Critical limitations on the efficiency of two-step thermochemical cycles

Previous models based on thermodynamic considerations have identified the properties desired for reactive oxides that can be used as oxygen storage materials in thermochemical cycles to produce fuel from sunlight. However, there are several important assumptions made in such models, such as the neglect of the energy required to preheat unreacted species and the assumption of constant vacuum pump efficiency. When these assumptions are relaxed, one comes to significantly different conclusions about the optimal reactor operating conditions. Furthermore, comparing two materials is not straightforward due to the high degree of coupling between material properties and reactor operating conditions. Herein, we describe a new framework for material comparison which employs a thermodynamic reactor model to predict the maximum possible efficiency of a given oxygen storage material. This model demonstrates how new materials can impact reactor performance and the limitations of such improvements. (C) 2015 Elsevier Ltd. All rights reserved.