An assessment of the performance of centrifugal screw pumps operating in reverse as turbines: A solution for fish-friendly micro-hydropower and pumping

The European Union’s Nature Restoration Law (NRL) aims to restore ecosystems, including reconnecting 25,000 km of rivers by 2030 through barrier removal/modification. However, it also poses a significant challenge to micro-hydropower (MHP) development as barriers are crucial for run-of-river MHP systems and modifying them involves high costs. A promising synergistic solution is the use of centrifugal screw pumps (CSP) as fish-friendly turbines, allowing energy generation while maintaining river connectivity. However, their performance in turbine mode remains unknown. This study investigated CSP-as-turbines (CSPAT) performance using theoretical and numerical approaches including Computational Fluid Dynamics (CFD) analysis, and new one-dimensional (1D) prediction equations. Two small CSPs, CSP-1 and CSP-2, were first analysed, showing peak efficiencies of 22.8% and 28.7% at 800 RPM, respectively. To enhance performance, a larger model, CSP-3, was assessed, achieved 33.1% efficiency and 12.5 kW output at 32.2 m head and 120 l/s flow. Based on the CFD results, new 1D pump-to-turbine prediction equations were developed, demonstrating low prediction errors (≤1.18%) and high accuracy (R2 = 0.979 for relative head and R2 = 0.973 for relative power). These findings enable the design of CSPAT for sustainable hydropower, pumping applications, and barriers modifications as part of nature restoration initiatives.