Solar distillation
Practical Action
area). Second, less feed water is in the still at any time and so the water is heated more quickly
and to a higher temperature. Simple wick stills are more efficient than basin stills and some
designs are claimed to cost less than a basin still of the same output.
Some designs have been developed which incorporate absorbent or film-type materials to increase
the surface area of evaporation – e.g. an article on the design developed by G.N. Tiwari of the
Indian Institute of Technology, New Delhi, was published in New Scientist.
Use of Reflector
The inside walls of the still can
incorporate a reflective coating,
such as aluminium foil, to increase
the reflection of heat energy onto
the evaporating water. It is not
known how far this has helped to
improve the efficiency of the still.
Air flow across
the condenser
Still
Sunlight
Inverted Absorber Solar Stills
Heat is absorbed from the
underside of the still to improve
efficiency. This allows that
condenser plate and the collector
plate to be separate. There are
several designs of inverted
absorber from the fairly simple to
more complex designs.
Glass cover
Secondary
reflector
Collector
Figure 1: An inverted absorber solar still. Illustration:
Neil Noble / Practical Action.
Spherical Still
In a design developed by the Thermal and Solar Laboratory at Claude Bernard University, Lyons,
France, a trough, where the saline water is placed, is positioned in the centre of a hollow
transparent plastic sphere. Distillate water condenses on the inside surface of the sphere and is
collected by a mechanical windscreen type wiper blade which forces the condensed water to fall to
the bottom of the sphere to be collected. There seems to be a small improvement in efficiency
compared with a conventional solar still, but the greater cost of this still might cancel out this
advantage. [World Water]
Inclined Stills
The aim of inclining a still is to increase the solar radiation, by catching it head on, rather than at
an angle as with stills which lie flat. To do this constantly, as the sun rises and sets, would need
someone to monitor the sun and turn the unit regularly, or a sophisticated automatic tracking and
turning mechanism.
Condensate Heat Recovery
Heat recovery from the energy given out when water vapour condenses has generally not been
attempted with small-scale solar distillation, unlike with larger-scale systems. It is known that the
Ben Gurion Institute, and more latterly the Technion Institute in Israel has undertaken some
experiments with heat recovery. In the simplest system, saline water is made to flow over the
outside of the condensation plate before entering the still, but then this would reduce the amount
of solar radiation passing through the plate. There may be scope for further research to overcome
current difficulties with attempting heat recovery from solar distillation.
Emergency still - To provide emergency drinking water on land, a very simple still can be made. It
makes use of the moisture in the earth. All that is required is a plastic cover, a bowl or bucket, and
a pebble.
Hybrid designs - There are a number of ways in which solar stills can usefully be combined with
another function of technology. Three examples are given:
Rainwater collection. By adding an external gutter, the still cover can be used for rainwater
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