Tidal stream generator

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Evopod - A semi-submerged floating approach tested in Strangford Lough. Evopod in Strangford Lough 2008.jpg
Evopod - A semi-submerged floating approach tested in Strangford Lough.

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. [1]

Contents

Tidal stream generators are the cheapest and the least ecologically damaging among the four main forms of tidal power generation. [2]

Similarity to wind turbines

Tidal stream generators draw energy from water currents in much the same way as wind turbines draw energy from air currents. However, the potential for power generation by an individual tidal turbine can be greater than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed. [3] Given that power varies with the density of medium and the cube of velocity, water speeds of nearly one-tenth the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where tide speed is at least 2 knots (1 m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 and 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.

Types of tidal stream generators

No standard tidal stream generator has emerged as the clear winner, among a large variety of designs. Several prototypes have shown promise with many companies making bold claims, some of which are yet to be independently verified, but they have not operated commercially for extended periods to establish performances and rates of return on investments.

The European Marine Energy Centre recognizes six principal types of tidal energy converter. They are horizontal axis turbines, vertical axis turbines, oscillating hydrofoils, venturi devices, Archimedes screws and tidal kites. [4]

Axial turbines

Bottom-mounted axial turbines Bottom Mounted Turbines.png
Bottom-mounted axial turbines
A cable tethered turbine Cable Tethered Turbine.png
A cable tethered turbine

These are close in concept to traditional windmills, but operating under the sea. They have most of the prototypes currently operating, including:

A 3D model of an Evopod tidal turbine Evopod lighter.jpg
A 3D model of an Evopod tidal turbine

Crossflow turbines

Invented by Georges Darreius in 1923 and patented in 1929, these turbines can be deployed either vertically or horizontally.

The Gorlov turbine [23] is a variant of the Darrieus design featuring a helical design that is in a large scale, commercial pilot in South Korea, [24] starting with a 1MW plant that opened in May 2009 [25] and expanding to 90MW by 2013. Neptune Renewable Energy's Proteus project [26] employs a shrouded vertical axis turbine that can be used to form an array in mainly estuarine conditions.

In April 2008, the Ocean Renewable Power Company, LLC (ORPC) successfully completed testing its proprietary turbine-generator unit (TGU) prototype at ORPC's Cobscook Bay and Western Passage tidal sites near Eastport, Maine. [27] The TGU is the core of the OCGen technology and uses advanced design cross-flow (ADCF) turbines to drive a permanent magnet generator located between the turbines and mounted on the same shaft. ORPC has developed TGU designs that can be used for generating power from river, tidal and deep water ocean currents.

Trials in the Strait of Messina, Italy, started in 2001 of the Kobold turbine concept. [28]

Flow augmented turbines

A shrouded turbine Sea Bed Turbine.png
A shrouded turbine

Using flow augmentation measures, for example a duct or shroud, the incident power available to a turbine can be increased. The most common example uses a shroud to increase the flow rate through the turbine, which can be either axial or crossflow.

The Australian company Tidal Energy Pty Ltd undertook successful commercial trials of efficient shrouded tidal turbines on the Gold Coast, Queensland in 2002. Tidal Energy delivered their shrouded turbine in northern Australia where some of the fastest recorded flows (11 m/s, 21 knots) are found. Two small turbines will provide 3.5 MW. Another larger 5 meter diameter turbine, capable of 800 kW in 4 m/s of flow, was planned as a tidal powered desalination showcase near Brisbane Australia.

Oscillating devices

Oscillating devices do not have a rotating component, instead making use of aerofoil sections which are pushed sideways by the flow. Oscillating stream power extraction was proven with the omni- or bi-directional Wing'd Pump windmill. [29] During 2003 a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. [30] The Stingray uses hydrofoils to create oscillation, which allows it to create hydraulic power. This hydraulic power is then used to power a hydraulic motor, which then turns a generator. [1]

Pulse Tidal operate an oscillating hydrofoil device in the Humber Estuary. [31] Having secured funding from the EU, they are developing a commercial scale device to be commissioned 2012. [32]

The bioSTREAM tidal power conversion system, uses the biomimicry of swimming species, such as shark, tuna, and mackerel using their highly efficient Thunniform mode propulsion. It is produced by Australian company BioPower Systems.

A 2 kW prototype relying on the use of two oscillating hydrofoils in a tandem configuration has been developed at Laval University and tested successfully near Quebec City, Canada, in 2009. A hydrodynamic efficiency of 40% has been achieved during the field tests. [33]

Venturi effect

Venturi effect devices use a shroud or duct in order to generate a pressure differential which is used to run a secondary hydraulic circuit which is used to generate power. A device, the Hydro Venturi, is to be tested in San Francisco Bay. [34]

Tidal kite turbines

A tidal kite turbine is an underwater kite system or paravane that converts tidal energy into electricity by moving through the tidal stream. An estimated 1% of 2011's global energy requirements could be provided by such devices at scale. [35]

History

Ernst Souczek of Vienna, Austria, on August 6, 1947, filed for a patent US2501696; assignor of one-half to Wolfgang Kmentt, also of Vienna. Their water kite turbine disclosure demonstrated a rich art in water-kite turbines. In similar technology, many others prior to 2006 advanced water-kite and paravane electric generating systems. In 2006, a tidal kite turbine was developed by Swedish company Minesto. They conducted its first sea trial in Strangford Lough in Northern Ireland in the summer of 2011. The test used kites with wingspan of 1.4m. [35] In 2013 the Deep Green pilot plant began operation off Northern Ireland. The plant uses carbon fiber kites with a wingspan of 8m (or 12m [36] ). Each kite has a rated power of 120 kilowatts at a tidal flow of 1.3 meters per second. [37]

Design

Minesto's kite has a wingspan of 8–14 metres (26–46 ft). The kite has neutral buoyancy, so doesn't sink as the tide turns from ebb to flow. Each kite is equipped with a gearless turbine to generate which is transmitted by the attachment cable to a transformer and then to the electricity grid. The turbine mouth is protected to protect marine life. [35] The 14-meter version has a rated power of 850 kilowatts at 1.7 meters per second. [37]

Operation

The kite is tethered by a cable to a fixed point. It "flies" through the current carrying a turbine. It moves in a figure-eight loop to increase the speed of the water flowing through the turbine tenfold. Force increases with the cube of velocity, offering the potential to generate 1,000-fold more energy than a stationary generator. [35] That maneuver means the kite can operate in tidal streams that move too slowly to drive earlier tidal devices, such as the SeaGen turbine. [35] The kite was expected to work in flows as low 1–2.5 metres (3 ft 3 in–8 ft 2 in) per second, while first-generation devices need over 2.5s. Each kite will have a capacity to generate between 150 and 800 kW. They can be deployed in waters 50–300 metres (160–980 ft) deep. [35]

Tidal stream developers

There are a number of individuals and companies developing tidal energy converters across the world. A database of all know tidal energy developers is kept up-to-date here: Tidal energy developers [38]

Tidal stream testing

The world's first marine energy test facility was established in 2003 to kick 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 currently testing at the site include Alstom (formerly Tidal Generation Ltd), ANDRITZ HYDRO Hammerfest, OpenHydro, Scotrenewables Tidal Power, and Voith. [39]

Commercial plans

RWE's npower announced that it is in partnership with Marine Current Turbines to build a tidal farm of SeaGen turbines off the coast of Anglesey in Wales, [40] near the Skerries. [41]

"The Skerries project located in Anglesey, Wales, will be one of the first arrays deployed using the Siemens owned Marine Current Turbines SeaGen S tidal turbines. The marine consent for the project was recently awarded, the first tidal array to be consented in Wales. The 10MW array will be fully operational in 2015." - CEO of Siemens Energy Hydro & Ocean Unit Achim Wörner. The project was shelved in 2016 after Marine Current Turbines was acquired by SIMEC Atlantis Energy. [42]

In November 2007, British company Lunar Energy announced that, in conjunction with E.ON, they would be building the world's first deep-sea tidal energy farm off the coast of Pembrokeshire in Wales. It will provide electricity for 5,000 homes. Eight underwater turbines, each 25 metres long and 15 metres high, are to be installed on the sea bottom off St David's peninsula. Construction is due to start in the summer of 2008 and the proposed tidal energy turbines, described as "a wind farm under the sea", should be operational by 2010.[ needs update ] Lunar Energy dissolved in 2019. [43]

Alderney Renewable Energy Ltd was granted a licence in 2008 and is planning to use tidal turbines to extract power from the notoriously strong tidal races around Alderney in the Channel Islands. It is estimated that up to 3 GW could be extracted. This would not only supply the island's needs but also leave a considerable surplus for export, [44] using a France-Alderney-Britain cable (FAB Link) which is expected to go online by 2020. This agreement was terminated in 2017. [45]

Nova Scotia Power has selected OpenHydro's turbine for a tidal energy demonstration project in the Bay of Fundy, Nova Scotia, Canada and Alderney Renewable Energy Ltd for the supply of tidal turbines in the Channel Islands. [46] OpenHydro was liquidated in 2018. [47]

Pulse Tidal are designing a commercial device[ when? ] with seven other companies who are expert in their fields. [48] The consortium was awarded an €8 million EU grant to develop the first device, which will be deployed in 2012[ where? ] and generate enough power for 1,000 homes. Pulse Tidal was liquidated in 2014. [49]

ScottishPower Renewables are planning to deploy ten 1MW HS1000 devices designed by Hammerfest Strom in the Sound of Islay.[ when? ] [50]

In March 2014, the Federal Energy Regulatory Committee (FERC) approved a pilot license for Snohomish County PUD to install two OpenHydro tidal turbines in Admiralty Inlet, WA. This project is the first grid-connected two-turbine project in the US; installation is planned for the summer of 2015. The OpenHydro tidal turbines that Snohomish County PUD will use are designed to be placed directly into the seafloor at a depth of roughly 200 feet, so that there will be no effect on commercial navigation overhead. The license granted by the FERC also includes plans to protect fish, wildlife, as well as cultural and aesthetic resources, in addition to navigation. Each turbine measures 6 meters in diameter, and will generate up to 300 kW of electricity. [51] In September 2014, the project was canceled due to cost concerns. [52]

Energy calculations

Turbine power

Tidal energy converters can have varying modes of operating and therefore varying power output. If the power coefficient of the device "" is known, the equation below can be used to determine the power output of the hydrodynamic subsystem of the machine. This available power cannot exceed that imposed by the Betz limit on the power coefficient, although this can be circumvented to some degree by placing a turbine in a shroud or duct. This works, in essence, by forcing water which would not have flowed through the turbine through the rotor disk. In these situations it is the frontal area of the duct, rather than the turbine, which is used in calculating the power coefficient and therefore the Betz limit still applies to the device as a whole.

The energy available from these kinetic systems can be expressed as:

where:

= the turbine power coefficient
P = the power generated (in watts)
= the density of the water (seawater is 1027 kg/m³)
A = the sweep area of the turbine (in m²)
V = the velocity of the flow

Relative to an open turbine in free stream, ducted turbines are capable of as much as 3 to 4 times the power of the same turbine rotor in open flow. [53]

Resource assessment

While initial assessments of the available energy in a channel have focus on calculations using the kinetic energy flux model, the limitations of tidal power generation are significantly more complicated. For example, the maximum physical possible energy extraction from a strait connecting two large basins is given to within 10% by: [54] [55]

where

= the density of the water (seawater is 1027 kg/m³)
g = gravitational acceleration (9.80665 m/s2)
= maximum differential water surface elevation across the channel
= maximum volumetric flow rate though the channel.

Potential sites

As with wind power, selection of location is critical for the tidal turbine. Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses. The following potential sites are under serious consideration:

Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term marine current power. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.

Environmental impacts

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 build up near the tidal device to severely affecting nearshore ecosystems and processes. [68]

One study of the Roosevelt Island Tidal Energy (RITE, Verdant Power) project in the East River (New York City), used 24 split beam hydroacoustic sensors (scientific echosounder) to detect and track the movement of fish both upstream and downstream of each of six turbines. The results suggested (1) very few fish using this portion of the river, (2) those fish which did use this area were not using the portion of the river which would subject them to blade strikes, and (3) no evidence of fish traveling through blade areas. [69]

Work is currently being conducted by the Northwest National Marine Renewable Energy Center (NNMREC [70] ) to explore and establish tools and protocols for assessment of physical and biological conditions and monitor environmental changes associated with tidal energy development.

See also

Related Research Articles

Hydropower energy derived from falling or running water

Hydropower or water power is power derived from the energy of falling or fast-running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of watermills has been used as a renewable energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.

Tidal power Technology to convert the energy from tides into useful forms of power

Tidal power or tidal energy is the form of hydropower that converts the energy obtained from tides into useful forms of power, mainly electricity.

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).

The Pelamis Wave Energy Converter was a technology that used the motion of ocean surface waves to create electricity. The machine was made up of connected sections which flex and bend as waves pass; it is this motion which is used to generate electricity.

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

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:

SeaGen architectural structure

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. It was successfully decommissioned by SIMEC Atlantis Energy Limited in summer 2019, having exported 11.6GWh to the grid since 2008.

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

Shrouded tidal turbine a type of tidal stream generator used in tidal power stations

The shrouded tidal turbine is an emerging tidal stream technology that has a turbine enclosed in a venturi shaped shroud or duct (ventuduct), producing a sub atmosphere of low pressure behind the turbine. The venturi shrouded turbine is not subject to the Betz limit and allows the turbine to operate at higher efficiencies than the turbine alone by increasing the volume of the flow over the turbine. Claimed improvements vary, from 1.15 to 4 times higher power output than the same turbine minus the shroud. The Betz limit of 59.3% conversion efficiency for a turbine in an open flow still applies, but is applied to the much larger shroud cross-section rather than the small turbine cross-section.

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 applications use tidal flows or rivers with a head of 20 metres (66 ft) or less to produce energy. These applications may not need to dam or retain water to create hydraulic head. Using the 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. Within low head hydropower you have forced and non-forced or free flow solutions. Most of low head hydropower is also known as small hydropower.

Evopod

Evopod is a unique tidal energy device being developed by a UK-based company Oceanflow Energy Ltd for generating electricity from tidal streams and ocean currents. It can operate in exposed deep water sites where severe wind and waves also make up the environment.

The Oyster is a hydro-electric wave energy device that uses the motion of ocean waves to generate electricity. It is made up of a Power Connector Frame (PCF), which is bolted to the seabed, and a Power Capture Unit (PCU). The PCU is a hinged buoyant flap that moves back and forth with movement of the waves. The movement of the flap drives two hydraulic pistons that feed high-pressured water to an onshore hydro-electric turbine, which drives a generator to make electricity. Oyster is stationed at the European Marine Energy Centre (EMEC) at its Billia Croo site in Orkney, Scotland.

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 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

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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