Warm Water Flowed Through Supercomputers to Cool Down Their Heat

IBM experiments with a liquid cooling system that moves heat away from sensitive computer components without chillers, cutting energy costs in half


Today's supercomputers run hot, thanks to power-hungry microprocessors that enable sophisticated scientific research and complex financial transactions to be performed in the blink of an eye. As these microprocessors have become smaller and more powerful over time, they are generating even more heat, a problem that data centers generally address expensively with air conditioning and chilled-liquid cooling systems.

Whereas datacenters have for decades used cold liquids to transfer heat away from central processing units (CPUs), a team of IBM researchers in Switzerland is experimenting with a micro network of copper tubes that run through smaller, clustered computer servers and whisk away heat with the help of warm water. Liquid cooling, even with warm water, is 4,000 times more effective than air cooling at removing heat, they say.

To prove the point IBM and the Swiss Federal Institute of Technology in Zurich (ETH) have created a supercomputer called the Aquasar, which uses 60-degree-Celsius water instead of air as its coolant to capture and remove heat. The researchers report in the April 16 issue of Science that early testing of the Aquasar indicates it uses half the electricity of a supercomputer cooled with cold water (which requires electric chillers to keep the water at about 15 degrees C).

These savings, as well as a sharp decrease in the amount of energy consumed, could be significant. In 2009, an estimated 330 terawatt-hours of electricity were needed to operate data centers worldwide—about 2 percent of global electricity production, according to International Data Corporation in Framingham, Mass.

Water does not necessarily need to be cold to work as a coolant; it just needs to be cooler than the microprocessor, says G. Ingmar Meijer, a researcher with IBM Research-Zurich in Switzerland. Using pipes that run alongside and around electrical components, 60-degree-C distilled water can keep microprocessors below their limit of 85 degrees C, above which components on the chip begin to malfunction, he adds. The water generally enters the cooling system at 60 degrees C and will exit at 65 degrees C. This can keep the microprocessor at about 75 degrees C.

Although it seems almost counterintuitive to use water, and warm water at that, to cool the inside of an electronic device, IBM's work highlights a trend to use liquid cooling in devices other than mainframes. Even some higher-end PCs from Dell and Apple have included liquid cooling systems.

The transistors inside a computer's microprocessors can be as small as 45 nanometers—roughly five times the thickness of a cell membrane—making it difficult to control the electric currents that leak out of these transistors. Such leakage can consume more power than the actual computational processes, according to Meijer. Microprocessors can dissipate anywhere from 100 watts and several hundred watts each, he adds.

Meijer acknowledges that cooler processors are able to run faster and last longer, but he points out that the challenge he and his colleagues are looking to address at this stage is a way to cut electricity consumption.

The Aquasar is made up of a rack of clustered IBM servers that use either IBM's Cell microprocessor developed by Sony, Toshiba and IBM or an Intel Core i7 microprocessor. The supercomputer has a peak performance of about 10 teraflops, or 10 trillion calculations per second.

As a prototype, Aquasar cost more to build than a comparable air- and liquid-cooled supercomputer without IBM's microchannel warm-water cooling system, with most of the added expense attributed to retrofitting copper tubing and heat sinks onto an existing computer server. Meijer estimates, however, that the cost savings on electricity consumption would make the prototype cost competitive with a normal supercomputer after a year and a half of operation.

If the Aquasar is a success for ETH, which is using the system to study fluidics, in time newer supercomputers could be designed with built-in pipes and heat sinks for liquid cooling systems. Within about five years, IBM hopes to have a way to run pipes directly through the microprocessors themselves (as opposed to alongside), cooling the silicon and providing even more efficient heat removal. IBM is now looking for business partners to help it build an even bigger prototype, 10 times the size of the Aquasar.

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