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.
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.
Mohammed Bah Abba a teacher in Nigeria, developed a small-scale storage “pot-in-pot” system that 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.
In Sudan, Practical Action and the Woman’s Association for Earthenware Manufacturing have been experimenting with the storage design of Mohammed Bah Abba. The aim of the experimentation was to discover how effective and economical the Zeer storage is in conserving foods. Zeer is the Arabic name for the large pots used. The results are shown in the following table. (click image to enlarge)
As a result of the tests, the Woman’s Association for Earthenware Manufacturing started to produce and market the pots specifically for food preservation.
A Bamboo Cooler
The base of the cooler is made from a large diameter tray that contains water. Bricks are placed within this tray and an open weave cylinder of bamboo or similar material is placed on top of the bricks. Hessian cloth is wrapped around the bamboo frame, ensuring that the cloth is dipping into the water to allow water to be drawn up the cylinder’s wall. Food is kept in the cylinder with a lid placed on the top.
An Almirah Cooler
The Almirah is a more sophisticated cooler that has a wooden frame covered in cloth. There is a water tray at the base and on top of the frame into which the cloth dips, thus keeping it wet. A hinged door and internal shelves allow easy access to the stored produce.
A Charcoal Cooler
The charcoal cooler is made from an open timber frame of approximately 50mm x 25mm (2″ x 1″) in section. The door is made by simply hinging one side of the frame. The wooden frame is covered in mesh, inside and out, leaving a 25mm (1″) cavity which is filled with pieces of charcoal. The charcoal is sprayed with water, and when wet provides evaporative cooling. The framework is mounted outside the house on a pole with a metal cone to deter rats and a good coating of grease to prevent ants getting to the food.
The top is usually solid and thatched, with an overhang to deter flying insects.
All cooling chambers should be placed in a shady position, and exposure to the wind will help the cooling effect. Airflows can be artificially created through the use of a chimney. For example using a mini electric fan or an oil lamp to create airflows through the chimney – the resulting draft draws cooler air into the cabinet situated below the chimney. The Bhartya cool cabinet uses this principle to keep its contents cool. Wire mesh shelves and holes in the bottom of the raised cabinet ensure the free movement of air passing over the stored food.