Hydroelectricity is electricity generated by hydropower, that is, power derived from the kinetic energy of falling or flowing water. It can be generated from streams, lakes and rivers or man-made structures such as dams, lagoons and reservoirs.
Hydroelectricity relies on water, a clean and renewable source of energy. Water that has been used to generate electricity is naturally recycled by the ‘water cycle’ through which water evaporates, forming clouds that then rain on the Earth, restarting the cycle. The technology is well-proven and reliable, there is little use of fuel in generation, and so emissions are low. However, the capital costs are high, it can require inundation of land and so cause loss of habitats, and in some cases, displacement of local populations.
Hydropower systems are generally dependent upon precipitation and elevation changes. Large elevation changes and high precipitation levels are necessary to generate substantial quantities of electricity. Generation depends on the gravitational force of flowing water moving downstream. Hydropower can be seen as the conversion of the potential kinetic energy of water at a higher level, falling to yield kinetic energy that drives a turbine, which in turn produces electricity.
This can require the creation of a large, high reservoir of water, sometimes by damming a river. In this case, water is then channelled through tunnels in the dams and energy from the water causes turbines to turn which generates electricity. Engineers can regulate the amount of water passing through the dam. The process of controlling this flow of water is called the intake system. Spillway structures allow water to flow directly into the body of water below the dam, preventing damage.
The amount of energy extracted from the water depends on the available water volume and the difference in height between the turbines and the elevated source. This height difference is known as the hydraulic head.
Hydroelectric systems include:
- Storage schemes. These make use of a dam that holds water in a reservoir.
- Run-of-river schemes. These use water's natural flow to drive a turbine.
- Pumped storage.This incorporates two reservoirs. Using electricity, water can be pumped from a lower reservoir to a higher one at times of low demand. This water is then released, driving the turbines to create power in order to meet peak demand requirements.
- Tidal schemes, which exploit the changing level of sea water as a result of tides.
Both storage and run-of-river schemes can be diversions, channelling water from lakes or rivers or dammed reservoirs to powerhouses containing turbines.
Three main categories define the output of hydroelectricity:
- Micro-scale capacity. A hydro plant producing less than 50 kilowatts (kW).
- Small-scale capacity. A hydro plant producing less than 5 megawatts (MW).
- Large-scale capacity. A hydro plant producing more than 5 megawatts (MW).
Hydro-electric systems below 100kW are sometimes considered to constitute microgeneration.
Once a hydroelectric system has been installed, it can last for 40-50 years, or longer if well maintained. Debris, floods and droughts can potentially cause damage to hydroelectric systems.
China is the largest producer of hydroelectricity, followed by Canada, Brazil, and the United States (Ref Energy Information Administration). According to the U.S. Geological Survey (USGS), approximately two-thirds of the economically feasible potential remains to be developed. The UK Government suggests that ‘The UK currently (2011) generates about 1.5% of its electricity from hydroelectric schemes. Although further large-scale development potential is limited, there is scope for exploiting our remaining small-scale hydro resources in a sustainable way.’ Ref Gov.uk Harnessing hydroelectric power.
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