Shadow-assisted solar still: Surface cooling via sub-basin heat exchanger

Abstract

Solar driven desalination has attracted significant attention amid the continuous demand of sustainable and low-cost freshwater production. In the present work, a Shadow Assisted Solar Still (SASS) is proposed to be integrated with a sub-basin heat exchanger to enhance the operation of open system evaporation condensing. Controlled surface shading is employed in the system to prevent high temperature rise in the basin water which serves to reduce heat losses and promote evaporation stability. At the same time, sub-basin heat exchanger helps to cool the surface and accelerate condensation process by stabilizing higher temperature gradient of water surface against glass cover. Results of experimental and/or theoretical studies indicate that besides increasing the quantity of freshwater produced, this double-effect contributes to its thermal performance compared with single-basin solar stills. In such arid and semi-arid regions with the scarcity of clean water supply, the proposed design is an economic and environment-friendly option to produce freshwater. The design freedom of a shadow-assisted solar still added with a sub-basin heat exchanger was found to improve the system productivity compared to the reference configuration. Surface shading decreased the BWT from peak solar hours towards intermittent heating periods, which caused a reduction in heat losses and an enhancement of condensation productivity. Concurrently, the thermal gradient was induced across the sub-basin heat exchanger to boost evaporation–condensation cycle. Consequently, there was a 40–60 % increase in the daily freshwater product at about performance ratio between 1.4 and 1.6 compared with those of the conventional still day yield. In addition, the thermal efficiency of the system was increased with increasing overall TES depth (Thermal Energy Storage Depth, The effective thickness of the thermal energy storage medium participating in heat storage and release) by enhanced usage of latent heat and less heat loss to the water from its surface. These enhancements indicate that a combination of surface cooling (solar shading) and inside heat recycling (sub-basin exchange) is beneficial to increase the productivity of the solar still at high temperatures.