Michael Brooks looks at the latest cool solution
The refrigerator of the future may be cooled by a semiconductor device no bigger than a credit card. There will be no buzz, no moving parts and, most important of all, it will do away with the need for the environment-destroying Freon gases used in conventional refrigerators.
The idea behind the fridge, which has already met with scientific approval, was recently published in American journal Physical Review Letters.
Gerald Mahan, a professor of theoretical physics at the University of Tennessee, has produced computer models of what he calls “thermionic refrigeration”, where current- carrying electrons in a semiconductor can be made to transport heat as well as electric charge.
Two hundred layers of a semiconductor-metal sandwich would give cooling power equivalent to a household refrigerator, all squeezed into a unit that would fit in your back pocket. Plug it into the mains and it will cool whatever it touches.
“The efficiency will be roughly the same as Freon compressor-based fridges – if you make something that takes more electricity people won’t buy it,” he says.
So far, Mahan has been funded by the United States Department of Energy’s Oak Ridge Laboratory and the American military, who are interested in using the technology to cool computer chips.
At the moment, they are cautious about their investment, but Mahan believes a practical demonstration of its cooling power will open their wallets: thanks to its simplicity, Mahan expects mass production would be possible for less than R160 per unit. A traditional unit currently costs about R120.
“Everybody’s in favour of getting rid of the Freon,” says Mahan.
There are approximately 64-million domestic refrigerator-freezers manufactured worldwide each year. All of them work by circulating a refrigerant liquid which evaporates as it absorbs heat from the fridge’s interior. It is then pumped into a compressor, where it condenses, and is cooled, ready for recirculating.
Chemicals known as chlorofluorocarbons (CFCs) have been widely used, but are known to cause damage to the earth’s protective ozone layer.
In the late 1980s, the chemical industry came up with an alternative: hydrofluorocarbons (HFCs), which do not destroy ozone. They do, however, contribute to global warming, estimated to be at least 1 000 times more potent than carbon dioxide, the main greenhouse gas. Their manufacture also releases toxic chemicals into the environment, and their reactions in the atmosphere are thought to produce poisons that return to the earth in acid rain.
In 1992 a technology known as Greenfreeze was launched, which uses isobutane (the liquid found in cigarette lighters) as a refrigerant.
Isobutane is thought to cause negligible environmental damage and the Greenfreeze technology is now used in 15% of new refrigerators.
Mahan’s idea is based on a phenomenon discovered in the 1830s: putting a current through a join between two different metals can cool the metals down or heat them up, depending on the direction of the current.
Moving from one metal to the other, the electrons have to cross an electrical “barrier” -rather like leaping across a stream (because of the difference between the metals) with the banks at different heights. Going one way, the electrons have to gain some “height”, and thus use up energy, cooling the metals. Going the other way, the electrons can afford to lose energy, which they give out as heat.
In the 1950s, researchers discovered the phenomenon also works with semiconductors. But despite intense research efforts, the efficiency and cooling power of this technique remained extremely poor, never challenging compressor-based refrigeration.
That inefficiency is mainly due to the fact that the electrons can only move short distances before they start to get in each others’ way. Only the electrons right on the edge of the material can make the jump across the barrier, and they need extra energy to do so. The electric current effectively pushes electrons into one another, and energy is transferred as they collide. But much of this energy is lost, so the electrons on the brink only gain enough energy to jump very gradually, slowing down the cooling process.
But, thanks to today’s improved understanding of semiconductors, Mahan has been able to design a much more efficient system. In a thin wafer of material, chosen to give electrons relative freedom of movement, most of the electrons are near enough to the edge to make the jump. The electric current thus forces many more of them across the barrier, without wasteful jostling between electrons.
Each jump between layers causes a drop in temperature. By building up multi-layer sandwiches of metals and semiconductors, Mahan believes he can produce a significant cooling effect. Two hundred layers, he says, will serve domestic refrigeration requirements.
Producing a prototype is now a matter of testing the materials that allow electrons the greatest freedom of movement. The hard work has already been done – Mahan’s research team has been busy identifying the right semiconductors for the job.
Mahan is confident that it won’t be long before they have a working model. “It may only take a couple of weeks,” he says.