Hybrid Two-Phase Cooling Technology For Next-Generation Servers: Thermal Performance Analysis

This paper advances the work presented at ITHERM 2020 where a novel passive two-phase technology was introduced for more efficient cooling of datacenters compared to air-based cooling solutions (e.g. In-Room, In-Row, etc.). The proposed hybrid two-phase cooling technology uses passive low-height thermosyphons to dissipate the heat generated by high power components inside the servers. The heat is then transferred to multiple rack-level thermosyphons, equipped with one or more overhead compact condensers, which in turn dissipate the total heat from the rack into a room-level water-based or air-based cooling loop. Air-cooling is still used for the low heat-generating components (e.g. memory units, secondary chips, etc.) to make the cooling technology cost effective. This study is focused on a thermal performance analysis of the hybrid two-phase cooling technology applied to a single server (2U HP ProLiant DL180 G6), where low-height thermosyphons are installed to cool the two CPUs, and the remaining server components are cooled via air in forced convection using server-level fans. The thermal performance of the low-height thermosyphons is extracted from previous experimental measurements carried out at Nokia Bell Labs and presented at ITHERM 2020. Additional measurements considering the effect of the refrigerant charge on the thermal resistance and associated chip case temperature are conducted. The air-cooling performance and flow distribution are investigated via CFD simulations performed in COMSOL that characterize the conjugate heat transfer between server components and cooling air. The experimental and numerical results presented here show that, at the maximum server IT load of 587 W, the hybrid two-phase technology ensures sufficient cooling for all the hardware components while significantly reducing the energy consumption of the fans. Specifically, the fan contribution is reduced from 24% (traditional air-cooling) to about 2% of the server effective IT load (hybrid two-phase cooling), demonstrating the potential of the proposed technology to scale existing hardware and build next-generation datacenters, while keeping low costs and being environmentally-friendly.