There is a great variety of solar vapour cycle refrigerators, including some
straightforward designs working on a 24 h cycle, but their use is not
widespread.
A better way to cool buildings in hot climates is again to use passive
designs (cf. Section 6.3). These either harness the natural flows of cooling
air (in humid areas), store coolness from night time or winter (in dry areas)
or in some cases automatically generate a cooling flow by convection.
A comprehensive account of the relevant design principles, with examples,
is given in the Manual of Tropical Building (Koenigsberger et al. 1974).
For cooling foodstuffs etc. offgrid, at least in small quantities, commercial
compression refrigerators and freezers are available, powered by solar cells
(see Section 7.9). At present these are economically attractive only in areas
remote from conventional electricity supplies.
6.6 Water desalination
To support a community in arid or desert conditions, potable, i.e. fit to
drink, water must be supplied for domestic use, and other water for crops
and general purposes. Many desert regions (e.g. central Australia) have
regions of salt or brackish water underground, and it is usually much
cheaper to purify this water than to transport fresh water from afar. Since
deserts usually have large insolation, it is reasonable to use solar energy to
perform this purification by distillation.
The most straightforward approach is to use a basin solar still
(Figure 6.8). This is an internally blackened basin containing a shallow
depth of impure water. Over this is a transparent, vapour-tight cover that
completely encloses the space above the basin. The cover is sloped towards
the collection channel. In operation, solar radiation passes through the cover
and warms the water, some of which then evaporates. The water vapour
diffuses and moves convectively upwards, where it condenses on the cooler
cover. The condensed drops of water then slide down the cover into the
catchment trough.
Example 6.4 shows that substantial areas of glass are required to produce
enough fresh water for even a small community.