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Hydropower

Hydropower, hydraulic power, hydrokinetic power or water power is power that is derived from the force or energy of falling water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as watermills, sawmills, textile mills, dock cranes, and domestic lifts. Since the early 20th century, the term is used almost exclusively in conjunction with the modern development of hydro-electric power, the energy of which could be transmitted considerable distance between where it was created to where it was consumed.

Another previous method used to transmit energy had employed a trompe, which produces compressed air from falling water, that could then be piped to power other machinery at a distance from the energy source.

Water's power is manifested in hydrology, by the forces of water on the riverbed and banks of a river. When a river is in flood, it is at its most powerful, and moves the greatest amount of sediment. This higher force results in the removal of sediment and other material from the riverbed and banks of the river, locally causing erosion, transport and, with lower flow, sedimentation downstream.

Calculating the amount of available power Edit

A hydropower resource can be measured according to the amount of available power, or energy per unit time. In large reservoirs, the available power is generally only a function of the hydraulic head and rate of fluid flow. In a reservoir, the head is the height of water in the reservoir relative to its height after discharge. Each unit of water can do an amount of work equal to its weight times the head.

The amount of energy, E, released when an object of mass m drops a height h in a gravitational field of strength g^[1] is given by

$\, E = mgh$

The energy available to hydroelectric dams is the energy that can be liberated by lowering water in a controlled way. In these situations, the power is related to the mass flow rate.

$\frac{E}{t} = \frac{m}{t}gh$

Substituting P for ^E⁄_t and expressing ^m⁄_t in terms of the volume of liquid moved per unit time (the rate of fluid flow, φ) and the density of water, we arrive at the usual form of this expression:

$P = \rho\, \phi\, g \, h$

A simple formula for approximating electric power production at a hydroelectric plant is:

$\,P = hrgk$

where P is Power in kilowatts, h is height in meters, r is flow rate in cubic meters per second, g is acceleration due to gravity of 9.8 m/s2, and k is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher with larger and more modern turbines.^[2]

Some hydropower systems such as water wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is the kinetic energy of the flowing water.

$P = \frac{1}{2}\,\rho\,\phi\, v^2$

where v is the speed of the water, or with

$\phi = A\, v$

where A is the area through which the water passes, also

$P = \frac{1}{2}\,\rho\, A\, v^3$

Over-shot water wheels can efficiently capture both types of energy.

5/4 kwh of energy generates 1 gallon of water

References Edit

↑ Standard gravity is 9.80665 m/s²
↑ Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, New York, 1978, ISBN 0-07020974-X pp. 9-3 and 9-4

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