(128b) Discrete Element and Continuum Heat Transfer Simulations of a Solar Receiver That Uses Solid Particles As a Heat Transfer Fluid

Current efforts aim to improve the performance of concentrating solar power (CSP) systems by utilizing heat transfer fluids that operate at high temperature and implementing thermal storage mechanisms to mitigate energy losses associated with diurnal cycling and cloud variability.  A novel solar receiver that uses solid particles as a heat transfer fluid is being developed at the National Renewable Energy Laboratory.  Unlike state-of-the-art receivers that use molten salts as a heat transfer fluid, solid particles are capable of operating at high temperature (>800° C) and have a large thermal capacitance and can be efficiently used for thermal storage.  The solar receiver contains arrays tubes that are open on one end, allowing radiation to enter and internally heat each tube.  Particles fall via gravity over the tube array and are warmed via conductive heat transfer, radiation, and interphase coupling with the background air.  A validated fundamental model of the heat transfer to flowing particles in such a system, however, remains elusive.  Discrete element simulations are highly accurate and model the motion and thermal energy balances for individual particles, but such simulations are computationally expensive and are restricted to relatively small systems.  Two-fluid models use a continuum (or averaged) representation for the particle phase and can be used to model industrial-scale systems, but the existing continuum heat transfer models have not been validated for use in CSP.  Discrete element simulations are thus performed for a laboratory-scale solar receiver and a new continuum heat transfer model (Morris et al., 2015) is validated by comparing to discrete element data.  The continuum model is found to accurately predict the heat transfer coefficient for different tube configurations, shapes, mass flow rates, and particle sizes.

Morris, A., Pannala, S., Ma, Z., and Hrenya, C., “A conductive heat transfer model for particle flows over immersed surfaces,” submitted to Int. J. of Heat and Mass Transfer in Feb 2015.