| Renewable energy such as wind energy,solar energy,and tidal energy has been developed rapidly in order to reduce greenhouse gas emissions and alleviate the dependence on fossil fuels.These renewable energy sources have good economic prospects and are conducive to environmental protection,but they have some fatal defects such as intermittent power generation,unstable supply and low energy density.These fatal defects may damage the normal operation of conventional power grids.Compressed air energy storage(CAES)technology as one of the most promising large-scale energy storage technologies can provide an effective solution for these defects.One promising development is to transform and reuse abandoned coal mine tunnels as CAES underground caverns.In the process of compressed air energy storage,frequent and rapid air charging and discharging will cause significant changes of air pressure and temperature in the underground caverns and thus affect the stability of the surrounding rock structure and the operation of compressed air energy storage.Therefore,it is of great scientific and engineering significance to investigate the coupling theory for the multi-physical process of the compressed air energy storage in underground caverns.In this study,both theoretical analysis and numerical simulation are used to comprehensively investigate the air leakage,the variation temperature and pressure in underground cavern during the compressed air storage process.Based on these studies,following main achievements are obtained:(1)A thermo-hydro-mechanical coupling model is established for the air leakage from the compressed air energy storage caverns.In this model,the thermodynamic equations for the temperature and pressure inside the cavern and the equation for the thermo-hydro-mechanical couplings in the surrounding rock are developed.A dynamic expression of permeability is established after considering the effects of volume deformation and temperature on gas seepage and the convective heat transfer between air and surrounding rock.This model was verified through the comparison of field test data,analytical solutions and numerical simulations.The results show that the temperature and pressure of the air inside the cavern changes periodically with a pattern of "up-down-down-up".The gas pressure in the cavern and the pore pressure in the surrounding rock interact each other,and are in a dynamic equilibrium.The air penetration range and pore pressure are also affected by periodic air injection and release.The permeability of rock is a key factor to affect the air leakage.The radius of the cavern and the mass flow rate of injected air also have a great influence on air leakage.(2)A thermodynamic coupling model is developed for the temperature and pressure in the compressed air energy storage cavern.This model couples the computational fluid dynamics with air heat transfer,and is used to study the variation of the temperature and pressure in the cavern.This model is established based on the mass conservation equation,the Navier-Stokes equation and energy conservation equation.Particularly,the energy conservation equation is expressed in terms of temperature and the convection term is introduced into simulations.Further,the heat conduction equation in the surrounding rock is established based on Fourier’s law of heat transfer.The k-? turbulence model is employed to describe the turbulent flow of air during compressed air storage and charging.The effects of natural convection and forced convection are explored.The results show that the periodic injection and release of air can cause periodic changes in temperature and pressure.During operation,gas velocity,temperature and pressure are unevenly distributed in the cavern space.The convection heat exchange is observed between the cavern air and the surrounding rock mass.This heat exchange has an impact on the operation of the compressed air energy storage system. |
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