Design and CFD Study of an ORC Radial Turbine for Multiple Working Fluids

Abstract

Organic Rankine cycle is an available solution for the conversion of low-grade thermal energy into electricity. In this way, it contributes to enhance the global plant efficiency hence to the reduction of the power plant carbon footprint (CO2 production). However, contrary to water Rankine cycle or Brayton cycle, in ORC the working fluid may change depending on the characteristics of hot and cold sources. Expander cost is estimated to be around half of the total cost of an organic Rankine cycle installation. Hence, developing a given turbine for multiple applications will help reducing the cost of ORC systems. In this work a radial turbine will be numerically investigated. The design and performance analysis of such a turbine will be analyzed for three working fluids taking into account real gas effect under expansion. An iterative process using preliminary design combined with meanline analysis allows the selection of the final geometry. Finally, 3D CFD simulations are computed on the obtained geometry with the selected working fluids for different operating conditions. Small deviations can be observed between the 3D CFD results and the prediction code. The different fluids have been selected based on safety (ASHRAE A1 and A2), environmental (GWP less than 150, ODP near to 0) and thermodynamic properties criteria (dry or isentropic fluid). The operating conditions have been selected to start the expansion in the low compressibility zone and featuring high rotational speed (up to 60000 rpm), low power (up to 9kW) and high maximum efficiency.