Air cooling in the automotive world has practically disappeared, with the last major hold-out - Porsche - having moved to water-cooling its cars' engines more than 10 years ago in order to reduce emissions.

Data centres are moving in the opposite direction but for precisely the same reason: they're big carbon emitters, with recent research showing that they consume around 1.5 per cent of the US's annual energy output and this figure is growing.

What's to be done? From a physical standpoint, a data centre is a box containing equipment that generates heat, a process that consumes energy. To remove that heat and cool the box to an acceptable temperature - around 18-22 degrees - in the traditional manner requires refrigeration, which also consumes energy.

But there is a better way of achieving that end, one which works in most areas of the globe, and which cuts the use of increasingly scarce and expensive fossil fuel resources, and so so helps reduce the data centre's carbon footprint.

As a result of cost pressures, designs for air cooling the data centre have been proliferating in recent years, and now include systems from major data centre infrastructure vendors such as APC, as well as from companies in related areas, including one - Bladeroom - whose provenance is building air throughput systems for hospital operating theatres.

Closed circulation

Air cooled designs have a number of degrees of commonality. Most are closed circulation systems, which means that the air inside is not drawn directly from outside but is recirculated, so eliminating external contaminants. Outside air is cooled using adiabatic cooling, a process that uses recycled water trickling over a series of corrugated membranes while filtered air is drawn over the membranes.

The cooled air is then drawn over a heat exchanger stage, consisting of high surface area pipes, where heated air from inside the datacentre is cooled - outside air never mixes with air from inside the facility. Exhaust heat is pushed out of the heat exchanger - ideally, to help heat a local building - while the cooled internal air is recirculated around the data centre.

This avoids expensive, energy-hungry refrigeration most of the time, but is not the only tool in the box. Data centres using air cooling are often warmer than those using conventional methods on the basis that modern equipment tolerates a warmer environment, allowing the facility to run at 24-27 degrees, which of course reduces the cooling requirement.

"Few places on the planet where you won't save energy and money"

Today's computers are also more thermally aware so, if the worst happens and the cooling fails, servers, storage devices and switches are more able to sense this and shut down, rather than overheating and damaging themselves.

As well running warmer, air-cooled facilities often make use of other energy-saving techniques such as closed aisles, so incoming cold and hot exhaust air cannot mix, reducing the volumes of cooling air required.

While the efficiency of air cooling depends on the temperature of the outside air, there are few places where it doesn't work at all, according to designers of air-cooled facilities. Singapore and places like it are among them due to their combination of heat and high humidity - averaging 84 per cent and often as high as 100 per cent. While conventional refrigeration will be needed in hot, dry climates, there are few places on the planet where you won't save energy and money by moving to air cooling.

The essential measure of the efficiency of air cooling is the PUE - power usage efficiency - which is the ratio between the power directly used by the computers, including networking and storage, and the power used to support those functions, including cooling.

A conventionally cooled data centre might not do much better than a PUE of around 2, indicating that it takes as much power to cool the facility as it uses for doing useful computing work. Modern air-cooled facilities have been returned PUEs of 1.3 or lower - which represents a huge energy saving.

The innate and understandable conservatism of data centre builders may slow the rate of adoption of air cooling, but economics seem likely to dictate that it's a technique that will become more widely adopted. ®