On the influence of net head and efficiency fluctuations over the performance of existing run-of-river hydropower plants

The power output over a typical year is a key feature to assess the performance of a run-of-river hydropower plant.In carrying out this analysis, net head fluctuations due to changes in the processed flow rate are frequently neglected, while variations in the efficiency of the electro-mechanical generation group are more often taken into account.To depict the combined effect of net head and efficiency fluctuations, the concept of Global Efficiency is introduced. Great emphasis is put on discharge conditions and an approximate method is put forward for correlating the residual energy in the discharge section with the processed flow rate.The proposed approach is applied to the existing Cavaticcio run-of-river plant, set in the historical centre of Bologna (Italy). Results show that fluctuations of the residual energy at discharge may affect the net head value even more than upstream losses. Moreover, although assuming a constant net head could provide an acceptable estimation of the annual energy production, yet the Best Efficiency Point in terms of Global Efficiency may significantly differ from what expected from the generation group efficiency curve.This may help decision makers to assess the actual suitability of an existing generation group for a given site.