Two-Phase Liquid Cooling Bubbles Up

Let’s take it as read that liquid cooling is better for your data centre than air cooling. But can you afford it, and is it mature enough yet?

Liquid cooling definitely wins on the energy budget. Liquid conducts away heat better, and a well designed system doesn’t need much pumping, compared with the amount of energy required to drive fans and air-conditioning systems to remove heat using air circulation. You also get to re-use the heat in the liquid, as it comes in a nice pumpable concentrated form.

Liquid cooling is specialised

But at the moment, liquid cooling systems are specialised, and built for a small market. This means they are more expensive. Possibly most significantly to people building data centres for the long term, they are very varied.

A variety of cooling systems is a sign of a market that hasn’t yet matured. Buy into the wrong one and your data centre may be left in a technological blind alley.

So we have various systems that use water to cool servers, sometimes simply running through the cabinet doors but in more advanced systems, closer to the electronics – but obviously well separated from them.

We also have systems which circulate inert fluid across the electronics. But here there are different approaches. You can have a specialist circulation system which takes the fluid through specialised blade server packages (like Britain’s Iceotope), or you can have an immersion cooling system (like Green Revolution Cooling) which immerses an entire rack in a bath of liquid.

Beyond that, in the research stage, there are people working on running the fluid through tiny capillaries which can be built inside stacks of chips, allowing much greater integration. That’s a very exciting idea, but not on the market any time soon, and still under development at universities, including the CMOSAIC project at EPFL in Lausanne.

Two-phase involves boiling fluid

Another difference in approach is between single-phase or two-phase cooling. Simply put, this is the choice between letting the fluid boil or not.

Allowing your cooling fluid to boil may sound like a bad idea. You don’t want that to happen in your car’s engine for instance. But a cooling system designed to operate with two phases removes heat more efficiently, because of the latent heat of vaporisation.

The CMOSAIC team has proven you can use two-phase cooling inside their tiny capillaries without allowing hotspots to develop.

And meanwhile, SGI has demonstrated a supercomputer called ICE-X which is cooled with a boiling fluid. 3M provides the fluid, and Intel is a partner, both of which lend cred to the project.

But this shows up how experimental this all is. It’s a new use for the 3M fluid, called Novec, even though the same stuff is already there in other cooling systems, including Iceotope’s which is – so far as I know – only a single-phase system.

It’s also Intel’s second look at immersion. Two years ago, it tested and approved the immersion system from Green Revolution which uses a different cooling fluid and isn’t (again as far as I know) a two-phase system.

The ICE-X system goes a long way to showing the practicality of liquid cooling. Buit it adds another variety to the liquid options, which must make data centre owners wary of a market that still has to fully mature.

A version of this article appeared on GreenDataCenterNews. 

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