Data centres Energy efficiency
Cooler
Designing and engineering data centres to be energy efficient is becoming increasingly challenging for professionals. Chris Beier looks at the key design issues and solutions surrounding these computer powerhouses
he design of new data centres has undergone rapid change and development over the past few years, with a significant increase in the energy load of computer-server racks. Up
to about five years ago a 3kW server rack load was considered high-density, but now loads in excess of 23kW are not unusual. These cannot be cooled by a conventional underfloor air distribution system that cools the whole room. This eightfold increase in energy load has been
driven by higher and faster data processing demands – which, in turn, have required the development of more innovative and energy efficient design solutions for data centre cooling and air distribution systems. Designers can no longer apply a ‘broad brush’ cooling
design load criterion of 750W/sq m with confidence for data centre halls. This was generally the accepted norm applied to these environments during the 1990s. It is also the average recommended IT load quoted in the CIBSE Guide B, 2005, Section 2.3.9.1, which is currently being updated to take account of new high-density server racks. Recognising the rapid advances in future high server-
rack loads, designers now need to be fully briefed, and able to obtain in advance from their clients the server- rack load expectation (both in terms of maximum load density and load density distribution) for a new or upgraded data centre. Other crucial factors that need to be considered
by designers at the outset are levels of system redundancy, system resilience, uninterruptible system
54 CIBSE Journal November 2010
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maintainability, power usage effectiveness (PUE), growth scalability, environmental considerations, CAPEX (capital expenditure) and OPEX (operating expenditure).
Hot and cold aisles Faced with high rack densities, designers have increasingly adopted the hot-aisle/cold-aisle design concept for data centre cooling. There are several variants of this theme but, most typically, the design incorporates computer room air conditioning (CRAC) units and raised floor as the cooling delivery infrastructure. Ideally, CRAC units are located outside the data hall in an adjoining service corridor, which allows for easy maintainability of the units, while also offering a ‘clean’ data hall for optimum server rack space planning. The concept focuses on separation of the server rack
inlet cold air and the exhaust hot air leaving the server rack. The fronts of the racks are normally configured to face each other; for a contiguous row of racks in this front-to-front arrangement, the space in between is known as the ‘cold’ aisle. For the ‘cold’ aisle to work properly it is physically
enclosed by end and top enclosures. Air from the ‘cold’ aisle (typically maintained at or a little above 20C – again, there are variations to this server delivery temperature) is induced through the high heat load server racks. Hot air is discharged from the rear of the server racks into the ‘hot’ aisle (typically 30C or higher). Air from the ‘hot’ aisle is then returned to the
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