Solar distillation
Practical Action
Some problems with solar stills which would reduce their efficiency include:-
Poor fitting and joints, which increase colder air flow from outside into the still
Cracking, breakage or scratches on glass, which reduce solar transmission or let in air
Growth of algae and deposition of dust, bird droppings, etc. To avoid this the stills need to
be cleaned regularly every few days
Damage over time to the blackened absorbing surface
Accumulation of salt on the bottom, which needs to be removed periodically
The saline water in the still is too deep, or dries out. The depth needs to be maintained at
around 20 mm
The cover can be either glass or plastic. Glass is preferable to plastic because most plastic
degrades in the long term due to ultra violet light from sunlight and because it is more difficult for
water to condense onto it. Tempered low-iron glass is the best material to use because it is highly
transparent and not easily damaged (Scharl & Harrs, 1993). However, if this is too expensive or
unavailable, normal window glass can be used. This has to be 4 mm think or more to reduce
breakages. Plastic (such as polyethylene) can be used for short-term use.
Stills with a single sloping cover with the back made from an insulating material do not suffer from
a very low angle cover plate at the back reflecting sunlight and thus reducing efficiency. It is
important for greater efficiency that the water condenses on the plate as a film rather than as
droplets, which tend to drop back into the saline water. For this reason the plate is set at an angle
of 10 to 20º. The condensate film is then likely to run down the plate and into the run off channel.
Brick, sand concrete or waterproofed concrete can be used for the basin of a long-life still if it is to
be manufactured on-site, but for factory-manufactured stills, prefabricated ferro-concrete can be
used. Moulding of stills from fibreglass was tried in Botswana (Yates, Woto & Tlhage, 1990) but in
this case was more expensive than a brick still and more difficult to insulate sufficiently, but has
the advantage of the stills being transportable.
By placing a fan in the still it is possible to increase evaporation rates. However, the increase is
not large and there is also the extra cost and complication of including and powering a fan in what
is essentially quite a simple piece of equipment. Fan assisted solar desalination would only really
be useful if a particular level of output is needed but the area occupied by the stills is restricted, as
fan assistance can enable the area occupied by a still to be reduced for a given output.
The Mexican still
In the Mexican still two stills such as the above are fixed together to form a triangular tent shape.
The glass plates can be supported from below at the apex where they join, but if they are not and
just lean against each other, fixed with sealant, this increases the fragility of the still and limits the
area even further of each of the glass plates.
The Brace Research Institute still
This is essentially a still as shown in the above drawing (Figure 1). However the stills are placed
next to each other over the width of say 10 metres of the distillation plant. Lengthwise, the unit
such as shown is built over a considerable distance, such as 15 metres. Glass plates are placed
along the length of the still and simply joined with sealant. Units of this size also have two small
weirs lengthwise to encourage saline water to flow along the full length of the still. A project of this
type was set up by the Brace Research Institute, McGill University, Canada in Haiti. The scale of
the unit requires caretakers to be trained in the maintenance of it, and maintenance requirements
are quite considerable.
Multiple-effect basin stills have two or more compartments. The condensing surface of the lower
compartment is the floor of the upper compartment. The heat given off by the condensing vapour
provides energy to vaporize the feed water above. Efficiency is therefore greater than for a single-
basin still typically being 35% or more but the cost and complexity are correspondingly higher.
Wick stills - In a wick still, the feed water flows slowly through a porous, radiation-absorbing pad
(the wick). Two advantages are claimed over basin stills. First, the wick can be tilted so that the
feed water presents a better angle to the sun (reducing reflection and presenting a large effective
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