Improving Stator and Rotor Regulation in Low-Head Axial Hydraulic Turbines through Simultaneous Rotational Speed Adjustment

Hydropower remains a dominant renewable energy source due to its high efficiency, energy density, and stable availability. For low-head applications, axial-flow turbines are necessary. On the other hand, low head hydroelectric plants are often run of river type power plants where regulation strategies play a crucial role, significantly impacting on efficiency. While Kaplan turbines utilize a combined distributor–rotor regulation mechanism for optimal performance, this approach may be too costly or complex for small-scale hydropower installations. As a result, mini hydropower plants frequently adopt simplified single-regulation strategies, adjusting either the distributor (stator) or the rotor blades. This study employs Computational Fluid Dynamics (CFD) simulations to evaluate the potential of the two regulation strategies, analyzing turbine overall performance and energy loss contributions. While single-regulation methods are cost-effective, they suffer from efficiency losses under off-design conditions. A potential strategy to improve mechanical single regulation approaches consists in an additional simultaneous variation of the turbine rotational speed, hence enabling a more flexible response to varying flow conditions. In this paper, this alternative approach— based on adjusting rotational speed simultaneously with guide vane or rotor stagger angles—is investigated in detail. The results provide valuable insights for the optimization of regulation mechanisms in low-head hydropower systems, balancing efficiency, cost, and operational flexibility.