Concept of integrating geothermal energy for enhancing the performance of solar stills

Solar stills have been coupled to several preheating systems to increase their productivity, but never before to a borehole exchanger. In this work, a coaxial type borehole exchanger has been integrated with a single basin solar still. Transient and steady-state heat transfer models are presented to assess the behaviour of the combined system for a four-month winter period corresponding to the Urmia Lake region in Iran. Because ground serves as a heat source during winters and a heat sink during summer, integration with a ground exchanger is seen to increase the solar still's productivity for the winter months, by about 126 % for the considered values of parameters. It is also seen that the steady model over calculates the total distillate collected, by about 7 % for ground exchanger integrated still and underestimates it by about 0.2 % for an independent still, respectively. It is observed that larger inner and smaller outer radius of the ground exchanger are desirable. Further, the water mass flow rate is seen to possess an optimal value at which the still's output gets maximized. This work is proposed to be a first step in analysing the usefulness of extracting geothermal energy via a borehole exchanger for desalination application via a solar still.