Water saving options in hydropower by means of variable speed operation: A prototype study in a mid-head Francis turbine

•The potentiality of using variable speed for saving water in a mid-head Francis Turbine is explored.•It is determined that Doubly Fed Induction Generator is generally more efficient than Full Size Frequency Converter for this purpose.•It is estimated that hydraulic efficiency could be improved around 3% for very low heads and partial load conditions.•For mid-head Francis Turbines, the averaged water saved in one year of variable speed operation could be 0.3%-0.5% less with respect to the synchronous generator operation.•Variable speed operation may be even a more interesting option in the near future with more common extreme droughts.