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|An Experimental Study of Floating Wick Basin Type Vertical Multiple Effect Diffusion Solar Still with Waste Heat Recovery
|Kaushal, Arvind Kumar
|Mittal, Madhup Kumar
|Solar still;Vertical multiple effect;Floating wicks;Heat recovery;Life cycle costing
|The present research work aims at reducing the unit cost of distillate produced in a solar still by incorporating suitable improvements in the existing basin type vertical multiple effect diffusion (VMED) solar still. Extensive literature survey was done to study the design innovations done by various researchers to enhance the distillate productivity of solar stills, particularly the basin stills. An improved and modified version of basin type VMED solar still, having multiple floating wicks in basin, and heat exchanger for waste heat recovery for feed water pre-heating (FW-BVMED-HR), has been developed. This floating wick basin type VMED solar still with waste heat recovery (FW-BVMED-HR) has double glass covers in its basin section. The partition plates had divided wick structure for uniform wetting, in which each of the wicks were fed by feed water from gravity flow, by individual feed tubes. Longitudinal rubber spacers were used to divide the cotton cloth wicks and maintain partition plate gaps, and also to effectively seal the partition cells on the border of partition plates. The basin section had internal stainless steel reflectors. The first partition plate was of copper and rest partition plates were of stainless steel. Another still identical in dimensions and features, to FW-BVMED-HR still, was fabricated. Without floating wick and heat exchanger, this still served as a reference still, for comparison experiments. The complete experiments were done in an order and hence the experimental set-up was also fabricated in stages. Initially, a basin still was fabricated, with copper plate as back wall, which was well insulated on the outer side. Few experiments were conducted on basin still to find out its productivity. The conventional basin still is then converted into basin type vertical single distillation cell (VSDC) solar still. In other words the conventional basin still is converted into basin type VMED still with two effects. Experiments were done on the basin type vertical single distillation cell (VSDC) solar still alone, for performance testing of the divided wick structure and individual wick feed water arrangement, at varying feed water rates. Its productivity was compared with the basin still. The productivity of the basin type VSDC still was found 15-20% higher than the basin still. Subsequently, the effect of heat recovery from waste feed water, on the performance of basin type vertical multiple effect diffusion solar still with heat recovery (BVMED-HR) was investigated. BVMED-HR still with four effects was made after adding, two more partition plates to the basin type VSDC still (two effect VMED still), and a heat exchanger for feed water pre-heating by heat recovery from waste feed water. The partition plate gap and basin water depth both were kept 10 mm, for both BVMED-HR and reference stills. The experiments were done at varying feed water rates. The experimental results showed that the distillate productivity of BVMED-HR increased by 10.6% as compared to that of reference still, under the same ambient and operating conditions. An empirical correlation was developed to predict the productivity of BVMED-HR still using the experimental database of the study. The principle weather parameters such as daily total insolation, daily average ambient temperature, daily average wind velocity and daily average relative humidity, along with feed rate and feed water temperature as an important operational parameter are taken into consideration for the development of correlation. A floating wick basin type VMED solar still with waste heat recovery (FW-BVMED-HR) was fabricated by adding multiple floating wicks in the basin of BVMED-HR still. In the next set of experiments, the performance of the FW-BVMED-HR still was compared with reference still under identical weather and operational conditions, by running them side by side simultaneously. Both the stills consisted of four effects and partition plate gap of 10 mm. The experiments were done at varying basin water depths and feed water rates. On a clear sunny day, the distillate productivity of FW-BVMED-HR still was found to be 21% higher than the reference still. High distillate productivity resulted, due to high convective heat transfer by humid basin air from high temperature float wick surface to first partition plate, feed water pre-heating from waste heat recovery, reduced basin bottom and side losses, and high night distillate productivity as a result of additional heat stored in multiple floating wicks.The higher temperature of first partition plate, due to the higher heat transfer from the high temperature float wick surface, as well as the pre-heated feed water from heat recovery, in case of FW-BVMED-HR still, increased the evaporation heat flux from the first partition plate towards the second partition plate and external environment through partition plates. Subsequently, parametric study on FW-BVMED-HR still alone, was done experimentally, under outdoor conditions. The effect of design parameters of number of effects (n) and gap between partition plates (δp) and operational parameters of feed water rate (f) and basin water depth (d), on the cumulative efficiency of the still, within a narrow band variation of solar radiation and other parameters, was studied. Optimum feed water rate for the still was found to be 0.27 g/m2/s, when the total solar radiation was in the range of 21-23 MJ/m2/day. The cumulative efficiency increased by a maximum of 33% when gap between partition plates was decreased from 16 mm to 10 mm, at constant feed rate of 0.27 g/m2/s. The cumulative efficiency decreases by a maximum of 25% as the feed rate increases from 0.27 g/m2/s to 0.38 g/m2/s, at a mean basin water depth of 2.25 cm. The maximum decrease of cumulative efficiency was observed to be 8.5%, when the basin water depth was increased from 1 cm to 3 cm, at constant feed rate of 0.21 g/m2/s. The cumulative efficiency showed a maximum rise of 58% when the number of effects were increased from 2 to 7, at constant feed rate of 0.27 g/m2/s. The data analysis of experimental results of FW-BVMED-HR still showed that the night productivity has almost linear dependence on day productivity. A productivity correlation has been proposed which can predict the productivity of FW-BVMED-HR still with reasonable accuracy. The proposed correlation predicts the experimental data within the deviation range of -17% to +14% with a mean deviation of 6.6%.The principle weather parameters such as daily total insolation and daily average ambient temperature, along with important operational and design parameters such as feed water rate, feed water temperature, number of effects and gap between partition plates, are taken into consideration for the development of correlation. Economic analysis of FW-BVMED-HR still, based on life cycle costing of the system, has been carried out. The capital cost of this still with increasing number of effects, from 2 to 7, is computed. The annual cost of operating this still based on a life cycle of 10 and 25 years is estimated. The average annual distillate and minimum unit cost of distillate, for increasing number of effects is determined. The minimum cost of distillate obtained for the 7 effect FW-BVMED-HR solar still is estimated to be Rs. 5.45/kg, for a life cycle of 25 years and for interest rate 0.16. With lower interest rate of 0.12, the cost of distillate reduces significantly to Rs. 4.52/kg. The still is found to be economically viable with low payback period. At large number of effects, the reduction in unit cost of distillate with further addition of an effect diminishes. It happens because the distillate gain becomes smaller in magnitude with the addition of each effect and hence addition of an effect beyond a reasonable number does not contribute much in reducing the unit cost of distillate. The unit cost of distillate reduces significantly with decrease in interest rate and/or increase in life cycle of the still, due to reduction in total annual cost resulting from reduced value of capital recovery factor.
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