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THE THERMAL PERFORMANCE OF SDHW - SYSTEMS MEASURED IN A SOLAR SIMULATOR
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Commission of the European Communities
n r g y
THE THERMAL PERFORMANCE
OF SDHW - SYSTEMS MEASURED
IN A SOLAR SIMULATOR
Report
EUR 10181 EN
Blow-up f rom mic ro f iche o r ig ina l
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Commission of the European Communities
n r g y
THE THERMAL PERFORMANCE
OF SDHW - SYSTEMS MEASURED
IN A SOLAR SIMULATOR
H. HETTING ER K.P. RAU
Commission of the European Communities
JOINT RESEARCH CENTRE
Ispra Establishment
1 21 2
Ispra Va)
Directorate-General Science, Research and Development
Joint Research Centre
1985 EUR 10181 EN
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Published by the
COM MISSION OF THE EUROPEAN COM MUNITIES
Directorate-General
Information Market and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Com missio n of the European Com munities nor any person acting on behalf
of the Commission is responsible for the use which might be made of the following
information
© E CS C - EEG—EAEC Brussels - Luxembourg 1986
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III
Abstract
An indoor test procedure for measuring the thermal performance
of solar domestic water heaters is proposed and applied to ten
different systems. This method is based on the ASHRAE STANDARD
95-1981.
The measurements are performed using a solar simula
tor with a climatic chamber. A complete performance test of
a SDHW-system is completed in less than ten
days.
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7/21/2019 The Thermal Performance of Sdhw - Systems Measured in a Solar Simulator
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ν
CONTENTS
Page
1. Introduction 1
2.
The Test Method 1
3. The Experimental Test Conditions 3
4.
The Experimental Installation 5
5. SDHW Systems in the Test 6
6. Results 8
6.1. Efficiency of SDHW Systems 8
6.2. Stratification in the Tank 12
6.3. Collector Efficiency 13
6.4. Auxiliary Heating 13
7. Discussion of Results 15
8. Conclusions 18
Acknowledgements 18
References 19
Figures 20
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1. INTRODUCTION
Many efforts to develop standard test methods for determining the
thermal performance of solar domestic hot water systems (SDHW) have
been under way for several years, especially in the USA, Canada and
Australia
/1,2,3/.
Although a solar hot water system is generally
a relatively simple device, no agreement on a standard procedure for
measuring and presenting the performance of those systems has been
obtained up to now. A first step to such an agreement is that the
ASHRAE STANDARD 95-1981 / l / is used as a basis for all dif
ferent rating standards of a certain importance. This indoor method
requires the use of a solar or thermal simulator together with a cli
matic chamber. Outdoor measurements with complete SDHW-systems over
very long time periods and under different climatic and draw condi
tions would give the best objective results. Unfortunately, manufac
turers and users of SDHW systems have to make prompt decisions and
cannot wait years for reliable data. For this reason an indoor test
procedure to obtain the performance of complete solar water systems
under standard solar input, using a standard draw pattern, could
provide usable information in a rather quick way. Along with climatic
data and results from outdoor experiments, it should be possible to
predict the long-time performance of SDHW systems.
In this report a simple method is described, which allows the compa
rison of different SDHW-systems using a solar simulator in a clima
tic chamber. This method is applied to ten domestic water heaters of
different designs. The time required to perform the test of one sys
tem is in the order of eight days.
2. THE TEST METHOD
The difficulty in developing a reasonable standard test method is that
the performance of a SDHW-system depends on many different combina
tions of operating conditions such as climate, weather, total load
demand, time of load demands during the day, temperature of inlet
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water,
temperature of hot water required, proper installation of the
system and numerous other factors. It is impossible to find a set of
test conditions which will cover all eventualities for all solar wa
ter heating systems and which would give results which represent an
annual saving of the systems. Test conditions must, therefore, be
chosen in a realistic manner so that they do not produce optimistic
or pessimistic results, which cannot be observed in normal operation,
and that they do not favour a certain system or adversely affect an
other.
The tests must also be performed in a reasonable space of
ti me
and
the results must be presented in such a way that they can be under
stood by everybody. In order to make a test procedure repeatable,
most of the standard test methods actually used require that the com
plete SDHW system be tested indoors, using either a solar or a thermal
simulator. The use of a thermal simulator will create some problems
when testing thermosyphon and Integral Collector Storage - systems
(ICS) and should be only considered when no solar simulator is avail
able.
The basis for the test method proposed in this paper is the ASHRAE
STANDARD 95-1981 Method of Testing to Determine the Thermal Perfor
mance of Solar Domestic Hot Water Heating Systems /l/ . This standard,
widely used in the USA, Canada and Australia, specifies only the
testing procedure, which consists in measuring the daily thermal per
formance of a system for some consecutive days (maximum 4
days),
until thermal equilibrium for the system is achieved. The meteorologi
cal and load conditions for the test are prescribed separately by
national or industrial rating associations, which take into account
the climatic conditions of the region where the SDHW-systems are used.
In the USA the Solar Rating and Certification Corporation (SRCC) has
prescribed a standard rating day /4,5/. The Air-Conditioning and Re
frigeration Institute (ARI) has proposed two standard rating days /6/.
Similar conditions concerning mainly the insolation
profile,
ambient
temperature, wind velocity, cold water inlet temperature and hot water
drawn pattern have been selected by the Canadian Standards Association
(CSA) /2/.
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The advantage of such selected rating conditions is that the perfor
mance of SDHW systems can be compared directly. The disadvantage is
that no accurate result about the relative performance of the system
is given when weather conditions have to be considered which are very
different from the standard conditions. Computer simulations which
utilize the results of long-term outdoor performance measurements
with characteristic SDHW systems along with site-specific meteorologi
cal data could be very helpful in the prediction of system performance
for all possible climatic conditions.
THE EXPERIMENTAL TEST CONDITIONS
As has already been mentioned in the previous chapter, the indoor test
procedure for SDHW-systems applied in the JRC Ispra is based on the
ASHRAE STANDARD 95-1981. The hot water system to be tested is installed,
according to the instructions given by the manufacturer, in the clima
tic chamber of the solar simulator LS -1 . Instruments are installed to
measure the ambient air temperature for the collectors and the hot
water storage tank, the cold water inlet temperature, the temperature
and flow rate of the hot water drawn from the system, the flow rate
and inlet and the outlet temperatures to and from the tank of the
heat transfer liquid in the collector circuit.
During the test the collectors are subjected to a daily solar radia
tion totalizing 16.8 MJ/m^ (summer day) and a second test with 10.4
MJ/m^ (winter day ). The simulated solar radiation profile is given in
Table 1.
A standard daily test load E^^ = 43200 kJ is taken in three draws at
9°°,
13°° and 17°° . Each draw is stopped when 14400 kJ are taken from
the tank or when the delivery water temperature falls below 25°C. The
outlet water flow is between 7 and 8 1/min. The test day is repeated
until convergence is reached but is limited to a maximum of 4 days for
each insolation level. For all systems tested so far the time period
of 4 days was sufficient to achieve thermal equilibrium.
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TABLE 1 - Simulated solar radiation during a test day
Time
9.00 - 10.oo
10.oo - 11.oo
11.oo - 12.oo
12.oo - 13.oo
13.oo - 14.oo
14.oo - 15.oo
15.oo - 16.oo
16.oo - 17.oo
"summer
kJ/m
2
900
1800
2500
3200
3200
2500
1800
900
16800 :
day"
kJ/m
2
"winter day"
kJ/m
2
_
900
1800
2500
2500
1800
900
-
10400 kJ/m
2
The ambient temperature in the climatic chamber is maintained at
T
a
= 20 ± 1°C. During the tests, an artificial wind blows across the
collectors with a speed of Ug = 4 ± 1 m/sec. The tank inlet tempera-
ture of the water during the three draws per day is Ti
n
■ 15 ± 1°C.
These experimental conditions are chosen for the testing of SDHW
systems designed for households with 3-5 persons. Such systems nor-
mally have a collector surface of about 3 m
2
and a storage tank in
the order of 200 1. All tests described in this paper are limited to
SDHW-systems of this order of size. As the solar energy converted to
the boiler is generally smaller than the load demand under the condi-
tions outlined above, the test will run to convergence after few days.
Thermal equilibrium or convergence is considered to be reached when
the ratio of the daily energy drawn and the daily solar energy received
by the collectors,
E¿[
raw
/ Eighty is within 2% of the value of the
previous day.
All SDHW-systems are tested as solar only or solar preheat systems.
This can be justified by the fact that we are mainly interested in
the solar performance of the systems. On the other hand, most manu-
facturers are installing electrical back-up systems which consist of
an electrical resistance heater in the upper part of the tank con-
trolled by a thermostat. If such a system is used uncritically, this
must lead to a considerable waste of energy. It is much more economic
to heat up the quantity of water drawn from the tank which is not
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sufficiently hot and not to keep the upper part of the tank at a high
temperature by use of the back-up system all day
/7/.
If such a simple
device must be used it should at least be used in an intelligent manner.
To keep a part of the tank at high temperature will not only increase
tank-losses,
but also decrease the collector performance because the
average working temperature will be higher (see chapter 6.4).
4. THE EXPERIMENTAL INSTALLATION
All measurements are done in the solar simulator LS-1 of the JRC
Ispra
/8/.
This installation allows us to perform measurements on a
test plane of 3x4 m
2
with uniform and uncollimated light under very
stable temperature conditions. It consists of two climatic chambers,
the first one contains the light source, the second one contains the
test support for solar collectors and the space for the installation
of complete SDHW systems (Figs. 1 and 2 . Both chambers are separated
by a glass pane. The light source consists of 296 discharge lamps
producing a uniform irradiance which may be selected between 250 and
1200 W/ m
2
. The spectrum of the simulated light is very similar to the
sun spectrum at sea level. For glass covered collectors it is not
necessary to apply any spectrum correction. The facility is equipped
with a large test loop, containing two thermostatically controlled
tanks,
one for hot and one for cold water. The inlet temperature to
the collectors and storage tank of the SDHW systems is adjusted and
controlled by motor-driven mixing valves. Flow measurements are per
formed with magnetic flow meters (system E.C.
Eckardt),
with an accu
racy better than 0.5 . All temperatures are measured with HP quartz
thermometers. High accuracy is especially required for ΔΤ measurements,
The precision achieved is about 10
-2
°C in ΔΤ. The light intensity is
measured with Kipp & Zonen pyranometers CM6. The surrounding wind
speed of ^ 4 m/sec is generated by radial fans. The data acquisition
and control of the measurements is achieved with a PDP-11/30 compu
ter (Table 2 ).
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TABLE 2 - LS-1 specifications
Light source
Type
Intensity
Stability
Test area
Light uniformity
Climatic chamber
Dimensions
Temperature range
Stability
Wind speed
: multiple lamp system, uncollimated 296 HQI-R 250 W
Osram Power Stars
: variable in 6 steps from 250 to 1100 W/m
2
: better than 1%
: 4x3 m
2
: ± 4% over test area
I i\; 5 m
w % 4 m
\
about 80 m^
h^¡4 ι
: -40°C <_T<_ 60°C
: ± 1°C
: 1 m/sec < ν < 5 m/sec
Collector test circuit
Heat transfer fluid
Working principle
System pressure
Number of loops
Temperature range
Ins trumentation
water, water + glycol, etc.
gravity
13 m water column
4
-20°C < Τ < 100°C
Light intensity
Ambient air temperature
Absolute fluid temperature
Differential fluid temperature
Fluid flow rate
Surrounding air speed
Data acquisition system
Principal application of LS 1
pyranometer (class 1)
quartz thermometer
quartz thermometer
quartz thermometer
magnetic flow meter
electronic anemometer
station computer and central computer
Thermal collector testing
Heat pump testing
Photovoltaic module testing
Thermal cycling of solar components
5. SDHW SYSTEMS IN THE TEST
The indoor test method is applied to 10 different SDHW systems from
8 manufacturers:
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System 1:
Name : SELF HELIOS
Manufacturer: SOLEFIL, Perpignan (France)
Type : pumped direct system
System 2:
Name : HELIOFIL
Manufacturer: SOLEFIL, Perpignan (France)
Type : pumped indirect system with heat exchanger in the tank
System S:
Name : KIT SOLAIRE
Manufacturer: CHAFFOTEAUX et MAURY, Montrouge Cedex (France)
Type : pumped indirect system with heat exchanger in the tank
System 4:
Name : FINTERM
Manufacturer: Joannes-Finterm, Torino (Italy)
Type : pumped indirect system with wrap-around heat ex
changer (double wall tank)
System 5:
Name : GIORDANO KSH 220
Manufacturer: GIORDANO, Vallauris (France)
Type : direct thermosyphon system
System 6:
Name : GIORDANO EUREKA
Manufacturer: Giordano, Vallauris (France)
Type : pumped direct system with thermovalve
System 7:
Name : DISCOTERM
Manufacturer: a.t.i. di Mariani & C., Cesena (Italy)
Type : integral collector system (ICS)
System 8:
Name : SOLAR EDWARDS L 180
Manufacturer: Edwards Hot Water Systems, Welshpool (Australia)
Type : direct thermosyphon system
System 9:
Name : ECOSOLAR EC/200
Manufacturer: ECOTERMICA, Trapani (Italy)
Type : integral collector system (ICS)
System 10:
Name : HOVAL THERMOMAX TS 100
Manufacturer: HOVAL, Carival S.p.A., Grassobbio (Italy)
Type : evacuated heat pipe collector with integrated boiler
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All these systems have a storage tank of about 200 1 and a collector
surface of 2-3 m
2
, except system No.10 where collector surface and
tank volume are smaller (see Table 4
.
6. RESULTS
6.1 Efficiency of SDHW systems
Table 3 shows the results of the measurement for SDHW-system 3 with
high insolation level. This system already reached thermal equilibrium
after two days of operation. With some experience, the initial condi
tions of the systems can be chosen in such a way that convergence can
be anticipated by at least one day. Four days for one test were never
exceeded. As each system has to be tested twice with different insola
tion,
the complete experimental procedure does not exceed 9 consecu
tive days, mounting and disassembly of the system included.
The results of the test campaign with 10 different SDHW-systems are
summarized in Tables 4a and 4b. The following definitions are used:
F [m
2
] = aperture area of the collector(s) or the ICS-system
V. ... [l] = volume of the water in the storage tank or in the ICS-system
Price [ECU ]=
purchase price of the system without installation costs
E [kJ] = the desired daily load of the SDHW system = 43200 kJ per day
l[kJ/m
2
l = daily light energy per m
2
. Two levels are used:
16800 kJ/m
2
and 10400 kJ/m
2
.
E.. .
t
[ k j ] = I x F
1 7
light coll
E, ...[kj] = daily energy transferred from the collector to the storage
tank by the working fluid =
= ƒ ¿C (T).(T
1
-T
2
)df.
o
F
This value is only measured for pumped systems. In thermo
syphon systems the installation of a flow meter would dis
turb the natural convection between collector and tank.
For ICS-systems
Ε - ι
is meaningless.
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TABLE 3 - Thermal
Date
Time
9.OO-10.OO
lO.oo-ll.oo
11.00-12.00
12.oo-13.oo
13.00-14.00
14.00-15.00
15.oo-16.oo
16.00-17.00
17.oo-18.oo
ΣΕ (kJ)
draw
E
light
draw
0
9236
10319
19555
efficiency test c
10.1
E
light
2278
4588
6392
8348
8473
6621
4634
2346
43680
.448
.85
E
boil
2099
3488
4377
5344
6219
4349
2517
147
340
28880
T
4-
out
13.7
34.6
35.6
-
f SDHW
draw
0
12774
10849
23623
.541
-system
11.1.
E
light
2284
4588
6410
8394
8478
6630
4587
2309
-
43680
(+21 )
3.
Insolation 16800 kJ/m
2
day
85
E
boil
1360
2856
3884
4914
6220
4281
2447
291
295
26548
T
4.
out
20.2
38.7
36.0
-
12.1.85
draw
0
12850
10863
23713
.54:
E E
light boil
2240
1343
4565
2881
5460 3930
8381 4931
8445 6232
6653 4306
4619
2478
2318 296
297
43680 26699
1
(+0.4 )
T
-
out
20.0
38.4
35.7
-
draw
0
12873
10972
23845
.546
13.1
E
light
2240
4580
6468
8379
8436
6653
4612
2312
-
43680
.85
boil
out
1348 19.8
2902
3966
4960
6244
38.4
4325
2482
292
304 35.8
26822
(+0.6 )
k£>
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TABLE 4a - Solar domestic hot water
System No.
System name
F .. (m
2
)
coll
V, . . (1)
boil
Price (ECU)
E¿]_
= standard daily
Tout 1
2 5 %
I (kJ/m
2
)
E
T ■ U4.
k J )
light
E, .. (kJ)
boil
E (kJ)
par
E^ (kJ) total
draw
E, (%) at 9.00
draw
E, (%) at 13.oo
draw
E^ (%) at 17.oo
draw
T (°C)
a
T. (°C)in
T ^ (°C) at 9.00
out
T (°C) at 13.oo
out
Τ , (°C) at 17.oo
out
draw dl
E /E
draw' light
E
boil
/E
light
1
SELF HELIOS
3.06
205
1363.-
system test
2
HELIOFIL
3.06
205
1185.-
load = 43 200 kJ taken in three
16 800 10 400
51 408 31 824
34 968 23 010
1 440 1 080
32 501 22 269
8 0
46 62
46 38
19.7 19.5
14.4 14.4
26.8 21.5
37.7 30.6
36.6 26.9
.75 .52
.63 .70
.68 .72
16 800 10 400
51 408 31 824
32 138 20 509
1 008 756
30 448 19 979
0 0
50 52
50 48
20.1 19.3
14.8 15.1
20.1 18.0
34.5 27.7
34.3 27.4
.70 .46
.59 .63
.63 .64
3
CHAFFOTEAUX &
MAURY
2.60
166
1220.-
draws of max 14
16 800 10 400
43 680 27040
26 822 17 169
1 008 756
23 845 15 595
0 0
54 52
46 48
20.4 20.6
14.1 14.1
19.8 19.4
38.4 32.2
35.G 31.1
.55 .36
.55 .58
.61 .63
— , . . . ι — - - . ■ ■
4
JOANNES-
FINTERM
3.05
195
1165.-
400 kJ at 9.00,
16 800 10 400
51 240 31 720
29 727 19 620
2 300 1 730
29 310 19 515
3 0
48 50
49 50
19.5 19.7
15.0 14.3
25.6 19.0
38.5 29.7
38.5 29.8
.68 .45
.57 .62
.58 .62
5
GIORDANO
KSH 220
2.06
235
783
13.oo a
16 800
34 608
-
-
19 166
0
55
45
20.7
15.5
22.7
33.2
32.1
.44
.55
id 17.00
10 400
21 424
-
-
11 939
0
49
51
20.1
13.9
22.2
29.0
28.8
.28
.56
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TABLE 4b - S o l a r do me s t i c ho t w a t e r
S ys t e m N o .
S ys t e m na me
F . . (m
2
)
c o l l
V, .. (1)
b o i l
P r i c e ( EC U )
E -^ = s t a n d a r d d a i l y
T > 25
o u t —
I ( kJ / m
2
)
E . . ( k J )
l i g h t
E, . . (kJ )
b o i l
E ( k J )
p a r
E
J
( k J ) t o t a l
d r a w
E , ( ) a t 9 . 0 0
d r a w
E , ( ) a t 13 . o o
d r a w
E„ ( ) a t 17 .oo
draw
T (°C)
T. (°C)
i n
T ( ° C ) a t 9 . 00
o u t
T ( ° C ) a t 13 . oo
o u t
T ( ° C ) a t 17 . o o
o u t
E /E
dr a w
7
d l
E
^ /
E
i · v. .
d r a w l i g h t
E
b o i l
/ E
l i g h t
6
GIORDANO
EUREKA
3 . 0 9
2 0 5
1 2 4 2 . -
s y s t e m t e s t
7
ATI DISCOTERM
2 x 1 . 1 0
2 x 1 2 5 . 5
8 0 5
l o a d = 4 3 2 0 0 k J t a k e n
16 80 0 10 400
51 91 2 32 136
- -
2 59 2 2 592
30 84 3 20 449
2 0
49 53
4 9 4 7
2 0 . 1 1 9 . 8
1 4 . 7 1 4 . 6
2 5 . 2 2 0 . 4
3 8 . 0 3 0 . 9
3 7 . 7 3 0 . 1
. 7 1 . 4 7
. 5 9 . 6 4
1
16 800
3 6 9 6 0
-
18 531
0
8 0
2 0
2 0 . 2
1 4 . 7
2 0 . 2
3 3 . 7
2 9 . 8
. 4 3
. 5 0
—
= 2 . 2 0
= 2 5 1
. -
i n t h r e e
10 400
22 880
-
10 749
0
39
6 1
1 9 . 7
1 4 . 6
2 0 . 0
2 8 . 8
2 8 . 8
. 2 5
. 4 7
™
8
SOLAR EDWARDS
1 . 8 3
176
1 6 0 6 . -
d r a w s o f m a x 1¿
16 800 10 40 0
30 744 19 032
— —
20 123 12 787
0 0
55 59
4 5 4 1
2 0 . 2 2 0 . 3
1 5 . 1 1 5 . 1
2 0 . 7 2 0 . 9
3 2 . 8 2 8 . 3
3 0 . 1 2 6 . 2
. 4 7 . 3 0
. 6 5 . 6 7
'
9
ECOSOLAR
E C / 2 0 0
2 . 6 3
192
8 6 8 . -
4 0 0 k J a t 9 . 0 0 ,
16 800 10 40 0
44 184 27 352
— —
16 03 0 9 132
0 0
53 50
47 50
2 1 . 1 2 1 . 3
1 5 . 1 1 5 . 2
2 2 . 7 2 2 . 8
3 3 . 8 2 9 . 8
3 4 . 6 3 0 . 7
. 3 7 . 2 1
. 3 6 . 3 3
10
THERMOMAX
TS 100
1 . 5 1
94
1 0 8 3 . -
1 3 . 0 0 a n d 1 7 . o o
16 800 10 400
25 368 15 704
~ —
18 140 11 539
0 0
53 53
47 47
1 9 . 8 1 9 . 5
1 5 . 3 1 4 . 5
1 9 . 5 1 9 . 2
4 0 . 4 3 2 . 1
3 8 . 6 3 0 . 8
. 42 . 27
. 72 . 73
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12
E [kj] = parasitic energy losses: the daily energy used to supply
power to pumps, controllers, etc. to operate the SDHW system
E ^
[kjl = daily energy delivered as hot domestic water =
fdraw
= ƒ mC (T)(T
out
-T
in
)dt
o
in three draws at 9.oo, 13.oo and 17.oo
T
out
L
25
°
c
'
fc
in - 15 ± 1°C
The maximum of one single draw is 14400 kJ.
T
&
[ c] = average ambient temperature during the test day
The ratio E /E is the relative performance of the SDHW system under the
draw dl
given test conditions. This is the most significant number of the test.
It shows how much energy is delivered as hot water by a certain system.
It can be easily influenced by changing the geometrical properties of
the system such as collector surface and/or tank, size.
Edraw/Eiight
c a n
Ì3e
defined as the thermal efficiency of a SDHW system.
This number shows the amount of solar energy received by the collector
which is converted into hot water energy. It depends mainly on the
collector efficiency and thermal insulation of the whole system. As
the collector efficiency and thermal losses of tank and tubes depend
on the operation temperature of the system, the relative performance
and the thermal efficiency will decrease with increasing temperature.
EjrjQ i/Ej ght gives the amount of energy originally transferred to the
storage tank and consequently is an indication of the thermal losses
of the tank. Unfortunately this number cannot be measured for all sys
tems in the test.
6.2 Stratification in the tank
As an additional measurement, the outlet temperature of the hot water
from the tank was registered during each draw. This gives information
on the stratification of the water in the tank or how much any possible
existing stratification is disturbed by the incoming cold water. A
storage tank with a good design will deliver water with a nearly con-
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13
stant high temperature over quite a long period and the temperature
will decrease with a step function to the inlet temperature of the
cold water when all the hot water is consumed. The use of a high tank
will favour the formation of a stratification and will soften down the
perturbation of the incoming cold water in the lower part of the boiler.
This is the reason why thermosyphon systems, where the tank is normal
ly disposed horizontally, and ICS-systerns without separate storage
tanks, have poor stratification.
In Fig. 3 the water outlet temperature of the different systems is
represented as a function of time for the draw at 13.oo where the
maximum amount of hot water is available.
6.3 Collector efficiency
For seven systems in the test, the solar collector efficiency was
measured in a separate experiment. For the two ICS and the evacuated
heat pipe system with integrated boiler, such a measurement makes no
sense.
As systems 1 and 2, and systems 5 and 6 use the same collectors,
only five different collector types had to be investigated for their
performance. The results of these measurements are given in Table 5
and Fig. 4. More details can be found in the corresponding Thermal
Collector Test Reports / 9/ and in /10/.
6.4 Auxiliary heating
Most of the SDHW-systems in the test are equipped with an auxiliary
heating device consisting of an electrical resistance and controlled
by a thermostat. In order to show the influence of such a supplementary
heater on the energy balance of a solar water heating device, system 4
was investigated. An electrical resistance heater of 2000 W was in
stalled in the upper part of the storage tank. The thermostat was set
to 50°C and the usual test method was applied. Thermal equilibrium of
the system was achieved after 3 days as for a solar water heater with
out auxiliary heating. Table 6 shows the principal results of these
experiments together with the data obtained without auxiliary heater.
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14
TABLE 5 - Instantaneous efficiency curves for collectors used in
SDHW tests
The instantaneous efficiency η is defined by η =
r r
cl
Q
u
: useful power extracted (W)
G : solar irradiance at collector aperture (Wm~
2
)
A
a
: aperture area (m
2
)
Linear performance characteristic η = η -aT
o
Λ 2
S e c o n d o r d e r f i t t o d a t a η = n
0
- a j T * - a
2
G (Τ )
Τ : r e d u c e d t e m p e r a t u r e
T =
Tm-Ta
s t a g
r e d u c e d s t a g n a t i o n t e m p e r a t u r e
a
l
s t a g
- 2 a
2
G
/ST
[2a
2
cJ
a
2
G
efficiency integral of collector
f
S t
ai *2 a
2
G
stag 2 stag
Τ
3 stag
C o l l .
F I N TERM
G i o r d a n o
S O L E F I L
CHAFFOTEAUX
M a u r y
S o l a r
E d w a r d s
F a b r .
N o .
3 2 7 1 1 3
3 2 7 3 7 5
7 2 7
7 3 7
Y
8 3
8 3
8 3
8 4
8 3
8 3
8 4
l i n .
0 . 8 1 6
0 . 8 1 6
0 7 54
0 8 04
0 7 69
0 7 73
0 7 24
f i t
a
7 . 2 3
7 . 0 1
5 . 2 3
6 . 3 8
8 . 3 7
8 . 4 2
6 . 8 9
2 n d
n
o
. 8 0 4
. 8 0 4
. 7 4 2
. 7 9 5
. 7 6 3
. 7 6 5
. 7 1 9
o r d e r
a
l
5 . 1 2
5 . 0 0
3 . 6 6
5 . 3 3
6 . 9 0
6 . 5 1
5 . 8 7
f i t
a
2
G
3 5 . 6
3 3 . 9
2 5 . 2
1 5 . 1
2 5 . 9
3 3 . 5
2 2 . 1
T
x
s t a g
. 0 9 4 7
. 0 9 7 0
. 1 1 3 7
. 1 1 3 0
. 0 8 4 1
. 0 8 2 5
. 0 9 1 2
J
E
. 0 4 3 1
. 0 4 4 2
. 0 4 8 4
. 0 4 8 5
. 0 3 4 6
. 0 3 4 7
. 0 3 5 6
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15
TABLE 6 - SDHW-system 4
"summer day" "winter day"
solar+aux. solar only solar
+aux.
solar only
E
light
^ i l
draw
E
aux
E
dl
(kJ)
(kJ)
(kJ)
(kJ)
(kJ)
51 240
26 937
38 001
13 608
43 200
51 240
29 727
29 310
-
43 200
31 700
17 287
37 391
23 598
43 200
31 700
19 620
19 515
-
43 200
System 4 delivered under those conditions 88% of the desired daily
load E¿i instead of 68% without electrical heating on a "summer day"
and 87% instead of 45% on a "winter day".
On the "summer day" 13 608 kJ electrical energy is consumed for the
extra production of 8 691 kJ = (AE
c
j
raw
) of hot water energy, which
results in an efficiency of the auxiliary heating of about 64%. From
the 4 917 k J = (E
aux
-AE
draw
) wasted, 2 790 k J = (ΔΕ^.^) or 56% are
due to the higher operation temperature of the collectors, whereas
the rest of 44% is due to higher tank losses. For the "winter day"
the efficiency of the auxiliary heating is increased to 76%. From the
6 722 kJ wasted, 41% are collector losses and 59% are tank losses.
In this case 58% of the energy delivered to the system is electrical
energy and it is debatable whether such a system can still be con-
sidered as "solar".
7.
DISCUSSION OF RESULTS
The most important numbers of Tables 4, i.e. the ratios E¿
raw
/E
dl
and E¿i
r
aw/
E
light'
a r e
represented in Figs. 5 and 6 along with the
system numbers. If we compare for example system 1 with system 3 we
can conclude that system 1 produces 75% and
52%,
respectively, of the
desired daily load, whereas system 3 only 55% and 36% at the two in-
solation levels, or system 1 performs better than system 3 by a fac-
tor 1.36 at high insolation and by a factor 1.44 at low insolation.
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16
This difference in performance is partly due to the difference of the
collector surfaces of the two systems. If we correct for
this,
we
obtain for system 2 a factor 3.06/2.60 = 1.18 which can only explain
abou t half of the difference in performance of the two systems. The
remaining difference is due to the difference in the thermal efficien
cy of the two systems, 0.63 and 0.55, the ratio of which is 1.15. On
the other hand, 1.36/1.18 = 1.15. At low insolation level it is
1.44/1.18
= 1.22 whereas the ratio of the two thermal efficiencies is
0.70/0.58 = 1.21. This difference in thermal efficiency can be attri
buted to a better collector efficiency, to a better heat exchange
(system 1 is a direct system)
to the tank size, tank insulation,
control strategy and many other factors. To separate all these effects
would be a very difficult undertaking.
If we consider that all systems are tested under two different insola
tion conditions we would expect a ratio of the relative performance
of 16800/10400 = 1.62 minus a factor which takes into account the
higher heat losses of the different systems at higher temperatures.
Table 7 shows the ratios for the 10 SHDW-systems.
TABLE 7
System
No.
E ,
(16.8)
draw
E (10.4)
draw
S y s t e m
No .
6
7
8
9
10
E „ ( 1 6 . 8 )
d r a w
E ( 1 0 . 4 )
d r a w
1 .51
1.72
1 .57
1 .76
1 .55
1
2
3
4
5
1.44
1.52
1.52
1.51
1.57
With this table we can state that the ratio of the two relative system
efficiencies obtained at two insolation levels seems to be a function
of the SDHW system type. Indeed, all the pumped indirect systems 2,3,4
and 6 have a factor 1.51 or 1.52, the two thermosyphon systems have a
ratio of 1.57 whereas the two ICS-systerns have the highest ratios of
1.72 and 1.76, respectively.
Whereas for 8 systems (5 pumped, 2 thermosyphon, 1 evacuated heat pipe)
the thermal efficiency is higher at the low insolation level, which is to
be expected, the opposite is true for the two ICS-systems No. 7 and 9 (Fig.6)
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17
Another unsolved problem is how to take account of the parasitic elec
trical energy Ep- ^ consumed by pumps, electronics, etc. in a system
performance test. The scale of proposals goes from not considering
them (because no manufacturer of, for example, oil fired heating sys
tems will mention the electrical energy consumed by the burner, or
valves, etc.), to a distinction of the original energy consumed to
produce the parasitic electrical energy. For the systems in our test
Epar ranges from 4% to 13% of the energy produced as hot water E ¿
r a w
.
The very high energy consumption of system 6 is due to the fact that
the manufacturer of this system had the idea of avoiding any control
of the system, with the consequence that the pump is in operation for
24 hours a day. This waste in electrical energy can easily be reduced
by the installation of an inexpensive time clock.
From Fig. 5 it can be seen that system No.l, a direct pumped SDHW-
system with a flat plate collector, is the best performing system in
the test. The system with the highest thermal efficiency can be
dis
tinguished in Fig. 6 as system 10, an evacuated heat pipe system with
integrated tank. A direct comparison of the two systems is not possible,
because system 10 has only half of the collecting surface and of the
storage tank volume of system 1. If we assume that two parallel
sys
tems of type 10 will have double the relative performance of one sys
tem, we can conclude that such an evacuated heat pipe system will
pro
duce 12% (0.84 : 0.75 = 1.12) and 4% (0.54 : 0.52 = 1.038 more hot
water energy than system 1 at the two insolation levels, respectively.
This example also shows the superiority of evacuated tube systems es
pecially at higher working temperatures.
In Tables 4 we have also listed the purchase prices of the different
SDHW systems, which include pumps, control units, thermosondes, valves,
etc. But these numbers do not say much about the real costs which are
needed to achieve a working system. For pumped systems, which have a
separate collector circuit, the installation expenses can exceed the
purchase prices even by a factor two, whereas for thermosyphon and ICS-
systems the installation costs can be near zero. It depends on so many
factors that a generally valid statement cannot be given here.
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18
8. CONCLUSIONS
It has been shown that the indoor test method proposed for the deter
mination of the thermal performance of SDHW-systems gives repeatable
results in a reasonable measuring time. It allows the comparison of dif
ferent systems and can be helpful for the user to make his choice of
a certain water heating system as well as for the manufacturer to see
the weak points of his product and to improve it. It is obvious that
these tests give only an indication about the thermal performance of
water heaters. They must be completed by rigorous quality and durability
tests. It is possible that the cost-effectiveness of a very simple
sys
tem (ICS, thermosyphon) is much better than that of a very sophisticated
installation with higher installation and maintenance costs. The safe
operation of a system under each condition (overheating, freezing)
must also be taken into consideration before making a decision.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Mr. M. Zanarella for the
accurate installation of the SDHW-systems. Thanks are due to Drs. Free
man A. Ford (FAFCO Inc., Menlo
Park),
Matthew W. Rupp (DSET, Phoenix, AZ),
Kent Reed (NBS, Gaithersberg,
MD),
and Arlen Reimnitz (SRCC, Washington
DC) for the informative discussions which were held before starting
this work.
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19
REFERENCES
1. ANSI/ASHRAE 95-1981. Methods of testing to determine the thermal
performance of solar domestic water heating systems. ASHRAE Publi
cations Sales Department, 1791 Tullie Circle, NE, Atlanta, GA 30329.
2. CSA Standard F 379-M 1982. Solar domestic hot water systems liquid
to liquid heat transfer. Canadian Standards Association, 178 Rex-
dale Boulevard, Rexdale, Ontario, Canada.
3. S.D. James, D. Proctor, Development of a standard for evaluating
the thermal performance of a domestic solar hot water system ,
ISES Meeting, Brisbane, 10-12 November 1982.
4.
SRCC Standard
200-82.
Test methods and minimum standards for cer
tifying solar water heating systems. Solar Rating and Certification
Corporation, 1001 Connecticut
Ave.,
N.W., Suite 800, Washington
DC 20036.
5. SRCC document OG-200 (1983). Operating guidelines for certifying
solar water heating systems, Solar Rating and Certification Cor
poration, 1001 Connecticut Ave., N.W., Suite 800, Washington DC
20036.
6. ARI standard 920 (1981). Standard for solar hot water systems.
Air Conditioning and Refrigeration Institute, 1815 North Fort Myer
Drive,
Arlington, Virginia 22209.
7. Gutierrez et al., Simulation of forced circulation water heaters;
effects of auxiliary energy supply, load type and storage capacity .
Solar Energy, 15, 287 (1974).
8. G. Blaesser et al., The solar test facility
LS-1 .
Internal re
port SE tp
07-78,
JRC Ispra.
9. H. Hettinger, K.P. Rau, Thermal Collector Test Reports:
TC
07/83,
TN 1.07.05.83.118; TC
05/84,
TN 1.07.Dl.84.39;
TC
07/84,
TN 1.07.Dl.84.81; TC 01/85, TN in preparation.
10.
H. Hettinger, comparison of collector performance measured in a
solar simulator . Report EUR 8347EN.
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PJ
3
> · *
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21
F i g . 2 - S o l a r c o l l e c t o r t e s t r i g i n t h e L S - 1 .
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draw [
m in
]
F i g . 3 - W a t e r o u t l e t t e m p e r a t u r e a s a f u n c t i o n o f t i m e .
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1 FINTERM
9
n- 0.804- 5.06T» - 34.8 Τ
2 GIORDANO .
η-
0.742- 3.66Τ» - 25.2 Τ*
IT.
2
SOLEFIL
η= 0.795- 5.33Τ* - 15
4 CHAFFOTEAUX MAURY
2
η- 0.764- β 71 T. - 29.7 T
5 SOLAR EDWARDS
9
η-
0.71Θ-5.87Τ» -22.1 T
1
2
3
4
5
T
0 0958
0 1138
0 113
0 0833
0 0912
^ »
INT.
0 0436
0 0484
0 0485
0 0346
0 0356
^
0 04
0 06
0 08
fr
0 1
SJ
OJ
\ :L¿i
Flg. 4 - Collector thermal efficiency curves.
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.80-
.70-·
.60--
.50-
.40-
.30-
.204
LU
.10+
LU O
- .10
System Nr.
X Epor
Edi
1 2
^ INSOLATION 16 8 M j /m
2
day
[ ] INSOLATION 10/
Mj /m day
8
10
F i g . 5 - Re la t iv e SDHW-sys tem pe r fo rm ance .
,·_.
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7 0 « ·
. 6 0 -
5 0 -
. 4 0 -
1.304
u i
2 0 +
o
■fe . 1 0 -
L Ü
O
1
S Y S T E M
N R .
-¿71
| | I N S O L A T I O N 16 8 M J
/ m d a y
Π INSOLATION IO,/» M j/m d a y
8
10
Fig. 6 - Thermal efficiency of SDHW-systems.
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