The Organic Rankine Cycle (ORC) is commonly accepted as a viable technology to convert from low to medium temperature geothermal energy into electrical energy. In practice, the reference technology for converting geothermal energy to electricity is the subcritical simple ORC system. Over time, geothermal ORC plants with more complex configurations (architectures) have been developed. In the open literature, a large number of advanced architectures or configurations have been introduced. An analysis of the scientific literature indicates that there is some confusion regarding the terminology of certain advanced ORC system architectures. A new categorization of advanced configurations has been proposed, with a special emphasis on the application of geothermal energy. The basic division of advanced plant configurations is into dual-pressure and dual-stage ORC systems. In this study, the real potential of advanced ORC architectures or configurations to improve performance as compared with the simple ORC configuration was explored. The research was conducted for a wide range of geothermal heat source temperatures (from 120 °C to 180 °C) and working fluids. Net power output improvements as compared with the basic subcritical simple ORC (SORC) configuration were examined. The ability to produce net power with different ORC configurations depends on the magnitude of the geothermal fluid temperature and the type of working fluid. At a lower value of geothermal fluid temperature (120 °C), the most net power of 18.71 (kW/(kg/s)) was realized by the dual-pressure ORC (DP ORC configuration) with working fluid R1234yf, while the double stage serial-parallel ORC configuration with a low-temperature preheater in a high-temperature stage ORC (DS parHTS LTPH ORC) generated 18.51 (kW/(kg/s)) with the working fluid combination R1234yf/R1234yf. At 140 °C, three ORC configurations achieved similar net power values, namely the simple ORC configuration (SORC), the DP ORC configuration, and the DS parHTS LTPH ORC configuration, which generated 31.03 (kW/(kg/s)) with R1234yf, 31.07 (kW/(kg/s)) with R1234ze(E), and 30.96 (kW/(kg/s)) with R1234ze(E)/R1234yf, respectively. At higher values of geothermal fluid temperatures (160 °C and 180 °C) both the SORC and DP ORC configurations produced the highest net power values, namely 48.58 (kW/(kg/s)) with R1234ze(E), 67.23 (kW/(kg/s)) with isobutene for the SORC configuration, and 50.0 (kW/(kg/s)) with isobutane and 69.67 (kW/(kg/s)) with n-butane for the the DP ORC configuration.

This paper is focused both on the thermodynamic and economic analysis of an organic Rankine cycle (ORC) based geothermal power plant. The analysis is applied to a case study of the geothermal field Recica near the city of Karlovac. Simple cycle configuration of the ORC was applied. Thermodynamic and economic performance of an ORC geothermal system using 8 working fluids: R134a, isobutane, R245fa, R601, R601a, R290, R1234yf, and R1234ze(E)], with different critical temperatures are analyzed. The thermodynamic analysis is performed on the basis of the analysis of influence of the operation conditions, such as evaporation and condensation temperatures and pressures, and evaporator and con-denser pinch point temperature difference, on the cycle characteristics such as net power output, and plant irreversibility. The economic analysis is performed on the basis of relationship between the net power output and the total cost of equipment used in the ORC. Mathematical models are defined for proposed organic Rankine geothermal power plant, and the analysis is performed by using the software package engineering equation solver. The analysis reveals that the working fluids, n-pentane and isopentane, show the best economic performances, regardless the evaporation temperatures, while the working fluid R1234yf and R290 have the best thermodynamic performances. In addition, each analyzed working fluid has its corresponding economically optimal condensation temperature (and condensation pressure). Economically optimal pinch point temperature difference of evaporator has different values, depending on the working fluid, while pinch point temperature difference of condenser has similar values for all analyzed working fluids. Analysis results demonstrate that the subcritical ORC geothermal power plant represents a promising option for electricity production application.