Calcium Chloride Composite Sorbent For Solar-driven Thermochemical Heat Storage

Energy transformation and clean energy utilization are the important methods to realize carbon neutralization.In our country,the place with abundant solar energy in summer needs heating in winter because of the cold weather.Therefore,it is necessary to transfer the abundant solar energy in summer to winter.Thermochemical heat storage technology based on the reversible hydration/dehydration reaction of hydrated salt and water vapor has the advantages of high energy density,without thermal insulation,and long-term storage,which is suitable for cross-season heat storage.The pair of Ca Cl₂/H₂O has high energy storage density,abundant resources,friendly environment and suitable temperature for heating.However,it has problems such as low reactivity,reduced stability,and heat and mass transfer resistance,which result in the actual heat storage density less than 30%of the theoretical heat storage density.Therefore,the research on the microstructure optimization,reaction dynamic enhancement,and physical/chemical stability of Ca Cl₂ sorption heat storage materials is carried out,and solar-driven thermochemical heat storage system is built based on the materials,which is beneficial for the development of advanced thermal storage and solar energy utilization technologies.The use of porous materials to support hydrated salt can relieve the problems of leakage and agglomeration caused by deliquesce of calcium chloride during the hydration process.The structural stability and mass transfer performance of the material can be improved and the key is the porous structure of the matrix.Here,the mineral diatomite was used as the matrix whose pore structure was optimized by acid and alkali washing to explore the influence of pore structure on the reaction performance and energy density of composite heat storage materials.The acid washing treatment can dredge the macroporous structure of the diatomite and provided space for loading calcium chloride.Alkaline washing treatment can produce more mesopores of 2-4 nm and 15-50 nm,which provides mass transfer channels for the diffusion of water vapor.Alkaline-washed diatomite loaded with calcium chloride has the highest water vapor sorption capacity,and its energy density reaches 808 J/g,which is 38%higher than that of unmodified diatomite loaded with calcium chloride.Subsequently,sepiolite(mesoporous),diatomite(macropores)and expanded perlite(foam-like macropores)are used as the matrix materials to explore the influence of typical pore structure of the matrix on the reaction performance,energy density and structural stability of the composite sorbent.Compared with sepiolite and diatomite,the foamy macroporous structure of expanded perlite is conducive to loading more calcium chloride,increasing the amount of water vapor adsorption and energy density.The leakage of the solution is prevented due to the surface tension of the macropores.The foamy macroporous matrix loaded with calcium chloride has the highest water vapor exchange capacity of 1.30 g/g,and the corresponding heat storage density is2179 J/g.Therefore,the high-porosity foam-like macropores can enhance the structural stability and energy density of the composite heat storage material at the same time.Conventional heat storage materials can not directly absorb solar energy,resulting in complex composition and difficult operation of heat storage system.Here,a honeycomb-like porous carbon material loaded with calcium chloride is obtained.Carbon materials have the excellent photothermal conversion efficiency.Moreover,the high-porosity macroporous structure provides space for the loading of hydrated salts.Natural cork is used to obtain a porous carbon material by carbonization,and the effects of carbonization temperature on the pore shape of the material and the load of hydrated salt are explored.Carbonization at 800? can produce carbon material with regular honeycomb-like pore structure,and the loading amount of hydrated salt is 34%.Subsequently,the effects of oxidation modification treatment on the surface polarity of the matrix and the loading of hydrated salt are explored.The oxidation treatment enhances the hydrophilicity of the matrix without damaging the macroporous structure,so that the compatibility between the matrix and the hydrated salt is improved and the loading amout of hydrated salts increases to 56%.It is found that the hydrated salt loading amount is positively correlated with the sorption capacity of the composite sorbent,which affects the energy density.The energy density of carbonized and oxidized matrix loaded with hydrated salt at 800? is the highest(1353 J/g),with the water exchange of 1.02 g/g.Under simulated natural light,the surface temperature of the composite sorbent can reach 70? due to the photothermal effect of the carbon material,which can realize the solar-drive desorption and thermochemical heat storage.The porous matrix loaded with hydrated salt can prevent the problems of liquid solution and leakage,but the open pore structure of the composite sorbent leads to a contradiction between structural stability and energy density.Here,a pomegranate-type carbon-coated calcium chloride composite sorbent is prepared.Calcium chloride nanoparticles are coated with a porous carbon framework,which ensures high hydrated salt loading and improves the structural stability of the material.In the case of the same salt content,the carbon-coated sorbent has higher structural stability than the porous matrix loaded with calcium chloride sorbent.The content of each component in the composite sorbent can be controlled by changing the carbon precursor amount.The calcium chloride content of Ca/CT40-700 is 89%with the sorption capacity was 1.13 g/g at 20?,RH 60%.The carbon shell plays a role in light-to-heat conversion,and the heat is transferred from the shell to the inside of the particles,realizing solar-driven desorption.After irradiation under simulated sunlight for 230 min,the volumetric energy storage density of Ca/CT200-700 is 254 kW·h/m³,with the water change of 0.81 g/g,which is at a higher level than other composite sorbents.Although the carbon-coated calcium chloride sorbent has the property for solar-driven thermochemical heat storage,the powdery form of the material leads to high pressure drop and mass transfer resistance in the fixed bed reactor.For the practical applications,a large-particle carbon-coated sorbent is prepared in batches.Calcium chloride is dispersed in the foamy macropores of the expanded perlite,and a layer of carbon is coated on the outside of the particles.The structure characteristics and heat storage performance of this material is studied.The macroporous structure of matrix ensures high hydrated salt loading and mass transfer performance,and the carbon content is controlled by changing the concentration of the carbon precursor solution.The carbon shell converts light energy into heat energy,and heat is transferred from the shell to the hydrated salt for the desorption.The sorption capacity of EP/Ca@C-0.25 is 1.22 g/g at 20? and RH 80%,and 84%of the water can be desorbed after illumination at 1 kW/m² simulate sunlight for 2 hours.The heat storage density in the sorption/desorption cycle is 1698 J/g.A solar-driven thermochemical heat storage system was proposed and lab-scale test was carried out with the large particle core-shell sorbent.The water-containing sorbent is placed in the outdoor for 2 hours for to achieve complete desorption of water vapor and efficient heat storage.The dried sorbent is placed on a fixed bed for the sorption exothermic test,and the gas temperature was increased by 5? for more than 10 hours,which proves the feasibility of the solar-driven thermochemical heat storage system.