Better Stoves for the Poor
Wednesday, January 28, 2009
IF USER demand were the sole driver of innovation, the biomass cooking stove would be one of the most sophisticated devices in the world. Depending on which development agency you ask, between two-and-a-half and three billion people?nearly half the world?s population?use a stove every day, in conjunction with solid fuel such as wood, dung or coal. Yet in many parts of the world the stove has barely progressed beyond the Stone Age.
The World Health Organisation (WHO) estimates that toxic emissions from cooking stoves are responsible for causing 1.6m premature deaths a year, half of them among children under five years old. In China 83m people will die from lung cancer and respiratory disease over the next 25 years, according to a recent report from Harvard University. Research from the University of California, Berkeley, on stoves in India, Guatemala and Mexico has found links between indoor air-pollution from stoves and increased incidence of pneumonia, cataracts and tuberculosis.
After an initial wave of stove design that sought to reduce deforestation through improved efficiency, scientists and engineers have turned their attention to stoves that minimise the levels of noxious emissions to which stove users?mainly women and children?are exposed. Crucially, they have also recognised the need to take account of the way in which stoves are actually used.
One of the principal problems the designer of a stove must solve is to optimise the thermodynamics. Typical stoves?including the basic ?three-stone fires? still used in many parts of the world?draw in too much air during the combustion process, which cools the fuel and means more of it is needed. Even with more advanced designs, poorly insulated combustion chambers can add to the cooling effect and thus to the inefficiency. The challenge, explains Bryan Willson of Envirofit, an organisation developing stoves for India (pictured above), is to optimise a stove?s air-fuel ratio and minimise heat transfer to improve combustion efficiency.
Envirofit?s latest stoves, introduced this year in a project sponsored by the Shell Foundation, use a carburettor design, with chimneys that draw air in through precisely calibrated inlets. Another model, the ?Oorja?, developed by BP and the Indian Institute of Science, has an integrated battery-powered fan to direct air to wood pellets in the combustion chamber, improving efficiency.
Better efficiency can reduce emissions, but does not solve the indoor-pollution problem. One solution is the ?rocket stove?, a simple design that diverts the smoke outside. Other options include stoves that run on propane gas or pellets, or reflect solar radiation onto a cooking vessel. But specialist fuels and constrained cooking times can restrict their appeal.
As well as being efficient, stoves must also meet the conflicting objectives of being transportable and being rugged enough to withstand the rigours of daily cooking. Combustion chambers therefore present a ?huge materials problem?, says Dr Willson, who as well as being Envirofit’s founder is also professor of mechanical engineering at Colorado State University. Mild steel is the natural choice for a stove chassis, but such stoves do not last for more than a few weeks in the field. Nickel-content alloys are a better choice for durability, but more expensive. For insulating the stoves, ceramic linings are the materials of choice in the lab, but they are difficult to ship and present problems of consistency for mass production. Materials are therefore often limited to those available locally.
