Why Data Centre Cooling Is Getting Simpler

Free air and evaporative cooling are far more simple than the chillers and refrigeration units they could replace, says Peter Judge

Data centres are getting warmer, and the technology used to cool them down is actually getting older and simpler.

End users are gradually becoming more comfortable with allowing their servers to get hotter – in order to spend less money cooling them down. That was a clear message from a webinar I chaired last week called Best Practices for Optimizing Data Center Efficiency and Sustainability (sorry about the multiple US spellings).

Data centre managers used to aggressively cool their servers down by blowing chilled air through them – heedless of the economical and environmental costs of the refrigeration and air-conditioning plant this required.

Most people used more electricity to cool the air they blew through their servers, than they did powering the processing and data storage in their IT kit. In the normal measure of data centre efficiency, this translates to a PUE (Power Usage Effectiveness) of more than 2.

These days, people are pushing for PUEs ever closer to the theoretical minimum, 1.0. In the process, they are raising the temperature of their servers, but keeping within the ratings published by manufacturers so as not to void their warranties. Temperatures are rising above 20C, and may go higher than 35C in future, providing cost savings in the energy used.

At the same time, different means of cooling are being used – and in fact this means using more traditional methods of cooling, instead of energy-guzzling refrigeration plants. Two of these ideas were combined last week in a modular cooling system for data centres launched in London.

“Free air” cooling and evaporative cooling, both featured in the launch of  EcoBreeze system from APC (full company name “APC by Schneider Electric”) – and both use simple technology that has been well known for hundreds of years.

Like a wet T-shirt

Though both systems use simple concepts, they have to be handled very carefully in the controlled environment of a data centre.

Free air, in a nutshell, is like opening the windows. For most of the world, on many days of the year, the outside air is cool enough to keep the servers at a sensible temperature.

It’s not as simple as just pumping outside air through the room’s hot aisle though, because the dust would kill too many of the servers. As APC’s John Bean Jr points out, killing just a small percentage of your servers would wipe out any savings from using less energy.

Instead, the air has to be carefully filtered or, as in EcoBreeze, actually kept in a closed circulating system, with the outside air only passing through a heat exchanger.

Evaporative cooling, Bean says, is “like a wet T-shirt”. Now, he might be giving us a suggestive picture opportunity (which, as a mostly-serious publication, we are  ignoring). But he’s actually got a point. Wearing a wet T-shirt on a hot day cools you down as the water evaporates, taking energy away, and lowering your body temperature to the “wet-bulb” temperature, which also depends on humidity.

The idea has been established and is quite widely used by Facebook among others, though Bean says APC’s version is a more fully productised instance of it. In the heat exchanger, the pipes carrying the internal air can be dampened using a water system, so the outside air cools it down to the wet-bulb temperature.

Again, though, it’s not as simple as just spraying water around. APC collects the sprayed water, to minimise the requirements of the system, cleans bugs out of it using electromagnetic pulses, and drains it out of the system when it’s not needed. You don’t want pipes full of water on a very cold day, as they will freeze and break!

Economisers rule

The idea of free-air cooling has been enshrined in the ASHRAE 90.1 standard from the American Society of Heating, Refrigeration and Air-Conditioning Engineers, which caused controversy by laying down free-air “economisers” as a technology that should be used.

In fact, the ASHRAE rules are not going to be rigidly enforced in the US, and the simple fact that economisers are a sensible solution is pulling them into the market, particularly in Europe, says Bean.The demand for these technologies is what has inspired the company to produce an off-the-shelf cooling system based on both, packaged in standard shipping containers which can be parked outside the walls of a data centre.

If EcoBreeze is a sign of things to come, then evaporative and free-air cooling could bring a breath of fresh air to the data centre.