The force of falling water generates hydroelectric power, or hydroelectricity. (Hydro comes from the Greek word for water.) Hydroelectric power is one of the cleanest, most reliable and cost effective sources of renewable energy, along with solar energy and wind power.
Water is the key ingredient necessary to run a hydroelectric power plant. In a hydropower plant, water is held behind a dam, forming an artificial lake, or reservoir. This is stored energy. When the water flows from the reservoir through the dam to spin the blades of a giant turbine it releases kinetic energy. The turbine is connected to a generator that makes electricity as it turns -- the generator is the heart of the plant. Two factors that control the amount of electricity generation are the volume of water flow and the amount of hydraulic head. The head refers to the distance between the water surface and the turbines. As the head and flow increase so does the electricity generated. The head depends on how much water is in the reservoir. After passing through the turbine, the water flows back into the river on the other side of the dam.
A pumped-storage plant uses two reservoirs -- an upper reservoir and a lower reservoir. The water in the upper reservoir flows through the hydroelectric power plant to generate electricity and then flows into the lower reservoir for storage. Water is pumped into the upper reservoir (from the lower one) during off-peak hours, such as nights and weekends. This ensures that enough water will be available for use during the weekdays when people need more electricity. The stored water can be freed to power the turbines and generate electricity as it flows back down to the lower reservoir.
The circulation and conservation of earth’s water is called the hydrologic cycle. As the sun heats liquid water, the water evaporates and forms water vapor. The sun heats the air, causing the air to rise in the atmosphere. The air is colder higher up, so as the water vapor rises, it cools, condensing into droplets. When enough droplets accumulate in one area, the droplets fall back to earth as precipitation. Some of the rain flows into the ground as groundwater. The rest flows through rivers back into the oceans. Hydroelectric plants depend on the hydrologic cycle. The only way electricity can be generated is if there is enough water available to flow through the plant and power the turbines.
There is no direct cost of fuel to run a hydroelectric power plant. Hydropower plants tend to last longer that fuel-fired power plants. Some hydro plants have been in service for 50 - 100 years. Hydro plants do not burn fossil fuels and do not produce carbon dioxide, a green house gas.
Once a dam is completed, hydroelectric power is a very efficient source of renewable energy. The direct and indirect costs can, however, be very significant. The environmental costs to plants, animals, local populations and the landscape should be considered in overall cost benefit analysis. An example is the Hetch Hetchy reservoir in California that was dammed in 1923 creating the O'Shaughnessy dam. Opponents advocate it's removal citing the natural beauty of the original valley and it's striking cliffs and rock formations similar to it's nearby valley cousin, Yosemite.
Hydroelectric power can disrupt surrounding aquatic ecosystems both upstream and downstream of the plant site. People must sometimes be relocated in order to build a reservoir. Changes in the amount of river flow will directly affect the amount of energy produced by a dam. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.
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