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close this book Boiling Point No. 36 - November 1995
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View the document Parameters for a Solar Cooker Programme by Richard C Wareham of the Sunstove Organisation
View the document The Sunstove Solar Box Cooker
View the document Sunstoves in the Republic of South Africa
View the document Gaining Ground in Solar Box Cooking in Kenya
View the document Solar Cookers - A Cause Worth Promoting
View the document Free Energy from the Sun
View the document A Solar Box Cooker with a Reflecting Lining
View the document The Solar Puddle - A New Water Pasteurization Technique
View the document Renewable Energy - A World Bank View

Free Energy from the Sun

by James Muriithi, Kenya Ministry of Energy/G7Z-SEP

Much of the earth's heat energy comes from the sun. In only one minute, more heat and light reach the earth from the sun than man can produce in a year. In spite of this energy being free, mankind has not made much use of it for his own purposes and most of the energy consumed in the Third World countries still comes from wood.

In the rural areas of Kenya 93 per cent of the energy requirements are met by wood, mainly in its primary form - firewood. Up to 83 per cent is used for cooking, 11 per cent for heating and 6 per cent for lighting. Although firewood is free in monetary terms in most rural areas, in other respects it is certainly not, as its continued use has grave consequences for several aspects of the environment.

In view of the increasing shortage of firewood for domestic cooking, the Ministry of Energy/GTZSEP programme and some local NGOs such as Trans-World Radio Nairobi and CITC-Kapsabet are exploring the possibilities of using solar energy for cooking. They have concentrated on the design and production of solar box cookers. Many such cookers have been designed and tried out in many countries over the last 30 or more years and seem to provide the best prospect of being accepted by users. A solar box (see Figure 1) is basically a box comprising of:

• an outer wooden box (wood is strong, easy to work and a poor conductor of heat)

• an inner box of sheet metal, without a lid and with the inside painted black/gray for maximum absorption of solar radiation

• insulating material such as cotton wool, straw, saw-dust etc, between the two boxes

• double glass lid (to reduce loss of heat by conduction)

• a reflecting sheet attached to the lid to direct the sun's rays into the box and on to the cooking pots

The sun's rays, direct and from the reflector, pass through the glass into the cooker and are converted into heat. The dark interior of the box absorbs the sun's energy while the glass prevents the heat rays from escaping (the greenhouse effect) and the insulation minimizes heat lost by conduction. The air inside the box may reach temperatures of up to 150-200°C. The food to be cooked is placed in darkened pots with dark lids.The solar box is directed at the sun and the reflecting lid is adjusted to direct the sun's rays into the cooker as the sun moves.


A pot of maize and beans, soaked overnight and cooked when there is maximum sunlight, might take about five hours to cook. Water placed in the cooker can reach average temperatures of 60 to 70°C and sometimes 100°C. Depending on the size and materials used, the costs of the solar cookers range (in 1995) from KSh 2,000 to 3,000 (US$33 to 50).

The advantages of the solar box cooker include:

• uses free energy

• smoke free

• very little or no attendance to food when cooking as it will not burn or stick to the pan and there is no need to stir

• food will remain hot when cooking is finished

The disadvantages include:

• cannot cook during the night

• cannot cook when the weather is not favourable i.e. when it is raining, cloudy etc

• takes a long time to cook a meal double the time of other cookers

• cooking has to be done outside and the solar cooker has to be moved now and then to follow the sun

• one person cannot handle the big size cookers

• cannot be used for roasting maize cobs

• cannot be used for space heating in the house

Up to the present, solar cookers have not been widely adopted in Kenya for the following reasons:

• the price of KSh 2,000 to 3,000 is too high when compared with the KCJ at about KSh 100

• the cookers are not strongly promoted

• people are disillusioned because they take too long to prepare a meal

• people are used to waiting by the stove whilst the meal is being prepared. They need to recognize that if they want to have the meal at 1.00 pm they should put the food into the solar box at about 8.00 am and then attend to their other activities.

• very few organizations or individuals are producing solar box cookers

Further development work and testing is being done on our solar box cookers to make them more suitable and affordable to our rural households.

Editorial note.

We look forward to receiving further information about the success of the project.

Kenya Ministry of Energy/GTZ-SEP,

PO Box 30582, Nairobi, Kenya