Free cooling = reduced bills
24-03-2010 - John Hatcher
Brian Taylor, technical manager, Weatherite Building Services, offers some advice on adopting a free-cooling' approach to data centres.
Since the early days of computer rooms, the environmental control within the room has been achieved using close control air conditioning systems which maintain tight tolerances on temperature and relative humidity conditions.
Typically these have been in the order of 22CdB +/- 1C and 50 per cent RH +/-5 per cent RH and these levels have been necessary due to the tolerances dictated by the computer equipment suppliers to ensure stable operation and reliable performance.
As IT equipment and servers have developed over the years, the resilience of the components has increased dramatically and most of today's server rooms are, surprisingly, very tolerant to wide ranges of temperature and humidity.
In fact, many of today's electronic switch and servers are rated for much higher operating temperatures and most have very wide humidity thresholds.
In many instances, however the environmental design within telecom and data halls has remained unaltered and clients are beginning to realise the net result is large bills for their air conditioning energy use and high service costs.
Energy savings opportunities
There is however, a growing realisation that temperature set points can be increased, which reduces cooling energy usage and gives scope for systems incorporating energy saving' opportunities.
The rate of change of temperature has a much more significant effect on servers than the actual temperature. If relative humidity conditions are relaxed to their full extent, then even greater savings can be made. Most equipment is happy between an RH band of 20 to 80 per cent and as long as the necessary precautions are taken to avoid static discharge when working on the kit then RH control can be minimised or even removed in many cases.
With the acceptance of wider operating conditions, the type of air conditioning solution can be changed to incorporate free cooling' from outside air.
With a chilled water system, higher chilled water temperature mean not only is the chiller generating device more efficient but the return water can be part or wholly cooled by a secondary heat exchanger with outside air passing across it. When you consider that in typical UK temperature profiles, the air is below 12C for about 54 per cent of the time thereby substantially reducing running time and cost of a normal chiller cooling method.
Of course using chilled water, there are efficiency losses through heat exchangers and the pump power. If however, direct DX packages are utilised, outside air can be used directly to cool the room (mixed with proportions of warm room air during very cold ambients), there are no such efficiency losses and the quantity of free cooling is increased dramatically.
The higher the accepted threshold of the room temperature, the longer the period of direct free cooling is achieved and massive energy savings can be realised. In addition to this, there is much less run time for compressors and the servicing costs are reduced with life expectancy increasing.
With multiple stand alone DX units, reliability is increased, compared to centralised chiller plant. Lower installed mains power is needed and lower associated costs with standby generators. Indeed, if short term high room temperatures are accepted, standby supply could be restricted to cover for only fans and controls to rely on outside air cooling only under such circumstances.
Further energy savings can also be made by utilising initiatives such as low energy EC fans, optimised component selection and well designed and commissioned control systems.
Weatherite Building Services has been involved with companies such as British Telecommunications since the mid 1980s and BT has pioneered the use of these direct free outside air cooling schemes, pushing the boundaries out on data centre control temperatures. They have thousands of Weatherite packaged DX free cooling units successfully operating throughout the whole of the UK.