Variable air gap membrane distillation for hybrid solar desalination

While a low temperature solar still for water production is simple in design, the accompanying productivity is quite low. This factor has necessitated development of enhancement techniques for water production. Alternatives suggested are the use of porous media in a solar still such as sponge, blackened jute cloth, pebble pieces, and floating carbonaceous material for absorbing solar energy. The use of a hydrophobic porous membrane in a solar still has not been investigated systematically thus far. The formation of a convex liquid meniscus alters vapor pressure across the pores of the membrane and is the origin of increased evaporation rates. The goal of the present study is to examine the performance of a solar still configuration towards water production in the presence of a surface-mounted hydrophobic membrane. Vertical arrangement is possible in a prototype but is inconvenient to setup in a laboratory-scale experiment. Hence, the device is compared with another without a membrane in a horizontal solar still configuration. Variable air gap is necessary to drain the condensed drop from an inclined condensing surface. The air gap membrane distillation system (AGMD) thus configured is selected for investigation and further comparison with a mathematical model. Experiments carried out in the present work show that a 40-70% increase in water production is obtained with the use of a membrane. The increase in evaporation rate is visible in the dropwise condensation patterns over the condensing surface. The present study thus confirms the utility of using a hydrophobic membrane in increasing water productivity of a solar still apparatus. Exergy analysis of the solar still when a chiller is used for the condensing surface is also presented.