State-of-the-art on thermal energy storage technologies in data center

•The necessity and uniqueness of TES for data centers are discussed.•TES units and TES integrated cooling systems in data center are summarized and classified.•The designs and performance characteristics of TES units and TES integrated cooling systems in data center are analyzed.•Main trends in TES developments and applications are summarized when utilized in data center. Data center consumes a great amount of energy and accounts for an increasing proportion of global energy demand. Low efficiency of cooling systems leads to a cooling cost at about 40% of the total energy consumption of a data center. Due to specific operation conditions, high security and high cooling load is required in data center. To achieve energy saving, cost saving and high security, novel cooling systems integrated with thermal energy storage (TES) technologies have been proposed. This paper presents an extensive overview of the research advances and the applications of TES technologies in data centers. Operating conditions, energy mismatch and requirement of high security in data center were overviewed. Principles and characteristics of TES technologies were discussed. Applications of passive TES coupled air flow and applications of active TES integrated cooling system are summarizes, and the design and performance of these TES integrated thermal systems are analyzed, with a focus on energy saving, cost savings and high security.