Thermal recuperation modeling of a solar thermochemical reactor

The United States spends over 700 billion dollars on foreign oil every year. A promising method to reduce this dependence, and be carbon-neutral as well, is concentrated solar thermochemical technology. Concentrated solar thermochemical technology has the potential to directly convert sunlight into a useable, carbon-neutral fuel that can be easily stored and integrated into our existing forms of energy demand such as transportation and heating fuels. Research is being performed by several groups at Sandia National Laboratories to fundamentally understand the complex physics and chemistry occurring within a solar thermochemical reactor prototype named the CR5 [counter-rotating-ring receiver/reactor/recuperator]. The objective of the work presented in this paper is to understand recuperative heat transfer within the CR5 as a function of reactor geometry and operational conditions. The CR5 vessel utilizes counter-rotating disks to provide for thermal energy recuperation, which is a necessity for an efficient reactor. Initially a simplified steady-state two-dimensional recuperation analysis was made to evaluate the relationship between the reactive material fin height and recuperation. The results from the simplified two-dimensional model indicate that recuperation is a strong function of fin height. Next, a more detailed, but still simplified, transient, three-dimensional model was developed. The initial three-dimensional simulations presented in this paper were performed to determine recuperator effectiveness and mesh density requirements for a generic case that had 2.5 kW energy input, fin height to gap ratio of 0.9, and finned reactor disks rotating at 1 rpm. A recuperator effectiveness of 88% and 85% was calculated from the finer and coarser meshes, suggesting that the coarser mesh (lumped in the fin thickness direction) is adequate for future parametric analysis simulations. This analysis will lead to a better understanding of recuperation as a function of reactor geometry, energy input, rotational speed, and thermophysical properties.