Heat Transfer Model of a 50 kW Solar Receiver–Reactor for Thermochemical Redox Cycling Using Cerium Dioxide

Abstract

This work reports on the development of a transient heat transfer model of a solar receiver–reactor designed for thermochemical redox cycling by temperature and pressure swing of pure cerium dioxide in the form of a reticulated porous ceramic (RPC). In the first, endothermal step, the cerium dioxide RPC is directly heated with concentrated solar radiation to 1500 °C while under vacuum pressure of less than 10 mbar, thereby releasing oxygen from its crystal lattice. In the subsequent, exothermic step, the reactor is repressurized with carbon dioxide as it cools, and at temperatures below 1000 °C, the partially reduced cerium dioxide is re-oxidized with a flow of carbon dioxide. To analyze the performance of the solar reactor and to gain insight into improved design and operational conditions, a transient heat transfer model of the solar reactor for a solar radiative input power of 50 kW during the reduction step was developed and implemented in ANSYS cfx. The numerical model couples the incoming concentrated solar radiation using Monte Carlo ray tracing, incorporates the reduction chemistry by assuming thermodynamic equilibrium, and accounts for internal radiation heat transfer inside the porous ceria by applying effective heat transfer properties. The model was experimentally validated using data acquired in a high-flux solar simulator (HFSS), where temperature evolution and oxygen production results from model and experiment agreed well. The numerical results indicate the prominent influence of solar radiative input power, where increasing it substantially reduces reduction time of the cerium dioxide structure. Consequently, the model predicts a solar-to-fuel energy conversion efficiency of >6% at a solar radiative power input of 50 kW; efficiency >10% can be obtained provided the RPC macroporosity is substantially increased, and better volumetric absorption and uniform heating is achieved. Managing the ceria surface temperature during reduction to avoid sublimation is a critical design consideration for direct absorption solar receiver–reactors.

Additive‐Manufactured Ordered Porous Structures Made of Ceria for Concentrating Solar Applications

“…Radiative heat transport within the RPC can be calculated using its effective radiative properties, e.g., effective extinction coefficient and effective scattering phase function, which in turn can be computed via pore‐level Monte Carlo (MC) simulation and other statistical approaches . When the macro‐porosity of the RPC is increased, its optical thickness decreases, radiation penetrates more deeply, which can improve the solar‐to‐fuel energy conversion efficiency to some extent, provided the apparent density—defined as the ceria mass per unit volume of the porous structure—is not reduced. On the other hand, the maximum pore diameter and/or minimum strut thickness of the RPC fabricated by the replica method with polyurethane foams is limited due to its mechanical stability …”

Perspectives of Perovskites for Solar Thermochemical Splitting of CO2 or H2O Molecules

“…Solar reactors for effecting this cycle include cavity receivers with rotating or stationary structures, [48][49][50][51][52][53][54][55][56][57][58][59][60] glass dome reactors, [61] aerosol flow reactors, [62] particle reactors, [63] and moving and fluidized bed reactors. [64,65] The most promising receiver/reactor designed to date is the cavity receiver containing a reticulated porous ceramic (RPC) foam made of pure CeO 2 .…”

“…[64,65] The most promising receiver/reactor designed to date is the cavity receiver containing a reticulated porous ceramic (RPC) foam made of pure CeO 2 . [51][52][53][54][55][56][57][58][59] A schematic of the receiver reactor is proposed in Figure 1. The RPC was directly exposed to concentrated thermal radiation at mean solar flux concentration ratios of up to 3015 suns.…”

Modeling of Ceria Reduction in a Solar Thermochemical Reactor via DEM Method

“…It was proved that the model has a higher agreement with the model of Zinkevich et al [32]. Zoller et al [26] used the thermodynamic equilibrium model to simulate a 50kW solar receiver-reactor, which obtained the solar-to-fuel efficiency larger than 10% with dual-scale reticulated porous ceramic. The most basic chemical kinetics reaction model is based on the Arrhenius equation proposed by Ishida et al [33].…”

“…The fuel production rate was enhanced by as much as 260% and 175% compared with traditional low porosity ceria and non-ordered mesoporous ceria. Oliveira et al [26] further studied the 3DOM ceria structure for solar thermochemical CO2 splitting in a tubular reactor. The result showed that the maximum fuel production rate is three times higher than the dual-scale porosity ceria reticulated porous foam.…”