Effect of PCM fraction and melting temperature on temperature stabilization of hybrid sensible/latent thermal energy storage system for sCO2 Brayton power cycle

Abstract

In order to maximize the operational efficiency of a power cycle, it is desirable to maintain operation as close to the cycle’s design point as possible. For the supercritical carbon dioxide Brayton cycle, this means a turbine inlet temperature of approximately 650 °C to 750 °C, depending on design, with a pressure of about 20–25 MPa. This work numerically explores the use of a hybrid sensible/latent heat thermal energy storage system intended to provide a stable discharge temperature of approximately 650 °C while maximizing the system’s exergy efficiency. Low-cost sensible heat storage in rocks provides the majority of the energy storage capacity, while relatively small fraction of phase change material at the top of the tank provides the temperature stabilization capability. The phase change materials (PCM) used are salts and their mixtures with melting temperatures between 657 and 680 °C, while the PCM quantity is varied from 0 to 100% by volume of the tank, with greater resolution at lower PCM fractions. It is found that 1) increasing the PCM fraction leads to longer stabilization durations but lower initial discharge temperatures; 2) increasing the melting temperature increases the stabilization temperature but reduces the stabilization duration; and 3) both effects 1 and 2 show diminishing returns.