Solar thermal energy
Practical Action
Solar drying
Controlled drying is required for various crops and products, such as grain, coffee, tobacco,
fruits vegetables and fish. Their quality can be enhanced if the drying is properly carried out.
Solar thermal technology can be used to assist with the drying of such products. The main
principle of operation is to raise the heat of the product, which is usually held within a
compartment or box, while at the same time passing air through the compartment to remove
moisture. The flow of air is often promoted using the ‘stack’ effect which takes advantage of the
fact that hot air rises and can therefore be drawn upwards
through a chimney, while drawing in cooler air from
below. Alternatively a fan can be used. The size and
shape of the compartment varies depending on the
product and the scale of the drying system. Large systems
can use large barns while smaller systems may have a few
trays in a small wooden housing.
Solar crop drying technologies can help reduce
environmental degradation caused by the use of fuel wood
or fossil fuels for crop drying and can also help to reduce
the costs associated with these fuels and hence the cost
of the product. Helping to improve and protect crops also
has beneficial effects on health and nutrition.
Solar wood kiln have been developed to season timber.
The Solar kiln is constructed using a wooden frame with
a glass roof placed over the top. The principles are
similar to that of a green house design. The simplest
have been modified greenhouses designed to take large
pieces of wood for drying. Air is passed through the kiln
to remove the moisture from the air and the cycle
continues.
Figure 7: Coconut drying in
Bangladesh.
Photo: Practical Action / Neil Cooper.
Solar cooking
Solar cookers fall into two main categories – solar ovens and direct solar concentrators. The
basic design for a solar oven is that of a box with a glass cover. The box is lined with insulation
and a reflective surface is applied to concentrate the heat onto the pots. The other approach is
to reflect the suns rays onto a put, often with a parabolic dish. The pots can be painted black
to help with heat absorption.
On a domestic scale the cookers have limitations in terms of only being effective during hours of
strong sunlight. Another cooking stove is usually required for the periods when there is cloud or
during the morning and evening hours. Cooking time is often a lot slower than conventional
stoves and cooking practice has to be adapted to suit. However, the main advantage to solar
cookers is that wood does not need to be purchased or collected, which is often a very time
consuming activity for women.
Many variations of solar cooker have been developed from the very basic reflective cardboard
sheet box to the very sophisticated large-scale institutional and commercial solar cookers now
being used in India.
Desalination /distillation
Basic solar stills can be used to purify water in remote regions where contaminated water is
present. They can be used to remove impurities such as fluoride and salts to produce drinking
water.
The basic still is made of a glass or transparent plastic cover and a shallow tray of water which
has a black backing to trap energy. When the sun heats the water up within the still water
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