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Hydropower or hydraulic power is the force or energy of moving water. It may be captured for some useful purpose.

Prior to the widespread availability of commercial electric power, hydropower was used for irrigation, and operation of various machines, such as watermills, textile machines, and sawmills. The energy of moving water has been exploited for millennia. In India, water wheels and watermills were built; in Imperial Rome, water powered mills produced flour from grain, and in China and the rest of the Far East, hydraulically operated "pot wheel" pumps that raised water into irrigation canals. In the 1830s, at the peak of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using inclined plane railroads. Direct mechanical power transmission required that industries using hydropower had to locate near the waterfall. For example, during the last half of the 19th century, many grist mills were built at Saint Anthony Falls, utilizing the 50 foot (15 metre) drop in the Mississippi River. The mills contributed to the growth of Minneapolis. Today the largest use of hydropower is for electric power generation, which allows low cost energy to be used at long distances from the water source.

Contents 1 Natural manifestations of hydraulic power 2 Types of water power 2.1 Hydroelectric power 2.2 Tidal power 2.3 Tidal stream power 2.4 Wave power 3 Physics 4 Small scale hydro power 5 See also 6 References 7 External links

In hydrology, hydropower is manifested in the force of the water on the riverbed and banks of a river. It is particularly powerful when the river is in flood. The force of the water results in the removal of sediment and other materials from the riverbed and banks of the river, causing erosion and other alterations.

There are several forms of water power:

• Waterwheels, used for hundreds of years to power mills and machinery
• Hydroelectricity, usually referring to hydroelectric dams or run-of-the-river setups.
• Tidal power, which captures energy from the tides in horizontal direction
• Tidal stream power, which does the same vertically
• Wave power, which uses the energy in waves

Main article: Hydroelectricity

Hydroelectric power now supplies about 715,000 MWe or 19% of world electricity (16% in 2003). Large dams are still being designed. Apart from a few countries with an abundance of it, hydro power is normally applied to peak load demand because it is readily stopped and started. Nevertheless, hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations are either already being exploited or are unavailable for other reasons, such as environmental considerations.^{[citation needed]}

Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO₂.

Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.

The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.

Not all hydroelectric power requires a dam; a run-of-river project only uses part of the stream flow and is a characteristic of small hydropower projects.

Main article: Tidal power

Harnessing the tides in a bay or estuary has been achieved in France (since 1966), Canada and Russia, and could be achieved in other areas with a large tidal range. The trapped water turns turbines as it is released through the tidal barrage in either direction. Another possible fault is that the system would generate electricity most efficiently in bursts every six hours (once every tide). This limits the applications of tidal energy.

A relatively new technology, tidal stream generators draw energy from currents in much the same way that wind generators do. The higher density of water means that a single generator can provide significant power. This technology is at the early stages of development and will require more research before it becomes a significant contributor.

Several prototypes have shown promise. In the UK in 2003, a 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon, and a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay.

The Canadian company Blue Energy has plans for installing very large arrays tidal current devices mounted in what they call a 'tidal fence' in various locations around the world, based on a vertical axis turbine design.

Main article: Wave power

Harnessing power from ocean surface wave motion might yield much more energy than tides. The feasibility of this has been investigated, particularly in Scotland in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity. Numerous technical problems have frustrated progress.

A prototype shore based wave power generator is being constructed at Port Kembla in Australia and is expected to generate up to 500 MWh annually. The Wave Energy Converter has been constructed (as of July 2005) and initial results have exceeded expectations of energy production during times of low wave energy. Wave energy is captured by an air driven generator and converted to electricity. For countries with large coastlines and rough sea conditions, the energy of waves offers the possibility of generating electricity in utility volumes. Excess power during rough seas could be used to produce hydrogen.

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 released by lowering an object of mass by a height in a gravitational field is

where 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.

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

For in watts, is measured in kg/m³, is measured in m³/s, (gee) is measured in m/s², and is measured in metres.

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.

where is the velocity of the water,

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

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

Small scale hydro or micro-hydro power has been increasingly used as an alternative energy source, especially in remote areas where other power sources are not viable. Small scale hydro power systems can be installed in small rivers or streams with little or no discernible environmental effect on things such as fish migration. Most small scale hydro power systems make no use of a dam or major water diversion, but rather use water wheels.

There are some considerations in a micro-hydro system installation. The amount of water flow available on a consistent basis, since lack of rain can affect plant operation. Head, or the amount of drop between the intake and the exit. The more head, the more power that can be generated. There can be legal and regulatory issues, since most countries, cities, and states have regulations about water rights and easements.

Over the last few years, the U.S. Government has increased support for alternative power generation. Many resources such as grants, loans, and tax benefits are available for small scale hydro systems.

In poor areas, many remote communities have no electricity. Micro hydro power, with a capacity of 100 kW or less, allows communities to generate electricity¹. This form of power is supported by various organizations such as the UK's Practical Action.

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

• Renewable energy
• Micro hydro
• Pico hydro
• Ocean thermal energy conversion
• Deep lake water cooling
• Blue energy

• Micro-hydro power, Adam Harvey, 2004, Intermediate Technology Development Group, retrieved 1 January 2005 from http://www.itdg.org/docs/technical_information_service/micro_hydro_power.pdf.
• Microhydropower Systems, U.S. Department of Energy, Energy Efficiency and Renewable Energy, 2005

• International Centre for Hydropower (ICH) hydropower portal with links to numerous organizations related to hydropower worldwide
• Practical Action (ITDG) a UK charity developing micro-hydro power and giving extensive technical documentation.
• National Hydropower Association
• British Hydropower Association
• microhydropower.net
• Congressional Research Service (CRS) Reports regarding Hydropower
• Hydro Quebec
• The Federal Energy Regulatory Commission (FERC) Federal Agency that regulates more than 1500 hydropower dams in the United States.
• Hydropower Reform Coalition A U.S.-based coalition of more than 130 national, state, and local conservation and recreation groups that seek to protect and restore rivers affected by hydropower dams.
• Small Scale Hydro Power