EVAPORATIVE
COOLING
Background
Much of the post-harvest loss of fruits and vegetables in developing countries is due to the
lack of proper storage facilities. While refrigerated cool stores are the best method of
preserving fruits and vegetables they are expensive to buy and run. Consequently, in
developing countries there is an interest in simple low-cost alternatives, many of which depend
on evaporative cooling which is simple and does not require any external power supply.
The basic principle relies on cooling by evaporation. When water evaporates it draws energy
from its surroundings which produces a considerable cooling effect. Evaporative cooling
occurs when air, that is not too humid, passes over a wet surface; the faster the rate of
evaporation the greater the cooling. The efficiency of an evaporative cooler depends on the
humidity of the surrounding air. Very dry air can absorb a lot of moisture so greater cooling
occurs. In the extreme case of air that is totally saturated with water, no evaporation can take
place and no cooling occurs.
Generally, an evaporative cooler is made of a porous material that is fed with water. Hot dry air
is drawn over the material. The water evaporates into the air raising its humidity and at the
same time reducing the temperature of the air. There are many different styles of evaporative
coolers. The design will depend on the materials available and the users requirements. Some
examples of evaporative cooling designs are described below.
Pot designs
These are simple designs of evaporative coolers that can be used in the home. The basic
design consists of a storage pot placed inside a bigger pot that holds water. The inner pot
stores food that is kept cool.
One adaptation on the basic double
pot design is the Janata cooler,
developed by the Food & Nutrition
Board of India. A storage pot is
placed in an earthenware bowl
containing water. The pot is then
covered with a damp cloth that is
dipped into the reservoir of water.
Water drawn up the cloth evaporates
keeping the storage pot cool. The
bowl is also placed on wet sand, to
isolate the pot from the hot ground.
Wet cloth
Storage pot
Earthenware bowl
filled with water
Mohammed Bah Abba a teacher in
Nigeria, developed a small-scale
storage “pot-in-pot” system that
Wet sand
Figure 1: A Janata Cooler
Illustration: Practical Action / Neil Noble.
uses two pots of slightly different size. The smaller pot is placed inside the larger pot and the
gap between the two pots is filled with sand. Mohammed won the Rolex 200 Award for
Enterprise for his design. Further details are in Number 4 Volume 27 Oct/ Dec 2000 of
Appropriate Technology.
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