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






n r g y

THE THERMAL PERFORMANCE

OF SDHW - SYSTEMS MEASURED

IN A SOLAR SIMULATOR

H. HETTING ER K.P. RAU


JOINT RESEARCH CENTRE

Ispra Establishment

1 21 2

Ispra Va)

Directorate-General Science, Research and Development

Joint Research Centre

1985 EUR 10181 EN



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



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.





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





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



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/.



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.



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



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 ).



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:



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



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.



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£>



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



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



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-



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.



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



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.



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)



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.



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.



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.



PJ

3

> · *



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 .



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 .



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.



.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 .

,·_.



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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The Thermal Performance of Sdhw - Systems Measured in a Solar Simulator

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