Tidal Energy

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.

Figure No 01 : Various design types for Tidal energy extraction 

Tidal Energy Generating Methods

There are currently three different ways to get tidal energy. They are tidal streams, barrages, and tidal lagoons. Brief introduction about above three methods can be mentioned as following manner

Tidal stream

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 inletand the Irish Sea. The tide can move at 4 meters (13 feet) per second across the strait.

Figure No 02 : Tidal stream concept 

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, silt ecosystems. 

Figure No 03 : Barrage 

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.

Figure No 04 : Tidal lagoon 

Tidal energy potential Potential

Worldwide potential for wave and tidal power is enormous. So this will be a next energy supplying resource for the world.

Figure No 05 : Tidal energy potential in the World 

Environmental Impacts

Unlike fossil-fueled power plants, wave and tidal energy facilities generate electricity without producing any pollutant emissions or greenhouse gases. Since the sea wave and tidal energy facilities are currently being deployed, the full environmental impacts of wave and tidal power remain uncertain but are projected to be small. Concerns include impacts on marine ecosystems and fisheries. Environmental impact studies are currently underway and several pilot and commercial projects are undergoing environmental monitoring. The East River tidal turbine pilot project includes a $1.5 million sonar system to monitor impacts on fish populations. Careful siting should minimize impacts on marine ecosystems, fishing and other coastal economic activities. Wave and tidal facilities also have little or no visual impact, as they are either submerged or do not rise very far above the waterline.

Advantages of Tidal Energy

• It is an inexhaustible source of energy.
• Tidal energy is environment friendly energy and doesn't produce greenhouse gases.
• As 71% of Earth’s surface is covered by water, there is scope to generate this energy on large scale.
• We can predict the rise and fall of tides as they follow cyclic fashion.
• Efficiency of tidal power is far greater as compared to coal, solar or wind energy. Its efficiency is around 80%.
• Although cost of construction of tidal power is high but maintenance costs are relatively low.
• Tidal Energy doesn’t require any kind of fuel to run.
• The life of tidal energy power plant is very long.
• The energy density of tidal energy is relatively higher than other renewable energy sources.
• Tides are totally predictable, enabling us to calculate when we can generate more, and at times when the generation is low, shift the load to some other source of electricity generation.
• Offshore turbines and vertical-axis turbines are not extremely expensive to build and do not have a large environmental impact. 

Disadvantages of Tidal Energy

• Cost of construction of tidal power plant is high.
• There are very few ideal locations for construction of plant and they too are localized to coastal regions only.
• Intensity of sea waves is unpredictable and there can be damage to power generation units.
• Influences aquatic life adversely and can disrupt migration of fish.
• The actual generation is for a short period of time. The tides only happen twice a day so electricity can be produced only for that time.
• Frozen sea, low or weak tides, straight shorelines, low tidal rise or fall are some of the obstructions.
• This technology is still not cost effective and more technological advancements are required to make it commercially viable.
• Usually the places where tidal energy is produced are far away from the places where it is consumed. This transmission is expensive and difficult. 
• Many birds rely on the tide uncovering the mud flats so that they can feed. Fish can’t migrate, unless “fish ladders” are installed
• There are only a few suitable sites for tidal barrages 

Tidal Energy is thus a clean source of energy and does no t require much land or other resources as in harnessing energy from other sources. However, the energy generated is not much as high and low tides occur only twice a day and continuous energy production is not possible.