Thermosyphon solar water heaters: effect of storage tank volume and configuration on efficiency

Pergamon PII:

Energy Coneers. Mgmr Vol. 38. No. 9. pp. 847-854, 1997 0 1997 Elsevier Science Ltd. All rinhts reserved Printed in &eat Britain 0196-8904/97 $17.00 + 0.00 s01%-8904(%)ooo994

THERMOSYPHON SOLAR WATER HEATERS: EFFECT OF STORAGE TANK VOLUME AND CONFIGURATION ON EFFICIENCY AFIF HASAN Mechanical Engineering Department,

Birzeit University, P.O. Box 14, Birzeit, West Bank, Via Israel (Received 25 November 1995)

Abstract-A thermosyphon solar water heater SWH is simulated using the TRNSYS program. The effect of the hot water storage tank volume and configuration on efficiency is investigated in this paper. The efficiency of the system can be improved by employing larger storage tanks. Horizontal and vertical storage tanks give similar efficiencies and useful solar heat. The performance of the SWH is better described by the supplied heat to the load rather than the gained useful solar energy. 0 1997 Elsevier Science Ltd Flat-plate collector

Solar water heater

Thermosyphon

TRNSYS simulation

NOMENCLATURE A, = Area of collector (ml)

a = Constant in efficiency equation (I) b = Slope in efficiency equation (I), (kJ/h’m*‘“C) DI, = Diameter of collector’s header (mm) D, = Diameter of collector’s risers (mm) f. = Solar fraction H, = Vertical distance between inlet and outlet of collector (m) HO = Vertical distance between bottom of tank and bottom of collector (m) H, = Height of collector return above bottom of tank (m) H, = Length of storage tank (m) I, = Solar insolation on tilted collector (kJ/m*.day) 15,= Length of collector’s header (m) Q1-d = Energy load of hot water (MJ) Q,ypp= Energy supplied to load (MJ) Q. = Useful solar energy (MJ) T, = Ambient air temperature (“C) T, = Collector’s average temperature (“C) CJA = Heat loss coefficient from tank (kJ/“C) j? = Collector’s tilt angle Q, = Latitude angle tt = Collector’s efficiency

INTRODUCTION

Water heating by solar energy for domestic use is one of the most successful and feasible applications of solar energy. Thermosyphon solar heaters can be used in remote regions even if there is no electricity supply. The efficiency of the solar collector can be used as an indication for the performance of the solar water heater (SWH) system. Flat-plate collectors are usually used in the SWH system. The efficiency of the SWH is a function of system structure and material. It is also affected by the volume of the hot water storage tank [l]. The performance of the system can be described through mathematical modeling of the system components. System simulation can be performed for various system design and operation parameters. The TRNSYS computer simulation program is one of the most widely used simulation programs for solar thermal systems. 847

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HASAN: THERMOSYPHON

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The performance of SWH systems depends on system design parameters as well as on weather conditions, such as solar insolation and ambient temperature. It is also a function of operation conditions, such as hot water withdrawal rates. In this paper, the performance of a SWH thermosyphon system is examined at a fixed load of 200 liters per day for various hot water storage tank volumes and configurations. In addition, the effect of the collector area on the performance will be examined as well. The TRNSYS program will be used for the simulation. SYSTEM

DESCRIPTION

Thermosyphon type solar water heaters are used in Palestine. A typical SWH installed in Palestine consists of two or three flat-plate collectors, each of 1.44 m2, and a hot water storage tank of 120-200 liters. The flat-plate collectors are connected in parallel, with a total area of 4.32 m2, and mounted facing due south with an inclination angle of 45”. The storage tank is mounted on a rig about 30 cm above the collector’s top header. The return from the collectors to the tank is placed at a point about two-thirds the height of the tank, while the cold water feed is placed at the bottom of the storage tank. Hot water is withdrawn from the upper part of the storage tank [2]. An auxiliary electric heater is optional and usually is added at the bottom of the storage tank. Vertical hot water storage tanks are used, while horizontal ones are sometimes used in inclined roofs. SYSTEM

SIMULATION

The TRNSYS program is used for simulation of the SWH system. In the TRNSYS program, mathematical models of system components called units are connected together to form the system 131. The TRNSYS deck was formed from the weather data reader, solar insolation processor, hot water consumption function and thermosyphon collector-storage subsystem units [4]. A weather process unit from the TRNSYS program was used to generate hourly weather data from the daily monthly mean values. Solar insolation for Bet-dagan, Israel [5] and ambient temperature from the Rammallh meteorological station were used for the simulation. The parameters characterizing the collectors, storage tank and connecting pipes are shown in Table 1. Such parameters were used for simulation of the SWH system. A typical hot water consumption pattern for Palestine was assumed in the simulation [4]. The useful solar energy gained by the collectors, energy supplied to the load and energy required to heat water from ambient to 55°C are all computed by the program at hourly intervals. The collector efficiency can be expressed in terms of solar insolation, collector and ambient temperature difference and collector design parameters, as given in equation (1) below. q = a - b(Tc - T,)/Zt. Table 1. Characteristic

parameters Palestine

Parameter A. (m*) Dh (mm) D, (mm)

Lh (m) Hc (m) I% (m) I% (m) I% (m) (uA)t V, (liter) a equation (1) b equation (1) (kJ/h’m*‘“C) cp B

of SWH system in Value 2.88, 4.32 25 12.7 4 1.27 1.0 1.57 1.2 3.4 5o-400 0.70 15.0 32” 45”

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SOLAR WATER HEATERS

0.6

0.5

6 .z {

0.4 d-V=

0.3

50 liters

+V=

150 liters

*V=

200 liters

+V=

300 liters

-+-V=

400 liters

0.2 0

2

4

6

8

10

12

month Fig. 1. Effect of storage tank volume on efficiency.

The efficiency, by definition, can be computed from the useful heat and solar insolation as given below: rl = Q”I‘G I,. (2) The solar fraction is defined as the solar energy supplied to the load from solar energy divided by the load, as shown in equation (3). fs = Qsupp/Q~w,.

(3)

The solar fractionf, is computed for each month and then averaged annually. Iffs is greater than one, it is set equal to one, assuming the excess energy is not stored from one month to the next.

RESULTS

AND

DISCUSSIONS

The monthly efficiency at different storage tank volumes is given in Fig. 1. It can be seen from this figure that the efficiency increases as the volume of the storage tank is increased. This is explained in view of equation (1) and the fact that the system temperature, collectors and tank, decreases as the tank volume is increased [l], and lowering the system temperature will decrease the heat lost to the surrounding, resulting in higher efficiency. Similar conclusions can be drawn from Fig. 2 where the annual efficiency is plotted versus the storage tank collector’s volume to area ratio. A large increase in efficiency, from 0.38 to 0.45, is obtained as v//‘lA,increases from 10 to 25, then the efficiency levels at 0.46. Vertical hot water storage tanks are more common in Palestine than horizontal ones. From the efficiency point of view, there is no difference between the two configurations, as seen in Fig. 3. Similarly, very little difference can be detected between the useful solar heat for the vertical and the horizontal tanks, as shown in Fig. 4. Figure 5 shows the daily behavior on April 1 of the horizontal and vertical tank systems, along with the required hot water heating load. The heat supplied for a given hot water consumption pattern during the 24-hour period shows that the vertical tank is slightly better than the horizontal one. The higher energy content from the vertical tank is due to the fact that hot water is being withdrawn from the top of the vertical tank which is at a higher temperature due to stratification in the vertical tank. Figure 5 also shows that the energy in the water withdrawn before noon is below the required load, as its temperature is less than the required 55°C.

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SOLAR WATER HEATERS

0.6

0.5

0.4

0.3

0.2 0

20

40

60

60

100

Fig. 2. Annual efficiency versus storage tank volume/collector area.

Typical solar water heater systems in Palestine consist of three flat-plate collectors with a total area of 4.32 m*, while a few systems contain two collectors with a total area of 2.88 m*. As shown in Fig. 6, the three-collector system covers the heating load for nine months, while the two-collector system covers the total load for six months only. Hence, more auxiliary energy is needed when using

0.6 c

0.5 -

6 8 ._ t

o.4 E 0.3

--c vertical tank ??

-

horizontal

0.2 r 0

2

4

6

6

month Fig. 3. Effect of storage tank configuration on efficiency.

10

12

HASAN:

THERMOSYPHON

SOLAR

WATER

HEATERS

851

1400

1400

tank’s configuration -

I

80012

vertical .._.._.... ho,izcntal

1

I

I

I

I

I

I

I

I

3

4

5

6

7

8

9

10

11

-800 12

MONTH Fig. 4. Useful heat for horizontal

and vertical tank systems.

the two-collector system. The daily supplied heat from the two-collector system for April 1 is below the required load throughout the day, while the three-collector system falls short of the load before noon only, as seen in Fig. 7. However, when comparing the monthly efficiencies for both systems, the small system gives a better efficiency as shown in Fig. 8. The higher efficiency in the smaller system might result from

6

6 Tank’s configuration

5-

-5

April 1 -------Q sup. vet-t.tank

5

-....-".. Q sup. horiz. tank -

P" 3 =

-4

Qload

3-

-3

G

-

E G

2--

I

l-

0

-2

-1

I

2

I

4

I,

6

8

10

12

14

16

18

20

22

HOUR Fig. 5. Daily performance

of vertical and horizontal

tank systems.

24’

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SOLAR WATER HEATERS

1400

1400

area

collector 1300-

/' ,/

,oo J

600

-

'.'......................,,,.,,.,,..,,,,,.... '\.

Area4.32

............... load -....-.... Area 2.88

I 12

.y 700

I

I

I

I

I

I

I

I

I

3

4

5

6

7

8

9

10

11

.600

12

MONTH Fig. 6. Monthly mean useful heat for SWH, area 4.32 and 2.88 m*.

the lower water temperature in the system, and hence, lower heat losses and higher efficiencies result, as seen in equation (1). Table 2 summarizes the annual performance of the typical three-collector system and for two-collector system. Annual efficiencies of 0.5 for the smaller system and 0.45 for the larger are obtained. The energy supplied in the consumed hot water is above the required load for 6

-6

Effect of collector’s area 5-

April 1

!S

-5

-------. Q sup. area 4.32 Q load -....-.... Q sup. area 2.88

P4 5

-

=

3-

&

-

E

2-

E

l-

o-

’

2

I 4

6

8

10

12

14

16

18

20

HOUR Fig. 7. Daily performance of SWH, area 4.32 and 2.88 m2.

22

2:

will the one the

HASAN: THERMOSYPHON

SOLAR WATER HEATERS

853

month Fig. 8. Monthly mean efficiency for SWH, area 4.32 and 2.88 m2.

typical system and less than the load for the smaller one, with a solar fraction of 0.86 for the smaller two-collector system. CONCLUSIONS

Based on the TRNSYS simulation of the thermosyphon following conclusions are reached:

solar water heater in Palestine the

1. Efficiency of a SWH system can be increased by using a larger hot water storage tank or smaller collectors area. 2. There is no difference between the performances of vertical and horizontal storage tank systems. 3. The small SWH system, consisting of two collectors, is not sufficient to meet the domestic hot water demand. 4. A good performance parameter for a SWH is neither the efficiency nor the useful heat gained, but the heat supplied to the load in the consumed hot water.

Table 2. Values of annual performance parameters for SWH systems in Palestine Parameter Annual Annual Annual Annual

efficiency useful heat (MJ) supplied heat (MJ) solar fraction

Two collectors 0.508 10,320 9608 0.86

Three collectors (vertical) 0.454 13,820 12,800 >I

Three collectors (horizontal) 0.450 13,720 12,620 >I

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REFERENCES 1. Shariah, A. M. and Ecevit, A., Energy Convers. Mgmt, 1995, 36, 289. 2. Hasan, A., Domestic water solar heating in West Bank. Proc. World Renewable Energy Congress. Reading, p. 1798, 1994. 3. Klein, S. A., et al. TRNSYS 13.1 User’s manual, Report 38-13. Solar Energy Lab., University of Wisconsin, 1990. 4. Hasan, A., Inr. J. Solar Energy, (in press), 1995. 5. Manes, A. and Lanetz, A., Solar irradiance on non-horizontal surface and optimum exposure of solar collectors. Ministry of Transport, Israel Meteorological Service Bet-dagan, 1980.