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Burning wood to power fridges

May 14, 2007 By Philip Ball This article courtesy of Nature News.

Project aims to bring high-tech device to developing world.

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A consortium of UK universities hopes to bring affordable domestic appliances to rural areas of developing countries by developing a device that acts as a refrigerator, cooker and power generator all in one, powered by locally available biomass fuels such as wood.

The SCORE (Stove for Cooking, Refrigeration and Electricity) project, led by the University of Nottingham in England, has been granted £2 million (US$4 million) to develop the device using a technology called thermoacoustics. The United Kingdom's Engineering and Physical Sciences Research Council have contributed 80% of this funding.

Thermoacoustics takes advantage of the way that sound waves can be produced when a gas is heated unevenly. In a thermoacoustic engine, such as the Stirling engine developed in the nineteenth century as an alternative to steam power, these pressure sound waves drive mechanical motion.

This process may also be run in reverse: the sound waves can be used to extract heat, pumping it from a cool source to a hot sink and thereby inducing cooling.

Thermoacoustic engines and refrigeration units have been used before in high-tech settings, as power sources or cooling units on spacecraft, satellites and military craft for example. But it needn't be limited to these top-end applications. "In principle, thermoacoustic devices are quite simple and should be able to be made very cheaply," says Scott Backhaus of the Los Alamos National Laboratory in New Mexico, a specialist on thermoacoustics who is assisting the SCORE team.

Sound system

In the SCORE device, says the project director Paul Riley at the University of Nottingham, UK, "burning wood heats a gas-filled pipe at one end. The gas moves from the hot part, where it expands, to the cold part, where it contracts. The pipe then resonates rather like an organ pipe." This produces acoustic pressure waves, which can be harnessed to produce electricity in the reverse process to how a loudspeaker turns electrical signals into vibrations.

The sound waves are also used to drive a second engine that operates as a heat pump to remove heat from a nearby refrigeration unit. And the heat from the burning wood can also be used for cooking in a conventional cooker stove. The fridge and the cooker are connected by "the pipes necessary to carry the hum," says Riley — but are kept apart so that the heat from the stove doesn't interfere with the cooling.

The system will only generate electricity and cool the fridge while it is operating as a stove.

One of the main attractions of SCORE stoves is that they don't need an external electricity supply. "The electric grid in developing countries basically only serves the cities," Backhaus explains. And many homes are already using a fuel source such as wood for cooking. "Over two billion people use open fires to cook," says Riley.

SCORE aims to be producing these fridge/stoves in significant numbers within five years.

Do it yourself

Backhaus thinks this goal is feasible, but admits that it is challenging. He cautions that the SCORE team "will have to stay focused on keeping the device inexpensive and not let a desire for technical perfection get in the way of the true goal: improving the living conditions in the developing world."

The research team recognizes that the science is only a part of the challenge: success will also depend on ensuring that local communities have enough expertise to maintain and ultimately to produce the devices themselves.

"One of the keys is to make this device simple enough so that it can be produced cheaply by the local population," says Backhaus. Local researchers and experts from the development charity Practical Action will advise on how to best introduce the new technology. The group is already discussing their project with governments of developing countries.

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