Integrated Liquid Cooling with Heat Reuse: A New Generation of Energy Efficient Computers and Photovoltaics

The topic of this doctoral thesis is energy efficient electronic cooling using hot water as coolant. The aim of the work is to provide a viable solution to reduce the energy spent for cooling supercomputers and allow energy reuse for secondary applications. A transition from air cooling to single phase liquid (water) cooling is proposed to master the challenge of ever increasing heat dissipation densities in electronic components. Due to its superior thermal characteristics over the traditional air cooling, single phase liquid cooling of electronic components is now a well-recognized and practically unavoidable alternative to address rising heat dissipation densities. The thermal conductivity of water is by a factor of 24 higher and its volumetric heat capacity is by a factor of more than 3000 larger than that of air. Hence, the thermal resistance in water cooled solutions is reduced and the necessary temperature differential for efficient heat removal is drastically lowered. The resulting reduction in the required temperature difference between the coolant and the electronics allows the use of hot water for efficient heat removal. The use of hot water enables heat removal by passive heat exchangers towards the ambient or a secondary user of the heat. Therefore energy intensive chillers previously required to pre-cool the air become obsolete and the overall energy spent for cooling is almost cut in half. A compact thermal model to determine junction temperatures of microchips is developed and experimentally verified. The model is used to demonstrate that the application of a flow-control feedback loop could achieve a further reduction of the energy spent for cooling purposes. A microchannel manifold heat sink is used to experimentally demonstrate the feasibility of hot water cooled electronics. The microchannel manifold heat sink under investigation is a realistic, scalable design of a water cooled heat sink which is already included in a prototype hot water-cooled IBM BladeCenter QS22 / HS22 cluster named Aquasar. Aquasar represents an important stepping stone toward energy-aware computing because it directly repurposes excess heat for the university buildings. It is shown that water temperatures as high as 60◦C are sufficient to cool microprocessors with over 90% 1st law (energy based) efficiency. However, using