A novel fish-friendly-twisted blade towards low-head pumped hydro storage: assessment of flow distortion-induced energy loss and operational instability

Pumped hydro storage is one of the most sustainable energy storage technologies, accounting for over 90% of global energy storage capacity. It faces challenges related to fish injury and mortality similar to those encountered by conventional pumping stations. In this study, the bending profile of the so-called fish-friendly-twisted blade, recognized as the key geometric parameter affecting fish safety, is mathematically characterized using a novel power function. The systematic assessment of the twisted blades with varying exponential factors is performed, focusing on flow distortion, energy loss and operational reliability. Results indicate that as the exponent decreases, the blade leading edge tends to be more inclined toward the fish trajectory near the blade tip, resulting in a significant reduction in strike velocity and improved fish friendliness. Compared to blade morphologies with exponents of 4, 2, 1 and 0.5, the configuration with an index of 0.25 produces a more confined vortical flow in both size and intensity, and achieves a higher level of flow uniformity. This bending profile consequently mitigates flow-induced instabilities, including radial and axial forces as well as blade loading, and decreases the vortex-induced energy losses, particularly those caused by the leading edge hub vortex and the tip leakage vortex. The outcomes of this research offer comprehensive guidance for the design and application of fish-friendly blades in pumps and turbines, promoting the development of eco-friendly hydraulic machinery.