IMPACT OF OFF-DESIGN PERFORMANCE OF TURBINES ON ORGANIC-RANKINE CYCLE SIMULATIONS FOR SOLAR-POWER APPLICATIONS

Organic Rankine Cycle (ORC) power systems offer a high potential for utilizing solar power for electricity generation. Inherently, the heat supply by solar power is time-dependent, and significant fluctuations occur during the day. Although some time-averaging of the heat input by thermal storage systems might be employed, the off-design performance of turbines is highly relevant for the mean thermal efficiency of such cycles. This study aims to analyze the effect of a hybrid power plant system obtained by combining an Organic Rankine Cycle (ORC) and a Linear Fresnel system (LFR) on the ORC turbine efficiency under part-load. The performance data of the LFR system is taken from an installed system designed to produce 20 kW of heat. The LFR-ORC system was modeled in detail, and thermodynamic calculations were performed using the cycle simulation program EBSILON. The local Direct Normal Irradiation (DNI) values for the selected day, the thermal efficiency of the system, and the temperature and mass flow rate of the thermal oil entering the system were used as inputs for ORC cycle. The ORC power plant was designed as a subcritical cycle with Novec 649 as the working fluid. The off-design performance of the ORC turbine used is compared with the nominal design operating performance using the Kroon and Tobiasz approach. The turbine efficiency as a function of the part-load operation was discussed within the framework of selected loss correlation. The analysis examined the thermal efficiency obtained from the turbine on the ORC side for varying thermal efficiency on the solar field side.