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Classically, if you have free flow (high level to low, no pump) between two vessels you want to control you use some restricting orifice, or flow over a variable height weir. Both dissipate energy.

Now you could have a turbine in the flow, attached to a generator. If you adjust the torque applied by the generator to the turbine, you can control the flow over some range and extract power. I think this is an old idea, I just don't know the corect term to properly google it.

There are many considerations why this may not be feasible or worth the trouble in a given instant (e.g. hydraulic efficency of turbine over the flow ranges required). The applications I'm thinking of would be wastewater treatment plants with flows 1-10m³/s and heads <1m - so single digit kW most likely. At these heads, Banki or axial flow type turbines would probably be used.

My question is about the possibility - Can you actually control flow by appying breaking torque to a turbine, what does this do to hydraulic efficiency?

Maybe I'll ask about the electrical side in a different question, while certainly not trivial it's out of scope for now.

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  • $\begingroup$ What do you want to do with the shaft power, or how are you going to adjust the torque? Gas turbines do something like that, but the shaft power is determined by the gas input. Hydroelectric turbines do that but the speed is determined by the grid-connected generator. $\endgroup$
    – ericksonla
    Commented Feb 8, 2017 at 4:21
  • $\begingroup$ Extra question: do you need to control flow or volume transferred? Because instead of limiting throughput, a more efficient solution would be pulsing it. Say, instead of constricting the flow to 75% of nominal (most energy-efficient for given turbine), run the 100% flow for 45 minutes, then none for 15 out of each hour. $\endgroup$
    – SF.
    Commented Mar 10, 2017 at 9:37

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Flow can be controlled using the various governing mechanisms of the turbines.
A generator generally does not have anything to do with the control system of turbine.
Usually the flow is controlled using the gate opening with the help of governing mechanisms which nowadays are automated.
Efficiency of the turbine if operated under constant head has maximum value at certain speed at certain gate opening.
Graphs have been plotted that give you a fair idea of the values of these parameters.
Efficiency increases with increasing flow rate but then it remains constant after a certain value of flow rate.
According to your application, space, budget and arrangements, you can go with kaplan or propeller.
Since you want to generate power you will probably run the turbine at a constant speed.
Try searching for operating characteristics and main characteristics of turbines.

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You can use A Kaplan turbine and control the pitch of the blades and/or the wicket gates or use a Francis turbine and use the wicket gates to control the flow rate. Pelton turbines can also be used to control flow by adjusting the spear valves(basically needle valves).

I know this because I have personally installed all three types in WTW in the UK and Ireland.

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