An Introduction to Hydropower

According to the National Renewable Energy Laboratory (NREL), hydropower plants produce 24 percent of the world’s electricity, supplying 1 billion people with power totalling 675,000 MW or the equivalenct of 3.6 billion barrells of oil. Hydroelectric plants include 2,000 plants in the U.S. and it represents one of the oldest and the most pragmatic alternative energy sources in the world.

The Basics of Hydropower Plants

Hydropower plants simply harness the energy produced by falling water and convert it to electricity by using thewater flowing through a dam to turn a turbine, which turns a generator. The basic components of a conventional hydropower plant includes:

• Dam: Creates a large reservoir.
• Intake: Once the gates on the dam are opened, gravity pulls the water through a pipeline, building up pressure as it flows, leading to the turbine.
• Turbine: The blades of the turbine turn as the water strikes them, turning at a rate as high as 90 revolutions per minute.
• Generators: The turning turbine blades cause the turning of a series of magnets inside the generator, moving electrons which produce alternating current (AC). Hydropower plants have multiple generators, each typically able to generate in excess of 125 MW of power.
• Transformer: Inside the powerhouse, the transformer takes the AC and converts it to higher voltage current.
• Power Lines: 3-phased lines plus a neutral (or ground) comes out of every power plant.
  

  Figure 1 - Hydroelectric Dam

• Outflow: Used water re-enters the river downstream.

There are also pumped storage hydropower plants where the outflow is pumped into a lower reservoir (rather than reentering the river downstream), and using a reversible turbine, the water can be pumped back to the upper reservoir, particularly useful during peak demand.

It’s All About the Water

Water in the reservoir is termed stored or potential energy but when the gates open it becomes kinetic energy. The amount of electricity actually generated is determined by:

• Volume of water, and
• Amount of hydraulic head. The head is determined by the the amount of water in the reservoir and it specifcally referrs to the verticaldistance between the water surface and the turbines.

The amount of electricity generated is determined by the increase in head and flow of water. Therefore, a full understanding of the hydrologic cycle and its relevance to specific geographic locations is important, as simply stated, lack of rain near a plant will preclude the collection of water upstream, resulting in less water flow and less generation of electricity.