Variable-speed Pelton turbine for an efficient exploitation of the reserved flow: an Italian case study

This paper presents an Italian case study of a small hydropower plant exploiting the reserved flow (RF) at the dam outlet. The significant variations of the water level in the reservoir and of the seasonal RF discharged in accordance with the Italian environmental laws determine extremely variable operating conditions for the hydropower plant that can be efficiently faced only resorting to a variable speed turbine. This breakthrough technology allows to optimize the machine performance at a wider range of operating conditions, but still presents several issues to be faced, for example the lack of standardized design criteria. This study was focused on the design optimization of the variable-speed Pelton turbine to be installed in the small hydropower plant. Numerical analyses were carried out by the commercial code Ansys CFX 14 to define a Pelton bucket geometry characterized by high efficiency values in the requested wide operating range. , A transient homogeneous model coupled with a SST turbulence model was applied to solve the two-phase flow field., Experimental analyses were also carried out on the optimized runner in an open water test rig of the Laboratory of Turbomachinery of the University of Padova to validate the numerical simulations results. The experimental test rig, equipped with a pressurized reservoir, was properly settled so as to verify the turbine performance in a wide operating range. Finally, the results of the combined numerical and experimental design optimization were verified by comparison with the performance of the model installed in the small hydropower plant of Forte Buso.