Sihwa Lake Tidal Power Station, located in Gyeonggi Province, South Korea, is the world's largest tidal power installation, with a total power output capacity of 254 MW.
Tidal power or tidal energy is the form of hydropower that converts the energy obtained from tides into useful forms of power, mainly electricity.
Although not yet widely used, tidal energy has potential for future electricity generation. Tides are more predictable than the wind and the sun. Among sources of renewable energy,
tidal energy 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
(e.g. dynamic tidal power, tidal lagoons) and turbine technology (e.g. new axial turbines, cross flow turbines), 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. The process of using falling water and spinning turbines to create electricity was introduced in the U.S. and Europe in the 19th century.
The world's first large-scale tidal power plant was the Rance Tidal Power Station in France, which became operational in 1966. It was the largest tidal power station in terms of output until Sihwa Lake Tidal Power Station
opened in South Korea in August 2011. The Sihwa station uses sea wall
defense barriers complete with 10 turbines generating 254 MW.
Principle
Variation of tides over a day
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. As the Earth rotates, this bulge of ocean water meets
the shallow water adjacent to the shoreline and creates 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 utilizes the Earth's internal heat, which comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).
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 (length of a day) 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.
Methods
The world's first commercial-scale and grid-connected tidal stream generator – SeaGen – in Strangford Lough. The strong wake shows the power in the tidal current.
Tidal power can be classified into four 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 or are entirely submersed, thus
avoiding concerns over impact on the natural landscape. 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.
Stream energy
can be used at a much higher rate than wind turbines due to water being
more dense than air. Using similar technology to wind turbines
converting energy in tidal energy is much more efficient. Close to
10 mph (about 8.6 knots in nautical terms) ocean tidal current would
have an energy output equal or greater than a 90 mph wind speed for the
same size of turbine system.
Tidal barrage
Tidal barrages make use of the potential energy in the difference in height (or hydraulic head)
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
Top-down diagram of a DTP dam. Blue and dark red colors indicate low and high tides, respectively.
Dynamic tidal power (or DTP) is an untried but promising[according to whom?]
technology that would exploit an interaction between potential and
kinetic energies in tidal flows. It proposes that very long dams (for
example: 30–50 km length) 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.
Induced tides (TDP) could extend the geographic viability of a new
hydro-atmospheric concept 'LPD' (lunar pulse drum) discovered by a Devon
innovator in which a tidal 'water piston' pushes or pulls a metered jet
of air to a rotary air-actuator & generator. The principle was
demonstrated at London Bridge June 2019. Plans for a 30m, 62.5kwh
'pilot' installation on a (Local Authority) tidal estuary shoreline in
the Bristol Channel are underway.
Tidal lagoon
A
new 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 pre-existing
ecosystem.
The lagoons can also be in double (or triple) format without pumping or with pumping
that will flatten out the power output. The pumping power could be
provided by excess to grid demand renewable energy from for example wind
turbines or solar photovoltaic arrays. Excess renewable energy rather
than being curtailed could be used and stored for a later period of
time. Geographically dispersed tidal lagoons with a time delay between
peak production would also flatten out peak production providing near
base load production though at a higher cost than some other
alternatives such as district heating renewable energy storage. The
cancelled Tidal Lagoon Swansea Bay in Wales, United Kingdom would have been the first tidal power station of this type once built.
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
(note: see map in reference). Nothing came of the study and it is
unknown whether Canada had been approached about the study by the US
Federal Power Commission.
In 1956, utility Nova Scotia Light and Power of Halifax commissioned a pair of studies into the feasibility of commercial tidal power development on the Nova Scotia side of the Bay of Fundy. The two studies, by Stone & Webster of Boston and by Montreal Engineering Company of Montreal
independently concluded that millions of horsepower could be harnessed
from Fundy but that development costs would be commercially prohibitive
at that time.
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 (Reassessment of Fundy Tidal Power) 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 (1550 MW), Cumberline Basin (1085
MW), and Cobequid Bay (3800 MW). These were never built despite their
apparent feasibility in 1977.
US studies in the twenty first century
The Snohomish PUD, a public utility district located primarily in Snohomish county, Washington State, began a tidal energy project in 2007; in April 2009 the PUD selected OpenHydro,
a company based in Ireland, to develop turbines and equipment for
eventual installation. The project as initially designed was to place
generation equipment in areas of high tidal flow and operate that
equipment for four to five years. After the trial period the equipment
would be removed. The project was initially budgeted at a total cost
of $10 million, with half of that funding provided by the PUD out of
utility reserve funds, and half from grants, primarily from the US
federal government. The PUD paid for a portion of this project with
reserves and received a $900,000 grant in 2009 and a $3.5 million grant
in 2010 in addition to using reserves to pay an estimated $4 million of
costs. In 2010 the budget estimate was increased to $20 million, half
to be paid by the utility, half by the federal government. The Utility
was unable to control costs on this project, and by Oct of 2014 the
costs had ballooned to an estimated $38 million and were projected to
continue to increase. The PUD proposed that the federal government
provide an additional $10 million towards this increased cost citing a
"gentlemans agreement".
When the federal government refused to provide the additional funding
the project was cancelled by the PUD after spending nearly $10 million
in reserves and grants. The PUD abandoned all tidal energy exploration
after this project was cancelled and does not own or operate any tidal
energy sources.
Rance tidal power plant in France
In 1966, Électricité de France opened the Rance Tidal Power Station, located on the estuary of the Rance River in Brittany. It was the world's first tidal power station. The plant was for 45 years the largest tidal power station in the world by installed capacity: Its 24 turbines reach peak output at 240 megawatts (MW) and average 57 MW, a capacity factor of approximately 24%.
Tidal power development in the UK
The
world's first marine energy test facility was established in 2003 to
start the development of the wave and tidal energy industry in the UK.
Based in Orkney, Scotland, the European Marine Energy Centre (EMEC)
has supported the deployment of more wave and tidal energy devices than
at any other single site in the world. EMEC provides a variety of test
sites in real sea conditions. Its grid connected tidal test site is
located at the Fall of Warness, off the island of Eday, in a narrow
channel which concentrates the tide as it flows between the Atlantic
Ocean and North Sea. This area has a very strong tidal current, which
can travel up to 4 m/s (8 knots) in spring tides. Tidal energy
developers that have tested at the site include: Alstom (formerly Tidal
Generation Ltd); ANDRITZ HYDRO Hammerfest; Atlantis Resources
Corporation; Nautricity; OpenHydro; Scotrenewables Tidal Power; Voith. The resource could be 4 TJ per year. Elsewhere in the UK, annual energy of 50 TWh can be extracted if 25 GW capacity is installed with pivotable blades.
Current and future tidal power schemes
- 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 (Race Rocks Tidal Power Demonstration Project) was installed at Race Rocks on southern Vancouver Island in September 2006. The Race Rocks project was shut down after operating for five years (2006-2011) because high operating costs produced electricity at a rate that was not economically feasible. The next phase in the development of this tidal current generator will be in Nova Scotia (Bay of Fundy).
- 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 1.2 MW 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 (South Korea) 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 was proposed by the South Korean government in 2009. Project halted since 2012 due to environmental concerns.
- The Scottish Government has approved plans for a 10 MW 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 50 MW 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 1.05 MW.
- Construction of a 320 MW tidal lagoon power plant outside the city of Swansea in the UK was granted planning permission in June 2015 and work is expected to start in 2016. Once completed, it will generate over 500 GWh of electricity per year, enough to power roughly 155,000 homes.
- A turbine project is being installed in Ramsey Sound in 2014.
- The largest tidal energy project entitled MeyGen (398 MW) is currently in construction in the Pentland Firth in northern Scotland
- A combination of 5 tidal stream turbines from Tocardo are placed in the Oosterscheldekering, the Netherlands, and have been operational since 2015 with a capacity of 1.2 MW
Issues and challenges
Environmental concerns
Tidal
power can have effects on marine life. The turbines can accidentally
kill swimming sea life with the rotating blades, although projects such
as the one in Strangford feature a safety mechanism that turns off the
turbine when marine animals approach. Even though, there is this
technology in place to turn off the turbines it is causing a major loss
in energy because of the amount of marine life that passes through the
turbines. 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 provides access to scientific literature and general information on the potential environmental effects of tidal energy.
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 electromagnetic fields
and acoustic outputs may affect marine organisms. 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 (particularly those who echolocate to communicate and navigate in the marine environment, such as dolphins and whales). 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 building 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
(suspended solids) 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. In August 2004, a humpback whale swam through the open sluice gate of the Annapolis Royal Generating Station at slack tide, ending up trapped for several days before eventually finding its way out to the Annapolis Basin.
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.
Cost
Tidal Energy has an expensive initial cost which may be one of the reasons tidal energy is not a popular source of renewable energy.
It is important to realize that the methods for generating electricity
from tidal energy is a relatively new technology. It is projected that
tidal power will be commercially profitable within 2020
with better technology and larger scales. Tidal Energy is however still
very early in the research process and the ability to reduce the price
of tidal energy can be an option. The cost effectiveness depends on each
site tidal generators are being placed. To figure out the cost
effectiveness they use the Gilbert ratio, which is the length of the
barrage in metres to the annual energy production in kilowatt hours.
Due to tidal energy reliability the expensive upfront cost of
these generators will slowly be paid off. Due to the success of a
greatly simplified design, the orthogonal turbine offers considerable
cost savings. As a result, the production period of each generating unit
is reduced, lower metal consumption is needed and technical efficiency
is greater. Scientific research has the capability to have a renewable resource like tidal energy that is affordable as well as profitable.
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.
