Liquid logic

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Sep 4th 2008
From The Economist print edition

Computing: Liquid cooling could improve the performance of computers, allow waste heat to be recycled and make solar cells more efficient


Illustration by Belle MellorREADERS 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 engines with water.

Something similar is happening 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, the processors in some high-powered computers will soon start melting. Moreover, the trend is to stack chips on top of one another, to improve communications between them. But this 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 in this way you generate heat at about two kilowatts per cubic centimetre—more intensely than in a nuclear reactor and ten times more than in any other man-made device. That would destroy an uncooled chip within a fraction of a second.

Water, however, can absorb 4,000 times as much heat as air, for a given rise in temperature. Mr Brunschwiler and his colleagues have therefore been experimenting with water-cooling of stacked chips. They have developed a stacked processor 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 methods. 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 its layers. The water absorbs the heat and carries it away.

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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 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 could be 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 could 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, in fact, as a high-performance 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 found that he can concentrate 2,300 times as much sunlight on a cell as nature normally provides, while maintaining that cell at a (relatively) cool 85°C. Without the cooling system, its temperature would rapidly exceed 1,500°C, 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.