Tidal+Energy

Rehana Jamani, Sarah Masotti and Molly Scott

=TIDAL ENERGY =

How Tides Occur
Tidal energy is a renewable form of hydropower. It is produced by changing tide levels. The tides are easy to predict therefore making tidal energy a reliable source of energy, more so than solar and wind energy. The gravitational attraction from the moon and sun cause changes in water level and tidal currents. The magnitude of the tides determines whether there is potential in tidal energy in a certain area or not.

As the earth turns around the common centre of gravity, a phenomenon called inertia causes centrifugal force, which creates a pull on earth’s oceans (called a bulge) on the side away from the moon. On the side of the earth closest to the moon another bulge is created by the gravitational pull of the moon. As the earth moves, these bulges create the tides. High tide occurs at points directly facing towards or away from the moon. Low tide occurs at points perpendicular to the moon. The sun also has an effect on tides but is less influential than the moon. The pull of the sun can either add to or detract from the effects of the moon.

Link: Animation of what causes tides

Generation of Electricity
The most common use of tidal energy is the old mill concept or the barrage method. This involves segregating off a section of the sea, known as a basin. A powerhouse is then built across the river. As the tide goes in and out, water goes into the tunnels in the dam. The velocity of the water is used to turn the turbines. The section of the powerhouse kn own as the sluiceway section serves as a barrier for the water. It open and closes are the tides come in and out. It is closed at high tide, this cause there to be a significant difference between the water level in the estuary (the blocked off portion of the sea) and the sea level. When the tide moves the Sluiceway is opened and the water goes through the turbo generators turning the turbines. The energy is created from the water going through the powerhouse at fast speeds. Another method of getting tidal energy is known as Dynamic Tidal power in which the dam is longer than the barrage. The dams are approximately 30- 50 kilometres, and they extend from the shore into the ocean. As a result of the dam being so long there is water pressure variation along it. Turbines on the ocean floor are used to acquire the energy.

Where Tidal Energy Can Be Harnessed
For a plant to be economically profitable, the tidal range must be seven metres or greater. Currently, there are facilities in Canada, China, and France. The area with the most potential for energy production is the Pacific Northwest especially the coastal areas of Alaska, British Columbia, and Washington. On the Atlantic side, Maine is most ideal.

Brief History of Technology
Using energy from the tides is not a new concept. Britain, France, and Spain were the first countries to have tidal mills, dating back to the 12th century. These mills are no longer in use, however, they are now preserved as historic sites. Early mills such as the mill built in Port-Royal, NS, in 1607 gave off only 20-75 kW of power or less. This amount of power is equal to the miniscule power available in modern compact cars. These early tidal mills used the mechanical force of the tides to turn waterwheels connected to grindstones. When a rising tide comes in to shore, water passes through washing gates in a dam, filling an enclosed body of water and powering hydro alternators. These gates are opened and closed so that the plant operators can trap huge masses of water. When the height of the water on outside of the dam drops considerably, the dam gates can be opened allowing the water to pour back into the turbines of the sea. Electricity can be generated each time water transverses the dam.

Advantages/Disadvantages
Tidal power is an interesting renewable resource. It has many advantages as well as disadvantages. Its advantages are that tidal energy is reliable and is fully predictable and plentiful. Tidal energy needs no external source of fuel and its operation is essentially pollution-free as it produces no greenhouse gases or other wastes. In addition, after construction, a tidal plant can be used for 75-100 years and therefore is long-lasting. Lastly, using tidal energy can reduce the amount of energy needed from other sources that are less advantageous.

However, there are also many disadvantages of tidal energy. For example, although tidal energy is fully predictable, it must be either stored or integrated with other sources of generation before it can be used, which causes complications. As well the tidal barrages built must be adjustable to accommodate the fluctuations of tidal generation.

Although once tidal barrages are built they are cheap to maintain, the original construction of the structures is very expensive as they must be built in a difficult, rigorous, saltwater enviornments.

There are also only a few places acceptable to build tidal turbines. Only about 20 sites in the world meet the factors acceptable for a tidal barrage. The conditions include an area of storage large enough for seawater at high tide. As well, it must be so situated within the estuary that the operation of the plant will not significantly change the original tidal resonant system. Finally, there must be a tidal range large enough to exploit the available tidal energy.

As well, the actual tidal barrages, which are extremely expensive to build, cause some environmental and social concerns. The environmental effect of tidal barrages varies depending on the location of the facility. One barrage may affect the environment for many miles upstream and downstream. Many birds rely on the tide uncovering natural mud flats so they can feed. When a barrage disrupts the natural flow of water, the birds are unable to reach these mud flats. Similarly, some fish require “fish ladders” to migrate because the tidal barrages make the water too shallow for natural migration. As well, tidal barrages are large structures and some people may object them as “eye-sores” disrupting their ocean view. Changing the flow of the tides can also affect navigation and recreational activities negatively.

Finally, after the large investment of building a turbine, the average electric power output is still quite small. Because there is only the twice-daily ebb and flow of tides, the power available is limited. The average output of tidal electricity is less than 40% of the installed generating capacity, meaning the production of power is significantly lower than electricity created from other river dams, averaging 70-100% installed capacity. It is for this reason that some say building a turbine for tidal energy is almost worthless.

Tidal Energy in Canada
 The only tidal energy facility in North America is the Annapolis Tidal Power Plant (completed in 1984) located in the Bay of Fundy in Nova Scotia. The plant acts as a large tourist attraction in the area.

Although there is only one tidal energy plant in Canada, there are 190 possible sites across the country. The majority of these are in British Columbia but Nunavut holds the most potential in terms of the amount of energy that could be harnessed.

There are also many Canadian developing companies involved in tidal energy production. These include New Energy Corp., Clean Current, and Blue Energy Canada.

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