PV-wirefree: Bringing PV systems back to their essentials 3 rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan Henk Oldenkamp OKE-Services,

PV-wirefree:Bringing PV systemsback to their essentials

3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan

Henk OldenkampOKE-Services, The Netherlands12 May 2003

Web: www.pv-wirefree.com

Overview presentation

• Why PV-wirefree?• What is PV-wirefree?• Design• Comparison of PV-wirefree and string

systems• Status and developments• Conclusions

PV-wirefree objectives

• To minimize BOS-costs of PV-systems– PV-wirefree will reduce the PV BOS costs

with 50%• To minimize costs electricity generated

by PV-systems– PV-wirefree will decrease the kWh costs of

PV with 25%

• To minimize energy pay-back time and optimize LCA of PV-systems

How to reach these objectives?Currently many components are used in grid-connectedPV-systems. At the DC-side we find the following components:

What is PV-wirefee?

How to reach these objectives?In this presentation we will show that all these components can be omitted and/or replaced by the only four essential ones:

What is PV-wirefee?

Back to basics approach

What are the essential components ofa PV-system?• PV-laminates consisting of PV-cells to

generate electricity• Conductor to carry the DC-current from

the PV-laminate to an inverter• Inverter to convert DC- to AC-power

What is PV-wirefee? – How to reach these objectives?

Ultimate integration of functions


The basics of PV-wirefree

• Large numbers of PV-laminates connected in parallel using a current carrying mounting frame (= mounting bus)

• Each group of PV-laminates connected to one set of mounting busses has its own inverter, and is called a subsystem


Design method

• To abolish so many components clear design choices affects and limits design freedom of the other components.

• Leading design choice: optimizing the most expensive component = PV-laminate

• Step 1: Optimize PV-laminate dimensions• Step 2: Design mounting bus• Step 3: Design module connector• Step 4: Design inverter

Design

Touch safety

• In PV-wirefree systems bare conductors are used

• International safety standards require:– In dry conditions: voltage < 60 V– In wet conditions: voltage < 30 V

• PV-wirefree open circuit voltage will be 21 V

Design

Energy hazard

• Power of PV-wirefree systems > 240 VA• International safety standards require

following measures:– Sufficient separation of bare conductors– European requirements: test finger of about

8 cm shall not be able to make a short circuit– American requirements: tool shall not be able

to make a short circuit• PV-wirefree uses spacing between

mounting busses > 0.50 m

Design

Design step 1: laminate

• PV-wirefree laminate

• Mounting bus

• Module connector

• Inverter

• Step 1• • •

• •

• •

• • • • •

Design – Method – Overview

Basic assumptions

PV-wirefree should be suitable for “nearly all” PV

market sectors.

The PV-laminate should be designed for:• Flat and sloped roofs, facades and desert plants

(VLS-PV)• Both multi and polycrystalline silicone cells• Cells of the following dimensions: 12,5 x 12,5;

15 x 15 and 20 x 20 cm.

Design – Method – PV-wirefree laminate

To meet our goals

• Minimize material use and costs replace frame by 4 points mechanical connection ( no pollution like moss-grow)

• Minimize material use and costs limit wiring to cell-to-cell wiring, omit junction box

• Minimize labour costs easy installation click and fit system

• Minimize costs use standard 4 mm PV-glass (heat strengthened)


To meet general standards

For heat strengthened glass a.o. the following figures apply:• According to IEC 61215 laminates should resist a

test wind load of 2400 Pa• Additional safety factors vary per country.

Maximum calculated stress must be in the range of 50-80 N/mm² at the actual design wind load.

• At most locations in Europe the maximum wind load is well below 1500 Pa

• Actual breaking will occur at 200 – 300 N/mm²• Working conditions allow a maximum weight per

PV module of about 10 kg


Design choices

The PV-wirefree laminate should:• Withstand a test wind load of 2400 Pa, while

the stress of the glass should not exceed 80 N/mm². This implies 50 N/mm² at a wind load of 1500 Pa.

• Use standard 4 mm heat strengthened glass (common for PV). For calculations 3,8 mm must be used to cope with tolerances of glass thickness

• Have an area of maximal 1 m² (not to exceed weight of about 10 kg per PV-module)


In search of the right dimensions

• Currently: 4 x 9 cells


Stress analysis: 4 x 9 cells laminate (1)

Laminate: • 4 x 9 cells 150 mm• 0.649 m x 1.414 m =

0.981 m²Fixing:• Stiff in the cornersResult:• Maximum stress in

laminate: approximately 800 N/mm² (at a test wind load of 2400 Pa)




0.981 m²Fixing:• Stiff at optimum

position: distance between fixings 0.774 m

Result:• Maximum stress in





0.981 m²Fixing:• Flexible at optimum



laminate: approximately 130 N/mm² (at at test wind load of 2400 Pa)


Stress analysisConclusions 4 x 9 cells laminate

• 4 points connection not feasible for standard 4 x 9 cells laminates (150 mm cells) since even for an optimized design maximum stress in glass is 130 N/mm² at a test wind load of 2400 Pa



• Let’s study a 5 x 7 cells module instead of a 4 x 9 cells module



PV-wirefree laminate: • 5 x 7 cells 150 mm• 0.802 m x 1.108 m =

0.889 m²Fixing:• Fixed at optimum
















• Optimized connectorResult:• Maximum stress in

laminate: approximately 80 N/mm² (at a test wind load 2400 Pa)


Stress analysisConclusions 5 x 7 cells laminate

• 4 points connection feasible for PV-wirefree laminate of 5 x 7 cells (150 mm cells).

• The maximum stress in glass is 80 N/mm² at a test wind load of 2400 Pa

• Provided a properly engineered connector located at optimal bus distance



• Other possible dimensions: 5 x 7 cells (125 mm) and 4 x 5 cells (200 mm)


Stress analysis: 4 x 5 cells laminate







Stress analysis: 5 x 7 cells laminate







Stress analysisoverall conclusions

• 4 points connection feasible for PV-wirefree laminates of– 5 x 7 cells laminates (150 mm cells)

80 N/mm² – 4 x 5 cells laminates (200 mm cells)

80 N/mm² – 5 x 7 cells laminates (125 mm cells)

52 N/mm²at a test wind load of 2400 PaNote: Provided a properly engineered

connector located at optimal bus distance

Design – Method – PV-wirefree laminate - Dimensions

Cell interconnection= Additional advantage

• 4 x 9 cells 5 x 7 cells• Wiring limited to cell to cell wiring• Increased safety


Cell interconnection= Additional advantage

• PV-wirefree laminate of 5 x 4 cells. • But wiring should be done properly


Top 3 PV-wirefree laminates5 x 7 cells125 mm

5 x 7 cells150 mm

4 x 5 cells200 mm

Dimensions[m x m]

0.677 x 0.933

0.802 x 1.108

0.849 x 1.052

Area [m²] 0.632 0.889 0.893

Bus distance [m]

0.533 0.648 0.612

Weight [kg] 6.9 9.8 9.8

Power [Wp]@ cell efficiency

88 @ 16% 118 @ 15% 88 @ 11%

Voc [V] 21 21 12

Vmpp [V] 17 17 9


Design step 2: mounting bus


• Mounting bus


• Inverter

• 5 x 7 cells (12,5 x 12,5 and 15 x 15 cm) and 4 x 5 cells (20 x 20 cm)

• 4 mm re-enforced glass• 4 points connection (no frame)• Optimum distance between mounting

busses

• Step 2•

• •

• • • • •


Design choices

The mounting bus should:• be strong enough to carry the weight of the PV-

laminates and the wind load on the PV-laminates• have a cross area sufficient to carry the current

of the subsystem of parallel connected PV-laminates

• be made of material suitable to provide a reliable electrical contact

• be able to be fixed easily to the roof• be rugged• be low cost

Design – Method – PV-wirefree mounting bus

Material selection

Aluminum

Steel Copper

Costs (€/kg)•Bulk•Sheet•Extruded

1.54.14

30.81

1.645

Physics•Specific mass [kg/m3]•Resistivity [m]

27000.03

78570.1

89000.017

Energy content [kWh/kg]•New•Recycled•Finishing

55-698

8-14

3.7-8

> 281.4


Material selection

Aluminum

Steel Copper

Required cross section [mm²]

30 100 17

Weight [kg/m] 0.081 0.786 0.151

Costs [€/m]•Bulk•Sheet•Extruded

0.120.330.24

0.640.250.76

Energy content [kWh/m]•New•Recycled

3 – 5.60.3 – 0.6

6.3 – 11 > 4.20.2


Comparison design of a wire with a resistance of 0.001 Ohm/m

Material selection: conclusions

• Energy content per amount of conduction of aluminum is lower than steel and comparable with copper.

• The costs per amount of conduction of aluminum is lower than steel and copper.

• Since aluminum is used as common building material and can easily be extruded in virtually any shape

material choice = aluminum


Dimensions

• In order to withstand mechanical loads from the PV-modules – caused by the wind load on the PV-modules (2400 Pa) – and to be rugged enough

A strength optimized profile of approx 150 mm² is required.

• Based on this profile the maximum PV-wirefree subsystem size is approximately 3-4 kW for the 5 x 7 cells laminates and 1 kW for the 4 x 5 cells laminates

• Aluminum used: 0.67 kg per module, resulting in a energy pay back time of 2 months (5 x 7 cells, 150 mm)


Design step 3: module connector


• Mounting bus


• Inverter



busses

• Cross section of > 150 mm²• Material aluminum

• Step 3•

• • • •


Design choices

The PV-wirefree module connector should:• Provide mechanical support by a 4-points

connection• Strong enough to withstand the wind loads at the

PV-laminate• Electrically connect the PV-laminate to the

mounting bus• Have a life time > 20 jaar• Provide a visually checking possibility for correct

mating• Provide suitable means for unmating

Design – Method – PV-wirefree module connector

Material selection

• Aluminum selected for the mounting bus • Avoid corrosion caused by the difference

in electrochemical potential of different metals.

Aluminum-Aluminum contact: all electrically connected outdoor metals should be aluminum– Including contacts– Including terminals coming laminate

• This also avoids costs of finishing.


Al-Al contact principle

A reliable outdoor contact

between Al-Al requires:• Stabilized normal

contact force of about 100 N to crush the thin aluminium-oxide layer, guaranteeing a reliable connection for > 20 years

• After mating the contact surfaces shall not move


Impressions module connector


Design step 4: inverter


• Mounting bus


• Inverter



busses


• Aluminum – aluminum connection• Stabilized normal force should be > 100 N

• Step 4• • • •


Design choices

The PV-wirefree inverter should:• Have a working input voltage matching the PV-

wirefree laminates of approx 15 V (35 cells laminate) or 8 V (20 cells laminate)

• Not load the PV-modules below Vmpp for longer periods

• Never short the PV-laminates even in single fault conditions, both inside and outside the inverter for longer periods

• Have an efficiency comparable with existing inverters

• Have redundant and rugged connections to the mounting bus

• Have a price per watt comparable with existing inverters

Design – Method – PV-wirefree inverter

No short-circuit

• To avoid an external short-circuit between the inputs of the inverter, these terminals should be properly separated.

• Therefore only the enclosure is grounded and PV-modules are floating, so a short circuit is not possible at single fault conditions


Latest results

• Topology developed meeting these requirements. Even under multiple fault conditions – like mosfet, diode or driver failure – the inputs will not be short-circuited

• Testing 350 W model has started

• The topology has acceptable efficiency

• Well scalable to the size required for PV-wirefree

• First model will be approx 700 Wac.


Overview PV-wirefree components and their properties • PV-wirefree

laminate

• Mounting bus


• Inverter



busses


• Aluminum – aluminum connection• Stabilized normal force should be > 100 N

• Operating input voltage of 15 V (8 V)• Efficiency > 92% (overall)• Protecting from short circuits in system• Ground fault detection• Redundant connection to mounting bus• Price: < 0.5 €/watt (large numbers)


Intermezzo:Why we can omit so many components?

PV-string systems

• A string of modules is illuminated while a few cells are shaded The illuminated cells force the shaded cells to

operate in reverse direction Excessive power will be dissipated in shaded cellthis will cause hot spots and may eventually lead to cell defects

• To avoid hot spots bypass diodes must be usedBypass diodes are not fail-safeWhen one fails the diode may become a short circuitExcessive back-feeding current in one string may

occur when more strings are connected in parallel.Overheating of PV-modules and over-current in the

wiring. Conclusion: bypass diodes introduced a new problem

• To avoid over-current fuses must be used– As diodes are not fail safe, fuses are mandatory to

limit the back-feeding current

Intermezzo – Partial shading and hot spots

PV-wirefree systems

• What will happen when one cell of a PV-module is shaded, while the other cells are illuminated?– Will it cause hot spots like in a series

connection? – Do we need bypass diodes to avoid these hot

spots?

Intermezzo – Partial shading and hot spots

No bypass diodesIntermezzo – Partial shading and hot spots – PV-wirefree systems

No bypass diodesIntermezzo – Partial shading and hot spots – PV-wirefree systems

PV-wirefree systems

• What will happen when one PV-module is shaded, while the other PV-modules are illuminated?– Will it cause too high back-feeding currents

like in a series connection?– Are fuses needed to avoid these back-feeding

currents?

Intermezzo – Partial shading and hot spots – PV-wirefree systems

No fusesIntermezzo – Partial shading and hot spots – PV-wirefree systems





Fuses and bypass diodes: conclusions

• One shaded cell in a PV-module while the other cells are illuminated– No significant reverse current in this cell no

hot spots no bypass diodes required• One shaded PV-module while the other PV-

modules are illuminated– No significant back-feeding current no string

diodes and/or fuses required• But only provided that the inverter does not load

the PV-modules below Vmpp for longer periods (no short-circuits)

Intermezzo – Partial shading and hot spots – PV-wirefree systems

Comparison PV-string systems with PV-wirefree systemsSubjects• Performance, which is determined by:

– Mismatch losses• Optimal conditions: all PV-laminates are equally

illuminated, having same temperature• Suboptimal conditions: PV-laminates are not

equally illuminated or partially shaded or partially polluted

• Single fault conditions• Multiple fault conditions

– Conduction losses• Reliability• Safety

– Touch safety– Energy hazard

• Costs– Installation costs– Material costs– Energy payback time

Comparison PV-string systems and PV-wirefree systems

PV-laminate definitions

PV-string modules• 36 cells of 150 x 150

mm• Dimensions: 0.649 m x

1.414 m • Area = 0.918 m²• Power = 122 Wp• Cell efficiency = 15%• Voc = 21.6 V• Isc = 8 A• Vmpp = 17.5 V• Impp = 7 A• Two bypass diodes per

PV-module

PV-wirefree laminates• 35 cells of 150 x 150

mm• Dimensions: 0.802 m x

1.108 m• Area = 0.889 m²• Power = 118 Wp• Cell efficiency = 15%• Voc = 21 V• Isc = 8 A• Vmpp = 17 V• Impp = 7 A


System definitions

PV-string subsystem• 8 modules in series

• P = 976 Watt• Area =7.344 m²• Voc = 175 V• Isc = 8 A• Vmpp = 140 V• Impp = 7 A• In total 16 bypass

diodes are used• Transformerless inverter

PV-wirefree subsystem• 8 PV-wirefree laminates in

parallel• P = 944 Watt• Area =7.112 m²• Voc = 21 V• Isc = 64 A• Vmpp = 17 V• Impp = 56 A• Mounting bus length =

6.556 m (20 mm spacing between the PV-laminates)

• Distance between mounting busses = 0.658 m


Optimal conditions

PV-substring

0

PV-wirefree subsystem

0

PV-wirefree subsystem does not suffer from mismatch losses due to initial differences between Impp of the PV-modules. Transformerless inverters may have a slightly higher efficiency than PV-wirefree inverters.

These effects are considered to have about the same effect on the output of the PV-substring/PV-wirefree subsystem

Comparison PV-string systems and PV-wirefree systems - Performance

Suboptimal conditionsComparison PV-string systems and PV-wirefree systems - Performance








Suboptimal conditions: conclusions

PV-substring

−


++

PV-substring suffers from mismatch losses due to differences in illumination, temperature etc. of the PV-modules in the string. Therefore each string needs bypass diodes to prevent being damaged under these conditions. When one bypass diode conducts 10% of the power of the 8 modules string is lost. When two bypass diodes are conducting the loss is 25%. PV-wirefree does not need bypass diodes.

PV-wirefree subsystem does not suffer from mismatch losses. The mismatch losses at system level are nihil, resulting in an increase of the annual yield in suboptimal conditions with 5-15%


Single fault conditions

Condition PV-substring PV-wirefree subsystem

Open circuit anywhere

-- The power of the complete string is lost

+ Loses only the power of one module

Bypass diode short circuit

- 10% loss of string power

++

PV wirefree does not use bypass diodes. No losses.

Cell short circuit

0 0 Approximately equal output losses in the subsystem of 0.35%


Conduction losses

• The high conduction losses in the dc-wiring has been one of the main reasons to connect PV-modules in series

• PV-wirefree system uses mounting frame for conduction. Due to the large cross section the losses in a PV-wirefree subsystem are overall comparable with a PV-substring.

• For small (< 1 kW) systems PV-wirefree is slightly better; for larger systems (> 2-3 kW) PV-substrings are slightly better.


Conclusions

Condition PV-substring


Optimal conditions 0 0

Suboptimal conditions 0 ++

Single fault conditions•Open circuit anywhere•Bypass diode short circuit•Cell short circuit

---0

+++0

Conductions losses 0 0


Reliability

Condition PV-substring PV-wirefree subsystem

All conditions

− In case one of the components fails, the power of the complete string is lostMany connections in series increases chance of failures.

No redundancy of connections

++ In case one of the components fails, only the power of one module is lostComponents that are likely to fail are omitted (diodes and fuses).Very rugged mounting bus replaces DC-wiring.All connections are made redundant.

Comparison PV-string systems and PV-wirefree systems - Reliability

Touch safety



Optimal conditions + + Comparable

Suboptimal conditions

+ + Comparable

Single fault conditionsDC-wire or connector isolation damagedLaminate isolation damagedLaminate glass broken

−−

−−

−−

++

++

++

In PV-substrings there is a serious chance of personal injuries

Comparison PV-string systems and PV-wirefree systems - Safety

Energy hazard



Optimal conditions + + Comparable

Suboptimal conditions

+ + Comparable

Single fault conditionsDC-wire or connector isolation damagedLaminate isolation damagedLaminate glass brokenBypass diodes failure (open circuit)

* Provided that it is detected and switched off by condition monitoring

0*0*0*−

++++ In PV-substring

protection against partial shading is lost; cannot be detected at system level. Hot spots may occur

Comparison PV-string systems and PV-wirefree systems - Safety

Costs

PV-substring


Installation costs − + Easy installation PV-wirefree requires less time

Material BOS costs −− 0 Number of components is reduced considerable

Energy pay back time − + Energy payback time of PV-wirefree is reduced

Comparison PV-string systems and PV-wirefree systems - Costs

Conclusions comparison PV-string systems with PV-wirefree systems• Performance: In suboptimal conditions PV-

wirefree outperforms PV-strings• Reliability: Chance on failures in PV-wirefree

are reduced an order of magnitude because of redundancy in connections.

• Touch safety: PV-wirefree remains touch safe under all circumstances, even in multiple fault conditions.

• Energy hazard: Comparable• Costs: PV-wirefree reduces costs

considerably


PV-wirefree projectTime schedule• April 2002:

– Start development• October 2002:

– First prototype presented at Rome conference• January 2003:

– Start “proof-of-principle” project• December 2003:

– Start field test of PV-wirefree system• December 2004:

– Ready for market introduction (expected)

Status and developments

PV-wirefree partners

• Bear Architects• Netherlands Energy Research

Foundation ECN• NKF Electronics• OKE-Services• Oskomera Solar Power Solutions BV• TNO Bouw

Status and developments

A challenge for you?

• PV manufacturers to participate• … and for other inverter manufacturers

• But more work needs to be done:– Adapting the current PV-standard– Support of the PV-community.– It will save lots of energy when all parties start

pushing in the right direction from today on.

Status and developments - The future

Overall conclusion

• PV-wirefree offers an opportunity to reach the minimum BOS costs of PV systems possible in due time

• And thus to contribute to the target making photovoltaic solar energy a viable option for producing electricity in a sustainable way

Status and developments - The future

Finally

• More information available at

www.pv-wirefree.com

Documents

PV-wirefree: Bringing PV systems back to their essentials 3 rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan Henk Oldenkamp OKE-Services,