1 Experimental Study of Solar Ponds with Different Geometry and Construction Materials M. A. A. H. K. Chowdhury, B. Salam and N. Nafis Department of Mechanical Engineering, Chittagong University of Engineering and Technology, Chittagong-4349 ABSTRACT: It is a global concern as well as a fact of urgent research for proper utilization of solar energy. The aim behind the design of solar pond under present study was to investigate the temperature rise in the solar pond. In this research work, two different solar ponds with different construction materials and dimensions were used and data were collected for determining temperature distributions in the ponds. One solar pond was made from sheet material of rectangular top inside surface area of 94 cm x 94 cm and depth 90 cm. Its inner side was insulated with polythene covered cork sheet of 5 cm. This pond was covered with 4 mm glass. Another solar pond was constructed with concrete material with a depth of 1.6 m and circular top surface area with inner diameter of 0.762 m. A thermocouple was used to measure the temperatures at different positions. The maximum temperature of 33 0 C in the storage zone was found in circular concrete solar pond and in the rectangular polyethylene-cork sheet insulated solar pond the maximum temperature of 35 0 C was recorded. Keywords: Solar pond, Construction materials; Experimental study. 1. INTRODUCTION A solar pond is a simple and low cost solar energy system which collects solar radiation and stores it as thermal energy in the same medium for a long period of time. For this reason, solar ponds attract the interest of researchers in this subject more than some alternative solar energy systems. Solar ponds generally consist of three regions, the upper convective zone (UCZ), the non-convective zone (NCZ) and the lower convective zone (LCZ), as shown in Fig.1. The UCZ is the topmost layer of the solar pond. It is a relatively thin layer which consists almost wholly of fresh water. The NCZ is just below the UCZ and has an increasing concentration relative to the UCZ, and it also acts as insulation on the LCZ. The LCZ is the layer in which the salt concentration is the greatest, and there is no concentration gradient in it (Fig. 2). If the concentration gradient of the NCZ is great enough, no convective motion will occur in this region, and the energy absorbed in the bottom of the pond will be stored in the LCZ. Water is a fluid which does not transmit infrared radiation, so only the visible light part of the solar energy spectrum reaches the bottom of the pond and is absorbed there. Because of the poor conductive capability of water, the nature of infrared radiation and the insulating property of the NCZ, the stored energy in the LCZ only escape from the pond with conduction. So, this action provides the solar pond as a collector at the same time as being a heat storage device [1, 2]. Fig. 1: The salt gradient solar pond configuration.
2 Fig. 2: Schematic diagram of concentration and temperature profiles of the solar pond. The principle and application potential of solar ponds were reported in the literature at the beginning of this century. The first recorded reference to a natural solar lake was that of Kalecsinsky, who described the Medve Lake in Transylvania (42 0 44'N, 28 0 45'E). He observed temperatures increasing up to 70 0 C at a depth of around 1.32 m during the end of summer. The minimum temperature was 26 0 C during early spring. These high temperatures were apparently due to the salinity gradient in the lake. Thereafter, natural temperature gradients occurring in lakes were reported in a number of countries. Anderson reported on a shallow saline lake in Washington State, showing temperatures up to 50 0 C in midsummer at a depth of 2 m. Wilson and Wellman reported on Lake Vanda in Antarctica where, despite the surface being covered with ice, the bottom temperature was 25 0 C, while the ambient was - 20 0 C at a depth of 60 m during December. In the same region, in Lake Bonney, temperature gradients were also found the same as Lake Vanda. The effects of direct solar radiation causing elevated temperatures in these Antarctica lakes have been well explained. At the bottom of a natural solar lake near Eliat in Israel, temperature increments up to 48 0 C were reported by Cohen et al. Systematic measurements performed in Castle Lake, in California, showed a temperature difference of about 20 0 C between the surface and a depth of 5 m [3, 4, 5]. The idea of solar energy collection by creating artificial solar ponds has been initiated with extensive pioneering research on various aspects including analytical model treatments, laboratory testing and construction and economic analysis in a number of countries, including Australia, India, Canada, Chile, Portugal, Russia, Kuwait, etc. Solar pond work has also been reported from various world countries. Despite the increasing international awareness of the subject and the suitability of the country in many aspects for solar pond utilization, few efforts have been made in Bangladesh. In this research, the performances of two different solar ponds have been studied both experimentally and theoretically. In both cases the overall efficiency of these small size salt gradient solar ponds built on the university campus was found below 2%. The aim was to build a solar pond with cheap material so that this solar pond will be a sustainable energy source in Bangladesh. 2. PHYSICAL MODEL In the first stage of this study, an experimental stand was located in the Chittagong University of Engineering & Technology (CUET). One solar pond was made from sheet material of rectangular top surface area of 94 cm x 94 cm and depth 90 cm. Its inner side was insulated with polythene covered cork sheet. This pond was covered with 4 mm glass as shown in fig. 3. Another solar pond was constructed with concrete material with a depth of 1.6 m and circular top surface area with inner diameter of 0.762 m as shown in fig.4. This pond had no cover to the top. In the experiments, sodium chloride (NaCl) salt solutions in different concentrations have been tested separately. The solar radiation intensity coming to the pond surface has been measured by a thermocouple. An artificial concentration gradient in the pond has been produced with the different concentrated solutions prepared before. The fluid temperatures in the solar pond changed with depth and time. The temperatures of the surroundings, soil and glass were measured by the system composed of Fe- Constantan thermocouples.
3 4mm Glass cover UCZ: 10cm, 0 kg/m 3 Insulator: Cork & polythene NCZ: 60cm, 115 kg/m 3 Steel Container UCZ: 10cm, 350 kg/m 3 Fig. 3: Geometry, Material, Insulator and salt concentration of rectangular glass cover solar pond. UCZ: 20cm, 0 kg/m 3 Concrete Insulator: Wood Dust NCZ: 100cm, 57.5 kg/m 3 UCZ: 40cm, 175 kg/m 3 Fig. 4: Geometry, Material, Insulator and salt concentration of circular solar pond without cover. 5 cm thick white cork sheet was used as insulator inside a rectangular steel box container (1m x 1m x 1m) to support the load of salt and water solution of the solar pond. Also thin black color polythene was used to make the pond leak proof so that salt solution cannot go outside the pond. And the pond was covered with 4mm transparent glass to protect it from foreign materials like birds, dusts, rain water, etc. as shown in fig. 3. This solar pond was installed on the roof of the two stored EME Building of CUET. Total 150 kg salt (NaCl) was used in the solar pond. After experiment it was found that the salt concentration in UCZ, NCZ and UCZ were 0 kg/m 3, 115 kg/m 3 and 350 kg/m 3 respectively. On the other hand, another pond was made of concrete material and half portion outside the pond was insulated with wood dust as shown in the fig. 4. The pond was installed on the ground near the Workshop of CUET campus. No glass cover was used on the top of the pond. Total 70 kg salt (NaCl) was used in the solar pond. After experiment it was found that the salt concentration in UCZ, NCZ and UCZ were 0 kg/m 3, 57.5 kg/m 3 and 175 kg/m 3 respectively.
4 3. DATA COLLECTION AND ANALYSIS Data were collected during the month of May, June and July of 2007. Table.1 contains the average temperatures of the solar ponds during this period. Fe-Constantan thermocouple was used to measure the temperatures of water in different depths. Table. 1: Three months average data of two solar ponds Depth from Top (cm) Pond-1: Temperature ( 0 C) Pond-2: Temperature ( 0 C) 0 31 31 5 35 31 10 34 31 15 34 31 20 34 30 25 32 29 30 31 29 35 31 29 40 31 28 45 31 28 50 31 28 55 31 28 60 31 28 65 32 29 70 34 29 75 36 29 80 38 29 85 29 90 28 95 29 100 29 105 29 110 30 115 31 120 31 125 31 130 31 135 31 140 31 145 32 150 33 155 34 160 34 UCZ NCZ LCZ Upper Convective Zone Non Convective Zone ( Heat Insulator) Lower Convective Zone (Heat Storage) Pond-1: Solar pond with dimension of 0.94m X 0.94m and height of 0.90m as shown in the figure 2. Air gap of 0.1m was given and the pond was covered with 4mm transparent grass sheet. Pond-2: Circular solar pond with inner diameter of 0.762m and height of 1.6m as shown in fig.3. No air gap was given and the upper surface was opened to the atmosphere.
5 Solar Pond Data 39 Temparature( 0 C) 37 35 33 31 29 27 25 0 20 40 60 80 100 120 140 160 Pond-1 Pond-2 -->Depth from Top (cm) Fig. 5: Temperature profile of two solar ponds. Table.1 contains the data of recorded temperatures in different depth with an interval of 5 cm of two different solar ponds. Inside water of solar pond-1 is 80 cm high and that of solar pond-2 is 160 cm high. In the case of solar pond-2, the UCZ, NCZ and LCZ are 20 cm, 100 cm and 40 cm respectively. But these regions are 10 cm, 60 cm and 20 cm in solar pond-1. In fig.5 the data are plotted in the same graph. In solar pond-2 the maximum temperature of the UCZ, NCZ and LCZ are 31 0 C, 31 0 C and 34 0 C respectively. But in solar pond-1, there are some fluctuations in UCZ and NCZ. The maximum temperatures of these zones are 31 0 C and 33 0 C respectively. And the temperature of the LCZ rises gradually with the maximum temperature 38 0 C. 4. RESULTS Table.2: Summary of result for solar pond-1 Zone Salt Density (kg/m 3 ) Height (m) Area (m 2 ) Volume (m 3 ) Mass (kg) Max Temp ( 0 C) T ( 0 K) Q=mS T (KJ) UCZ 0 0.1 0.8836 0.08836 0 31 0 0 NCZ 115 0.6 0.8836 0.53016 60.9684 33 2 379.223448 LCZ 350 0.2 0.8836 0.17672 61.852 38 7 1346.51804 Total 0.9m Efficiency (%) 2.25 Table.3: Summary of result for solar pond-2 Zone Salt Density (kg/m 3 ) Height (m) Area (m 2 ) Volume (m 3 ) Mass (kg) Max Temp ( 0 C) T ( 0 K) Q=mS T (KJ) UCZ 0 0.20 0.45580 0.068370 0 31 0 0 NCZ 57.5 1.00 0.45580 0.433015 24.8983 31 0 0 LCZ 175 0.40 0.45580 0.182322 31.9063 34 3 297.68659 Total 1.60m Efficiency (%) 0.98 5. DISCUSSION It is infer that the efficiency of solar pond-1 is more than that of solar pond-2 though the size of the pond is smaller than the other one. The salt concentration plays an important role in solar energy absorbing and storing. Solar pond with high salt concentration can store more energy than the solar
6 pond with low salt concentration and if insulation is properly given by using cheap materials heat loss can be minimized over traditional materials. So cheap material like polythene and cork sheet can be used to minimize the cost of construction of solar pond and the solar pond will be a sustainable source of energy. 7. CONCLUSION Solar pond is a proven technology and a reliable source of energy. Initial cost is high because it requires valuable land area and construction materials. But the maintenance cost is low. So small size solar pond with high salt concentration can be made with cheap construction and insulating materials like cork sheet and polythene. Also this small type solar pond can be installed on the roof of the building. Thus solar pond can be a cheap energy source. Solar ponds can be effectively used as replacements in industries that use fossil fuel to generate thermal energy. Solar ponds can also be used for process heating, refrigeration, water desalination, production of magnesium chloride, bromine recovery from bittern, and enhancement of salt yield in salt farms. It will be the future energy source. Now Bangladesh is facing energy crisis and the demand of energy is rising day by day. Solar ponds can be a one of the solutions to face the energy crisis. REFERENCES 1. Solar Pond Conception-Experimental and Theoretical Studies; Huseyin Kurt, Fethi Halici, A. Korhan Binark. Energy Conversion & Management 2000, September 1999, Vol. 41, pp 939-995. 2. A Solar Pond Model with Insulated and Glass Covered Surface (IGCSP); Nuri Ozek, Mchmet Karkilcik, Nalan Cicek Bezir. Bulgarian Journal of Physics, December 2000, Vol. 27, pp 67-70. 3. Experiment and Analysis of Particle scale Solar Pond Stabilised with Sait-Gradicnt; K. Kanayama, H. Inaba, H. Baba and T. Fukuda. Solar Energy, December 1991, Vol. 46, pp 353-359. 4. Correlations for the Yearly and Monthly or Seasonally Optimum Salt Gradient Solar Pond in Greece; K. A. Antanopoulus and E. D. Raddakis. Solar Energy, December 1993, Vol. 50, pp 417-424. 5. Usage of the Empirical Functions Derived for Air and Soil Temperatures for the Southern Part of Turkey in a Model Development for Rectangular Solar Pond; R. Kayali, S. Bozdemir and K. Krymac. Solar energy, December 1998, Vol. 63, pp 345-353