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

Abstract

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 highefficiency power cycles while simultaneously maintaining stability of the heattransfer medium and thermal performance of the receiver. Solidparticlebased CSP systems are one alternative projected to have significant cost and performance advantages over current nitratebased molten salt systems. NREL is developing a design that uses gas/solid, twophase flow as the heattransfer fluid (HTF) and separated solid particles as the storage medium. A critical component in the system is a novel nearblackbody (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, particleflow conditions, and heattransfer 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 twofluid modeling techniques. Heattransfer 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. Particleflow visualization experiments confirm particle/wall separation, but also exhibit complex periodic behavior and flow instability that create intermittent sidewall contact and enhance heat transfer above that predicted by the theoretical simulations.