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

Abstract

Liquid cooling is now a mainstream approach to boost energy efficiency for high performance computing systems. Higher coolant temperature is usually considered to be 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 20-30° temperature difference between the coolant and electronic parts of a server. Under the stress-test conditions the coolant temperature rose up to 65 °C while server operation remained. We also studied power efficiency (expressed in floating point operations per watt) dependence on the coolant temperature (19-65 °C) on the individual server level (based on Intel Grantley platform with dual Intel Xeon E5-2697 v3 processors). The power performance ratio shows moderate (≈10%) efficiency drop from 19 to 65 °C due to increase of leakage current in chipset components and reduction of processor frequency which 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.