Taking the Temperature

11-04-2012 - John Hatcher

While energy consumption continues to be a secondary consideration to maintaining the functionality and resilience of a data centre, the whole issue of energy efficiency and carbon footprint is certainly rising up the agenda. Consequently there is growing pressure to find ways of reducing energy consumption without compromising core functions.

In parallel, of course, as server processing power increases so too does the cooling load, making the whole situation even more challenging. However, at the same time, newer servers are becoming more tolerant of higher temperatures, typically up to 26°C without any impairment to their function. Therefore, designing to the ‘standard’ set-point conditions of 22°C at 50% relative humidity may not be necessary in newer installations.

Clearly this is a decision that has to be made on a project-by-project basis and many data centre managers may be nervous of straying from the ‘norm’. However, given the very high heat loads in a typical data centre, a major cooling failure will lead to such a rapid temperature increase that a few degrees difference in set-point will make very little difference.

The advantage of allowing set-point temperatures to be higher is that it gives more flexibility to configure the cooling systems for higher energy efficiency. For example, where it is possible to operate with higher temperatures there is an opportunity to use higher chilled water temperatures; perhaps 10°C/16°C flow/return, rather than the traditional 6°C flow/12°C return temperatures. Here, it will often be necessary to increase air volumes as well, resulting in an increase in fan power, but the overall energy savings will still be significant.

Another advantage of working with higher chilled water temperatures is that there is more scope to use outside air for cooling the water to the required temperature – which is known as ‘free cooling. To that end there are now computer room air conditioning (CRAC) units and chillers that have been optimised to maximise free cooling – even at outdoor temperatures as high as 15°C.

So at 15°C the outdoor air can be used to pre-cool the water, which is then cooled further by running the chillers to reach the set-point flow temperature. The lower the outdoor temperature, the higher the contribution free cooling can make, so that even at traditional chilled water temperatures 100% free cooling can often be achieved at ambient temperatures of 0°C. Consequently, working with higher chilled water temperatures maximises the potential for free cooling.

Clearly this offers significant energy-saving opportunities. Indeed, field trials with plant optimised for free cooling, operating for 24 hours a day – as would be typical for a data centre – have shown that around 35% energy savings can be achieved compared to traditional designs.

The high level of resilience required for data centres means that they often have more CRAC units than are needed to meet the cooling loads at any one time. This safeguards against breakdowns and enables units to be taken off line for maintenance. However, it also means that the units may be running at part-load for much of the time, which is potentially inefficient. Here, using variable speed fans will help to optimise the efficiency of individual units while maintaining the spare capacity to meet resilience requirements.

Another consequence of high resilience is the need for strict maintenance regimes, which opens the doors for wider use of water-cooled chillers with cooling towers, which are considerably more energy-efficient than air-cooled chillers. In most commercial environments air-cooled chillers are favoured because of their lower maintenance requirements, but this is less of a barrier in a high maintenance environment.

Needless to say, any measures that are taken to improve energy efficiency and reduce carbon emissions must not compromise the required level of resilience for the facility. Therefore, the measures outlined here, and others that could be introduced, need to be considered in the light of the individual requirements of each data centre. The key is not to be hide-bound by what has always been done in the past, but to look at the possibilities that are available today with an open mind and explore all of the opportunities.