| Abstract |
Geothermal is an abundant, ubiquitous and sustainable energy, which is stored in subterraneous, aquifer and other geologic body from ground surface to centrosphere. Co-axial closed-loop geothermal system is a novel method using a continuously closed wellbore without water exchange with. The closed system can avoid many troubles in traditional geothermal development methods such as reservoir blockage, heat transmission fluid leakage. Recently, Using CO2 as a heat transmission fluid to extract geothermal energy has attracted many attentions. As a novel heat transmission fluid, the thermophysical properties of CO2 are quite different from those of water. CO2 has many advantages such as larger mobility and buoyancy resulted from the lower density and viscosity. This is an excellent characteristic for circulating refrigerants in terms of flow and heat transfer. However, there are still some disadvantages for CO2 as the heat transmission fluid, such as small heat capacity, leading to a less heat at the same mass flow rate. Therefore, how to design appropriate parameters to avoid these shortcomings to use CO2 for heat transfer has broad prospects. In this work, wellbore-reservoir coupling model is built to simulate the running processes of co-axial closed-loop geothermal system. The difference of productivity between geothermal systems using water and CO2 as heat transmission fluids has been studied. Besides, we investigated the fluid flow and thermal processes of supercritical CO2 and water along the wellbore and the heat-extracting mechanism. By comparing the two heat transmission fluids in terms of heat extract in performance and heat transfer mechanism, advantages and disadvantages of using CO2 is identified. This work also provides some theoretical reference for the selection of heat transmission fluid in the Engineering of the geothermal development. |