Analysis of a CO2 blended fluid based EGS cycle and its implication for highly efficient low grade geothermal power generation system

Abstract

The economy of water based Enhanced Geothermal System affect it’s utilization compared to other renewables largely due to the phys-chemical properties of water. CO2 captured from energy industry is a promising substitution to replace water. A novel design approach is proposed to maximize the thermodynamic efficiency of Enhanced Geothermal System tapping into low grade geothermal reservoirs. CO2 regenerated from stripper tower is condensed and collected into a subcritical, liquid form, it absorbs heat from geothermal reservoir and becomes supercritical. In the combined effect of geothermal and gravitational field, it rises back from the production well and is blended with another working fluid with unequal pressure. Undergoing heat and mass transfer to another saturated or subcooled working fluid (primarily an organic working fluid) in direct contact, the supercritical CO2 expands while the secondary liquid vaporizes, creating a binary species fluid expanding across the expander(s). Heat transfer between both species may bear the potential of raising power output significantly, and results in a polytropic process with a polytropic index closer to 1 compared to that of an adiabatic process. Simple calculation with separate flow method has demonstrated that majority of the power generated comes from the latent heat of the second working fluid experiencing phase change. And design of the compression stage coupled with gravity of CO2 effectively reduces energy consumption to bring CO2 back to liquid state, while making enough room for separation of fluids. Such novel design schematics is able to generate power from very low temperature, e.g., 50?scCO2, in large scale deep wells, while taking care of CO2 geological storage simultaneously. The proposed scheme can improve the economy of geothermal power system.