Data centres Energy efficiency
> A low PUE
will become an increasingly important design objective for clients
factory-engineered and assembled product is far better than the quality control that can be achieved on site. Adiabatic cooling is now recognised as a safe design option and, in recent years, has become increasingly favoured by designers. In this option, outside air is admitted to large air- handling units (capable of delivering air volumes of 15 to 25 cu m/s), where it is filtered and cooled by passing air through spray washers. Water treatment is highly controlled and monitored. Reserve or supplementary CHW or DX cooling coil units are normally provided in the AHU, although these units may only need to operate for very short periods annually. Multiple fans are arranged in parallel operation, normally with 20% redundancy (for example, twelve fans, two of which are redundant). As this solution uses large volumes, potentially
100%, of outside air, free cooling can be used for the majority of the year. This is particularly the case if the conditions in the ‘cold’ aisle can be permitted to extend on a high safe side to, say, 25C Dry Bulb and 70% RH. ASHRAE offers a recommended tolerable environmental envelope of 18C to 27C Dry Bulb and 40% to 80% RH. This application can yield an impressive PUE of
condenser water system). This arrangement can be used in the event of chiller failure or to use condenser water optimiser ‘free’ cooling operating regimes.
Conclusion It is unlikely that server rack load densities will increase signifcantly above their current levels without a major revolution in microprocessor chip design: it is hard to see how server-rack densities can comfortably exceed a peak of 35kW in the foreseeable future. At this rack load density, the application of in-row cooling units that brings the cooling source in close proximity to the cooling demand is likely to be the most, possibly the only, effective cooling solution. It is also likely that data hall capacity/footprint will need to increase to facilitate further growth, at least in the short to medium term. A larger data hall facility footprint will also improve
(dilute) unit rack infrastructure CAPEX through economies of scale. Future higher energy and operating costs will mean that the achievement of a low PUE (l CIBSE is planning to publish guidance on data centres next year. More details will be available in coming months
Chris Beier BSc(Hons), CEng, FIEI, FCIBSE, FIMechE
www.cbass.ie
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