Tidal power, also called tidal energy, is a form of hydropower that converts the energy
of tides into useful forms of power, mainly electricity.
Although not yet widely used, tidal power has potential for future electricity generation.
Tides are more predictable than wind energy and solar power. Among sources of renewable
energy, tidal power has traditionally suffered from relatively high cost and limited availability
of sites with sufficiently high tidal ranges or flow velocities, thus constricting its
total availability. However, many recent technological developments and improvements, both in design
and turbine technology, indicate that the total availability of tidal power may be much
higher than previously assumed, and that economic and environmental costs may be brought down
to competitive levels. Historically, tide mills have been used both
in Europe and on the Atlantic coast of North America. The incoming water was contained
in large storage ponds, and as the tide went out, it turned waterwheels that used the mechanical
power it produced to mill grain. The earliest occurrences date from the Middle Ages, or
even from Roman times. It was only in the 19th century that the process of using falling
water and spinning turbines to create electricity was introduced in the U.S. and Europe.
The world's first large-scale tidal power plant is the Rance Tidal Power Station in
France, which became operational in 1966.
Generation of tidal energy
Tidal power is taken from the Earth's oceanic tides; tidal forces are periodic variations
in gravitational attraction exerted by celestial bodies. These forces create corresponding
motions or currents in the world's oceans. Due to the strong attraction to the oceans,
a bulge in the water level is created, causing a temporary increase in sea level. When the
sea level is raised, water from the middle of the ocean is forced to move toward the
shorelines, creating a tide. This occurrence takes place in an unfailing manner, due to
the consistent pattern of the moon’s orbit around the earth. The magnitude and character
of this motion reflects the changing positions of the Moon and Sun relative to the Earth,
the effects of Earth's rotation, and local geography of the sea floor and coastlines.
Tidal power is the only technology that draws on energy inherent in the orbital characteristics
of the Earth–Moon system, and to a lesser extent in the Earth–Sun system. Other natural
energies exploited by human technology originate directly or indirectly with the Sun, including
fossil fuel, conventional hydroelectric, wind, biofuel, wave and solar energy. Nuclear energy
makes use of Earth's mineral deposits of fissionable elements, while geothermal power taps the
Earth's internal heat, which comes from a combination of residual heat from planetary
accretion and heat produced through radioactive decay.
A tidal generator converts the energy of tidal flows into electricity. Greater tidal variation
and higher tidal current velocities can dramatically increase the potential of a site for tidal
electricity generation. Because the Earth's tides are ultimately due
to gravitational interaction with the Moon and Sun and the Earth's rotation, tidal power
is practically inexhaustible and classified as a renewable energy resource. Movement of
tides causes a loss of mechanical energy in the Earth–Moon system: this is a result
of pumping of water through natural restrictions around coastlines and consequent viscous dissipation
at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth
to slow in the 4.5 billion years since its formation. During the last 620 million years
the period of rotation of the earth has increased from 21.9 hours to 24 hours; in this period
the Earth has lost 17% of its rotational energy. While tidal power will take additional energy
from the system, the effect is negligible and would only be noticed over millions of
years. Generating methods
Tidal power can be classified into three generating methods:
Tidal stream generator
Tidal stream generators make use of the kinetic energy of moving water to power turbines,
in a similar way to wind turbines that use wind to power turbines. Some tidal generators
can be built into the structures of existing bridges, involving virtually no aesthetic
problems. Land constrictions such as straits or inlets can create high velocities at specific
sites, which can be captured with the use of turbines. These turbines can be horizontal,
vertical, open, or ducted and are typically placed near the bottom of the water column.
Tidal barrage
Tidal barrages make use of the potential energy in the difference in height between high and
low tides. When using tidal barrages to generate power, the potential energy from a tide is
seized through strategic placement of specialized dams. When the sea level rises and the tide
begins to come in, the temporary increase in tidal power is channeled into a large basin
behind the dam, holding a large amount of potential energy. With the receding tide,
this energy is then converted into mechanical energy as the water is released through large
turbines that create electrical power through the use of generators. Barrages are essentially
dams across the full width of a tidal estuary. Dynamic tidal power
Dynamic tidal power is an untried but promising technology that would exploit an interaction
between potential and kinetic energies in tidal flows. It proposes that very long dams
be built from coasts straight out into the sea or ocean, without enclosing an area. Tidal
phase differences are introduced across the dam, leading to a significant water-level
differential in shallow coastal seas – featuring strong coast-parallel oscillating tidal currents
such as found in the UK, China, and Korea. Tidal lagoon
A newer tidal energy design option is to construct circular retaining walls embedded with turbines
that can capture the potential energy of tides. The created reservoirs are similar to those
of tidal barrages, except that the location is artificial and does not contain a preexisting
ecosystem. US and Canadian studies in the twentieth century
The first study of large scale tidal power plants was by the US Federal Power Commission
in 1924 which if built would have been located in the northern border area of the US state
of Maine and the south eastern border area of the Canadian province of New Brunswick,
with various dams, powerhouses, and ship locks enclosing the Bay of Fundy and Passamaquoddy
Bay. Nothing came of the study and it is unknown whether Canada had been approached about the
study by the US Federal Power Commission. There was also a report on the international
commission in April 1961 entitled " Investigation of the International Passamaquoddy Tidal Power
Project" produced by both the US and Canadian Federal Governments. According to benefit
to costs ratios, the project was beneficial to the US but not to Canada. A highway system
along the top of the dams was envisioned as well.
A study was commissioned by the Canadian, Nova Scotian and New Brunswick Governments
to determine the potential for tidal barrages at Chignecto Bay and Minas Basin – at the
end of the Fundy Bay estuary. There were three sites determined to be financially feasible:
Shepody Bay, Cumberline Basin, and Cobequid Bay. These were never built despite their
apparent feasibility in 1977. Current and future tidal power schemes
The first tidal power station was the Rance tidal power plant built over a period of 6 years
from 1960 to 1966 at La Rance, France. It has 240 MW installed capacity.
254 MW Sihwa Lake Tidal Power Plant in South Korea is the largest tidal power installation
in the world. Construction was completed in 2011.
The first tidal power site in North America is the Annapolis Royal Generating Station,
Annapolis Royal, Nova Scotia, which opened in 1984 on an inlet of the Bay of Fundy. It
has 20 MW installed capacity. The Jiangxia Tidal Power Station, south of
Hangzhou in China has been operational since 1985, with current installed capacity of 3.2
MW. More tidal power is planned near the mouth of the Yalu River.
The first in-stream tidal current generator in North America was installed at Race Rocks
on southern Vancouver Island in September 2006. The next phase in the development of
this tidal current generator will be in Nova Scotia.
A small project was built by the Soviet Union at Kislaya Guba on the Barents Sea. It has
0.4 MW installed capacity. In 2006 it was upgraded with a 1.2MW experimental advanced
orthogonal turbine. Jindo Uldolmok Tidal Power Plant in South
Korea is a tidal stream generation scheme planned to be expanded progressively to 90
MW of capacity by 2013. The first 1 MW was installed in May 2009.
A 1.2 MW SeaGen system became operational in late 2008 on Strangford Lough in Northern
Ireland. The contract for an 812 MW tidal barrage near
Ganghwa Island north-west of Incheon has been signed by Daewoo. Completion is planned for
2015. A 1,320 MW barrage built around islands west
of Incheon is proposed by the South Korean government, with projected construction starting
in 2017. The Scottish Government has approved plans
for a 10MW array of tidal stream generators near Islay, Scotland, costing 40 million pounds,
and consisting of 10 turbines – enough to power over 5,000 homes. The first turbine
is expected to be in operation by 2013. The Indian state of Gujarat is planning to
host South Asia's first commercial-scale tidal power station. The company Atlantis Resources
planned to install a 50MW tidal farm in the Gulf of Kutch on India's west coast, with
construction starting early in 2012. Ocean Renewable Power Corporation was the
first company to deliver tidal power to the US grid in September, 2012 when its pilot
TidGen system was successfully deployed in Cobscook Bay, near Eastport.
In New York City, 30 tidal turbines will be installed by Verdant Power in the East River
by 2015 with a capacity of 1.05MW. Construction of a 240 MW tidal power plant
in the city of Swansea in the UK, estimated to begin in Spring 2015. Once completed, it
will generate over 400GWh of electricity per year, enough to power roughly 121,000 homes.
Completion is scheduled for 2017, and the project has a projected 120 year lifespan.
Tidal Power Issues Environmental concerns
Tidal power can have effects on marine life. The turbines can accidentally kill swimming
sea life with the rotating blades. Some fish may no longer utilize the area if threatened
with a constant rotating or noise-making object. Marine life is a huge factor when placing
tidal power energy generators in the water and precautions are made to ensure that as
many marine animals as possible will not be affected by it. The Tethys database seeks
to gather, organize and make available information on potential environmental effects of tidal,
wave and offshore wind energy development. Tidal turbines
The main environmental concern with tidal energy is associated with blade strike and
entanglement of marine organisms as high speed water increases the risk of organisms being
pushed near or through these devices. As with all offshore renewable energies, there is
also a concern about how the creation of EMF and acoustic outputs may affect marine organisms.
It should be noted that because these devices are in the water, the acoustic output can
be greater than those created with offshore wind energy. Depending on the frequency and
amplitude of sound generated by the tidal energy devices, this acoustic output can have
varying effects on marine mammals. Tidal energy removal can also cause environmental concerns
such as degrading farfield water quality and disrupting sediment processes. Depending on
the size of the project, these effects can range from small traces of sediment build
up near the tidal device to severely affecting nearshore ecosystems and processes.
Tidal barrage Installing a barrage may change the shoreline
within the bay or estuary, affecting a large ecosystem that depends on tidal flats. Inhibiting
the flow of water in and out of the bay, there may also be less flushing of the bay or estuary,
causing additional turbidity and less saltwater, which may result in the death of fish that
act as a vital food source to birds and mammals. Migrating fish may also be unable to access
breeding streams, and may attempt to pass through the turbines. The same acoustic concerns
apply to tidal barrages. Decreasing shipping accessibility can become a socio-economic
issue, though locks can be added to allow slow passage. However, the barrage may improve
the local economy by increasing land access as a bridge. Calmer waters may also allow
better recreation in the bay or estuary. Tidal lagoon
Environmentally, the main concerns are blade strike on fish attempting to enter the lagoon,
acoustic output from turbines, and changes in sedimentation processes. However, all these
effects are localized and do not affect the entire estuary or bay.
Corrosion Salt water causes corrosion in metal parts.
It can be difficult to maintain tidal stream generators due to their size and depth in
the water. The use of corrosion-resistant materials such as stainless steels, high-nickel
alloys, copper-nickel alloys, nickel-copper alloys and titanium can greatly reduce, or
eliminate, corrosion damage. Mechanical fluids, such as lubricants, can
leak out, which may be harmful to the marine life nearby. Proper maintenance can minimize
the amount of harmful chemicals that may enter the environment.
Fouling The biological events that happen when placing
any structure in an area of high tidal currents and high biological productivity in the ocean
will ensure that the structure becomes an ideal substrate for the growth of marine organisms.
In the references of the Tidal Current Project at Race Rocks in British Columbia this is
documented. Also see this page and Several structural materials and coatings were tested
by the Lester Pearson College divers to assist Clean Current in reducing fouling on the turbine
and other underwater infrastructure. Structural Health Monitoring
The high load factors resulting from the fact that water is 800 times denser than air and
the predictable and reliable nature of tides compared with the wind makes tidal energy
particularly attractive for Electric power generation. Condition monitoring is the key
for exploiting it cost- efficiently. See also
Hydroelectricity Ocean energy
Thermal energy World energy resources and consumption
Structural health monitoring Offshore wind power
Wave power Marine energy
Notes
References
External links Enhanced tidal lagoon with pumped storage
and constant output as proposed by David J.C. MacKay, Cavendish Laboratory, University of
Cambridge, UK. Marine and Hydrokinetic Technology Database
The U.S. Department of Energy's Marine and Hydrokinetic Technology Database provides
up-to-date information on marine and hydrokinetic renewable energy, both in the U.S. and around
the world. Tethys Database A database of information
on potential environmental effects of marine and hydrokinetic and offshore wind energy
development. Severn Estuary Partnership: Tidal Power Resource
Page Location of Potential Tidal Stream Power sites
in the UK University of Strathclyde ESRU—Detailed
analysis of marine energy resource, current energy capture technology appraisal and environmental
impact outline Coastal Research - Foreland Point Tidal Turbine
and warnings on proposed Severn Barrage Sustainable Development Commission - Report
looking at 'Tidal Power in the UK', including proposals for a Severn barrage
World Energy Council - Report on Tidal Energy European Marine Energy Centre - Listing of
Tidal Energy Developers -retrieved 1 July 2011
Resources on Tidal Energy Structural Health Monitroring of composite
tidal energy converters