Tidal Barrage
Why Tidal Energy?
Tidal barrage is a form of tidal energy conversion, which converts energy from tidal flows into electrical energy. Tidal energy has many advantages compared with other forms of renewable energy. The main advantage is that tidal motion is well-understood, especially in coastal areas, and thus tidal predictions have a high degree of accuracy. Secondly, tidal flow is periodic, and occurs in only two directions, the direction of flood (rising tide) and ebb (falling tide), so it is much easier to design equipment that maximizes this source of energy. Contrast tidal movements with the variation and unpredictability associated with the weather, and tidal energy is a more attractive option compared to solar and wind power.
What Causes Tides?
Tides result from the interaction between the earth, moon, and sun. The attraction force exerted by the moon or the sun on a molecule of water can be calculated as f =(K*M*m)/d^2, where f is the attraction force, K is the universal constant of gravitation, M is the mass of the moon or sun, m is the mass of a water molecule, and d is the distance from a water molecule to the moon/sun. Despite the greater mass of the sun, the moon's attractive force is 2.17 times greater due to its proximity to earth. As a result, the moon has the most influence on the earth's tides. If we imagine the earth as an aqua planet, we would see a bulge of water at the point closest to the moon, and a corresponding bulge on the opposite side. These bulges correspond to high tides. Since the earth rotates about it axis faster than the moon rotates about the earth, the result is the location of the bulges changes, leading to the tidal ranges we experience. Additionally, periods of greater tidal ranges, known as spring tides, coincide with a full or new moon, while periods of lesser tidal ranges coincide with quarter moons (halfway between new/full moon). When other tidal effects occur due to the coastline, estuary funneling, and tidal resonance, the result can be large tidal ranges. The most well-known example is the Bay of Fundy.
Horizontal component of the tidal force on earth when the tide-generating body is above the Equator at Z. For an aqua planet, high tide corresponds to point Z, and low tide to the dashed line (Credit: Stewart, 2010).
|
Time-lapse video of the Bay of Fundy, located between Nova Scotia and New Brunswick, Canada. The Bay of Fundy has the largest tidal range in the world, with parts of the bay having a tidal range of over 43 ft (13 m) (Credit: bayoffundy.com, Nova Scotia Webcams (original footage)).
|
How Does Tidal Barrage Work?
Tidal barrage involves constructing a large dam-like structure called a barrage. The barrage has a sluice or gate to allow water to flow past it into a basin during a rising tide. At high tide, the sluice is closed, retaining the water within the basin. After a period of time, the basin is emptied emptied out past a row of turbines, which generate electricity. This process is known as ebb generation because generation occurs as the tide ebbs. Ebb generation is the primary method for generating energy via tidal barrage. The process can be reversed, with the sluice gates closed as the tide is flooding (rising) on the ocean side. Once the difference in height between ocean and estuary is sufficient, the sluice gates and the generators start to. Flood generation has a lower efficiency compared to ebb generation, increased ecological risk, and reduced shipping access. The diagram on the right shows ebb generation (Credit: Encyclopædia Britannica).
|
Your browser does not support viewing this document. Click here to download the document.
Two-way generation combines ebb and flood generation, allowing power generation to occur as the tide both ebbs and floods in each cycle. Two-way generation extracts more energy than ebb or flood generation alone. However, practically, it cannot significantly improve the efficiency of tidal barrage for several reasons.
|
Where Does Tidal Barrage Come From?
Tidal barrage is based on tidal mills, which were prominent on the Atlantic Coast during the Middle Ages, especially in England and France, where they primarily provided power for grinding grain. Tidal mills trapped water in a pond while the tide was rising. During a falling tide, the water was released, driving a waterwheel and thus creating energy. Tidal mills were also used in America during the colonial period and were important to the early Industrial Revolution. In 1813, the world's first integrated spinning and weaving factory, located in Waltham Massachusetts, was powered by a water mill.
Tidal mills fell out of favor in the 19th century as they could not keep up with the energy demands of the Industrial Revolution, and also led to problems with pollution and silt build-up. However, with the advent of turbines, using tidal flows to generate electricity once again became feasible. In 1966, the first tidal barrage power plant was completed in La Rance, France, generating 240 MW. In 2011, the Sihwa Lake Power Station was built in South Korea. With a capacity of 254 MW, it is the largest tidal barrage power power plant in the world. |