Liquid Cooling Gets Inside Chips

Researchers in Lausanne believe that liquid cooling should be used on a smaller scale than ever before, with tiny capillaries interwoven in the silicon chips at the heart of servers. The group demonstrated their ideas at the launch of the  €2.9 million EU-funded GreenDataNet project, which aims to reduce data centre energy demands by 80 percent.

A set of initiatives based at Ecole Polytechnique, Federale de Lausanne (EPFL) proposes to reduce the size of server electronics radically, by using many layers of silicon, with tiny channels sandwiched in-between that will carry a coolant liquid.

Coolant that goes through the chip

The CMOSAIC project suggests greater integration of chips in 3D with memory stacked on top of processors. This could enable higher powered systems which would operate faster because, for instance their memory is closer to the processor, and signal delays are reduced – but the concentrate of electronics would have a heavy power demand – something like 100W per square centimetre in each layer, so that a cubic metre of such a material would need something like 1GW, according to calculations by the project leader, Professor David Atienza.

To remove heat more efficiently, the group is working on two-phase cooling, where some of the liquid is vapourised as it passes across the hot electronics. This carries away much more heat, but results in a flow of liquid containing bubbles which can easily become turbulent or create dry, hot areas. A large part of the group’s work has focused on creating micro-channels, thinner than a human hair, that can carry two-phase flow without turbulence or back-flow.

“Two-phase cooling is more efficient because there is no temperature gradient,” said post-doctoral researcher Sylwia Szczukiewicz, who has captured high-frame rate videos of fluid travelling through microchannels in a heatsinnk, attached to a device which simulates the heat output of  a processor chip. As a result, the team has developed an input nozzle with a lip which eliminates the turbulence.

Liquid cooling will be included in the GreenDataNet project, as it produces waste heat in high concentration, compared with air cooling. This allows the energy to be reclaimed, increasing overall efficiency. This is an important factor in the European region, where population density is quite high, so data centres tend to be in populated areas where the heat can be re-used to warm offices and homes, explained  Babak Falsafi, the director of EPFL’s EcoCloud unit, which manages CMOSAIC and other projects.

Reducing energy use still further, the group is working on gravity-fed circulation of the coolant, to eliminate the use of pumps.

This won’t be an overnight success of course. CMOSAIC has been underway for some years and the group had no direct commercialisation plans to share when TechWeek Europe visited last week. However, given the inevitable move to liquid cooling and the obvious benefits of embedding that cooling within the chips, this looks like a good bet for a future technology direction.

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