Influences of variable porosity on CaO/Ca(OH)2 thermochemical energy storage characteristics in direct/indirect heated reactor

Abstract

• Influence of variable porosity on CaO/Ca(OH) 2 TCES characteristics is investigated. • Variable porosity significantly influences permeability and heat conductivity. • Constant porosity over/underestimates DHR/IHR performance in dehydration by 9%. • Constant porosity underestimates DHR/IHR performance in hydration by 19%. In the present work, the influences of variable porosity of CaO/Ca(OH) 2 , which is normally regarded as constant, on the thermochemical energy storage characteristics in direct/indirect heated reactor (DHR/IHR), are comprehensively investigated by establishing a transient multi-physics coupled model. It is found that, unlike the traditional constant porosity assumption, the variable porosity shows significantly two-fold competitive impacts. On the one hand, increasing/decreasing porosity increases/decreases the permeability of reaction bed, which leads to the drop/rise in pressure field and further the local reaction equilibrium temperature drop/rise. Thus, the local reaction rate increases/decreases correspondingly. On the other hand, increasing/decreasing porosity also decreases/increases the heat conductivity of the reaction bed, which leads to the drop/rise in transient temperature field. Thus, the transient reaction rate decreases/increases instantaneously. The results indicate that the constant porosity assumption may over/underestimate the performance of DHR/IHR during the charging (dehydration) process by around 9%, while underestimate the performances of both DHR and IHR during discharging (hydration) process by up to around 19%. It is also found that the variable porosity demonstrates greater influence on the discharging (hydration) process in DHR and on the charging (dehydration) process in IHR.