Tidal stream generator


A tidal stream generator, often referred to as a tidal energy converter, 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 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, 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. shown in the T100 model as applied in Den Oever. Currently, 1 river model and 2 tidal models 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˚.
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 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 successfully completed testing its proprietary turbine-generator unit 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 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 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.
Pulse Tidal operate an oscillating hydrofoil device called Pulse generator in the Humber Estuary. 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.