A study of the optimal operating conditions in the organic Rankine cycle using a turbo-expander for fluctuations of the available thermal energy

The organic Rankine cycle is widely used to obtain electric power from renewable energy sources, such as solar energy, geothermal energy, and waste thermal energy. In a typical ORC, a turbo-expander or volumetric expander is applied to convert the thermal energy to mechanical energy. The turbo-expander is widely used for large-scale output power because it has merits when used with large mass flowrates; the scroll expander is used for small-scale output power. In ORCs that produce small-scale output power, the available thermal energy as a renewable heat source usually cannot be supplied continuously. For fluctuating levels of available thermal energy, positive displacement machine has difficulty in adjusting the mass flowrate. In order to regulate the mass flowrate for varying thermal energies, a small-scale radial-type turbine and supersonic nozzles were designed specifically for this study. R245fa was used as the working fluid, and the thermodynamic properties of the working fluid in the cycle were predicted on the basis of the designed turbine blade and nozzle shape even though the mass flowrates were varied. The output powers at the off-design operations were predicted for the full range of 30 kW according to the number of nozzles used.