Experiment and analysis of practical-scale solar pond stabilized with salt gradient

Solar Energy Vol. 46, No. 6, pp. 353-359, 1991

0038-092X/91 $3.00 + .00 Copyright © 1991 Pergamon Press plc

Printed in the U.S.A.

EXPERIMENT A N D ANALYSIS OF PRACTICAL-SCALE SOLAR POND STABILIZED WITH SALT GRADIENT KIMIO KANAYAMA,HIDEO INABA,* HIROMU BABA, and TAKEYUKIFUKUDA Department of Mechanical Engineering,Kitami Institute of Technology, Koen-Cho Kitami, Hokkaido, 090 Japan Abstract--A large-scale solar pond with salty water was constructed in the suburbs of Kitami in 1985. Its performance has been measured and analyzed by the authors after that. The solar pond body is circular of 44 m diameter, and the pond water is of 3 rain total depth. After, 15 months, the depth of the salt gradient zone (S.G.Z.) was thinned by l0 cm in the top and by 20 cm in the bottom due to convection of the top and bottom zones. The temperature in the convective storage zone (C.S.Z.) reached 70°C, its maximum, at the beginning of September in 1985, however, it was not as high in 1986 due to contamination of the pond water. The temperature of the storage zone was reduced from November to April due to ice covering on the pond surface. The collected heat yielded largelyand the collection efficiencyreached more than 30% in summer, but decreased to negative values in winter. The thermal performance of the solar pond was predicted by a simulation calculation, and the calculated result compared well with the measurements. !. INTRODUCI'ION As a simple method utilizing the large amount of solar energy for a long period, the salt gradient solar pond has been studied by many researchers in various countries of the world. However, the most of those researches and developments on the solar pond has been conducted in U.S.A.. Through the experiments indoors and outdoors, the analysis and simulation of the performance, and designing of the construction, the subject of the solar pond is advancing on the practical stage [ 1,2 ]. On the other hand, in Japan, a history of the study for the solar pond is not so long, and there are a only few researches. For instance, M. Taga [ 3 ], Kinki University, experimented on a small scale solar pond with a salt gradient and a polymer solar pond. K. Kinose and K. Sakurai[4-6], National Research Institute of Agricultural Engineering, carried out a study on the stability of stratified salty water and the analysis of a convective storage zone by using a trapezoidal solar pond with 7 m × 7 m surface and 3 m depth. N. Isshiki[7] experimented on a special solar pond with fresh water by floating a lot of vacuumed glass-tubes on the surface. The study for the solar pond at Kitami Institute of Technology[8-10] was started in cooperation with several companies in 1981. After performing a reference survey, inspecting several solar ponds oversea, and experimenting on a model solar pond indoors and outdoors, a practical-scale solar pond with salt gradient was constructed in Abashiri (44°01'N,144°17'E), a suburb of Kitami, as a national project in 1984. The measurement of the performance, and the maintenance and control for the solar pond were entrusted to our laboratory substantially after that, and an experimental Work sponsored by Japan Small Business Corporation, MITI. * Now Professor at Okayama University, 3-Chome, Tsushima-Naka, Okayama, 700 Japan.

and analytical study for the practical-scale solar pond was carried out by ourselves. As the result of these investigations, it was clarified that the major factors which affect the solar pond performance were how to hold the transmittance in high quality and the salt gradient of pond water in good condition. In this report, the performance of the solar pond such as temperature of the storage zone, the collected, stored and extracted heats, and the collection efficiency, are obtained by processing the measured results. Using the measurements of the spectral transmittance of the salty water and the solar radiations, the performance of the solar pond is predicted from a view point of the transmitted and absorbed energy in the pond water. 2. OUTLINE OF THE SYSTEM To extract heat from the storage zone in which solar energy is collected and stored as warmed salty water, the heat is converted into warmed fresh water through an external heat-exchanger, stored in a tank temporarily, and supplied to the user as the occasion demands. Composition of the system is as follows: proper body of the solar pond, heat extraction equipment, water control equipment to purify the pond water and to maintain the salt gradient, heat supply equipment combined with a heat pump, and measurement equipment needed for the experiment. The heat pump supplies hot water for heat demand of a process heat in a few factories near there, after increasing temperature and thermal energy. However, no piping work for the heat supply was completed. Figure 1 shows a plan of the solar pond body, and Fig. 2 shows its cross section. The solar pond body is circular with diameter of 44 m and depth of 3.5 m, created with a concrete wall for the periphery and with asphalt on the bottom. The inside of the wall and the bottom is insulated with 80-mm-thick urethane foam and is waterproofed with a synthetic rubber sheet. Depth of the pond water is 3.0 m in total, of which

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the bottom, of 1.5 m depth, is the convective storage zone (C.S.Z.) with 20% salt concentration; the middle, of 1.3 m depth, is the salt gradient zone (S.G.Z.) of 0% to 20%; and the top, of 0.2 m depth, is the upper convective zone (U.C.Z.) of fresh water. Filling salty water into the pond vessel was started on the beginning of June, 1985, and the stratification was completed on June 24th. The salt gradient zone was stratified by using Zangrando method [ 11 ]. Kingsalt [ 12 ], commercial name of a kind of sodium-chloride, was used for the salt of the pond water. Fresh water was put on the U.C.Z. by a pump to flow out floatings on the surface and to decrease salinity of the top of pond water. Items of the measurement for the solar pond performance are the temperature and concentration in the water layer, soil temperature of the surroundings, ambient temperature, solar radiation and wind velocity, and so on. Mark × in Fig. 1 indicates the measuring point for temperature and concentration in each water layer and also the sampling point of the salty water of a specimen for the spectral transmittance.

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Thermopile thermometer sensors were permanently fixed at five points with defined depth of the pond water, and the temperature of each layer of the pond water were automatically recorded for a long period. In addition, a thermocouple thermometer and an electromagnetic concentration meter were lowered into the pond water on a specified day, the temperature and concentration of the salty water were measured in each point from the bottom, and distributions of the temperature and concentration were obtained as profiles as shown in Fig. 3. Figure 3 (a) represents the profiles of the temperature and concentration just after completing the stratification of the S.G.Z. and the temperature of the C.S.Z. had already reached about 28°C, due to temperature rising also during the stratifying period. It can be seen from the concentration profile that all the convective storage zone of 1.5 m, the S.G.Z. of 1.3 m, and the U.C.Z. of 0.2 m are clearly made up. Figure 3(b) represents the measured result when the temperature of the C.S.Z. attained the maximum of 70°C. From depth of the bending points at both profiles, it can be seen that the thickness of the S.G.Z. decreased at the under and upper parts due to erosion by convecting force of the C.S.Z. and the U.C.Z., respectively. In this figure, the most stabilized distribution of the temperature and concentration are recognized from both the profile curves within the gradient zone. In the upper layer and the lower layer of the pond water, however, the profile of the concentration curve is inversely increasing with height. The former may be caused by an instantaneous turbulence in the layer in relation to fresh water feed and wind sweep on the water surface, and the latter may be caused by dragging lower salinity water of the gradient zone into the bottom by a convective motion due to increasing temperature of the storage zone. In the next year, as shown in Fig. 3(c), the decreasing trend of thickness of the gradient zone can been seen, and the temperature of the storage zone did not rise as high as before due to contamination of the pond water. Temperature drop near the bottom seems to be caused by cooled salty water returned from the heat extraction process.

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The erosion of the upper part of the S.G.Z. might to be mainly due to a carrying motion of the fresh water, which was sometimes poured in the surface layer to keep its concentration to be as low as possible. Since the restoration of the upper layer of the gradient zone would be difficult, no make-up water was added to counter the slow diffusion of salt upwards. Figure 4 shows the monthly average temperatures in the convective storage zone, the upper convective zone, and the surroundings during one half year (July 1985 to December 1986) after stratification of the salty water. Figure 5 shows the monthly values of the solar radiation, the collected heat, the stored heat normalized with mean ambient temperature, extracted heat, and the collection efficiency. The collected and stored energy in the storage zone is extracted from the upper layer and returned to the lower layer of the storage zone as hot salty water to warm fresh water through a heat-exchanger outside. As comparison of Fig. 4 with Fig. 5, temperature of C.S.Z. reached its maximum of 70°C at the begin-

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ning of September of 1985, and heat of the C.S.Z. was extracted after that, because convective motion which causes erosion of the under part of the S.G.Z. was forced. The rate of temperature rising of the storage zone and the attainable maximum temperature are higher at the initial period from July to September in the this year than those of the next year, however, after that those values go down quickly because of heat extraction and contamination of the pond water. No solar radiation penetrated into the pond water due to the ice cover from November in this year to April in the next year, and the temperature of the C.S.Z. was lower than during the summer. The rate of temperature rising and the attainable maximum temperature of the storage zone in 1986 was not high as much during the preceding year because of contamination of the pond water and the heat extraction, and variation of the temperature of the storage zone became flat from June to October in 1986. The stored energy based on temperature difference At' was always largely positive, and even during winter it kept one third of the value in

VARIATION OF MONTHLY AVERAGE TEMPERATURE OF C. S. Z. AND U. C. Z.

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summer. Here, the amount of the extracted heat in each month during July to September, 1986, corresponds to l0 to 13 per cent of solar radiation in the preceding month. During 18 months from June of 1985 to November of 1986, total energy of the solar radiation incident upon the water surface was 8240 G J, and that of the extracted heat was 495 GJ, so that the rate of heat extraction to the incident radiation was only 6% in total average. Collection efficiency is positive in spring and summer, and sometimes exceeds 30 per cent in a monthly average. In autumn, however, it becomes negative because the solar radiation decreases, and it retains a negative value during the freezing period in the winter. Both heat losses from the wall and the bottom of the solar pond are few due to the thick insulation layer; for instance, an average value of the latter is 11.1% of the total heat stored and extracted for a period of all the experiments.

4. E S T I M A T I O N O F T H E S O L A R P O N D P E R F O R M A N C E

4.1 Equation o f heat balance A solar pond model is shown in Fig. 6. The performance factors can be calculated from the heat balance equation under the following assumptions: 1. Upper convective zone is independently treated because there are reflection of the incident radiation, heat transfer and evaporation into the atmosphere at the surface.

2. Salt gradient zone, which is controlled by heat conduction, is divided into N layers ( = 15 ) because the absorption of solar radiation and the temperature of the salty water are changeable with depth of the pond water. 3. Temperature and concentration of the convective storage zone are uniformly constant. 4. Heat flow from the wall is dependent on heat conduction in the surrounding soil and heat flow from the bottom is also dependent on heat conduction transferred into the unchangeable underground. Heat balance on the U.C.Z. (No. 1 layer): dTl CiMl"-~ = Qj.AI .S- QE.S-

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(4) To carry out the calculation from eqn ( 1 ) to eqn (3), the hourly values of the horizontal-total solar radiation [ 13 ] and the spectral transmittance [ 12 ] of the salty water of which both were measured at our laboratory were used. Absorption rate A of incident radiation in each layer within the S.G.Z., was obtained by integrating the product of the spectral transmittance of salty water by the spectral solar radiation separated into the direct-scattered components applying Bird's model[14]. Contamination of the pond water is incorporated into the model by reducing the rate of the transmittance, assuming the transmittance decreases uniformly in each of fifteen layers so that its total effect can be given by the product of the exponential terms of them. The rate of heat extraction is given by hourly value of which the total heat is distributed over the concerned month. As a procedure of the calculation, the initial conditions such as surface area, shape and depth of the solar pond, existent of heat extraction, and period of

the calculation are settled first of all. Next, after the weather conditions such as outside temperature, amount of evaporation, wind velocity and the circumferential conditions are given, and then the data of solar radiation are put into the computation. Continuously, heat balance between the water layers each other or between the water layer and its surroundings or atmosphere are calculated every four hours. Thus, the incident solar radiation, collected heat, stored heat, collection efficiency, heat loss, and temperatures in each part are calculated every five days as a unit. 4.2 Calculated results Real conditions under the experiments and physical properties of the substances were referred to the calculation of the solar pond performance. That is, progressing degree of contamination in the pond water was given by the decreasing rate of the transmittance based on the measurement[15 ], and an initial temperature of the pond water was set in average value on the site. The statistic data of precipitation observed in Sapporo [16] was used for evaporation rate of fresh water from the pond surface in Abashiri, where the

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K. KANAYAMAel al.

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solar pond is located, because we have no observation of the precipitation there. Temperature of the circumferential soil at three points on the vertical line of 0.5m distance from the pond wall was measured as a reference. However, the monthly temperature change represented by a sine curve evaluated from the average ambient temperature TA and fixed ground temperature Tc was used for the calculation. With respect to thermal conductivity, the measured value [ 17 ] was used for salty water of the solar pond, and the empirical value was used for soil of the underground. As a result of the calculation, the temperature variation of the C.S.Z. is shown in Fig. 8, comparing the calculated with the measured. The calculated value agrees well with the measured value for an initial period in the first year. However, after the temperature of the storage zone attained its maximum, the former becomes higher than the latter because of neglecting the decrease of thickness of the S.G.Z. in the calculation. If the effect of decreasing thickness would be taken into account, the program and procedure of this calculation might be considered to be satisfactory to conduct the following simulation calculation under the various conditions. Assuming that the reduction of the transmittance of the salty water varies from 0% to 80%, and heat is extracted by 10% of the amount of solar radiation in the preceding month when temperature of the C.S.Z. exceeded 50°C, the variation of temperature in the storage zone is predicted as shown in Fig. 9. We can see obviously the influence of the transmittance reduction to the temperature rise of the storage zone.

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Acknowledgments--The authors express their hearty appreciations to Japan Small Business Corporation, MITI, to allow the publication of the results of this "Solar Pond" national project, to the late Prof. I. Oshida, Dr. T. Noguchi and Prof. S. Tanaka who were the committee members to promote this program, and also to Nippon Kokan Inc., Taisei Kensetsu Inc., and Ebara Seisakusho Inc. who achieved this work in cooperation with us. NOMENCLATURE C M G T

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Based on a lot of data measured for a practicalscale solar pond, the experimental results were successfully processed and analyzed to clarify the working performance of the solar pond, and to predict the performance of any solar pond by the simulation program under various conditions. The operation and the experiments of the solar pond have been closed at the end of 1986, because the maintenance costs very much to repair the thickness decreasing of the salt gradient

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A practical-scale solar pond stabilized with salt gradient

359

REFERENCES

1. H. Tabor, Solar ponds, Solar Energy 27, 181 (1981). 2. J. R. Hull, C. E. Nielsen, and P. Golding, Salinity-gradient solar ponds, CRC Press, Boca Raton, FL (1989). 3. M. Taga, On the solar pond, Journal of JSES 5(3), 50 (1979) (in Japanese). 4. K. Kinose and K. Sakurai, Bull. of National Research Inst. of Agricultural Engng., No. 19 (1980) (in Japanese). 5. K. Kinose and K. Sakurai, Bull. of National Research Inst. of Agricultural Engng., No. 21, (1981), (in Japanese). 6. K. Kinose and K. Sakurai, Tech. Report of National Research Inst. of Agricultural Engng., B, No. 49 ( 1981 ) (in Japanese). 7. N. lsshiki, Journal ofJSME, Vol. 84, No. 757, 55 ( 1981 ) (in Japanese). 8. K. Kanayama and H. Baba, Proc. 25th National Heat Transfer Symposium of Japan, 343 ( 1988 ) (in Japanese). 9. K. Kanayama and H. Baba, Proc. First KSME-JSME Thermal and Fluids Engng. Conf., Vol. 1, 1-288 ( 1988 ).

10. K. Kanayama and H. Baba, Proc. 26th National Heat Transfer Symposium of Japan, 190 (1989) (in Japanese). 11. F. Zangrando, A simple method to establish salt gradient solar ponds, Solar Energy 25, 467 (1980). 12. K. Kanayama and H. Baba, Proc. ASME-JSME-JSES Solar Energy Conf., 166 (1987). 13. H. Baba and K. Kanayama, Estimation of horizontaltotal and normal-direct insolations from sunshine hours, and frequency distributions of continuous cloudy day, Trans. of JSME. 53(496), 3780 (1987)(in Japanese). 14. E. Bird, A simple, solar spectral model for direct-normal and diffuse horizontal irradiance, Solar Energy 32, 461, (1984). 15. K. Kanayama and H. Baba, Proc. 8th Japan Symposium on ThermophysicalProperties, 105 (1987) (in Japanese). 16. Monthly Report of The Japan Meteorological Agency, Meteorological Observations for 1981-1986 (in Japanese). 17. K. Kanayama and H. Baba, Proc. 5th Japan Symposium on ThermophysicalProperties, 113 (1984) (in Japanese).