Tidal energy is produced by the surge of ocean waters during the rise and fall of tides. Tidal energy is a renewable source of energy.

During the 20th century, engineers developed ways to use tidal movement to generate electricity in areas where there is a significant tidal range—the difference in area between high tide and low tide. All methods use special generators to convert tidal energy into electricity.

Tidal energy production is still in its infancy. The amount of power produced so far has been small. There are very few commercial-sized tidal power plants operating in the world. The first was located in La Rance, France. The largest facility is the Sihwa Lake Tidal Power Station in South Korea. The United States has no tidal plants and only a few sites where tidal energy could be produced at a reasonable price. China, France, England, Canada, and Russia have much more potential to use this type of energy.

In the United States, there are legal concerns about underwater land ownership and environmental impact. Investors are not enthusiastic about tidal energy because there is not a strong guarantee that it will make money or benefit consumers. Engineers are working to improve the technology of tidal energy generators to increase the amount of energy they produce, to decrease their impact on the environment, and to find a way to earn a profit for energy companies.

Tidal Energy Generators

There are currently three different ways to get tidal energy: tidal streams, barrages, and tidal lagoons.

For most tidal energy generators, turbines are placed in tidal streams. A tidal stream is a fast-flowing body of water created by tides. A turbine is a machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is much more dense than air, tidal energy is more powerful than wind energy. Unlike wind, tides are predictable and stable. Where tidal generators are used, they produce a steady, reliable stream of electricity.

Placing turbines in tidal streams is complex, because the machines are large and disrupt the tide they are trying to harness. The environmental impact could be severe, depending on the size of the turbine and the site of the tidal stream. Turbines are most effective in shallow water. This produces more energy and allows ships to navigate around the turbines. A tidal generator's turbine blades also turn slowly, which helps marine life avoid getting caught in the system.

The world's first tidal power station was constructed in 2007 at Strangford Lough in Northern Ireland. The turbines are placed in a narrow strait between the Strangford Lough inlet and the Irish Sea. The tide can move at 4 meters (13 feet) per second across the strait.


Barrage
Another type of tidal energy generator uses a large dam called a barrage. With a barrage, water can spill over the top or through turbines in the dam because the dam is low. Barrages can be constructed across tidal rivers, bays, and estuaries.

Turbines inside the barrage harness the power of tides the same way a river dam harnesses the power of a river. The barrage gates are open as the tide rises. At high tide, the barrage gates close, creating a pool, or tidal lagoon. The water is then released through the barrage's turbines, creating energy at a rate that can be controlled by engineers.

The environmental impact of a barrage system can be quite significant. The land in the tidal range is completely disrupted. The change in water level in the tidal lagoon might harm plant and animal life. The salinity inside the tidal lagoon lowers, which changes the organisms that are able to live there. As with dams across rivers, fish are blocked into or out of the tidal lagoon. Turbines move quickly in barrages, and marine animals can be caught in the blades. With their food source limited, birds might find different places to migrate.

A barrage is a much more expensive tidal energy generator than a single turbine. Although there are no fuel costs, barrages involve more construction and more machines. Unlike single turbines, barrages also require constant supervision to adjust power output.

The tidal power plant at the Rance River estuary in Brittany, France, uses a barrage. It was built in 1966 and is still functioning. The plant uses two sources of energy: tidal energy from the English Channel and river current energy from the Rance River. The barrage has led to an increased level of silt in the habitat. Native aquatic plants suffocate in silt, and a flatfish called plaice is now extinct in the area. Other organisms, such as cuttlefish, a relative of squids, now thrive in the Rance estuary. Cuttlefish prefer cloudy, silty ecosystems.

Tidal Lagoon
The final type of tidal energy generator involves the construction of tidal lagoons. A tidal lagoon is a body of ocean water that is partly enclosed by a natural or manmade barrier. Tidal lagoons might also be estuaries and have freshwater emptying into them.

A tidal energy generator using tidal lagoons would function much like a barrage. Unlike barrages, however, tidal lagoons can be constructed along the natural coastline. A tidal lagoon power plant could also generate continuous power. The turbines work as the lagoon is filling and emptying.

The environmental impact of tidal lagoons is minimal. The lagoons can be constructed with natural materials like rock. They would appear as a low breakwater (sea wall) at low tide, and be submerged at high tide. Animals could swim around the structure, and smaller organisms could swim inside it. Large predators like sharks would not be able to penetrate the lagoon, so smaller fish would probably thrive. Birds would likely flock to the area.

But the energy output from generators using tidal lagoons is likely to be low. There are no functioning examples yet. China is constructing a tidal lagoon power plant at the Yalu River, near its border with North Korea. A private company is also planning a small tidal lagoon power plant in Swansea Bay, Wales.

tidal energy
Tidewater swirls into a turbine.

DTP
Dynamic tidal power (DTP) is one of the newest proposals to harness the power of tides. Using DTP, enormous dams (as long as 50 kilometers (31 miles)) would extend straight from the shore into the open ocean.

aquatic
Adjective

having to do with water.

barrage
Noun

a low dam.

Noun

body of water partially surrounded by land, usually with a wide mouth to a larger body of water.

Noun

natural or artificial line separating two pieces of land.

breakwater
Noun

a manmade wall rising from the sea floor that protects a harbor or beach from the force of waves.

coastline
Noun

outer boundary of a shore.

commercial
Adjective

having to do with the buying and selling of goods and services.

complex
Adjective

complicated.

construct
Verb

to build or erect.

consumer
Noun

person who uses a good or service.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

cuttlefish
Noun

marine organism (mollusk) related to squid and octopuses.

dam
Noun

structure built across a river or other waterway to control the flow of water.

decrease
Verb

to lower.

dense
Adjective

having parts or molecules that are packed closely together.

disrupt
Verb

to interrupt.

Noun

community and interactions of living and nonliving things in an area.

effective
Adjective

useful or able to perform a task.

electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.

enclosure
Noun

area surrounded by a wall, fence, or other physical boundary.

energy
Noun

capacity to do work.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

English Channel
Noun

strip of the Atlantic Ocean between southeast England and northwest France.

enthusiastic
Adjective

excited.

environmental impact
Noun

incident or activity's total effect on the surrounding environment.

Noun

mouth of a river where the river's current meets the sea's tide.

expensive
Adjective

very costly.

experiment
Verb

to try or test an idea.

extinct
Adjective

no longer existing.

fluid
Noun

material that is able to flow and change shape.

freshwater
Noun

water that is not salty.

fuel
Noun

material that provides power or energy.

function
Verb

to work or work correctly.

generate
Verb

to create or begin.

generator
Noun

machine that converts one type of energy to another, such as mechanical energy to electricity.

guarantee
Verb

to promise or confirm.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

harness
Verb

to control or guide for a specific purpose.

high tide
Noun

water level that has risen as a result of the moon's gravitational pull on the Earth.

infancy
Noun

very early stages.

inlet
Noun

small indentation in a shoreline.

investor
Noun

a person or organization that gives money in order to gain a future advantage.

legal
Adjective

allowed by law.

low tide
Noun

water level that has dropped as a result of the moon's gravitational pull on the Earth.

marine
Adjective

having to do with the ocean.

migrate
Verb

to move from one place or activity to another.

minimal
Adjective

the lowest or least.

navigate
Verb

to plan and direct the course of a journey.

Noun

large body of salt water that covers most of the Earth.

penetrate
Verb

to push through.

potential
Noun

possibility.

power plant
Noun

industrial facility for the generation of electric energy.

predator
Noun

animal that hunts other animals for food.

predictable
Adjective

regular or able to be forecasted.

profit
Noun

money earned after production costs and taxes are subtracted.

reasonable
Adjective

logical.

renewable energy
Noun

energy obtained from sources that are virtually inexhaustible and replenish naturally over small time scales relative to the human life span.

rock
Noun

natural substance composed of solid mineral matter.

salinity
Noun

saltiness.

severe
Adjective

harsh.

significant
Adjective

important or impressive.

Noun

small sediment particles.

squid
Noun

marine animal (cephalopod) with eight arms and two tentacles.

stable
Adjective

steady and reliable.

Noun

narrow passage of water that connects two larger bodies of water.

submerge
Verb

to put underwater.

surge
noun, verb

sudden, strong movement forward.

technology
Noun

the science of using tools and complex machines to make human life easier or more profitable.

thrive
Verb

to develop and be successful.

tidal constituent
Noun

force that helps create a tide.

Noun

energy produced as ocean waters surge in and out with tides.

tidal energy generator
Noun

machine for turning tidal energy into electricity humans can use.

tidal force
Noun

gravitational pull exerted by one object, such as the sun or moon, that raises tides on another object, such as the Earth.

tidal lagoon
Noun

pool of ocean water that is partially cut off from the ocean by a barrier. Often used as a source of hydroelectric power.

tidal range
Noun

the difference in height between an area's high tide and low tide.

tidal river
Noun

river whose flow is affected by ocean tides.

tidal stream
Noun

an ocean current produced by the tide.

Noun

rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.

turbine
Noun

machine that captures the energy of a moving fluid, such as air or water.

Noun

kinetic energy produced by the movement of air, able to be converted to mechanical power.