Graph of Power versus Speed for a reaction turbine (Francis and Kaplan) 
It would appear that optimizing the electrical power generated for any particular gate opening (water flow) would also optimize the efficiency. If this is true it would greatly simplify the controls – no need to know the exact position of the gate opening, the water flow through the turbine or the head. The firmware in the inverter could simply periodically optimize the torque on the generator shaft to optimize electrical power generated. This may only be possible on a hydro turbine because of the relatively slow response time of the turbine and the slowly changing river conditions. A wind turbine must respond much more quickly to changes in wind speed and would not have time to optimize the torque. A predetermined lookup table of turbine performance would be used. Would such an optimizing scheme work for a Kaplan hydro turbine ? Has anyone got experience of optimizing the power from a variable speed hydro turbine ?