A generic solar-thermochemical reactor model with internal heat diffusion for counter-flow solid heat exchange

Kurzfassung

For nonstoichiometric redox reactions that produce CO and H2 from CO2 and H2O, heat recuperation from the solid phase is a promising mechanism to improve the cycle efficiency. Many different approaches to heat recuperation and gas separation have been presented in solar thermochemical reactor concepts recently. To describe the many possible degrees of freedom in the reactor design, a generic reactor model is described for two-step redox reactions of solid pieces of reactant moving in counter flow between reduction and oxidation chambers. The reactive material is assumed to be porous ceria, where heat recuperation from the solid phase is achieved through radiation heat exchange between reduced and oxidized elements moving in opposite directions. A separation wall prevents gas cross-over and provides structural support. Heat transfer by radiation in the porous material is modeled with the Rosseland diffusion approximation and by conduction with the three resistor model. The model can be adapted to a wide range of reactor concepts. A study of crucial design parameters shows that heat diffusion in the reactive material can have a significant influence on the performance of the heat exchanger. If the time required for heat diffusion is large with respect to the total residence time in the heat exchanger, the material thickness can be decreased to enhance the share of the material actively participating in the heat exchange process. Furthermore, at the relevant temperatures, radiation dominates the heat exchange within the porous structure, thus the overall heat exchange can be enhanced through an increase of porosity. Heat exchanger length and residence time are correlated, allowing different combinations of these two variables at constant heat exchanger efficiency. In general, efficiencies close to 70% are possible with an adequate parameter combination. However, the achievement of the maximum heat exchanger efficiency requires a minimum number of chambers and thus physical length, as irreversibilities are reduced for a larger number of intermediate temperature levels. The presented generic model includes the description of heat diffusion within the reactive material, is a valuable tool for the design of heat exchangers, and can be used to identify technically interesting reactor concepts for the achievement of high energy conversion efficiencies.