Configuration of Organic Rankine Cycles for Geothermal Applications: An Industrial Perspective

Geothermal resources represent a valuable option to reduce fossil fuel-based power production because they feature an unmatched capacity factor among other Renewable Energy Sources (RES). Geothermal resource availability reduces with the temperature. Therefore, developing cost-effective solutions to exploit low-temperature geothermal energy is mandatory to expand technology utilisation. The standard solution for converting low-temperature thermal sources into power is Organic Rankine Cycles (ORC). ORC basic layout (subcritical) is well-known, but the more advanced alternatives, such as transcritical and two-pressure level cycles, are much less widespread, and it is unclear whether the higher efficiency justifies the higher capital cost. The paper focuses on the exploitation with ORC of geothermal resources (hot water) with a temperature lower than 200°C and mass flow rates between 200 and 1400 m3/h for a target power production ranging from 3 to 30 MW. The paper compares three ORC layouts, subcritical, transcritical and two pressure-level, from thermodynamic and economic points of view to map the most cost-effective solutions in the investigated size ranges. The techno-economic comparison considers the impact of the operating conditions and fluid on the machine’s expected performance and the heat exchangers’ size. As expected, more complicated layouts yield higher conversion efficiencies, with the two pressure-level cycles achieving the highest power output for the same geothermal source conditions. However, the economic analysis showed that the most efficient solutions are not always preferable when considering the cost-efficiency trade-off.