Optimization of geothermal power plant design for evolving operating conditions

The main goal of this work is to determine optimal geothermal power plant designs by taking into account the transient evolution of the plant/reservoir system. To do so, a geothermal reservoir model is developed, where the permeability of the ground is represented by a series of parallel pipes inside which the underground water can flow. The reservoir model is coupled to an evolving Organic Ranking Cycle (ORC), where the pressure at the condenser adapts to the conditions in the geothermal reservoir (temperature of the brine and mass flow rate) based on the Stodola equation. The system is then optimized in order to maximize the total energy output of the power plant over its lifetime. A series of parametric analyses was performed for relevant design parameters (e.g., overall conductance of the heat exchanger at the evaporator, turbine sizes, number of years of operation, etc.), while other parameters were optimized, namely the working fluid to geofluid mass flow rate ratio, the pressure at the evaporator, and the geofluid mass flow rate. The optimal values that were found were values that yielded viable cycles over the entire exploitation period of the plant and that did not deplete the thermal reservoir prior to the end of the plant lifetime. ORC cycles that were optimized by considering the time evolution of the system were then compared against cycles optimized under the assumption of constant geothermal reservoir properties. It was also demonstrated that by allowing key design parameters to change over the course of the exploitation of the plant, it was possible to further increase the plant performance.