Pilot-Scale Testing of a Transcritical CO2-Based Pumped Thermal Energy Storage (PTES) System

Abstract As the world moves toward an electrical generation system that relies heavily upon non-dispatchable resources such as solar photovoltaic and wind power, reliable, low-cost means to store electrical energy and dispatch it as supply and demand fluctuate are vital. Pumped thermal energy storage (PTES) consists of a reversible heat pump / heat engine cycle, where thermal energy is transferred between two reservoirs, one at high temperature and the other at low temperature. During the charging phase of operation, thermal energy is extracted from a low temperature reservoir (LTR), increased in temperature, and then stored in a high temperature reservoir (HTR) using an electrically driven heat pump cycle. During the generating process, the stored heat in the HTR is converted back into electricity using a heat engine cycle — essentially the heat pump operating in reverse — and the residual low-temperature thermal energy is stored in the LTR. A simplified thermodynamic analysis of a general PTES system provided insights into the governing physics of the system, which were used to select the cycle architecture, working fluid and thermal reservoirs. A pilot-scale (c.a. 100 kW) PTES system using CO2 as the working fluid was then designed, built and tested, including two HTR concepts and an ice/water slurry LTR. Data from this test sequence was used to validate component and system steady-state performance and transient system models. Further, this experience has informed the conceptual design of a 100 MW, 12 hour PTES system that is planned for deployment in 2028.