Analysis of a graphite foam–NaCl latent heat storage system for supercritical CO2 power cycles for concentrated solar power

Abstract

A latent heat thermal energy storage (LHTES) system that operates at high temperature was analyzed for applications to supercritical CO2 (s-CO2) power cycles for a concentrated solar power (CSP) plant. Because the operation temperature of the s-CO2 power cycles is high (650–700°C), sodium chloride (NaCl), with a melting point of 800°C, was selected as the phase-change material (PCM) for energy storage. Due to the low thermal conductivity of salt materials (usually <1W/mK), use of graphite foam was chosen to improve the overall thermal conductivity of the graphite foam–PCM combination. Three-dimensional (3-D) heat transfer simulations were conducted for the envisioned full-scale LHTES system. The anisotropic thermal conductivities of graphite foam were considered in the simulations. The thermal performance and the exergy efficiency were investigated for the full-scale LHTES system to study the improvements due to the graphite foam. The results show that this material improves the heat transfer performance in the LHTES system and, therefore, significantly reduces the total number of the heat transfer fluid (HTF) pipes needed in the storage system by a factor of 12 compared to a PCM-only system. Furthermore, the graphite foam helps to increase the exergy efficiency of the LHTES system considerably. The system parameter (i.e. HTF inlet temperature, flow velocity) effects on the thermal performance and exergy efficiency of the graphite foam–PCM LHTES system were analyzed in the paper. Moreover, the graphite foam–NaCl system matches the temperature requirements of the s-CO2 power cycles for the CSP plant.