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 performance and rates of return on investments. Some of the many companies and turbines tested are summarised in
development of tidal stream generators. The
European Marine Energy Centre recognizes six principal types of tidal energy converters. They are horizontal-axis turbines, vertical-axis turbines, oscillating hydrofoils, venturi devices, Archimedes’ screws and tidal kites.
Axial turbines These are similar in concept to traditional windmills but operate under the sea. They have most of the prototypes currently under design, development, testing or operations. The SR2000, a prototype 2MW floating turbine developed by
Orbital Marine Power in Scotland, was operated at the
European Marine Energy Centre,
Orkney, from 2016. It produced 3,200 MWhs of electricity in 12 months of continuous testing. It was removed in September 2018 to make way for the Orbital O2, the production model, completed in 2021. Tocardo, a Dutch-based company, has been running tidal turbines since 2008 on the Afsluitdijk, near Den Oever. Typical production data of tidal generator shown in the T100 model as applied in Den Oever. Tocardo were declared bankrupt in 2019. QED Naval and HydroWing have joined forces to buy tidal turbine business Tocardo in 2020. The AR-1000, a 1 MW turbine developed by Atlantis Resources Corporation, was successfully deployed at the EMEC facility during the summer of 2011. The AR series are commercial-scale, horizontal-axis turbines designed for open ocean deployment. AR turbines feature a single rotor set with fixed-pitch blades. The AR turbine is rotated as required with each tidal exchange. This is done in the slack period between tides and held in place for the optimal heading for the next tide. AR turbines are rated at 1 MW at 2.65 m/s of water flow velocity. The
Kvalsund installation is south of
Hammerfest,
Norway, at a 50-meter depth of sea. Although still a prototype, the HS300 turbine, with a reported capacity of 300 kW was connected to the grid on November 13, 2003. This made it the world's first tidal turbine delivering to the grid. The submerged structure weighed 120 tonnes and had gravity footings of 200 tonnes. Its three-blades were made in glass fibre-reinforced plastic and measured 10 metres from hub to tip. The device rotated at 7 rpm with an installed capacity of 0.3 MW.
Seaflow, a 300 kW periodflow marine current propeller type turbine, was installed by
Marine Current Turbines off the coast of
Lynmouth,
Devon, England, in 2003. The 11-meter-diameter turbine generator was fitted to a steel pile which was driven into the seabed. As a prototype, it was connected to a dump load, not to the grid. In April 2007,
Verdant Power began running a prototype project in the
East River between
Queens and
Roosevelt Island in New York City; it was the first major tidal-power project in the United States. The strong currents pose challenges to the design: the blades of the 2006 and 2007 prototypes broke and new reinforced turbines were installed in September 2008. Following the Seaflow trial, a full-size prototype called
SeaGen was installed by Marine Current Turbines in Strangford Lough in Northern Ireland in April 2008. The turbine began to generate at full power of just over 1.2 MW in December 2008, is reported to have fed 150 kW into the grid for the first time on July 17, 2008, and has now contributed more than a gigawatt hour to consumers in Northern Ireland. It is currently the only commercial-scale device to have been installed anywhere in the world. SeaGen is made up of two axial flow rotors, each of which drive a generator. The turbines are capable of generating electricity on both the ebb and flood tides because the rotor blades can pitch through 180˚. A prototype semi-submerged floating tethered tidal turbine called
Evopod has been tested since June 2008 in Strangford Lough,
Northern Ireland at 1/10 scale. The UK company developing it is called Ocean Flow Energy Ltd. The advanced hull form maintains optimum heading into the tidal stream and is designed to operate in the peak flow of the water column. In 2010, Tenax Energy of Australia proposed to put 450 turbines off the coast of
Darwin, Australia, in the
Clarence Strait. The turbines would feature a rotor section approximately 15 metres in diameter with a slightly larger gravity base. The turbines would operate in deep water well below shipping channels. Each turbine is forecast to produce energy for between 300 and 400 homes. Tidalstream, a UK-based company, commissioned a scaled-down Triton 3 turbine on the Thames in 2003. It can be floated to its site, installed without cranes, jack-ups, or divers, and then ballasted into an operating position. At full scale, the Triton 3 in 30–50 m deep water has a 3 MW capacity, and the Triton 6 in 60–80 m deep water has a capacity of up to 10MW, depending on the flow. Both platforms have man-access capability both in the operating position and in the float-out maintenance position. European Technology & Innovation Platform for Ocean Energy's (ETIP OCEAN) 2019 report "Powering Homes Today, Powering Nations Tomorrow" makes note of record volumes being supplied through tidal stream technology.
Crossflow turbines Invented by
Georges Darreius in 1923 and patented in 1929, these turbines can be deployed either vertically or horizontally. The
Gorlov turbine is a variant of the Darrieus design featuring a helical design that is in a large-scale, commercial pilot in South Korea, starting with a 1 MW plant that opened in May 2009 and expanding to 90MW by 2013. Neptune Renewable Energy's Proteus project 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. 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.
Flow augmented turbines 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 that are pushed sideways by the flow. Oscillating stream power extraction was proven with the omni- or bi-directional Wing'd Pump windmill. During 2003 a 150 kW oscillating hydroplane device, the
Stingray tidal stream generator, was tested off the Scottish coast. 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. Having secured funding from the EU, they are developing a commercial-scale device to be commissioned 2012. The bioSTREAM tidal power conversion system uses the
biomimicry of swimming species, such as sharks, 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 called
oscillating wing tidal turbine 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.
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.
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. ;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 called the
Deep Green Kite 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.). Each kite has a rated power of 120 kilowatts at a tidal flow of 1.3 meters per second. ;Design Minesto's kite has a wingspan of . 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. The 14-meter version has a rated power of 850 kilowatts at 1.7 meters per second. ;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. That maneuver means the kite can operate in tidal streams that move too slowly to drive earlier tidal devices, such as the
SeaGen turbine. The kite was expected to work in flows as low 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 deep. == Tidal stream developers ==