Some of the heat from solar radiation converts water to water vapour, or steam, that rises in the atmosphere to form clouds. When this water precipitates as rain or snow in upland areas, the waters stored in rivers above sea level are like a storage of potential energy. This potential energy changes to kinetic energy when the stored water is channelled to turn the blades of a turbine to generate electricity. Electric power generated in this way is called hydro power.

For centuries water wheels have been used to convert the kinetic energy of moving water into mechanical energy, used in flour mills and other machinery. The water was usually taken from a fast-flowing river. Sometimes dams were constructed to store water, or control the flow rate to the mill. The two basic kinds of water wheel are the undershot wheel and the overshot wheel. The overshot wheel produces much more power.

The first hydroelectric power stations were small and were placed at waterfalls near towns because transmitting electrical energy over long distances was not economical. The best places to generate hydroelectricity are in mountainous regions where few people live. Today, electric power can be transmitted inexpensively over long distances to make hydro power inexpensive. Transmission over long distances is carried out by means of transmission lines that can transmit large quantities of electricity cheaply and efficiently.

Unlike coal-fired power stations, hydroelectric power stations can begin generating electricity very quickly. They can respond to sudden increases in demand for electricity. Regular time for high demand for electricity is called ‘peak demand’, for example when people wake up and cook breakfast; when businesses and factories start work; and when people cook their evening meal. Hydro stations need only small staffs to operate and maintain them, and no fuel is needed.

A hydroelectric power plant uses a renewable source of energy that does not pollute the environment. However, the construction of dams to enable hydroelectric generation may cause significant environmental damage. Also, water used to drive the power plant could have other uses at other times, for example, for irrigation or town water supply. 1

Norway, Sweden, Canada, and Switzerland rely heavily on hydroelectricity because they have industrialized areas close to mountainous regions with heavy rainfall. The U.S., Russia, China, India, and Brazil get a much smaller proportion of their electric power from hydroelectric generation.

The capability to produce and deliver electricity for widespread consumption was one of the most important factors in the surge of American economic influence and wealth in the late nineteenth and early twentieth centuries. Hydroelectric power, among the first and simplest of the technologies that generated electricity, was initially developed using low dams of rock, timber, or granite block construction to collect water from rainfall and surface runoff into a reservoir. The water was funneled into a pipe (or pen-stock) and directed to a waterwheel (or turbine) where the force of the falling water on the turbine blades rotated the turbine and its main shaft. This shaft was connected to a generator, and the rotating generator produced electricity. One gallon (about 3.8 liters) of water falling 100 feet (about 30 meters) each second produced slightly more than 1,000 watts (or one kilowatt) of electricity, enough to power ten 100-watt light bulbs or a typical hairdryer.

There are now three types of hydroelectric installations: storage, run-of-river, and pumped-storage facilities. Storage facilities use a dam to capture water in a reservoir. This stored water is released from the reservoir through turbines at the rate required to meet changing electricity needs or other needs such as flood control, fish passage, irrigation, navigation, and recreation. Run-of-river facilities use only the natural flow of the river to operate the turbine. If the conditions are right, this type of project can be constructed without a dam or with a low diversion structure to direct water from the stream channel into a penstock. Pumped-storage facilities, an innovation of the 1950s, have specially designed turbines. These turbines have the ability to generate electricity the conventional way when water is delivered through penstocks to the turbines from a reservoir. They can also be reversed and used as pumps to lift water from the powerhouse back up into the reservoir where the water is stored for later use.

During the daytime when electricity demand suddenly increases, the gates of the pumped-storage facility are opened and stored water is released from the reservoir to generate and quickly deliver electricity to meet the demand. At night when electricity demand is lowest and there is excess electricity available from coal or nuclear electricity generating facilities the turbines are reversed and pump water back into the reservoir. Operating in this manner, a pumped-storage facility improves the operating efficiency of all power plants within an electric system. Hydroelectric developments provide unique benefits not available with other electricity generating technologies. They do not contribute to air pollution, acid rain, or ozone depletion, and do not produce toxic wastes. As a part of normal operations many hydroelectric facilities also provide flood control, water supply for drinking and irrigation, and recreational opportunities such as fishing, swimming, water-skiing, picnicking, camping, rafting, boating, and sightseeing. 2

Attention has recently focused on environmental and social issues, particularly the resettlement of people due to the construction of large reservoirs. By following available guidelines of the International Hydro Association, large schemes can be developed with mitigated and acceptable impacts. One constraint on development is the geographical separation between potential supply and demand, which involves the development of costly infrastructure (long transmission lines, roads, etc.). GHG emissions from reservoirs in tropical areas can be significant, due to emissions of methane from biomass contained in the reservoir. However, overall emissions are significantly below those of fossil fuels. Studies for northern countries show that emissions are 100 times lower than for coal. 3

There are however some social, ecological and hydrological effects that have to be taken into consideration when planning a hydroelectric power station. These effects can be enormous if the system is very large.

• Hydropower is only suitable for sites with large volumes of flowing water. Decreased rainfall, due to climate change, would reduce the electricity available.
• Considerable capital investment is required, especially for large schemes.
• Dams cause large areas upstream to be flooded. This may cause displacement of people and will destroy animal habitat and flora.4

Water power is perhaps the oldest form of harnessing renewable energy. Today kinetic energy from water movement creates modern hydroelectric power. There are valid arguments questioning its efficiency on a large scale, but small scale schemes have also been embraced in countries such as the UK, Georgia, Armenia, China and India. 5


1. Power for a Sustainable Future, Fact Sheet 9: Hydro Power,
3. World Business Council for Sustainable Development (WBCSD), Hydro: Facts and Trends
4. Australian Institute of Energy, Fact sheet 6: Hydro Electricity, p.6,
5. P. Brown, ‘Global Warning: The last chance for change,’ Dakini Books NP (2006) p.308