First-principle study of CaO/Ca(OH)2 thermochemical energy storage system by Li or Mg cation doping

Abstract

Thermochemical energy storage can accomplish the need of long-term and long-distance storage and thus it is very important in many industrial applications, such as waste heat recovery, solar energy utilization, etc. In this study, we have investigated the micro-mechanism of chemical heat storage, as well as the micro-mechanism of Li or Mg catalysis in the heat storage, by using the first principle method and transition state theory. Thermodynamic analysis shows that the doped materials are relatively stable. Furthermore, through the micro scale comparison between the undoped and doped systems in terms of the transition state, energy barrier and electronic density of states (DOS), the effects of doping on macroscopic heat storage process are inferred. The changes of molecular structures of transition state imply that the chemical reaction kinetic is modified. The reduction of the energy barrier from 0.40eV to 0.11eV further indicates the dehydration reaction (i.e., heat storage process) can be accomplished at a lower temperature with Li doping. The narrowed pseudogap width in the DOS means that with Li doping the OH bonds of Ca(OH)2 can be more easily broken and the dehydration speed is higher at the same temperature. In contrast, we conclude that the Mg cation doping has little impact on the heat storage process, since the results of the micro-mechanism analysis with Mg doping remain almost unchanged. The experimental results show that the time needed for dehydration process is reduced after Li doping, which agree well with our analysis of the energy barrier and electronic density of states.