Hydropower

Hydropower is energy obtained from flowing water. Energy in water can be harnessed and used, in the form of motive energy or temperature differences. The most common application is the dam, but it can be used directly as a mechanical force or a thermal source/sink.

Physics

A hydropower resource can be measured according to the amount of available power, or energy per unit time. The power of a given situation is a function of the hydraulic head or rate of fluid flow. When dealing with water in a reservoir, the head is the height of the water level in the reservoir relative to its height after it has left, since hydrostatic pressure at the base is a function of height only.

Related Topics:
Power - Head - Rate of fluid flow - Hydrostatic pressure

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The amount of energy E released by lowering an object of mass m by a height h in a gravitational field is

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:E = mgh where g is the acceleration due to gravity.

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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.

Related Topics:
Hydroelectric dams - Mass flow rate

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:rac{E}{t} = rac{m}{t}gh

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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 phi) and the density of water, we arrive at the usual form of this expression:

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:P = ho phi g cdot h

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For P in watts, ho is measured in kg/m³, phi is measured in m³/s, g (gee) is measured in m/s², and h is measured in metres.

Related Topics:
Watt - Gee - Metre

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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.

Related Topics:
Water wheel - Kinetic energy

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:P = rac{1}{2} hophi v^2 where v is the velocity of the water,

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or with phi = A cdot v where A is the area through which the water passes, also

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:P = rac{1}{2} ho A v^3.

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

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