Experimental study on a novel rack-level integrated cooling system driven by a compressor and liquid pumps

Abstract

Conventional air-conditioners generally operate in vapor-compression (VC) cycles year-round for data center cooling, resulting in high energy consumption. An integrated cooling system, composed of a VC cycle and a pump-driven heat pipe (PHP) cycle, allows the VC cycle to be turned off when the cold source temperature is low, which can reduce energy use. However, the PHP cycle only operates efficiently under specific low-temperature conditions, and this restricts the annual energy-saving potential. This paper proposes a novel rack-level integrated cooling system to enhance energy-saving potential, which operates in three modes including liquid-pump-driven (LPD), integration-driven (ID) and vapor-compressor-driven (VCD) modes, corresponding to low, medium, and high cold source temperatures, respectively. Compared with a representative PHP/VC system, a gas–liquid separator near evaporator outlet, which operates in all modes, and a subcooler are introduced here. In the tests, the evaporating temperature was set within the range of 23.5–24.0 °C to maintain a server room temperature of 27 °C in accordance with ASHRAE recommendations. The proposed system exhibited better performance than the PHP/VC system. When the condenser inlet water temperatures were 14.5–18.5 °C, 9.8–13.5 °C, and below 9.8 °C, the energy efficiency ratio ($EE{R}_{\text{cp}}$) values of the proposed system were 10.0 %–22.3 %, 2348.5 %, and 26.5 % higher than those of the PHP/VC system. Furthermore, during transient tests under conditions with severe cooling load fluctuations across multiple parallel evaporators, the proposed system effectively supplied adequate refrigerant to prevent excessive overheating at the outlet of the evaporator with the highest cooling load.