Server level liquid cooling: Do higher system temperatures improve energy efficiency?

Liquid cooling is now a mainstream approach to boost energy efficiency for high performance computing systems. Higher coolant temperature is usually considered as an advantage, since it allows heat reuse/recuperation and simplifies datacenter infrastructure by eliminating the need of chiller machine. However, the use of hot coolant imposes high requirements for cooling equipment. A promising approach is to utilize coldplates with channel structure and liquid circulation for heat removal from semiconductor components. We have designed a coldplate with low heat-resistance that ensures effective cooling with only 2030° temperature difference between coolant and electronic parts of a server. Under stress-test conditions the coolant temperature was up to 65 °C while server operation was undisturbed. We also studied power efficiency (expressed in floating point operations per watt) dependence on the coolant temperature (19-65 °C) on theindividualserverlevel (based on Intel Grantley platform with dual Intel Xeon E5-2697v3 processors). иThe power performance ratio shows moderate (≈10%) efficiency drop from 19 to 65°C due to increase of leak age current in chipset components and reduction of processor frequency resulted into proportional reduction of DGEMM benchmark performance. It must be taken into account by datacenter designers, that the amount of recuperated energy from 65 °C should be at least≈10% to justify the choice of high temperature coolant solution.