Solar desalination

Solar desalination is the desalination of water using solar energy. Solar desalination in the modern era extends back to the early 1950's when simple solar stills were studied for remote desert and coastal communities. The cheap availability of water pumps and pipelines and declining energy costs in the 20th century, ensured these were not competitive for community-scale projects.

In general, two different principles are considerable: Electrically / mechanically driven systems and thermally driven systems. Reverse Osmosis is currently the favoured technology for large scale desalination where electricity is cheaply available, being the most cost-effective in this case. For small to mid scale desalinations systems, being dependent on renewable energy input (e.g. solar thermal, photovoltic, wind), thermally driven installations are feasible and make economic sense. The relatively cheap availability of solar thermal heat using thermal solar collectors makes such systems preferable for remote applications.

Recently, there is evidence of emerging research interest in the field (European Solar Thermal Technology Platform, ESTTP ESTTP). This is prompted by growing energy costs, demand growth in the face of depleted fossil water stores, and the growing human pollution of many communities' water supplies.

HDH - Solar Humidification/Dehumidification
The Multiple Effect Humidification / Dehumidification process, in some applications also called Solar Multistage Condensation Evaporation Cycle (SMCEC) or Multiple Effect Humidification (MEH)- is a technique for thermal solar desalination using natural convection in a vertical chimney. It uses a natural chimney effect to draw the outgoing heated water vapour past condenser plates (through which the incoming water runs), thereby pre-heating the incoming water and increasing the overall system efficiency. The HDH-Process is an example for a Solar Humidification-Dehumidification process (comp. Solar Humidification).

Problems
There are two inherent design problems facing any solar desalination project. Firstly, the system's efficiency is governed by preferably high heat and mass transfer during evaporation and condensation. The surfaces have to be properly designed within the contradictory objectives of heat transfer efficiency, economy and reliability. This is content of the present research work in this field. Secondly, the heat of condensation is valuable. It takes large amounts of solar energy to evaporate water and generate saturated, vapor-laden hot air. This energy is, by definition, transferred to the condenser's surface during condensation. With most forms of solar still, this heat of condensation is ejected from the system as waste heat. The challenge still existing in the field today, is to achieve
 * 1) the optimum temperature difference between the solar generated vapor and the sea water cooled condenser.
 * 2) maximal reuse of the energy of condensation.
 * 3) minimising the asset investment.