Shrouded Tidal Turbines: How Ducted Turbines Work, What They Promise, and What the Evidence Shows

A shrouded tidal turbine is an underwater turbine placed inside a duct, usually a funnel-like ring that narrows toward the rotor and widens behind it. The idea is to use the shape of the duct to draw more water through the rotor than would pass through it in open water, and to lower the pressure behind the rotor so the flow speeds up. In principle, that lets a small rotor produce more power than the same rotor could on its own.

Shrouded designs are also called ducted turbines, diffuser-augmented turbines, or, in some company literature, venturi turbines. The Wikipedia article on the subject uses the term "ventuduct" for the venturi-shaped shroud. Several developers built and tested such machines in the 2000s and 2010s, including a famous open-centre design from the Irish company OpenHydro and a ducted unit installed by Clean Current Power Systems off Canada's Vancouver Island.

The topic is interesting partly because of the physics and partly because the claims about it vary so much. Published and promotional figures range from modest gains of 15% to claims of three or four times more power. Whether those numbers are impressive depends heavily on what they are compared against, and later academic work has been much more cautious. This article explains the background of tidal stream energy, the physics of ducts and the Betz limit, the main designs and projects, the engineering trade-offs, worked examples, and the misconceptions that often surround the subject.

Tidal stream energy in brief

There are two broad ways to get electricity from the tides. The older approach is a tidal barrage: a dam across an estuary that holds back water as the tide changes and releases it through low-head turbines, like a hydroelectric plant. Brian Kirke of the University of South Australia, in a review of ducted water current turbines, describes the 240 MW installation on the Rance estuary in France, completed in 1966, as the best-known example. Barrages require large civil works and can disrupt shipping and ecosystems.

The second approach is tidal stream generation: placing turbines directly in fast-moving tidal currents and extracting the kinetic energy of the water, much as a wind turbine extracts energy from moving air. Wikipedia's article on tidal stream generators says these machines were first conceived in the 1970s during the oil crisis. Kirke notes that the concept was investigated by Reading University in the UK in 1979, and by researchers in Canada and Australia around the same time, and that it was used on a small scale in Africa in the early 1980s to extract energy from river currents.

Why water is attractive

Water is roughly 800 times denser than air. The power carried by a moving fluid through a given area is:

P = ½ × ρ × A × v³

where ρ (rho) is the fluid density, A is the area the flow passes through, and v is the flow speed. Wikipedia's tidal stream article uses a seawater density of 1,027 kg/m³ in this equation. Because of the high density, a tidal turbine can produce significant power at much lower speeds than a wind turbine. The same article notes that water speeds near one-tenth of wind speeds give the same power for the same turbine size, and that practical use generally needs tidal speeds of at least about 1 m/s (two knots), even near neap tides. At EMEC's Fall of Warness test site in Orkney, currents reach up to 4 m/s during spring tides.

Tides are also predictable years in advance, because they are driven by the motions of the Moon and Sun. That predictability is one of the main selling points of tidal energy compared with wind and solar.

The main families of tidal turbines

Wikipedia's tidal stream article lists several families of device:

Shrouded turbines are the flow-augmented branch. Most of them are axial rotors in a ring-shaped duct, but ducted crossflow designs also exist.

The physics of the shroud

The Betz limit for an open rotor

Any discussion of shrouded turbines starts with the Betz limit. In 1919 the German physicist Albert Betz published the result that no turbine in open flow can capture more than 16/27, about 59.3%, of the kinetic energy flowing through the area its rotor sweeps. Wikipedia notes that Frederick Lanchester reached the same maximum in 1915 and Nikolay Zhukovsky published it in 1920, so it is sometimes called the Lanchester–Betz–Joukowsky limit.

The reasoning goes like this. A turbine extracts energy by slowing the fluid down. But the fluid cannot be stopped completely, or it would pile up in front of the rotor and nothing more could flow through. If the turbine slows the fluid too little, little energy is extracted; if it slows it too much, the flow spills around the rotor instead of through it. There is an optimum in between.

Using an idealized "actuator disk" model (a thin, frictionless disk that extracts energy uniformly), the optimum occurs when:

• the fluid far downstream moves at one-third of the upstream speed;
• the fluid at the disk moves at two-thirds of the upstream speed;
• the outgoing flow keeps 1/9 of its original kinetic energy, so 8/9 is removed from the stream that passes through the disk.

Measured against the kinetic energy flux through the disk's own area at the upstream speed, the result is 16/27, or 59.3%. Kirke notes that Newman later showed a corresponding limit of 16/25, or 64%, for a "double actuator disk" such as a crossflow turbine, where the flow passes through the rotor twice.

The key point is in the assumptions: open flow, with the streamlines free to expand around the rotor. As the turbine slows the flow, the stream tube passing through it widens. That expansion is what limits how much energy can be extracted.

What a duct changes

A duct around the rotor changes the flow boundaries. Kirke explains that in a ducted turbine, streamline expansion is limited by the duct's shape, and energy is extracted primarily through a pressure drop, so the turbine behaves more like an ultra-low-head hydro turbine than a wind turbine.

If the downstream part of the duct is shaped as a diffuser, widening gradually, it recovers some of the velocity of the water leaving the rotor as pressure. This lowers the pressure just behind the rotor. The larger pressure difference across the rotor draws more water through the duct than would pass through the same rotor in open flow. The rotor sees both a higher flow rate and a larger pressure drop.

Kirke summarizes the theoretical limit for a ducted turbine as depending on two things: the pressure difference that can be created between the duct inlet and outlet, and the volume of flow through the duct. Both depend on the duct shape and on the ratio of duct area to rotor area.

Can a shrouded turbine "beat" Betz?

This is where the subject gets slippery. Wikipedia's shrouded turbine article states that shrouded turbines are not subject to the Betz limit because they increase the flow through the turbine, but that the 59.3% limit still applies when measured against the larger cross-section of the shroud rather than the rotor. Wikipedia's Betz law article makes the same point more generally: claims of beating Betz with nozzles or diversion devices usually measure only the rotor area rather than the total airflow that feeds the device.

Kirke reports a theoretical result attributed to Riegler: the maximum power coefficient for a diffuser-augmented turbine, based on rotor area, is 1.96, about 3.3 times the Betz limit. (Check: 1.96 ÷ 0.593 ≈ 3.3.) A power coefficient above 1 sounds impossible, but it simply means the rotor is extracting more power than the kinetic energy flux through its own small area, because the duct is pulling in water from a much larger upstream area.

So two statements are both true:

1. Measured per unit of rotor area, a well-designed ducted turbine can in theory far exceed 59.3%.
2. Measured per unit of the whole device's frontal area, including the duct, the gains are much smaller, and may be zero or negative in practice.

Which comparison matters depends on what is expensive. If the rotor and generator are the costly part and the duct is cheap, the rotor-area comparison is relevant. If the duct is just as expensive per square meter, or if the site's usable area is the constraint, the whole-device comparison is what counts.

Claimed and measured performance

Wind tunnel and early water results

Much of the early experimental evidence came from diffuser-augmented wind turbines. Kirke describes work by Gilbert and Foreman, who after extensive wind tunnel experiments reported a power augmentation factor of 4.25: a turbine with a diffuser produced 4.25 times as much power as the same turbine in open flow. They used slots in a short, wide-angle diffuser to draw in high-energy air from outside and keep the flow attached to the diffuser walls (boundary layer control).

Kirke adds an important caveat. That 4.25 figure is not inconsistent with Riegler's theoretical 3.3 because the test turbine had untwisted, low-aspect-ratio blades, so its performance in open flow was well below the Betz limit to begin with. A poor rotor can be "augmented" a lot because it starts from a low baseline.

Kirke's own experiments with Tidal Energy Pty Ltd of Palm Beach, Queensland, progressed in stages: flume tests, flow-table visualization, tow-tank tests and pontoon prototypes. In tow tests of a 280 mm axial turbine (the impeller of a desk fan), a slotted diffuser gave about a 70% increase in output compared with the bare turbine. His abstract reports that the water turbine work had "so far achieved an augmentation factor of 3 at an early stage of the investigation." Wikipedia's article cites a peer-reviewed claim associated with this work of a 3.84 (384%) efficiency increase.

Company claims

The Wikipedia article summarizes several company claims:

Wikipedia states the overall range of claims as output improvements of 1.15 to 4 times that of the same turbine without a shroud, and adds that many figures have not been independently verified because companies closely guard their technologies.

Later academic work: a more sobering picture

Independent research in the 2010s and 2020s compared ducted and open turbines more carefully, often using the whole device's diameter as the reference.

• Belloni (Oxford, 2013). Clarissa Belloni's doctoral thesis, Hydrodynamics of Ducted and Open-Centre Tidal Turbines, found, according to its catalogue summary, substantial decreases in power generated by ducted and open-centre turbines relative to a bare turbine of equal total device diameter in axial flow.
• Borg and colleagues (Strathclyde, 2022). A numerical study of a ducted high-solidity tidal turbine in yawed flow, published in Renewable Energy, cites earlier work establishing that cylindrical bi-directional ducts exhibit no performance improvement once the duct exit area is considered.
• Ducted Betz limit studies. A 2026 paper in the Proceedings of the Institution of Mechanical Engineers, Part C, applying actuator disk theory to a bi-directional ducted tidal turbine, derived a maximum power coefficient of 0.385 based on the maximum duct area, well below the open-rotor Betz value of 0.593. Its experiment, at a blockage ratio of 0.105, reached a power coefficient of 0.093, which the authors treated as a benchmark rather than a failure of the theory.
• Flanged diffusers. Not all findings are negative. Rezek and colleagues, in a 2020 Energies paper on micro-hydrokinetic turbines, reported an 81% increase in maximum power coefficient with a brimmed (flanged) diffuser, measured against the turbine section area. A Hong Kong Polytechnic University thesis on flanged-duct tidal turbines found that a design with an inlet-arc flap and a curved flange could achieve a duct-area-based power coefficient close to the Betz–Joukowsky limit.

The pattern is consistent. Ducts can greatly raise power per unit of rotor area, especially with carefully shaped, asymmetric, flanged or slotted diffusers. But for the symmetric, bi-directional ducts that suit reversing tides, the gain largely disappears per unit of total device area, and can become a loss. Bontempo and Manna's 2020 review in Applied Energy of theoretical models for diffuser-augmented wind turbines is a useful background on how these comparisons depend on modeling assumptions.

Worked examples

The following examples are hypothetical and use simple physics to show why the reference area matters. They use the seawater density of 1,027 kg/m³ from Wikipedia's tidal stream article.

Example 1: the cube law

Take a (hypothetical) open rotor with a diameter of 6 m.

• Swept area: A = π × (3 m)² ≈ 28.27 m².
• At 2 m/s: kinetic power flux = ½ × 1,027 × 28.27 × 2³ = 0.5 × 1,027 × 28.27 × 8 ≈ 116,100 W, or about 116 kW.
• At 3 m/s: ½ × 1,027 × 28.27 × 27 ≈ 392,000 W, or about 392 kW.

Increasing the current speed by 50% raises the available power by a factor of 3.375 (because 1.5³ = 3.375). This is why site selection dominates tidal energy economics, and why a shroud's ability to speed up the flow is so tempting: a small increase in velocity at the rotor means a large increase in power.

If the rotor achieves a power coefficient of 0.40 (below the Betz maximum of 0.593, which no real rotor reaches), it delivers about 0.40 × 116 = 46 kW at 2 m/s, and about 0.40 × 392 = 157 kW at 3 m/s, before generator and conversion losses.

Example 2: rotor area versus device area

Now put the same 6 m rotor inside a (hypothetical) duct whose outer diameter is 9 m. The duct's frontal area is π × (4.5 m)² ≈ 63.62 m², which is 2.25 times the rotor area (because (9/6)² = 2.25).

Suppose the duct doubles the rotor's power output compared with the bare rotor, an augmentation factor of 2, which is within the range of claims discussed above.

• Ducted 6 m rotor at 2 m/s: 2 × 46 kW = 92 kW.

Measured against the rotor area, that is a power coefficient of 0.80, which seems to "beat Betz." But compare it with an open rotor as wide as the whole device:

• Open 9 m rotor at 2 m/s, power coefficient 0.40: 0.40 × ½ × 1,027 × 63.62 × 8 ≈ 0.40 × 261,300 W ≈ 104.5 kW.

In this illustration, the 9 m open rotor produces more power than the 6 m ducted rotor occupying the same frontal area. Measured against the duct's area, the ducted machine's power coefficient is 92 ÷ 261.3 ≈ 0.35, below the open rotor's 0.40.

This does not mean ducts are pointless. The ducted machine has a smaller, faster-spinning rotor, which can mean a smaller generator and gearbox, as Kirke points out. It may also be easier to protect from debris. But it shows why an augmentation factor quoted against rotor area cannot be read as free extra energy.

Example 3: the loads

Ducts also add drag. Wikipedia's article states that loads on a shrouded turbine are 3 to 4 times those on an open turbine. In the hypothetical example above, if an open 6 m rotor experienced a peak thrust of some value T, the ducted version might need a foundation and mooring rated for 3T to 4T. Since foundations and installation are often a major share of the cost of a tidal project, that multiplier can outweigh the extra energy.

Design elements of a shrouded turbine

The duct shape

A typical shroud has three parts:

1. An inlet (sometimes called a nozzle or concentrator) that guides water toward the rotor.
2. A throat, the narrowest point, where the rotor sits.
3. A diffuser that widens behind the rotor and recovers pressure.

The diffuser is the most important part. If it widens too quickly, the flow separates from the walls and the diffuser stops working. Slots, flaps and flanges can help keep the flow attached, as Gilbert and Foreman showed with their slotted short diffuser. A flange or brim at the exit creates a low-pressure wake that pulls more water through, which is why flanged designs show some of the best results in research.

Bi-directional ducts for reversing tides

Tidal currents reverse roughly every six hours. A wind turbine can yaw to face the wind; many tidal turbines are fixed. A duct that is optimized for flow from one direction works poorly when the flow reverses. Developers responded in several ways:

• Symmetric, bi-directional ducts, used by Clean Current, OpenHydro and Lunar Energy, work in either direction but cannot have the asymmetric diffuser that gives the biggest augmentation. Research cited above suggests this is the main reason bi-directional ducts struggle to show net gains.
• Yawing ducts, which rotate to face the current. Wikipedia mentions active yaw devices and passive alternatives, such as a pontoon on a swing mooring or an underwater "kite" design. Tidal Energy Pty Ltd's design used a pivoting base and turntable that yawed passively.
• Wide-angle diffusers, which Lunar Energy used to capture flow not perfectly aligned with the turbine axis.

Misaligned and turbulent flow

Real tidal flows are not perfectly steady or aligned. Wikipedia notes that shroud performance depends on geometry, and that misaligned flow from eddies and swirl reduces efficiency. The 2022 Strathclyde study specifically examined yawed flow because the angle between the current and the device axis changes over the tidal cycle and from site to site.

Open-centre rotors and rim generators

Some ducted designs, notably OpenHydro's, put the generator in the rim of the duct rather than in a central hub. Kirke describes the related idea of a rotating rim joining the blade tips, and notes that Teamwork Technology's Tocardo turbine incorporated magnets in the blades and stator windings in the duct, so the rotor itself formed part of a permanent-magnet generator, eliminating gearing. OpenHydro's turbine was a direct-drive design with an open centre and no gearbox. Fewer moving parts can mean less maintenance in an environment where every repair requires a vessel and divers or remote vehicles.

Twin rotors

Wikipedia mentions a more recent research direction: a shrouded twin-rotor turbine that adds a second coaxial rotor inside the duct. Kumar and Saha, writing in Renewable Energy (2019, vol. 134), used actuator disk theory to estimate significant power augmentation in both turbine-turbine and turbine-fan configurations. As with all such theoretical estimates, the practical results depend on real losses and costs.

Advantages claimed for shrouded turbines

Kirke and Wikipedia between them list several practical advantages beyond raw power:

Kirke contrasts this with the wind industry, where Dörner argued that the extra cost of a diffuser outweighs its advantages because a diffuser cannot be feathered in a storm. Kirke notes that the New Zealand company Vortec built a 7 m diffuser-augmented wind turbine based on Gilbert and Foreman's findings before going into liquidation, and argues the criticism applies less underwater.

Disadvantages and engineering challenges

Historical development and projects

Early concepts

Kirke credits the French engineer Darrieus with proposing that turbines be placed in ducts to augment the power from a given-size rotor. Blue Energy Canada proposed ducted crossflow turbines and a "tidal fence" that forces all flow through turbines across a strait, with an ambitious scheme to produce up to 2,200 MW. Kirke reports that Blue Energy estimated from flume tests in Ottawa in 1981–82 that an augmentation factor of up to about 5 could be achieved in a ducted crossflow turbine, but notes it had not published experimental data in the literature.

Clean Current at Race Rocks (2006)

One of the first grid-relevant ducted tidal turbines was installed at Race Rocks, off the southern tip of Vancouver Island in British Columbia. It was a joint project of Pearson College, EnCana and Clean Current Power Systems, enabled by a $3 million investment from the EnCana Environmental Innovation Fund.

The machine was a 65 kW, bi-directional, ducted, horizontal-axis turbine with a direct-drive, variable-speed permanent-magnet generator. It was a 1:5 scale model with a 3.5 m blade diameter. It was installed between July and September 2006 (sources disagree on the exact water depth), and its purpose was to replace diesel generators supplying the Race Rocks ecological reserve, while supporting education and environmental monitoring. Clean Current redeployed it in October 2007. A Government of Canada news release later described it as the world's first ducted free-stream tidal power project, and the turbine was eventually displayed at a technology park.

OpenHydro (2004–2018)

OpenHydro Group Ltd, founded in 2004 and based in Dublin with a factory in Greenore, became the best-known developer of ducted tidal turbines. Its design was an open-centre, horizontal-axis turbine in a ducting shroud with a direct-drive rim generator and no gearbox.

EMEC, Orkney. In December 2006, OpenHydro completed the installation of the first tidal turbine at the European Marine Energy Centre's Fall of Warness test site off the island of Eday. It was a 6 m diameter, 250 kW unit on a test rig. In May 2008 it became the first tidal device connected to the grid in Scotland, and EMEC describes it as the first to generate electricity to the UK national grid, feeding power through the Caldale substation on Eday. The turbine was removed in 2022, and in December 2024 the rest of the platform was decommissioned. EMEC called it the first tidal turbine platform ever to provide electricity to the UK national grid.

Paimpol-Bréhat, France. With the French utility EDF, OpenHydro tested a 250 kW turbine from 2013 to 2014. Two 500 kW turbines were installed in 2016 but never connected to the grid; they were removed for repair in 2017 and not reinstalled, and the project was cancelled in 2018.

Bay of Fundy, Canada. With Emera, through the Cape Sharp Tidal joint venture, OpenHydro tested turbines at the FORCE site in Nova Scotia. A 1 MW turbine deployed in 2009 was damaged after about 20 days. A 2 MW turbine was grid-connected from 2016 to 2017, and a second 2 MW unit installed in 2018 was left on the seabed after the developer's collapse. The province of Nova Scotia holds a $4.5 million bond for its retrieval.

Ownership and collapse. The French naval group DCNS (later Naval Energies) acquired OpenHydro in 2013. In July 2018, Naval Energies withdrew from tidal energy to focus on floating wind and ocean thermal energy, and OpenHydro went into liquidation with debts of about €280 million. Plans that never materialized included a 14 MW Normandie Hydro project approved for French funding in 2018 and a 300 MW array at Alderney, announced in 2014, envisaged as 150 turbines of 2 MW each.

OpenHydro's history shows both the promise and the risk of the technology. Its turbines generated power to two national grids, but the Bay of Fundy experience in particular showed how punishing very fast, turbulent sites can be.

Lunar Energy and the Korean plan

Lunar Energy designed the Rotech Tidal Turbine (RTT), a bi-directional ducted unit with a wide-angle diffuser. Around 2008 the company announced an agreement with Korea Midland Power (KOMIPO) for a planned 300 MW tidal farm in the Wando Hoenggan waterway in South Korea, to consist of 300 RTTs of 1 MW each, with Hyundai Samho Heavy Industries selected for fabrication and installation, at a reported cost of $600 million and a completion target of January 2015. Reports described each 1 MW unit as having an 11.5 m turbine diameter and weighing over 2,500 tonnes fully ballasted; another described a 1.5 MW version with a 21 m duct intake. A 1 MW commercial prototype power train was reported to have synchronized to a simulated grid in dry testing, and deployment at EMEC was expected. This article could find no confirmation that the Korean farm or the EMEC deployment was completed.

Tocardo and other ducted designs

Kirke cites Teamwork Technology's Tocardo turbine from the Netherlands as an example of a ducted axial-flow turbine, with magnets in the blades and windings in the duct. Wikipedia's tidal stream article notes that Tocardo T1 turbines are suitable for currents as low as 0.4 m/s, illustrating the niche of small turbines in slow or shallow water.

How shrouded designs compare with the projects that succeeded

The tidal stream projects that have operated longest at megawatt scale have mostly used open rotors. Wikipedia lists SeaGen at Strangford Lough in Northern Ireland, which reached just over 1.2 MW at full power in December 2008; the MeyGen project in Scotland, whose Phase 1A has four 1.5 MW turbines in operation; and Orbital Marine Power's floating SR2000 (2 MW), which produced 3,200 MWh in 12 months, followed by the O2 production model completed in 2021. Wikipedia classes SeaGen and Orbital's machines as axial, open-rotor designs, and none of these flagship projects uses a shroud. That does not prove shrouds cannot work, but it shows that the industry's mainstream has so far gone in a different direction.

Environmental considerations

All tidal stream turbines raise similar environmental questions. Wikipedia's tidal stream article lists blade strike and entanglement of marine animals, electromagnetic fields and noise that may affect marine mammals, and effects on water quality and sediment. It also reports that a study of the RITE project in New York's East River found very few fish in the area and no evidence of fish passing through blade areas.

Shrouded designs have some specific features:

• Screening. A grid over the inlet, as Kirke suggests, could keep out larger animals and debris, though it would also add drag and could clog.
• Larger structures. Ducts add bulk on the seabed, with more area for marine growth and potentially more effect on local flow and sediment.
• Retrieval risk. The turbine left on the Bay of Fundy seabed after OpenHydro's collapse shows that abandoned hardware is a real environmental and financial issue; Nova Scotia's $4.5 million bond exists for that reason.

Wikipedia notes that one manufacturer described its shrouded technology as "eco-benign," but that is a company characterization, not an independent assessment.

Common misconceptions

"Shrouded turbines break the laws of physics by beating the Betz limit." No. The Betz limit applies to a single actuator disk in open flow, measured against the disk's own area. A duct changes the flow conditions, so the rotor can exceed 59.3% of the energy passing through its own area. When the whole device's area is used, the advantage shrinks or disappears.

"A shroud multiplies power by three or four." Some tests and company claims reported factors that large, but often against a weak baseline rotor, or measured against rotor area only. Independent research on bi-directional ducts suited to tides found little or no net gain per unit of device area.

"A shroud makes the water flow three or four times faster." Wikipedia records this as a claimed advantage, but because power rises with the cube of velocity, a true tripling of speed at the rotor would mean 27 times more power, far beyond any measured augmentation. Claims of this kind should be read carefully and checked against power measurements.

"Ducts are always cheaper because the rotor is smaller." The rotor and drivetrain can be smaller, but the duct itself, its foundation and its mooring must withstand loads 3 to 4 times higher, according to Wikipedia. The overall cost balance depends on the design.

"Shrouded turbines failed because the idea was wrong." OpenHydro's collapse came when its owner withdrew from tidal energy, and the industry as a whole has struggled with costs. The technical lessons are mixed rather than a simple verdict.

"Tidal energy is always the same as tidal barrages." Tidal stream turbines, shrouded or not, sit in the current without damming it. Barrages like La Rance work on a different principle.

Frequently asked questions

What is a shrouded tidal turbine? An underwater turbine surrounded by a duct, often shaped like a venturi or diffuser, that concentrates flow through the rotor and lowers the pressure behind it to increase power from a given rotor size.

How much more power does a shroud give? Claims range from 1.15 to 4 times the power of the same rotor without a shroud. Lunar Energy claimed 15% to 25%; Kirke's early water tests reported an augmentation factor of about 3. Measured against the whole device's area, independent studies of bi-directional ducts found little or no gain.

What is the Betz limit? The maximum fraction of kinetic energy, 16/27 or about 59.3%, that an ideal single rotor in open flow can extract from the fluid passing through its swept area. It was published by Albert Betz in 1919, with earlier and parallel results by Lanchester and Zhukovsky.

Why do tidal turbines need to handle flow in two directions? Tidal currents reverse with the tide. A fixed turbine must either work in both directions, rotate to face the flow, or accept reduced output on one tide.

What was OpenHydro? An Irish developer founded in 2004 that built open-centre ducted tidal turbines. Its turbine at EMEC in Orkney was the first tidal device connected to the grid in Scotland, in May 2008. It was owned by Naval Energies from 2013 and went into liquidation in 2018.

What was the Race Rocks turbine? A 65 kW, bi-directional ducted turbine by Clean Current Power Systems, installed in 2006 near Race Rocks off Vancouver Island, with a 3.5 m blade diameter, to replace diesel generation at an ecological reserve.

Are shrouded turbines used commercially today? The largest operating tidal stream projects cited in this article, such as MeyGen and Orbital's O2, use open rotors. Shrouded designs reached grid-connected demonstration stage, but their best-known developers did not reach sustained commercial operation.

Can a shroud help in rivers and slow currents? That is one of the more promising niches. Wikipedia notes that smaller shrouded units can be seabed-mounted in shallow rivers and estuaries, and research on micro-hydrokinetic turbines has found substantial gains with flanged diffusers.

How is power from a tidal turbine calculated? Available power is ½ × water density × area × velocity³; a real turbine captures a fraction of this given by its power coefficient, minus drivetrain losses. Because of the cube, a 50% increase in current speed gives 3.375 times more available power.

Summary

Shrouded tidal turbines are an appealing idea: put a rotor in a carefully shaped duct, pull more water through it, and get more power from a smaller, cheaper machine. The physics is real. A diffuser can raise a rotor's output well beyond what the same rotor achieves in open water, and per unit of rotor area it can exceed the Betz limit. Early experiments by Gilbert and Foreman in wind, and by Kirke and others in water, showed large augmentation factors.

The difficulty is in the comparisons and the engineering. Measured against the whole device's frontal area, and especially for the symmetric, bi-directional ducts that suit reversing tides, much of the advantage disappears in independent studies. Ducts add size, weight and loads that are reported to be three to four times higher than for open turbines. The leading developer, OpenHydro, reached real milestones, including the first grid-connected tidal device in Scotland, before collapsing in 2018. The tidal stream projects that have lasted at megawatt scale have so far used open rotors.

Shrouded designs may still find their place in particular niches: shallow rivers, slow currents, sites where debris protection matters, or optimized flanged designs. The lesson for readers is to ask, whenever a performance figure is quoted, what area it is measured against.

Sources

• Shrouded tidal turbine (Wikipedia)
• Tidal stream generator (Wikipedia)
• Betz's law (Wikipedia)
• Kirke, B. "Developments in ducted water current turbines," University of South Australia (via PNNL Tethys)
• Belloni, C. Hydrodynamics of Ducted and Open-Centre Tidal Turbines, University of Oxford thesis
• Borg et al. (2022), A numerical performance analysis of a ducted high-solidity tidal turbine in yawed flow conditions, Renewable Energy
• Ducted Betz limit for a bidirectional ducted tidal turbine, Proc. IMechE Part C (2026)
• Rezek et al. (2020), Energies 13(17):4560, diffuser-augmented micro-hydrokinetic turbines
• OpenHydro (Wikipedia)
• EMEC: The first grid connected tidal device in Scotland
• EMEC: Successful installation of first tidal turbine
• Race Rocks Tidal Power Demonstration Project (Wikipedia)
• Government of Canada: SDTC and museum unveil tidal turbine technology park
• Renewable Energy World: Lunar Energy seals tidal deal in Korea