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.
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.
Tidal power can be classified into four generating methods:
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 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 (or DTP) 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 (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.
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.
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.
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.
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%.
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.
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.
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. 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.
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.
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.
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.
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.
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 (1 kilowatt hour = 1 KWH = 1000 watts used for 1 hour).
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.
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.
The European Marine Energy Centre (EMEC) Ltd is a UKAS accredited test and research centre focusing on wave and tidal power development based in the Orkney Islands, UK. The Centre provides developers with the opportunity to test full-scale grid-connected prototype devices in unrivalled wave and tidal conditions. The operations are spread over five sites:
Billia Croo wave energy test site, Mainland (wave power)
Fall of Warness tidal energy test site, off the island of Eday (tidal power)
Scale wave test site at Scapa Flow, off St Mary’s Bay
Scale tidal test site at Shapinsay Sound, off Head of Holland
Stromness (office and data facilities)EMEC was established by a grouping of public sector organisations following a recommendation by the House of Commons Science and Technology Committee in 2001. In addition to providing access to areas of sea with high wave and tidal energy potential, the centre also offers various kinds of support regarding regulatory issues, grid connection and meteorological monitoring as well as local research and engineering support.Kalpasar Project
The Kalpasar Project envisages building a 30 km dam across the Gulf of Khambat in India for establishing a huge fresh water coastal reservoir for irrigation, drinking and industrial purposes. A 10 lane road link will also be set up over the dam, greatly reducing the distance between Saurashtra and South Gujarat.Spending began in earnest in 2004, but no work has started amid questions about feasibility.List of power stations in South Korea
The following page lists power stations in South Korea.List of tidal power stations
This page lists most power stations that run on tidal power. Since tidal stream generators are an immature technology, no technology has yet emerged as the clear standard. A large variety of designs are being experimented with, with some very close to large scale deployment. Hence, the following page lists stations of different technologies.Lists of hydroelectric power stations
The following are lists of hydroelectric power stations based on the four methods of hydroelectric generation:
List of conventional hydroelectric power stations, hydroelectric generation through conventional dams
List of pumped-storage hydroelectric power stations, hydroelectric generation through pumped-storage
List of run-of-the-river hydroelectric power stations, hydroelectric generation through run-of-the-river hydropower
List of tidal power stations, hydroelectric generation through tidal powerMarine energy
Marine energy or marine power (also sometimes referred to as ocean energy, ocean power, or marine and hydrokinetic energy) refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in the world’s oceans creates a vast store of kinetic energy, or energy in motion. Some of this energy can be harnessed to generate electricity to power homes, transport and industries.
The term marine energy encompasses both wave power i.e. power from surface waves, and tidal power i.e. obtained from the kinetic energy of large bodies of moving water. Offshore wind power is not a form of marine energy, as wind power is derived from the wind, even if the wind turbines are placed over water.
The oceans have a tremendous amount of energy and are close to many if not most concentrated populations. Ocean energy has the potential of providing a substantial amount of new renewable energy around the world.Rance Tidal Power Station
The Rance Tidal Power Station is a tidal power station located on the estuary of the Rance River in Brittany, France.
Opened in 1966 as the world's first tidal power station, it is currently operated by Électricité de France and was for 45 years the largest tidal power station in the world by installed capacity until the South Korean Sihwa Lake Tidal Power Station surpassed it in 2011.Its 24 turbines reach peak output at 240 megawatts (MW) and average 57 MW, a capacity factor of approximately 24%. At an annual output of approximately 500 GWh (491 GWh in 2009, 523 GWh in 2010), it supplies 0.12% of the power demand of France. The power density is of the order of 2.6 W/m2. The cost of electricity production is estimated at €0.12/kWh.
The barrage is 750 m (2,461 ft) long, from Brebis point in the west to Briantais point in the east. The power plant portion of the dam is 332.5 m (1,091 ft) long and the tidal basin measures 22.5 km2 (9 sq mi).Renewable energy in Bangladesh
Renewable energy in Bangladesh refers to the use of renewable energy to generate electricity in Bangladesh. The current renewable energy comes from biogas that is originated from biomass, hydro power, solar and wind.Renewable energy in France
Under its commitment to the EU renewable energy directive of 2009, France has a target of producing 23% of its total energy needs from renewable energy by 2020. This figure breaks down to renewable energy providing 33% of energy used in the heating and cooling sector, 27% of the electricity sector and 10.5% in the transport sector. By the end of 2014, 14.3% of France's total energy requirements came from renewable energy, a rise from 9.6% in 2005.The outlook for renewable electricity in France received a boost following the publication in October 2016 of the "Programmation pluriannuelle de l'énergie", showing a commitment to re-balancing the electricity mix towards renewables. According to the report, renewable electricity capacity is planned to grow from 41 GW in 2014 to between 71 and 78 GW by 2023. Historically the electricity sector in France has been dominated by the country's longstanding commitment to nuclear power. However, the report emphasizes that by 2025 more than half of France's nuclear power capacity will come from stations that will be 40 years or older, and subject to closure or refurbishment to extend their operation. Thus, there is a need to look to other sources, including renewables, to meet the expected generating-capacity shortfall.A key component of France's renewable target is the commitment to greatly increase energy efficiency, particularly for buildings and thermal insulation. Heat wastage is targeted to be reduced by 38% by 2020. The renewable targets are also intended to stimulate new trades and changes to existing trades to enable green growth.
The PPE plan targets the reduction of the consumption of primary fossil energy by 22% in 2023 from 2012 levels (reference scenario) or a fallback scenario of an 11% reduction under less-favorable conditions (variant scenario).
In terms of the reduction in primary consumption, petroleum products are targeted to fall by 23% between 2012 and 2023 (reference scenario) or 9.5% (variant scenario), gas by 16% (9% variant scenario) and coal by 37% (30% variant scenario).In the transport sector, France has a range of initiatives designed to promote renewable energy use and increase efficiency.
These include changing transport behavior, such as a target of 10% of tele-worked days by 2030 to reduce consumption.
By 2023, the country aims to have a fleet of 2.4 million rechargeable electric and hybrid vehicles and for 3% of heavy-duty applications to use natural gas vehicles (NGVs).
Biofuels blended with petrol are set for 1.8% in 2018 and 3.4% in 2023, and for diesel 1% in 2018 and 2.3% in 2023.
By 2030, non-road freight transport is targeted to reach 20% of all goods.
Initiatives to increase walking and cycling are also being undertaken. Car pooling and digital services will be promoted to increase occupancy rates to between 1.8 and 2 people by 2030. The country is also pursuing research and development of autonomous vehicles, particularly in public transport.Renewable energy in Pakistan
Renewable energy in Pakistan is a relatively underdeveloped sector; however, in recent years, there has been some interest by environmentalist groups and from the authorities to explore renewable energy resources for energy production in light of the energy crises and power shortages affecting the country. Most of Pakistan's renewable energy comes from hydroelectricity.Renewable energy in Russia
Renewable energy in Russia mainly consists of hydroelectric energy. The country is the sixth largest producer of renewable energy in the world, although it is 56th when hydroelectric energy is not taken into account. Some 179 TWh of Russia's energy production comes from renewable energy sources, out of a total economically feasible potential of 1823 TWh. 16% of Russia's electricity is generated from hydropower, and less than 1% is generated from all other renewable energy sources combined. Roughly 68% of Russia's electricity is generated from thermal power and 16% from nuclear power.While most of the large hydropower plants in Russia date from the Soviet era, the abundance of fossil fuels in the Soviet Union and the Russian Federation has resulted in little need for the development of other renewable energy sources. There are currently plans to develop all types of renewable energy, which is strongly encouraged by the Russian government. Russian Prime Minister Dmitry Medvedev has called for renewable energy to have a larger share of Russia's energy output, and has taken steps to promote the development of renewable energy in Russia since 2008.Renewable energy in Scotland
The production of renewable energy in Scotland is an issue that has come to the fore in technical, economic, and political terms during the opening years of the 21st century. The natural resource base for renewable energy is extraordinary by European, and even global standards, with the most important potential sources being wind, wave, and tide.
At the start of 2019, Scotland had 10.9 gigawatts (GW) of installed renewable electricity capacity. Renewable electricity generation in Scotland was 26,708 GWh in 2018, making up 74% of gross electricity consumption. Scottish renewable generation makes up approximately 25% of total UK renewable generation. In 2015, Scotland exported over 28.9 per cent of generation.In 2015, Scotland generated 59% of its electricity consumption through renewable sources, exceeding the country's goal of 50% renewable energy by 2015. Moving forward, the Scottish Government's energy plan calls for 100% of electricity consumption to be generated through renewable sources by 2020, and 50% of total energy consumption (including transportation) by 2030.Continuing improvements in engineering and economics are enabling more of the renewable resources to be utilised. Fears regarding peak oil and climate change have driven the subject high up the political agenda and are also encouraging the use of various biofuels. Although the finances of many projects remain either speculative or dependent on market incentives, it is probable that there has been a significant, and in all likelihood long-term change, in the underpinning economics.In addition to planned increases in large-scale generating capacity and microsystems using renewable sources, various related schemes to reduce carbon emissions are being researched. Although there is significant support from the public, private and community-led sectors, concerns about the effect of the technologies on the natural environment have been expressed. There is also an emerging political debate about the relationship between the siting, and the ownership and control of these widely distributed resources.SeaGen
SeaGen was the world's first large scale commercial tidal stream generator. It was four times more powerful than any other tidal stream generator in the world at the time of installation.The first SeaGen generator was installed in Strangford Narrows between Strangford and Portaferry in Northern Ireland. Strangford Lough was also the site of the very first known tide mill in the world, the Nendrum Monastery mill where remains dating from 787 have been excavated.Severn Barrage
The Severn Barrage refers to a range of ideas for building a barrage from the English coast to the Welsh coast over the Severn tidal estuary. Ideas for damming or barraging the Severn estuary (and Bristol Channel) have existed since the 19th century. The building of such a barrage would constitute an engineering project comparable with some of the world's biggest. The purposes of such a project has typically been one, or several of: transport links, flood protection, harbour creation, or tidal power generation. In recent decades it is the latter that has grown to be the primary focus for barrage ideas, and the others are now seen as useful side-effects. Following the Severn Tidal Power Feasibility Study (2008–10), the British government concluded that there was no strategic case for building a barrage but to continue to investigate emerging technologies.
In June 2013 the Energy and Climate Change Select Committee published its findings after an eight-month study of the arguments for and against the Barrage. MPs said the case for the barrage was unproven. They were not convinced the economic case was strong enough and said the developer, Hafren Power, had failed to answer serious environmental and economic concerns.Severn Estuary
The Severn Estuary (Welsh: Môr Hafren) is the estuary of the River Severn, the longest river in Great Britain. It is the confluence of four major rivers, being the Severn, Wye, Usk and Avon, and other smaller rivers. Its high tidal range, approximately 50 feet (15 m), means that it has been at the centre of discussions in the UK regarding renewable energy.The Skerries, Isle of Anglesey
The Skerries (Welsh: Ynysoedd y Moelrhoniaid) (grid reference SH268948) are a group of sparsely vegetated rocky islets (skerries), with a total area of about 17 hectares (42 acres) lying 3 kilometres (1.9 mi) offshore from Carmel Head at the northwest corner of Anglesey, Wales. The islands are important as a breeding site for seabirds, and they attract divers, who come to visit the numerous shipwrecks. The Skerries Lighthouse sits atop the highest point in the islands.
The islands can be visited by charter boat from Holyhead. The individual islets are accessible from one another at low tide and by small bridges.
The name "Skerry" is the Scottish diminutive of the Old Norse "sker", and means a small rocky reef or island. The Welsh name for these islands, 'Ynysoedd y Moelrhoniaid', means "Islands of the Seals". An alternative name provided by some English-language sources is 'St Daniel's Isle'.Tidal barrage
A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces.Instead of damming water on one side like a conventional dam, a tidal barrage allows water to flow into a bay or river during high tide, and releases the water during low tide. This is done by measuring the tidal flow and controlling the sluice gates at key times of the tidal cycle. Turbines are placed at these sluices to capture the energy as the water flows in and out.Tidal barrages are among the oldest methods of tidal power generation, with tide mills being developed as early as the sixth century. In the 1960s the 1.7 megawatt Kislaya Guba Tidal Power Station in Kislaya Guba, Russia was built.Variable renewable energy
Variable renewable energy (VRE) is a renewable energy source that is non-dispatchable due to its fluctuating nature, like wind power and solar power, as opposed to a controllable renewable energy source such as hydroelectricity, or biomass, or a relatively constant source such as geothermal power or run-of-the-river hydroelectricity.Wave power
Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC).
Wave power is distinct from tidal power, which captures the energy of the current caused by the gravitational pull of the Sun and Moon. Waves and tides are also distinct from ocean currents which are caused by other forces including breaking waves, wind, the Coriolis effect, cabbeling, and differences in temperature and salinity.
Wave-power generation is not a widely employed commercial technology, although there have been attempts to use it since at least 1890.In 2000 the world's first commercial Wave Power Device, the Islay LIMPET was installed on the coast of Islay in Scotland and connected to the National Grid. In 2008, the first experimental multi-generator wave farm was opened in Portugal at the Aguçadoura Wave Park.