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Sunseed solar cooker - Tanzania trials 1995

Summarised from an article for Boiling Point by G. Beaumont Sunseed Desert Technology, Apdo. 9, 04270 Sorbas, Almeria, Espana

Designed at Sunseed Desert Technology, a British registered, charitable research centre, this ultralow cost solar cooker was built at MVUMI, Tanzania, largely from mud and straw (adobe). It is claimed to be the first truly affordable yet robust, family sized cooker designed for construction and maintenance by the users.

Intended for on-site building each cooker costs about £8 equivalent (depending on local prices). Of the 'solar cooker' type, it uses the greenhouse effect to trap solar heat inside a glazed, insulated box. Unlike other box cookers, however, it is a fixed installation with sides facing due east/west; the independent reflector is adjusted to maximise sunlight entry.

As the cookers are fairly labour intensive to build, only 10 have been made, usually in places where they would be used daily.

The cooker is a shallow, insulated, adobe box with a ground level side-opening door and a fixed, nearly horizontal, transparent cover of plastic or glass. Reflective inner walls direct sunlight onto a black metal baseplate, and extra sunlight is captured by a steel or aluminium moveable reflector panel controlled by guy ropes. Under the baseplate an adobe slab stores heat, for cloudy periods. It is capable of cooking for 10 people in clear sky conditions, assuming a daily individual requirement of 0.4 kg of dry food. The interior space is 93cm square and 20cm deep, which allows for a pot 35cm diameter x 15cm deep.

Materials currently imported (polyester window and metallized polyester reflector) cost about £4 per cooker and need yearly replacement. The cooker accepts the cheap aluminium cooking pots ('isufurias') commonly available in east Africa. Having a fixed position, it is most suitable for locations within 10 degrees of the equator as efficient tracking of the winter sun by the reflector is not convenient elsewhere.

We revised cooking procedures to take account of the local water quality. As in much of Africa. water is often contaminated with salmonella and similar species' and food once wetted should be heated quickly to 70°C, to avoid breeding conditions. The cookbook instructs that water should be preheated for I hour in the cooker before the food is put in the water; this heats the food


The Sunseed Adobe Solar Cooker quickly and should avoid problems.

Construction Details

The base is supported over an insulating air space by crossed sticks, topped with thin layers of straw, aluminium foil and polythene. The cooker walls are insulated by a 10cm layer of tightly bound straw, wrapped in polythene to prevent wetting by steam from inside the cooker, or occasional rainfall.

The large, erect reflector is made from a thin (25 micron) sheet of metallized polyester film stretched inside a rectangular wood or cane frame braced against both twist and bending. The reflector also appears flimsy, and is exposed to the wind.

In clear conditions the cooker can bring a 4 litre load of water to cooking temp. (80°C) in 60 to 70 minutes, and cooking can be maintained to within an hour of sunset. In mid December 1994 a prototype brought 2 litres of water to over 80°C in Spain, with a maximum sun angle of 30 degrees and ambient temperature of 25°C.

Follow-up report (Visit November-December 1995)

Following pilot trials of the Sunseed ultra low cost solar cookers in Tanzania, their condition was checked to see how many of the cookers survived and-were being used for cooking.

Of the 10 cookers started, 5 were completed by the Sunseed team, I lacked only the reflector, and I lacked only the window, as the team left. 3 were completed in terms of the ground structure and window after the team left, and 3 never had reflectors made up. 1 had made little progress and no further work done at all in the absence of the team. Overall 9 cookers were completed in terms of ground structure; of these 6 survived with windows intact and in such condition that cooking could take place.


Wind, cows & children

Five cookers were still in use at the time of the visits, and 3 others had been in use for heating water until the windows were broken (between Sept and Nov). Of the 5 still in use, 2 had replacement windows following incidents involving livestock. The repairs had been carried out effectively, and in one cases the replacement window was made from two pieces joined with silicon sealant.

Six reflectors had been completed, of which 5 had been in regular use. 4 of these survived in good condition at the visits, and one had the reflective panel destroyed after being caught in a gust when left out overnight. (The unused reflector is in storage). I cooker was essentially complete, but was never put into use, due to lack of black paint for the base plates.

Conclusions

Overall, the survival rate of the cookers was felt to be quite respectable for such a cheap design on its first outing in the 'real world'. It is now clear that completely inexperienced rural people can use and maintain the cookers successfully if they are well motivated and trained. Cookers should not be regarded as complete until there is some kind of protection around them against animals and children. We tried to ask as many people as possible about fuel saving, but it was very difficult to get reliable answers.

Given these factors, it seems that the success rate can be very high' and each cooker may give significant fuel saving of the order of I tonne per year. Fully demonstrated cookers have been used routinely and successfully for a variety of staple dishes. They appear to be valued both for their convenience and economy, and we hope the design will soon be taken up by other agencies for larger scale dissemination.

About Sunseed

Set up in 1986 near Almeria. southern Spain, Sunseed Desert Technology (SDT) is the research and demonstration centre of the Sunseed Trust. SDT researches into solar cooker technology as part of an overall strategy to combat desertification, and sustainably improve the living standard of poor people in decertified areas. A solar cooker was designed and tested, see Results of the Second Comparative Solar Cooker Test in Almeria.

Synopsis, Route d'Olmet, F34700 Lodeve, France, 1994. Field trials of the stove were carried out in 1995.