Performance analysis of a solar still driven by a packed bed thermal storage tank during off-sunshine period

Abstract

During cloudy or rainy days, when sunlight is absent, a solar still cannot operate in its natural mode. In such circumstances, the still can be run using energy stored in thermal storage systems during sunshine hours. This work presents an analytical investigation of such a scenario in which a packed bed sensible thermal storage tank is used to provide the input energy for desalination via a double basin solar still during off sunshine season. Closed form solutions are derived for the temperature profile in storage tank, and of temperatures in both upper and lower basins of the still. Using these solutions, a closed form solution is obtained for the net mass of distillate produced until the cessation of operation. Parametric studies reveal that the present system can produce appreciable quantity of pure water that remains unchanged with any variation in mass of water in ether basin of the double basin still, mw1, mw2. The net distillate increases linearly with an increase in the initial tank excess temperature,θ0. Further, the analysis tells that there is optimum mass flow rate of the working fluid that delivers heat from storage tank to the still,m˙athat maximizes net distillate production. The reason of existence of such an optimum mass flow rate is discussed in detail. For the values considered, it is seen that a storage tank of 1 m3 vol, at an initial temperature of 50 °C above ambient, can produce 11.69 kg of potable water, yielding a thermal efficiency of about 68%. This work is a first ever investigation of the viability of a packed bed thermal storage tank for operating a solar still, and can serve as a guiding study for further, more comprehensive research work in this direction.