C _J._ -T-
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INVERTER . TRANSFORMER CONTACTOR PA~1..
Figure 5-12. Power Conversion System Interface
exponentially. A sensitive relay or solid state logic circuit, C, closes the circuit
for 'the ac input contactor B. AC line voltage L1 and L2 will then operate the dc
input contactor, A, permitting the inverter to transfer power. If either L1 or
L2 should fail, contactor A will open, thus, shutting the inverter down. Auxi1-
i1ary logic and timing controls will verify a net power output on startup. If
there is not a net power output, the inverter will be shut down again for a preset
short period to avoid rapid cycling. When the sun sets, the inverter will shut
down for the night due to net power less than zero. This evening shutdown and open
ing of the ac line contactor eliminates night time no load losses on the trans
former.
Maximum Power Contro1-- The operation of the maximum power tracking controller is
based upon the photovoltaic power voltage characteristic,shown in Figure 5-13,
derived from the photovo1taic IV characteristic, also shown, A simplified block
diagram for this tracking controller is shown in Figure 5-14. The basic elements
include:
1. A wattmeter circuit that continuously measures the power level and provides
a signal output proportional to actual power.
2. Two sample and hold circuits, controlled by a timer, that alternately sample
the wattmeter signal output and hold it for comparison with the next sample.
3. A comparator that works in combination with a logic circuit to determine if a
given sample represents a power level that is greater or smaller than the
previous sample.
4. A flip-flop circuit that changes state whenever a new sample is smaller
than the preceding one, but remains in the same state if a new sample
is larger than the preceding one, thus representing an increase in
power level.
5-25
VOLTS INPUT
III +-' +-' ro ~
~
S-<lJ :;: 0 c...
WATT METER
AMPERES
I-V
(/) c... ~ c:( .. I-Z W c::: c::: => u
VOLTAGE, VOLTS
Figure 5-13. PV Power Characteristics
I , I
1
">-----4 LOG I C
I 2 ,- '"-'----r----'
: COMPARATOR I SAMPLE AND I FLIP
I HOLD CIRCUITS I FLOP B---------- ___ ..J-.II
INPUT INTEGRATOR
Figure 5-14. Simplified Block Diaqram of Gemini Maximum Power Tracking Controller
5-26
OUTPUT
5. An integrator circuit that provides a constantly changing output, whose
direction of change is increasing for one state of the flip-flop and
decreasing for the other state of the flip-flop.
Operation of the system is as follows. The output of the integrator is the
signal which controls the input voltage level to the Gemini. It has limits which are
set so that it does not attempt to control voltage beyond the tracking limit set
points. The integrator will constantly change its output in a direction determined
by the state of the flip-flop. If, for example, the flip-flop is forced to remain
in a state that causes a constantly increasing output of the integrator, the Gemini
will sweep through its voltage range starting at a low level and increasing to
maximum as determined by its preset limit. If the state of the flip-flop is
changed and forced to remain in the opposite condition, the integrator will cause
the Gemini to sweep down the curve toward zero.
In automatic operation, the flip-flop is not held in any particular state,
but is allowed to change as determined by the interpretation of successive samples.
The sample rate is adjusted so that many samples are taken in the time it would
take the Gemini to make a complete sweep from minimum to maximum or from maximum
to minimum.
Assume that the Gemini starts at zero voltage and is moving in a direction
toward maximum voltage. As each sample is taken, the logic circuit and the
comparator compare it to the preceding one. On the short-circuit current side
of the maximum power point as the voltage increases, each sample of power is
larger than the one before it, and the flip-flop does not react. The voltage,
therefore, continues to rise. Eventually the voltage reaches the value corres-
5-27
ponding to maximum power point and passes through it. When this happens, the
next sample of power indicates to the logic circuit and the comparator that the
power has decreased from the preceding sample, and the flip-flop changes state.
This causes the voltage to stop rising, reverse movement to a downward direction,
and return toward the maximum power point.
From this time on, the voltage will cycle back and forth around the maximum
power point, reversing direction each time it moves far enough to indicate a de
crease in power. If the source characteristics or output changes, an automatic
readjustment restores the Gemini voltage control to the new optimum point. To
provide minimum voltage swings in the area of maximum power and to allow optimum
stability, the rate of voltage change as well as the time between samples, is
adjustable.
De Input Filter and Transformer Specification
The remaining two packages of the pes are the input filter and the transformer.
The filter inductor serves the function of maintaining current flow through the
conduction portion of each Thyristor commutation cycle. This, in turn, reduces
the current harmonics to levels acceptable for use or further filtering if requir
ed. The Gemini provides the filter package to match the inverter and power ratings.
The third package of the pes is the transformer. The transformer serves the
dual function of providing isolation of dc array output and the ac line and matching
of the ac line voltage to the dc voltage for proper commutation. Isolation of the
sources' is a mandatory circuit requirement when ~rounded dc bus is used in the array
design, The negative bus is grounded in this design, thus requiring an isolation
transformer. For this design, the turns ratio will be nearly unity, but for other
5-28
roof sizes and module types, the dc/ac voltage mismatch may be severe enough
to require a transformer matching even if no dc bus grounding and isolation is
required.
The transformer will be rated substantially higher than the inverter bridge
and PCS as a whole to accommodate the out of phase ac voltage and current. A
nominal rating of 15 kVA for the transformer will be suitable for accommodating the
anticipated power factor of operation.
PV Array Electrical Interface to Conventional House System
The basic approach for the design of the interface arrangements is to employ con
ventional wiring runs and equipment in a manner to maximize safety considerations.
This approach provides full recognition to National Electric Code requirements,
and should facilitate acceptance by local regulatory authorities. With this
approach, it was determined that the PV array source should be treated in much the
same manner as the conventional utility service entrance to the residence.
The PV service entrance, therefore, was located at the same exterior wall as the
utility service, as shown in Figure 5-15, using a stan~ard weathp.rhead and drop loop
configuration. This visually separates the residence from one having only a normal
overhead service and should alert any regulatory agency, utility company personnel
or fire protection agency that two systems are in use. It is assumed that the
residence, due to its self-generation capabilities, will have been fully explained
and discussed with all those having an interest, such as local fire companies.
5-29
The National Electric Code requires that service entrance conductors be ter
minated at an overcurrent device at the nearest accessible point of entry of service
to the building. Since the equipment room is located at an exterior wall, the PV
disconnect could have been located inside the equipment room, along with the utility
service disconnect. It was felt, however, that the exterior location of the dis
connecting means was preferable as it afforts an external visible break in the PV
supply.
The requirement for this external switch is debatable and its use is discussed
in more detail in the System Equipment Concerns and Alternatives Section. As PV
systems become widely installed and model codes address PV systems, this switch
can be eliminated in the design. The disconnect switch includes fuses for over
current protection due to the Code requirement for such devices on a service. The
PV array will generate less than 110% of full load current even if shorted.
The same philosophy of visibility was employed in the method of running the
negative dc conductor from the bus location to the weatherhead also shown on Figure
5-15. The raceway could have been run within the soffit, but to ensure complete
visibility from bus to the disconnect switch for initial installations, it was run
exposed.
The negative bus is grounded at the external main disconnect on the array side
so the ground is always in existence even if the disconnect is open. This detail
is shown in the wiring of diagram of Figure 5-16. The green insulated grounding
system also originates at the main disconnect and interconnects all enclosures.
This equipment grounding cOhductor is also green insulated and connects directly to
the house ground bus.
5-30
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GENERAi. _ ELECTRIC .~ .. c. P,,,' •• ON
Detailed Residential P. V. System Preferred Designs Sandia Lab. Contract Doo.#13-8779
, , .
Johnson & Stover, inc. ' Electrical Engineers 127 Taunton St. Middleborough, Mass. 02346 617-947-8464
rnas~~~;sian ~
Archlwcu Met Planners Inc. 138 Mt. Auburn St./ c.mbrldgl, M ... 02138/617·491·0961
SOuthWest All-Electric House WithP. V. Only Job No, 9515 Revised
EXr~ ELEVAlfDrJ Sheet No,
Off,V.4LJ1lur1~KVk:{5 TI~~~ Scale w
Figure 5-15. PV Electrical Exterior Cabling
5-31/5-32
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GEN ERAL e ELECTRIC ... "c. g.V •• ,ON
Detailed Residential P.V. System Preferrad~ Sandia Lab. COntract Doc.#13-8779
Johnson & Stover, Inc. Electrical Engineers 127 Taunton 5t. Middleborough, Mass. 02346 617-947-8464
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Figure 5-16. PV System Schematic Wiring Diagram
5-33/5-34
An additional disconnecting means (unfused) is provided in the equipment room
entrance to serve as_ an lsolation switch in conjunction with the 60-amp breaker
in the main panel should it be necessary to perform any maintenance on the AC/DC
inverter equipment. The positive leg of-the dc system has been provided with
surge protection on the line side of the switch to protect the equipment even
if the interior switch is open. The green isolated grounding lead from the Varistor
Surge Arrestor is run directly to the house ground bus. The dc input is carried
through a raceway system to the inverter enclosure. The ac output of the inverter
passes through the isolating transformer, the output contactor, and connects b,y means
of a 2-pole circuit breaker to the normal house panel board all through a raceway
system. The output contactor is controlled from the inverter internal electronics
which insures that the contactor is open if either the utility service fails or if
the PV array power capability is below the no load system loss level.
The house service panel is provided with a standard residential type lightning
arrestor which is grounded directly to the panel board ground bus. The neutral of
the utility service is also grounded to the house service panel board ground bus
within the panel board enclosure. The house service panel board ground bus is then
connected to the house ground bus.
An exposed copper house ground bus has been located directly below the house
service panel board to provide a visible and accessible means for terminating all
grounding conductors mentioned above. Properly labelled grounding connections to
the bus should insure that no accidental elimination of grounds occurs. The house
ground bus is connected to the incoming water service as prescribed by the National
Electric Code. An additional ground rod has been provided should the water service
ground be unsuitable.
5-35
UL-labelled materials have been used throughout the installation. The loca
tion and quantity of disconnect switches indicates a conservative design which should
satisfy even the most critical regulatory authorities. All equipment will be label
led and copies of the one line diagram and equipment room plans will be provided in
the equipment room. Appendix C provides a detailed electric system installation
specification which can be used for obtaining detailed installation cost estimates.
Miscellaneous Considerations
The National Electrical Code (NEC) applies to matters of safety of persons
and property due to hazards arising from the use of electricity. The NEC
has been recognized by all major model building codes and most municipal codes.
Since photovoltaic systems are electrical in nature, compliance with the require-
ments of the NEC must be assured. However, the NEC was written without consideration
for photovoltaic generators and these requirements do not yet exist. Efforts are
currently underway to draft performance criteria for photovoltaic modules and systems.
Until such time as these efforts are reflected in a revision to the NEC which covers
photovoltaic systems, it will be necessary to seek code interpretations for these
devices from local building officials, manufacturers, architect/engineers, underwriters,
and users. The ultimate classification of photovoltaic modules as a IIPre-manufactured
Item with Internal Wiringll would permit the greatest flexibility of use while still
preserving the necessary safety requirements. Such a product approval will require
certification by a national testing laboratory such as Underwriters Laboratories.
It is anticipated that such approval will take several years to achieve and in
the near term, it is likely that photovoltaic modules will operate on buildings
at the discretion of local code enforcement officials.
5-36
Equipment Room Layout
The equipment room has 66 ft2 of floor area (12 feet by 5.5 feet) as shown on
the floor plan of Figure 4-2. It is located on the west end of the house. The
normal utility line and the PV array power enters the equipment room as shown in
Figure 5-15. The room is slightly larger than required for the electrical equip-
ment, but the extra area can be used for additional equipment or storage.
Internally within the room, the PV power enters as shown on Figure 5-17. Table
5-5 lists the major equipment in the room along with the legend numbers shown in
Figure 5-17. Most of the equipment is wall mounted. The internal disconnect switch,
the input filter and the inverter are all mounted on the west exterior wall. The
normal house circuit breaker panel board is mounted on the south-facing wall in the
room on a 3/4" plywood backboard while the isolation transformer is mounted on the
floor. All of the wiring runs are shown in Figure 5-17 in the two sectional views. TABLE 5-5
Major Equipment List - -
Legend # Equipment Description
1 Fused 2-pole 60-ampere, 240-volt I
disconnect switch.
2 4" sq. X 2" deep outlet box.
I 3 Nonfused 2-pole, 60-ampere, 240-volt disconnect switch.
4 DC I n put Fi lter
5 10 kVA Inverter Enclosure.
6 15 kVA Isolation Transformer.
7 Lightning Protector. I
8 AC Contactor. I
9 Normal Residence Circuit Breaker Panel
5-37
The National Electric Code contains requirements which have an effect on
the location and size of the equipment room,
Section 230~72 of the Code, sub~paragraph ec) location, dictates that the
service disconnecting means be installed either inside or outside the building
at a readily accessible location, nearest the point of entrance of the service
entrance conductors.
Section 230~44 is also applicable and indicates that conductors would be
considered outside the buildin9 if installed under at least 2" of concrete
(below a slab) or encased with not less than 2" of brick or concrete.
Section 110-16, working space about electrical equipment, specifies that
three (3) feet clearance is required in front of electrical equipment operating
at 150 volts to ground.
The basic decision was made that the PV array should be considered as a
"service" similar to the utility supplied service, This decision brought the
code rules into effect, especially with respect to "Service Disconnecting Means",
To be in compliance with Section 230-72, the main disconnecting means had
to be located at the nearest accessible point of entrance to the building. The
location of the visible disconnect on the exterior wall satisfies this require
ment regardless of the location of the equipment room) but this exterior dis
connect actually is not strictly required to be installed and is indicated solely
to satisfy safety concerns and identify the residence as "out of the ordinary",
5-38
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GENERAL _ ELECTRIC Johnson & Stover, Inc. Electrical Engineers 127 Taunton St. Middleborough, Mass. 02346 617-947-8464
."'''0. DIY'.ION Detailed Residential P.V. System Preferred Designs Sandia Lab. Contract Doc.#13-8779
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5-17. Equipment Room Layout
Sheet No.
5-39/5-40
It is expected that as the Code is modified to keep pace with PV technology, the
redundancy of disconnect switches will be eliminated.
As additional practical experience is developed, it is expected that the ex
terior switch will most likely be deleted. This would require the inside disconne(
to be the service disconnecting means. This would result in a requirement to
either locate the equipment room on an exterior wall or to run the PV array conduci
below or enclosed in the 2" of concrete or masonry. This tends to favor the exter·
ior wall location of the equipment room. The exterior wall location also reduces
the length of run of the PV array conductors based on the busoar arrangement as
designed. Further designs may eliminate this benefit, so realistically there is
no reason that the location of the equipment room has to be fixed on the exterior
wa 11 .
The dimensions of the room are generous and are intended to satisfy the three
(3) foot clearance rule dictated by Section 110-l6. The invertor package is the
largest item of equipment, and its assumed depth is twelve (12) inches. The in
ductor enclosure is approximately 16" deep and the isolating transformer 13" deep.
In order to make effective use of space within the equipment room, it was decided
that double loading with equipment on opposite walls was the most effective. This
requi res a minimum wi dth of room to be approximately 5' -6" if allowance is made fOI
12" depth of eqUipment on the opposite wall.
The ultimate layout of the Southwest House indicates equipment on adjacent
walls. However, the 5'-6" dimension was retained as being most workable in re
lation to the other residential equipment to be installed, which includes hot
water heater and storage and the heat pump. Some reduction in inverter enclosure
5-41
dimension may be possible as the technology develops, and future projects mayor
may not have the isolating transformer. The disconnecting means enclosure and
panel board enclosure dimensions are expected to remain constant.
Lightning Protection
The isokeraunic level (thunderstorm days per year) for an area is indicative
of the potential for lightning damage in the area. Lightning damage is created b
both direct strike effects and induced electrical effects in nearby wiring and
circuits. The generally low profile of single family residential buildings in
an area profile including other building types, trees, utility lines and contour
levels decreases the probability of residential direct strikes below the general
direct strike probability in an area. Residential direct strike protection in
the form of air terminals with lightning down conductors is an uncommon feature
in modern residential design practice, except for extremely unusual combinations
of exposure factors_
The isokeraunic level ranges from 0 to 100 across the continental United Sta
The Southwest region is in a low to moderate portion of the range with a level of
26 in Phoenix, 47 in Albuquerque and 28 in El Paso. These levels are indicative
of residential direct strike probabilities in the order of once every few thousan(
years. No direct strike protection has been included in the Southwest residential
design since the isokeraunic conditions are low.
The Southwest isokeraunic levels above are also indicative of one direct
strike within a 100 meter radius circle once every 10 to 30 years. For this
type of probability, nominal surge protection for the residential interior
electric equipment can be employed. Varistor surge arrestors have been specif
ied across the ac and dc service entrance cables to limit the conduction of
5-42
exterior line or array induced lightning effects on interior electrical equipment
including the power- conversion system. The design calls for surge arrestor instal
lation on ac service lines at the main service panel ahead of the 200 amp main
breakers, and on the positive dc line on the array side of the interior discon-'
nect dwitch. Service ground bus connection wiring has been provided for the
varistors to assure positive ground shunting of lightning-induced high-voltage
transients.
Electrical Equipment Components
The electrical equipment components consist of all the electrical equipment
required to connect the solar array to the residential service panel. This in
cludes, in addition to the power conversion system discussed in the previous
section, the cabling, terminations, switches, fuses, contactors, breakers and
approved electrical enclosures for the devices. The interconnecting power runs
are specified as #4 gauge wire run in 111 PVC conduit. NEMA approved devices and
enclosures suitable for the intended service have been specified. The electrical
equipment included from the 1/8 11 x 5/8 11 array positive and negative busbars to the
house service panel are:
• Busbar cable bolted pressure terminals
• Exterior array disconnect fused switch
• Interior PCS isolation disconnect switch with surge supressor
• Power conversion system
• AC system tare loss elimination contactor
• Service panel PCS isolation and protection circuit breaker and surge
suppressor
• Power and ground cabling in conduit
• Positive ground components including ground bus and ground rod
5-43
System Equipment Concerns and Alternatives
Several design details and design approaches with regard to additional equip
ment selection and equipment arrangement have been included in this residential
detailed design to' insure a conservative approach for both personnel safety and
equipment integrity. These details include grounding of the negative array bus,
multiple disconnects and protective devices between the array and the service
panel, the use of an isolation transformer, and a single point system ground.
The following sections discuss some of the design considerations.
Negative Busbar Grounding--The array negative bus is located near and parallel to
the roof eave line. After installation, it is embedded in an insulating coat of
Pliobond 20 on top of the roof wood sheathing and roofing felt. The busbar is then
covered with both the negative termination modules and the first course of SCM
modules . With this installation, the negative bus is judged to be at least as well
insulated and as secure from a safety aspect as normal residential insulated service
cable. Any personal contact with the negative bus under these circumstances must
be by deliberate attempt, and no built-in safety arrangement can protect against
deliberate hazardous exposure. Visible exterior array cabling is protected in a
manner identical to normal utility service cable. The negative array busbar located
near the gutters at the eave of the roof is subjected to a potential ice buildup
from the gutter under a combination of environmental circumstances in northern
installations. Water from this ice backup could conceivably penetrate the insulation
of the busbar or a slender tool being used to clean debris or the ice blockages
. could come in contact with the busbar providing a potential to ground. The ad
ditional safety precaution, therefore, has been provided of a wired connection to
earth ground for the negative busbar. This connection creates the requirement for
an isolation transformer in the power conversion system. While an isolation trans-
5-44
former is not a mandatory system element except for array bus grounding, its presence
can provide a convenient location for adjustments of system output to the required
house service rating. This feature provides design flexibility in array circuit
layout for different roof designs. The transformer, however, adds economic penalties
both in initial cost and in net power loss over the life of the system operation.
Elimination of array bus grounding and the associated isolation transformer,
providing the array output matches the house service rating, can increase the
system output at negligible decrease of safety.
Disconnect Switches--Examination of the power path from the array drop cable, see
Figure 5-16, shows a fused exterior switch,interior entry disconnect switch, in
verter on/off switch controlling a dc contactor, internal inverter dc and ac
fusing, an ac contactor and service panel interconnection through a circuit
breaker. In some measure, the number of devices again, tends towards a conserva
tive design approach. The additional devices and installation, however, are
only minor additional initial economic penalties. Nonfunctional redundancy in
this chain may be eliminated, such as one of the initial switches, as experience
with system installations is gained and NEe codes for photovoltaic installations
are developed.
Lightning Protection--Due to the relatively low isokeraunic level in the south
west, and the low probability of residential building severe lightning effects,
surge arrestor varistors have been provided only on the dc input side of the
power conversion system and on the utility line input. In most locations, even
this much protection may not be needed . For severe lightning areas, such as
Florida, additional surge protection for drop cables may be required as a min
imum with the possibility of a complete residential mast and grounding system
5-45 -
in some cases.
Equipment Grounding--Improved isolation and filtering of transients and spikes
between the power conversion system, residential loads and utility lines may
become a requirement in the future. The single point grounding arrangement
designed for this system, in addition to the safety provided, will assist in
minimizing any potential problems in this area. With the elimination of
ground loops, detection of any potential problem will be simplified, and solution
of difficulties encountered will be eased.
Fire Safety--Fire safety is an important issue that could not be specifically
addressed in the design other than for general considerations. It is likely
that the module installations will be considered as a roof covering by local
building code officials. In this case, the module installation must receive
a fire rating classification in conformance with the requirements of UL-790,
"Test for Fire Resistance of Roof Covering Materials".
Local fire companies would have to be alerted and instructed in regards to
the potential danger of the energy-producing roof. The location of the dc
power line adjacent to the normal utility line and the exterior disconnect switch
should alert fire personnel that the house has an additional power supply. How
ever, fire personnel must be informed that disconnect at the switch still leaves
the roof as an active generator in the daylight and only interrupts power supply
to the house.
5-46
SEcnON 6
SYSTEM DESIGN ALTERNATI,VES
J'ntroduction
This section dtscusses several design alternatives that can impact the photo
voltaic system design including the extension of the Southwest design to the
Northeast and Southeast regions of the country;reduced system size based on ec
onomic assumptions; and alternatives which relate to changing the system load.
The latter alternatives require a change in the HVAC or DHW system. For example,
small sized solar thermal domestic hot water systems and hot water recovery units
currently being marketed can be implemented for reducing the hot water demand.
In addition, there is potential for residential water source heat pump systems
which may operate at a higher COP resulting in a reduced space conditioning electri
cal demand. Each of these options will be discussed briefly in the following
sections with detailed simulations to be addressed in subsequent designs.
System Performance for the Southeast and Northeast
The all electric PV only system with utility feedback is a system that showed
economic viability in each of the three regions of interest. The basic system
design would be similar in each of the regions since it is a relatively simple
system. This effort was intended only to project system performance estimates
for the Southeast and the Northeast based on the detailed system design for
the Southwest. Discussion of system changes are included but no detail design
information was developed. Specifically in the analysis of the Northeast, the
same single story house design was assumed, however, that a roof slope of 400
instead of 260 was used in the simulation analysis. This type of slope on the
house design probably would not be architecturally acceptable. The Northern
two story house design, which will be described in the second detailed design
6-1
report, would require a completely different PV shingle matrix circuitry due
to the difference in roof layout. With these considerations, the performance
analyses were extended to Miami and Boston climates.
Array Sizing
The TMY weather tapes for the respective cities were used for the hourly
insolation data. Electrical house demands are similar to those described in
Appendix 0 and shown in Figure 0-2 with domestic hot water adjustment factors
for Miami and Boston. The annual totals are tabulated again below.
COMPONENT
Baseload (average) Cooking and clothes drying Domestic hot water (average)
Adjustment factors Miami = .75 Boston = 1.08
LOAD, kWh
4917
2070 4384
The space conditioning thermal loads used are those listed in Table 6-1.
Again, it should be noted that the thermal space conditioning demands for
Boston were calculated for the Northern house and not the Southwest design.
The resultant heat pump monthly electrical requirements are shown in Figure 6-1.
All of the economic assumptions, system cost estimates and energy price esti-
mates as described in Appendix E were used for this analysis. The array area
optimization for both Miami and Boston as a function of the cost-to-benefit
ratio is shown in Figure 6-2. It is noted, again, that an array slope of
400 was used for the Boston analysis. The trends are similar to the Phoenix
and Albuquerque results. The optimum array size for a sell back to buy ratio
of 0.5 is the maximum available roof area of 92.9 m2. The cost-to-benefit
ratio for Boston for the economic assumption used was slightly above one.
6-2
TABLE 6-1
Single Family Monthly Load Profi1e~
SITE LOCATIONS
BOSTON MIAMI (KWH) (KWH)
MONTH COOLING HEATING COOLING HEATING
JANUARY a 2280 75 52
FEBRUARY 1 a 1422 91 11 I I
MARCH a 1382 989 I a
APRIL a 486 1035
I a
MAY 13 214 1351 a
I JUNE 71 15 I 1687 a I , JULY 1019 0 1814 I 0 I
! ! j
AUGUST 886 0 1986 ! 0 I
i
I SEPTEMBER 388 0 1923 a
OCTOBER 15 178 1497 0
NOVEMBER 1 829 1191 0
DECEMBER a 1919 140 22
TOTAL 2293 8725 13779 85
6-3
1000 B 1000
I B
~ 800 ~ 800
.. .. A A
~ ~ ~
600 ~ 600 A A
~ ~ 0"1
p:: ~ ~ , Z Z +:» ~ ~
~ 400 ~ 400 u
t-I t-I p::
!\ p::
.H E-i ·U U ~ ~ ~ ~ ~ .200 ~ fOO
o I I I I I I II I I II I .0 1 , ,
A'M'j.JA S·ON J F MA M J J A S 0 N D J F M D
ANNUAL TOTALS ANNUAL TOTALS
• SPACE LOADS • SPACE LOADS - HEATING 9.8 MWh - HEATING O.lMWh - COOLING 2.2 MWh - COOLING 13.8 MWh
• HEAT PUMP LOAD 4. 7 IvR-Jh • HEAT PUMP LOAD 4.8 MWh • AVERAGE HP COP 2.54 • AVERAGE HP COP 2.92
fIGURE 6~1. Heat Pump Electrical Requirements for Single Family Residence
B B 1.B 1 B
. SELL BACK RATIO
1. 6 ..L SELL BACK 1. 6 PSjPE RATIO PSjPE ---- 0.3
1.4 1.4 I ~ - I 0.3 ~ .0--j 0.5
: 1.2- ~ 0.5 : 1.2 ~I H 0 7 H 0.7
O'l r:... • r:... I ~ ~
(J'1 ~ 1.0 ~ 1.0 I ~ ~
o 0 H H H .B H .B 00 00 o 0 u u
.6 .6
.4 J DESIGN PO'~NT------I .4 I DESIGN POIwr--------1
I t I .2 .2
o 20 40 60 BO 100 0 20 40 60 BO 100
COLLECTOR AREA, M2 COLLECTOR AREA, M2
FIGURE 6-2. COST TO BENEFIT RATIO AS A FUNCTION OF COLLECTOR AREA
Table 6-2 lists the annual simulation results for all four of the sites studied.
In addition, Figure 6-3 shows the monthly design performance for Boston and
Miami.
In Boston, the total electrical load peaks in winter and is dominated by
the space heating demand; whereas, in the summer, the air conditioning demand is
relatively small, thereby causing the total load to decline. The PV system
output profile is out of phase with the load, having its best performance in
summer. This mismatch results in a relatively low (27%) portion of the load supplied
directly by the PV system.
Miami, on the other hand, has a summer peaking load proftle and the PV system
output can more closely match the load profile. The results in 44% of the load
being supplied by the PV system directly. It is also interesting to note that
the net system output is less than the total load and the feedback energy to the
utility is less than the utility makeup requirements for Boston and Miami. This
is the reverse of characteristics for the Southwest system performance.
Inverter Characteristics
The pes size would be expected to remain in the 8-10 kW range since the array
size is the same as for the Southwest design. However, the 8 kW size may be ac
ceptable for the Southeast and Northeast as opposed to the 10 kW size for the
Southwest as will be discussed below. An evaluation of the array input voltage
to the inverter also shows two different sets of characteristics for the South
east and the Northeast as compared to the Southwest. Figures 6-4 and 6-5 show
the integral distribution of the array maximum power versus the maximum power
6-6
TABLE 6-2
Summary of System Performance for Four Regions
NET SYS. UTILITY FEEDBK ELECTRICAl EFFECTIVE PV AREA OUTPUT MAKEUP ENERGY LOJl.D UTIUZATION
A pv Eo Eum Es Le U
CITY r\12 kWh kWh kWh kWh PS/PE=0.5
ALBUQUERQUE 92.9 19174 8928 13336 14766 .652 I • t ALBUQUERQUE 73.3 14953 9184 9371 14766 .687 ~ I
j ALBUQUERQUE 48.9 9654 9688 4576 14766 .763 1 .I ~ I I , PHOENIX 92.9 17455 8735 10254 15937 .709 I :1 tt,
~ PHOENIX 73.3 13580 9021 6664 15937 .752
,
I PHOENIX 48.9 8663 9941 2667 15937 .839 I
j rvJIAM I 92.9 13523 8405 6892 15036 .745
MIAMI 73.3 10481 8847 4282 15036 .796 , I
MIAMI 48.9 6627 9840 1431 15036 .892 I I
92.9 10956 12058 6585 16429 .700 I
BOSTON
BOSTON 73.3 8467 12392 4430 16429 .738
BOSTON 48.9 5333 13025 1929 16429 .819 -.... "'-.'"""""~-....-......'- ... '-- .... --~--. - -~'.
........
* U = 1 - (1 - P /P ) E IE - S E S 0
where PS/PE is the sellback to buy price ratio of electricity.
6-7
0) I co
2.5 EJ 2.0
LOAD ___ -----16.4 MWh --....
~ 1.5+ .- \ 'ttT"C'f"J1 f"J1n \ I
ft
~ 1 § 1.0
/
0.5+ PV SYSTEM~ OUTPUT 11.0 MWh
o I I I I I I I I I I I I I J F M A M J J AS 0 N . D
UTILITY MAKE-UP 12.1 MWh UTILITY FEEDBACK 6.6 MWh % OF LOAD SUPPLIED
DIRECTLY - 27%
~ ft
~ P::
.r:Z::I 'z
r:z::I
2.0
1.5
LO-
0.5
o
G
J FMA MJ JA SOND
UTILITY MAKE-UP 8.4 MWh UTILITY FEEDBACK 6.9 MWh % OF LOAD SUPPLIED
DIRECTLY - 44%
FIGURE 6-3. PV System Monthly Performance Summary
22
20
f- X 18 => :;0: "- ~ f-
~,,-16 => 0
:E >- ::-<.!l V '" uJ 14 W z <.!l w c(
-l ~ 12 <:( 0 => ::-z: 5 10 c(
:3: >- 0
~ 0-8 r:>: ~
<:( :::> ~
'" ..... 6 c( ><
-l ~ 0 Vl
f- 4 ex:
2
0
ARRAY CONFIGURATION 25 S by 19 P
BLOCK IV SHINGLE 225.8 VDC
~~\'~~----~------+-----~--~~------~-----r----~~----~ 0 150 160 170 180 190 200 210 220 230
SOLAR ARRAY MAXIMUM POWER VOLTAGE ~VMP' (VOLTS)
15.2 NWH
IGURE 6-4. Integral Distribution of Solar Array Maximum Power Point Voltage for Miami
22
20
....... 18 :; ::t: :?-
:l.. ::;: -=> 16 ::> 0... :E
>- ::-:.!:l " :>:: 14 .... w z <.!l .... c(
f--' ...I. 12 <:( 0 => :> z z r.r:: 10 <:( w
:?->- 0
~ 0...
ox: ::;: 8 <:( => :.;: ox: ..... 6 <:( >< -l ~ ::> til
f- 4 ex:
2
0 O~ I
170
ARRAY CONFIGURATION 25 S by 19 P
BLOCK IV SHINGLE
I
180 I
190
(PHOENIX HOUSE)
200 210 220 230
SOLAR ARRAY t·1AXIlUI POWER VOLTAGE ...... VMP • (VOLTS)
240
251.5 VDC , I
___ ' ___ 12.3 MWH
250 260
FIGURE 6-5. Integral Distribution of Solar Array Maximum Power Point Voltage for Boston
6-9
power voltage for Miami and Boston respectively. The Miami profile shows a
narrow voltage range of 180 to 225 Vdc, narrower than all the sites studied.
This is mainly a result of the narrower ambient temperature swing for Miami.
Boston, on the other hand, shows a large voltage operation range from 194
to 251 Vdc, similar to Albuquerque . As a matter of fact, the array never
operates at the NOCT voltage of 183 volts. Again this can be attributed to
the wider ambient temperature and lower insolation conditions in Boston
allowing th e array to operate at a much cooler temperature .
A review of the integral distribution of the array maximum power versus the
maximum power output in Figures 6- 6 and 6-7 shows the maximum power output for
Miami as 9.6 kW and 10.1 kW for Boston as compared to 9.8 kW for Phoenix and
11 . 2 kW for Albuquerque. However, both plots have a very flat profile between
the 8 and 10 kW range resulting in only a small portion of the annual array
output in this range. This characteristic is further shown in the power dura
tion curves of Figures 6-8 and 6-9 for Miami and Boston respectively. In Miami,
the system maximum power generation is above 8 kW for approximately 80 hours.
In Boston, the power output is above 8 kW approximately 200 hours. If an 8 kW
inverter subsystem was specified, the system would have to operate off maximum
power at the 8 kW level for these hours. In a detailed design, an economic
tradeoff would then be required to assess the reduced cost of the smaller sized
inverter versus the loss in total system output.
5kW System Design Details
System sizing for reduced sellback to buy ratios for energy fedback to the
utility results in a 48 m2 array size as discussed in the collector sizing sub
section of Section 3. This corresponding array layout results in a shingle
circuit networ k of 25 seri es modu l es by 10 parallel ci r cuits . Thus,
6-10
t-=> a.. t-=>~ o ::c
::;: >-::;:: <.!) --cc w z a.. w \J ...J -' .::t: lLJ ~ :;> ZL.J.J ;;:: -' ""0:; >- W·
~6 cc a.. q:
f-cr; c:(
"" ...J 0 VI
f-=> a.. f-~
::::> .r: o~ >- --l!! cr: . wa.. fjV -' -' c:( w => :> 2W· Z -'
""cr; >- w ""::;: c.: c:::>
." "-"" cr;!;:(
"" ...J 0 VI
22
20
18
16
14
12
10
8
6
4
2
AR~\Y CONFIGURATION 25 S by 19 P
BLOCK IV SHINGLE 9.6 KW
I
15.2 MWH
0 ~----~----~------~----~------~----~------~----~------~---.~ 0 2 3 4 5 6 7 8 9 10
SOLAR ARRAY ~IAXH1UM POWER OUTPUT"" p. (KW)
FIGURE 6-6. Integral Distribution of Solar Array Maximum Power Point for Miami
22
20
18
16
14-
12
10
8
G·
4 -
2.
ARRAY CONFIGURATION 25S BY 19P
BLOCK IV SHINGLE
(PHOENIX HOUSE)
10. ~ kW
I I
~ __ - ______ I- - - -12.3 MWh
--~-----~2------3~1----'--4rl------5r!------6rl------7rl------~1------9~1 ----~lt~----~l~
SOLAR ARRAY MAXIMUM POWER OUTPUT"" P (kW)
FIGURE 6-7. Integral Distribution of Solar Array Maximum Power Point Power for Boston.
6-11
3: -'-<
0..
0: w ::>:: 0 0..
~ n. ? 0:: uJ
6 n.
12.,..
11
10 - .. -- 9.6 kW
9
8
7
6
5
4
3
2
1
01 ~------+I------~----~.---4--· -----r-------~I--------I~-----0 500 1000 1500 2000 2500 3000 3500
OURATION HOURS AT PO\oJER > P
Figure 6-8. Power Durati on Plot for PV Array for Mi ami
12
11-
10 -- 10.2 kW (PHOENIX HOUSE)
9
8
7-
6
5 -
4-
3
2 -
I -
0 I ----0 500 1000 1500 2000 2500 3000
DURATION HOURS AT POHER > P
Figure 6-9. Power Duration Plot for PV Array for Boston
6-12
-r--
3500
system operating voltage remains the same since the series circuit length remains
the same but the SOC power output rating is reduced to 4.3 kWp at NOCT due to the
reduction of 9 parallel circuits. The system current level is also reduced to the
maximum power NOCT value of 23.5A. The same system configuration can be utilized
to accommodate the reduced array size with respecification of subsystem components
such as the PCS, cabling and circuit breakers.
Roof Array Layout
Reduction of the size of the PV array on the house roof provides several
architectural variations in the house design. The requirement for a long rectangu-
lar roof is relaxed and, thus, breaks in plane can provide variations.
could be located on the west end of the roof as shown in Figure 6-10.
The array
The cabling
to the equipment room would then remain identical to the full roof area design.
The 10 parallel module circuit results in an array length of 8.7 m which takes the
array beyond the roof break on the north elevation. The new construction house
design could be modified easily to have the north roof line break correspond to the
array length which would then allow a south roof plane break and the potential for
adding a clerestory to the house.
The conventional roof shingles would be selected to match or balance the array
texture. This was planned for the full roof array at the roof overhang on the south
elevations.
PCS Sizing
With the reduced array size, the PCS size would also be reduced. Figures6-11
and 6-12 show the integral distribution of max power point voltage and power with
annual array energy for Phoenix. Note that the voltage range in Figure 6-11 is
6-13
~
::r: f_ ;x ::> :E "- ...... I-::> 0 0..
E >- ::-<!l cr:: V LU LU Z <!l w «
I--' -' « 0 :::J ::-z z ex: « LU
3: >- C>
~ "'-ex: § «
:.E ex: ...... « >< -' « 0 :E Vl
f_ «
I-:::> a.. I-:::> 0
::r: >- ;x <!l ;:;: ex: w ;.~ . w o.
-' 'I « .J :::J LU ;,: ::-;;;: LU
-' >- 0: « w cr:: ;x ex: 0 « a.. er:: I-« « -' 0 Vl
11
10
9
8
7
6
5
4
3
2
1
NOTES 1. ARRAY CONFIGURATION
25 S BY 10 P BLOCK IV SHINGLE
1. 7 MWh I~
/
NOCT Hax Po\·/er Point Voltage 183 Volts
227 V 1 1
__ --' _ -'- .10.4 MWH
o 0 \r--:t::-:----t---~::::.-.---+--!.-----.--~f-I ---+---+-1 ----11 150 1 0 170 180 190 200 210 220 230
FIGURE 6-11. Integral Distribution of Solar Array_Maximum Power Point
Voltage for Phoenix Single Family Residence 5 kW System
5.2 kW 11
10.2 rMh -- - 10.4 MWh 10 - - - - - - -9
8
7
6 1 flOeT Max POl1er
5 ~ Point 4.3 k~1
NOTES 4
1. ARRAY CONFIGURATION 25 S 3Y 10 P
3 BLOCK IV SHINGLE
2 / , 1 /
.I I a I f --+- 1 --t- I .
0 1 2 3 4 5 6
tr,URE fi-12. Integral Dist"ribution of Solar Array Maxfmuin Power
Point Power for Phoenix Single Family Residence 5 kW System
6-14
/ 1"* F~I"
IC.. eAJ1:ftl e>f:RM --> I .'----',~~=============~~~~----------- - ____ I
I '
NcrzrH ELEVAlION
':X)ulH ELEVl\l1oN
I---____________ IJ
1Y:v;~ / .t:.mM
R Do-Fl ~IFI I K_","-> U U UU 1 ______ ---------_~~~~~~~~~~~~"'--'-"-"---"
en-II PL~I?
-SIPINt.. 12
LJ I
----------------~
GIN ERAl eELECTRIC 'Johnson & Stover, Inc. Electrical Engineers 127 Taunton St. Middleborough, Mass. 02346 e17-947-8464
:: II:
rn!assae~n'-Southwest All-Electric House With P. V. Only
ELEVA1iorfj ... AC. DIYI.JON
Detailed Residential P.V. System PrefemKI Designs Sa1dia Lab. COntract Doc.#13-8779
Archlt8ctl M1d ,,-.nn.n Inc. 138 Mt. Auburn St.1 Clmbridte. M-. 02138/611-'91-0961
Job No. 9515' Revised Scale .I'l!e' ---Sheet No.
6-10. North and South Elevations for 5 kW Array
6-15/6-16
identical to the Phoenix curves for the 8 kW system, Figure G-2. The annual energy
output, however, is reduced. Review of Figure 6-12 shows the peak array power out
put is 5.2 kW or 20 percent above the NOCT value of 4.3 kW. This is the same ratio
as the 8 kW system for Phoenix as noted in Table G-1. Therefore, the Albuguerque
peak power output would be expected to be 40% above 4.3 kW or 6 kW. The annual
energy generated above the rated power is small and therefore, a 5 kW PCS size is
recommended.
If a 5 kW size would not be a standard inverter size in large scale production,
a 6 kW size would be specified. The inverter specification in Appendix A would also
apply to the reduced size PCS with the appropriate modifications of reference to size
and background data.
System Performance
System performance calculations were performed for the 48.9 m2 array. Figure
6-13 presents the monthly energy performance for the system in Phoenix and Albuquer
que. Annual totals are listed beneath the curves for comparison of the 5 and 10 kW
systems. Note that the energy fedback to the utility is significantly reduced and
from 55 to 70 percent of the energy generated is utilized in the house. There is
no net flow of energy to the utility during any monthly period. The overall utility
make-up requirements for the two locations only increase slightly for the 5 kW system.
6-17
..c 3: ::E:
0"1 >-I c.!J - et:: co L1.J z L1.J
>--I :z: .-z 0 ::E:
r PHOENIX
2.0
1. 5 + / \
1.0 V 0.5
PV SYSTEM OUTPUT
o I~~~ __ ~ __ ~~~~~~~~~ J F M A M J J A SON D
ANNUAL TOTALS • Load 15.9 MWh • PV System Output 8.6 MWh • Utility t~ake-Up 9.9 MWh • Sell Back 2.7 MWh
10 KWSYS 15.9 MWh 17. 5 r~Wh 8.7 MWh
10.3 MWh
..c: 1.5 3: ::E:
>-c.!J c::: L1.J 1.0 + z L1.J
>--I :z: .-:z: 0 :E:
0.5
o
r~ ALBUQUERQUE
~ \ / . '" LOAD
J F M A M J J A SON D ANNUAL TOTALS
• Load 14.8 MWh • PV System Output 9.7 MWh • Utility Make-up 9.7 MWh • Sell Back 4.6 MWh
10 KWSYS 14.8 MWh 18.3 MWh 9 MWh
12.6 MWh
FIGURE 6-13. Monthly Performance Summary for 5 kW System
Electrical Equipment Modifications
Table 6-3 summarizes the key electrical comparisons of the two systems.
TABLE 6-3
Comparison of Design Parameters'and Component Sizes
PARAMETER
Array Size
Full Load, System Current amps
Fault System Current, amps
Positive Bus Bar Length, feet
Negative Bus Bar Length, feet
Bus Bar Size, inches
Positive Service Entrance Cable Length, feet
Negative Service Entrance Cable Length, feet
Service Entrance Cable Gauge, AWG
Inverter Size, KW
Breaker Size, amps
..
4.3KW SYSTEM
25S x lOP
23.5
26
28.4
28.4
1 x 0.010
26
47
10
5
40
8KW SYSTEM
25S x 19P
44.6
50
54
54
5/8 x 1/8
26
47
4
10
60
The service entrance cabling can be reduced from #4 AWG to #10 AWG while the
busbar sizes are reduced to 1" x .01". This busbar size still allows sufficient
surface area for good and durable electrical connections to the shingle module foil
leads. An advantage to using this busbar size is that it can be purchased in 100 ft.
Ilroll form" lengths. Installation of the busbar can be in one continuous length
=liminating the "cad welding" of the standard 12 ft. busbar length required for
6-19
the 8 kW system. Even though the'" x 0.01" busbar is less rigid than the 5/8" x 1/8"
version, similar attachment processes of the busbar to the service entrance cable can
be employed. All of the electrical wiring would proceed as indicated in Figure 5-16.
Solar Thermal Hot Water System
An option for the all electric/utility feedback system for the Southwest is
the addition of a small solar thermal system for domestic hot water (DHW). Since
hot water heating solar systems are currently available, their availability and
state of development in the 1986 time frame should make them an attractive
option. This section discusses this option in general terms and detailed
analyses of this option are planned during the second design effort.
Figure 6- 14 shows a simple schematic of a separate PV/Thermal system for
domestic hot water heating. Since DHW loads are a year round load, it pro
vides a good solar thermal application. The PV output would be used for
normal electrical house loads plus space conditioning with the heat pump and
would provide backup energy to the DHW tank. The schematic shows an over
sized DHW tank which also acts as the thermal storage tank. The use of a
separate thermal storage tank and a normal sized DHW tank would be a design
detail requiring tradeoff analyses. In addition, a double walled heat ex
changer is shown between the collector loop and the potable water. Previous
studies of side-by-side thermal systems in Reference 1, where the thermal
output was used for both space heating and domestic hot water heating, showed
economic potential for this system in the Southwest. This system assumed a
6-20
SOLAR THERMAL COLLECTOR
PHOTOVOLTAIC COLLECTOR
FIGURE 6-14.
DHW TANK
UTILITY BACKUP AND FEEDBACK
r----t;lJQo--... ·1I100F HOT WATER DEMAND
COLO WATER SUPPLY
OTHER ELECTRICAL LOADS
PV-Solar/Thermal nomRstic Hot W~t-~M ~ . t ~ h - 0,_, -)ys em -.>c emati ~
eparate hot water storage tank. A review of that analysis for Phoenix in
icated that with approximately 6% of the 95 m2 roof area covered with solar
~ermal collectors, 86% of the useful thermal output went to the DHW demand
ld 95% of the hot water lead was supplied.
Some additional preliminary analysis was performed for Phoenix varying the
6-21
percent of total roof area covered with thermal collectors. Cost estimates for
the thermal collectors and balance of system were developed for 1986 in 1980$
applying the two markups as discussed in Appendix E. Collector costs of $86.07/m2
($8/ft2) or $113.83/m2 ($10.58/ft2) after markups were assumed. Balance of system
costs which included the thermal storage tank, pumps, valves, solar controller,
heat exchanger and air separator were estimated at $196.23/m2 ($18.24/ft2). The
cost to benefit ratio results are shown in Figure 6- 15. The optimum thermal
collector area for Phoenix is still low since 95% of the DHW load can be supplied
with approximately 5.5 m2 of collectors. Since the solar thermal output
cannot be used other than for DHW heating in the summer, higher collector areas
are less economically attractive. This has only been a preliminary analysis
within the time constraints of the first design effort. More detailed simulation
will be provided on the second detailed design.
HVAC Heat Recovery Options
As energy conservation measures become more seriously considered by the home
owner and by industry, more devices and options for waste heat recovery will be
employed in the home. One type of device currently on the market is a Hot Water
Bank (HWB) Heat Recovery Unit. This unit is designed to recover waste heat from
the home air conditioning unit to preheat domestic hot water in the summer. Sim
ilar units for heat pump systems are also being considered which could reduce the
year round domestic hot water demand for the residence. Figure 6-16 presents a
simplified schematic of the HWB heat recovery unit. Basically, the HWB is an
auxiliary heat exchanger that acts like an extra condenser.
- 6-22
0 ....... l-c:1: 0:::
l-....... LL.. w :z: w CD
0 f--
t---U1 C) u
1.2 t i i
1.0 i t I
i
I .8 i
.6 t I
.4
.2
o
t 95% OF DHW LOAD SUPPLIED
i----+---__+_ --+--
10 20 30
PERCENT THER~1AL
ROOF AREA
40
PHOENIX
• FEEDBACK PV (PS = .5 PE)
• SAME ECONOMIC ASSU~PTIONS
• THERMAL COLLECTOR COSTS COLLECTOR $113.80/M2
B-O-S $196.23/r,12
• TOTAL AREA - 92.9 M2
• THER~~AL STORAGE 10. 88 GAL/~~2
FIGURE 6-15. Preliminary Economic Analysis for Solar
Thermal Hot Water System
r--------------, DHW _----, LOAD
DOMESTIC HOT
WATER TANK
HOT WATER
SIGNAL FROM DHW ALLOWS HWB TO OPERATE ON DEMAND
1 COLD
WATER SUPPLY
I EVAPORATOR
CONDENSER PUMP
I ----------------
FIGURE 6- 15. Hot Water Bank (HWB) Heat Recovery Unit
6-23
The heat exchanger consists of two concentric tubes. The refrigerant which
has absorbed heat during the functioning of the central air conditioning system
circulates through a heat exchanger. Water from a conventional water heater is
circulated in the heat exchanger space between the tubes. When the circulating
water is at a lower temperature than the gas, heat is transferred through the
copper walls of the tubing, thereby heating the water up to l60oF.
Application of the Hot Water Bank can be made with air conditioning equipment
in one-ton through five-ton range . The system capacity is approximately 9.2
gallons-per-hour per ton of rated cooling system capacity, and thus, a five-ton
unit could provide up to 46 gallons of heated water per hour.
Among the system's components are a sealed motor/pump providing circulation of
the water, a temperature-sensing switch that controls the motor/pump operation, and
a temperature-sensing valve that controls the temperature of the water leaving the
Hot Water Bank and prevents overheating hot water. The circulation pump is poten
tially energized whenever the thermostat calls for cooling, and its operation or
lack of operation is controlled by the water temperature in the tank. The
complete unit is compact in size (18~" x ll~" X 6 3/4") with all components
completely self enclosed. Except for piping connections, the existing house
central air conditioning and electric hot water heater systems remain unchanged .
The unit has an additional advantage over solar systems in that it provides useful
energy whenever the heat pump or air conditioner is run including nighttime and
cloudy days. It also provides a slight increase in the air conditioning COP since
it provides additional subcooling of the refrigerant which gives increased capacity
without additional compressor power .
6-24
In a heat pump installatio~ when the heat pump is in the heating mode, the
HWB reduces the capacity of the unit for household heating requiring the heat pump
to operate longer to satisfy the space conditioning loads. A benefit or energy
savings, however, can result since the heat pump is operating at a higher COP than
the domestic hot water utilizing r2R, which has a COP of 1.
The units are currently in residential use in Florida on air conditioning
systems. Potential savings of 30 to 60% of the kWh demand are possible depend
ing on the climate area. Minimum benefits would occur where extended lengths of
non-heating and non-cooling days are apparent. Typical manufacturers are Carrier
and General Electric. Installed cost estimate range is approximately $600 to
$750. Therefore, this design option appears to provide significant energy sav
ings at a low initial cost. The impact on the PV system is that the house load
requirements would be reduced, and according to the tr~llds shown in Figure 3-5,
a higher cost to benefit ratio may result or different optimum system sizing may
be required. Furthermore, if savings of 60% in DHW requirements are achieved,
the market for solar thermal hot water systems costing in the range of $1600 to
$2500 would be severely affected. Detailed simulations of the HWB are planned to
be included on the third detailed design.
Ground Source Heat Pumps
The last design option which may impact the PV system sizing is a residential
ground source heat pump HVAC system. Ground source heat pumps have been around
for a long time as evidenced by the discussions in Reference 10 which was writ
ten in 195Q. Applications to residential homes, however, have been practically
non-existent. The recent Solar Energy Intelligence report of July 30, 1979
5-25
discussed the current interest in this type of system, termed II hydrothermal II
systems, in Ohio. In fact, legislative procedures are underway to make such
systems exempt from state sales taxes and real property tax. The heat pumps
use the constant water table temperatures or pump water through artificial
aquifiers to take advantage of constant ground temperatures. The resultant
average COP is over 3 as compared to current air-to-air heat pumps which average
around 2. There are over 1000 installations currently in Ohio. Dayton Power
and Light Company is currently monitoring several of the installations and they
appear to be providing savings over conventional heat pump systems but no
quantified economic savings have been identified to date.
Additional information on the systems is being gathered. The obvious impact
on PV systems is again in the area of a reduced space conditioning demand. Suf
ficient information details on these systems may preclude their inclusion in
any of the detailed designs.
6-26
SECTION 7
REFERENCES
"Final Report - Regional Conceptual Design and Analysis of Residential
Photovoltaic Systems", Report No. SAND78-7039, General Electric Company,
January, 1979.
"Final Report System Design of a Photovoltaic Flat-Panel Applications
Experiment at Busch Gardens, Tampa, FL", Report No. DOE/ET /23056-l.
i. "Final Report - Development and Testing of Shingle-Type Solar Cell Modules",
Report No. DOE/JPL-954607-79/4.
IIFinal Report - Conceptual Design and System Analysis of Photovoltaic Systems,
Report No. ALO-3686-01, General Electric Company, March 1977.
"Final Report - Residential Photovoltaic Module and Array Requirements Study",
Report No. DOE/JPL 955149-70/1, June, 1979.
"Solar Energy Thermal Processes", J. A. Duffie and W. A. Beckman, Wiley 1974,
Page 76.
"Thermal Performance Testing and Analysis of Photovoltaic Modules in Natural
Sunlight", J. W. Stultz and L. C. Wen, JPL Document No. 5105-31, July 29,1977.
Personal Communication with R. Smith, Sandia Laboratories, October, 1979.
IITypical Electrical Bills 1977", prepared by the Federal Power Commission,
Bureau of Power, U.S. Government Printing Office, Stock No. 015-000-00364-2.
"Heat Pump Applications", E. N. Kemler and S. Oglesby, Jr., McGraw-Hill Book
Co., Inc., 1 950.
"Residential Photovoltaic Module and Array Requirement Study", Report No.
DOE/JPL-955149-79/1, Burt Hill Kosar Rittelmann Associates, June, 1979.
"Photovoltaic System Sizing Analysis", G. Jones & E. Mehalick, Paper Presented
at the Fourteenth IEEE Photovoltaic Specialists Conference, San Diego, CA,
January 7-10, 1980.
7-1/2
APPENDIX A
POWER CONVERSION SYSTEM SPECIFICATION
The current "state of the art" technology for power conversion devices does not
provide for off-the-shelf "catalog" packages to meet all the needs of residential
photovoltaic applications. This situation is changing with the development of
photovoltaic application designs, and several conversion devices are currently
being developed. This specification is provided as a set of requirements for
a power conversion device supplier so that a matched set of components can be
designed and supplied as a unit of three packages with suitable installation,
adjustment and operation instructions.
The specification has been reviewed by Windworks of Mukwanago, Wisconsin, the
marketing agent for the Gemini Inverter. The power conversion system can be
supplied by Windworks as a design variation of existing catalog items with some
standard and special options. As photovoltaic applications proliferate, it is
anticipated that several suppliers will have catalog items that will meet all
of the requirements specified herein.
A-I
SPECI FI CATION
FOR A
RESIDENTIAL PHOTOVOLTAIC
POWER CONVERSION SUBSYSTEM
FOR THE SOUTHWEST
A-2
Section
1
2
3
3. 1 3. 1 . 1 3. 1.2
3.2 3.2. 1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7 3.2.8 3.2.9 3.2.10 3.2.11 3.2. 12
3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7 3.3.8 3.3.9 3.3.10
4
4.1 4.2
CONTENTS
Scope
Applicable Documents
Requirements
Functional Requirements General Power Conversion System
Performance Requirements Input Power AC Line Voltage Presence Startup/Shutdown Voltage Maximum Power Operation Short Circuit Protection Undervoltage and Overvoltage Protection Open Circuit Protection Effi ci ency Harmonics Power Factor Controls and Indicators Data Interface
Design and Construction Physical Characteristics Service Conditions Grounding Li fe Electrical Safety Electromagnetic Interference Audible Noise Thermal Dissipation and Cooling Maintenance Documentation
Background Information
Photovoltaic Array Characteristics PCS Startup/Shutdown
A-3
Page
A-4
A-5
A-6
A-6
A-8
A-ll
A-15
SECTION 1
SCOPE
This specification establishes the requirements for the performance, design,
construction and testing of a single-phase, three-wire, sixty hertz, utility
line, voltage-controlled, photovoltaic power conversion subsystem (PCS) for
resid~ntial application.
A·4
SECTION 2
APPLICABLE DOCUMENTS
The following documents form part of this specification to the extent
specified herein. In the event of conflict between this document and the
referenced documents, the more stringent requirement shall apply.
NFPA 70 National Electrical Code
3.1 FUNCTIONAL REQUIREMENTS
3.1.1 GENERAL
SECTION 3
REQUIREMENTS
The PCS shall invert the variable dc output of a solar photovo1taic array to supply
residential ac electrical service in parallel with utility electrical service.
The PCS shall be operated as a current source with an output ac voltage control
led by the utility line voltage. The PCS shall regulate its dc input to maximize
the dc power obtained from the array. The PCS shall be capable of continuous
operation and its rated input power.
The PCS shall include the major functional components of a dc filter, inverter and
transformer as shown in Figure A-1. The Inverter shall include the inversion
components, control logic for automatic operation and power maximization, pro
tective components, control switches, adjustments, and indicators.
3.1.2 POWER CONVERSION SYSTEM
The PCS shall provide the interface between the solar array dc output and the
residential electrical service. Power output in excess of instantaneous residen
tial demand will be fed back to the utility lines. The PCS will automatically
start operation and connect the solar array to the service panel whenever the solar
array dc voltage exceeds the specified minimum value. The Contactor in Figure A-I
between the transformer and the service panel will operate upon receipt of the dc
minimum voltage activation signal. The PCS shall automatically disconnect the
solar array in the event of loss of utility line voltage or a solar array power
A-6
::t:> I
-...J
urlL . ~C
IT'( IN
--; VOUTSIDE WALL.
UTILITY r-METER
'240/12.0 V AC., 60 H!.. SIH~LE PHAS ":.1 3 WI R E -" ,...
---0 ~ ,:- - - - - - - - - - - - -1 {
'-
I I
I I START UP/ SHUTDOWN
\,SURGE I I PROTECTOR > I
J ~ ? TRANHORMER I r- \ I 60A I I :1 ': I~ + I n I
1 I 1 I ~OA 1 DC 0-r- ~ ~
I I ...... '" ~ ,... - '"
,.,. '" Il -'" - ~ - ~ ~
I
{~
150LATION FILTER INVERTER I D6CO'.,eT 11 I
L _ ____________ - ___ - - _I
POWER CoNVERSION S.YSTe.M
Figure A-I. Interconnection Diagram
N
L2.
LI
5E RVI"CE . PANEL V
,OOA ( (
GOA -~ 0-
60A
~
I ()
SURGE PROT£C OR.
f--
I:U f--
0
>
ISA :--0--
I -- }
T(P,CAl . ~R/~N(1i
w~ r:.p. PIPE. -.~ 6 I
SERVICE ~RGUND I (,ROUNG ROLJ
drop below the minimum specified level. The interface logic for startup and shut
down control is shown in Figure A-2.
3.2 PERFORMANCE REQUIREMENTS
3.2.1 INPUT POWER
The PCS shall be capable of continuous operation at an input dc level of 200 volts
and 50 amperes.
3.2.2 AC LINE VOLTAGE PRESENCE
The PCS shall not initiate or continue operation whenever the L1 or L2 ac voltage
lines are outside the voltage limits required for normal correct commutation.
3.2.3 STARTUP/SHUTDOWN VOLTAGE
The PCS shall automatically initiate the signal for closure of the output contac
tor and the inversion process whenever the dc input voltage rises above 180 Vdc.
The PCS shall automatically terminate the inversion process whenever the real
output power is less than zero. Sequencing or timing of the startup and shutdown
process shall insure correct PCS operation. Once the shutdown sequence has been
initiated, the PCS shall continue in the "off" mode for 5 minutes.
3.2.4 MAXIMUM POWER OPERATION
The PCS shall dynamically control its operation to operate at the instantaneous
maximum power point operating voltage of the solar array within ~ 1 percent over
the input voltage range from 180 to 220 Vdc. The PCS shall operate at 180 Vdc
whenever the maximum power point is less than 180 Vdc, and it shall operate at
220 Vdc whenever the maximum power point is greater than 220 Vdc ..
r-\..:l
TOD( FILTlJ
i"
CDt
AU TO t.1A,. 1'- OPERATION FOR VOc..., MINlMUf'\ AuTOMATIC SHUTDOWN 0/'1 AC LINE LO~$ NO At LOAD IN OFF CONDITION
--, r~-'-
VOC)MIN I I
S 1 (ON/OFF CONTROL) I
L04ICOR. -, ....... -' C _-'-_
EQUIVALENT -T-
I "I ___ J I '------ I I
r~ I @t ~ '----y--J AUTOMATIC 120VAC. OPERATION SERVIC.E WITH LINE WHEN
®" CONNECTION VOC )MIN
./ ,
A @t 1-1 ,.. l A"OMA", AUTOMATIC
POWER INPUT LINE WITH LINE CONNECT'OW -< ___ J 'O,"EmO'
WITH VDC}MIN
'-
INVERTER
-
-
FIGURE A-2. Power Conversion System Interface
TRANSFORMER
®~ { B .
I
I I
I)
" " @
Ll -- -I -
B !--
V-r~"-- ~ Ita !
,--0---;. --- )
J l~ ~r'R\'I( .. t
. CONTACTOR f'''~tl
3.2.5 SHORT CIRCUIT PROTECTION
The PCS shall be equipped with protective circuits or devices which will prevent
damage to the PCS due to short circuit of the input or the output.
3.2.6 UNDERVOLTAGE AND OVERVOLTAGE PROTECTION
The PCS shall not be damaged by undervoltage or overvoltage conditions at the PCS
input or output.
3.2.7 OPEN CIRCUIT PROTECTION
The PCS shall not be damaged by open circuit conditions appearing at the dc input
or the ac output or both simultaneously.
3.2.8 EFFICIENCY
The PCS shall have an efficiency greater than 92% at the rated input specified in
Section 3.2.1. Operating power losses shall not exceed:
o 300 Watts-Tare loss
o 3 Volts-Thyristor bridge loss
o 0.125 Ohms-Resistive loss
3.2.9 HARMONICS
The PCS RMS total harmonic content of the output current shall be less than 30%
of the fundamental at the rated input specified in Section 3.2.1.
3.2.10 POWER FACT0R
The PCS power factor shall be greater than 60% at the rated input specified in
Section 3.2.1.
A-IO
3.2.11 CONTROLS AND INDICATORS
The following controls and indicators shall be provided on the front panel of
the Inverter enclosure.
1. Inverter On/OFF switch
2. Input current meter
3. Input voltage meter
4. PCS ON indicator light
3.2.12 DATA INTERFACE
The PCS shall provide as an option, controls and terminations for manual solar
array IV characteristic IV trace, and continuous signals to a data system during
both manual or automatic operation. Analog (0-5 Vdc range) signals shall include
1. dc input current,
2. dc input voltage
3. maximum power operation on/off
4. output current
5. output voltage
6. ac output real power
3.3 DESIGN AND CONSTRUCTION
3.3.1 PHYSICAL CHARACTERISTICS
The PCS shall be housed in three wall or floor mounting enclosures meeting the
requirements of the National Electrical Code. Enclosures shall provide access
for installation, service, and maintenance. The three enclosures shall have
maximum dimensions and weights as specified in Table A-I.
A-ll
Enclosure
DC Fi Her
Inverter
Trans former
Height (in)
15
30
20
TABLE A-I
PCS
Width (in)
12
24
17
3.3.2 SERVICE CONDITIONS
Operating
Non-Operating
Ambient temperature Relative humidity
Barometric pressure
Ambient temperature
Relative humidity Barometric pressure
3.3.2.3 Shock and Vibration
Depth (in)
12
9
15
00 to 400 C
Weight (lb)
120
60
300
up to 95% noncondensing
520 to 790 mm Hg.
-25 to 600 C
up to 95% noncondensing
520 to 790 mm Hg.
The PCS shall be constructed to withstand normal handling and transportation
environments. Special packaging or restraints required for shipment shall be
provided by the supplier.
3.3.3 GROUNDING
Input and output circuits shall not be grounded within the PCS enclosures.
A-12
Enclosure safety ground connections for interconnection to service ground as
shown in Figure A-I will be provided and identified by the supplier. Neutral
wiring for the transformer secondary and primary may be specified as required.
3.3.4 LI FE
The PCS shall be designed for a 20-year life with a minimum of maintenance.
Any required preventive maintenance requirements shall be identified.
3.3.5 ELECTRICAL SAFETY
The inverter design and construction shall conform to the applicable require
ments and practices of the National Electrical Code (NFPA 70).
3.3.6 ELECTROMAGNETI,C INTERFERENCE
Good design practices shall be followed to mini~ize electromagnetic interference
and susceptibility. Conducted interference of the PCS shall be less than 200 ~fV-./
between 5 kHz and 3 MHz.
3.3.7 AUDIBLE NOISE
Audible noise from the PCS shall be less than 50 dB one meter from the equipment
when mounted in accordance with installation instructions.
3.3.8 THERMAL DISSIPATION AND COOLING
The PCS shall be designed to operate in the environment defined in Paragraph
3.3.2 without external cooling devices. Integral fans or blowers, if required,
shall utilize ambient air. Interlocks shall be provided to shut the inverter
down in case of failure of internal cooling devices.
fc-13
3.3.9 MAINTENANCE
Accessibility
The PCS shall be designed to allow ready access for installation, adjustment, and
maintenance. Insofar as possible, plug-in cards and modules shall be utilized to
facilitate troubleshooting and repair.
Replacement
A listing of parts required to support the unit will be provided in the Operation
and Maintenance Manual. This list shall identify those items required for opera
tional support and maintenance.
3.3.10 DOCUMENTATION
Drawings
Schematics, wiring diagrams, and significant assembly drawing information shall be
included in the Operations and Maintenance Manual.
Operations and Maintenance Manual
An Operations and Maintenance Manual shall be provided.
foll owi ng:
It shall consist of the
1. General Description (a brief overall description of function and per
formance)
2. Installation Instructions (mechanical mounting and electrical connections)
3. Adjustment Procedures (set-up and calibration information)
4. Operating Instructions (description of operating controls and sequences)
5. Parts List (a list of applicable parts and replacement items)
Provision shall be made for permanent storage of the Manual in the inverter enclosure
A-14
SECTION 4
BACKGROUND INFORMATION
4.1 PHOTOVOLTAIC ARRAY CHARACTERISTICS
The dc input power to the PCS shall be supplied by a series parallel matrix
array of solar photovoltaic cells. The power output of the array is a variable
dependent upon the level of solar irradiance and solar photovoltaic cell tempera-I
thre. The static peak power rating of the array at an irradiance level of 1 kW/m2
and a nominal operating cell temperature of 640 C is 8 kW at 183 volts as shown by
the array current voltage (IV) characteristic curve of Figure A-3. The array IV
characteristic curve at the reference cell temperature of 280 C shows a peak power
point of 9.66 kW.
The dynamic characteristics of the array over the course of a year with variable
insolation, ambient temperature, and wind velocity are displayed in Figures A-4,
A-5, and A-6. These curves are plots of the peak power point and voltage at the
peak power point. Figure A-4 shows the distribution function of the array peak
power point with respect to the annual energy produced by the array when operated
continuously at its peak power point. Figure A-5 shows the distribution function
of the peak power voltage with respect to the annual energy. Figure A-6 shows the
duration of the peak power output over the hours of a year.
The curves of Figure A-4, A-5, and A-6 have been prepared from calculations of
the hourly environmental conditions seen by the array. The data base used for the
hourly conditions was the Albuquerque TMY SOLEMT record supplied by the National
Climatic Center. This data represents a set of conditions that produces array
performance near the high end of continental U.S. performance for this roof array
design.
A-15
"f" I-' cr.
IV = CONSTANT ~
50, 0c _
en a:..
40
~ 30 « I-Z w a: a: ::::> 20 u
10 I
VMP = 1f-·3
I I
64°C (NOCT)
NOTES
1. ARRAY CONFIGURATION 25S BY 19P
2. VALUES REFLECT BUS BAR AND CABLING LOSSES
3. INSOLATION = 100 mW/CM2
o I I
o 50 100 150 200 250 300
VOLTAGE, VOLTS
FIGURE A-3. I-V Characteristics at Reference
Conditions for Inverter Input
I-:::> Q.
I-:::> 0 >-~ (!):t: a:~
» w~ I Z_ -' W -....J ...J Q.'
<t" :::>...J Zw Z> <tw >-...J <ta: a: w a:~ <to
Q.
a: I-<t<t ...J 0 en
22 r 20.1 MWH ARRAY CONFIGURATION
20 ,- - - -. - - 25 S BY 19 P - . - -~ --.-.- .-~
BLOCK IV SHINGLE I 8.0 KW
18
16
14
12
10
8
6
4
2
0 0 1 2 3 4 5 6 7 8 9
SOLAR ARRAY MAXIMUM POWER OUTPUT.-vP (KW)
FIGURE A-4. Integral Distribution of Solar Array
Maximum Power Point Power for Albuquerque
11.2 KW I I
20.7 MWH
10
J:
.~ ~ o..~ l- e. ::> o E
);> I
» C)V
C:) a: w wC) z<{ wI-...J...J <{o ::» za: ZW <{~ >0 <{o.. a:~ a:::> <{~ a:x <{<{ ...J~ 01-CI)<{
22
20
18
16
14
12
10
8
6
4
NOTES
1. ARRAY CONFIGURATION 25 S BY 19 P
BLOCK IV SHINGLE
1. 9 MWH
2L--------
SOLAR ARRAY MAXIMUM POWER VOL TAGE,...Vmp (VOLTS)
FIGURE A-5. Inteqral Distribution of Solar Array
Maximum Power Point Voltage for Albuquerque
245.5 V I I
20.7 MWH
P I -' 1.0 -~
~
0.. ~ ex: w ~ 0 0..
12
11
6 I 789 HOURS I
5 , I I I I I I I I . I I J:::.... -o·~------------~------------~--------~--------~------~ o 500 1000 1500 2000 2500 3000 3500 4000
DURATION HOURSAT POWER:> P
~IGURE A-6. Power Duration Plot for PV Array .:, AI buquerqlle
I
4500
It should be noted that the power and voltage limits specified in Section 3 of
this specification will limit the peak power output to 10 kW and voltage to
200 + 10%. These limits will result in approximately a 1% reduction of the
available annual energy from the array under the Albuquerque conditions. The
limits will cause the array to be operated at a point on the characteristic IV
curve other than the maximum power point when limit conditions are reached.
4.2 pes STARTUP/SHUTDOWN
The logical sequence specified in Section 3.2.3 above is based upon array open
circuit voltage conditions. Alternate sequences based upon array short circuit
current conditions that meet the requirements for automatic startup when enabled
by the pes on/off switch with automatic shutdown on either ac line failure or no
net power output will be acceptable.
A-20
APPENDIX B
PHOTOVOLTAIC SHINGLE INSTALLATION INSTRUCTIONS
Introduction
The photovoltaic shingle module is designed to be installed with a procedure
similar to conventional shingles. Electrical solder connections however. are re-
quired for attachment of the first row of active PV shingles to the negative bus
bar at the roof eave and for the last row to the positive busbar at the roof ridge.
Figure B-1 shows the different type of shingles used for a complete instal
lation and lists the part numbers (Gl, G2, G4, G5, G6, G7, G7, and Gll and the
Solar Cell Module, SCM) referred to in the installation instructions. This appendix
initially specifies installation 'safety procedures to be followed and then presents
detailed installation instru·ctiQns,.
B-1
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B-2
General Safety Procedures
The photovoltaic array can be installed during daylight hours but several
safety precautions must be enforced. Installation on a cloudy but dry day would
be preferred, but safety procedures must still be enforced. Each SCM module can
generate a maximum potential of 12 volts DC (similar to a standard automotive
battery). The series connection of SCM modules up the slant height of the roof
will provide a potential of 300 volts DC from eave to ridge. CAUTION must be
exercised to insure that no conductive path is provided across SCM terminals by
installation personnel or equipment. The following list itemizes key safety
requirements.
• No metal ladders or scaffolding should be used.
• Installation should proceed only on a completely dry roofing surface
• The negative array ground terminal should not be connected until the
electrical installation is complete
• The modules should be installed one course at a time from roof edge to
roof edge and not in any staggered pattern
• No electrically conductive material should be laid over the roofing surface
during the installation
• The installation should be accomplished working from the roof surface as
much as possible
• Grounded components, such as plumbing standard pipes should not be touched
and preferrably they should be covered with an electrical insulating cover
during the installation
• Care should be exercised not to make physical contact with module termina
tions across a multiple of installed shingles
B-3
Installation Details
The following pages provide the suggested installation instructions. The
instruction details are described for the 25 series by 19 parallel module matrix
for the current design. This results in an odd number of series rows and odd
number of parallel strings. In general, if the PV roof design ended with even
numbered series rows or parallel strings, the instruction and appearance for the
last row at the top of the roof and the last parallel string at the right roof edge
would be different (ORPosite hand) than that described and sketched.
B-4
1. LAY OUT BASELINE AND PERPENDICULARS
Strike chalkline parallel with bottom of roof 12 7/8" from the eave. Run perpendiculars . from Working Points #1 and #2 to the ridge, being sure both lines lie at least 1/2" in from all points along the overhang (rake) of the roof. Also be sure the lines are 52'- 7 3/4" apart. Total length of roof along slope is shown on the drawings.
Length up the roof must be 22' 4" from eave
~rc;>~",~ --------
to ridge. If roof is too small, adjust the WOIiO£IHG WOO<INGr • eave to be sure that top shingle does not R:>,·..J1 *" I ~HAt.K LINe: 'I Po""'"'T project above ridge. If roof is too "b" i,-4oe-...... • /. ---.....;. large, adjust ridge and/or eave. Roof '15-¥--""1-_--.:~7a'_r------------+_\ must be within 2" of slope dimension required.
2. SET BOrrO!>! RON OF DUMMY SHINGLES
Starting with the left side of the roof (the same side as the service entrance) set a Gl shingle along the eave line. Top of shingle should be " below chalkline. Set shingles 32.4" apart. End row on right side (opposite service entrance) with G-6 shingle. ~ail shingles in with roofing nails driven only through nail spotters.
3. PREPARE CABLE END OF BUS BAR
Drill 1 1/2" hole through roof just outside of a line extended from wall of house. Hole should be as close as possible to the top of the G-l shingle. Hole should be drilled through entire overhang perpendicular to roof.
Through the hole, bring up the cable end to which has been fastened the compression connection lug. Draw through about 2 feet of cable for easy working.
Drill 1/4" diameter hole through end of first section of busbar. Sl~p 2 feet of shrink-fit insulation over the busbar. Then bolt lug to busbar, and slip insulatioi. down over the lug, partially covering the cable.
Bend busbar so that lug is entirely exposed below roof overhang when turn-down end is poked through the hole in the roof. This should leave at least 6" of insulation on the flat part of busbar.
3-5
~~ _______ :;:ou,..::..I~~ __ ._ ... _._ ... _ .. _._
I . ~
4. MOUNT NEGATIVE (LOWER) BUSBAR.
Poke the turn-downed end of busbar back through the roof. Attach sections of busbar together with fusion weld connections, using temporary nails to keep bars from falling off the roof. An asbestos insulating sheet should be placed beneath each weld connection during the welding process. Cut last bus 8" short of right end of roof.
5. MOUNT NEGATIVE TERMINATION SHINGLES G-2 and §::2
52.'· 7Y."
Begin the next row from the right, so that G-2 shingles stagger relative to G-l shingles. Lay the shingles upside down above the busbar, so that the foil tabs extend over the busbar. Wr-~~--Shingle should be at 32.4" spacing, exactly 11r---~===~;iJ~===~~~:ta3_--Jj one-half module over from row below.
Clean underside of foil tab and top of bar at each joint w:i.th steel wool. Plate insulating sheet beneath solder jgint. Coat cleaned area lightly with non-corrosive flux. Place preformed solder strip on bar within cleaned and fluxed area. Position foil tab over solder, and hold down with special wire tool supplied with shingles Apply torch to top of foil on bus bar till solder flows out around edges of foil onto bar. Remove torch and continue firm hold with tool until solder sets up. Torch time is about 10 seconds; holding time after soldering is about 10 seconds.
After soldering is completed, bed the busbar in position about 1/4" above the top of I the first row of shingles. Bed it in a • heavy coat of PI iobond. TIlen cover the busbar II' thoroughly with Pliobond, to form insulation. ~U< ELe-c..r"'CAi,..
/.. ,Nt!..-. 1lirc~."f~'_"_' .A_:'_--~I __ ~:I Now turn the shingles over carefully. Beginning -:: i ~ :~ , .. . \j . at the right, align the right hand shingle YI I ~ to e .. __ ~~ ~r:t ..
at the right edge, while also aligning the I ___ -\1'1- ___ _ _ __ -\,r- ~ __ i'
two terminal holes over the chalkline. ~lake I G'z... ~, 6'~ ~ \ sure the shingle is parallel with the line, ~G~'_7~-J~ ____ ~~ ____ ~ ________ ~~~~J and secure with nails driven through the nail \ .
\ ~ I spotters marked on the shingle. ~c:::~~~~ ~ Using the spacing tool provided with the shingles, . ~ "C:>PA-:'IN~ iOC,,-align the next G-2 shingle at the proper dis-tance, and with the two terminal holes over the chalkline. Continue for entire row.
At end of row, attach a dummy G-7 shingle.
6. MOUNT COLLECTION MODULES (SCM)
The modules containing the solar cells are automatically positioned when they are screwed into the module below. Start with an SCM module at the left side of roof, and set two screws per module into G-2 and G-7 modules below, using 10-15 pounds torque.
When shingle is positioned, nail in place with roofing nails driven through the 2 nail spotters on each shingle.
Finish first row with G-S right end dummy shingle.
Continue with successive rows. Second row begins on the right with SCM shingle, and ends on the left with left end dummy shingle G-4. Each row screws into the terminals in the row below.
Final row of SCM shingles has a right end dummy. Top of last row should be at least 5 1/4" from the ridge.
7. PREPARE CABLE END OF BUS BAR
As in step 3, drill a 1 1/2" hole through the overhang of the roof, just outside the end of the building. Since this hole is close to the ridge pole, dril the hole perpendicular to the ground, going through the overhang at an angle.
Attach the' compression connector lug to the cable, and draw lug through the hole, pulling through enough cable to allow easy working.
Slip on shrink-fit insulation, drill busbar, attach lug to busbar as before. ~fuen bending bar down, also twist bar so that it can pass through the hole in the overhang without causing the busbar on the roof to twist. Then slip the shrink-fit insulation over the lug and shrink it in place. Be sure that the lug is entirely exposed below soffit of roof overhang (the hole is longer than at the bottom, since it is at an angle).
Temporarily support the busbar on nails, and fusion weld the sections together as in Step 4. Cut right end of bus bar back 2' from edge of roof.
B-7
W?I
8. MOUNT POSITIVE TERMINATION SHINGLES (G-9, G-ll)
Beginning at the right end, layout positive tennination shingles G-ll on the module. Lay shingles upside down with the tabs overlapping the busbar as in Step 5. Shingles will lie on opposite slope of roof, and may need temporary support.
G-ll shingles have two foil tabs and each should be attached to the busbar except for the last module at the right end of the roof. For this shingle, cut off right end tab and only solder left tab to busbar.
Solder tabs to busbar exactly as described in Step 5.
Upon completion of soldering, set busbar in Pliobond, and completely cover with Pliobond, to form insulation.
Carefully turn the shingles over, and align them with terminals below. Top of 3:1L;'bl(~s should lie along ridge. Finish rowan left end with G-9 left end dummy shingle.
9. MOUNT TOP ROW OF DU~!/IIY SHINGLES (G-IO and G-12)
Align top of shingle with top of last row (G-9's & G-ll's). Start with G-12 shingle at left, to lap joints in G-9/G-ll row. Nail through spotters.
G-12 G-12
10. MOUNT EDGE ANGLES
Nail special hold-down anchor clips at every other row of shingles along both ends (rakes) of the roof, to hold cover angle. Snap pre-painted aluminum angle into clips. Run continuous bead of silicon sealant between top flange of angle and shingles, to form a water tight joint. Run angle up to ridge, before mounting ridge cap.
11. MOUNT RIDGE CAP
Nail special hold-down anchor clips at 18" O. C. along ridge, and snap prepainted aluminum ridge cap into clips. Cap should cover last row of nails in shingles.
12. FINISHING SOFFIT HOLES
Close up holes in soffit of overhang with sealant to prevent insects from nesting.
13. REPLACEMENT
To replace a defective or broken shingle, force a wedge carefully beneath the cell portion of the shingle directly above. Insert a flat offset Phillips head screwdriver and unscrew the connections directly above the shingle. Do the same with the connections directly to the side of the shingle. Now, insert a slate na il cutter (see illustration) under the shingles on each side of the shing'le to be removed, and hook slot around nail. Strike cutter with hammer ~"here arrows indicates and shear the nail shank. Do the same -with the nails above the shingle. At this point, the shingle will slide out. Insert the new shingle and make all four electrical connections by prying up adjacent shingles and inserting the flat ratchet head screwdriver. The replaced shingle does not have to be nailed. Follow installation safety precautions for the replaceme~t of a defective module. Open the system DC contactor and remove the gorund to the negative terminal of of the array.
B-9/10
<
SLltTC /AIL. CVTTC~ '100L.
APPENDIX C
PART 1: GENERAL
RESIDENTIAL PHOTOVOLTAIC ELECTRICAL
INSTALLATION SPECIFICATIONS
1.1 REFERENCES
A. Cooperate and coordinate all electrical work with other con
tractors on site.
B. The complete residence with all facilities will be constructed
under another contract. The utility company service will be in
place along with the house circuit breaker panelboard and the
customary house wiring system. The roof will be constructed and
covered -with roofing felt ready for shingle installation.
1.2 SCOPE OF WORK
A. Provide all labor, materials, equipment and supervision necessary
to complete the electrical work associated with a roof mounted
photovoltaic (PV) system, including all equipment identified up
to the point of attachment to the regular house service panel
board.
1.3 WORK NOT INCLUDED
A. Normal house AC wiring system;
B. Construction of house including roof underlayrnent and roofing felts.
1.4 QUALITY ASSURANCE
A. The work shall be executed in strict conformity with the latest
edition of the National Electric Code and all local regulations
that may apply. In case of conflict between contract documents
and a governing code or ordinance, the more stringent standard
shall apply.
B. Unless otherwise specified or indicated, materials and workmanship
shall conform with the following standards and specifications
(latest edition):
1. National Electric Code
2. Occupational Safety and Health Act
3. Standards of Underwriters Labs (UL)
C-l
-2-
4. National Fire Protection Association (NFPA)
S. National Electrical Safety Code
6. Local codes
C. Carry out tests, secure permits, pay fees and arrange for all
inspection of regulatory agencies for work under thi~ section.
1.S SUBMISSION DATA
A. Submit six (6) copies of all equipment to be incorporated into
the work, for approval prior to ordering same.
1.6 PRODUCT DELIVERY, STORAGE, HANDLING
A. All equipment, upon receipt, shall be inspected for damage and
shall then be stored and protected from damage until project
completion.
1.7 OPERATING INSTRUCTIONS AND MAINTENANCE MANUALS
A. Provide operating instructions to designated persons with respect
to operation functions and maintenance procedures for all equip
ment and systems installed.
B. At project completion provide two (2) copies of bound brochures
including all shop drawings, maintenance manuals and spare parts
lists.
1.8 ELECTRICAL CHARACTERISTICS
A. The PV array will produce an output of approximately 183 volts
DC, which will then be inverted to a nominal voltage of 120/240
volts single phase. Capacity of the system is approximately
8 KW.
1.9 TEMPORARY LIGHT AND POWER
A. Provide temporary electricity as required to allow completion of
the work. Remove any temporary wiring when no longer required.
C-2
-3-
1.10 RECORD DRAWINGS
A. Maintain two (2) copies of the documents on site and record any
revisions on one set as the job progresses. At project com
pletion, transfer all changes to the other set.
C-3
-4-
PART 2: PRODUCTS
2.1 RACEWAYS AND FITTINGS
A. Conduit - General
1. No conduit shall be used smaller than 3/4" diameter. No
conduit shall have more than four 90° bends in anyone run
and where necessary, pull boxes shall be provided.
2. Rigid PVC conduit shall be Schedule 40 UL listed for 90°C.
All fittings shall be solvent connected. Provide threaded
fittings where connected to metallic boxes. PVC conduit
shall be equal to Carlon. PVC conduit shall be used for
exterior work and for raceways enclosing ground conductors.
3. Thin wall conduit (EMT), zinc coated steel, conforming to
industry standards shall be used for all interior raceway
systems. Fittings for EMT shall be compression type or set
screw. EMT shall be equal to Pittsburgh Standard Conduit
Company, Republic Steel Tube or Youngstown Sheet and Tube
Company.
4. Conduit fittings
a. Insulated bushings shall be provided on all raceways
larger than 3/4".
b. Access fittings: shall be type LL, LR or LB as required,
and shall be equal to Appleton, Crouse-Hinds or RACO.
B. Outlet, Pull and Junction Boxes
1. Each outlet box shall have sufficient volume to accommodate
the quantity and size conductors entering the box in accordance
with the requirements of the National Electric Code. Outlet
boxes shall be pressed steel as manufactured by Steel City,
RACO or Appleton.
2. Pull boxes or junction boxes shall be constructed of code
gauge sheet metal of a size not less than required by the
National Electric Code if no size is indicated on the drawings,
and shall have hinged doors.
C-4
-5-
2.2 SUPPLEMENTARY STEEL, CHANNEL AND SUPPORTS
A. Furnish and install all supplementary steel, channel and supports
necessary for the proper mounting and support of all equipment.
Provide minimum of 3/4" thick plywood backboards for mounting of
all equipment in the Mechanical Room.
B. All supplementary steel, channel and supports shall be UL approved,
be galvanized steel and be as manufactured by Steel City, Unistrut,
Power Strut or Kindorf.
2.3 CONDUCTORS
A. All conductors shall be stranded copper of the size indicated on
the drawings. All conductors shall be type THWN rated 90°C for
dry locations and 75°C for wet locations.
B. Conductors for use on the DC system shall be color coded red for
positive and black for negative. AC conductors shall be color
coded black for phase A, red for phase B, white for neutral and
green for ground.
C. All conductor terminations shall be made up by standard lug con
nections on equipment having same. Terminations made up for
attachment to positive or negative roof bus bars
or the DC input inductor I shall be bolted type compression
lugs as manufactured by Burndy, Thomas & Betts or Panduit of tin
plated copper. Bolts shall be 1/4-20 silicon bronze.
D. All terminations, other than located within enclosures, shall be
insulated.
2.4 BUS BARS
A. Positive and negative bus bars shall be minimum of 99% pure copper
of the size indicated on the drawings; standard lengths of 12 feet.
Joining of the sections of bus on the roof shall be by means of a
fusion welding process such as cadwelding or thermoweld. Bus shall
be set in and, after terminations to shingle tabs are complete,
covered with an insulating compound equal to Pliobond 20. The
portion of the bus within 4 inches of the turn-down into the roof
and for the rest of its length within the roof overhang to the
compression lug shall be insulated by means of heat-shrink tubing
as manufactured by 3M Company or Thomas & Betts.
C-5
-6-
2.5 SAFETY SWITCHES
A. Safety switches shall be general duty 2-pole or 3-pole fused or
non-fused in NEMA 1 or NEMA 3R enclosures as indicated on the
drawings and shall be capable of being padlocked. Switches
shall be equal to General Electric, Westinghouse or Square D.
2.6 A.C. CONTACTOR
A. A.C. Contactor shall be 60 amperes 2-pole, 600 volts, normally
open, magnetically held and rated for all classes of load. It
shall have 3-wire control and shall be mounted in a NEMA 1
enclosure. Contactor shall be equal to Automatic Switch Company
Bulletin 1035, Catalog number 3554C.
2.7 GROUNDING SYSTEM
A. There shall be four (4) isolated ground systems as follows:
1. Grounded Neutral - The house wiring system neutral shall be
grounded only at the house panel by means of a #6 AWG
connection between the panel neutral bus and the panel
ground bus. The panel neutral block shall be isolated from
the panel enclosure.
2. Equipment Ground - A #6 AWG green insulated ground shall be
looped between all equipment and shall be connected to each
piece of equipment by means of a ground lug on the equipment.
This conductor shall terminate at the house ground bus.
3. Varistor Ground - The #14 AWG green insulated ground conductor
from the Varistor shall be connected through a separate con
duit system. This ground shall terminate at the house ground
bus below the house service panel.
4. Array Negative Bus Ground - Provide a #6 AWG green insulated
ground conductor from the line side of negative bus terminal
at the exterior mounted overcurrent device to the house ground
bus below the house service panel.
5. House Ground Bus - Provide a 12" long 1/8" x 1" copper ground
bus on standoff insulators 12" above the floor below the house
AC panel. There shall be four (4) ground connections to the bus
as follows: #6 AWG ground to house panel ground bus, #6 AWG
ground to equipment ground system, #6 AWG ground at array negative
bus and #14 AWG ground for Varistor grounding . Provide a #6
C-6
-7-
AWG ground from the bus to the entering water system in
accordance with code. In addition, provide a #6 AWG ground
to a driven 3/4" x la' long Copper Weld ground rod. This
ground rod shall be located within the Equipment Room and
shall extend 4" above the slab.
6. Varistor Surge Protection
a. Furnish and install a Varistor for surge protection, which
shall be mounted in a junction box sized as required on the
drawings. The junction box shall be provided with an
insulating mounting block and terminal bar isolated from the
metal structure. A #14 AWG ground wire shall be tap con
nected to the positive bus conductor which is to be protected
by means of Burndy Servit type KS split bolt connector.
This connector shall be insulated by taping. The load side
#14 AWG ground from the Varistor shall be interconnected
to the house ground bus located below the house panel.
b. Varistors on the positive DC side of the invertor shall be
General Electric catalog number V275 LA40B.
B. Furnish and install a General Electric Lightning Arrestor nippled
to the side of the house service panel board. Unit shall be
catalog number TLP 175 and shall be connected with black leads to
line busses and white to ground.
2.8 PHOTOVOLTAIC EQUIPMENT
A. The photovoltaic equipment including all shingle modules as shown
on drawings E-l and E-2 and including the interconnecting screws,
edge angles, ridge cap and mounting hardware are available from
General Electric Company. See installation manual and drawing
E-3 for suggested method of erection of the roof array.
B. The DC input inductor, the invertor package and the isolating
transformer which form an integral part of the generating
and conversion system shall also be secured according to the
inverter specification document.
C. Upon receipt of this equipment, each of the several parts shall
be carefully examined to identify any possible shipping damages.
C-7
-8-
PART 3: EXECUTION
3.1 WORK COORDINATION AND JOB OPERATIONS
A. Be responsible for all equipment necessary for erection of the
roof array including staging. Commencement of array erection
signifies acceptance of the surface upon which the shingles
are to be installed. Refer to installation manual for suggested
erection process.
B. Coordinate all work prior to starting with all existing con
ditions of the structure.
3.2 PLANS AND SPECIFICATIONS
A. The drawings showing layout of equipment, especially within the
Equipment Room, show a suggested layout. Carefully check dimensions
of all equipment and make any adjustments necessary to accommodate
any variations.
B. Post the schematic diagram and equipment plan and elevation
(drawings E-5 and E-6) at a reduced scale under glass in the Equip
ment Room.
3.3 SYSTEM IDENTIFICATION
A. Provide screwed-on phenolic nameplates (black with white engraving)
on all equipment.
3.4 WORKMANSHIP AND INSTALLATION METHODS
A. All work shall be installed in a first class manner consistent with
best current trade practices. All materials and equipment shall be
securely installed plumb and/or level.
B. All equipment capable of vibration (iron core swinging inductor,
invertor cabinet and isolation transformer) shall be mounted
using Korfund vibration pods. All wiring connections to this
equipment shall be in flexible metal conduit.
C. All raceways shall be properly aligned, grouped and supported at
right angles to or parallel with the principal building members.
D. Any holes drilled through structure shall be neatly made and
properly sealed after equipment installation.
C-8
-9-
E. All wiring in panelboards and enclosures shall be neatly formed
and grouped.
F. Be responsible for all safety precautions and rubbish removal.
Leave site in clean condition.
C-9/10
APPENDIX D
PERFORMANCE SIMULATION MODEL AND INPUT DATA
Thp. simulation model is an adaptation of an analytical model develooed durina
previous DOE-sponsored studies. The model permits the assessment of system
performance on an annual basis using hourly SOLMET TMY data tapes as the input.
The program calculates, for each daylight hour, the solar array maximum power
operating point voltage and power using an iterative calculation procedure based
on models for current-voltage characteristics, array temperature, and insolation
as described below. The losses in the inverter are then determined based on
solar array output power.
, Solar Array Electrical Model
The synthesis of the solar array current-voltage characteristic, as a function
of the total insolation on the surface and the solar cell temperature, is modeled
based on a single cell characteristic which is represented by the following
relationship:
Where
I =
v =
=
C =
=
=
=
2 Cell output current (Amperes/cm )
Voltage across cell terminals (Volts)
Illumination current (virtually equal to short-circuit current)
(Amperes/cm2)
Ratio of the total insolation incident on the solar cells to the
reference insolation for the basic cell characteristics (lOOmW/cm2)
Shunt resistance of the cell (Ohms-cm2)
Reverse saturation current of the ideal diode characteristics
exp (K V oc)-exp (K Rs ISC)
D-1
K
Y
where
T
y
=
=
=
=
Coefficient of the exponential (Volts-I)
Series resistance of the cell (Ohms-cm2)
Cell open circuit voltage (Volts)
Encapsulation loss or gain expressed as the fraction of bare cell
short-circuit current.
1SC ' RS' Rp' K and Voc are represented by polynominals of the form: 3 4 5 6 aO + al T + a2T + a4T + a5T + a6T =
= Solar cell temperature (oC)
= Dependent variable (1 SC ' RS, Rp' K, or Voc )
The values of the coefficients are selected to represent the characteristics of
the solar cells used in the shingle module as shown in Figure 4-6. The total
solar array output characteristics are calculated based on the single cell
characteristic by multiplying the voltages and currents by the number of cells in
series and parallel, respectively. In addition, the series resistance of panel
wiring is accounted for in the array characteristic.
Insolation Model
The value for total insolation (direct plus diffuse) on the sloped surface is
obtained from the values for the direct and diffuse components of the insolation
on a horizontal surface as ootained from the SOLMET TMY data tape by applying the
following relationship:
=
where
=
=
H~1~ =
HDIR Rum + HDIF [ (1 + ~os 8) + p (1 - ~os ~ ) ]
Total insolation on the sloped solar array surface
Diffuse component of the solar flux incident on a horizontal surfa l
Direct components of the solar flux incident on a horizontal surfac
0-2
P = angle between horizontal and solar array surface
p = the reflectance of the surrounding ground
(a value of 0.4 was assumed in the analysis)
The value of ROIR is the ratio of the cosine of the solar angle of incidence
(9;) on the tilted solar array surface to the cosine of the solar angle of
incidence (Qh) on a horizontal surface. The value of cos 8; is determined as
a function of the day of year, time of day and surface location and orientation
in accordance with the following relationship:
where
Q. 1
¢
()
{3
y
W
cos 9. = sin 0 [ sin ¢ cos P - cos y cos ¢ sin P ] 1
=
=
=
=
=
=
+ sin y sin p cos 0 sin w
+ cos 0 cos W [ cos y'sin ¢ sin 13 + cos ¢ cos {3 ]
Angle of incidence of beam radiation measured between the beam
and the normal to the solar array surface
Site latitude (north is positive)
Solar declination angle
Angle between horizontal and solar array surface
Solar array surface azimuth angle (zero is due south, west of
south is positive)
Hour angle (zero is solar noon)
For a horizontal surface this expression reduces to:
cos ,\ ;:: sin () sin ¢ + cos 0 cos W cos ¢
Array Thermal Model
The temperature of a roof-mounted solar cell module was calculated based on
natural convective cooling from the front surface of the module installation.
Under this condition the heat balance equation for the solar cell modules is giver
by:
[asp + 0.3 (l-Pl] HTOTAL = ho (TCell - Tamb ) + «7 (Tce1l4
- TSk/) + k (TCell- 1
0-3
where
HTOTAL
P
Tamb
Tsky
TR
Tcell
h o
k
a
=
=
=
=
=
=
=
=
=
=
=
=
total insolation on the sloping solar array surface (W/m2)
ratio of solar cell area to total module area
solar absorptance of solar cells
ambient temperature (oK)
sky temperature (oK)
temperature of the living space under solar array (oK)=296oK
solar cell module temperature (oK)
convective film coefficient on the exposed module surface (w/m2 oK)
0.213 (W/m2 oK)
hemispherical emittance of the front surface of the solar cell modules
Stefan-Boltzmann constant -8
5.6697 x 10 W/m2 °K4
The sky temperature, Tsky ' is calculated, using the relationship given in
Reference 6:
= o 0552 (T ) 1.5 . amb
The film coefficient, ho' is calculated using the relationship given in Reference
ho = 1.247 [(Tcell - Tomb) cos Il] 1/3 + 3. SlV
where
{3 = slope angle of roof (measured from horizontal)
v = wind speed (m/s) The heat balance equation given above was solved using an iterative technique
to yield the results shown in Figure 0-1.
0-4
U 0
I-2 w al ~ « w > a OJ « w en a: w a: :::> I-« a: w Q..
~ w I-
WIND AMBIENT SPEED TEMP (OC) (m/s)
SOLAR CELL as = 0.88
+ + INTERSTICE as = 0.3 70
ROOF SLOPE = 35°'
1~ } ROOF fh = 0.8 0
30 60
o} 50
1 30 .
~} 2 40
37'?C - -- !'vIEASURED DATA POINT AT JPL @ Im/s AND
!) 20°C 30 4
20
10
10
o ~----~------~----~------~----~ o 0.2 0.4 0.6 0.8 1.0
TOTAL INSOLATION ON ARRAY SURFACE (kW/m2)
FIGURE D-1. Thermal Model Prediction of Solar Cell Operating Temperature
. 0-5
Recent data obtained by JPL is shown in this figure and indicates that the
analytical model overpredicts cell temperature, with the result that predictions
of cell output contain a conservative bias.
Inverter Loss Model
The power losses in a line-commutated inverter can be approximated by the
following model:
where
PL = K1 + K2 I + K3 12
PL = inverter losses (Watts)
K1 = constant losses (Watts) associated with logic circuitry, coils,
transformer and inductor
K2 = constant voltage drops (Volts) associated with the SCR bridge
K3 = resistive losses (Ohms) associated with the transformer, inductor and
wiring
I = current (Amperes)
The values for these constants we well as the values of their component loss
mechanisms are listed in Table 0-1 . Inyerter losses and efficiency oyer the full
power range with this model are shown in Figures 0-2 and 0-3.
TABLE 0-1
Components of Inverter Loss Model
Kl = 345 Watts (Transformer~lOOW; Inductor~95W, Logic~lOOW; Coils~50W)
K2 = 3 Volts (SCR Bridge)
K3 = 0.171 Ohms (TransformerJ'-0.120 Ohms; Inductor""O.031 Ohms; Wiring-0.020 Ohms)
0-5
1.0
~ 0.8
~ -~ 0 ..J a: w 0.6 :c 0 A. a: w to-a: w > 0.4 !:
0.2
o
.95
i! > u .90 z w (3 u.. u.. w a: w .85 I-a: w > z
o
INPUT VOLTAGE (VOLTS)
2 4 $ 8
INPUT ?OWER (KW)
FIGURE D-2. Inverter Losses
INPUT VOLTAGE (VOLTS)
2 4 6 8
INPUT POWER (KW)
FIGURE D-3. Inverter Efficiency D-7
10
180
200
220
240
240 220 200 180
10
INPUT DATA
The PV system simulation model requires input data, refpresenting the PV system
characteristics and hourly histories of local weather, insolation,and residential
electrical and space conditioning loads. Each of these input items is discussed
below.
PV System Data
This data provides a description of the system in terms of its electrical
characteristics. It includes such information as the number of solar cells in
each series circuit and the number of parallel circuits. For this design, a 25
series by 19 parallel circuit was modeled with 19 100 mm cells per module. Cabling
resistance and module interconnection resistance are also input for determining power
losses from the DC output to the inverter input. A series resitance of 28 milliohms he
been calculated for the current.
Weather Data
The National Climatic Center has recently made available rehabilitated combined
hourly solar radiation and meteorological data for twenty-six sites across the
United States. These SOLMET data sites have had the solar radiation values corrected
to reduce some of the major data errors and gaps, and to provide non-measured direct
radiation data on the basis of the most recently developed correlation techniques with
measured total radiation data. The SOLMET records provide up to a twenty-three year
record for the selected sites.
Sandia has recently prepared a synthetic Typical Meteorological Year (TMY) for each
of the twenty-six SOLMET sites. These TMY data tapes are based upon a selection
of the most typical January, February, etc. available in the years of record for
each site. Typical months were selected by a weighting technique for insolation,
temperature and wind speed. These TMY SOLMET records for all 26 sites are available
D-8
at GE. They provide a real weather data base for solar system performance evaluation
and comparison.
The TMY weather tapes for Phoenix and Albuquerque were used in this analysis.
Electrical Load Data
Electrical loads for other than space conditioning were divided into three categories.
The first categoy was that of diversified or base load demand and includes lights
and many miscellaneous applications in the modern home. The second group was
cooking and clothes drying and the third grouping was heating of water for domestic
use. Profiles of the integrated amount of energy used during each hour of the day for
the various categories of energy usage were developed for 1977 by utilizing a wide
variety of references. From the data examined, it was hypothesized that an annual
usage of 5540 kWh would be representative of the diversified energy usage by a
typical family of four persons in 1977. Since lights and some appliances (e.g.,
refrigeration equipment and HVAC auxiliaries) do have seasonal variations in their
use, profiles for four seasons were developed. No significant regional variations
were found. The cooking and clothes drying profile was found to have negligible
seasonal and regional variations, and their annual load was determined to be 2220
kWh for the 1977 time period. An annual hot water load was similarly determined
at 4940 kWh but a regional correction was applied to both the hourly profile and
the annual value to account for ~ifference in ground temperature across the
:ountry. Seasonal variations were also incorporated.
\11 the profiles and annual values developed for 1977 were modified for projected
usage trends in 1986 according to the rationale indicated in Table D-2.
0-9
TABLE D-2
Seasonal Reqional and Projection Adjustments of Loads
SEASON.~l REGIONAL 1986 PROJECTION EFFECTS VARIATIONS DIFFERENCES FROM 1977 BASE YEAR
B.;SE LOAD PROFILES FOR NO CORRECTIONS USAGE DECREASED BY DEH4ND 4 SEASOHS USED 1 1/4% PER. YEAR
(ll 1/4%)
COOKING NONE NO. CORRECTIONS COOKING REDUCED B,Y 1%11'2, Cl_OWES DRYII~G SIGNIFICANT USED DRYING REDUCED· BY lI2ZiYR,
(TOTAL 63/4%)
DO;';ESTIC PROFILES FOR 4 REG roNAL ADJUSTMENT REDUC~ BY 1 1/4%/YR. HOT .HATER SEASONS IDENTIFIED FACTOR IDENTIFIED (TOTAL 11·1/4%)
( ."75 .. T01.:16)
D-10
The 1986 electrical load profiles were developed by applying the projected energy
reduction factors from Table 5-1 to each of the corresponding components of the
total electrical load for 1977. Profiles of the components of the average daily
electrical load (exclusive of space conditioning) for 1986 are shown in Figure 0-2.
The annual total of each component is tabulated below.
Com~onent Load, kWh
Baseload 4917
Cooking and clothes drying 2070
Domestic hot water(average) 4384 Adjustment factors
Phoenix = 0.88 Albuquerque = 1.05
Space Conditioning Loads
Space heating and cooling loads for the residence were computed on an hourly basis
utilizing General Electric's Building Thermal Transient Load (BTTL) program.
Program inputs included hourly weather and insolation data, building usage schedules.
and a thermal model of the house. The thermal model was developed from the
architectural plans and analytically represented the significant heat flow paths,
thermal capacitances, and heat generating elements of the house. The model assumed
two thermostatically independent zones: one the living area; the other, the bed
room area. In winter, the living area thermostat was set at 20°C (68°F) during
the day and at 17.2°C (63°F) at night; while the bedroom thermostat was maintained
at 17.2°C (63°F) day and night. During the summer, the living and bedroom areas
thermostats were set at 25.6°C (78°F) day and night.
The house was assumed to be occupied by a four-member family. The internal
sensible and latent heat generated by occupants, lighting, cooking and miscellaneous
appliances was derived from the hourly profiles of electrical loads and cooking loads
0-11
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
TOTAL ELECTRICAL LOAD
DOMESTIC HOT WATER
\ (12.0 KWH/DAY)
'\
" I \ ~ '" \ I 'vI \ .1\ \ I ~ \ \ /. ~-I
BASE LOAD (13.5 KWH/DAY)
9 10 11 N 1 2 3 4 6 6 HOUR OF DAY
-SPACE HEATING AND COOLI LOADS NOT INCLUDED
-LOADS SHOWN REFLECT 198 PROJECTION AND TOTAL 11,400 KWH PER YEAR
-BASE AND DOMESTIC HOT W, LOADS REPRESENT AVERAG FOR FOUR SEASONS
-HOT WATER LOADS ADJUSTI FACTORS; PHOENIX· .88, ALBUQUERQUE 1.06
COOKING AND CLOTHES DRYING
(6. 7 KWH/DAY)
FIGURE 0-2. ' Average Daily Electrical Load Profile
D-12
established in the previous section. The latent portion of the load was estimated
from any showers, boiling of water or other evaporative type processes occurring
ithin the residence besides that due to human presence.
The infiltration rate through window and door leakage was assumed to be a function
Jf wind speed. Since air flows are due to changes in air-stream static pressure, which,
Jver the surfaces of buildings, are approximately proportional to the square of wind
speed, the following equation was used in calculating infiltration gain of the building:
Infiltration = 0.25 + 0.5 x (m/se~3~1~d speed)2, air change per hour
onthly space heating and cooling load profiles for the single family residence in
hoenix and Albuquerque are listed in Table 0-3. It is important to note that these
,pace conditioning loads are loads which are satisfied by the heating and cooling
~quipment. In the all electric house the heat pump supplies these loads and creates
In electrical demand equivalent to the space conditioning load divided by the heat
Jump COP. The monthly profiles of heat pump electrical requirements are shown in
Figure 0-3 for Phoenix and Albuquerque. Both the space conditioning loads and the
heat pump electrical requirements reflect the geographic differences between Phoenix
and Albuquerque which are less than 350 miles apart. Phoenix has a ,high summer time
peak whereas Albuquerque has a winter and summer peak with the winter demands slightly
higher. The space conditioning demands are calculated hourly during the simulation
and these plots only provide the monthly summaries.
0-13
TABLE 0-3
Sunbelt Single Family Monthly Load Profiles
Site Loca-tions Phoenix Albuquerque
(KWH) ( KWH) Month Cooling Heating Cooling Heating
January 28 414 0 1468
February 32 208 2 1046
March 77 76 0 782
April 72 11 0 232
May 1358 0 17 60
June 2099 0 710 0
July 2637 0 1098 0
August 2390 0 913 0
September 1952 0 28 3
October 874 0 31 108
November 155 32 3 474
December 64 271 0 1091
TOTAL 11739 1013 2802 5264
0-14
X ~ ~
c:i z « :E w C > CJ a: w Z w ..J « u a: l-U w ..J W
1200
1000
800
600
400
200
PHOENIX
JFMAMJJASOND
ANNUAL TOTALS
-SPACE LOADS
- HEATING - COOLING
1.0 MWH 11.7 MWH
-H EA T PUMP LOAD 5.1 MWH
2.51 -AVERAGE HP COP
X ~ ~
c:i z « :E w C > CJ a: w Z W ..J « u a: I-(J W ..J w
1200 I ALBUQUERQUE I
1000
800
600
400
200
O~~~~~-L-L~~~~
J F M A MJ J A SON 0
ANNUAL TOTALS-
-SPACE LOADS
- HEATING - COOLING
-HEAT PUMP LOAD
-~VERAGE HP COP
5.3 MWH 2.80MWH
3.2 MWH-
2.6 MWH
Figure 0-3 Heat Pump System Electrical Requirements For Sunbelt Single Family Residence
D-15/16
APPENDIX E ECONOMIC MODEL AND ASSUMPTIONS
Economic Methodology
Many of the simple investment evaluation techniques, such as payback time or simple
return, suffer from two major drawbacks: life of the investment is not considered,
and uneven costs and/or benefit streams cannot be handled.
The second item is of critical importance to alternate energy systems since virtually
any economic scenario projects rising energy prices and therefore a steadily increas
ing benefit stream from an alternate energy system. For this reason, considerable
effort has been made to stress "life cycle costing" for energy systems. The follow
ing sections describe the life cycle costing model used in the analysis for system
sizing.
Levelized Annual Cost
True life cycle cost analysis must necessarily consider the timing of costs and
benefits as well as the magnitude. A method employed in previous General Electric
solar and wind energy programs is to compare Levelized Annual Benefits (LAB),
representing system energy savings, with the Levelized Annual Cost (LAC), the levelized
dollar amount required to own, operate, and maintain a system during each year of
the life of the system. Specifically, the levelized annual cost accounts for:
1. "Paying off" system capital costs (mortgage principal)
2. Paying mortgage interest
3. Paying property taxes and insurance
4. Paying operating and maintenance expenses.
For cost evaluation and comparison of systems for future implementation, it is
appropriate to express the levelized annual cost (LAC) referenced to a particular
year, e.g., 1980. The result is the levelized annual cost in constant (base year)
dollars given by:
CRF' LAC (constant $) = CRF x FCR x I + AOC
E-1
where I is the capital cost of the solar system and AOC is the annual system
operating cost which includes operation and maintenance and insurance. The parameter
FCR is the fixed charge rate and represents the yearly cost of ownership, expressed
as a percent of the capital cost, I. These costs consist of mortgage interest,
principal and property taxes. The parameter CRF is the capital recovery factor,
defined as the uniform periodic payment (as a fraction of the original principal)
that will fully repay a loan (including all interest) in yearly periods over the
loan lifetime at a specified yearly interest rate. The interest rate r used to
calculate CRF is called the discount rate and for the homeowner is equal to the
after-tax interest rate of the mortgage.
The relation expressing CRF as a function of r and system lifetime N is given as
CRF =
The parameter CRF ' is the corresponding capital recovery factor in constant (base
year) dollars. CRF ' is based on the real (or inflation adjusted) discount rate,
r', defined as
rl = 1 + r _ 1 1 + g
where g is the general inflation rate. The equation for CRF also applies for CRF '
with rl replacing r.
It should be noted that the LAC equation applies only to those systems without
storage batteries since no replacement costs are necessary. For systems with storage,
an additional term of the form
CRF ' --'---....,.- X II (l+r)ll B
E-2
must be added to account for the replacement of the battery in the eleventh year
(e.g., for a battery life of 10 years). Here, liB is the cost of the replacement
battery in constant 1980 dollars.
Levelized Annual Benefits
The comparison of the energy cost savings of the solar system to the levelized
annual cost is accomplished by computing the levelized annual benefits (LAB) for
the energy savings. LAB is inherently a function of present and projected energy
prices and may be expressed by
CRF I
LAB (constant $) = CRF x M x Po x Eo
where Eo represents the annual energy saved by the solar system, M is an energy
saving multiplier which is defined as the levelized value of an encalating cost
stream which accounts for the rate of energy price escalation over the lifetime
of the system, and Po is the energy price in year zero (for a 1986 start, year zero
becomes 1985). For stand-alone systems, minimum monthly energy charges are
included as a benefit in the computation of LAB.
The multiplier M is a function of energy price escalation rate (f), system lifetime
(N), and discount rate (r), and is expressed as
M = r (1 + f) r - f
[ (1 + r)N - (1 + f)N J* (1 + r)N - 1
The energy price in year zero (po) is related to the energy price in constant (base
year) dollars per energy unit (p) through the expression
* When r = f: M = CRF . N
E-3
where ~ is the number of years from the base year to year zero (value of 5 was used
for a 1986 start with a base year of 1980).
The economic viability of a system can be measured through the use of the cost-to-
benefit ratio, which is defined as the ratio of the levelized annual cost to the
levelized annual benefit. The system can be economically viable when the cost-to
benefit ratio is less than unity. The break-even system cost occurs when the ratio
is exactly ~nity, i.e., when LAC and LAB are equal.
Economic Assumptions
This subsection spells out the basic assumptions that will be used in the economic
analysis of PV residential systems. These assumptions concern the price of e1ectrici
in the designated regions for the 1986 time period and system capital cost. Also
considered are assumptions regarding inflation, interest, and tax rates insofar
as they will affect economic comparison results.
Model Assumptions
In order to utilize the cost-to-benefit ratio for system sizing studies, a set of
economic assumptions were developed. These assumptions are summarized in Table E-1.
Most of these assumptions are consistent with assumptions utilized in previous
residential studies as Reference 1. All of the economic calculations were completed
for a 1986 start in constant 1980$. The overall average inflation rate of 5 percent
was assumed through the time frame of the analysis. This value is low according
to current rates but since the cost-to-benefit ratio analysis becomes independent
of the inflation rate and to maintain similarity to previous work, this value was
E-4
TABLE E-l.
Economic Assumptions for Optimization
• 1986 Sta rt • Sell Back To Buy Ratio: 0.3, 0.5 & 0.7
• General Inflation Rate: 5% • System Life: 20 Years • Insurance: 0.5% Of Capital Cost • Maintenance: $lOO/Yr. • Electricity Price Escalation: • 1980$
4% Over Inflation
• Mortgage Rate: 10% • Tax Bracket: 35% • No property Tax
used. In addition, a resultant homeowner mortgage rate of 10 percent for a 20-year
loan was assumed with a marginal income tax rate of 35 percent for the homeowner.
This is equivalent to a taxable income of approximately $25,000 at present tax rates
A 20-year life will be assumed for the solar systems of interest. Since
several states and local communities have already exempted solar systems from
property tax, no additional property tax will be assumed. These figures imply
an annual cost or "fixed charge" of about 9.1% of the initial photovoltaic
system cost. Annual operating costs are assumed to be $lOO/year for operation
and maintenance and 0.5% of system cost for insurance. All components of the
solar system are assumed to have the same lifetime as the system (20 years) except
for batteries (10 years), where' applicable.
An electricity price escalation of 4% over inflation was also used in the
analysis, although the cost-to-benefit ratio analysis allows the extension to
other escalation rates and system lifetime assumptions by a constant adjustment
factor as done in Reference 1. The sell back price of the energy to the
utility was varied between .3 and .7 of the buy price to show the effects
of these values on system sizing.
E-5
Capital Cost Estimates
To calculate the LAC, an estimate of the system capital cost is required.· These
cost estimates were made assuming 1986 price projections for equipment or 1986
National PV Program cost goals as for the array. In addition, estimates for
installation costs and all remaining equipment costs were estimated as part of
the balance of system costs. These latter values are only estimates made prior
to final system design selection since the intent was to use these values in a
relative comparison for system sizing and tradeoffs. The detailed design data
within this report now can be used for obtaining detailed costs for installation
and all small equipment costs. All the costs are in 1980$ and include in general,
two 15% markups for distribution and contractors. These markups are probably low,
but are used in this analysis until more detailed numbers are available for dis
tribution network.
The system capital costs were divided into an array cost and balance of system
costs. The balance of system costs were further separated into a fixed cost
(however small a PV installation, a minimum amount of equipment, and thus, cost,
is required independent of system size) and a variable cost based on system size.
The variable, or area-related costs of a photovo1taic system include the cost of
modules, array installation, and a portion of the power conditioning. The fixed
costs obviously include the roof to load wiring, switchgear and power condition
ing installation. There are other less obvious fixed costs, however.
For example, the cost of a power conditioning unit will include basic labor,
cabinet and parts costs independent of size. In the area of operation and main
tenance, a large fraction of the cost will not be area related. This would include
almost all PCU and switchgear maintenance. For any given system, more or less
fixed costs may be present but the general contributors will remain. Appendix G
E-6
evaluates the effects of varying the level of system fixed and variable costs.
The array cost assumed is the National PV Program goal of 70¢/peak watt or
$700/kWp factory price. Including the markups, the cost is $925/kWp on site or
approximately $82/m2 for the Block IV shingle. The balance of system costs
were made up of array installation estimates, power conditioning subsystem costs
and remaining equipment costs. The array installation cost estimate was based
on labor and material ~stimates from the 1978 Building Cost File Index for
conventional asphalt shingle installation and increased by an assumed factor
of 3 to account for the additional complexity of the PV shingle installation.
The resulting cost is $35.35/m2 per unit array installation.
Labor and material credit for weather-tight replacement of the conventional
shingles was then given at $12.29/m2 for a net array installation cost of 2 $23.06/m. These values are consistent with residential array installation
cost estimates developed by Burt Hill Kosar Rittleman Associates, Reference 11,
where non-optimized flat PV panel installation costs were approximately $40/m2
and roof credits for integrated arrays were approximately $11/m2~
The power conditioning subsystem cost estimates were based on cost projections
from several inverter manufacturers assuming high production levels in 1986 ob
tained from Reference 8. These projections were of the same magnitude as assumed
in Reference 1 at $144/kVA in 1975$ or $202/kVA in 1980$. Several current cost
quotes for the inverter, DC input filter and isolation transformer for 4, 8, and
10 kVA sized systems showed a fixed cost independent of system size and variable
cost dependent on system size. Based on this data, the $202/kVA cost for 1986
E-7
was separated into a fixed and variable cost and then adjusted with two 15%
markups resulting in the PCS system cost estimate of $689 + $181/kVA or ap
proximately $689 + $15.60/m2 in 1980$.
The remaining fixed balance of system cost estimates include costs for junction
boxes, disconnect switches, varistors, busbars, cabling and miscellaneous connec-
tors, wire and tape and installation labor. Applying the markups resulted in a
fixed cost of $1090. All of these system cost estimates are summarized in
Table E-2. The costs listed in Table E-2 are basically direct system costs,
however, imbedded in the assumed values are also the indirect costs associated
with system design and installation. Some of the indirect costs could include
architect fees, real estate fees, interest during construction, project manage-
ment costs and the cost of the contingency and spares. It is difficult to
TABLE E-2. Array Installation and Balance of System Costs
SHINGLE ARRAY INSTALLATION ESTIMATES
Labor + Material Credit For Conventional Shingles
POWER CONDITIONING COSTS
(Based on Projected High Production Estimates For 1986) Includes Inverter, Input Filter and Transformer
Cost = $689 + $181/kVA 2 Or Approximately $689 + $15.60/m
B-O-P EQUIPMENT COST ESTIMATES
(Includes Switches, Junction Boxes, Varistors, Busbar, Cabling)
SUMMARY
ARRAY VARIABLE COSTS FIXED COSTS
E-8
$925/kWp
$38.66/m2
$1779
1980$
2 $35.35/m2 -$12.29/m $23.06/m2
$689 + $15.60/m2
$1090
estimate all of these indirect costs and therefore, they are included in general
terms. The table also does not separately list the fixed costs associated with
operation and maintenance for the system. These costs are normally added in the
LAC calculation as a levelized annual value of $lOO/yr. This value relates back
to a fixed capital cost of $1590 for the economic assumptions listed in Table £-1.
No attempt was made to break out a fixed and variable portion of the operation
and maintenance costs for this analysis.
Energy Price Estimates
Strong regional differences exist in energy pricing especially in electricity.
The previous General Electric study of residential photovoltaic system per
formance (Reference 1) used electrical cost data developed from typical elec
trical bills as reported by the Federal Power Commission for 1975 (FPC is now
a branch of DOE), Reference 9. An effort was made during this present study
to establish the 1979 rates for electricity on a similar basis so that meaning
ful comparisons with the results of the previous study might be possible. Lack
of the latest available data for 1979 during this ~nalysis effort led to an
a lternate approach for estimati ng current el ectri ca 1 pri ces. Therefore, the
electrical rates for single family residences used in this current analysis
were obtained directly from the local utility companies. These rates should
be consistent with the type of data published by the Federal Power Commission,
and were used for the simulation studies. The rates obtained directly from
the local utilities include all costs (e.g. minimum service charge and fuel
adjustment charge). These rates were used for monthly bills typical of the
loads discussed in Appendix D and an average rate developed and adjusted to
1980 dollars as shown in Table E-3 for 4 cities: Phoenix, Albuquerque,
E-9
TABLE E-3.
Electrical Cost Estimates for Residential Service
COST CITY 1980$/kWh UTI LITY
Al buquerque .042 Pub 1 i c Service of New Mexico
Boston .062 Boston Edison Co.
Miami .050 Flori da Power and Light Co.
Phoenix .057 Arizona Public Service Co.
Miami, and Boston. The Miami and Boston values were obtained for extended
system simulation analyses as discussed in Section 6. As updated typical
utility bill data becomes availab1e, it will be compared with the rates
obtained directly from the utilities. If significant discrepancies exist
between the two cost values, adjustments to these current cost analyses will
be used in subsequent designs.
E-I0
.'
APPENDIX F
PV SHINGLE MODULE CONSTRUCTION DETAILS
The construction details of the shingle module are shown in Figure P-1, which
is a section taken through the laminated assembly at the location designated
by A-A on Figure 5-1. A description of each major component of the construction
follows.
Coverpl ate
The glass coverplate, which is the rigid exposed portion of the shingle module,
is 4.0 mm thick ASG SUNADEX glass. This embossed low-iron soda-lime glass
is cut to the required hexagon shape and thermally tempered to achieve a mean
modulus of rupture in bending of 138 MPa (20,000 psi). The solar cells are
bonded to the embossed surface of the glass.
Outer Substrate Skin
The outer substrate skin is B. F. Goodrich FLEXSEAL which is 6 x 6 polyester
scrim reinforced white HYPALON roofing membrane. HYPALON is a synthetic
rubber with excellent weathering characteristics, low moisture vapor
transmission rate, good oil and chemical resistance, and good abrasion and
puncture resistance. The scrim reinforcement provides the excellent tear
resistance needed to prevent roofing nail tear-out under wind loading conditions.
COVER PLATE
SOLAR CELL
CELL BOND I NG ADHES IV E MODULE
ENCAPSULANT
OUTER SUBSTRATE SK IN FOAM CORE
SUBSTRATE ADHESIVE
REAR COVER
FIGURE F-l. Block IV Modul~ Construction F-l
Foam Core
The foam core of the shingle substrate is 4.8 mm (0.188 inch) thick L-200
closed cell polyethylene foam manufactured by Rodgers Foam Corporation.
This foam provides a low-cost, low-density filler material to maintain a
nearly uniform shingle thickness.
Rear Cover
The rear cover, which covers the entire rear surface of the shingle module,
is cut from 1.5 mm thick "Pan-L-Board" manufactured by Mead Paperboard
Products. Pan-L-Boara is a weather-proofed, fire-resistant, pressed
paperboard panel which Mead claims has endured 17 years of outdoor weathering
in Wisconsin. This rear cover provides a low cost barrier against the entry
of water and moisture from the underside and also provides some degree of
protection against penetration by sharp objects during handling.
Cell Bonding Adhesive
The cell bonding adhesive is GE534-044, a clear silicone compound, manufactured
by General Electric, Silicone Products Department.
Module Encapsulant
The module encapsulant is a white silicone construction sealant which is
ident,fied by the GE Part No. SCS 1202.
Substrate Adhesive
The adhesive which is used to laminate the substrate layers is M6338 Super
White Silaprene manufactured by Uniroyal. The Silaprene formulation is a
blend of high solid elastomeric compounds which provides excellent bond
strength to most surfaces without priming, heating or mixing. It is resistant
to water, ozone, salt and many chemicals.
Bus Stri ps
The bus strip conductors within the flexible shingle substrate are fabricated
F-2
by folding 25.4 mm (1.00 inch) Wide strips of 0.13 mm (.005 inch) thick
soft copper foil in the pattern required to connect the cell terminals to
the four output terminals of the module. These copper foil strips are
sandwiched between the foam core and the pressed board rear cover. The
single cross-over point of these two strips is insulated with a 31.8 mm
(1.25 inch) square of 0.13 mm (.005 inch) thick Kapton H. film.
Solar Cells-
The solar cells are procured from ARCO-So1ar per the requirements of GE
Specification No. SVS-10011. The required minimum shipping lot average
electrical output for these cells is 0.966 watts or 2.10 amperes at 0.460
volts.
Modu1e-to-Modu1e Interconnectors
The electrical connection between the two negative terminals on one module
to the two positive terminals of the lower course is made by means of a
screw and washer. The preload force of this fastener into a nylon nut, which
is held captive within the positive terminal copper boss, assures a high
contact pressure between projections on the copper boss and the solder-coated
copper foil strip which forms the negative terminal.
Electrical Performance
The shingle solar cell module electrical performance is based on the minimum
average bare cell performance as discussed above for the ARCO-So1ar cells.
This bare cell performance is modified by an enhancement factor to account for
the following: (a) a covering gain which occurs by virtue of the more favorable
index of refraction match at the air/glass interface and at the glass/adhesive/
~ce11 interface as opposed to the air/cell interface of the bare cell, and (b)
the gain due to reflected light from the white interstices of the module. A
gain factor of 3 percent has been conservatively used to account for the
covering of a cell. The enhancement due to the reflected light from the white
F-3
interstices of the module has been experimentally determined to be a function
of the module packing factor as shown in Figure F-2.
With a packing factor of 76.3 percent for the third-generation shingle module
design, Figure F-2 gives an enhancement factor of 9 percent due to reflected
light from the interstices. Thus, the total resultant enhancement in short-
circuit current, relative to bare cell performance, is 12 percent.
Using this short-circuit current enhancement, Figure F-3 represents the
calculated average module I-V characteristic at 280 C and at the expected NOCT
of 64oC.
12r I \
0-Z w
i c:: c:: :l U I 0-
S I u I a: u 1.1L ... a: 0 :r
'" w ~ ... <[ ...J w a:
1.0
0.1
• fSOLAR CELL AREA) PACKING FACTOR
MODULE AREA
Figure F- 2. Enhancement of Module Output With an Embossed Glass Coverplate and \vnite Interstices
F-4
Ci) w a: w Q.
:E ~ I-2 w a: a: ::>
2.0
u 1.0
o
Max iQJum Power Point
17.14 Wp
• 100m W/cm2
a Temperature Coefficients C1 = .0007 AfOc
Cv = -. 042 V 1°C
5
v = 7.3 v
VOLTAGE (VOLTS)
10
Figure F-3 Block IV Shingle Module I-V Characteristics
F-5/6
15
APPENDIX G
DESIGN TRADEOFF DETAILS
Shingle Module Arrangement
The arrangement of the photovoltaic modules on the residential roof surface
is influenced by several key considerations which include
• System performance effects
• Simplicity of design and construction
• Aesthetic app~arance
• Safety of residents and service personnel
Each of these considerations were reviewed in selecting the final roof instal
lation. Operating performance of the system will be affected by the voltage
level which in turn is a function of the number of modules connected in series.
The initial design approach is to use the highest voltage (i.e., longest array
series branch circuit) possible within the electrical constraints of equipment
dielectric strengths, customary voltage levels for the service interface matching
and equipment ratings. This approach will minimize resistive power losses and
wire sizing for a given power level. The length of the branch circuit is con
strained, however, by physical factors such as the module size and available roof
area.
Operating performance may be improved by direct coupling of the array and power
conversion equipment to the residential utility service line. This technique el
iminates both the initial cost of a transformer in the power conversion system
and the operating power losses associated with the transformer. Use of direct
coupling circuit techniques eliminates the possibility of grounding an array bus,
and requires a relatively narrow range of dc input voltage with a fixed relation-
G-l
ship to the ac utility service" voltage for effective conversion. This narrow
dc input voltage range may not be achievable on all roof designs limiting this
design approach. For large market penetrations, design layouts and installation
procedures will be simplified and improved if standardized approaches are used
in the arrangement of array modules on residential roofs. This suggests that
end point locations of branch circuits should not be customized for each roof
size to achieve a closely specified voltage level. In addition, voltage levels
and ranges for a specific array arrangement and size will vary from location to
location due to the local combinations of insolation, ambient temperature and
wind. Therefore, direct coupling of the array was not ultimately selected for
this design.
Symmetrical module arrangements on the roof with connecting cable runs at
usual house locations are deemed to be more acceptable than alternative layouts
for residential houses both from installation simplicity and aesthetically.
In addition, until enough installation and operation experience is gained to
prove the pragmatic safety of photovoltaic roof array installations, array
buses and cabling should be as far out of normal contact as possible, and should
be safely grounded to the extent possible.
In general, the options available for roof module branch circuit layouts can
be categorized as top to bottom, side to side, and custom series voltage selected
lengths which will terminate in asymmetric non-linear bus arrangements. Review
of the advantages and disadvantages associated with each of the considerations
discussed above with the layout options has resulted in the conclusion that the
branch circuit running from top to bottom of the roof is the best choice. Poten
tial roof slant height distances in the normal range of 15 ot 25 feet provide
G-2
normal branch circuit voltages in the range of 130 to 220 volts. These levels are
reasonable for residential service, associated equipment ratings, and acceptable
power losses. This type of layout entails one array bus at the ridge line of the
roof, far removed from contact for safety, and the other bus at the eave line.
The bus at the eave line can be grounded for safety. With buses at the top and
bottom, cable connections can be made at one end of the house to the busses, and
routed in a normal drop line manner for entrance into the residence interior.
The top to bottom layout maximizes the use of available rectangular area, and
provides a symmetric appearance. Design approaches and installation procedures
are simplified to the extent possible, and are uniform from house to house with
no speCial considerations for individual house and size installations. Variations
in power and voltage levels will be accommodated by adjustments and size modifi
cations in the power conversion system. This roof array layout arrangement philos
ophy was therefore used in the system design. The specific Southwest residential
preferred design array size dictated by the shingle module size, the rectangular
roof area available and the choices for shingle module arrangement result in a
matrix of 25 series modules by 19 parallel branch circuits.
G-3
Power Conversion Subsystem Tradeoffs
Three key selections were made for the power conversion subsystem. These
were the inverter type, the power conversion system sizing and the connection to
the residential utility interface. The first of these selections concerned the
type of inverter technology to be used. Available options included line com
mutated inverters and self-commutated inverters. Since this residential design is
intended to operate in a utility connected backup mode with feedback of excess
solar energy to the utility, a line commutated type was selected. The self
commutated inverter wi]l be used for one of the future system designs. Among
the inverter types that are line commutated, two general types are available.
One type based upon SCR thyristor components in the inverter bridge circuit is
based upon commercial direct motor drives systems with regenerative feedback.
The second type is based upon power transistor technology widely used for small
dc to ac power supplies in a wide variety of commercial applications. For equival
ent power ratings, the thyristor type inverters are generally more economical than
transistor types, but generally produce a power quality ac output. A thyristor
type inverter was selected for this first residential design.
The second selection for the power conversion subsystem was the rated size
of,the system. As discussed in Section 5, the size was based upon the annual
distribution of the array maximum power point and its associated voltage. Al
buquerque insolation and meteorological variations were used to establish the
array output parameters. Albuquerque provided the largest array power output
and close to the maximum voltage among the four locations (Boston, Miami, Phoenix,
and Albuquerque) examined. Figures G-l and G-2 show the array maximum power
G-4
PHOENIX SINGLE FAMIL V RESIDENCE 9.8KW I
19.6 MWHI 20 _______________________ -....::-::-:-----...J---19.8 MWH
2
ARRAY CONFIGURATION 2GUY 19P 'J.QeK IV SHtNQLJ
I 8.1 KW I I ,
o ~----L---~~--~--__ ~ ____ ~ ____ ~ ____ -L ____ -L~ __ ~ __ ~ o
20
18
16
14
12
10
8
6
4
2
., 2 l 4 5 6 7
SOLAR ARRAY MAXIMUM POWER OUTPUT ,... P (KWI
FIGURE G-l. Integral Distribution of Solar Array
Maximum Power Point Power for Phoenix
PHOENIX SINGLE FAMILY RESIDENCE
ARRAY CONFIGURATION 25 S BY 19P BLOCK IV SHINGLE
8 9 10
227V I I
----'---19.8 MWH
OL-~--~------~-----~----~L-.---~------~-----~----~----~ o 160 1S0 190
SOLAR ARRAY MAXIMUM POWER VOLTAGE
FIGURE G-2. Integral Distribution of Solar Array
Maximum Power Point Voltage for Phoenix
8-5
point and maximum power voltage annual distribution functions for Phoenix weather
variations. Similar curves were presented in Section 5 for Albuquerque and in
Section 7 for the residential array output with.Miami and Boston weather con
ditions. For the three southern locations the roof slope was maintained at 26.60
or a 1:2 standard construction ratio. For the Boston location, the roof slope
was raised to 39.80.
Annual variations of maximum power point voltage and maximum power point
with the annual energy output are summarized for the four locations in Table G-I.
Review of the curves and the data in the table shows two items of interest with
respect to the actual annual array location dependent performance and the array
rating based upon standard operating conditions (SOC). For example, the shingle
array layout design has a rated maximum power output of 8 kW at 183 volts at
NOCT of 640C as shown in the IV curve in Section 3. In all regions, actual peak
TABLE G-I
Annual Design Array Output Variations
Energy Vo ltage Range Maximuin Output
Location Volts Power kW . MWh -
Albuquerque 203 + 21% 11.2 20.7 .-
Boston 223 + 13% 10.1 12.3 -
Miami 203 + 11% 9.6 15.2 -
Phoenix 198 + 15% 9.8 19.8 -
power exceeds array rated power, but the annual incremental portion of energy
produced above the array rated power is a small portion of the total annual
energy produced. In all cases, also, the bulk of the annual energy is produced
at maximum power voltages in excess of the array rated voltage. These results
S-G
are attributed to the actual site distributions of solar irradiance, temperature
and wind speed as opposed to the standard conditions for array rating.
Limitation of the voltage, as discussed in Section 5, to a narrow band
causes a negligible annual energy loss, and maintains a better efficiency and
power quality for the inverter type selected. Limitation of the peak power,
while causing only a negligible energy loss, does not contribute any real
advantages as long as the power level is within the rating of the SCR thyristors
used in the inverter. Thus, rating values were chosen for the power conversion
subsystem in the Southwest regions at 10 kW (50 amps) and maximum power voltage
tracking limits of 200 + 10% volts. It should also be noted, however, that for
specified sites within a general region, a smaller sized PCS may suffice.
The third consideration for the power conversion system was the method of
coupling the array to the residential utility interface. The array output can
be directly coupled to the utility line or it can be isolated from the utility
by an isolation transformer. If the array positive and negative buses are not
grounded, the array inverter output can be directly connected to the residential
utility interface within specified voltage constraints. In a single phase PCS,
the dc input voltage must be less than 90% of the commutating ac line voltage on
the bridge to achieve commutation. Since the utility single phase residential
service is normally 240 Vac, array voltage would have to be constrained to a
maximum of 216 Vdc without a transformer. It is desirable to operate the array
at the highest possible voltage for best efficiency but within the constraints
of module dielectric strength, residential application voltage limits and module
series buildup on a residential roof area. The selected design has a specified
voltage limit of 220 Vdc, as mentioned in the previous paragraph, which is quite
. r .. ?
close to the maximum allowable of 216 Vdc for direct coupling. The 220 Vdc
limit could be adjusted downward, with negligible loss, to match the direct
coupling requirements. An additional margin, however, would also be required
to accommodate tolerance and swings on the nominal 240 V service. Direct coup
ling also has the advantages of reducing system power losses and costs associa
ted with transformer coupling of the inverter output to the interface. Thus,
for an ungrounded dc array systems direct coupling is a viable system option.
When a grounded dc bus is selected, as is the case for this design, the
use of an isolation transformer is mandatory to allow proper inverter operation.
The use of a transformer also permits matching of the array voltage levels and
the utility ac line voltage for maximizing the efficiency of operation and power
output quality. This may become important where array layout constraints and
module type require dc voltage levels not matched to the utility line voltage
for proper inverter operation.
G-B
Busbar and Cabling Design Tradeoffs
This section delineates the design and selection process of the busbar and service
entrance cables used in the residential photovoltaic shingle system for the South
west single family dwelling. The design constraints and system requirements used
for this analysis are specified below as:
Positive Busbar Length
Negative Busbar Length
Postive Service Cable Length
Negative Service Cable Length
Full Load System Output
Full Load System Current
Fault System Current
19 Parallel Shingle Branch Connections per Busbar of Current, each
54 feet
54 feet
26 feet
47 feet
8141.5 watts
44.6 amps
50 amps
2.35 amps
Some of these values were initial system estimates but were close to final
specifications. Practical design considerations along with power losses and costs
were used to determine the final design selections.
The general assumptions used in the design analysis consisted of:
1. Both busbar and service entrance cable operating temperatures are 90 a C.
2. System ambient operating temperature is 50 a C.
3. Busbar and service entrance cables are copper of 100% conductivity.
4. Busbar losses are prepared from a linearly distributed model.
Busbar Sizing
There were three primary considerations for busbar sizing:
1. The size of the busbar must be selected to allow sufficient surface area
for good and durable electrical connections to the shingle foil leads and
the service cables. The" busbar must be pliable enough for lIin field ll
modifications (i.e., bending and twisting for installation through the
roof) .
2. The busbar must be sufficiently sized to meet the current carrying require
ments for the system.
3. The busbar must be selected from available standard sizes."
The standard busbar profiles considered were 111 x 1/811 and 5/8 11 x 1/811, (width x
thickness). Each of these bus bars were of sufficient width to provide more than
adequate surface area for good electrical (solder) connections of the shingle foil lead
The 5/8 11 x 1/811 busbar was best suited for this shingle application since it lends
itself more easily to lIin field ll modifications than the wider bar. The 1/811 thickness
for the busbar was chosen since it is less than the 3/16 11 shingle thickness and there
for easily covered by the shingle array. The use of a thinner busbar was discounted,
since installation and handling requires a durable, semi-rigid body that will not kink
during installation or deform during the soldering process of the shingle foil leads.
The 5/8 11 x 1/811 busbar was also capable of meeting the system current carrying
requirements and is available in standard twelve foot sections.
Service"Entrance Cable Sizing
The initial selection basis for the cable size was the ability of the cable to safely
carry the fault system current of 50 amps. The performance of three wire guages --
2 AWG, 4 AWG and 6 AWG--were examined at operating temperatures of 90°C and ambient
conditions of 50°C. From the National Electrical Code, these wire gauges have
adjusted current carrying capacities (at operating temperatures of 90°C and ambient
temperatures of 50°C) of 96 amps, 72 amps, and 56 amps respectively for the 2 AWG,
4 AWG and 6 AWG wire. The 6 AWG wire current capacity of 56 amps left little
G-IO
design margin for the fault system current of 50 amps and therefore was dropped
from consideration. The selection between the 4 and 2 AWGwire was then made based
on losses and initial costs. The resistance for 2 AWG wire at 2SoC as referenced
from the National Electrical Code is 0.162 ohms per 1000 feet and for the 4 AWG
wire it is 0.259 ohms per 1000 feet. Correcting the resistance fo~ an operating
temperature of 90°C resulted in a 0.203 ohms resistance per 1000 feet for 2 AWG
and 0.324 ohms per 1000 feet for 4 AWG.
The total length of cable required for service entrance as listed in the system
requirements was 73 feet. Thus, the total system cable resistance for the 2 AWG
wire was 0.015 ohms and for the 4 AWG wire was .024 ohms; Table G-2 summarizes
the calculated cable power losses for the full load current of 44.6 amps in watts
and percent of system rated power of 8141.5 watts. Also shown in the table are
the installed costs. Although, there is a slight improvement in performance
with the use of 2 AWG service cable, 4 AWG cable is more than adequate to meet
the current carrying system requirements without adding the additional cost of
a heavier gauge wire.
TABLE G-2
Summary of Cable Power Losses
TOTAL CURRENT INSTALLED CABLE PERCENT OF CAPACITY, WIRE
WIRE OHMS PER RES I STANCE, CABL( POWER SYSTEM AMPS @ 50° COST/STEEL GAUGE 10001 @ 90°C OHMS LOSS, WAITS lOSS AMBIENT CONDUIT
2 0.203 0.015 30 0.37 96 $131 4 0.324 0.023 46 0.5'l 72 $103 6 0.513 0.032 64 0~17 56 * -
* Wire Gauge 6 was eliminated from previous considerations
'::-11
Busbar Losses
The busbar losses were computed from a linearly distributed model as shown in
Figure G-3. Individual branch currents, represented by IB, feed the busbar in a linear
distribution at 19 locations. Each IB feed was equally spaced by LBB/19 feet. LBB
represents the total length of the busbar. An equivalent sectional resistance, REQ
was determined for the busbar at an operating temperature of 90°C. Thus, the
equation for power losses can be shown as
19 Power Loss = ~
M=l
Utilizing this model resulted in the power loss shown in Table G- 3 for both the
positive and negative busbar. The table also shows the cabling losses and the total
losses for the busbar and cabling which result in a loss of less than 1% of rated
system power.
Selected Busbar i and Wi re Gauge
Summa rv of
TABLE G-3
Busbar and Cabling Losses Total I Total
Resistance, Losses, Ohms Watts
i Percent of System Loss
.. - ._--- - - ---- --j---------+------+--- ---
4 AWG Cable 0.024
0. 014
48
10 .4
~~C~a~b~le~a~n~d~B~u~s~b~ar~ __________ ~0~. ~03~8~ __ ~ __ ~58 . 4
0.59
0.13
0 . 72
~ I I-' vJ
I~ .. -----.---.. -.
1,'1
'f"'-.... Service ~ "" Entrance Cable
1(8
-Lss--
-4
13
Susbar
Power Loss = 19 2 L [MIs] REQ M=l
where: REQ = Equivalent Sectional Resistance
Lss = 54 feet IS = 2.35 amps I, = IS' 12 = 2I S' ... , 119 = 19I 5
Figure G-3. Busbar Model
... J
.11
I~ -4 ~ 1,
REQ (Typical 19X)
Hot-Spot Heating Analysis
The large number of parallel elements in the series/parallel interconnection matrix
of the proposed solar array configuration makes this design immune to the
detrimental effects of what is called IIhot-spot li heating. Figure 6-4 shows
a hypothetical case involving the complete shadowin'g or open-circui,t failure of
three modules in parallel within the same course on the roof. The resulting I-V
characteristic for the unaffected portion of the array is represented by the curve
labeled 1124S x 19pII while the characteristic for the remaining modules in parallel
with the affected modules is labeled Ills x l6pll. In the latter case a typical I-V
characteristics in the reverse voltage direction is also drawn since it will become
important in the determination of the resultant composite I-V characteristic. This
composite array characteristic is obtained by adding the voltaqe contribution due to
each part of the circuit for constant values of array current. The resulting
curve is shown in Figure G-5. If a short circuit condition would occur for the array
as indicated by point A on the figures, the power dissipation at this point is
7650 watts (since the unshadowed array is operating at 45 amps current and 170 volts)
or 320 mw/cm2 of cell area. This is a high dissipation rate, however, still within
the operati ng capabil i t.Y of the array. The shadowi ng or open ci rcuit of three modlJl es
in the same course is a relatively worst case situation. If only one random module
were open, the resulting heat dissipation would be negigible.
G-14
-400
SHADOWED OR OPEN MODUL.ES
-350 -300 -250 -200 -150
60
40
tfl 30 0:: w 11. ::;; « IZ w 0:: 0:: :J
20
U 10
-100 -50 0 VOLTAGE (VOLTS)
• • •
• • •
o INSOLATION = Ik~/m2 o CELL TEMPERATURE = 64 C
50
IS x 16p
SHADOWED OR OPEN COURSE
100 150 200
Figure G-4. Separate I-V Characteristics for Three Shadowed Modules In Parallel
c:-15
250
-en w 0::: w c.. :E 30 < -I-Z w 0::: 0::: 20 ::J U
10
o o so
COMBINED CIRCUIT I-V CHARACTERISTICS
100 1 SO 200
VOLTAGE (VOLTS)
UNDER SHORT CIRCUIT CONDITIONS AT POINT A
250
POWER DISSIPATION = (45 AMPS) X (170 VOLTS)
POWER DISSIPATION = 7650 WATTS
(OR 320 mw Icm2 CELL AREA)
f FIGURE 8-5. Combined Array I-V Characteristics for Three Shadowed Modules
':>16
Array Sizing Tradeoffs
Array sizing for the design condition was discussed in Section 3. As
noted in the Array Sizing subsection, low energy sellback rates to the utility
tends to imply smaller system sizes, however, system sizing is also sensitive
to the assumptions for the fixed and variable portions of the system capital
cost. Therefore, several variations in the assumptions for fixed and variable
costs were made and t~e cost-to-benefit ratios determined. Table E-2 of
Appendix E listed the:fixed cost of $1779, and a total variable cost of
$118.31/m2 per unit array area initially assumed for the tradeoffs. For the
92.9 m2 array size the system capital cost, I, becomes
I = FC + VC x Array Area
I = $1779 + $118.31/m2 x 92.9 m2 = $12,769
where FC is the fixed system cost and VC is the variable system cost. For this
analysis, the Operation and Maintenance cost are not included in the base value
of $1779 but are included in the LAC calculation as discussed in Appendix E.
A ratio of the fixed to variable costs (FC/VC) equals 15 for the base case.
This ratio was then reduced to 50% and 10% of this value and increased to 70%
above the base case. Holding the system capital cost fixed at $12,769 for the 2 92.9 m array area and then using the revised FC and VC values corresponding to
the different FC/VC ratios assumed,affects the cost-to-benefit ratio variation
with array area. These results are shown in Figure G-6 for the three sellback
to buy ratios of 30, 50 and 70%. The curve labeled 3 on the figure represents
the base case as shown in Figure 3-4. The trends shown in Figure G-6 indicate
that if the fixed cost assumption is increased, the optimum array size remains
at the full roof area even for sell back rates down to 30%. As the fixed cost
assumed is reduced, high sellback rates imply full roof arrays, low sellback
G-17
G") I 1.. j f-' CO
1. 2 l
1.0
0
S ... ~
.8 .... z .... "" . 1 0 ... .6
1 ... V> 0 u
.4 ~
0"'L 0
4
3 PS/PE = .3 2
1.0 0 ;: ~ ... - .8 ... .... z ... "" I 0 ... ... .6 I V>
8 I MINIMUM POINT
.4 I .. . 0
0 20 40 60 80
. \ 3 ,
AREA. mZ
\ \ .
: ~\ PS/PE =·5
, ~
20 40 60 80 100
AREA. Ii
100
I = SYSTEM COST = FC + VC X AREA
BASE CASE FC = $1779 vc = $118.31/M2 I
CUP-VE Fc/ve = $12 J 769
1 2 3 ..
10% BASE CASE 50% BASE CASE . .
BASE CASE = 15 1. 7 X BASE CASE
o
S !: ... ... z ... "" ~ ... 8
1.4
1.2
1.0
.8
;Ii
.4
4
3
2
PS/PE = .7
or · 1 Ii 1 1 o 20 40 0 80 100
AREA, .. 2
FIGURE G-6. Variation in Cost to Benefit Ratios for Fixed Cost Variations
rates tend to optimize around a 50 m2 roof array and at a 50% sellback rate
the cost-to-benefit ratio is insensitive to array size. Thus, the sellback
rate and the assumed fixed to variable system costs values can affect the system
sizing but array sizes between 50 and 92 m2 appear to cover the range.
G-19/20
DISTRIBUTION LIST:
P. D. Maycock (25) Director, Photovoltaic Systems Division DOE/C&SE 600 E Street NW Washington, DC 20004
Attn: A. S. Clorfeine R. F. Santopietro
MIT-Lincoln Laboratory (100) P. O. Box 73 Lexington, MA 02173
Attn: Miles Russell
New Mexico Solar Energy Institute New Mexico State University Box 350L Las Cruces, NM 88003
Attn: John Schaefer
General Electric Co. (20) Advanced Energy Programs P. O. Box 8661 Philadelphia, PA 19101
Attn: E. M. Mehalick
4719 Gary J. Jones (234 )
(100)
