Optimizing Binary Cycles in Radial Inflow Turbines

Abstract

Historically axial type turbines adapted from steam turbine designs have been widely used in the binary cycles. 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 provides an ideal fit 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 to determine what is the best thermal efficiency achievable and to estimate which fluid is the most suitable for radial turbine operation. Fluids have been ranked using a performance factor which allows for the proper selection of a fluid that gives the best compromise between efficiency and turbine size. The advantages of operating a binary cycle above or as close as possible to the critical pressure and with lighter organic fluid than usual provides for an increase to the recovered electrical power while decreasing the expander frame size and lowering the initial project CAPEX. A substantial benefit can be realized by optimizing the binary cycle process fluid together with a radial inflow turbine design to achieve the best net cycle efficiency. Using a radial inflow turbine in standard execution with variable inlet nozzles allows for the ability to smooth seasonal variations inherent to geothermal process. Variable inlet nozzles are used to control the flow through the expander eliminating wasteful throttling. All the expansion energy in the nozzles and wheel is recovered at near constant isentropic efficiency throughout the year allowing the user to net a realized additional income over the plant lifetime.