MULTISTAGE, INDIRECTLY HEATED SOLAR-STILL

A solar still was built based on the principle of a stacked tray array for tandem distillation and heat recovery. Heat is supplied to the lowermost tray, coutaining seawater like all the rest, and a diffusion distillation occurs. Evaporation from the hottest tray causes condensation onto the upper, colder one, thus producing distilled water and a flow of heat upwards. As this exchange proceeds, the thermocline travels upwards. This procedure is found to be dependant on the partial pressure difference of water vapor, just as in greenhouse solar stills. A simple mathematical model is evolved and calibrated with field data to make it fit adequately experimental results gathered along some 14 months of continuous operation. Conditions can now be identified for which the added cost of particular design features are adequately compensated by the return of distilled water from the sea. © 1990.