Performance Study On The Ca（OH）₂/CaO Thermochemical Energy Storage System

Solar energy is one of the most promising renewable energy sources,but it is unstable and intermittent.Thermal energy storage technology,which can effectively reduce the cost of concentrated solar power generation,plays a crucial role in bridging the gap between energy supply and demand.In addition,thermal energy storage subsystem can improve performance and reliability of the whole energy system.Most solar thermal power generation systems,currently demonstrated and operated in china,adopt the method of sensible thermal energy storage.In contrast,thermochemical energy storage is a relatively new concept,which is still in the stage of basic test and verification.The main thermochemical energy storage systems include redox system,metal hydride system,carbonate decomposition system,ammonia decomposition system,methane reforming system and inorganic hydroxide system.Among them,the typical gas-solid reaction Ca（OH）₂/CaO is considered to be one of the most promising medium-high temperature thermochemical energy storage systems,with cheap raw materials,environmentally friendly and high heat storage capacity.In this paper,an indirect heating fixed bed reactor is selected,and an experimental device which could adjust the operating environment is designed.The reactivity and heat storage properties of Ca（OH）₂/CaO system,separately in N₂ and air atmosphere in 20 cycles were studied.In order to explore the operating temperature of hydration and dehydration process,the representative temperature of 5 points（T₁,T₂,T₃,T₄,T₅）in the reaction bed was monitored.In N₂ atmosphere,the average molar reaction fraction x_h and x_d are 0.939 and 0.061 respectively,in 20 cycles.The average total heat output q_h was 1241.48kJ/kg_{Ca（7）OH（8）2},accounting for 88.43%of the theoretical value(1403.70kJ/kg_{Ca（7）OH（8）2}).And the average total heat storage capacity q_d is 1233.20kJ/kg_{Ca（7）OH（8）2},accounting for 87.85%of the theoretical value.In the cycle process,the downward trend of q_h and q_d is similar.Overall heat release/storage capacity of the system is very high,and cycle stability is good.In the air atmosphere,the x_h decreased to 0.648 and x_d increased to 0.326,after 20 cycles.Activities of CaO and Ca（OH）₂ in the reaction bed decreased significantly.At the 20th cycle,only 492.70kJ/kg_{Ca（7）OH（8）2}was left in q_h,which merely accounted for 35.1%of the theoretical value.qd is 451.99kJ/kg_{Ca（7）OH（8）2},accounting for only32.2%of the theoretical value.Heat storage and release properties of the system decreased seriously and the cycle life of the system was short.The negative effect of CO₂ on the whole system increases after each cycle.Therefore,preventive measures should be taken to avoid CO₂pollution when using this technology in open systems.Reactant circulated 20 times in the air atmosphere was calcined under 1000℃.It turned out that x_h recovered from 0.648 to 0.982,and x_d decreased from 0.326 to 0.005.Reaction activity was well recovered.qh increased from 492.70kJ/kg_{Ca（7）OH（8）2}to 1378.43kJ/kg_{Ca（7）OH（8）2},and qd increased from 451.99kJ/kg_{Ca（7）OH（8）2}to 1371.41kJ/kg_{Ca（7）OH（8）2}.Heat storage performance of the Ca（OH）₂/CaO system was also well restored.In practical application,periodic calcinations can be considered for restoring properties of the system.This study can provide guidance for reducing negative effect of CO₂ on the Ca（OH）₂/CaO thermochemical energy storage system,and maintaining the stability of the system.