Performance evaluation of two-phase direct-to-chip liquid cooling combined with air cooling for data centers

The increasing power density of modern data centers presents significant cooling challenges, as traditional air cooling methods approach their physical and economic limits for artificial intelligence and high-performance computing applications. Integrating direct liquid cooling with existing air-cooled infrastructures offers a promising solution to enhance thermal management and energy efficiency. This study investigates the performance of a hybrid cooling system employing two-phase direct-to-chip liquid cooling, retrofitted to nine air-cooled Open Compute Project servers equipped with Intel Xeon E5 v3 microprocessors. Experiments were conducted in a controlled server wind tunnel environment to evaluate thermal and power characteristics under inlet air temperatures ranging from 25 to 45 °C and varying IT workloads. Results demonstrate substantial CPU temperature reductions, from 93 °C with air cooling to 56 °C with hybrid cooling at 25 °C inlet under full load. As inlet temperature increases on the air side, the two-phase system captures a greater proportion of the heat load, reaching up to 90% at 45 °C. These findings demonstrate the potential of hybrid cooling to extend the thermal envelope of existing air-cooled data centers beyond conventional ASHRAE A1 class limits, reduce operational costs, and facilitate waste heat recovery for secondary applications. • Two-phase direct-to-chip retrofit extends thermal envelope of air-cooled DCs. • Hybrid cooling enables higher air inlet temps without CPU throttling. • Significantly reduced air cooling demand and enhanced heat recovery with hybrid cooling. • Memory throttling under fixed fan speeds highlights airflow optimization needs with hybrid retrofitting. • Experimental validation of two-phase cooling in controlled wind tunnel setup.