Study on the thermal storage performance of high-temperature molten salt spheres packed bed using different heat transfer fluids

Abstract

The high-temperature packed bed latent heat thermal energy storage system (PBLHTES) system is regarded as the key to the efficient operation of concentrating solar power (CSP) system. The heat transfer and storage materials determine the thermal performance of the thermal energy storage (TES) system. In this study, a discrete scheme for the PBLHTES system has been programmed, and a one-dimensional non-equilibrium model has been established. The numerical results are in good agreement with the experimental findings of the references. Originally, the effects of different fluids' thermodynamic characteristics on thermal performance are studied. Three widely used heat transfer fluids for high temperature working conditions, namely mixed nitrate salt, mixed chloride salt, and thermal oil, are applied for the established packed bed system, respectively. Then, three systems are numerically simulated, and sensitivity analyses of the Reynolds number, Stefan number, and sphere diameter are performed. According to the results, raising the Reynolds and Stefan numbers improves the energy storage rate but leads to an increase in heat transfer temperature difference and a decrease in exergy efficiency. Increasing Stefan number enhances the thermal storage capacity of three systems but causes irreversible loss. Differently, increasing the particle diameter not only extends the thermal storage time and increases the heat transfer temperature difference in all three systems but also decreases the energy storage rate and exergy efficiency. Furthermore, the decreasing slope of exergy efficiency with an increase in Reynolds and Stefan numbers is slowing down in systems 1 and 2 but growing in system 3. With the increase in sphere diameter, the decline slope is decreasing in all three systems. Overall, it is critical to choose the suitable HTF in a traditional or novel PBLHTES system to optimize thermal storage performance.