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Thermodynamic Analysis of Full Liquid-cooled Data Centers
Keywords: data center, direct liquid cooling, hybrid liquid-air
Data centers currently account for 1.5% of the total electric consumption in the United States, and according to the U. S. Department of Energy, this amount is expected to grow by 12% every year based on current trends (Anderson, 2010; Brown et al., 2008). One way to model the thermodynamic efficiency of a data center is to apply a second-law analysis through exergy calculations of the individual components. Recently, the in-house data center modeling tool Villanova Thermodynamic Analysis of Systems (VTAS) was used to determine the influence of various hybrid liquid-air technologies on the overall exergy destruction in a data center (Bhalerao, Wemhoff, & Ortega, 2014). Multiple configurations of cooling equipment, including computer room air handling (CRAH) units, rear-door heat exchangers (RDHXs), in-row coolers (IRHXs), and overhead coolers (OHXs), were compared. The results showed that using hybrid liquid-air technologies resulted in a lower overall exergy destruction compared to legacy air cooling. It was also shown that configurations containing CRAH units yielded the highest overall exergy destruction, suggesting that configurations without CRAH units are preferable for better thermodynamic efficiency. This result is expected since the heat removal occurs at higher temperatures in the hybrid liquid-air coolers compared to the CRAH units. To study the effects of direct liquid cooling on data center performance, VTAS simulations were run for a configuration of one row containing ten racks with twelve servers each, a coolant distribution unit (CDU), a chiller, and a cooling tower. Preliminary results show that using direct liquid cooling is thermodynamically more favorable compared to hybrid liquid-air and legacy air cooling strategies, which is expected since the direct liquid cooling provides heat removal at the chip surface, which represents the largest temperature in the system. Specifically, the application of direct liquid cooling reduces the overall exergy destruction by over 40% compared to a configuration with only one CRAH unit, by 21% compared to a configuration with either one IRHX or one OHX, and by 14% compared to a configuration with one RDHX. Future work will continue to evaluate the thermodynamic effects of direct liquid cooling on data centers by studying configurations involving liquid cooling coupled with hybrid liquid-air technologies.
Anish Bhalerao,
Department of Mechanical Engineering, Villanova University
Villanova, PA

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