(263h) Simulations of Heat Transfer to Flowing Particles Used for Long Duration Thermal Energy Storage

Energy storage systems are crucial to transform the world’s energy reliance from coal-based energy production to clean energy production. The National Renewable Energy Laboratory (NREL) is leading an effort to develop the Economic Long-Duration Electricity Storage by Using Low-Cost Thermal Energy Storage and High-Efficiency Power Cycle (ENDURING) system that aims to provide clean energy at low cost. In the ENDURING system, silica sand is the heat transfer fluid as well as the thermal storage material because it is a relatively low-cost material that is reliable at high temperatures. To warm the particles, an array of electric heating elements is powered by excess energy from the transmission grid and used to heat the flowing sand particles. In our work, we study how to maximize the heat transfer from the electric heating elements to the sand particles. To investigate the heat transfer problem, we use MFiX, a Discrete Element Modeling (DEM) computational solver developed by the National Energy Technology Laboratory (NETL). The particle-particle and particle-wall friction coefficients, surface angle, and particle surface roughness were the key parameters investigated in this study. Also, an analytical model of the heat transfer process was developed to extrapolate the small-scale DEM results to a large-scale prototype. After exploring the parameter spice, we found that a relatively shallow surface angle of 20° increases the residence time and heating rate of particles without incurring jamming. The simulations also reveal that friction can enhance the heat transfer by improving thermal contact, but cases with high friction can restrict the flow and result in worse heat transfer. The simulation results are discussed in detail and configurations for optimal performance of the particle heater system are presented.