Cool ways to save energy
Tom Wilkie admires the ingenuity of the engineers who are devising so many ways to keep supercomputers cool
C
ooling has become interesting– almost, in a manner of speaking, it’s cool. Hardly a week
goes by now, without some announcement of a specially engineering cooling system being installed in a data centre or HPC site. At the end of June, for
example, the Centre for Biological Sequence Analysis (CBS) at the Technical University of Denmark, where the HPC system is ranked 121 in the Top500, replaced traditional air cooling with liquid cooling from CoolIT Systems to provide heat not just for the adjacent buildings but also for the nearby town of Roskilde. Cray’s recently announced machine for the UK Met Office, the company’s largest sale outside the USA, will be cooled by Motivair’s ChilledDoor heat exchanger system that fits on to the rear-doors of the racks. Te range of cooling options
now available is testimony to engineering ingenuity. HPC centres can choose between air, oil, dielectric fluid, or water as the heat-transfer medium. Opting for something other than air means that single or two- phase flow could be available, opening up the possibilities of convective or evaporative cooling and thus saving the cost of pumping the fluid round the system. Some systems were originally
developed for computer gamers, to keep overclocked
processors cool as they drove the game faster. Some trace their antecedents to the electrical power engineering industry. Te engineering design of others has been driven by the exacting demands of HPC, but their developers hope this may open the way to an even larger market in commercial data centres for the likes of Facebook and Google.
One question: many answers All these engineering solutions exist because there is no single, right answer to the question: ‘What is the best way to cool a supercomputer?’ Due to differences in the price of energy, the answer will be different in Germany compared to the USA.
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Sometimes the answer depends on the temperature of the inlet water to the facility – if chillers have to be employed, then that will drive up the overall cost – so the answer may well be different in California compared to Sweden. Sometimes the answer
depends on another, entirely non-technical question: ‘Who is paying?’ Oſten the budget for the
26 SCIENTIFIC COMPUTING WORLD
facility – the cost of the building and its associated plant and machinery – is separate from the budget allocated to pay for the IT itself. Sometimes, capital costs may come from a different ‘pot’ than operating expenditure. Or there may be a need to install a new machine in the building that housed its predecessor, limiting the options that can be deployed because of the infrastructure that already exists.
Pumping without pumps One of the most technologically imaginative and elegant solutions for taking heat away from the processor is that from the Belgian company Calyos. Although it was set up in 2011, its technology derives from applications developed for the power electronics industry since the 1970s. To circulate the dielectric coolant without the need for any pumps, it makes use of two principles from physics: capillary attraction and two- phase flow,. A highly engineered heatsink or cold-plate sits in contact with the processor. It is made from a metal powder that has been compressed and sintered to take the form of a porous metallic foam, so it is honeycombed internally with tiny capillary ‘tubes’ through which the fluid is driven by capillary attraction. As it absorbs heat from the processor, the fluid then changes phase, and the vapour passes along a pipe to the
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heat exchanger at the edge of the server where the fluid condenses and the whole cycle continues. One of the critical parameters
in cooling technology is the heat transfer coefficient. For air, this parameter typically has a value between 10 and 100 W/(m2
K),
whereas water is more effective, with a coefficient of 500 to 10,000 W/(m2
K). According to Maxime
Vuckovic, sales and marketing manager for Calyos: ‘We can
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