Francis turbines can be designed for a wide range of heads and
flows and are the most common water turbine in use today. The
Francis turbine has the characteristics of simple structure, small
size, low cost, high efficiency at full capacity (typically over
90%), stable in running, applicable to a wide range of water heads.
The Francis turbine is a type of reaction turbine, a category of
turbine in which the working fluid comes to the turbine under
immense pressure and the energy is extracted by the turbine blades
from the working fluid. A Francis turbine consists of the following
main parts: Spiral Casing, Guide or Stay Vanes, Runner Blades,
Draft tube, etc..
Francis turbines are almost always mounted with the shaft vertical
to keep water away from the attached generator and to facilitate
installation and maintenance access to it and the turbine.
Wicket gates around the outside of the turbine's rotating runner
adjust the water flow rate through the turbine for different water
flow rates and power production rates.
In addition to electrical production, Francis turbines may also be
used for pumped storage, where a reservoir is filled by the turbine
(acting as a pump) driven by the generator acting as a large
electrical motor during periods of low power demand, and then
reversed and used to generate power during peak demand. These pump
storage reservoirs, etc. act as large energy storage sources to
store "excess" electrical energy in the form of water in elevated
reservoirs. This is one of only a few ways that temporary excess
electrical capacity can be stored for later utilization.