| Thermal energy storage technology can effectively improve the utilization efficiency of renewable energy,solve the defects of intermittent and unstable solar energy,reduce the environmental problems caused by the burning of fossil raw materials,and provide a method for the storage and continuous stable supply of thermal energy.There are three ways to store thermal energy:sensible heat storage,latent heat storage,and thermochemical energy storage.Due to the advantages of large energy storage density and long storage time,the thermochemical energy storage system can be widely used in waste heat recovery and energy transmission systems.Among the dozens of thermochemical energy storage systems,Ca（OH）2/CaO system has the advantages of cheap and easily available raw materials,safety and non-toxicity,and energy storage density.It is one of the current high-temperature thermochemical energy storage systems with development potential.This paper builds a fixed-bed reactor,prepares composite materials CE and CEL,and explores the reaction performance of composite materials in the process of dehydration and hydration.The composite material CE is prepared by doping expanded graphite in Ca（OH）2.The effects of bed temperature,reaction mole fraction,energy storage changes,and different dehydration temperatures and hydration pressures on the endothermic/exothermic process of composite materials CE were investigated.During the endothermic reaction,the bed temperature first increased and then stabilized.As the mass mixing ratio of the expanded graphite increases,the reaction rate increases.The composite material CE with w=0.25undergoes dehydration for 75 minutes,and the bed temperature tends to be stable;while the pure calcium hydroxide remains stable at 494℃after 140 minutes of reaction.In addition,the thermal storage capacity of the composite material CE is higher than that of pure Ca（OH）2.Pure calcium hydroxide absorbs 801.1 k J/kg of thermal energy during the dehydration process,while the composite material CE of w=0.25 absorbs 983.8 k J/kg.During the exothermic process,water vapor enters the reaction bed and reacts quickly with calcium oxide,and the bed temperature rises rapidly.With the increase of the EG mass mixing ratio,the maximum value of the bed temperature increases,and the hydration reaction rate increases.When w=0and 0.25,the heat release capacity accounts for 80.8%and 85.8%of the maximum energy storage capacity corresponding to the theoretical enthalpy change of pure Ca（OH）2/CaO,respectively.The composite material CEL is prepared by doping LiBr in the composite material CE.Observe the mass and heat transfer performance and cycle stability of the composite material CEL during the dehydration-hydration-dehydration cycle reaction process.The results show that the addition of LiBr to the composite material CE has no significant effect on the reaction bed temperature and molar reaction fraction during the endothermic reaction.During the hydration reaction,as the LiBr molar mixing ratio increases,the hydration molar fraction increases and the release energy capacity increases.The molar reaction fraction at the end of hydration of CEL with n=0.100 is 0.1118 higher than that of pure Ca（OH）2.The heat release capacities of CEL with pure Ca（OH）2,CE and LiBr molar mixing ratio of 0.100 are 1132.8k J/kg,1191.8 k J/kg and 1298.6 k J/kg,respectively.Whether it is pure Ca（OH）2,CE composite or CEL composite,during the dehydration process,as the dehydration temperature increases,the reaction rate increases;during the hydration process,as the hydration pressure increases,the hydration reaction rate increases;As the number of cycles increases,the particle size of the reactants increases,but the molar reaction fraction fluctuates within a small range and has good cycle stability.In terms of numerical simulation,Gambit software was used to establish an indirect heat transfer reaction bed model,which was imported into FLUENT software for numerical calculation to explore the dehydration performance of Ca（OH）2and the hydration performance of CaO,respectively.The research shows that the numerical simulation is not much different from the experimental results during the dehydration process.However,during the hydration process,indirect heat transfer fluids were added to the numerical simulation,and as the HTF flow rate increased,the hydration reaction rate increased.The partial pressure of water vapor affects the rate of hydration reaction.With the increase of hydration pressure,the maximum value of the bed temperature increases.The lower the porosity of the reaction bed,the longer the hydration reaction time and the greater the energy storage density. |
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