Compressor Development for CO₂-Based Pumped Thermal Energy Storage Systems

Pumped thermal energy storage (PTES) offers a cost-effective means to store electrical energy for long duration by utilizing a heat pump cycle to transfer thermal energy from a low temperature reservoir (LTR) to a high temperature reservoir (HTR). A key component of the PTES system is the heat pump compressor, which represents a significant driver to the cost, performance, and operating characteristics of the PTES system. At the power rating of the charging process (>100 MW), traditional compressor scaling charts indicate that the operating conditions needed would be best served by a multistage axial compressor. While frame gas turbine compressors at these power ratings exist and operate at higher pressure ratios than needed for the CO₂ PTES system, the inlet pressure and fluid density of this application exceeds experience values. The conceptual design of a large-scale CO₂ axial compressor was completed, including mean-line estimates of the compressor performance at full power conditions. At steady-state, full power operation, the isentropic efficiency and mechanical efficiency of the compressor have significant impact on the cycle design and round trip efficiency of the PTES system. The results of the conceptual design were used to refine the PTES cycle design, and updated operating conditions provided for further aero design optimization. An important characteristic of the PTES system is its ability to charge at variable rate, which provides significant challenges on compressor operability, especially for a compressor that will be coupled to a fixed-speed synchronous motor. Cycle studies of variable charging rate processes have been conducted, and the impact of compressor operating map characteristics explored. Based on initial modeling studies, single compressor operation can be achieved down to at least 50% of rated power, with further reductions possible depending on the characteristics of the compressor map speedlines.