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.
Developed by James B. Francis in Lowell, Massachusetts, around 1848, the Francis turbine is a reaction turbine: the runner is fully submerged and pressurised, and water gives up energy both through its pressure drop and its change in velocity. Water enters a spiral casing (volute), passes adjustable wicket gates that regulate flow and set the swirl, turns through the runner and leaves via a draft tube that recovers kinetic energy.
Key figures:
- Head range: about 30–700 m, with most units between 40 and 400 m (hydraulic head)
- Peak efficiency: 93–96% for large units; efficiency drops markedly below about 40–50% of rated flow, where vortex rope and pressure pulsations in the draft tube appear
- Unit size: from under 1 MW to 1,000 MW; Baihetan (China) uses 16 Francis units of 1,000 MW each; Three Gorges and Itaipu use 700 MW-class units
Francis machines dominate global installed hydro capacity because they suit the medium-head dams typical of large reservoir schemes. A reversible Francis pump-turbine is the standard machine for pumped hydro storage, running in one direction as a pump and the other as a turbine.
Design challenges include cavitation, controlled by setting the runner below tailwater level, and part-load instability, which matters more as hydro plants are asked to balance variable wind and solar output.
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.
- 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.
- 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.
- Pumped hydro storage
Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.
- 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.