Variable-speed Pelton turbine for an efficient exploitation of the reserved flow: an Italian case study Santolin A. Tamanini Hydro S.r.l. Pavesi G. Department of Industrial Engineering – University of Padova Cavazzini G. Department of Industrial Engineering – University of Padova Ardizzon G. Department of Industrial Engineering – University of Padova Abstract 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. Introduction A relatively new measure in the Italian legislation [1] requires that also big hydropower plants have to ensure a Minimum Ecological Flow (MEF) at the foot of dams. This legislation defines the MEF to be proportional to a few parameters, connected to the river basin under the dam, such as average flows of tributaries, and it imposes a non- constant discharge during the year. The company that operates the plant decided to submit a project for the MEF release that foresees the installation of a mini hydropower plant instead of just a dissipating device to implement a significant energy recovery, fully respecting the environmental parameters. The dam already had an intake to be used as output for MEF discharge. For safety reasons, the water discharge had to be set a few hundred metres downstream the dam foot, creating a fixed head of about 35m, to be added to the basin’s height. The basin level varies between summer and winter of about 50%. As a consequence, the hydraulic turbine to be installed had to consider both the head variation during the seasonal operation, and the plant’s flow parameters. Given the head and the maximum capacity, the design choice led to a Pelton turbine that, however, did not adapt well to operation with low basin level. Pelton turbines, in fact, have very short operation efficiency and do not adapt to big head variations. The plant’s designers decided to suggest a variable-speed solution. This way it was possible to operate the turbine more efficiently, even with high head variations [2].