Editing Water turbine (section)

== Theory of operation ==
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  [[Mechanical work|work]]). In this way, energy is transferred from the water flow to the turbine.

Water turbines are divided into two groups: [[reaction (physics)|reaction]] turbines and [[Impulse (physics)|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 ===
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 laws of motion#Newton's third law|Newton's third law]] describes the transfer of energy for reaction turbines.

Most water turbines in use are reaction turbines and are used in low (<{{convert|30|m|sigfig=1|disp=or|abbr=on}}) and medium ({{convert|30|-|300|m|sigfig=1|disp=or|abbr=on}}) head applications. In reaction turbine, pressure drop occurs in both fixed and moving blades. It is largely used in dam and large power plants.

=== Impulse turbines ===
Impulse turbines change the velocity of a water jet. The jet pushes on the turbine's curved blades which changes the direction of the flow. The resulting change in momentum ([[Impulse (physics)|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. An impulse turbine is one in which the pressure of the fluid flowing over the rotor blades is constant and all the work output is due to the change in kinetic energy of the fluid.

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.

Impulse turbines are often used in very high (>300m/1000&nbsp;ft) head applications.

=== Power ===
The [[Power (physics)|power]] available in a stream is;

<math>P=\eta\cdot\rho\cdot g\cdot h\cdot\dot q</math>

where:

*<math>P=</math>     power (J/s or watts)
*<math>\eta=</math>  turbine efficiency
*<math>\rho=</math>  density of fluid (kg/m<sup>3</sup>)
*<math>g=</math>     acceleration of gravity (9.81&nbsp;m/s<sup>2</sup>)
*<math>h=</math>     head (m). For still water, this is the difference in height between the inlet and outlet surfaces. Moving water has an additional component added to account for the kinetic energy of the flow. The total head equals the ''pressure head'' plus ''velocity head''.
*<math>\dot q</math>= flow rate (m<sup>3</sup>/s)

=== Pumped-storage hydroelectricity ===
Some water turbines are designed for pumped-storage hydroelectricity. They can reverse flow and operate as a [[pump]]<ref name="Roman helix-turbine mill" /> to fill a high reservoir during off-peak electrical hours, and then revert to a water turbine for power generation during peak electrical demand. This type of turbine is usually a [[Deriaz turbine|Deriaz]] or [[Francis turbine]] in design.

This type of system is used in El Hierro, one of the Canary Islands: "When wind production exceeds demand, excess energy will pump water from a lower reservoir at the bottom of a volcanic cone to an upper reservoir at the top of the volcano 700 meters above sea level. The lower reservoir stores 150,000 cubic meters of water. The stored water acts as a battery. The maximum storage capacity is 270 MWh. When demand rises and there is not enough wind power, the water will be released to four hydroelectric turbines with a total capacity of 11 MW."<ref>{{cite web|url=https://www.greenbiz.com/blog/2014/03/03/how-small-spanish-island-became-renewable-energy-pioneer-el-hierro|title=How a small Spanish island became a renewable energy pioneer|first=Laurie|last=Guevara-Stone|date=3 March 2014|website=greenbiz.com|access-date=4 May 2017|archive-date=3 October 2017|archive-url=https://web.archive.org/web/20171003032051/https://www.greenbiz.com/blog/2014/03/03/how-small-spanish-island-became-renewable-energy-pioneer-el-hierro|url-status=dead}}</ref><ref>{{cite web|url=http://euanmearns.com/an-independent-evaluation-of-the-el-hierro-wind-pumped-hydro-system/|title=An Independent Evaluation of the El Hierro Wind & Pumped Hydro System|first=Benjamin|last=Jargstorf|date=23 February 2017|website=euanmearns.com/}}</ref>

=== Efficiency ===
Large modern water turbines operate at [[mechanical efficiency|mechanical efficiencies]] greater than 90%.