Low-Pressure Heat Transfer Fluids for Pumped Two-Phase Cooling

Abstract

The increasing heat flux from chips, and high server- and rack-level heat densities in high performance computing infrastructure have resulted in the advent of advanced cooling technologies, such as cold plates and immersion cooling. Water-based heat transfer fluids are used in cold plate systems. It is widely debated if it will be effective and energy efficient for cooling of chips with heat flux exceeding 500 kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Two-phase cooling with dielectric fluorinated fluids, such as hydrofluorocarbons (HFC), hydrochlorofluoroolefins (HCFO), hydrofluoroolefins (HFO), hydrofluoroethers (HFE), in cold plates may be more suited to sustain the ever-increasing heat density. The need to cool devices with ever-increasing power density, the push towards lower global warming potential fluids, and the occupational and equipment safety requirements has motivated a search for suitable two-phase heat transfer fluids. This paper introduces the environmental and occupational safety aspects, and effect of global regulations on the selection of fluorinated heat transfer fluids for use in data centers. This study presents drop-in performance evaluation of low-pressure fluid options, namely, HCFO-1233zd (E), HFC245fa and HFE-7000, in pumped two-phase cooling system for heat flux up to 640 kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Performance is also compared with single-phase water cooling. The performance is reported in terms of thermal resistances of the cold plate and the system. Compared to water cooling, two-phase cooling achieved lower junction temperatures and more uniform cooling at much lower flow rate. System level thermal resistance was lower for two-phase cooling, except with HFE-7000, which had considerably high thermal resistance in the condenser. Unlike HCFO-1233zd(E), HFC-245fa will be eventually phased-down globally, therefore it is not a sustainable long-term option for the industry. This study shows that HCFO-1233zd(E) is a feasible low-pressure heat transfer fluid option for thermal management of high performance computing infrastructure.