Tidal power

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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. Sihwa Lake Tidal Power Station 01.png
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 obtained by converting energy from tides into useful forms of power, mainly electricity using various methods.

Contents

Although not yet widely used, tidal energy has the 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. [1] The earliest occurrences date from the Middle Ages, or even from Roman times. [2] [3] The process of using falling water and spinning turbines to create electricity was introduced in the U.S. and Europe in the 19th century. [4]

Electricity generation from marine technologies increased an estimated 16% in 2018 and an estimated 13% in 2019. [5] Policies promoting R&D are needed to achieve further cost reductions and large-scale development. 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. [6]

Principle

Variation of tides over a day Tide type.svg
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. [7] 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 seafloor and coastlines.

Tidal power is the only technology that draws on energy inherent in the orbital characteristics of the EarthMoon 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%). [8]

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; [9] in this period the Earth has lost 17% of its rotational energy. While tidal power will take additional energy from the system, the effect[ clarification needed ] is negligible and would only be noticed over millions of years.[ citation needed ]

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. SeaGen installed.jpg
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 the wind to power turbines. Some tidal generators can be built into the structures of existing bridges or are entirely submersed, thus avoiding concerns over the 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. [11]

Stream energy can be used at a much higher rate than wind turbines due to water being denser than air. Using similar technology to wind turbines converting the 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 the turbine system. [12]

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 the 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. [13] 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 colours indicate low and high tides, respectively. DTP T dam top-down view.jpg
Top-down diagram of a DTP dam. Blue and dark red colours indicate low and high tides, respectively.

Dynamic tidal power (or DTP) is a theoretical 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. [11] The lagoons can also be in double (or triple) format without pumping [14] or with pumping [15] 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 baseload 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. [16]

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 southeastern 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. [17]

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. [18]

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), Cumberland Basin (1085 MW), and Cobequid Bay (3800 MW). These were never built despite their apparent feasibility in 1977. [19]

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; [20] in April 2009 the PUD selected OpenHydro, [21] 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 "gentleman's agreement". [22] When the federal government refused to provide the additional funding with 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 [23] 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. [24] The resource could be 4 TJ per year. [25] Elsewhere in the UK, annual energy of 50 TWh can be extracted if 25 GW capacity is installed with pivotable blades. [26] [27] [28]

Current and future tidal power schemes

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. However, this feature causes a major loss in energy because of the amount of marine life that passes through the turbines. [49] 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. [50] In terms of Global Warming Potential (i.e. carbon footprint), the impact of tidal power generation technologies ranges between 15 and 37 gCO2-eq/kWhe, with a median value of 23.8 gCO2-eq/kWhe. [51] This is in-line with the impact of other renewables like wind and solar power, and significantly better than fossil-based technologies.

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 far-field water quality and disrupting sediment processes. [52] 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. [53]

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. [53] 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. [54]

Tidal lagoon

Environmentally, the main concerns are blade strike on fish attempting to enter the lagoon, the acoustic output from turbines, and changes in sedimentation processes. However, all these effects are localized and do not affect the entire estuary or bay. [53]

Corrosion

Saltwater 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 number 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 are relatively new technology. It is projected that tidal power will be commercially profitable within 2020[ needs update ] 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. [55]

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. [56] 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, make tidal energy particularly attractive for electric power generation. Condition monitoring is the key for exploiting it cost-efficiently. [57]

See also

Related Research Articles

Electric power systems consist of generation plants of different energy sources, transmission networks, and distribution lines. Each of these components can have environmental impacts at multiple stages of their development and use including in their construction, during the generation of electricity, and in their decommissioning and disposal. We can split these impacts into operational impacts and construction impacts. This page looks exclusively at the operational environmental impact of electricity generation. The page is organized by energy source and includes impacts such as water usage, emissions, local pollution, and wildlife displacement.

Wave power Transport of energy by wind waves, and the capture of that energy to do useful work

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).

Cobscook Bay bay

Cobscook Bay is located in Washington County in the state of Maine. It opens into Passamaquoddy Bay, within the Bay of Fundy. Cobscook Bay is immediately south of the island city of Eastport, the main island of which straddles the two bays. In the 1930s, Cobscook Bay was part of the aborted Passamaquoddy Bay Tidal Power Project to generate electricity from its large tidal range.

Severn Estuary estuary and Site of Special Scientific Interest in Wales

The Severn Estuary 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.

Severn Barrage range of ideas for building a barrage from the English coast to the Welsh coast over the Severn tidal estuary

The Severn Barrage is any of 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 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.

Rance Tidal Power Station Dam in Brittany

The Rance Tidal Power Station is a tidal power station located on the estuary of the Rance River in Brittany, France.

Marine currents can carry large amounts of water, largely driven by the tides, which are a consequence of the gravitational effects of the planetary motion of the Earth, the Moon and the Sun. Augmented flow velocities can be found where the underwater topography in straits between islands and the mainland or in shallows around headlands plays a major role in enhancing the flow velocities, resulting in appreciable kinetic energy. The sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with minimal changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines. Other effects such as regional differences in temperature and salinity and the Coriolis effect due to the rotation of the earth are also major influences. The kinetic energy of marine currents can be converted in much the same way that a wind turbine extracts energy from the wind, using various types of open-flow rotors.

Renewable energy in Scotland Wind, wave, tide and other renewable sources

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.

Wave farm installment of one or several wave power devices in one place

A wave farm – or wave power farm or wave energy park – is a collection of machines in the same location and used for the generation of wave power electricity. Wave farms can be either offshore or nearshore, with the former the most promising for the production of large quantities of electricity for the grid. The first wave farm was constructed in Portugal, the Aguçadoura Wave Farm, consisting of three Pelamis machines. The world's largest is planned for Scotland.

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:

Marine Current Turbines Ltd (MCT), a Siemens business, is a United Kingdom-based company which is developing tidal stream generators.

Severn Tidal Power Feasibility Study is the name of a UK Government feasibility study into a tidal power project looking at the possibility of using the huge tidal range in the Severn Estuary and Bristol Channel to generate electricity.

New Zealand has large ocean energy resources but does not yet generate any power from them. TVNZ reported in 2007 that over 20 wave and tidal power projects are currently under development. However, not a lot of public information is available about these projects. The Aotearoa Wave and Tidal Energy Association was established in 2006 to "promote the uptake of marine energy in New Zealand". According to their 10 February 2008 newsletter, they have 59 members. However, the association doesn't list its members.

Low head hydropower refers to the development of hydroelectric power where the head is typically less than 20 metres, although precise definitions vary. Head is the vertical height measured between the hydro intake water level and the water level at the point of discharge. Using only a low head drop in a river or tidal flows to create electricity may provide a renewable energy source that will have a minimal impact on the environment. Since the generated power is a function of the head these systems are typically classed as small-scale hydropower, which have an installed capacity less than 5MW.

Marine energy Energy stored in the waters of oceans

Marine energy or marine power 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.

Tidal farm

A tidal farm is a group of multiple tidal stream generators assembled in the same location used for production of electric power, similar to that of a wind farm. The low-voltage powerlines from the individual units are then connected to a substation, where the voltage is stepped up with the use of a transformer for distribution through a high voltage transmission system.

Tidal stream generator a type of tidal power generation technology

A tidal stream generator, often referred to as a tidal energy converter (TEC), is a machine that extracts energy from moving masses of water, in particular tides, although the term is often used in reference to machines designed to extract energy from run of river or tidal estuarine sites. Certain types of these machines function very much like underwater wind turbines, and are thus often referred to as tidal turbines. They were first conceived in the 1970s during the oil crisis.

Tidal barrage dam-like structure used to capture the energy from masses of water

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.

Ocean Renewable Power Company company in Portland, United States

Ocean Renewable Power Company is a marine renewable energy company based in Portland, Maine. The company develops technologies which generate electricity from tidal, river, and ocean currents. The turbines are a cross-flow design in the helix shape of DNA with the axis of rotation perpendicular to the flow of water and work on the same principle as water wheels. As the tide comes and goes, the turbine foils spin in the same direction producing mechanical power that a permanent magnet generator converts to electricity and then sends to the electrical grid via an underwater power cable and onshore power station. The TidGen® power system and RivGen® power system are the company's trademarked systems.

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Further reading