Model of an integrated solar thermochemical reactor/reticulated ceramic foam heat exchanger for gas-phase heat recovery

The efficiency of solar thermochemical cycles to split water and carbon dioxide depends in large part on highly effective gas phase heat recovery. To accomplish this goal, we present the design and analysis of the thermal and hydrodynamic performance of a counter-flow, tube-in-tube alumina heat exchanger operating at temperatures of 1500 °C and integrated with a solar thermochemical reactor for isothermal production of syngas via the ceria redox cycle. The heat exchanger tubes are filled with alumina reticulated ceramic to enhance heat transfer. The effects of foam morphology and heat exchanger size on heat transfer, pressure drop, and process solar-to-fuel efficiency are explored by coupling a computational fluid dynamic model of the heat exchanger, including radiative transport, with the overall reactor energy balance. We examine foam pore densities of 10, 20 and 30 PPI, and porosities of 65-90%. The 10 PPI foam yields the best heat transfer performance and lowest pressure drop, as the larger pores enhance radiative heat transfer and decrease fluid phase drag forces. Although lower porosity is preferred to improve solid phase conduction in the RPC, the tradeoff in heat transfer and pressure drop point to use of higher porosity foam. Optimization for solar-to-fuel reactor efficiency is achieved with 85-90% porosity, 10 PPI RPC.