High-temperature solar thermochemical systems are designed for maximum solar-to-chemical energy conversion efficiency and fast process rates. High temperatures are achieved by applying high-flux irradiation of solar reactors obtained with concentrating solar collectors. Solar thermochemical systems typically include solid–gas heterogeneous media at temperatures exceeding several hundred degrees Celsius, and in some applications reaching more than 2000 C. Such media serve multiple purposes. They absorb high-flux irradiation (absorption is predominantly by the solid phase as direct gas absorption is ineffective for length scales of a solar reactor) and transfer the heat to a chemical reaction. In directly-irradiated reactors, radiation is absorbed by a solid that provides surface to a chemical reaction. In indirectly-irradiated reactors, radiation is absorbed by a solid and then transferred to a chemical reaction by conduction, convection, and/or radiation through an intermediate heat transfer medium (solid, fluid, or multiphase medium).
Thermal transport processes are an area where physics has extensively been studied at discrete spatial levels varying from nanoscale to microscale to macroscale. However, accurate characterization and simulation techniques connecting highly disparate spatial and temporary scales of solar thermochemical systems require significant advancements to become computationally effective. Such techniques are desired for direct design and optimization of reactors featuring prescribed materials of unknown continuum characteristics. Advancements in materials and computational sciences have enabled, however, an even more intriguing but less explored approach of materials-by-design, in which materials of prescribed continuum characteristics indicated by predictive reactor-level models are inversely engineered by targeted identification of suitable composition and morphology. Both direct thermal characterization and materials-by-design approaches are useful for understanding and optimizing the complex thermal transport processes and, consequently, for achieving the ultimate high efficiencies The desired characteristics of an absorbing active material must allow for (i) efficient absorption of incident concentrated solar radiation, (ii) rapid heat transfer between the absorption and process sites, (iii) confinement of the emitted thermal radiation in the close vicinity of the process site, and (iv) minimum heat losses from the absorbing medium by conduction and convection. While a particular material design satisfying these conditions strongly depends on a specific application, any design and optimization efforts for high-temperature solar thermochemical systems can be summarized as minimization of irreversibilities associated with heat and reactant transport to the reaction site and transport of products from the reaction site.
The basic approach to modeling coupled heat and mass transfer in reacting media of high-temperature solar thermochemical systems such as solar reactors and simplified setups exposed to concentrated solar radiation involves simultaneous solutions of the radiative transfer equation (RTE) along with the mass, momentum, and energy conservation equations. The interactions between thermal radiation and chemical kinetics are of special interest. Transient variation of radiative properties typical to such systems requires determination of the radiative contribution to the energy equation at any instant of the solution, leading to high computational cost. The complexity of solution further increases for systems for which gas radiation effects cannot be neglected, requiring application of accurate but computationally expensive spectral methods.
In this study, advances in thermal sciences pertinent to development of solar thermochemical systems are systematically reviewed. Examples of studies of heat and mass transfer in solar reactors, reactive media and media features such as individual reacting particles are discussed together with recent developments in direct numerical predictions of thermal transport and optical properties of heterogeneous materials, from nano- to micro- to macrostructures.