Design and Evaluation of Low Specific Speed Expander for Geothermal Energy Application
The increased interest in harnessing geothermal energy requires more attention to exploring and optimizing the design space for expanders with a specific focus on efficiency and reliability under off-design conditions. This study focuses on the design of a low-specific speed radial expander with a targeted shaft power of 0.5 MW at 22,000 rpm for the geothermal application utilizing CO2 as a working fluid at 150°C. The design analysis includes exploration of the effects of loading and flow coefficients on low-specific speed expanders and iterative design optimization using coupled 3D design and CFD simulations. The 3D optimization shows that designing a turbine with higher loading enables optimal efficiency point (92% isentropic) with the desired off-design performance, however, with increased reaction thrust as the consequence of increased loading. Further CFD simulations of the entire expander control volume including front and back seals show that thrust force in the direction from outlet to inlet (upthrust) increases as the flow rates and rotational speeds reduce. The study concludes with a discussion about the control scheme for turbine start-up and the performance of low-specific speed turbines for geothermal applications.
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