The Effects Of Injection Parameters On The Thermodynamic Performance Of An Enhanced Geothermal System（EGS） Using Supercritical CO₂ Cycle

Geothermal resource is rich in China,the development of geothermal energy is an important way to achieve the goal of carbon neutrality,among which utilizing the power of hot dry rock to generate electric is a research hotspot currently.Enhanced geothermal systems（EGS）using supercritical CO₂ cycles are promising technologies for energy recovery from hot dry rocks.The injection pressures,temperatures and flow rates of CO₂ determine the distribution profiles of CO₂ temperature and pressure,and the thermophysical properties along the down-hole well,then influence the heat transfer in wellbore and reservoir,and thus affect the production wellhead parameters.They are key parameters influencing cycle thermodynamic performance.Based on the characteristics of the hot dry rock resource at Gonghe Basin,Qinghai,a downhole well heat transfer model was developed and the impact mechanisms of CO₂ injection parameters on the parameters of production wellhead were analyzed,and then the cycle thermodynamic performance were also studied.When CO₂is injected downhole in a subcritical state,increasing CO₂ subcooled temperature can increase the CO₂ density which leads to higher pressure along the injection well and thus the production wellhead pressure is increased.The increase in pressure ratio of turbine can lead to higher system net power output,W_net.For higher condensing pressure,the circulating pump power consumption is lower and increasing the subcooled temperature has a greater effect on the improvement of W_net.For injection temperatures lower than 23.5℃,decreasing the injection pressure leads to higher turbine power output,but the increase of circulating pump power consumption is larger,thus W_net first increases and then decreases.The maximum net power output is reached for an injection temperature of 20℃and a pressure of 6.29 MPa.When CO₂ is injected downhole in a supercritical state,decreasing injection temperature can increase CO₂ density and pressure along the injection well.And then the production wellhead pressure,p₁,is increased which leads to higher pressure ratio of turbine and W_net.The density increases sharply due to the decrease in temperature for injection pressure is around 8.2 MPa,and p₁ changes more widely.Therefore,the increase in p₁ brought by the decrease in injection temperature first increases and then decreases with increasing injection pressures.And thus the increases in pressure ratio of turbine and W_net follow the same rules.As CO₂ is compressed before injection,the maximum net power output is reached for a turbine exhaust pressure of 8.2 MPa and an injection pressure of 9.75 MPa.Compared to CO₂ is injected directly,compressing CO₂before injection can improve net power output,especially for lower turbine exhaust pressure.When turbine exhaust pressure is 7.6 MPa,the increase rate of W_net is up to51.9%,and the thermal efficiency can increase by 14.9%relatively.The increase in flow rates can increase the wellbore frictional pressure loss and reduce the pressure of injection well bottom,and the pressure drop in production well is also increased,thus the production wellhead pressure decreases.When saturated liquid or supercritical CO₂ of 35℃is injected,for every 5 kg/s increase in production flow rate,W_netincreases by an average of 8.0%and 7.7%respectively.The CO₂ sequestration rate per electrical power generation,ω,first decreases and then increases with the increase of production flow rates.When CO₂ is injected in a supercritical state,ωis larger and the average values are 2.02-2.26 kg·s^-1/MW.