SOLAR DISTILLATION
Introduction
Solar distillation is a relatively simple treatment of brackish (i.e. contain dissolved salts) water
supplies. Distillation is one of many processes that can be used for water purification and can use
any heating source. Solar energy is a low tech option. In this process, water is evaporated, using the
energy of the sun then the vapour condenses as pure water.This process removes salts and other
impurities.
Solar distillation is used to produce drinking water or to produce pure water for lead acid batteries,
laboratories, hospitals and in producing commercial products such as rose water. It is
recommended that drinking water has 100 to 1000 mg/l of salt to maintain electrolyte levels and
for taste. Some saline water may need to be added to the distilled water for acceptable drinking
water.
Solar water distillation is a very old technology. An early large-scale solar still was built in 1872 to
supply a mining community in Chile with drinking water. It has been used for emergency situations
including the introduction of inflatable stills for life boats by the navy.
There are a number of other approaches to desalination, such as photovoltaic powered reverse-
osmosis, for which small-scale commercially available equipment is available; solar distillation has
to be compared with these options to determine its appropriateness to any situation. If treatment of
polluted water is required rather than desalination, slow sand filtration is a low cost option.
Energy requirements for water distillation
The energy required to evaporate water, called the latent heat of vaporisation of water, is 2.26
Megajouls per kilogram (MJ/kg). This means that to produce 1 litre (i.e. 1 kg as the density of
water is 1 kg/litre) of pure water by distilling brackish water requires a heat input of 2.26 MJ. This
does not allow for the efficiency of the system used 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 2.26 MJ/kg or 2260 kJ/kg is required to evaporate water, to
pump water through 20 m head requires only 0.2 kJ/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
Rainwater
catchment
channel
Condensate runoff
channel to outlet
Glass or plastic plate
Saline solution
Base incorporating insulation
Blackened surface
Overflow
Figure 1: Shows a single-basin still. Illustration: Martin Bounds.
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