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Data centre energy consumption has grown significantly in recent years; cooling energy forms a substantial part of this and presents a significant opportunity for efficiency improvements. With best practice air management in place, it is possible to reduce airflow and increase supply air temperatures. This can be achieved through separation of hot and cold air streams by cold aisle, hot aisle or rack exhaust containment systems. Once server air inlet temperatures are within a narrow range, close to the supply temperature from cooling units, it is possible to increase temperature set points with the confidence that IT equipment receives air at an acceptable temperature. This allows energy savings to be realised through more efficient refrigeration cycle operation and increased opportunities to benefit from free cooling; in many cases it may be possible to remove refrigeration altogether. There are several different methods for free cooling using economizer cycles in data centres, which can be used instead of / in conjunction with traditional refrigeration for full or partial free cooling. These include direct and indirect air side and water side free cooling systems. The typical overall approach (difference between ambient wet bulb condition and data hall supply air temperature) is given for each of the free cooling methods, together with the projected maximum design ambient temperatures in different locations and the resulting maximum data hall supply air temperatures. The results indicate the extent to which zero refrigeration is possible for each type of free cooling system throughout the USA, when supplying air into the data hall within the ASHRAE recommended and allowable ranges. Zero refrigeration solutions result in a significant saving on capital cost due to the reduction in mechanical plant and associated supporting electrical infrastructure i.e. switchboards, generators, power distribution etc. In many US climates 100% free cooling (zero refrigeration) is possible and significant operational and capital cost savings can be realised. [ABSTRACT FROM AUTHOR]