Comparative analysis of two-phase expansion and sub-critical organic Rankine cycle systems for solar and geothermal applications

For low to medium temperature heat sources (up to about 300 °C), the organic Rankine cycle technology is a cost-effective option for power generation. In addition to conventional sub-critical organic Rankine cycle systems, cycle architectures characterized by two-phase expansion, like, trilateral and partial evaporation organic Rankine cycles, have been proposed. These cycle architectures results in a higher utilization of the available heat and subsequently in a higher exergy efficiency of the system compared to that of sub-critical organic Rankine cycle systems. However, these configurations have a higher cost of the power system, and lower isentropic efficiency of the expander compared to the sub-critical organic Rankine cycle based configurations. This paper presents a techno-economic comparison among trilateral, partial evaporation and sub-critical organic Rankine cycle systems considering concentrated solar power and geothermal heat sources, respectively. A selection method for these configurations is proposed, helping to select the optimal power cycle for concentrated solar power and geothermal plants. The results suggest that the subcritical organic Rankine cycle system is the preferred choice over the trilateral and partial evaporation organic Rankine cycles for concentrated solar power plants. As for the geothermal energy powered plants, the results indicate that for 10 % penalty in expander isentropic efficiency, the specific cost of the trilateral organic Rankine cycle system should be equal or lower than that of the subcritical organic Rankine cycle system in order to become favourable. In addition, high values of the condensation temperature are favourable for the trilateral and partial evaporation organic Rankine cycles.