Performance Evaluation of a 40-kWth Prototype Counterflow Particle-sCO2 Fluidized Bed Heat Exchanger

Abstract

Particle-sCO₂ heat exchangers (HXs) can couple particle-based thermal energy storage for concentrating solar power (CSP) plants with recompression closed Brayton power cycles (RCBC) to enable continuous dispatchable renewable electricity. RCBC firing temperatures >700°C require HXs with expensive Ni-based alloys, requiring HX designs to reduce mass with high overall heat transfer coefficients U_HX to meet CSP primary HX cost. To enhance U_HX, mild bubbling fluidization with downward particle flows and upward fluidizing gas flows can achieve particle-wall heat transfer coefficients h_T,w >800 W m^-2 K^-1 with CARBOBEAD HSP 40/70. To assess how high h_T,w with mild particle fluidization impacts U_HX, a 40-kW_th particle-sCO₂ HX with 12-parallel narrow-channel fluidized beds was assembled and tested to particle inlet temperatures up to 530 °C. Tests show reliable steady-state HX operation by maintaining fluidized particles in a freeboard zone above the parallel channels, but axial dispersion mixes partially cooled particles up from the fluidized channels with particles fed into the freeboard zone from the feed hopper. This mixing lowers the effective bed temperatures and the driving force for heat transfer to counterflowing sCO₂ in microchannelled walls. Measured U_HX based on particle inlet temperature never exceeded 205 W m^-2 K^-1. The trade-off between increased h_T,w, and increased vertical dispersion with gas flows resulted in only small improvements in U_HX after the onset of fluidization. Dispersion was incorporated into HX design and performance models that used h_T,w correlations fitted to single-channel heat transfer tests. Model results show that reducing dispersion leads to higher particle wall heat transfer.