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This paper summarizes the concept design and hardware build efforts as part of a US Department of Energy cost shared grant, two year project (2010-2012) that was undertaken to develop highly energy efficient, warm liquid cooled servers for use in chiller-less data centers. Significant savings are expected in data center energy, refrigerant and make up water use. The technologies being developed include liquid cooling hardware for high volume servers, advanced thermal interface materials, and dry air heat exchanger (chiller-less with all year “economizer”) based facility level cooling systems that reject the Information Technology (IT) equipment heat load directly to the outside ambient air. Substantial effort has also been devoted towards exploring the use of high volume manufacturable components and cost optimized cooling designs that address high volume market design points. Demonstration hardware for server liquid cooling and data center economizer based cooling has been built and is operational for a 15 kW rack fully populated with liquid cooled servers. This design allows the use of up to 45 °C liquid coolant to the rack. Data collection has commenced to document the system thermal performance and energy usage using sophisticated instrumentation and data collection software methodologies. The anticipated benefits of such energy-centric configurations are significant energy savings at the data center level of as much as 30% and energy-proportional cooling in real time based on IT load and ambient air temperatures. The objective of this project is to reduce the cooling energy to 5% or less of a comparable typical air cooled chiller based total data center energy. Additional energy savings can be realized by reducing the IT power itself through reduced server fan power and potentially less leakage power due to lower device temperatures on average for most locations. This paper focuses on the server liquid cooling, the rack enclosure with h- at exchanger cooling and liquid distribution, and the data center level cooling infrastructure. A sample of recently collected energy-efficiency data is also presented to provide experimental validation of the concept demonstrating cooling energy use to be less than 3.5% of the IT power for a hot summer day in New York.