Radial Inflow Turbines for Kalina and Organic Rankine Cycles

Abstract

Cryostar has been a world leader in radial inflow turbine technology for more than 30 years but is a relative newcomer in the geothermal business. Experience gained during the design and production of two turbo-expanders coupled to a generator for binary cycles has shown that standard expanders can easily be used for geothermal or heat recovery applications. The Main concern of the corrosion resistance of turbine elements against a corrosive working fluid like ammoniawater mixture can be solved with a rigorous selection of materials. The other concern is about sealing gas management in a close cycle loop. The Dry Gas Seal solution allows the seal gas flow to be decreased to very low values. The recompression of the leaking gas back into the cycle offers a zero leakage system. 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. Other losses in gear boxes, generator and bearings typically do not exceed in total 10% of the total isentropic enthalpy drop. For this reason the larger units can recover almost 85% electricity compared to the total isentropic enthalpy drop. 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. In fact, 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. The advantages of operating at higher pressure levels 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 optimise the binary cycle process data together with the turbine design to offer the best net cycle efficiency.