The propeller turbine looks like the screw of a ship, but its mode of operation is the reverse of the ship's propulsion unit. In a ship a motor turns the propeller which pushes against the water, forcing the ship to move. In the hydropower plant, by contrast, moving water drives the propeller turbine to generate power.
Propeller turbines are most useful for low-head applications such as slow running, lowland rivers. Their efficiency drops off rapidly when the water flow drops below 75% of the design rating so plant designers often use multiple propeller turbines in parallel, shutting down some when the water flow drops in order to keep the remaining turbines operating at their optimum efficiency.
In some cases multiple turbines will be inappropriate, even though flows are not steady. Under these circumstances, a single turbine can provide better performance under variable flow conditions if the angle of the blade on the turbine can be varied. This is the principle of the Kaplan turbine.
Another variant is the bulb turbine, used for extremely low-head conditions. In this design the turbine and a watertight generator are enclosed in a bulb-shaped container. The turbine rotor can have fixed or variable blades. Water flows into one end of the bulb-shaped container and out the other, with no change of direction. The bulb turbine has been used in tidal power plants.
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