PAT efficiency variation with design parameters

The use of pump operating as turbine (PAT) instead of traditional turbines is an effective source of reducing the equipment cost in small hydropower plants. However, the lack of information on the PATs performances represents a limit for their wider diffusion. Furthermore, while for the design of traditional hydropower plants the knowledge of the best efficiency point of the turbomachine is sufficient, additional difficulties arise under variable operating conditions for the exploitation of hydraulic power within water distribution networks: a procedure for the design of the optimal machine operating under variable hydraulic conditions, named variable operating strategy (VOS), has been recently developed, and for its implementation a full set of characteristic curves of several different PATs rotating at different speeds is needed. In order to face the hydraulic variability, variable operating strategy considers either a hydraulic or an electric regulation. The hydraulic regulation system consists in a series-parallel circuit with a PAT and two regulating valves, while for the electric regulation the PAT generator is connected to an inverter which modifies the rotational speed of the machine. If the performances curves of a single PAT are known, all the information needed for the application of VOS can be obtained by the application of the affinity law of turbomachines, wich allows to extent the given information to a complete set of similiar machines. This paper focuses on the study of the affinity law for the evaluation of the behaviour of a single machine under variable speed. A large database of experimental curves of several PATs operating at different speeds is available and the experimental data are compared with the results of the application of the affinity law herein. (C) 2013 The Authors. Published by Elsevier Ltd.