Numerical Modelling of Concentrating Solar Thermochemical Processes: A Brief Review
Morteza Hangi, Vincent Wheeler, and Wojciech Lipiński*
Research School of Engineering
The Australian National University, Canberra, ACT 2601, Australia
Key words: solar, thermochemistry, thermal transport, multi-scale, numerical
The potential for concentrated solar systems to drive thermochemical processes was recognized in the early 1980’s  (Gregg 1980) if not earlier. Many processes under study hold tremendous potential for environmentally benign solutions to traditionally fossil fuel intensive industrial production of transportation and stationary-application fuels as well as material commodities including lime and cement, metals, and ammonia [2, 3] (Bader) (Romero 2012). Syngas production processes including methane reforming, water and carbon dioxide splitting, and gasification of carbonaceous materials are particularly interesting—many approaches have been surveyed here:  (Kodama 2003). Process heat from the sun is a clean and abundant but comes with the drawback of being extremely transient.
These processes are realized by reactors driven by (often tremendously) concentrated sunlight. They are designed for maximum solar-to-chemical energy conversion efficiency and fast process rates. Reactors typically utilize solid–gas heterogeneous media at temperatures ranging from several hundred to more than 2000ºC. The reactive materials are often porous solid structures or particles that feature characteristic sizes many orders of magnitude smaller than the reactor itself. Accurate and computationally effective characterization and simulation techniques connecting the highly-disparate spatial and temporal scales of solar thermochemical systems are required to advance the understanding and optimize the complex thermal transport processes, and to guide design of efficient processes, components and systems.
Cite perspective by floudas on multi-scale simulation in energy and environment: (Floudas 2016)
Thermo-chemical simulations of solar-driven reacting systems are generally highly multi-physical and often multi-scale; radiative, conductive and convective heat transfer, mass transport of multiple species and reactions all must be considered. Interactions between thermal radiation and chemical kinetics are of special interest since the conversion of incident solar irradiation to the creation or destruction of chemical bonds is the fundamental process that underlies the efficiency of these processes. Transport and material properties are either known when a system is being modelled to evaluate optimal operating conditions, optimal geometry, or the like or unknown when the properties themselves need to be extracted from a validated model.
Modelling of particle based reactors is particularly challenging, especially when the reactor uses a fluidized bed. Numerical methods and high performance computing now make it possible to model these systems at the particle level, resolving each particle individually. Consult  (Deen 2007)
Ergun’s equation is an expression for the friction factor in terms of relevant packed bed parameters to estimate the pressure drop across the bed.
Important features of a solar driven thermochemical reactor that require acute modelling attention:
(i) Chemically reacting solid–gas interface, (ii) multi-component, often multiphase, mass transport, (iii) high flux solar irradiation. Items (i) and (ii) constitute a “classical” chemical reactor system where the process heat is supplied by conduction or convection. Each represents a field of study in its own right.
Also discuss the field of combustion and the modeling approaches found there. We will have to differentiate between what is reviewed here and what is done there. Jacobsen gives a reference to a viskantha paper reviewing combustion modeling. Check that out. Perhaps the inclusion of high intensity SOLAR radiation is the key difference. Thermal radiation will be important for both.
Two established fields are bridged by the study of high-temperature solar thermochemical reactors—classical heat-cycle-centric concentrating solar thermal (CSP) engineering and chemical reactor engineering. Solar driven chemical reactors feature the
This short review aims to outline the challenges unique to high-temperature solar thermochemical reactor modelling, highlight the existing literature in the field and their attempts to overcome these hurdles, and guide the reader to relevant studies depending on material/chemical process and physical reactor configuration.
In the following, we will