AS chillers and close control units are optimised to make more effective use of free cooling, there is a need for specifiers to take a fresh look at their designs. Dean Ward of Walter Meier (Climate UK) explains how these principles apply to data centres
Operators of data centres, and the building services engineers that specify equipment for these applications, are currently facing quite a challenge.

On the one hand, the majority of such operators are trying to reduce their energy overheads and associated carbon emissions. On the other hand, as computer manufacturers pack more processing power into ever smaller servers, the electrical power requirements of data centres are increasing rapidly. As a result, and despite attempts to reduce the heat output of modern servers, heat loads for data centres are increasing rapidly.

In tackling this problem, it’s clear that the cooling for these applications needs to be made more energy efficient. This can be achieved by a two-pronged approach that includes the manufacturers of chillers and computer room air conditioning (CRAC) units making their equipment more efficient – and system designers thinking about how best to exploit those increased efficiencies.

Free cooling presents an obvious opportunity and there are now CRAC units and chillers on the market that have been optimised to take full advantage of even relatively high ambient temperatures – up to 15°C. They are designed to gradually reduce levels of mechanical cooling as ambient temperature falls, so that 100% free cooling can often be achieved at ambient temperatures of 0°C.

Design conditions

In fact, field trials have shown that this approach will deliver around 35% energy savings compared to traditional designs, when operating 24 hours a day – such as would be typical in a data centre application. In order to take full advantage of this equipment it makes sense to start with the general design conditions, acknowledging the fact that modern servers are more tolerant of elevated temperatures than earlier models. So it often isn’t necessary to design to 22°C at 50% relative humidity – most modern servers can tolerate temperatures up to 26°C without a problem.

This, in turn, opens the door for using higher chilled water temperatures – especially if higher air volumes are also used. For instance, the traditional 6°C flow/12°C return temperatures commonly used in general applications aren’t always appropriate for data centres. A flow temperature of 10°C and return temperature of 16°C will often be perfectly adequate, while increasing the potential for free cooling considerably – especially when the design also includes higher room temperatures.

As noted above, increasing chilled water temperatures will necessitate higher air volumes, so there will be an increase in fan power but this will almost certainly be less than the energy saved by maximising free cooling. It’s also worth bearing in mind that most data centres have more CRAC units than they need at any one time, to allow for maintenance, so they are usually running at part capacity.

Using variable speed fans will take advantage of the fact that a 20% speed reduction will result in a 50% reduction in energy consumption because of the cuboidal relationship between fan speed and fan power consumption. In addition, if space allows, CRAC units with larger fans can be used to reduce fan power consumption even further.

Of course, this entire approach needs to be backed by appropriate levels of resilience (2N or 2N+1) to ensure that if one unit fails the remaining plant is still able to maintain design conditions. And, of course, in the very unlikely event that all the plant failed at once, the temperature of an average data centre (with perhaps a heat load of 1500W/m2) will rise so quickly that it wouldn’t really matter if the space temperature is set to 22°C or 26°C.

Cooling towers

On the subject of system resilience, it’s worth pointing out that data centres are geared up to operate strict maintenance regimes so the arguments that are often used against water cooled chillers in some applications don’t really apply here. And if water cooled chillers are used it’s possible to achieve a much higher Seasonal Energy Efficiency Ratio (SEER) compared to perhaps 4 for an air cooled chiller. Plus, any free cooling will be based on the ambient wet bulb temperature – which will be lower than the dry bulb temperature.

Another area to consider is the humidification requirement in winter. Traditionally, electrode boiler humidifiers have been located in the CRAC units but rather than treating both fresh air and recirculated air it makes more sense to focus on the fresh air. And it will be more efficient to humidify this using a cold water spray humidification system than a steam boiler.

In taking this strategic view, considering all of the options and perhaps consulting specialists in this field it is possible to strike the right balance between protecting the end client’s investment and meeting their sustainability requirements.

Walter Meier (Climate UK) 0800 801 819