Tinkering with the cook stove

It has been estimated that 100 million people in developing countries unable to obtain sufficient fuel-wood to meet their needs and that a further one billion are affected by occasional shortages. In the Sudan, where I recently completed some work on development and testing of cook-stoves, there are few places in the northern part of the country where tree stocks have not been drastically depleted. Annual fuel consumption 1981 was about 13 million tons, per cent of this going to the domes. sector. mainly for cooking (there little need for home heating in the Sudan). Only about 10 of every 100 hectares of timber cut are replaced. High population growth rates and the clearing of land for agriculture have meant that the Centre, north, and east the country are increasingly dependent on southern regions for their supply of fuelwood. The long distances involved have meant that charcoal. being easier to transport than wood, has become a dominant fuel. Per caput consumption averages about 1 kg annually; that of firewood is cubic metre. Even so. shortages inevitable, and fuel prices double during the rainy season, when roads become impassable. Low-income families in semi-urban areas may spend as much as 20 per cent of their income on fuel, with the expected effects on their standard of living. Poorer families often cook only the midday meal, reheating food for supper and breakfast the following morning. In traditional families where women eat separately, a shortage of fuel may mean that they wild not cook again for themselves and their children, but will simply eat leftovers from the meals prepared for the men. Several households sometimes prepare food together in order to economize on fuel.

Recently, substantial amounts of research have been undertaken to find means of reducing demand for fuelwood in developing countries. Avenues explored have included the development of solar cookers, the production of briquettes from agricultural wastes' and improvement in the efficiency of traditional stoves.

Various types of solar cookers have been devised, but their slow cooking time is a major technical problem for most. To obtain heating comparable to that from a gas ring requires from two to two and a half square metres of collection area, which is impractical for daily use. The high cost of durable materials also makes them prohibitive for the rural poor, although they may have potential as a complementary method for cooking lighter foods in the semiurban areas.

The production of briquettes from agricultural waste is a relatively unexplored option for meeting energy needs for cooking. The briquetting process includes collection of the biomass, drying, particle-size reduction and homogenization, addition of a binding agent such as manure or ash. pressing, and cooling. Pressing can be undertaken by motor or manually. Pulverized biomass is fed through a nozzle under pressure so that the briquettes acquire a cylindrical shape with a diameter between 2.5 and 10 cm. The length varies between 10 and 40 cm. The briquettes have a density about twice that of acacia Senegal, the wood commonly used for fuel in the Sudan. and their energy content (4.3 kcal/g) is about half that of charcoal.

Briquette production from agricultural waste has potential in the Sudan. Millet stalks and cotton, groundnut, and sorghum waste are available for processing. A motor-driven press with a capacity of 75 kg per hour would replace the charcoal requirements of about 200 families. A hand-operated press with output of five kg per hour would provide about 20 families with their the fuel they need. This suggests that although the estimated cost of energy from agro-waste briquettes is likely to be somewhat cheaper than an equivalent amount of energy from charcoal. production of briquettes at village level would require a substantial number of presses to supply the needs of the whole community. However. large-scale production of briquettes in the agricultural areas of central and western Sudan could provide the fuel scarce northern areas with another convenient source of energy.

Whatever fuel is used. it can be burned more efficiently. Traditional charcoal stoves are made of rectangular empty four-litre oil tins with a grate attached four or five centimetres below the top. A large draft opening is cut in one side. These stoves range between 10 and 18 per cent in efficiency, compared with 50 per cent for a gas ring, but much depends on how they are used. Very often, the whole of the grate area is covered with charcoal for a relatively small amount of cooking. which means that heat losses are substantial. Cooking in Sudan is a hot, uncomfortable task and to hasten the process women like to bring food quickly to the boil, and the fire is often kept burning at a high rate.

Improving the efficiency of the Sudanese charcoal stove required a systematic approach. The average Sudanese stove requires about 350 grams of charcoal to cover the grate area. A bed of charcoal about five centimetres deep will absorb all the oxygen it can get whereas a thicker bed will burn incompletely and form carbon monoxide. To bring about 1.5 litres of water to the boil takes about 18 minutes and uses about 150 grams of fuel. A further 100 grams of fuel is used for the first half hour of simmering; a nominal quantity of fuel will keep the water simmering for a second half hour.

Many variables could be tested in the design of a metal stove. But, since our goal was a major improvement in fuel efficiency, we concentrated on the effects of altering a few simple design parameters.

The first of these was the size and position of the draft hole. Experiments showed that the hole was usually too large and had an adverse cooling effect on the air entering the stove. It was also found that it was better for the hole to be placed near the grate. The grate itself, we found, should be placed 10-12 cm inside the stove in order to facilitate even combustion at the boiling stage. This provided a slight chimney effect with the fire sucking air through the grate at a fast rate similar to forced convection, reducing boiling time to about 10 minutes. Once the food reached cooking temperature a small door could be used to shut down the air flow. Small holes could then be punched around the sides of the stove near the grate to allow secondary air to enter.

Our most useful innovation in improving fuel consumption was a ring cut from the crown of an empty paint tin. The ring. with triangular slits cut in its side, was slightly smaller than the base of the cooking pot so that the flames could encircle the base and sides of the pot similar to a gas ring. With this method, the initial fuel requirement was reduced to 200 grams. Slightly less than 100 grams was used to reach the boiling stage and another 70 for the first half hour of simmering. The water was maintained at a gentle simmer for 87 minutes, which should be sufficient for a moderate amount of cooking. A fuel saving in the range of 30 to 40 per cent could be realized.

Responses among Sudanese women who tried the redesigned stove were very favorable: "It is fast to get the charcoal alight," "It uses less charcoal." Most noted the economical used of fuel, while one commented that the stove gave "a quiet, slow burning fire". Another said. "All the heat is concentrated in the pan

Two more stoves were built were tested. The "gauze-in-tin" model consisted of a cylinder made from chicken wire inside a shiny, open- topped tin with a draft door. A few centimetres below the top of 1 cylinder, a piece of chicken wire triple thickness was 'sewn" in to form a grate with plenty of air spaces. The use of chicken wire minimized heat losses by conduction and the inside of the shiny tin re-radiated heat back onto the fire. It also trapped connective heat. Once the draft door was closed, air could enter at the top. This stove took 14 minutes to boil 1.5 litres of water and kept the water at a gentle simmer for 76 minutes with 200 grams of fuel

Reaction to the gauze-in-tin" model was also favorable. although one woman thought it was a bit small. More typical comments were: "It fast to burn," 'It uses less charcoal," and '`It kept the heat inside." Another model was the brick basket" made from a double layer of chicken wire filled with broken bricks. The grate, some 10 cm above the bases. was also made from layered chicken wire. The basket sat on a circle of five bricks, one of which could be removed to provide a draft hole. Since bricks in Sudan are very light. the stove was portable. but could not be easily knocked over by children. The brick basket did not need tin and had better insulating properties than the tin stoves and worked just as well as they did. With 200 grams of fuel, it took 12 minutes to boil 1.5 litres of water and kept it simmering for about 70 minutes.

Sudanese women liked the brick basket because the outside of the stove kept cool, because it was fast to cook, and because it "kept the heat inside " Women also found it easy to use and noted that there were no sparks flying from the fire. Heat retention and speed of boiling were mentioned more frequently than fuel economy.

Overall, the three stoves performed similarly. Women generally claimed that they could save as much as a third of their fuel. Their perceptions of economy, however, were much influenced by the ease of lighting the fire' the fast boiling times. and various heat-retention attributes. Attempts to quantify fuel consumption showed that there was considerable variation in day-to-day cooking needs.

These stoves could be fuelled by agro-waste briquettes if these were available in the agricultural areas of the Sudan. A suitable briquette would have a diameter of 2.5 to 5 cm and a length of 5 to 10 cm, but their lower calorific value might mean that women would have to refill the stove at least once during a cooking period. Even with charcoal, women generally claimed to add one or two pieces to maintain the cooking process.

As for marketing the improved stoves, it is usually the men who manufacture, sell, and buy stoves. Thus future promotional work may need to be directed toward men. Once they realize the economies achieved by improved stoves, there would be some incentive for stove makers to produce them on a large scale .