How hydroelectric power plants work

Hydroelectric power — using the flow of water to perform work — represents one of the oldest methods humanity has used to tap renewable energy sources. Currently, 15% of the world’s electricity comes from hydroelectric power (Renewable Energy for the 21st Century, “Renewables 2007: Global Status Report”). While this number is impressive, new technologies will help hydroelectric power play a larger role in meeting global energy needs in the future.

How does hydropower work? What new methods of generating electricity using the kinetic energy of flowing water are under development?

How Moving Water is Used to Make Electricity

Hydropower yields electricity in a fundamentally different way than fossil fuels. For example, heat from burning coal boils water to produce steam. The steam pushes a turbine, which rotates a wire coil in a magnetic field, inducing a flow of electrons (electricity).

In contrast, hydropower involves a flow of water directly pushing a turbine, which rotates a coil of wire in a magnetic field to produce electricity. Because no fuel is combusted, hydroelectric power plants are more efficient than conventional power plants; in fact, hydropower plants approach 90% efficiency (U.S. Department of the Interior, Reclamation Department website, viewed on November 1, 2009).

Because a constantly-recharging flow of water provides the energy that hydroelectric power plants exploit to make electricity, and water is not used up in the process, this is a clean and renewable energy source (U.S. Department of Energy web page, viewed on November 1, 2009).

Types of Hydroelectric Power Plants

The three methods for making electricity using a flow of water are:

“Traditional” hydroelectric
Tidal power
Wave energy.
Of these three, “traditional” hydro power plants are the most widely used.

The Design of “Traditional” Hydroelectric Power Plants

These plants use the flow of a river to push turbines and make electricity. A dam may be constructed to house the turbines and, by slowing down the river, produce a reservoir. Water is slowly released from the reservoir, flowing over the turbines to steadily make electricity.

Other “traditional” hydropower plants do not employ a dam. Instead, a small, narrow channel is built to divert water from the river. The diverted water flows over turbines, and then flows back into the river downstream. This design is typically used only for plants that supply small amounts of power (up to 100 kilowatts) locally to a ranch or village (U.S. Department of Energy, “Types of Hydropower Plants” web page, viewed on October 31, 2009).

Advantages of “Traditional” Hydropower

Totally renewable, clean source of energy
Reliable electricity source (as long as the flow of water is uninterrupted)
Reservoir also provides year-round supply of drinking water (if plant uses a dam)
Dams also help with flood control in the rivers they are built in (U.S. Department of Energy web page, viewed on November 1, 2009).

Disadvantages of “Traditional” Hydropower

Dams change river flow from faster, colder, and clearer to slower, warmer, and siltier, which affects fish and other species (U.S. Department of Energy web page).
Reservoirs associated with dams displace people, and destroy vegetation and the habitat of animals when they are constructed.

In fact, Diane Raines Ward claims in her book Water Wars: Drought, Flood, Folly, and the Politics of Thirst (Riverhead Books, 2002) that “…large-scale hydro projects are… wildly capital intensive and subject to decisions by people who profit from their building. They also destroy massive amounts of land and dislocate thousands of people, almost always the poorest of the poor”.

How Tidal Power Plants Work

According to the Ocean Energy Council, just one major power plant in the world exploits the ocean’s tides to make electricity — the La Rance facility on the northern coast of France. Several smaller tidal power plants do exist (for example, in Nova Scotia, Canada, and Murmansk, Russia) but they are not yet common (Ocean Energy Council, “Tidal Energy” web page, viewed on November 1, 2009).

How do these power plants work? Unlike “traditional” hydropower, a dam is built across the mouth of a bay that experiences very high and very low tides. In fact, a height difference of at least 7 meters (~ 21 feet) between high and low tide levels is required. At high tide, water flowing into the bay through the dam pushes the turbines to make electricity, while at low tide, water flowing out of the bay pushes the turbines (Ocean Energy Council web page, viewed on November 1, 2009).

Advantages of Tidal Power

Large growth potential (there are many suitable areas that have not been exploited yet)
Tides are reliable and predictable

Disadvantages of Tidal Power

Power is produced only twice per day (because tides rise and fall on a 12.5 hour cycle)
Damming a bay could negatively affect aquatic organisms, according to the Ocean Energy Council.

Using Waves to Make Electricity

Currently, full-scale wave energy power plants remain theoretical. However, pilot projects have been constructed in several countries, including Scotland and Japan. Although all of these projects convert kinetic energy from the up-and-down motion of waves into electrical energy, they differ in their details (Ocean Energy Council web page, viewed on November 1, 2009) and locations (for example, some are installed on shorelines while others are offshore).

The pilot project currently operating on the Isle of Islay, Scotland, is an oscillating water column system. In this system, the ocean flows into hollow tubes on the shore that house turbines. As a wave rolls in, the air it pushes spins the turbines, generating electricity. As the wave retreats, it sucks air back with it, spinning the turbines to make electricity (Research Institute for Sustainable Energy web page, viewed on November 1, 2009).

Predicted Advantages of Wave Power

Renewable, clean energy source that will never be depleted.
Can be employed on many coastlines or offshore locations.

Disadvantages of Wave Power

Wear and tear on equipment is significant
Technology is still in its infancy.

Hydroelectric Power Will Help Meet Future Energy Needs

Hydropower is currently the world’s most relied-upon alternative energy source, and will become even more important as new technologies for making electricity using ocean tides and waves mature. In particular, wave power has huge growth potential because it can be employed virtually anywhere.