Mechanism And Properties Of Potassium Carbonate-based Thermochemical Sorption Thermal Energy Storage

As an emerging technology with wide application prospect,thermochemical energy storage(TCES)system can be used to solve the mismatch between energy supply and demand in time,space and intensity,which helps to improve the comprehensive utilization rate of renewable energy and promote the process of energy decarbonization.In the temperature range of low-grade solar energy and industrial waste heat,the typical working pairs for TCES systems are mainly inorganic salt/water and inorganic salt/ammonia.Considering safety and cost factors,studies applied to thermal energy storage in buildings have mainly focused on the inorganic salt/water.However,inorganic salt/water pairs are still facing many problems in practical TCES applications,such as slow reaction rate(poor heat storage and exothermic capacity),volume change(expansion or contraction)and morphological change(agglomeration and deliquescence of particles)of materials,which eventually lead to deterioration of the material properties and instability of the reaction performance.These macroscopic problems are essentially amplifications and diffractions of problems at the microscopic level.Therefore,the key to move toward the widespread application of the low-temperature TCES system is to understand the microscopic mechanism of inorganic salt/water pairs in the thermochemical sorption process and optimize them at the macroscopic level of the TCES system in a targeted way to make them operate efficiently and stably.This paper reviewed and compared the common inorganic salt/water working pairs,and finally choosed potassium carbonate/water working pair(K₂CO₃/H₂O)as the representative to study the hydration mechanism and hydration kinetics characteristics from the micro level,select the suitable matrix material type according to the micro research results,complete the preparation,optimization and characterization of the composite sorbents for TCES.Based on the above results,a numerical model of TCES system was established and verified by building an experimental platform of the TCES system,and a multifactor analysis of the performance of the TCES system was performed by using numerical simulations.The specific work contents are as follows:(1)Firstly,the current status of domestic and international research was summarized and sorted out.Based on the material classification,the literature review of material properties,reactor performance and system performance optimization were conducted for different kinds of materials,and finally the K₂CO₃/H₂O working pair was selected as the TCES material in this paper.(2)The microscopic hydration behavior of K₂CO₃surface was investigated using a first-principles method based on density functional theory(DFT)to reveal the deliquescence mechanism of water molecules sorbed on the K₂CO₃surface.The sorption configurations,electronic structure properties,thermodynamic properties and hydrogen bonding properties of water molecules sorbed on the specific crystalline surface of K₂CO₃ at different water coverages were obtained by theoretical calculations.The results showed that the surface hydration of K₂CO₃ is chemisorbed and the water molecules tends to sorb on the vacancies of K₂CO₃(001)surface through hydrogen and ionic bonds.The Mulliken population analysis revealed the presenc of a small amount of electrons transferred from K atoms on the K₂CO₃(001)surface to water molecules.The thermodynamic properties of the H₂0/K₂CO₃(001)surface systems at different water coverages showed that the K₂CO₃(001)surface remained hydrophilic even when the coverage of water molecules increased,verifying that K₂CO₃ has a strong hygroscopicity.Due to the good water-surface and water-water interactions the sorption configuration of the H₂0/K₂CO₃(0 01)surface system at 1.0 ML coverage is the most stable,and the four water molecules in this configuration form a head-to-tail water chain through hydrogen bondings.The distances between adjacent molecular layers increase with increasing water molecule coverage,but the differences between distances become smaller until they are close to equal,and these changes indicate that K₂CO₃swells during early hydration,explaining the experimentally observed phenomena from a microscopic scale.(3)Microscopic hydration kinetic properties of K₂CO₃ surface were investigated by molecular dynamics(MD)simulations based on Reax FF force field,and relative concentration(RC)distribution,radial distribution functions(RDF),the number of water molecules sorbed on the surface,conversion rate,mean square displacement(MSD),self-diffusion coefficient(SDC),and hydrogen bonding properties of water molecules sorbed on the specific crystalline surface of K₂CO₃ at different temperatures and coverages were obtained.The results showed that the orderliness of water molecules on the K₂CO₃(001)surface decreases with increasing coverage,and local dissolution occurs on the K₂CO₃(001)surface at high coverage.The minimum and maximum values of the SDC of water molecules were obtained at the coverages of 0.5ML and 1.0 ML,respectively.When the water coverage is constant,lowering the temperature is favorable to increase the conversion rate.It was also found that two types of hydrogen bonds exist in the H₂O/K₂CO₃(001)surface system,and they compete with each other in the surface hydration of K₂CO₃.To improve the sorption capacity and avoid deliquescence,the formation of HB₁-type(O_S...H_W)hydrogen bonds should be promoted and the formation of HB₂-type(O_W...H_W)hydrogen bonds should be inhibited as much as possible.The findings provide a theoretical basis for the selection of suitable substrates for the construction of K₂CO₃ composites for TCES.(4)Baed on the conclusion obtained from microscopic studies,expanded vermiculite(EV)was selected as a matrix material and EV-K₂CO₃(EVPC)composites for TCES were prepared by solution impregnation method,and their hydration and dehydration processes were experimentally investigated.The results showed that the EVPC composites could solve the problems of deliquescence and overhydration of the monomer K₂CO₃ salt,while maintaining the properties of the monomer K₂CO₃ salt,so the performance of the EVPC composites can be analyzed by the analytical method of the monomeric K₂CO₃ salt.Simultaneously,the TCES properties based on EVPC composites were evaluated comprehensively by three methods:DSC measurements,MD simulations,and theoretical calculations(TC),and the best composite was finally obtained as EVPC₄₀(salt content of 67.9%),with the water uptake of 0.68 g/g,average mass energy storage density of 0.49 k Wh/kg and bulk energy storage density of 173k Wh/m³ at a temperature of 30°C and relative humidity of 60%,indicating that the EVPC₄₀ composite is a TCES material with development potential.(5)The microstructure,porosity,pore size,particle size distribution,thermal conductivity,specific heat at constant pressure,equilibrium sorption performance,sorption kinetics,and cycling stability of the EVPC₄₀ composite for TCES were investigated using a series of characterization methods(e.g.,scanning electron microscope(SEM),fully automatic mercury intrusion piezometer(MIP),intelligent gravimetric analyzer(IGA),thermal conductivity meter,and differential scanning calorimeter(DSC)).Basd on the experimental results and theoretical analysis,the relevant expressions for the key parameters such as equilibrium sorption amount,heat of hydration and sorption kinetic model were obtained,which provides the basic data for the subsequent numerical simulations and applications.(6)Based on the concept of open system,the mathematical models of mass transfer,momentum transfer,heat transfer and sorption kinetics were established for the study of a packed bed reactor.The coupling problem of heat and mass transfer in the process of low-temperature TCES system based on the EVPC₄₀ composite was solved by using COMSOL multi-physical field simulation software.The validity of the numerical model was also verified by building a small-scale laboratory TCES system.(7)Based on COMSOL numerical simulations of,the sorption performance of the open TCES system with the EVPC₄₀ composite were investigated under different external conditions(e.g.,inlet air temperature,relative humidity and wind speed)and internal conditions(e.g.,porosity,particle size and attenuation coefficient).On this basis,a feasible application model was proposed,and the TCES performance of EVPC₄₀composite under two modes of seasonal thermal storage and short-term thermal storage was simulated and analyzed using TRNSYS software for a single-family house as an example.