As people discovered centuries ago, the flow of water represents a huge supply of kinetic energy that can be put to work. Water wheels are useful for generating motion energy to grind grain or saw wood, but they are not practical for generating electricity. Water wheels are too bulky and slow.
Hydroelectric plants are different. They use modern turbine generators to produce electricity, just as thermal (coal, natural gas, nuclear) power plants do, except they do not produce heat to spin the turbines.
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How a Hydropower Plant Works
A typical hydropower plant is a system with three parts:
- — a power plant where the electricity is produced;
- — a dam that can be opened or closed to control water flow; and
- — a reservoir (artificial lake) where water can be stored.
To generate electricity, a dam opens its gates to allow water from the reservoir above to flow down through large tubes called penstocks. At the bottom of the penstocks, the fast-moving water spins the blades of turbines. The turbines are connected to generators to produce electricity. The electricity is then transported via huge transmission lines to a local utility company.
- Water in a reservoir behind a hydropower dam flows through an intake screen, which filters out large debris, but allows fish to pass through.
- The water travels through a large pipe, called a penstock.
- The force of the water spins a turbine at a low speed, allowing fish to pass through unharmed.
- Inside the generator, the shaft spins coils of copper wire inside a ring of magnets. This creates an electric field, producing electricity.
- Electricity is sent to a switchyard, where a transformer increases the voltage, allowing it to travel through the electric grid.
- Water flows out of the penstock into the downstream river.
Head and Flow
The amount of electricity that can be generated at a hydro plant is determined by two factors: head and flow. Head is how far the water drops. It is the distance from the highest level of the dammed water to the point where it goes through the power-producing turbine.
Flow is how much water moves through the system—the more water that moves through a system, the higher the flow. Generally, a highhead plant needs less water flow than a low-head plant to produce the same amount of electricity.
One of the biggest advantages of a hydropower plant is its ability to store energy. The water in a reservoir is, after all, stored energy. Water can be stored in a reservoir and released when needed for electricity production.
During the day when people use more electricity, water can flow through a plant to generate electricity. Then, during the night when people use less electricity, water can be held back in the reservoir. Storage also makes it possible to save water from winter rains for generating power during the summer, or to save water from wet years for generating electricity during dry years.