Finite element method simulations of heat flow in fixed bed solar water splitting redox reactors

Abstract

An improved design for radiation absorption and heat flow into materials with low thermal conductivity is demonstrated. The design was developed for application in fixed bed two step solar water splitting redox reactors. The fixed bed was assumed to be made from porous ceramic. The low thermal conductivity of the porous ceramic redox material is compensated for by changing the profile of the fixed bed. The profiling used was wedges cut into the material which allows concentrated solar radiation to be incident on a larger area of redox material than a flat monolith design. The design is demonstrated to efficiently transfer heat to the bulk and greatly reduce re-radiation. For a wedge 9 cm in depth and 1.6cm wide at the opening, heated with 500 Wm 2 s −1 incident radiation for 300 seconds approximately double the amount of radiation is absorbed. The effects of thermal conductivity, emissivity and scaling of the design were investigated. The radiation absorption performance improved when scaled up. The improvement of the design over a flat plain increases for lower emissivity. The improvement provided by the wedge design was found to decrease for increasing thermal conductivity, and eventually for high conductivity values it reduced performance. Using this method a larger amount of material with low thermal conductivity can be heated with the same power input and reduced radiation losses. Finally a concentrated solar cavity reactor based on the design is proposed.

Analytical Model of CeO2 Oxidation and Reduction

“…The model predicts a somewhat faster reaction, which could be attributed to the assumption that the entire pellet is uniformly heated. In reality, the bottom of the pellet is heated at a lower rate as the pellet is a porous ceramic, which has poor heat-transfer properties . This will reduce the rate.…”

“…In reality, the bottom of the pellet is heated at a lower rate as the pellet is a porous ceramic, which has poor heat-transfer properties. 27 This will reduce the rate. The final value in the model is that of equilibrium, but in our experiment the final values were all roughly 30% lower than predicted by the model.…”

“…The lack of such a simple analytical model for the reaction kinetics of ceria reduction makes the modeling of the performance of the above reactions difficult. If one has a simple model, which depends only on the concentrations of the reactants, the temperature, and the oxygen partial pressure, then it can easily be linked to heat flow and diffusion simulations . This will greatly improve reactor design capabilities and allow for more accurate assessment of this proposed fuel production technology.…”

Thermodynamics of CeO2 Thermochemical Fuel Production

“…A number of reactor designs for such fuel conversion have been proposed, − and some prototypes have been constructed and tested. ,,, So far, fuel conversion efficiencies have been very low, with values typically in the region of or less than 1%. Thermodynamic studies of such cycles suggest that solar to fuel efficiencies approaching 30% could be achievable. , …”