Turbomachinery for a Supercritical CO2 Electro-Thermal Energy Storage System

This paper presents analysis of CO2 turbomachinery for the electro-thermal energy storage (ETES) concept for site-independent bulk (grid-scale) electric energy storage. In charging mode, ETES operates as a transcritical CO2 heat pump, consuming electric energy which is converted into thermal energy stored in the form of hot water and ice on the hot and cold side of the cycle, respectively. On demand, the CO2 cycle is reversed for discharging during which ETES operates as transcritical CO2 power generation plant, consuming the stored hot and cold sources. The target capacity of the ETES system is of the order of units of MW electric to ∼100 MW electric, with typical daily cycles and 4 to 8 hours of storage. The estimated electric-to-electric round trip efficiency of ETES is about 60%. A companion paper [1] presents the control concept of the ETES plant and discusses several issues specific to the ETES plant design and operation. This paper analyzes these particular requirements from the perspective of the CO2 turbomachinery required for the storage plant, presenting the selection of the turbomachinery types and their shaft arrangement suitable for the ETES. The expected performance, main design features and challenges are discussed, together with questions related to the scalability of the turbomachines towards high power targets. Impacts of the turbomachinery designs on the ETES system performance, such as the sensitivity of the system electric-to-electric round trip efficiency on the turbomachinery efficiency are discussed.