Main Machine Types
There are a range of water turbines in use, all of which have different strengths and features. Different heads of water create different pressures, and there are different configurations of machine to deal with this.
Pelton Wheels are an old and well tested machine. They are excellent for large heads, and widely used.
Francis Turbines have been widely used for medium head sites, and are an old design. They can be efficient, but tend to be less efficient when there is less water available than their design flow.
Propellers are used for lower heads, and are somewhat like a boat propeller, sometimes with control of the pitch of the blades (then known as Kaplan Turbines).
Crossflow are a more modern design, and are applicable to a range of heads. They have a good efficiency, and can sustain this over a wide range of flows. This is often useful, as abstraction licences may require smaller amounts of water to be used.
More information on the above machines can be found on the BHA website.
Archimedean screws are a new idea in power generation, and are excellent for low and very low heads (as low as 1m). They are simple and relatively cheap to install, compared with conventional Kaplan turbines.