Extreme energy efficiency using water cooled servers inside a chiller-less data center

The paper summarizes part of a project that was undertaken to develop highly energy efficient warm liquid cooled servers for use in chiller-less data centers that could save significant data center energy use and reduce data center refrigerant and make up water usage. One of the key concepts developed as part of this project is the Dual-Enclosure-Liquid-Cooling (DELC), which comprises of a 100% liquid cooled server cabinet and an outdoor dry cooler unit for heat rejection to the ambient and this configuration is used to reject the Information Technology (IT) equipment heat load directly to the outside ambient air without the use of a chiller. Demonstration hardware for server liquid cooling and a chiller-less data center was built and is operational for a 15 kW rack fully populated with liquid cooled servers which has been designed for use for up to 45°C liquid coolant to the rack. The anticipated benefits of such energy-centric configurations are significant energy savings of as much as 25% at the data center level. This paper builds on recent work that focused on the server liquid cooling, the rack enclosure with heat exchanger cooling and liquid distribution, and the data center level cooling infrastructure and which also presented sample data from experiments in support of the DELC concept. This paper presents experimental data related to the novel data center loop in a new manner, which is used to create a simplified thermodynamic model using curve-fit of surfaces of heat exchanger approach temperatures and power use of cooling devices. The model is validated using experimental data for a 22 hour test that was conducted in August of 2011. Subsequent to model validation, the simplified model is then used to make projections for DELC prototype performance (thermal and energy) under different conditions including different simple control schemes and weather conditions in the US. Weather data from nine different US cities is analyzed for a single day in August and realizable energy and energy cost savings over traditional chiller based data center cooling designs are presented. The results show that the new innovative data center cooling configuration presented could reduce cooling energy use to be less than 3.5% of the IT power for most US locations even in warm summer times of the year.