Most traditional stoves consist of three stones surrounding a fire. The pots are made to

Transcription

1 Cookstoves Most traditional stoves consist of three stones surrounding a fire. The pots are made to be held in the three stones. The fire burns under the pot in the open or inside the dwelling. In the latter case, health problems such as pneumonia can arise from extended breathing of the smoke. In addition, objects burning indoors can release carbon monoxide from incomplete combustion. If the house is relatively tight, the people living in it could die. Fig. E A traditional cookstove made of three stones surrounding a cooking pot suspended over an open fire; such stoves are used commonly all over the undeveloped world, from South America to Africa to Asia. Biofuels are often easy to gather or find, though it is estimated that the poor spend around 20% of their disposable income for fuelwood, (132) and the stoves (Fig. E23.3.1) are simple to construct. They provide very attractive (usually cheap) sources of warmth and useful for cooking food. Manuel claims that fuels based on biomass account for more than one-half of all energy use in many developing countries and for as much as 95% of all energy use in some of the poorest nations. (133) More than two billion people currently use biomass energy as their primary energy source. ( ) As usual, the poor are most exposed to the deleterious impacts of energy use. (138) Often women bring the fuel home balanced above their heads ( headloading ), and this can cause injuries to the head, neck, and back. (133) Smoke gets in the eyes of everyone in a household using a traditional stove, leading to increased incidence of eye complaints. (139,140) There can be increases in ear infections. (132)

2 Energy, Ch. 23, extension 3 Cookstoves 2 The effects are not all such direct physical effects; exposure to smoke weakens resistance to diseases of the respiratory tract. Leukocytes in the blood are changed by exposure to smoke from cooking fires. (141) In Guatemala, women cooking using traditional stoves are exposed to levels of particulates of size under 2.5 µm (PM 2.5 ) greater than or equal to 520 µg/m 3, more than eight times greater than would have been permitted in the U.S. (the EPA average 24-hour exposure standard for PM 2.5 is 65 µg/m 3 ). (142) In Costa Rica, the largest peak exposure during cooking was measured to be 8170 µg/m 3 (310 µg/m 3 was the smallest peak value measured). (143) The PM 10 can actually be higher than 30 mg/m 3 (30,000 µg/m 3 ) during cooking. (133) One study found in excess of 50 mg/m 3 near the fire itself, and other pollutants such as carbon monoxide and benzo[a]pyrene are found as well. (133,138,128) In southeastern India, researchers found the average 24-hour exposure of cooks to be 231 ± 109 µg/m 3. (144) Even those not involved in cooking had average 24-hour exposures above the EPA level, 90 ± 21 µg/m 3. (144) The authors of Ref. 144 note that these data show the greater risk borne by the very young and the elderly, who are most likely to be indoors during cooking times, and who in any case are at greater risk than adults. In Kenya, the same is true. It is estimated that over two million people die every year from exposure to these pollutants, many of them under 5, half a million of them in India alone. (145) In India, around 5% of the total health burden is due to biomass combustion. (146) Not only the cook, but elderly men and women in homes cooking with biomass were found to have heightened rates of asthma compared to those using cleaner fuels, i.e., kerosene or gas. (147) Ezzati and Kammen found substitution of charcoal for biomass resulted in improvement of roughly 25% in complaints of acute lower respiratory tract infection in infants and children under 5 years of age. (134,148) Low birth weight is also linked to

3 Energy, Ch. 23, extension 3 Cookstoves 3 maternal exposure to pollutants, and the primitive stoves give rise to many such pollutants. (137) And, of course, even in developed countries there is ample evidence of health effects from polluted indoor air. (136) Gathering wood for feeding the stoves involves a commitment of a lot of time. In many developing countries, women are bearing the brunt of this work. They cannot do their other important jobs if they are occupied in gathering wood to burn. In addition, because women are desperate to find anything that can be burned instead, scraps such as plywood and plastic are often gathered and added to the fire. These materials can release hazardous chemicals such as dioxin when burned. In addition to their health hazards, traditional open fires yield only 5% to 10% of the energy stored in the wood. (130) Doubling or tripling this by replacing this with practically any stove is a gain; using an innovative stove design such as the jiko stove of Kenya would appear feasible, (131) and would greatly reduce the pressure on forests and woodland. Much of the wood burned in open fires or stoves is burned to boil water so that it is safe for drinking. So one way to save time, decrease energy use, and decrease deforestation is to provide clean water. The ultraviolet germicidal lamp mentioned in Ch. 9 is one way to achieve this aim. Even if imported fuel has to be burned to supply the purified water, overall there will be considerable savings. As Rosenfeld says: (149) We find it remarkable that a device costing only a few hundred dollars, with a service life of ~10 years, has the lifetime potential of saving tonnes of carbon. In terms of deforested hillside, the life-time mass of avoided firewood is equivalent to more than 10,000 tons!

4 Energy, Ch. 23, extension 3 Cookstoves 4 In addition, in tropical regions dependent on the monsoons, normal solar energy is of minimal use for providing water purification, even if photovoltaics are available. (149) Furthermore, even if they are fueled by renewable energy such as wood and crop residue, the cookstoves are contributing to global warming. In some cases, the global warming commitment from cooking a meal could be much greater than that of the fossil fuels. (150,151) Biogas as a cookstove fuel is an ideal match. It is therefore important to provide better cookstoves, better fuels, and other ways of achieving the ends desired by those using the cookstoves. Designers need a knowledge of both local cooking styles and various other uses of stoves (for example, space heat or insect control). (152) The three-stone open fire can be 15% to 20% efficient; traditional stoves with no chimney, about 20% efficient; improved fuelwood stoves, 15% to 22% efficient, and portable stoves 25% to 45% efficient. (152) Experts agree that the focus of stove introduction programs should shift to urban areas, and that stoves should be mass produced and portable. (130) The original emphasis was on the rural population, but experience has shown that the stoves are often quickly abandoned for lack of qualified repair people or because the stoves seem too complicated to the user. Also, much experience on mud stoves (heavy-weight clay stoves made of natural materials) shows that use does not diffuse well, and the stove s proper use would involve changes in traditional methods of preparation. Despite such problems, stove upgrading or replacement holds great promise for reducing wood consumption worldwide and for minimizing exposure to toxic pollutants indoors. What is most important is indigenization of the stoves, as has been the case of the jiko ceramic cookstove in Kenya. (131,132) Local artisans have adapted and disseminated the stoves and are producing in excess of 13,000 every month. About 700,000 efficient

5 Energy, Ch. 23, extension 3 Cookstoves 5 cookstoves are saving energy, retarding greenhouse warming, and providing an enhanced quality of life for many families. (131) The stoves were originally created and supported by international funding, but, as appropriate, that is no longer needed. Exceeding the threshold has made the cookstoves truly Kenyan. (131) Fig. E The ceramic metal-covered cookstoves known variously in different African countries as Jambar stoves (Senegal), Sewa stoves (Mali), and Jiko stoves (Kenya). In Mali, consumers saved nearly $148,000 in 2000 from use of such stoves. Use of the Sewa stoves can reduce charcoal consumption by as much as 44%. Source: US Agency for International Development The expansion of cookstoves not only benefits the health of the people obtaining them, but, combined with biogas digestion also reduces the amount of greenhouse gases emitted. Studies show that greenhouse gases are released by biomass burning. (138,151) For example, a stove burning biogas has less than 1% the global warming effect of a stove burning dung. (150) But not all cookstoves are equal vented Indian cookstoves based on ceramic materials outperform those based on mud. Unfortunately, Indian emphasis has been on improving mud cookstoves rather than replacing them with ceramic vented cookstoves. (150)

6 Energy, Ch. 23, extension 3 Cookstoves 6 Emissions Clearly, given the pressing and immediate health problems attendant on use of biomass, the problem of global warming looks less of an impediment to distribution of safer cookstoves (and in any case, the emissions of greenhouse gases is below that of an open fire). (137,151) A study in China of 28 different types of stove revealed huge differences in emissions characteristics. Given stoves of equal safety, it would be good if on the basis of expected emission of greenhouse gases that certain designs would be favored. (138,153) The fuel matters a lot: Zhang et al. (Ref. 153) find that emissions decrease and efficiencies increase in the following order: crop residues, brushwood, fuelwood, kerosene, and gas. Bailis, Ezzata, and Kammen examined cooking fuel alternatives in sub-saharan Africa (excluding Namibia and South Africa) in terms of greenhouse gases emitted. They found that traditional charcoal production would generate a total of 6.7 Gt of carbon over 2000 to 2050, about 6% of regional emissions during that interval and comparable to current yearly global emissions. (a) A transition to higher-efficiency charcoal production would essentially cut these cumulative emissions by half. (a) An attempt to quantify biofuel emissions implied that biomass burning is responsible for one-sixth of global CO 2, one-eighth of global CO, and one-sixteenth of global NO. The estimate was based on extrapolation from measurements of more than one hundred domestic fires in Zimbabwe. (154) Based on their experience, the researchers assumed that what is being burned in these domestic cooking fires is mostly (four-fifths) wood, with the remaining 20% apportioned to agricultural residue (15%), dung (2.5%), and charcoal (2.5%). Ludwig et al. (Ref. 154) assume that roughly 85% of worldwide emissions from domestic fires occur in the less developed countries of Asia, Africa, and Latin America. According to Ref. 154, the proposed emission estimates have a high degree of

7 Energy, Ch. 23, extension 3 Cookstoves 7 uncertainty. Although present knowledge does not allow an uncertainty analysis that is statistically defensible, the major error bounds can be made apparent. They go on to point to the major uncertainty biofuel consumption, which is estimated at between 1086 and 6192 billion kilograms of carbon! The numbers of Ref. 154 are in reasonable agreement with the estimates of releases by others, but (admittedly) are highly uncertain. (151,153,155) It is easy to say that Kenyans, Indians or Chinese have adopted newer stoves in large numbers (greater than 100,000 in each country), (135) or dissemination of over 120 million stoves in China, (132) but one should be careful about believing the numbers. A careful study found that the Indian government s statements as to the penetration of newer chulhas (stoves) to be greatly overstated. (140) Fig. E Human exposure to air pollution occurs predominmantly in developing countries. (Ref 126, Fig. ES 1) There are undeniable benefits to these stoves for energy use and for health, especially given the huge exposure of many in the less developed countries (Fig. E23.3.3).

8 Energy, Ch. 23, extension 3 Cookstoves 8 Individuals who have them express relief and gratitude for the change. For example, one Mexican recipient of a stove is quoted to have gushed It s much easier to cook on... We had to cut four horseloads of wood a week. Now I can burn scraps, anything. (156) Substitution of charcoal for biomass does reduce individual exposure, but the method of preparing charcoal can lead to increases in overall pollution. (148) Gas is a much better fuel than biomass directly, and digesters show great promise in reducing health and environmental impacts of cooking. There are many types of gas stoves, and some are much more efficient than others; the use of flat-bottomed pans coupled with welldesigned gas jets provides significant savings. (157) One problem in Bangladesh is that round-bottomed pans are more popular than flat-bottomed pans. The reduction of flow rate to optimal rather than use of the high setting can lead to additional savings. (157) In China, similarly, there are many different stove designs, and some are much more efficient than others. Many wood cooking stoves are also hazardous and inefficient. Domestic smoke exposure can lead to bronchial problems, as noted. New designs must consider ways to reduce exposure to pollutants. Solar cookstoves Research is also ongoing into solar cookers, which use no fuel and emit no pollutants. Three popular models (Philippine, Chinese, and IME) were compared and determined that the former two, using a Fresnel arrangement of reflectors (the reflectors are circular tilted rings, as if a curved surface were cut into strips, arranged on a flat surface, as in Fig. E23.3.4), were better at distributing heat for cooking. (158)

9 Energy, Ch. 23, extension 3 Cookstoves 9 Further development led to the SPRERI cooker, a rather massive 24 kg, which performed well and shows promise of relatively cheap cookstoves in the future. Cooking times are long, typically over an hour for most applications (boiling water takes 1.5 h, but heating to 90 C takes only about 20 min). (159) a. b. Fig. E The principle of a Fresnel arrangement. The circular curved segments are made as if from a curved surface and mounted flat. The segments give the same reflective effect as the curved surface. a. The diagram shows the side view of a sphere. b. A top view of the sphere, showing how the strips would be cut from the sphere. (In actual practice, the curved surface used would be parabolic, not spherical.) One novel idea is to use a simple wood or cardboard box with a glass window and a reflector (perhaps made of aluminum foil). The oven costs about $10. (160) This oven achieves temperatures of 180 C (350 F) even without direct sunlight. It cooks the way a crock pot does. Cooking times are, however, twice as long as in a conventional oven. Some cooks like it and some don t. On a final note, even this good old idea can be improved. An added reflector, when properly oriented, gave increases in common box-type solar cookers of anywhere from 40 to 100%. (161) References in addition to those listed for this chapter are shown in red in the text, and listed below: a. R. Bailis, M. Ezzati, and D. M. Kammen, Mortality and greenhouse gas impacts of biomass and petroleum energy futures in Africa, Science 308, 98 (2005).