Solar Water Purification at Night: design of a suitable heat pump

This paper is about a method to enhance solar water still productivities we build on previous efforts by the Solar Engineering team at our university and on those by others elsewhere, to introduce a design that enables sustained solar distillation even in cloudy and night time conditions. Water is a most essential requisite for life as we know it on this planet. It plays a crucial role in the welfare of societies around the globe and affects the sustenance and stabilization of every living thing. However, despite the fact that it covers 70% of the Earth’s surface, only 2.5% of what is available, is suitable for human consumption. The balance 97.5% is either too salty or too contaminated. Even the 2.5% can be subject to micro-bio contamination during the collection, distribution and storage stages. It is therefore imperative to decontaminate it before drinking it. Because previous work on solar water purification relied only on the thermal effect of sunlight, it suffered serious loss of productivity during cloudy and night time conditions. Also, the purifiers had big thermal inertias because of the high specific heat capacity of water, leading to delayed onset of distillation when the sun reappeared. In the present work, we address these problems by use of supplementary heating from stored solar energy in form of electricity, but this time enhanced by the heat pump effect of refrigeration systems. Moreover, the design introduced, significantly lowers the top glazing temperature, increasing distillation rate, while also reducing energy losses to the atmosphere. Preliminary results from the prototype show a more than 100% rise of daily pure water collection at a heat pumping coefficient of performance varying between 2.3 and 4.1. It is concluded that this is a significant contribution to the technology of solar water purification.