Data centres Energy efficiency
Faced with high server-rack densities, designers have increasingly adopted the hot-aisle/cold-aisle concept
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CRAC units for reprocessing and then returned again to the ‘cold’ aisle. Floor supply air grilles are located in the ‘cold’ aisle only, and return air grilles are located at high level in the ‘hot’ aisle. The only permissible airflow path from ‘cold’ aisle to ‘hot’ aisle is through the server racks. Any other airflow path is deemed to be undesirable short-circuiting or leakage, compromising overall system efficiency. This same concept can be applied to the use of in-row cooling units in lieu of CRAC units. In-row cooling is discussed again later. There should be no concern about the ‘hot’ aisle
reaching temperatures as high as 35C: this simply demonstrates that the system is extracting large amounts of heat from the racks, which is what it is supposed to do. What is critical is the server rack intake temperature.
This is governed by the server manufacturer, and it is the designer’s responsibility to ensure that this reservoir of chilled air is available in adequate and filtered quantities, at the right temperature and relative humidity.
Glossary/definitions
COP: Coefficient of performance = Refrigeration Capacity / Compressor Power (a COP>3.0 is preferred)
PUE: Power utilisation effectiveness = total data centre power / IT power (a PUECRAC: Computer room air conditioning unit
Cold aisle: A contained space where air (ideally at 18C to 25C) is delivered directly to server racks
Hot aisle: A space (generally the volume of the data centre hall excluding the cold aisles’ volume) which receives hot exhaust air from server racks and is then returned to CRAC’s or other cooling units for reprocessing
Adiabatic cooling: Cooling achieved by passing air through a water spray washer – this maintains a constant wet bulb temperature and reduces the dry bulb temperature
It is essential that this reservoir of expensively
generated chilled air is not permitted to go anywhere other than through the server racks. Otherwise, the cost of chilling and distributing the air is wasted, and the risk of ‘hot spots’ occurring is greatly increased. The emergence of ‘hot spots’ in an already established and populated data centre is not easily remedied.
Chilled-water temperatures The cold aisle temperature will ultimately depend on the chilled water flow and return temperatures to and from the CRAC units. Chilled water flow and return (CHW F&R) temperature selection may be governed by:
1. Existing site-wide system CHW F&R conditions/ constraints;
2. Avoidance of latent cooling (dehumidification) and therefore avoidance of re-humidification. Where possible this inefficient and costly process should be avoided;
3. Optimisation of chiller coefficient of performance (COP); thereby PUE optimisation; and
4. Using ‘free cooling’ where chillers have ‘free cooling’ heat exchangers that can avail of low ambient temperatures without the need for chiller compressor operation. This again will also yield PUE credits.
CHW F&R temperatures that range between 15C
and 21C can offer the greatest potential for free cooling in the UK and Ireland for several months of the year, when ambient air temperatures fall below 13C. Even above this ambient temperature, some free cooling is available. An example of potential cost savings for a data centre
with a 1,000 kW cooling load using ‘free cooling’ by air cooled chillers is given here:
56 CIBSE Journal November 2010
www.cibsejournal.com >
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