Nowadays, in order to mitigate the global warming effects, there is a need of renewable energy generation for the objective of carbon neutrality. In this context, hydropower plays a key role not only because the amount of renewable energy generated but also because it is a fundamental player to ensure the stability of the electrical grid, as one of the main dispatchable sources. Nevertheless, hydropower is facing nowadays with the climatological problem of the extreme droughts, that are expected to be more common in the upcoming years. Therefore, it has become more important than ever to use the water flowing through the rivers properly. In this paper, the potentiality of using variable speed operation with the aim of increasing the overall efficiency of Francis turbines, which are the most widely used hydro turbines worldwide, is numerically explored. This implies to use less water to produce the same amount of electrical power. The study is based on an accurate modelling with real prototype data which has been made available for this study. It is shown that variable speed could improve the overall efficiency of the unit with respect to the constant speed generator, typically used in hydropower. While this idea has been mentioned in some previous studies, in this paper we also consider the electrical efficiency decrease of the variable speed technologies and restrictions of the unit regarding the electrical power generated. Results show that when the unit operates at some specific operating conditions, namely low heads at part load operations and high heads at maximum power, variable speed technologies could be used to save more than 2% of water with respect to the fixed speed unit. Main results and models of this paper can be used as a reference for future studies with similar type of units.

First of all, Authors would like to acknowledge XFLEX Hydro [20]. This study is a direct output of the technologies being studied in this project. The Hydropower Extending Power System Flexibility (XFLEX HYDRO) project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 857832. Alexandre Presas would like to acknowledge, Nicolas Hugo from ALPIQ, Thomas Hildinger from VOITH and Josep Bordonau from UPC for his expertise and valuable comments regarding the FSFC and DFIG models and also Jo˜ao Delgado for his valuable contributions and ideas during part of this project. Alexandre Presas and David Valentín acknowledge the Serra Húnter program of Generalitat de Catalunya.