We are all aware of solar distillation where the sun evaporates the water from lakes, rivers, oceans and other surface waters leaving salts and other minerals behind. This evaporated water eventually reaches the upper atmosphere where it recondenses as clouds and precipitates back to the land. This is the basic principle behind the use of solar energy for distillation.
As mentioned in the introduction, one of the limiting sources in desalination is the energy. What better energy source can be utulized than solar radiation? It is an intermittant, low-intensity, and very abundant energy source. Nearly 65 Btu are recieved by one square mile of land on a sunny, summer day. This amount is roughly equivalent to 15,000 barrels of petroleum. Heat or energy can be generated from solar energy to be used to operate a desalination process. It can also be used directly to distil water in equipment which both absorbs solar energy and serves as a distillation process.
The solar distillation method is fairly simple and is pretty much self-operating. Saline water is supplied either continuously or intermittantly to a pool ranging in depths of approximately 1 inch to 1 foot. The bottom of the pool has a black surface which absorbs solar energy. The discardes salts exit through a drain. A transparent cover composed of glass sheets or plasic film is supported above. These are arranged so that the surfaces slope downward into small troughs at their lower edges. These troughs are connected to channels or piping which transport the condensate to storage.
A majority of the solar energy is absorbed in the basin bottom with a small amount being absorbed by the salt water itself. Heat is absorbed by the salt water from the basin bottom, raising the temperature and vapor pressure of the water. Partial vaporization occurs and these vapors are transported upward to the transparent cover by convection covers. The cover is generally 10 to 30 degrees F cooler than the vapors and therefore condensation occurs. The condensation flows down the slope and collects in the troughs. The heat of condensation is transported through the cover and into the atmosphere. Only about half of the original feed is evaporated to prevent salt deposition onthe bottom of the tank. The rest goes to waste.