In many developing countries there is an important need for clean, pure drinking water. Often the water sources contain dissolved salts or harmful bacteria and therefore cannot be used for drinking. This means that there are many coastal locations with a lot of water that is unfortunately un-available. Pure water is also useful for batteries and in hospitals and schools.
How can we over come this obstacle you ask? Well one way of doing this is distillation. It is one of many processes that can be used for water purification. This method requires an energy in-put. Heat, from solar radiation can be this source of energy. This process involves the water being evaporated; this separates the water vapours from dissolved matter, which condenses as pure water.
Solar water distillation is a very old technology, with a very long history. Installations were built over 2000 years ago; however they were used to produce salt rather than drinking water. Evidence of the use of stills dates back to the sixteenth century, and in 1872 a large scale solar still was built to supply a mining community in Chile with drinking water. Mass production happened for the first time during the Second World War when 200,000 inflatable plastic stills were made to be kept in life-crafts for the US Navy.
There are quite a lot of other approaches to water purification, such as desalination, such as photovoltaic powered reverse-osmosis.
Energy requirements for water distillation
The energy required to evaporate water is the latent heat of vaporisation of water. This has a value of 2260 kilojoules per kilogram (kJ/kg). This means that to produce 1 litre (i.e. 1kg since the density of water is 1kg/litre) of pure water by distilling brackish water requires a heat input of 2260kJ. This does not allow for the efficiency of the heating method, which will be less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.
It should be noted that, although 2260kJ/kg is required to evaporate water, to pump a kg of water through 20m head requires only 0.2kJ/kg. Distillation is therefore normally considered only where there is no local source of fresh water that can be easily pumped or lifted.
How a simple solar still works.
The main features of operation are the same for all solar stills. The solar radiation is transmitted through the glass cover and is absorbed as heat by a black surface in contact with the water to be distilled. The water is then heated and gives off water vapour. The water vapour condenses on the glass and runs down into a gutter from where it is fed into a storage tank.
Design objectives for an efficient solar still for high efficiency the solar still should maintain:
A high feed (undistilled) water temperature
A large temperature difference between feed water and condensing surface
Low vapour leakage.
A high feed water temperature can be achieved if:
A high proportion of incoming radiation is absorbed by the feed water as heat. Hence low absorption glazing and a good radiation absorbing surface are required
heat losses from the floor and walls are kept low
the water is shallow so there is not so much to heat.
A large temperature difference can be achieved if:
the condensing surface absorbs little or none of the incoming radiation
condensing water dissipates heat which must be removed rapidly from the condensing surface by, for example, a second flow of water or air, or by condensing at night.
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Posted under Climate, How To's & Guides, Renewable Energy
This post was written by Xavier Tikadar on July 30, 2008