Kaplan turbine
A propeller-type reaction hydro turbine with adjustable runner blades and wicket gates, suited to low heads of about 2 to 40 m and large flows.
Invented by Viktor Kaplan in 1913, the Kaplan turbine is an axial-flow machine resembling a ship's propeller. Its defining feature is double regulation: both the wicket gates upstream and the runner blades themselves can be rotated. Coordinating the two (the "cam curve") keeps the flow angle matched to the blades across a wide range of flows and heads, giving high efficiency from about 30% to 100% of rated flow.
- Head range: roughly 2–40 m, occasionally up to 70–80 m
- Peak efficiency: 92–95% for large units
- Specific speed: high, allowing compact units for very large flows; runner diameters exceed 10 m at big river plants
Variants include:
- Propeller turbines: fixed blades, cheaper but efficient only near design flow
- Bulb turbines: generator housed in a submerged bulb in the water passage, standard on large low-head rivers and at the La Rance tidal barrage
- Pit, S-type and straflo designs: different generator layouts for small hydro
Kaplans dominate run-of-river plants on large lowland rivers such as the Danube, Rhine, Columbia and Madeira, as well as tidal range schemes. "Fish-friendly" designs with fewer blades, minimal gaps and lower rotational speed reduce injury to fish passing through, a key licensing issue. For very low heads under about 10 m, Archimedes screws compete in small-hydro applications.
Related terms
All terms →- Hydraulic head
The vertical height difference between the water surface at a hydropower intake and the turbine outlet or tailwater, which together with flow determines available power.
- Francis turbine
A reaction-type hydro turbine with fixed blades in which water enters radially and leaves axially; the most widely installed design, used for medium heads of about 30 to 700 m.
- Pelton turbine
An impulse hydro turbine in which high-velocity water jets strike double-cupped buckets on a wheel; the standard choice for high heads above roughly 250 m.
- Run-of-river hydro
Hydropower that diverts or uses the natural flow of a river with little or no reservoir storage, so output follows river flow rather than demand.
- Tidal range power
Electricity generated from the height difference between high and low tide, by impounding water behind a barrage or lagoon and releasing it through low-head turbines.