Experimental study of operating characteristics of a novel rack-level hybrid cooling system for data centers.

Conventional data center air-conditioners consume excessive energy. Rack-level hybrid cooling systems combining a pump-driven heat pipe (PHP) and a vapor-compression (VC) cycle provide a promising alternative but often encounter refrigerant starvation in evaporators. Therefore, we previously proposed a novel rack-level hybrid cooling system adopting gas–liquid separation in all modes and a subcooler. However, prior work studied its energy efficiency ratio (EERcp) only at fixed vapor quality, leaving key operating characteristics unexplored. To address these gaps, this paper experimentally investigated the novel system’s response characteristics under varying vapor quality, condenser-side disturbances, and non-uniform cooling loads, including variations in mode-switching temperatures, EERcp, and refrigerant state. Experiments were conducted on a prototype setup with five plate heat exchangers representing rack-level evaporators, at evaporating temperatures of 23.5–24.0 ◦C to match the ASHRAE-recommended 27 ◦C server room. Results show that reducing the evaporator outlet vapor quality from 0.76 to 0.58 decreased EERcp by 13–30 %, shifted mode-switching temperatures by 2–4 ◦C, and increased evaporator inlet subcooling by 1.6–2.9 ◦C. The 6 ◦C disturbances in the condenser inlet water temperature (Tcon,win) caused transient fluctuations in evaporator inlet subcooling, yet it still remained above 0 ◦C to maintain the flow distribution among evaporators. Under severe non-uniform cooling loads, the system effectively prevented local hotspots, with EERcp decreasing by only 0.1–0.2 in vapor-compressor-driven and hybriddriven modes, and remaining 127 in liquid-pump-driven mode. Although based on prototype experiments, these findings provided useful insights into strategies for operating the novel system in practical applications.