Optimizing Binary Cycles Thanks to Radial Inflow Turbines

Historically expansion turbines used for binary cycles were axial type turbines adapted from steam turbine design. They expand high-molecular-weight working fluid like isopentane which condenses at atmospheric pressure. Thus pressure ratio is limited across the turbine if a single stage design is considered. Process data for binary cycles are ideal for radial inflow turbines: pressure ratios, flows and temperatures ensure an operation very close to the maximum achievable isentropic efficiency. An extensive survey of different working fluids has been conducted: not only to determine what is the best thermal efficiency achievable but also to estimate which fluid is the most suitable for radial turbine operation. Fluids are ranked by a performance factor which enables one to select the fluid giving the best compromise between efficiency and turbine size. Then advantages of operating the binary cycle as close as possible to the critical pressure or above and with lighter organic fluid than usual are explained: It increases the recovered electrical power, whilst decreasing the expander frame size and its price. In most of the cases there is a large benefit to optimize the binary cycle process data together with the turbine design to offer the best net cycle efficiency. Another benefit in using radial inflow turbine in standard execution with variable inlet nozzles is the ability to smooth seasonal variations inherent to geothermal process. This device can be used to control the flow widely through the expander without wasteful throttling. All the expansion energy in the nozzles and wheel is recovered almost at constant isentropic efficiency throughout the year leading to very important additional income over the plant lifetime.