THERMAL MODELLING AND PERFORMANCE STUDY OF MODIFIED DOUBLE SLOPE SOLAR STILL

Abstract

Solar distillation is a process of producing purified drinking water from brackish water by using the heat of solar radiation as the feed to evaporate the impure water which on condensation gives the pure water. It is independent of electricity, a major portion of which is generated from fossil fuel causing environmental pollution and relies completely on renewable source of energy like solar radiation, thus making it environment friendly. In this paper a modified double slope solar still (modified DSSS) has been designed by using Transparent Acrylic and opaque Fibre Reinforced Plastic (FRP) as its body material with two toughened glass covers. The basin and north wall of modified DSSS have been made by using FRP of thickness (0.005 m), whereas, its three sides (East, West and South walls) are made of transparent Acrylic sheet of thickness (0.003 m) equivalent to that of FRP for the same heat loss, which results in increased input solar radiation inside the solar still and improved performance but with low cost. It is evident that, the inside space of solar still is filled with air and vapour molecules which can come in contact with inner surfaces of walls and glass covers. The vapour molecules close to the walls strike it due to molecular collisions and stick to it to release its heat for phase change from vapour to liquid during condensation process. Hence, five troughs (distillate collecting channels) have been placed at inside surfaces of all its walls and glass covers. The yield has been collected from all the sides of the solar still except north wall which acts heat absorber. The molecules which come in contact with north wall get additional heat from it and get evaporated. In this paper, a thermal model has been developed to predict theoretically the performance of MDSSS for the climatic condition of MNNIT, Allahabad, India on 22 nd May 2014. Expressions for water and glass temperatures and hourly yield for the modified double slope solar distillation system have been derived analytically. It has been found that the total yield obtained from the MDSSS in a period of 24 hour is 16 Kg of purified water from 25 Kg of brackish water which is about twice of that obtained from conventional solar stills. Also the effect of solar radiation on the productivity of solar still has been analyzed.