Granular Flow and Heat-Transfer Study in a Near-Blackbody Enclosed Particle Receiver

Concentrating solar power (CSP) is an effective means of converting solar energy into electricity with an energy storage capability for continuous, dispatchable, renewable power generation. However, challenges with current CSP systems include high initial capital cost and electricity price, and advances are needed to increase outlet temperature to drive high-efficiency power cycles while simultaneously maintaining stability of the heat-transfer medium and thermal performance of the receiver. Solid-particle-based CSP systems are one alternative projected to have significant cost and performance advantages over current nitrate-based molten salt systems. NREL is developing a design that uses gas/solid, two-phase flow as the heat-transfer fluid (HTF) and separated solid particles as the storage medium. A critical component in the system is a novel near-blackbody (NBB) enclosed particle receiver that uses an array of absorber tubes with a granular medium flowing downward through channels between tubes. Development of the NBB enclosed particle receiver necessitates detailed investigation of the dimensions of the receiver, particle-flow conditions, and heat-transfer coefficients. This study focuses on simulation and analysis of granular flow patterns and the resulting convective and conductive heat transfer to the particulate phase using Eulerian–Eulerian two-fluid modeling techniques. Heat-transfer coefficients in regions with good particle/wall contact are predicted to exceed 1000 W/m² K. However, simulations predict particle/wall separation in vertical flow channels and a resultant reduction in heat transfer. Particle-flow visualization experiments confirm particle/wall separation, but also exhibit complex periodic behavior and flow instability that create intermittent side-wall contact and enhance heat transfer above that predicted by the theoretical simulations.