Rack-level cooling technologies for data centers – A comprehensive review

With the growth of information technology applications, the high energy consumption issue of data center cooling systems has drawn more attention. Existing cooling systems in data centers mostly adopt room air conditioners, which can easily cause local hot spot issues with low energy efficiency. By contrast, the rack-level cooling technology, which adopts on-demand direct cooling, is regarded as a promising solution. Aiming at rack-level cooling technology, some studies have been carried out from the aspect of design, performance analysis, and operation regulation. However, there is still a lack of systematic reviews to analyze existing deficiencies and to explore future development tendencies. Firstly, the different types of existing rack-level systems are investigated in detail. Then, the impact factors and operation performance are analyzed based on the existing studies. Furthermore, limitations and opportunities of the rack-level cooling technology are demonstrated in aspects of operation control, reliability, and energy-saving retrofit. Finally, technological limitations including the determination of optimal filling ratio, matching control for inhomogeneous load distribution, multi-rack integration model for solving redundancy problem, and opportunity in energy-saving retrofit are pointed out for promotion and application of rack-level cooling technology. This review is expected to contribute significantly to achieving green, sustainable, and energy-efficient data centers.