Comparison of Overhead Supply and Underfloor Supply with Rear Heat Exchanger in High Density Data Center Clusters

The power trend for server systems continues to grow thereby making thermal management of data centers a very challenging task. Although various configurations exist, the raised floor plenum with computer room air conditioners (CRACs) providing cold air is a popular operating strategy. In prior work, numerous data center layouts employing raised floor plenum and the impact of design parameters such as plenum depth, ceiling height, cold isle location, tile openings and others on thermal performance of data center was presented. The air cooling of data center however, may not address the situation where more energy is expended in cooling infrastructure than the thermal load of data center. Revised power trend projections by ASHRAE TC 9.9 predict heat load as high as 5000 W per square feet of compute servers' equipment footprint by year 2010. These trend charts also indicate that heat load per product footprint has doubled for storage servers during 2000-2004. For the same period, heat load per product footprint for compute servers has tripled. Amongst the systems that are currently available and being shipped, many racks exceed 20 kW. Such high heat loads have raised concerns over limits of air cooling of data centers similar to air cooling of microprocessors. A hybrid cooling strategy that incorporates liquid cooling along with air cooling can be very efficient in such situations. The objective of this paper is to study and compare the performance of hybrid cooling solution in two widely used air supply configurations namely overhead supply and underfloor supply focusing on rack inlet temperature. The numerical models of a representative data center employing overhead and underfloor supply with hot aisle-cold aisle arrangement are constructed using a commercial CFD code. The effect of these configurations on rack inlet temperature is discussed.