Axial turbines

Bottom-mounted axial turbines

A cable-tethered turbine

These are close 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.^[5][6]

Tocardo,^[7] a Dutch-based company, has been running tidal turbines since 2008 on the Afsluitdijk, near Den Oever.^[8] Typical production data of tidal generator shown of the T100 model as applied in Den Oever.^[8] 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 0.4 m/s.^[9] Tocardo were declared bankrupt in 2019.^[10] QED Naval and HydroWing have joined forces to buy tidal turbine business Tocardo in 2020.^[11]

The AR-1000, a 1MW turbine developed by Atlantis Resources Corporation that 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 1MW @ 2.65 m/s of water flow velocity.^[12]

The Kvalsund installation is south of Hammerfest, Norway at 50m depth of sea. Although still a prototype, the HS300 turbine with a reported capacity of 300 kW was connected to the grid on 13 November 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.^[13]

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.^[14] The 11m 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^[15] 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.^[16] 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.^[17][18]

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^[19] and is reported to have fed 150 kW into the grid for the first time on 17 July 2008, and has now contributed more than a gigawatt hour to consumers in Northern Ireland.^[20] It is currently the only commercial scale device to have been installed anywhere in the world.^[21] 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˚.^[22]

A 3D model of an Evopod tidal turbine

A prototype semi-submerged floating tethered tidal turbine called Evopod has been tested since June 2008^[23] in Strangford Lough, Northern Ireland at 1/10 scale. The UK company developing it is called Ocean Flow Energy Ltd.^[24] 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.^[25]

Tidalstream, a UK-based company, commissioned a scaled-down Triton 3 turbine in the Thames in 2003.^[26] It can be floated to its site, installed without cranes, jack-ups or divers and then ballasted into operating position. At full scale the Triton 3 in 30-50m deep water has a 3MW capacity, and the Triton 6 in 60-80m 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 and innovation platform for ocean energy (ETIP OCEAN) Powering homes today, Powering nations tomorrow report 2019 makes note of record volumes being supplied through tidal stream technology.^[27]

Crossflow turbines

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

The Gorlov turbine^[28] is a variant of the Darrieus design featuring a helical design that is in a large scale, commercial pilot in South Korea,^[29] starting with a 1MW plant that opened in May 2009^[30] and expanding to 90MW by 2013. Neptune Renewable Energy's Proteus project^[31] 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.^[32] 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.^[33]

Flow augmented turbines

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.^[34]

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.^[35] During 2003 a 150 kW oscillating hydroplane device, the Stingray tidal stream generator, was tested off the Scottish coast.^[36][37] 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 called Pulse generator in the Humber Estuary.^[38][39] Having secured funding from the EU, they are developing a commercial scale device to be commissioned 2012.^[40]

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.^[41]

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.^[42][43]

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.^[44][45]

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.^[46]

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.^[47] 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.^[46] 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^[48]). Each kite has a rated power of 120 kilowatts at a tidal flow of 1.3 meters per second.^[49]

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.^[46] The 14-meter version has a rated power of 850 kilowatts at 1.7 meters per second.^[49]

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.^[46] That maneuver means the kite can operate in tidal streams that move too slowly to drive earlier tidal devices, such as the SeaGen turbine.^[46] 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.^[46]

Turbine power

Tidal energy converters can have varying modes of operating and therefore varying power output. If the power coefficient of the device "${\displaystyle C_{P}}$" 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:

${\displaystyle P={\frac {\rho AV^{3}}{2}}C_{P}}$

where:

${\displaystyle C_{P}}$ = the turbine power coefficient

P = the power generated (in watts)

${\displaystyle \rho }$ = 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.^[66]

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:^[67][68]

${\displaystyle P=0.22\,\rho \,g\,\Delta H_{\text{max}}\,Q_{\text{max}}}$

where

${\displaystyle \rho }$ = the density of the water (seawater is 1027 kg/m³)

g = gravitational acceleration (9.80665 m/s²)

${\displaystyle \Delta H_{\text{max}}}$ = maximum differential water surface elevation across the channel

${\displaystyle Q_{\text{max}}}$= maximum volumetric flow rate though the channel.