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 the run of a 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 stream generators are the cheapest and least ecologically damaging among the four main forms of tidal power generation.
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 a 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 speeds. 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 two knots (1 m/s), even close to neap tides. Furthermore, at higher speeds in a flow between two and three meters 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 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. Currently, 1 river model (R1) and 2 tidal models (T) are in production, with a 3rd T3 coming soon. Power production for the T1 is around 100 kW and around 200 kW for the T2. These are suitable for tidal currents as low as 0.4 m/s. 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˚.
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