Investigations on the cost-effective design and operation of water-based thermal energy storage systems for data centers

Thermal energy storage systems (TES) can be employed to address energy supply disruptions and supply and demand mismatches typically due to climate change and weather extremes. With the advent of concerns about increasing energy costs, TES systems are also attracting attention for lowering peak demand and flattening load profiles and managing the cooling supply and demand to take advantage of low-cost electrical energy periods. This study aims to explore how TES systems, particularly water tanks primarily designed as emergency cooling systems in data centers, can be used to reduce operational costs. In a typical data center with a 1000 kW information technology (IT) equipment load, a TES system is integrated into the data center cooling infrastructure. TES satisfies part of the cooling load during peak hours and chillers handle the remainder of the load running at reduced capacity. Initial sizing of the TES system follows ASHRAE recommendations, which are not intended for optimal operation. The thermodynamic model of the integrated system provides insight into the dynamic behavior of the data center cooling infrastructure and the effect of various control parameters. These include the flow entering the TES tank during the charging and discharging processes as well as the chiller set-point temperature and the number of operational chillers. Controls ensure safe thermal environmental conditions for the IT equipment, meet the charging criteria as well as reduce operational costs. The coupled design-And-operational optimization is based on a cost objective function, which includes operational cost as well as the initial cost of the TES system based on its size. The results of the study indicate various sizes of recommended TES designs and control strategies depending on operational variables.