Solar distillation
Practical Action
The main features are the same for all solar stills. The solar radiation is transmitted through the
glass or plastic cover and captured by a black surface at the bottom of the still. A shallow layer of
water absorbs the heat which then produces vapour within the chamber of the still. This layer
should be 20 mm deep for best performance.
The vapour condenses on the glass cover, which is at a lower temperature because it is in contact
with the ambient air, and runs down into a gutter from where it is fed to 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.
Stills
Single-basin stills have been much studied and their behaviour is well understood. The efficiency
of solar stills which are well-constructed and maintained is about 50% although typical efficiencies
can be 25%. Daily output as a function of solar irradiation is greatest in the early evening when
the feed water is still hot but when outside temperatures are falling. At very high air temperatures
such as over 45ºC, the plate can become too warm and condensation on it can become
problematic, leading to loss of efficiency.
a) Basin type solar still (Battelle
design)
b) Inflated plastic cover design
c) V-shape plastic cover design
d) Inclined glass/stretched plastic
cover design
e) CSIRO – Australia design
Figure 2: Examples of solar sill designs. Illustration: Martin Bounds.
2