Hydroelectric power (HEP), or hydroelectricity, is electrical power that is generated through the energy of flowing water. It has the advantage of using only renewable resources, and not producing harmful waste or byproducts, and is therefore viewed by many as an environmentally friendly alternative to the burning of fossil fuels that still provides the bulk of the electricity consumed throughout the world. HEP schemes, however, often meet with considerable opposition on the grounds that they cause major environmental problems of their own.
People have been harnessing the energy of moving water for centuries. In Ancient Greece and China, waterwheels were installed in rapidly flowing rivers to turn millstones and other equipment, and they continue to be used today in some parts of the world. In effect, these devices are using gravity, as water flows from a higher to a lower area.
Hydroelectric power plants rely on the same basic principle, but instead of using the kinetic energy directly, the force of the water drives a turbine which in turn powers a generator, producing electricity which can be used on-site or transported to other regions. As with other turbine-powered methods of generating electricity, magnets in the turbine move relative to static conductors. This results in an electrical current flowing through the conductors.
There are a number of different designs for hydroelectric power plants, but the most common one involves constructing a large dam across a lake at a relatively high elevation to form a reservoir of water, and allowing water to flow from it, in a controlled way, downhill to the generator, which is located at a lower elevation. The flow of water can be maximized at periods of high demand for electricity, and reduced when demand is lower. In most cases, the water is replenished by rainfall, but some power plants may use excess electricity to pump water back up to the reservoir at times of low demand.
The main advantages of hydroelectric power are that it is clean and renewable. Since it does not involve the burning of fossil fuels, such as coal, oil, and natural gas, it does not produce any emissions of harmful pollutants such as soot and sulfur dioxide. Equally importantly, it does not produce any carbon dioxide, the gas that causes the “greenhouse” effect and which is thought to be responsible for climate change, through power generation. It has the advantage over nuclear power that it does not generate dangerous waste products that present long-term disposal problems.
HEP is also renewable, in that it does not use a resource that is limited. While coal, oil, natural gas, and even, eventually, uranium, will run out, water is inexhaustible. Once the power station has been built, the water it requires is available at no cost, in contrast to the fuels that have to be extracted from the ground. The supply of water is continually renewed in most areas by rainfall, which in effect means that the Sun is providing the energy, as its heat evaporates water from the surface that falls as rain.
Although HEP is clean and renewable, it is not necessarily environmentally friendly. Large-scale projects capable of providing significant amounts of electricity usually require the damming of lakes or rivers, with consequent flooding of large areas of land. This can result in people being displaced, and destruction of habitats. Drastic alterations to the flows of rivers can also have a profound impact on wildlife, ecosystems and, in some cases, agriculture. Migrating fish, such as salmon, may have their routes blocked by dams, although this can be mitigated by installing a fish ladder — a series of steps that fish are able to negotiate.
The locations that are most suitable for hydroelectric power schemes are often of great scenic beauty, and may lose some of their value due to new roads, pylons, and other constructions associated with an HEP project. Some projects have had unintended consequences. For example, the Aswan dam in Egypt, which was very successful in providing power, also controlled the annual flooding of the river Nile downstream. This was at first thought to be another advantage, but the floods had been laden with nutrient-rich silt which made it possible to farm along the Nile, and once the dam was installed, all of that silt collected uselessly in the reservoir, depriving the land downstream of necessary nutrients. Another possible problem, according to some studies, is that decaying vegetation in areas flooded to provide HEP may give off significant amounts of carbon dioxide, offsetting the advantages of clean power generation.