Study of Vertical Multiple Effect Diffusion (VMED) Solar Still Integrated With Mini Solar Pond

Abstract

The aim of the present research work is to enhance the diurnal distillate output and produce nocturnal distillate from the solar still by incorporating suitable modifications in existing conventional basin type vertical multiple-effect diffusion (VMED) solar still. Several floating wicks were used in the basin to enhance the diurnal distillate output of the still and mini solar pond was used to supply heat to basin water in batch mode during night to produce nocturnal distillate from the modified basin type VMED solar still. The present study was done in a proper sequence and hence the complete research work was done in three phases. In the first phase of this study, a mini solar pond, having trapezoidal shape and salt gradient along pond depth, was fabricated and investigated. In order to determine the advantage of trapezoidal shape over the cuboid shape of pond, the shading areas in upper convective zone (UCZ), non-convective zone (NCZ) and lower convective zone (LCZ) of trapezoidal solar pond was theoretically computed and compared with shading areas in corresponding zones of cuboid solar pond having same top surface area and depth. It was found that the shading area of trapezoidal solar pond was always lower than that of cuboid one in all the three zones. The yearly average shading area in UCZ, NCZ and LCZ of trapezoidal solar pond was computed to be 54%, 56.2% and 43.5% respectively less than the yearly average shading area in corresponding zones of cuboid solar pond. In order to determine the effective area, which receive direct solar radiation, sunny area ratio has also been calculated and it was found that the trapezoidal solar pond and cuboid solar pond have highest sunny area ratio of 94.12% and 84.20% respectively in the month of June. The top surface of solar pond has been covered with double glass cover in order to reduce the evaporative and convective losses from the top. This also results in increase of temperature even in the top zone of the solar pond and leads to more volume utilization for heat storage in the pond. A reflector made of mirror finished aluminium sheet has been used to enhance the solar intensity on the solar pond during sunny hours. A procedure, to determine optimum tilt angle of reflector in order to utilize maximum amount of solar energy at noon, has been proposed. The use of reflector enhanced the average solar intensity on the top surface of solar pond by 22%. The maximum average temperature of trapezoidal solar pond with glass cover and reflector has been observed to be 70.5 °C. Experiments were also conducted to investigate the thermal performance of trapezoidal salt gradient solar pond (TSGSP) for three cases, i.e., case-1: without double glass cover and without reflector, case-2: with double glass cover and without reflector, and case-3: with double glass cover and with reflector. The thermal efficiencies of LCZ, NCZ and UCZ for the trapezoidal solar pond with double glass cover and reflector have been estimated to be 32.73%, 23.22% and 5.30%, respectively and found to be most thermally efficient among the three cases of trapezoidal solar pond. Hence, trapezoidal solar pond with double glass cover and reflector was used to supply additional heat to still for further experimental work. The second phase of the study was started with fabrication of two conventional stills having four effects. Then one of these two stills was modified by placing several floating wicks on basin water and coupling it with a mini solar pond. In the present study, the basin type VMED still having floating wicks in basin and coupled with mini solar pond has been referred as modified still or BVMEDS-SP and the other still has been referred as conventional still. The experiments were conducted on both the stills, placed side by side, under similar weather and operating conditions in order to determine performance improvement of modified still over conventional still. Before starting the actual experiments to compare the performance of modified still with conventional still, several trial runs were performed on modified still by circulating the basin water through heat exchanger of pond during night in order to gain insight into the variation of basin water temperature during circulation of basin water. On the basis of trials, it was decided to supply additional heat two times during 9:00 pm to 9:30 pm and 3:00 am to 3:30 am from solar pond to modified still. After deciding appropriate time duration of heat supply and number of times of heat supply to basin water from solar pond during night through trial runs, the experiments were performed on both modified (BVMEDS-SP) and conventional stills, simultaneously, to determine performance improvement of modified still over conventional still. The diurnal, nocturnal and overall total productivity of modified still was found to be 49.87%, 71.21% and 56.92% respectively, more than the corresponding productivities of conventional still. The daily cumulative efficiency of modified and conventional still was found to be 80.29% and 59.6% respectively, when the total solar radiation on glass cover was 23.1 MJ/m2/day, average ambient temperature was 25.3 °C and feed rate was 0.13 g/m2/s. The cost analysis was also done to compare unit cost of distillate produced from conventional and modified still. The distillate production cost of modified 4-effect still was found to be 6% lesser than that of conventional 4-effect still. In the third and last phase of the study, the performance of BVMEDS-SP was tested for 24 h by varying feed water flow rate, feed water temperature and number of effects. It was found that the daily cumulative efficiency of BVMEDS-SP having 4 effects was increased by 13.94% as the feed rate was decreased from 0.32 g/m2/s to 0.13 g/m2/s. As temperature of feed water was increased from 25.65 °C to 32.4 °C, the daily cumulative efficiency of BVMEDS-SP was found to be increased by 28%. The daily cumulative efficiency showed a rise of 93.15% when the number of effects was increased from 2 to 7 at constant feed water rate of 0.13 g/m2/s. The night distillate output of BVMEDS-SP was found to be 30-42% of the total daily distillate output. The cost analysis has also been done to study the effect of number of effects on the cost of distillate produced from BVMEDS-SP. The cost analysis revealed that cost of distillate for 2-effect still was Rs. 4.22/kg which reduced to Rs. 3.40/kg for 7-effect still. The BVMEDS-SP is found to be economically viable with low payback period of 2.3 years, for 7 effect still and for interest rate of 0.12, assuming a selling price of distillate at Rs. 10/kg. A productivity correlation has been developed to estimate the daily productivity of BVMEDS-SP by using the experimental database of present study. The accuracy of proposed productivity correlation has been verified by comparing the estimated productivities obtained from proposed correlation with those measured experimentally in the present work. It has been found that the proposed correlation estimates the experimental data within the deviation range of -5% to +10% with a mean deviation of 3.24%. Therefore, it can be concluded that the estimated productivities by the proposed correlation are in good agreement with the experimentally measured productivities. The proposed productivity correlation can be used to estimate the productivity in order to carry out techno-commercial feasibility studies before installation of similar type of solar desalination units in any part of the world.