Performance of a portable mini solar-pond

M.A. Tahat a,*, Z.H. Kodah, S.D. Probert b, H. Al-Tahaineh a

aDepartment of Mechanical Engineering, PO Box 3030, Jordan University of Science and Technology

(JUST), Irbid, JordanbSchool of Mechanical Engineering, Cran®eld University, Bedfordshire MK43 OAL, UK

Abstract

For the present experiments, such a pond was located at JUST, i.e. at 32�N latitude. Its

walls were inclined at 45� to the horizontal. The pond was built of galvanized steel (1.44 mmthick) with a circular surface area and total depth of 1 m2 and 500 mm, respectively. Thee�ects of the solar-pond's depth and its water's salinity on the store's temperature distribu-

tions were determined experimentally and compared with theoretical predictions. A dimen-sional analysis was carried out to show the e�ect of the mini solar-pond's size on its thermalbehaviour. # 2000 Elsevier Science Ltd. All rights reserved.

1. The challenge

Solar energy can be collected and stored e�ectively as sensible heat in a solar pondcontaining dissolved salt (NaCl), the latter helping to create a stable density-gradient[1]. The storage capacity depends on the pond's depth: the salt concentrationincreasing with depth in the solar pond. It has been realised recently that solarponds can be an economically-viable source of heat. Seebaluk and Russol [2] testeda small-scale salt-gradient solar pond, with a depth (below ground level) of 0.94 mand a diameter of 1.65 m. The optimal thickness of the gradient zone, in order toachieve the desired storage-temperature, was estimated [2].The aim of this investigation is to study, both experimentally and theoretically, the

performance of a portable inverted truncated-cone, mini solar-pond (see Fig. 1) atIrbid for harnessing insolation and storing the resulting hot water for subsequentdomestic use. The e�ects of various parameters on the ponds's stability, water salinityand temperature gradient have been measured.

Applied Energy 66 (2000) 299±310

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Nomenclature

As Surface area, m2

cp Speci®c heat of stored water, J/kg Kd Pond's water depth, mdlcz Lower convective zone vertical extent, mdncz Non-convective zone vertical extent, mducz Upper convective zone vertical extent, mE Eckert numberH:

Daily total insolation incident upon a horizontal surface, (J/m2 h)h(z) Fraction of solar radiation penetrating to the depth z in the pondI0 Hourly insolation incident upon the surface of the pond, (W/m2)Kw Stored-water's thermal conductivity, (W/m K)m Mass of water in the store, (kg)Q:

in Rate of energy input to the pond, (W)Q:

lost Rate of energy lost from the storage zone to the ambient environment, (W)Q:

stored Rate of energy storage in the lower convective zone (W)R:

t Ratio of the hourly to daily incident total insolationsS Salt concentration in the pond, % by mass.Ta Ambient temperature, (�C)Ts Storage-zone's average temperature (�C)t Time, (sec)ts Solar time, (h)u Velocity of the solar-pond current, m/sUtop Overall top-surface's heat-loss transfer coe�cient (W/m2 K)W Diameter of base of pond, (m)

�T Temperature di�erence in the storage-zone after a time interval of �t,(�C)

�t Time intervals, (sec)� Declination angle, (�)�pc Pond's e�ectiveness, (%)� Density of the water in the storage zone, (kg/m3)� Non-dimensional size parameter� Latitude of the pond's location, (�) Non-dimensional performance parameter! Hour angle, (�)!s Sunset-hour angle, (�)� Density of the water in the storage zone, (kg/m3)�p Pond's e�ectiveness, (%)

LCZ Lower convective zoneNCZ Non-convective zoneSGSP Salt-gradient solar pondUCZ Upper convective zone

300 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310

1.1. Non-convective salt-gradient solar pond

A salt-gradient solar-pond collects and stores insolation as heat in a single unit.The stability of the mini solar-pond, when so heated, is normally maintained by thepresence of the salt. To a ®rst approximation, both the UCZ and LCZ strata (seeFig. 2) have uniform and constant temperatures and salt concentrations, whereasthe temperature and the salt-concentration increase with depth in the NCZ layer. Inthe pseudo steady-state:

Rate of heat �input� � Rate of heat stored in the lower convective zone

� rate of heat losses:

Fig. 1. Schematic diagram of the inverted truncated-core, mini solar-pond and its support structure,

which rests on horizontal ground.

M.A. Tahat et al. / Applied Energy 66 (2000) 299±310 301

i.e.

Q:

in � Q:

stored �Q:

lost �1�

The temperature of the storage zone (LCZ) at the end of the period, Tt+�t is

Tt��t �As h�z�I0 � kwTa

dncz

� �� mcpTt

�t

� �� �mcp

�t

h i� Askw

dncz

� �� � �2�

Fig. 2. Schematic vertical section through the mini solar-pond.

Fig. 3. Observed temperature pro®le along the vertical axis of the mini solar-pond, at noon on 20th

October 1997.

302 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310

Fig. 4. The predicted average annual storage-zone temperature vs the LCZ thickness; d=500 mm and

ducz =75 mm.

Fig. 5. The predicted average annual storage-zone temperature vs the NCZ thickness; d=500 mm and

ducz=75 mm.

M.A. Tahat et al. / Applied Energy 66 (2000) 299±310 303

1.2. Pond's performance

The thermal e�ectiveness, Zp, of the solar pond can be de®ned as the ratio of theuseful energy stored to the amount of insolation transmitted to the storage zone ofthe pond, both in the same period, �t, [3].

Zp �mCp

�t�Tt��t ÿ Tt�

� �AsIoh�z� �3�

2. Experimental rig

2.1. The pond

The mini solar-pond was constructed from galvanized steel sheet of 1.44 mmthickness, as shown in Fig. 1. The areas of the water surface and the base of thepond were 1 and 0.013 m2 respectively, both being circular. The pond, an invertedtruncated cone, had sidewalls inclined at 45� to facilitate insolation capture withrelatively little shading.

Fig. 6. Predicted energy loss from the pond over a period of 1 year; d=500 mm and ducz=75 mm.

304 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310

The overall depth of the mini-pond was 0.50 m and its sides were insulated toreduce the heat loss through the walls. The top surface of the pond was coveredthroughout the experiment with a transparent nylon material in order to reduce thewind e�ect on the stability of the pond. The inner surfaces of the wall were paintedwith matt-black to increase the rate of solar radiation absorbed.

Fig. 7. Predicted e�ect of the NCZ thickness on the storage zone temperature, d=500mmand ducz=75mm.

Table 1

E�ect of the salt concentration in the LCZ zone on the storage-zone mean temperature

S=20%

dlcz (m) 0.10 0.15

Tlcz (�C) 40.0 36.0

S = 30%

dlcz (m) 0.10 0.15

Tlcz (�C) 40.4 36.2

M.A. Tahat et al. / Applied Energy 66 (2000) 299±310 305

2.2. Temperature distributions

The temperature pro®les were measured with eighteen copper±constantan ther-mojunctions distributed within the solar pond. The distance between successivethermojunctions was 2 cm.

Table 2

Comparison of data for three di�erent ponds

Pond Present data Ref. [6] Ref. [2]

As (m2) 1 57 2.14

d (m) 0.5 2.5 0.94

�T (�C) 12.4 1.4 12.5

�t (days) 0.3 7 57

� 4 8.96 2.422

1.722�1014 3.43 �1014 1.4366�1018

Fig. 8. Predicted e�ect of the LCZ thickness on the amount of stored energy; d=500 mm and ducz=75

mm.

306 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310

Fig. 9. Predicted energy storage e�ectiveness of the solar pond over a period of 1 year; d=500 mm and

ducz =75 mm.

Table 3

Recent average mean weather data for Irbid city

Month Insolation, H.(MJ/m2/day) Average ambient temperature (�C)

January 10.040 9.0

Feburary 10.579 9.9

March 18.635 12.2

April 23.956 16.2

May 26.193 20.3

June 28.978 23.7

July 29.282 26.0

August 26.830 27.6

September 22.830 23.9

October 18.189 21.5

November 12.404 16.2

December 8.438 10.4

M.A. Tahat et al. / Applied Energy 66 (2000) 299±310 307

3. Experimental procedure

3.1. The pond

A calculated amount of salt (NaCl) was placed on the base of the pond and thenfresh water added until the pond is half full. Mixing was carried out in order toencourage the salt to dissolve in the water and become homogeneous. The pond wasthen ®lled continuously with fresh water. In order to maintain the required stablegradient in the mini solar-pond, it was necessary periodically to inject brine into theupper and lower convective layers and wash the upper zone with fresh water.

3.2. Salinity measurements

The existence of a salt gradient in the pond is desirable because it gives the pondthermal stability and hence improves its performance. To measure the density pro®le

Fig. 10. The measured temperature, the total energy stored and the solar-pond e�ectiveness for 20th

October 1997.

308 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310

in the pond, known-volume samples were taken from the three di�erent zones andweighed. During this investigation, the water salinity was maintained by recyclingthe salty water removed from the pond's surface and then injecting it into the solarpond, near its base, as recommended by Newell and Boehm [4].

4. Observations

The horizontal boundaries of the UCZ, NCZ and LCZ layers in the salt- gradientsolar-pond can be located from the measured temperature-pro®le within the pond,as shown in Fig. 3. The surface-layer's thickness ¯uctuated throughout each test dueto the e�ects of wind and upwards salt-di�usion.As the dlcz was increased, the storage-zone's temperature fell, as shown in Fig. 4.

As the thickness (dncz) of the non-L convective zone increased, the temperature ofthe storage zone rose (see Fig. 5) The behaviour of the solar pond is as shown inFigs. 6±11. Increasing the salt concentration in the LCZ had little e�ect on the per-formance of the solar-pond (see Table 1).

Fig. 11. The e�ect of the total energy input and energy stored on the solar-pond's e�ectiveness for the

period 1st to 20th October 1997.

M.A. Tahat et al. / Applied Energy 66 (2000) 299±310 309

5. Deductions and correlations

The larger the pond, the greater its e�ectiveness [5]. A dimensional analysis wascarried out, for three di�erent-size ponds, to correlate the experimental results (seeTable 2)The correlating dimensionless parameters employed in Table 2 are de®ned by

� �Tcp �t� �2d2

� �Tcp

�d=�t�2 ��Tcp

u2� 1

E�4�

and

� � As=d2 �5�

The performance parameter for the solar pond is de®ned as the ratio of thethermal energy to the viscous energy, which is equal to 1/E, where E is the Eckertnumber. For the mini solar-pond to be stable, the viscous energy generation, due tosalt di�usion from the LCZ to UCZ, should be small compared with the thermalenergy stored, i.e. E should be small. The variation of with � indicates that theremust be a critical shape and size for the pond to operate e�ectively, (see Table 2) [6].

6. Conclusions

The behaviour of the mini solar-pond is in¯uenced by its size, the e�ectiveness of itsthermal insulation and the thickness of its three zones. Salt-gradient solar ponds canbe used to store large amounts of heat while operating at relatively low temperatures.The performance of the domestic salt-gradient solar pond in Irbid is encouragingprimarily due to the high levels of solar radiation experienced there, as shown in Table3. So it can be economically viable. Other advantages of the mini solar-pond are itsportability, its low space-requirement and its environmental friendliness.

References

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[2] Seebaluk D, Russol MS. A small-scale solar pond's performance at Reduit. PROSI Magazine, N

344, September 1997.

[3] Keren Y, Rubin H, Atkinson J, Priven M, Bemporad GA. Theoretical and experimental comparisons

of conventional and advanced solar pond performances. Solar Energy 1993;51(4):255±70.

[4] Newell TA, Boehm RF. Gradient zone constraints in a salt-gradient solar pond. Solar Energy

Engineering, 1982;104:285.

[5] Jabri AJI, Rasheed AM. An investigation of solar pond capability in providing space heating for

residental building in Baghdad/Iraq. JIMEC 97 1997;1:200±17.

[6] Khashhan SA. Computer simulation of a solar pond, MSc thesis, JUST, 1993.

310 M.A. Tahat et al. / Applied Energy 66 (2000) 299±310