Spontaneous downward heat transport comparison tests of an improved system

Solar Energy Vol. 50, No. 1, pp. 27-34, 1993

0038-092X/93 $5.00 + .00 Copyright © 1992 Pergamon Press Ltd.

Printed in the U.S.A.

SPONTANEOUS D O W N W A R D HEAT TRANSPORT COMPARISON TESTS OF AN IMPROVED SYSTEM G. DE BENI* and R. FRIESEN Commission of European Communities--JointResearch Centre, Institute for Advanced Materials-Ispra Establishment, 21020-ISPRA(Varese), Italy Abstract--A previous spontaneous downward heat transport solar hot water system has been improved by the adoption of a double control float valve. This component, derived from an industrial product, enhances the performances of this system. Comparison tests have been made between two hot water solar systems, one conventional with a differential thermostat and circulation pump, and the other one fitted with a spontaneous downward heat transport system. The parameters varied during the tests were the setting of the differentialthermostat, the amount of daily water draw-off, and the threshold temperature of the water extracted. In most working conditions the spontaneous system had a higher efficiency.Advantages of this system are the independence from electrical energy availability, greater reliability, and the insensitivity to cold climates.

other systems have been proposed, based on vapor bubble formation [ 3 ], or on pressure cycles for circulating the heat transport fluid [ 4,5 ]. A precursor of the system described in this paper, also based on pressure cycles, was presented a few years ago in this journal[ 6 ]. That system was successfully applied to solar collectors in a circuit intended to do quantitative measurements of the system's efficiency[7 ], then it was applied in a practical test associated to l0 m 2 of solar collectors, transferring the heat against a height of about 8 m[ 8,9 ], and it was also applied to two other plants installed at mountain refuges (one of them at an altitude of 3650 m in the Italian Alias) for snow melting and hot water production [ l0 ]. The first proven advantage of a spontaneous downward heat transport system, in comparison with a forced circulation system, is its reliability, due also to the independence from other energy forms, to the high quality of the float valve (the 10 m 2 solar plant has been in operation for more than five years, i.e., since its construction, with 100% availability), and to the insensitivityto very low temperatures. The comparison between this and the conventional system, however, must be made not only on reliability but also on the overall efficiency. To that purpose, two equal plants, one with forced circulation and the other with the spontaneous heat transport, were built and operated. The first results have already been published [ 11]; they showed that the forced circulation plant produces slightly better results when the water store is relatively cold (<35 °C), but that the situation is reversed when the temperature of the water store increases. These resuits are in agreement with the considerations expressed in the previous article [ 6 ].

1. INTRODUCTION Conventional solar systems, with a circulation pump and a control device, do the job of transporting heat downward from the collectors to the heat store. Experience has shown, however, that they are subject to failures of various origin: mechanical (motor, pump), thermal (freezing, boil out in case of stagnation), and electrical (damage to electronic control from transient overvoltage). Thermosyphon units, that is, units relying in the density variation of the heat carrier fluid for the transport of heat, avoid all these inconveniences, and this explains why these units are now so popular. Due to their principle of heat transport, however, these units must have the heat store located above the collectors, which means large weights on the roofs of buildings, and increased heat losses. Moreover, the circulation of the heat carrier fluid usually requires a large AT between collectors and water store entraining a reduced collector efficiency. In recent years, a certain number of systems capable of transporting heat downward in a spontaneous way, that is, without the use of pumps for circulating a heat carrier fluid, have been proposed. Due to their mode of operation, these systems, like the thermosyphons, do not require the availability of electrical or mechanical energy; they do not have moving parts like motors or rotating shafts, and in principle they do not require assistance or maintenance and should be trouble-free. These systems can be applied to any heat source, but they are particularly interesting in connection with the solar thermal applications, when the collectors, to prevent shadowing from the surrounding structures, are installed on the roofs while the utilisation of the collected heat is at a lower level. Systems for spontaneous downward heat transport have already been reviewed in the past[ 1,2]. Since then,

2. THE EVOLUTIONOF THE SYSTEM The reasons for this behavior are in a parasitic increase of the temperature of the auxiliary reservoir (see Fig. 1 for a scheme of the system) in which the condensed working fluid is collected. This is due to the

* ISES member. 27

28

G. DE BENIand R. FRIESEN

(a)

3

SOLAR COLLECTOR

/_

C.V.

SEPARATO~

t HEAT EXCHANGER

(b)

__•

COLLECTOR

C.V.

C.V.

SEPARATOR HEAT EXOHAIt3ER

-x/VV~Fig. I. Scheme of the system with the single control float valve. (a) Heat transport; (b) liquid recharge.

periodic input of hot vapors during the transfer of the working fluid from the auxiliary reservoir to the separator. At higher temperatures the increased heat losses from the auxiliary reservoir to the environment negate this effect. A quantitative analysis of the influence of the amount of heat released into the auxiliary reservoir on the efficiency of the heat transport system has been made by Neeper [ 12 ] on an analytical model very similar to the scheme of Fig. I. His conclusions were that a definite amount of heat must be withdrawn from the auxiliary reservoir in order to reach the highest efficiency. One of the assumptions made for the theoretical analysis was that during the fluid recharging phase (float valve open), there was available an unlimited amount of vapor from the collectors. This condition is just what is avoided in the system that has been subsequently developed[13]. In Fig. 2 there is a sketch of the operation of this improved system. The difference from the previous scheme is that now the float valve controls not only the output of vapors from the separator to the auxiliary reservoir, but also the input from the solar collectors [ Figs. 3, 4]. The float valve action is such that the input/output nozzles are respectively open/closed during the period of heat transport (liquid level "high" in the separator), and closed/open during the very short period of liquid recharging phase (liquid level "low" in the separator). In this way, the equilibration of pressure between aux-

iliary reservoir and separator, necessary for the liquid recharge, is obtained with only the small amount of vapour available in the separator and from the residual liquid. Therefore, the temperature increase of the auxiliary reservoir is much reduced, approaching the working conditions foreseen by Neeper [ 12 ], but without the loss of heat. To prevent back-flow from the collectors to the separator during the liquid recharge phase, a checkvalve must be inserted in the liquid feed line (valve A in Fig. 2). The pressure increase in the collectors, however, will be kept small if the sizing of the components is such that the liquid recharge phase becomes short, approximately 20 to 25 s. Moreover, check-valve "A" has a calibrated small hole, which provides for pressure relief. A back-flow of liquid takes place during the liquid recharge phase, but of an amount negligible in comparison to the liquid coming down from the auxiliary reservoir. Besides the improvement of efficiency, the adoption of the valve with double control has favourable consequences on reliability and safety of the system. We said that the previous system has a very high reliability. However, it is possible that a leak may reduce the working fluid to such an extent that the float valve does not operate properly. In this situation a continuous energy input would increase the temperature of the collectors and reservoirs, and the pressure of the whole circuit, until the stagnation temperature

"

(b)

SOLAR

IATOR HEAT EXCHAN6ER

Fig. 2. Scheme of the system with the double control float valve. (a) Heat transport; (b) liquid recharge.

Spontaneous downward heat transport

29

Fig. 3. View of the double control float valve.

of the collectors is reached. In the case of the double control valve, some liquid is fed to the separator through the hole of the check-valve until the float valve can close, keeping the system in operation. Moreover, in the extreme case of some internal mechanical failure (a very unlikely event), the bleeding of liquid from the collectors would continue until they become dry. The collectors would reach the stagnation temperature, but the pressure would remain at a low level, a favourable situation as far as safety is concerned. 3. THE EXPERIMENTAL

SET-UP

The experimental set-up used for testing the performance of a solar circuit fitted with the double control float valve is essentially the same as was used earlier for testing the single control valve[11]. In Fig. 5 there is a scheme of that installation, conceived for making comparison tests. In brief, it is composed of two equal solar circuits, one with the spontaneous heat transport system, and

the other with circulation pump and control. Each circuit comprises 4 m E of solar collectors and a water reservoir of 200 dm 3 placed about 9 m below the collectors. The collectors, according to the manufacturer, are suitable either for liquid heat exchange media or for boiling fluids. Provision is made for measuring various temperatures, the solar energy, and the energy extracted from the two water stores. As the purpose of the experimental installation is more a comparison between the two circuits than the determination of absolute values, a sophisticated instrumentation has not been used. However, the energy extracted is measured by a magnetic flowmeter and two platinum resistance thermometers. The double control float valve is derived from an industrial product. In this modified version, the valve is suitable for circuits containing up to 10 to 12 m 2 of solar collectors. Being an industrial product, the valve has been fully tested by its manufacturer, and its functionality is assured for a very large number of cycles,

30

G. DE BENtand R. FRIESEN

Fig. 4. Close-up view of the mechanism.

corresponding to a life of more than 50 years for a plant with 10 m 2 of solar collectors. The auxiliary reservoir has a volume of 18 dm 3, and the circuit is charged with 20 dm 3 of working fluid. The installation is computer controlled. Readings are taken every 25 s; the mean values of the temperatures and the integrated values of the solar energy are recorded on disc and printed out every 30 rain. Water is extracted from the water stores at a rate of about 65 cm 3/s. Water draw-off is terminated when a required volume of water or a required amount of energy has been extracted, provided that the outlet water temperature is higher than a required threshold value. Also during water draw-off, readings are taken every 25 s; all flow and temperature readings and the integrated energy values are continuously confronted with the set of parameters chosen for the test: volume, energy, and temperature. These parameters are described in the following section.

4. EXPERIMENTALRESULTS For the comparison tests the two solar circuits are run exactly in the same way, and the comparison is made on the amount of energy extracted. A single run lasts a m i n i m u m of 2 weeks and can go up to 5 weeks. In this way, the effect of the changing weather conditions is taken into account. However, for the comparison of the results obtained with different sets of parameters, one should consider also the seasonal effect on solar irradiance and ambient temperature. The parameters varied during the tests are mostly inherent to the water extraction profile, and the setting of the differential thermostat: it was set at 3°C, 5°C or 9°C. The water extraction profiles were selected to simulate real situations. We have done 1, 2, or 3 water draw-offs during the day, at the given times, irrespective of the weather conditions. The threshold temperature of the water extracted was put at 25°C, 30°C, 35°C, and

Spontaneous downward heat transport

Table 2. Runs with water draw-offat 11 am (T > 25°C), at 1 pm ( T> 25°C), and at 6 pm (T> 14°C)

AUXILIARY RESERVOIR ANO FLOAT VAL~

/7 //il

Thermostat (AT)

DIFFERENTIAL IHE~'IAT CONTROL

1

31

4

FLONNETER

FLOWI4ETER

Fig. 5. Scheme of the installation for comparison tests.

40°C. For some runs a further draw-offwith a threshold temperature of 14°C was done at the end of the day, mainly for restoring constant conditions for the next day. The inlet water temperature is always around 10°C. Most of the runs had a threshold only on the outlet temperature; for few of them we put also a limit in the amount of water extracted: In this case it was limited at half store ( 100 dm 3) for each of the two daily drawoffs. The runs with three draw-offs were made for testing the conditions of maximum energy extraction. In practice, only some of all the possible combinations of these parameters were tested. Tables 1 to 4 show the run number, the number of days taken into account, and the respective period of the year for the tested combinations. The number of days of a single run may not correspond with the full length of a given period, because in the final computation days in which the solar energy integrated over 30 min never reached the value of 50 W h / m 2 were excluded.

3

5

9

Run number Days Period

38 33 10/13-11/22

36 20 8/30-9/18

37 26 9/18-10/13

Run number Days Period

48 23 9/3-9/25

50 19 10/18-11/6

49 13 10/1-10/17

For the runs referring to Table 3, the maximum daily amount of water extracted is 200 dm 3, that is, the volume of the water store. This maximum is not reached during the cloudy days if the threshold temperature of 35°C is not always attained. For all the other runs, the volume of water extracted every day is limited only by the water temperature, and it becomes much larger than the tank volume for the runs of Table 2, where there are three water drawoffs per day with a relatively low threshold temperature. Table 4 refers to runs with progressively increasing threshold temperature of the water extracted. Tables 5 to 8 report the results obtained for the runs of Tables 1 to 4. For each run the following mean daily values are reported: G = solar irradiance ( W h / m2), T m = diurnal ambient temperature. For computing the mean value only the periods of 30 min during which the integrated solar energy was higher than 50 W h / m 2 are considered, E = gross energy of the draw-off water (Mj), Min = minutes of operation of the circulation pump, WT = equivalent thermal energy for the circulation pump (i.e., three times the electrical energy, in Mj), E~ = energy net; for the spontaneous system equal to E (for the system with circulation pump: E. = E - WT, in Mj), V25 = volume of water drawn offwith T > 25°C (dm3), T25 = temperature of V25 (°C), V14 = volume of water drawn offwhen 25 > T > 14 (dm3), Tin4 = temperature of Vl4 (°C), V = total water volume (dm3), T = temperature of V (°C), R = ratio energy of the spontaneous system/ gross energy of the circulation system, R n = ratio energy of the spontaneous system/net energy of the circulation system, S stands for "spontaneous" system, and C stands for "circulation" system.

Table 1. Runs with water draw-off at 6 pm (T > 25°C), and at 7 pm (T > 14°C) Thermostat (AT)

Table 3. Runs with water draw-off at 12 (noon) and 6 pm (T> 35°C and V< 100dm 3)

3

5

9

Run number Days Period

27 20 1/26-2/14

35 20 8/11-8/30

32 15 5/22-6/5

Run number Days Period

33 30 6/15-7/14

/ / /

/ / /

Thermostat (AT)

Run number Days Period

3

5

9

39 20 11/22-12/14

40 21 12/14-1/10

41 15 1/12-1/30

32

G. DE BENI and R. FRIESEN Table 4. Runs with water draw-offat 1 P.M. and 6 P.M. Thermostat (AT) Threshold

3

30

35

40

5

9

28 17 2/14-3/7

/

31 25 4/28-5/22

Run number Days Period

43 13 3/22-4/4

/

42 27 2/23-3/22

Run number Days Period

30 32 3/23-4/28

29 14 3/7-3/23

34 25 7/18-8/11

Run number Days Period

Table 5. Mean daily results for the runs of Table 1 Run number

G

Tm

27

3741

8.0

33

5098

24.3

35

5307

25.8

32

4759

19.9

S C S C S C S C

E

Min

WT

En

V2~

T25

VI4

Z14

V

T

R

R,

18.4 20.7 27.9 30.4 32.9 32.2 25.1 24.7

-217 -277 -172 -112

-1.8 -2.2 -1.4 -0.9

18.4 18.9 27.9 28.2 32.9 30.9 25.1 23.8

123.8 144.7 168.0 190.8 186.0 193.3 165.2 181.4

40.7 38.4 47.9 46.5 51.3 48.8 44.1 40.3

33.6 52.6 52.8 56.2 59.6 64.5 50.3 56.8

17.9 17.7 17.4 17.4 17.1 17.5 17.7 17.6

157.3 197.3 220.8 246.9 245.6 257.8 215.6 238.2

35.8 32.9 40.6 39.9 43.0 41.0 37.9 34.9

0.88

0.97

0.91

0.98

1.01

1.06

1.01

1.05

system causes a loss o f efficiency, a n d the ratio o f the energy extracted goes in favour of the forced circulation system. T h e situation is completely reversed for the runs o f Table 7, where there was a limit o f half store in the water extraction (twice a day) a n d a t e m p e r a t u r e threshold o f 35°C. These tests were done during the winter season; the resulting large A T b e t w e e n the water store a n d the o u t d o o r t e m p e r a t u r e plays in favour o f the s p o n t a n e o u s system. Moreover, the more pron o u n c e d t h e r m a l stratification in the water store o f the s p o n t a n e o u s system is a n o t h e r advantage; all this is

T h e runs o f Table 5, with the extraction of all the energy at the end o f the day, show that the two systems have nearly the same efficiency; there is consistently a lower v o l u m e of water extracted from the s p o n t a n e o u s system, b u t with a higher m e a n temperature. In Fig. 6 a typical i n p u t / o u t p u t diagram for r u n 35 is shown. The runs o f Table 6 are those with the highest volu m e o f water extracted; for b o t h systems this v o l u m e is m u c h larger t h a n the capacity o f the water store. This is a n unusual situation a n d causes a relatively low water temperature. In these conditions the relative increase o f the collector t e m p e r a t u r e o f the s p o n t a n e o u s

FILE

35

45 4O

-i-

35

g 0. I.--

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15 10 .X .

5 0 0

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200o

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300o

4oao

~oo

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70oo

8c0o

IRRADIATION

Fig. 6. Input/output diagram: + = spontaneous, × = circulation pump.

33

Spontaneous downward heat transport Table 6. Mean daily results for the runs of Table 2

Rtln number

G

T,~

38

3296

13.1

48

4511

21.7

36

3773

20.2

50

2397

12.9

37

4438

19.1

49

3729

18.1

S C S C S C S C S C S C

E

Min

WT

En

V25

T25

Vi4

Ti4

V

T

17.2 22.7 30.7 37.7 24.6 28.8 14.1 15.5 28.3 31.3 25.8 26.4

-226 -316 -165 -90 -144 -115

-1.8 -2.6 -1.3 -0.7 -1.2 -0.9

17.2 20.9 30.7 35.2 24.6 27.4 14.1 14.8 28.3 30.1 25.8 25.5

177.3 248.0 269.0 386.6 222.0 299.5 136.3 141.8 275.5 362.0 231.6 268.8

32.7 30.1 36.2 31.9 36.4 31.9 32.9 30.7 35.1 30.3 35.0 30.0

46.5 86.5 39.9 67.1 75.0 101.8 39.8 96.2 73.2 96.2 41.5 109.7

17.4 18.4 17.6 18.0 16.6 18.2 17.0 18.0 17.0 19.0 17.9 17.7

223.8 334.5 308.9 453.7 297.0 401.3 176.1 237.9 348.7 458.2 273.2 378.5

R

2 9 . 5 0.75 27.1 33.8 0 . 8 1 29.8 31.4 0.85 28.4 29.3 0 . 9 1 25.6 3 1 . 3 0.90 27.9 32.4 0.97 26.5

R~ 0.82 0.87 0.89 0.95 0.94 1.01

Table 7. Mean daily results for the runs of Table 3 Run number

G

Tm

39

2811

4.5

40

2771

3.9

41

3168

5.9

S C S C S C

E

Min

WT

En

V

T

R

Rn

8.2 7.1 8.0 4.7 12.7 8.0

-112 -58 -52

-0.9 -0.5 -0.4

8.2 6.2 8.0 4.2 12.7 7.6

73,3 64,9 69,8 39.6 99.7 61.3

37.8 37.2 37.4 37.9 39.2 39.8

1.14

1.31

1.70

1.89

1.58

1.67

reflected in a ratio m u c h in favour o f the spontaneous system. The runs o f Table 8 are characterized by a progressively increasing threshold temperature o f the water extracted, with two draw-offs during the day. There is a clear tendency to a better behavior o f the spontaneous system with the increase o f the draw-off temperature. 5. CONCLUSIONS Although the energy actually extracted from the forced circulation system is the "gross energy," one cannot forget that a certain a m o u n t o f electrical energy has to be spent for harvesting the solar energy, and has to be taken into account. Different j u d g m e n t s can be

made, but we believe that the ratio Rn is the suitable figure o f merit for the comparison o f the thermal performance o f the two systems. O n this basis one can conclude that the spontaneous d o w n w a r d heat transport system fitted with the double control float valve has slightly less efficient performances only until the water is utilized at low temperatures, and that it becomes more efficient than the forced circulation system when the water temperature goes over 30 to 33°C. The results would be more in favour o f the spontaneous system if the vertical distance were smaller than the 9 m o f this study, or if the absolute pressure o f the working fluid would be higher or its density lower.

Table 8. Mean daily results for the runs of Table 4 Run number

G

Tm

28

4410

10.2

43

3682

14.7

30

4004

13.7

29

4713

14.0

31

5347

19,1

42

4728

14.1

34

5205

25.2

S C S C S C S C S C S C S C

E

Min

WT

En

V

T

R

R,

22.1 24.5 17.7 18.3 18.6 17.5 23.8 21.6 31.7 31.1 24.9 19.7 34.1 30.3

-260 -180 -182 -143 -139 -95 -130

-2.1 -1.5 -1.5 -1.2 -1.1 -0.8 -1.0

22.1 22.4 17.7 16.8 18.6 16.0 23.8 20.4 31.7 30.0 24.9 18.9 34.1 29.3

175.2 200.9 130.4 135.6 119.7 108.4 151.5 139.5 225.4 254.8 185.4 145.0 217.2 201.0

38.0 37.0 41.8 41.3 46.9 48.3 46.0 45.4 44.3 39.9 41.3 41.2 48.4 46.8

0.90

0.98

0.96

1.05

1.06

1.16

1.10

1.16

1.01

1.05

1.26

1.31

1.12

1.16

34

G. DE BENI and R. FRIESEN

The working fluid used for these tests was the R114; RI 1 is also suitable. These fluids have been chosen because their vapor pressure at the stagnation temperature o f the collectors is lower than the pressure that the collectors can withstand. Environmental constraints will exclude the future use of these refrigerants. There is now an effort underway for finding and producing alternate refrigerant fluids. The only requirement for a fluid to be suitable for this application is the vapor pressure. The higher the vapor pressure the higher the performances, the practical limit being dictated by the mechanical characteristics of the solar collectors. An advantage for finding an alternate refrigerant for this application, when compared with refrigeration industry, is that there are no electrical motors or rotating parts, so there are no constraints as far as compatibility with lubricating oils or insulation lacquers of motor winding. Some of the fluids actually on the market like R123, R142b, and R124 could be conveniently utilized. Moreover, if a solar circuit is completely outdoor, requirements regarding toxicity and flammability become a lesser concern. The main advantages of this system are the extremely high reliability, the independence from the availability of electrical energy, and the insensitivity to very cold climates.

Acknowledgments--Thanks are due to J. Hofman, J. R. C. Ispra for the assistance in electronics and instrumentation.

REFERENCES

1. G. P. Watchell, Self-controlling, self-pumping heat circulation system study, Franklin Research Center report C4772-2, final report for D.o E. contract EG-77-C-024488 (July 1978).

2. C.C. Roberts, Jr., A review of the heat pipe liquid delivery concepts, D. A. Reay (ed.), Proceedingsof the Fourth Int. Heat Pipe Conference, Advances in Heat Pipe Technology, September 7-10, 1981, Pergamon Press, Oxford, 693697 (1981). 3. W. Soerensen, The bubble pumped solar domestic hot water heating system, Proceedings of the ISES-Solar World Congress, 1987, September 13-18, 1987, Hamburg, Germany, Book of abstracts 2.9.20 (1987). 4. D. A. Neeper and J. C. Hedstrom, A self-pumping vapor system for hybrid space heating, Proceedingsof the ISESIntersol '85, June 23-29, 1985, Montreal, Quebec, Canada (1985). 5. J. H. Davidson, H. A. Walker, and G. O. G, L/Sf, Experimental study of a self-pumping boiling collector solar hot water system, Journal of Solar Energy Engineering 111, 211-218 (1989). 6. G. De Beni and R. Friesen, Passive downward heat transport: Experimental results of a technical unit, SolarEnergy 34, 127 (1985). 7. G. De Beni and R. Friesen, Experimental results of a solar hot water system with spontaneous, downward, nonfreezing heat transport system, Energy Conversionand Management 27, 293 (1987). 8. G. De Beni and R. Friesen, Impianto solare per la produzione di acqua calda di processo con trasporto spontaneo del calore verso il basso: Fase l--Progetto e costruzione, Energie Alternative H. T.E. 47, 205 ( 1987 ). 9. G. De Beni and R. Friesen, Impianto solare per la produzione di acqua calda di processo con trasporto spontaneo del calore verso il basso, Fase II--Risultati sperimentali, Energie Alternative H.T.E. 59, 201 ( 1989 ). 10. G. De Beni and R. Friesen, Impianto solare termico innovativo per i rifugi montani, EnergieAlternative H. T.E. 67, 331 (1990). 11. G. De Beni and R. Friesen, Spontaneous downward heat transport system: Quantitative determination of the overall et~ciency in comparison to a conventional circuit with circulation pump, Proceedings of the ISES Solar World Congress, 1987 September 13-18, 1987, Hamburg, Germany, 899-904 (1987). 12. D. A. Neeper, Analytical model of a passive vapor transport heating system, Solar Energy 41, 91 (1988). 13. G. De Beni and R. Friesen, Passive heat transfer device, U.S. Patent 4754906 (May 24, 1988).