Influence of geothermal water with N₂ and O₂ as major gas components on organic Rankine cycles using mixed method

One of the important issues for geothermal systems during utilization is the special physical properties and non-condensing characteristics of non-condensable gases (NCGs). Because most studies focus on CO₂-rich NCGs, the impacts of N₂ and O₂ NCGs on thermal cycles such as organic Rankine cycles (ORCs) have limited understanding. Thus, we present an appropriate method named ‘mixed method’ to study the impacts of physical properties of geothermal water with N₂ and O₂ as major gas components on the performance of basic, reheated, and regenerative ORCs. Results show that variations in thermal utilization characteristics are similar among all studied systems. Energy and exergy efficiency change nonlinearly as inlet temperature, volume flowrate variation, or water vapor content increases; Enthalpy and exergy difference change linearly in this case. Variations in all thermal utilization parameters are linear when the inlet pressure, reinject pressure, or total gas content rises. When the reinject temperature rises, energy and exergy efficiency changes are nonlinear and near-linear. Variations of enthalpy and exergy difference are linear and nonlinear. Considering the influence, we suggested that, except for inlet and reinject pressure, other physical properties should be monitored and controlled in the geothermal system for safe, steady thermal utilization. When working fluid proportion in turbine 2 of regenerative ORC varies between 0.4 and 0.6, regenerative ORC does not always have the highest energy and exergy efficiency. The proposed method and results would guide the thermal design and utilization of geothermal systems.