Mechanism And Experimental Study Of CaO/Ca（OH）₂ Thermochemical Heat Storage System

Energy plays an increasingly important role in modern society.With the depletion of fossil energy,the energy problem is becoming very serious,so research on the solar energy utilization and industrial waste heat recycling has become particularly important.The intermittent and non-uniform distribution of solar energy and waste heat resources restrict the utilization of these thermal energy.Therefore,heat storage and reutilization are essential for realizing solar energy utilization and waste heat recycling.Compared to sensible and latent heat storage approaches,thermochemical heat storage has two obvious advantages:high heat storage density and long term storage at ambient temperature even without any insulation.From the microcosmic view and using first principle methods as the breakthrough point,CaO/Ca（OH）₂ thermochemical energy storage system is investigated by the density functional theory（DFT）in order to fill gaps in the microscale mechanism studies.This paper investigates the micro-mechanism of CaO/Ca（OH）₂ heat storage system,including the transition state,energy barrier and electronic density of states（DOS）.Then the thermodynamic process of heat storage system is studied through experimental and theoretical analysis.The kinetic equation of the dehydration process of pure Ca（OH）₂ and Li-doped Ca（OH）₂ is obtained.Subsequently,the CaO/Ca（OH）₂ test-bed is set up to investigate the heat storage and release processes.The experiments under different conditions are analyzed,including different temperatures and different vapor pressures.Finally,this study investigates the side effects of CO₂ on the cyclic stability of CaO/Ca（OH）₂ heat storage systems.The effects under different conditions are studied,including different temperatures and different CO₂ levels.The micro-mechanism of chemical heat storage,as well as the micro-mechanism of Li or Mg catalysis in the heat storage,was studied by using the first principle method and transition state theory.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 0.11 ev 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,it is concluded 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 results can be used to explain the catalytic effect of Li or Mg cation on heat storage process.Based on the micro-mechanism investigation,Li-doped Ca（OH）₂prepared by ball grinding is compared with pure Ca（OH）₂ to study the influence of Li doping amount on heat storage speed by thermogravimetric（TG）experiment.Furthermore,the heat storage capacity（including the reaction enthalpy and specific heat capacity）with Li doping is studied,and the related kinetic control equation is derived.The results show that the heat storage capacity remains almost unchanged with Li doping,but the kinetic process of Ca（OH）₂ heat storage with Li doping is divided into two stages which can be analyzed from the molecular structure.A good agreement between theroretical and experimental results is achieved.Based on the above research,a CaO/Ca（OH）₂ test-bed is set up to investigate the heat storage and release processes.In the heat storage process of pure Ca（OH）₂,higher dehydration temperature results in a faster heat storage speed.The results of the experiment show that the air can seriously affect the heat release process in a fixed-bed test.Therefore,the method of pumping out the air is used in this study to improve the heat release process.According to the results of the heat release experiment,a lower initial sample temperature can lead to a higher extent of the dehydration reaction.Furthermore,a higher vapor pressure obviously improves the heat release capacity.Finally,the heat storage process of Li-doped Ca（OH）₂ can obtain a faster heat storage speed and more thermal energy than that of pure Ca（OH）₂,which are consistent with the studies of micro-mechanism and kinetics.This paper finally investigates the effect of CO₂ on Ca（OH）₂/CaO heat storage system.The experimental results show that CO₂ reacts with CaO in the water vapor that appears during the heat release process.Therefore,in the design of Ca（OH）₂/CaO systems,CO₂ should be cleared from the system.This study indicates that carbonic acid（H₂CO₃）could easily react with CaO/Ca（OH）₂ to form CaCO₃ during heat release processes.In addition,the experimental results show that carbonate shell does not exist in rehydration for the CaO samples,but the influence of CO₂ on the entire process increases after each cycle.