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The combination of electrical generators and hydraulic turbines allows hydropower systems to convert the potential energy of dammed or flowing water into storable electrical output. Although this conversion relies on relatively simple mechanical properties, the system employed to achieve it is often complex in its design and capabilities. Harnessing the motion of water to power machines and mechanical processes is one of the oldest methods of power generation currently in use. Today, there are thousands of hydropower plants in the United States, providing a notable percentage of the country’s electricity supply.
Most hydroelectric power is derived from water moving downhill and flowing through a dam where it causes a turbine to rotate, which in turn drives a electric generator. A large volume of moving water can generate an enormous amount of force, and the ability to regulate the rate of flow allows hydropower systems to channel the potential and kinetic energy involved. Hydroelectric power is advantageous for yielding a reduced amount of waste in its operations, and while there is debate regarding its sustainability and level of environmental impact, hydropower remains an important segment of the global energy industry.
A generator is the heart of a hydropower plant, and it is necessary to understand how it functions in order to grasp the other principles of hydroelectric energy. In a generator, electromagnetic charge is created by applying direct current to copper wiring attached to an assembly of magnetic steel. These steel field poles are positioned on the edge of a rotor, which is linked to a rotating turbine. As the rotor moves the field poles around the conductors embedded within an external wheel, electricity flows and generates voltage at the generator’s output centers. The generator is usually housed within a protective structure, and its stored energy can be fed into power lines. Larger hydroelectric plants often have multiple generators. The Hoover Dam, for example, has seventeen separate generators that can produce up to 133 megawatts of power.
Hydropower Plant Functions
The majority of hydroelectric plants depend on a dam that forms a barrier to collect a large amount of water in a reservoir. While most power plants rely on a single reservoir whose water flows through the system before being channeled downstream, a pumped-storage plant may have two reservoirs. The upper reservoir works like the reservoir in a conventional hydropower plant, but the lower reservoir collects the water that would normally flow downstream and pumps it back up to refill the first reservoir, restarting the flow cycle. This process allows pump-storage plants to generate more energy during higher consumption periods. The stages in a typical generating process include:
• Intake: When the dam opens its entrances the water flows into a pipeline, also known as a penstock, that channels it toward the turbines and builds up pressure as the water moves.
• Turbine Rotation: A turbine has vertical propeller blades set along a shaft linked to the plant’s generator. When the water reaches the blades, it causes the turbine to turn along its axis.
• Current Production: The rotating turbine creates a corresponding rotation of magnets around the conductors located within the generator, providing an alternating current.
• Conversion: Inside the generator building, a transformer changes the alternating current into electrical voltage that can be stored and used.
• Distribution: Most hydroelectric plants have attached power lines that correspond to the differing levels of voltage and allow energy to be carried out of the plant.
• Outflow: After the water’s motion has been harnessed, pipes carry it out of the plant where it continues to flow downstream or is re-circulated into the lower reservoir.
A hydroelectric plant’s capacity for producing energy partly depends on the volume of water available, the rate at which it flows, and the height from which it travels into the plant. Building from a high dam allows the water to accumulate more potential energy to be transformed into mechanical energy when it reaches the turbine. The distance between the water’s surface and the turbine’s blades is known as the hydraulic head, and it is used as one of the measurements for determining a plant’s generating efficiency.
Hydropower stations do not burn fuel, resulting in lower operations costs and fewer emissions. Waste-disposal problems are minimal, and the cycles of water flow and rainfall provide an inexpensive power source that is reliable over long periods of time. However, building a hydroelectric plant can be an expensive initial investment, and in some cases, hydropower systems can alter the conditions for fish and other wildlife. Likewise, short-term fluctuations in energy consumption can be difficult to address if precipitation patterns do not allow for it. For more information on the various advantages and disadvantages of hydroelectric power, see the U.S. Geological Survey’s assessment.
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