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Tidal Power
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Tides rise and fall twice a day in coastal areas around the world. The sometimes dramatic rise and fall of the water level is a result of the gravitational pull of the moon and sun. The difference in water levels created by tides is also affected by weather and the topography of the land. The world's more extreme tides (including that of the Bay of Fundy in New Brunswick) have tidal ranges of ten metres or more. The movement of water that results from the rise and fall of tides can be used as a source of energy to generate electricity. There are two approaches to harnessing the power of tides: the tidal barrage system, and the tidal current system.
Tidal barrage system
This system is very similar to the generation of hydroelectricity. A dam, called a barrage, is built across the area where there is a big difference in water level between high and low tides. The water is allowed to flow through tunnels near the bottom of the barrage. This increases the pressure of water flow, because large amounts of water are forced through small tunnels. Turbines near the tunnels are then turned by the power of the flowing water resulting from the flood (rise) or ebb (fall) of the tide. In order to ensure cost-effective electricity generation with the barrage system, the difference between high and low tide must be at least five metres.
Tidal current system
This approach makes use of the water currents created by tides. A series of submerged turbines are turned by water currents, and the turbines are connected to a generator that produces electricity. There are two types: the tidal fence, and the tidal turbine. Both form a line of submerged turbines, and do not dam water as the barrage method does. Because no dam is needed, they can be placed between non-confined areas (for example, between two islands where there is open water on both sides). The fence uses vertical turbines that move like turnstiles, while the tidal turbine is like a submerged wind turbine.
Tidal power makes up only a small part of Canada's total electricity generation. It is restricted to coastal areas with large tides, but Canada, with coastal land in both the east and west, has the potential for more tidal power. For example, the Bay of Fundy, with the greatest tidal rise in the world, is a major potential site. At present, the Annapolis Basin Tidal Barrage generating station in Nova Scotia is North America's only source of tidal power, and provides enough electricity to meet the needs of 4,500 homes each year.
What are the benefits of tidal power, and what are its drawbacks and environmental impact?
Tidal power does not cause air pollution or greenhouse gas emissions
- Tidal electricity generation does not release GHGs or other air pollutants, though construction of the barrages and turbines is fossil fuel intensive and does release GHGs.
It is a free electricity source
- The flow of tidal water is a free source of kinetic energy for electricity generation.
It's a reliable electricity source
- Tides are predictable. They are a natural process and occur twice a day, all year round.
- Tidal turbines do not greatly obstruct the natural flow of the tides.
DRAWBACKS AND ENVIRONMENTAL IMPACT
Barrage and fence systems disrupt local ocean ecosystems and migration routes
Tidal power is an expensive technology- Barrage systems may alter local tides, because dams (rather than nature) now controls the rise and fall of water level.
- They alter the time mudflats are exposed, which affects birds and other wildlife.
- Barrages alter the daily movement of aquatic animals, as well as their migratory patterns.
- They also alter tidal flows, which may have an impact on shipping and water transportation.
- Tidal gates may trap and harm larger marine animals.
- Building barrages and turbines is costly.
- The price of tidal electricity is not competitive with fossil fuel generated electricity, nuclear power, or conventional hydroelectricity.
For more information:
Centre for Energy (2010).Nova Scotia: Tidal Power. Available here.
The Pembina Institute. Tidal Power. Available here.
Pollution Probe (2003). Primer on the Technologies of Renewable Energy. Available here.
U.S. Energy Information Administration. Energy Kids: Tidal Power. Available here.
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