Energy Efficiency Analyses and Comparison of Air and Water Cooled High Performance Servers

This paper presents a study that illustrates the energy efficiency benefits of using water cooling for server thermal management versus utilizing air-cooling for high performance server applications. In 2008 IBM introduced the water cooled Power 575 Supercomputing Node, which is packaged in a super-dense 2U (88.9 mm) form factor. A fully configured system can dissipate as much as 72 kW with 80% of the heat load going to water. A fully air cooled version of this system, developed by Hitachi, operates 3.5 GHz and dissipates 61 kW at the frame level. In this paper, numerical analyses is first carried to help provide a context for the data center air-conditioning energy consumption, necessary to cool comparable performance clusters for both the air and water cooled racks, respectively. The room level CFD calculations are then extended to the facility level, by including the energy consumption of the refrigeration chiller, the building and condenser water pumps, and the cooling towers. Recent innovations that enable the use of water side economizers as well as 100% water cooling of racks are also discussed. The results show that there can be as much as a 45% reduction in the facilities power to cool the water cooled system compared to the energy required for the air cooled equivalent. When 100% of the server heat load is transferred directly to the chilled water supplied to the racks, there can be a 50% energy savings compared to traditional air-cooling. Lastly, use of 100% water cooling in conjunction with water side economizers can yield more than 90% cooling energy savings compared to the air-cooled configurations.