Heat And Mass Transfer Mechanism And Experimental Study Of Gas-solid Thermochemical Heat Storage

Due to the intermittence of renewable energy and the mismatch between energy supply and demand,heat storage technology plays an increasingly important role in the application of renewable energy.Thermochemical heat storage has the advantages of high heat storage density and long-term heat storage,which is an important means of heat storage technology.In this paper,the mechanism of heat and mass transfer in gas-solid thermochemical storage,the development of high-efficiency composite materials,the numerical simulation of thermochemical fixed bed reactor,and the operation of the high-pressure experimental system are studied.In this paper,the fractal algorithm is used to reconstruct the microstructures of the thermochemical heat storage materials.The reconstructed fractal model considers the porosity,fractal dimension,specific surface area.Then,based on the lattice Boltzmann method,the mass transfer process inside the heat storage materials is studied.Porous media with larger fractal dimension have more diffusion resistance,resulting in lower effective diffusivity.Further study shows that the thermochemical heat storage materials prepared at higher temperature have lower heat release rate due to larger fractal dimension.The relationship between effective gas diffusion coefficient and porosity of porous media is given.The results show that the method proposed in this paper is better than the Maxwell model when considering the characteristics of heat storage materials.Aiming at the heat transfer performance of fixed bed reactor,a multi-scale reconstruction algorithm(F-RSPP)is proposed in this paper.The algorithm considers many factors such as particle size distribution,different scale thermal resistance and so on.Compared with other single scale heat transfer models,the multi-scale heat transfer model constructed in this paper has the highest accuracy,and single scale numerical models are too high or too low to evaluate the effective thermal conductivity of materials.In addition,it is found that the effective thermal conductivity decreases with the increase of porosity,and the gas phase plays a restrictive role in the heat transfer performance.Then considering the application of gas-solid heat storage technology and the defects of natural heat storage materials,this paper developed a granular thermochemical heat storage composite material,which has superior performance in heat storage rate,heat storage density,mechanical properties,etc.At last,the performance and operation control of gas-solid thermochemical heat storage based on the fixed bed reactor are studied from the macroscopic point of view.Firstly,we used numerical method to study the direct fixed bed reactor.Because of the long heat storage time,we focused on the heat storage process of the system.The numerical results show that the heat transfer efficiency of the direct reactor is better.Moreover,the internal temperature distribution of reactor is uniform.It is found that for the direct thermochemical heat storage system,the partial pressure of vapor has little effect on the heat storage process,and the temperature becomes the limiting factor of the heat storage performance.In addition,the effects of reactant particle size,flow rate of heat exchange fluid and porosity on the heat storage power of direct thermochemical heat storage system are investigated.Then,based on the fixed bed reactor,this paper designed and built a high-pressure thermochemical heat storage test platform,explored the mechanism of using thermochemical heat storage to achieve the energy grade improvement.It is found that in the reactor level,the same material can achieve the energy grade improvement within the pressure controllable range.Specifically,in this study,7.65% energy grade is improved by using the same material in the heat storage and heat release process.In addition,it is found that the increase of pressure can not only increase the exothermic temperature,but also increase the exothermic rate.With the increase of decomposition temperature,the decomposition rate of heat storage materials increases.