Jul 10, 2008

Tech - How to stop chips from melting

How to stop chips from melting—and save the planet, too

READERS of a certain age may remember that Volkswagen Beetles once had air-cooled engines. That made for simplicity. But, eventually, increasing power meant that even VW conceded the point and started to cool its motors with water.
Something similar is about to happen with computer chips. Each flipping of a “one” to a “zero”, or vice versa, generates heat—and the flipping is happening so fast that if nothing is done chips will soon start melting. Moreover, the trend is to stack chips on top of one another, to improve communications between them. The adverse side of stacking is that it makes shedding the heat even harder, because of the falling ratio of the processor's surface area (through which heat can escape) to its volume (the amount of material generating heat).
According to Thomas Brunschwiler, a researcher at IBM's laboratory in Zurich, when you build processors this way you generate heat at about one kilowatt per cubic centimetre—more intensely than in a nuclear reactor and ten times the record of any other man-made device. That would destroy an uncooled chip within a hundredth of a second.
Water, however, conducts heat 4,000 times faster than air, and can also carry more much of the stuff in a given volume. Mr Brunschwiler and his colleagues have therefore been experimenting with water-cooled chips. They have developed a stacked processor that is permeated by a network of channels. These channels, which have a diameter of 50 microns (about the width of a human hair) are etched using standard silicon fabrication. They enable water to be pumped in a network that runs between the horizontal layers of a stack and the thousands of vertical interconnections that carry information between these layers. The water collects the heat and carries it away.
IBM Cooler than it looks
That, in the prototypes at least, is enough to keep the chips from melting. But in these days of environmental awareness, not to mention high energy prices, it seems a waste simply to throw the heat thus collected into the atmosphere. If chips are as hot as power stations, the thinking goes, why not use them as such?
In practice, not enough heat is generated to make a useful amount of electricity. But heat is useful stuff in its own right. It might, for instance, be used to warm buildings. The Zurich laboratory has already constructed a prototype that feeds the water from the chips into a heat-exchanger. The next stage is to link this exchanger to a district-heating system so that it can be pumped into central-heating. Bruno Michel, manager of advanced thermal packaging at the laboratory, reckons the heat from a medium-sized data centre—one consuming a megawatt of power—would be enough to warm about 70 houses within a range of 3km. IBM hopes to build such a centre within five years.
If it works, the potential is huge. At the moment, the world's data centres are estimated to consume about 14 gigawatts of power, and to be responsible for 2% of global carbon-dioxide emissions—roughly the same as air traffic.

Water-cooling of this sort may also make a more direct contribution to the reduction of greenhouse-gas emissions, by promoting the use of solar energy. Solar cells are also made of silicon, and the latest fashion is to concentrate sunlight on them using mirrors. That means you need less silicon to make a given amount of electricity, but it also makes the silicon very hot—as hot as a commercial microprocessor.
By cooling such devices with liquids, IBM reckons it can increase the amount of sunlight that can be focused on them without destroying them, thus increasing the amount of electrical energy they produce. Supratik Guha, a researcher at IBM's Thomas J. Watson Research Centre in Yorktown Heights, New York, has put this to the test and found that he can concentrate 2,300 times more sunlight on a cell than nature would provide, while maintaining that cell at a (relatively) cool 85°C. Without the cooling system, its temperature would rapidly exceed 1,500º, causing it to melt. With cooling, the cells can manage an output of 70 watts a square centimetre—a record, according to IBM, and a demonstration that plumbing, too, can be a high-tech form of engineering.

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