A water turbine is a rotary engine that takes energy from moving water.
Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.
Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine.
Water turbines are divided into two groups; reaction turbines and impulse turbines.
The precise shape of water turbine blades is a function of the supply pressure of water, and the type of impeller selected.
Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.
Newton's third law describes the transfer of energy for reaction turbines.
Most water turbines in use are reaction turbines. They are used in low and medium head applications.
Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.
Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.
Newton's second law describes the transfer of energy for impulse turbines.
Large modern water turbines operate at mechanical efficiencies greater than 90% (not to be confused with thermodynamic efficiency).