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Solar cooking
by Philip King

Our energy comes from the sun, but we mostly use it indirectly. Fossil fuels are made from plants and animals which millions of years ago used the sun's energy to grow; modern bio-fuels are obtained more directly from growing plants; wind energy originates partly from the action of the sun warming the oceans and atmosphere; and solar panels create electricity from sunbeams falling on photovoltaic cells.

We can also use the sun's power more directly and, although we sometimes get precious little sunshine in south London, we can still use a good sunny day to cook in a solar cooker. People in sunnier places can use solar cooking as a real practical alternative to other fuels, and in some developing countries solar cooking can help reduce dependency on dwindling supplies of costly and dangerous biomass fuels and kerosene. All that is needed is a little home-made equipment and a clear blue sky.

There are many different designs of solar cooker, but all follow a few basic principles: capture sunlight and direct it to where it is needed, usually by means of some sort of reflector; absorb the sunlight to create heat, usually with dark cooking pots; and retain the heat so that it builds up to the required temperature, usually by exploiting the greenhouse effect or insulation. These principles are used in different ways in different cooker types, depending on what type of cooking is to be done.

There are also some limitations which need to be considered, principally that solar cooking requires strong sunshine, meaning it cannot be used reliably every day, and in the UK it is only practical from May to September (unless huge reflectors are used). Other limitations are that it can be slow and that the best time for cooking is in the morning, not evening. That said, it is still practical as an occasional cooking method as well as being fun and educational.

The principal types of solar cooker are box cooker, panel reflector and parabolic reflector, and each have their strengths and weaknesses. Box cookers use reflectors to direct sunlight into an insulated box containing the cooking pots. The opening of the box is covered with glass, allowing sunlight to enter and preventing most of the heat from escaping. Heat builds up inside the box, cooking like in an ordinary oven. These cookers can be very effective, even in a UK summer. There are commercially made box cookers which are very efficient, but it is also possible to make a usable box cooker from simple materials such as cardboard and cooking foil. Panel reflector cookers use flat panels to direct sunlight onto a cooking pot, which is usually enclosed in a transparent cover providing some sort of insulation. The pot should be black and non-reflective to absorb sunlight. Examples of this cooker include the Indian Parvati, in which a shallow cone of panels directs sunlight onto a central pot, and the simpler CookIt cooker developed for use in rural Africa. Both are easy to make from cardboard and foil and the Parvati is especially usable in an English summer. Parabolic reflectors are curved dishes which concentrate sunlight to a focal point, providing the most powerful heat source of any solar cooker. A parabolic cooker used in strong sunshine can work like a grill or barbecue or can be used to heat a hot-plate to be used for frying. There are several commercial versions of this type but they are harder to make at home than the other types. They are also potentially more dangerous to use than the others due to the concentrated sunlight being reflected.

All types need to be turned to follow the sun. The box cooker is least sensitive, needing to be turned only occasionally; the parabolic is the most sensitive, requiring constant adjustment to keep the focus on the cooking vessel. Advantages of solar cooking include food that tastes better and uses less water because cooking is often done in containers or pots which are tightly closed, but a disadvantage is that it's not possible to stir or add ingredients without losing precious heat. Once in the pot, food has to stay untouched until it is taken out to be eaten, meaning that recipes need to be chosen carefully. Solar cooking is quite different from using a kitchen stove, so it is best to experiment slowly and try to get a feel for how to cook with the sun. Someone who is doing a lot of this, although with better sunshine than we have in London, is Arizona's Solar Oven Chef, who has vowed to solar-cook a meal every sunny day.

Philip has been a regular contributor to the Urban Green Fair, Brixton. Contact the author regarding solar cooking: 07905 863 263. More about building and using solar cookers here and Solar Oven Chef blog

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