A trip to your next data centre trade show or conference will bring into sharp focus the desire of the data centre industry to create a data centre model that requires little to no mechanical cooling.
ASHRAE’s 2011 Thermal Guidelines for Data Centre Processing Environments drastically expanded the allowable temperature range for data centre operation. In particular, the A3 and A4 classification looks at scenarios when the data centre operator and design team are planning for a true non-mechanical cooling system data centre and using only air side economisation.
The Pro's and Cons of Fresh Air Cooling.
The lack of capital outlay for expensive chilled-water systems or air-cooled CRAC units dramatically reduces the initial cost to build a new data centre. In addition, the continued operational cost of the space is much lower, due to the lower energy cost of operating an air-side economised data centre cooling system versus a large mechanical plant, even one with the most efficient design.
Good arguments also suggest that an air-side economised data centre could prove to be more reliable and better able to handle variable loads introduced to the data centre space by newer IT equipment. The improvement in reliability can come from the instantaneous startup capability of an air-side economisation system. This can provide immediate or continuous cooling during a power failure, as larger chilled-water plants typically cannot offer this capability during switchover to generation, due to compressor time outs and other factors. The ability to handle variable-load profiles has also improved, since high levels of step loading or load shedding that may wreak havoc on mechanical cooling systems are simply not an issue with an air-side economisation system.
The Other Side of the Coin
The biggest drawback identified in conversation revolves around the people issue. If a data centre is to operate in an A3 or A4 classification environment, “cold” aisle temperatures could reach 113F (45C). Then, depending on the delta T of the IT equipment, which could exceed 50F (28C), hot aisle temperatures could exceed 163F (73C). Clearly, these aisle temperatures are extremes, but inside the allowable classifications, however those hot aisle temperatures are quite possible, even in temperate climates. To operate in these environments, standard “cold”/”very hot” aisle arrangements will not work, even with physical aisle containment.
Planning for Extremes
A strategy for delivery of outside air in a separate plenum, ideally below the floor, to the IT equipment, and then return in a separate plenum in the form of a drop ceiling, would protect the data centre operator and any others from these extreme environments. The remainder of the data centre space could be comfort cooled like any other building space, allowing for comfortable people and efficiently cooled IT equipment.