Theoretical Analysis of Continuous Heat Extraction from Absorber of Solar Still for Improving the Productivity

Numerical Study on Continuous Heat Extraction from Absorber of Solar Still 2018 62 3 Abstract This paper communicates the theoretical analysis of continuous waste heat extraction from the other side of absorber plate. For theoretical analysis two conditions are determined one is the mass of water in the absorber and another one is mass flow rate of water around the absorber plate. Results indicated that the water temperature is reached maximum at 10 kg of mass and 5 kg/hr mass flow of water and the heat extracted from the absorber is higher at optimum mass flow of 5 kg/hr. Also, the higher temperature difference between the water and the collector cover is found during the off-shine period. The maximum achievable hourly productivity of 0.9 and 0.5 kg is found for the solar still with and without circulation respectively. The yield from present model with continuous heat extraction is increased from 3 to 5.5 kg/m2. As the approached method is more new to the society it may be determined by AgouzNagarajan-Sathyamurthy (ANS) model.


Introduction
Greater growth of population and industries results in shortage of fresh water. Earth is almost covered with a larger water source and smaller land mass. People used to drink large amount of water for their thirsty need. Water available in the form of rivers, lakes cannot be consumed directly without pre-processing, as these need some purification process in order to remove bacteria and undissolved salts. In early days the possible method of getting pure water is heating the brackish water and condensing them back to get fresh water by burning fossil fuels. Due to the exhaustion of coal, crude oil and increases in global warming, the evolution of using renewable energy for getting fresh water evolved during the late 20th century. One of the best energy source applied is solar energy. Source of energy from sun is mostly environmental friendly and non polluting clean energy source. Basin type still is the most aged process of producing fresh water [1][2][3][4][5][6][7][8][9][10]. The fresh water conversion rate from the solar still system is predominantly depends on the solar intensity. The basin water temperature increases continuously due to heat energy emitted from the sun in the form of solar intensity. After reaching the boiling point of water, the water evaporates and it condenses on the solar still condenser cover. The various techniques employed in solar still, to raise the temperature of water and hence increases the productivity was studied from the detailed literatures reviews of Ravishankar et al. [11], Kabeel et al. [12] and Manokar et al. [13][14][15]. From the detailed literature studies it is clear that water temperature can be improved by either feeding waste warm water or pre heat the water by incorporating the different solar collectors or concentrators. Gupta et al. [16] theoretically analyzed the intermittent flow of waste warm water to the double basin solar still similarly same analyses was done by Yadav et al. in single basin solar still [17]. Sodha et al. [18] and Tiwari et al. [19] experimentally investigated the performance of solar still utilizing the warm water from the industry. Tiwari et al. [20] keep up a higher temperature difference between the water and collector cover by increasing the water temperature by flowing waste hot water in the basin and decrease the collector cover temperature by flowing the cold water above the surface of collector cover. This would results in increase in both evaporation and condensation rate and hence higher yield. Sinha and Tiwari [21] identified a new solar still by coupling concentrating parabolic collector to a solar still. Similarly B. Prasad and Tiwari [22] analysed a dual basin solar still with Flat Plate Collector (FPC). Extensive studies are also carried out on single slope solar still by reflecting the solar radiation through the bottom absorber as inverted basin studied by Dev et al. [23] and Suneja et al. [24,25]. Several researchers investigated the integration of active solar still with FPC [26][27][28][29][30][31][32][33][34], Compound Parabolic Concentrator [35][36][37][38], Parabolic shaped concentrator [39], Parabolic Trough collector [40,41], Cylindrical parabolic concentrator [42,43], Evacuated Tube Collector [44][45][46][47], Parabolic Dish concentrator [10,48,49], Concentrator with crescent absorber [50], Solar pond [51][52][53][54], Solar still integration [55][56][57], Hybrid PV/T integrated solar still [58][59][60] for improving the yield.
In the previous studies, cover cooling technique was used to improve the condensation rate and yield from the solar still and hence only 5% of the heat was recoverable. The maximum amount of heat utilized by water and absorber were found to be 5% and 90% respectively. In this work a novel method is identified by circulating water around the basin to extract the waste heat. Also, in the present study water mass (M w ) inside the basin and flow rate of water (M f ) around the basin were optimized for better improvement in fresh water yield. Fig. 1 shows the schematic drawing of a novel method system with continuous extraction of waste heat around the basin. In the present method water is initially fed around the basin instead of feeding it directly into the basin. The inlet water flow is controlled by via flow control valve. With the help PV Powered DC pump water is re-circulated into the still basin. Insulation can be provided around the basin for reducing the heat loss to the surroundings. Excess water can be drained from the basin with the provision provided at the bottom of basin. Glass trough is used to collect the condensate water and it is measured by using fresh water collecting jar. Table 1 shows the parameters used for theoretical analysis. Also, Environmental parameters used for theoretical calculation is given in Fig. 2.

Theoretical approach
The basin temperature is determined as, The outlet water temperature is determined as, The water temperature is determined as [66][67][68], (3) Where "f (t)", "a" and "M equ " values are Also, Simplifying for basin water temperature after feeding hot water temperature with water maintained in basin, the temperature of water is given by, The specific heat capacity of saline water is determined as Where Y is the salinity level in water C p(w) = The seawater specific heat at constant pressure expressed as J/kg K.
The glass temperature is determined as, Convective heat transfer coefficient is determined as [60],   Fig. 3 (a), (b) and (c) shows the variation of water, basin and collector cover temperature of modified still with continuous flow of water around the basin for complete heat extraction by changed the M w from 10 to 50 kg. From the theoretical analysis, it is found that the maximum temperature of water, basin and collector cover were found as 110, 120 and 107 °C respectively at a M w of 20 kg and with a continuous constant M f of 5 kg/hr around the basin. During the morning hours the heat gained by lower M w has the highest temperature, whereas, heat thermal energy is stored in the saline water with higher M w . During the afternoon with a drop off solar intensity, the temperature of water decreased quickly at lower mass. The water temperature increase with increased M w as it is due to the storage of thermal energy by saline water inside the basin.

Results and Discussions
It is observed that the optimal M w in the basin is 20 kg as it produced the maximum water temperature. Fig. 4 shows the variation of temperature difference between the water and the collector cover. With an increased mass of water, the temperature difference is lower in the morning hours. It is also observed that from 7 AM to 9 AM there is a negative value in the temperature difference. This clearly indicates that thermal energy is stored in the saline water at higher M w . The M w with 10 kg has the maximum temperature difference during the sunshine hours and falling thereafter as the solar intensity is decreasing. The maximum temperature difference of 6.2 °C, 5.5 °C, 4 °C, 3 °C and 2.5 °C is obtained between the water and the collector cover during the off shine period with M w = 50 kg, M w = 40 kg, M w = 30 kg, M w = 20 kg and M w = 10 kg respectively. Fig. 5 shows the variation of yield from solar still under different M w with a constant M f of 5 kg/hr around the basin. It is found that the productivity of water under M w = 20 kg is higher and the maximum value is found to be 0.9 kg. Similarly the fresh water yield from M w = 10 kg is lesser during the off shine period and decreases by 86 % as compared with M w = 50 kg. As previously it is clearly discussed in the previous literatures, the energy storage by saline water increases the productivity and the optimum M w is found as 20 kg. The percentage difference between the yield of fresh water produced during off shine period with increase in M w from 20, 30, 40 and 50 kg are found as 10, 12.5, 20 and 26 % respectively. Fig. 6 (a), (b) and (c) shows the variations of water, basin and collector cover temperature of continuous water circulation around the basin with constant mass at different flow rate. It is found that the temperature of water without circulation is  at the higher flow rate of saline water, the gaining of heat from the basin will be higher. From Fig. 6 (b) it is found that the temperature of basin is higher and maximum (120 °C), as the heated water is feed into the basin and there is a larger possibility in rejection of heat to the surrounding. With these convection layer is formed between water-basin and basin-flowing water.
Due to the continuous evaporation of vapour inside the basin of solar still, the temperature of inner cover rises. It is found that, a maximum temperature difference of 5 °C with a deviation of 5-10 % is achieved for all cases (Fig. 7). In the off shine time the maximum temperature difference for M f of 5 kg/hr is found as 6 °C with a maximum yield of 0.2 kg/m 2 . By extracting the remaining heat from the basin the productivity of the system is improved by 78 % than the still without heat extraction. Fig. 8 shows the variation of productivity from the solar still with and without flow of water around the basin at constant M w of 20 kg in the basin. Due to the continuous flow and heat extraction, the productivity of fresh water is improved by 28 and 52 % with constant M f of 2.5 and 5 kg/hr respectively around the basin. The peak yield from solar still with M f of 2.5 and 5 kg/hr were found to be 0.7 and 0.9 kg/hr respectively.

Conclusions
From the present theoretical study the following identifications are arrived: • Instead of using parabolic trough collector and flat plate collector nearly 50 % of energy can be recovered from the present model with continuous circulation of water around the basin. • The yield of continuous waste heat extraction from an absorber of solar still was improved from 3 to 5.5 kg/m 2 .
• Water temperature inside the basin is higher in the case of m w = 50 kg during the off-shine period with a constant circulation of water around the basin (m f = 5 kg/hr). • Temperature difference between the saline water and glass was negative for m w = 40 and 50 kg and showing that energy is stored in the saline water. • The optimized M f and M w in the basin is found as 5 kg/hr and 20 kg respectively.

FPC Flat Plate Collector M w
Water mass M f flow of water