Solar distillation
Practical Action
collection to supplement the solar still output.
Greenhouse-solar still. The roof of a greenhouse can be used as the cover of a still.
Supplementary heating. Waste heat from an engine or the condenser of a refrigerator can be
used as an additional energy input.
Output of a solar still
An approximate method of estimating the output of a solar still is given by:
Q=ExGxA
L
where:
Q = daily output of distilled water (litres/day)
E = overall efficiency
G = daily global solar irradiation (MJ/m²)
L = The latent heat of vaportisation of water = 2.26 MJ/kg
A = aperture area of the still ie, the plan areas for a simple basin still (m²)
In a typical country the average, daily, global solar irradiation is typically 18.0 MJ/m² (5 kWh/m²).
A simple basin still operates at an overall efficiency of about 30%. Hence the output per square
metre of area is:
daily output = 0.30 x 18.0 x 1
2.26
= 2.34 litres (per square metre)
Performance varies between tropical locations but not significantly. An average output of 2.3 to
3.0 litres/m²/day is typical, the yearly output of a solar still is often therefore referred to as
approximately one cubic metre per square metre, 1 m³/m²/year.
Experience
Despite a proliferation of more sophisticated designs such as TERI’s solar desalination unit with
offset collectors, the single-basin still has the best track record in the field. Hundreds of smaller
stills are operating, in Africa and India.
The cost of pure water produced depends on:
the cost of making the still
the cost of the land
the life of the still
operating costs
cost of the feed water
the discount rate adopted
the amount of water produced.
An example of costs of a solar still in India is Rs. 28000 for 15 m² approximately $575.00 for 15
m², or $38.3 per m². The price of land will normally be a small proportion of this in rural areas, but
may be prohibitive in towns and cities. (See the TNAU website for details)
The life of a glass still is usually taken as 20 to 30 years but operating costs can be large especially
to replace broken glass.
It is important that stills are regularly inspected and maintained to retain their efficiency and
reduce deterioration. Damage, such as breakage of the collector plate, needs to be rectified.
Some companies, e.g. in the United States, Russia, India and South Africa, sell solar stills, largely
for household use to produce up to about 50 litres per day.
Would a solar still suit your needs?
People need 1 or 2 litres of drinking water a day to live. The minimum requirement for normal life
in developing countries (which includes cooking, cleaning and washing clothes) is 20 litres per day
(in the industrialised countries 200 to 400 litres per day is typical). Yet some functions can be
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