de:Wasserkraft nl:waterkracht sv:Vattenkraft Hydropower (or waterpower) harnesses the energy of moving or falling water. This is usually in the form of hydroelectricity from a dam, but it can be used directly as a mechanical force. The term refers to a number of systems in which flowing water drives a water turbine or waterwheel.

Prior to the widespread availability of commercial electricity, hydropower was widely used for milling, textile manufacture, and the operation of sawmills. In the 1830s, at the height of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using the technology of inclined plane railroads.

Hydropower can be far less expensive than fossil fuel or nuclear energy. Areas with abundant hydropower attract industry with low cost electricity. Increased environmental concerns have begun to outweigh cheap electricity in some countries.

Contents

1 Physics 2 Small scale hydro power. 3 References 4 See also 5 External Links

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, and rate of flow, and sometimes stream velocity. 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.

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

<math>E = mgh<math> where <math>g<math> is the acceleration due to gravity.

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.

<math>\frac{E}{t} = \frac{m}{t}gh<math>

Substituting <math>P<math> for <math>E/t<math> and expressing <math>m/t<math> in terms of the volume of liquid moved per unit time and the density of water, we arrive at the usual form of this expression:

<math>P = \rho\dot{V}gh<math>

For <math>P<math> in Watts, <math>\rho<math> is measured in <math>kg/m^3<math>, <math>\dot{V}<math> is measured in <math>m^3/s<math>, <math>g<math> is measured in <math>m/s^2<math>, and <math>h<math> is measured in meters.

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.

<math>P = \frac{1}{2}\rho\dot{V}v^2<math> where <math>v<math> is the velocity of the water.

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

Small scale hydro power.

In poor areas of the world, many remote communities still do not have access to electricity. Micro hydro power, between 5kW and 100kW allows such communities to generate their own electricity¹. This is a form of power which is supported by various organisations such as the UK's Intermediate Technology Development Group.

Micro-hydro power can be used directly as "shaft power" for many industrial applications. Alternatively, the preferred option for domestic energy supply is to convert to electricity either through the use of a custom generator or through a reversed electric motor which, while often less efficient is more likely to be available locally and cheaply.

References

• Micro-hydro power (http://www.itdg.org/docs/technical_information_service/micro_hydro_power.pdf), Adam Harvey, 2004, Intermediate Technology Development Group, retrieved 2005/01/01 from http://www.itdg.org/docs/technical_information_service/micro_hydro_power.pdf.

See also

• Hydroelectricity
• Water turbine

External Links

• Intermediate Technology Development Group (http://www.itdg.org/docs/technical_information_service/micro_hydro_power.pdf) a UK charity developing micro-hydro power and giving extensive technical documentation.
• British Hydropower Association (http://www.british-hydro.org/)
• microhydropower.net (http://microhydropower.net/index.php)