Facility Level Energy and Cost Analyses for Water Cooled Supercomputer Systems

This paper describes the facility level design of future water cooled supercomputers with an emphasis on the energy efficiency opportunities related to the use of water cooled economizer based facility cooling in lieu of traditional refrigeration chiller architectures. A hybrid cooled 80kW server rack with both air and water cooled components is considered for the analyses. In this system, 90% of the rack heat load is rejected directly to pumped water lines via CPU cold plates and conduction cooling and the remaining 10% rack power is rejected to the room ambient air and is subsequently transferred to the water loop via air to liquid heat exchanger based devices. Air to water heat exchangers are not included in the server racks. The facilities cost for water cooling of high power computer racks has been analyzed for data centers situated in two different geographic locations. The study takes into account both the energy savings resulting from the loop design as well as the duration of time for which these savings can be realized at the selected locations. Test data for the water cooled CPU modules is used to project the maximum chip junction temperature for different chilled water temperatures which are in turn determined using the outdoor air temperatures for different times of the year. After calculating the 5 year energy consumption savings over the traditional plant design for each location, for the two different geographic locations in the US considered, it was found that the outdoor weather permitted the use of the economizer based configuration for 53–76% and 60–80% of the time for A and B locations, respectively, yielding per rack energy related savings of $6–21k and $17–26k, respectively, for the thermal conditions considered.