How tidal stream turbines make electricity
Follow moving seawater through a yawing seabed turbine, checked gearbox and sealed induction generator. Onshore equipment converts alternating current (AC) to direct current (DC) and back to AC. Change current independently of the lesson view and account for each loss on the way to the grid.
In 60 seconds: how it works
A tidal stream turbine works like an underwater wind turbine. The regular movement of the tides turns its blades, and a generator makes electricity.
- The tide moves large amounts of seawater past a turbine fixed to the seabed.
- The moving water turns the rotor blades. The turbine turns to face the current.
- A gearbox and generator turn the slow rotation into electricity.
- A cable carries the power to shore, where converters change it into grid-quality AC.
- A transformer raises the voltage so the power can travel to the grid.
Want the engineering detail? Explore the 3D model, then read the engineering notes and the types and variations.
By Power Stations Worldwide · Related reading updated · Preparation and review scope. Educational illustrations; no independent engineering peer review is claimed.
Explore the equipment in 3D
Rotate equipment, follow energy flows and inspect each component. Read the steps below at any time.
Choose a step to read the process. Activate the 3D model or start a guided tour to use the animation controls.
Yawing seabed tidal-stream turbine, geared squirrel-cage induction generator (SCIG) and onshore full converter. Original pressure-housing/drivetrain/converter cutaways; nominal 2 m/unit, enlarged internals and compressed shore distance. Signed current and a separate finite-yaw clock are educational, not a tide forecast or validated controller. Visible shaft motion is 1/20 physical speed before the display multiplier; yaw is independent of viewer speed. Motion is slowed for inspection. Coloured overlays reveal flow through enclosed equipment; gold pulses show net electrical energy transfer, not individual electrons.
How it works
- Current at rotor depth
- Rotor and gearbox
- Induction generator
- Variable-frequency AC subsea cable
- Onshore AC–DC–AC conversion
- Transformer, switchgear and grid
Tidal energy uses the movement of seawater associated with the tides. This tidal-stream turbine extracts kinetic energy from a current flowing past its blades. The rotor and gearbox drive an induction generator; variable-frequency three-phase alternating current (AC) travels through a service loop, J-tube and subsea cable to shore. Onshore input voltage adaptation and filtering feed back-to-back AC–DC–AC conversion, followed by a transformer, switchgear and the grid. Its short direct-current (DC) link is on shore, not the subsea cable. The nacelle turns to face a reversing current; other designs accommodate reversal or locate conversion differently.
The astronomical drivers of tides make their timing predictable, but predictable does not mean constant output. Current speed changes through the tidal cycle, and a stream turbine produces little or no electricity during slack water. Site measurements are needed to understand the usable flow at rotor depth. Tidal-range schemes work differently: a barrier holds water back so a difference in water level can drive turbines. Wave energy uses surface-wave motion and is a separate marine-energy technology, not a tidal-turbine variation.
Follow the tidal-stream power path
A seabed-mounted, yawing horizontal-axis turbine drives a geared squirrel-cage induction generator. Variable-frequency generator AC travels ashore; the onshore converters decouple it from the grid. This is one illustrative architecture, not an exact Hammerfest product or a universal tidal layout. [2][3][5]
Cyan direction aids = water-current motion; grey shaft = mechanical motion; A/B/C labels and solid/dashed/dotted conductors = three-phase AC; DC+/DC− = shore converter rails; gold pulses = mean generated-energy flow. Structure and electrical conductors are distinct. Pulses are qualitative teaching aids, not electron speed or calculated water velocity. The rotor is displayed at 1/20 physical speed before the viewer’s speed multiplier; yaw follows the separate physical clock.
Whole-system overview
Select equipment above or in the viewer. The selection highlights both representations without changing current, yaw or simulated elapsed time.
1. Current, not tide height
Moving water contains kinetic energy. This stream turbine extracts part of it without impounding a basin. Changing the lesson view does not change the current. [1][2][4]
Engineering details — Current, not tide height
The model assumes a uniform, quasi-steady channel current at rotor depth. Resource power scales with swept area and the cube of speed magnitude. Real flow includes shear, turbulence, waves and non-tidal contributions; offshore currents can rotate. Slack need not coincide with high or low sea level. [1][2][4]
2. Hydrofoils turn water motion into torque
Lift on the three hydrofoil blades supplies useful torque to the hub and low-speed shaft. Much of the resource remains in the water; the turbine cannot capture it all. [2]
Engineering details — Hydrofoils turn water motion into torque
A real power coefficient depends on blade pitch, tip-speed ratio and hydrodynamics. Here Cp = 0.40 and a tip-speed-ratio target of 5 are illustrative below-rating assumptions, not measured blade performance. Shaft power and RPM are capped at a derived rated point; reduced capture above rating is imposed, not a simulated pitch controller. [2]
3. A checked gearbox ratio
The low-speed shaft turns a carrier holding three planet gears. They roll inside a fixed ring and drive the central sun gear, making the generator shaft turn eight times faster in the same local direction. [2][3]
Engineering details — A checked gearbox ratio
This original single-stage teaching gearbox has sun/planet/ring tooth counts 12/36/84 at a common module. The geometry closes because 84 = 12 + 2×36. With the ring fixed, no-slip pitch-circle kinematics give ωsun = (1 + 84/12)ωcarrier = 8ωcarrier. Planet rotation relative to the carrier is −7/3 of carrier speed. Teeth are illustrative profiles, not a manufactured gear/contact or fatigue design. Commercial multistage ratios differ. [2][3]
4. Induction generation inside a sealed nacelle
The geared shaft turns a conducting cage inside a fixed wound stator. The onshore generator-side converter establishes the rotating magnetic field and supplies magnetizing current. Mechanical work becomes electrical output. [2][5][6]
Engineering details — Induction generation inside a sealed nacelle
The selected squirrel-cage induction generator (SCIG) is not a permanent-magnet synchronous machine. Its rotor runs above synchronous speed in generating operation: slip s is negative. An imposed s = −0.02 and 8 pole pairs give fgen = 8ngen/[60(1−s)]. This is a kinematic teaching relationship, not a solved equivalent circuit or torque controller. Bearings, rotary seals, a dry air gap and closed cooling remain distinct from the surrounding seawater. [2][5][6]
5. Yaw once; do not reverse twice
When the current changes direction, the nacelle turns through 180° to face it. The rotor is parked during this finite turn. Once aligned, it turns in the same local direction as before. [3]
Engineering details — Yaw once; do not reverse twice
Manual signed current requests 0° for flood and 180° for ebb. The separate physical clock advances yaw at 3°/s. The generation interlock avoids pretending a cos³ yaw-loss curve is validated. Current arrows reverse, but net generated-energy arrows still point toward shore. There is no unlimited cable winding or slip ring in this finite-yaw example. [3]
6. A supported marine load path
The nacelle is supported by a yaw bearing and turret. Braced tripod legs carry loads into ballast blocks on the prepared seabed. The electrical cable is not a structural leg. [3]
Engineering details — A supported marine load path
Rotor thrust, torque reactions and environmental loads reach the foundation through supports and bearings. Nominal scale is 2 m/unit; the 9 m rotor radius leaves about 5.4 m above the flat bed and 11.6 m below the flat surface. Exported envelopes are checked separately. These geometric clearances are not a wave/tidal-level envelope, scour analysis, ballast sizing, pressure-hull calculation or certified structural design. [3]
7. Follow the cable all the way ashore
Three AC conductors leave the generator through a flexible service loop, then descend in a fixed protective J-tube. The seabed cable connects them to the shore input equipment. [3][5]
Engineering details — Follow the cable all the way ashore
A constant-length service coil inside the cutaway hollow turret accommodates finite yaw, with separate moving/fixed attachments and strain relief. Its diameter is enlarged for visibility. Arc length, bend radius and clearance are geometry checks, not cable torsion analysis or manufacturer installation approval. The cable carries variable-frequency three-phase AC—not the short DC link located ashore. Its 98% transfer factor is an illustrative lumped loss; electrical route length, current, ampacity, harmonics and resonant overvoltage are not calculated from the compressed shoreline. [3][5]
8. Onshore AC → DC → AC
Input voltage adaptation and filtering feed a generator-side converter. A capacitor-supported DC link joins it to a grid-side inverter. Output filtering smooths the switching waveform before grid connection. [5]
Engineering details — Onshore AC → DC → AC
Each illustrative voltage-source converter (VSC) has three two-switch legs. Controlled switching can manage generator torque/magnetizing current independently of the selected 50 Hz grid output. The DC capacitor is an energy buffer, not a steady-state source of extra power. This page solves only steady powers and mechanical/yaw kinematics—not pulse-width modulation, DC ripple, reactive power, dead time, resonance or a validated time-domain electrical simulation. [5]
9. Transformation, switchgear & net export
The grid transformer adapts the voltage and switchgear connects or isolates the path. Shore output is lower than captured shaft power because each stage loses energy. [5]
Engineering details — Transformation, switchgear & net export
The six losses are deducted sequentially from their own upstream inputs; they are not all percentages of the original resource. Net shore generation is positive for either aligned direction and zero at slack/interlock. A 1 MW net export cap and fixed efficiency factors are example assumptions. Standby imports, protection, fault current and an AC network solution are excluded, so ‘zero generation’ is not a claim of zero site consumption. [5]
One consistent calculation model
Assumptions: uniform aligned quasi-steady current; density 1025 kg/m³; radius 9 m; Cp = 0.40; tip-speed target 5; cut-in 0.5 m/s; maximum speed 3 m/s; shore export capped at 1 MW. All are illustrative, not specifications of a real turbine. Fixed stage efficiencies are 96%, 95%, 98%, 99%, 97% and 99% in the order below. No measured performance curve, hydrodynamic load, electrical transient, cable resistance or harmonic solution is implied. The power relationship is book-supported; these parameter values are not. [2]
- Presource = ½ρA|v|³; A = πR²
- ρ = water density (kg/m³); A = rotor swept area (m²); R = rotor radius (m); v = signed current (m/s). Resource power is the kinetic flux through that area, not all recoverable energy.
- Pshaft = min(Cp Presource, Prated,shaft)
- Cp = dimensionless capture coefficient; P = power (W). This equation is active only above cut-in and after yaw alignment. Rating limits captured power, not the available water resource.
- Pout,i = ηi Pin,i; Ploss,i = Pin,i − Pout,i
- ηi = each stage’s assumed efficiency; i identifies the sequential stage. Shore output is the final output, always nonnegative. Overall shaft-to-shore efficiency is Pshore/Pshaft where shaft power is positive.
- λ = ωrotor R/|v|; n = 60ω/(2π); τ = Pshaft/ωrotor
- λ = tip-speed ratio; ω = angular speed (rad/s); n = revolutions per minute (rpm); τ = shaft torque (N·m). Below rating, target λ sets speed. Above rating speed is capped. At slack λ is not defined; the parked shaft torque is set to zero.
- ngen = 8nrotor; s = 1 − ngen/nsync; fgen = pngen/[60(1−s)]
- ngen = generator rotor speed; nsync = synchronous-field speed; s = slip, here imposed −0.02; p = pole pairs, here 8 (16 total poles); fgen = stator fundamental frequency (Hz). The receiving grid remains selected 50 Hz. Slip is accounted for, not confused with a synchronous generator. [6]
- Current resource
- 2,037.740 kW
- Captured shaft power
- 815.096 kW
- SCIG fundamental
- 13.870 Hz
- Shaft-to-shore efficiency
- 84.97%
- Tip-speed ratio
- 5.000
- Effective capture coefficient
- 0.4000
| Term | Power (units shown) |
|---|---|
| Available kinetic resource | 2,037.740 kW |
| Resource not captured | 1,222.644 kW |
| Gearbox loss | 32.604 kW |
| Generator loss | 39.125 kW |
| Subsea cable loss | 14.867 kW |
| Onshore input TX / filter loss | 7.285 kW |
| Full converter / output filter loss | 21.636 kW |
| Grid transformer loss | 6.996 kW |
| Total six-stage loss | 122.513 kW |
| Shore-generated export | 692.583 kW |
| Resource − uncaptured − losses − shore | 0.000000 W |
‘Uncaptured’ is not an electrical heat loss: it includes resource left in the flow and imposed curtailment/interlock. Standby imports are excluded. Fixed losses do not imply fixed real plant efficiency at every current. No switching waveform is shown as a measured or validated electrical simulation.
Checked worked example — aligned flood at +2.5 m/s
Area π×9² = 254.469 m². Resource = ½×1025×254.469005×2.5³ = 2,037.740 kW; shaft = 0.40×resource = 815.096 kW. Rotor = 13.263 rpm; generator = 8×rotor = 106.103 rpm; imposed −2% slip gives 13.870 Hz.
Shore = shaft×0.96×0.95×0.98×0.99×0.97×0.99 = 692.583 kW. The six losses total 122.513 kW; uncaptured resource is 1,222.644 kW. Resource = uncaptured + losses + shore, with unrounded residual 0.000000 W. Rounded display terms can differ slightly. At aligned ebb −2.5 m/s, the same positive shore output is obtained; at slack it is zero. This is an invented operating point, not a manufacturer rating.
Tradeoffs & common misconceptions
Astronomical tidal forcing is predictable, but stream output is not constant and site current is not simply tide height. Marine access, seals, corrosion, subsea cable engineering, scour, environmental effects and converter maintenance are real constraints. Neither zero visible emissions nor a lack of a barrage means zero environmental impact. This page does not assert a universal cost advantage or a fully validated marine design. [1][2][3][5]
Check your understanding
References, access & review scope
Source passages checked . Original explanations, schematic and Blender equipment. Software and geometry checks are not independent engineering peer review, standards compliance or installation approval.
Main book references
- Roger H. Charlier & Charles W. Finkl. Ocean Energy: Tide and Tidal Power. Springer, 1st ed., 2009. DOI 10.1007/978-3-540-77932-2.
Actual lawful chapter 8 preview, pp.141–142, §§8.1–8.2 checked for stream/range and current-height distinctions. Remaining chapter/full book is subscription-only; inaccessible passages were not checked. Historical cost/resource projections are not adopted.
- Faisal Wani, Jianning Dong & Henk Polinder. Tidal Turbine Generators, in Amir Ebrahimi (ed.), Advances in Modelling and Control of Wind and Hydrogenerators. IntechOpen, 2020. DOI 10.5772/intechopen.90433.
Actual open peer-reviewed technical book chapter: §1 Equation1/TSR, §2/§2.1 geared versus direct drive, §3 sealed air-gap nacelles/seals/ingress checked. Its later flooded-PM design is not the SCIG shown here. Model parameter values are not taken from its indicative curves.
Supplementary technical sources
- ANDRITZ HYDRO Hammerfest. Renewable energy from tidal currents, historical brochure code 04.12.
Actual printed pp.6–7,9 checked for rotor, pitch/yaw/speed control, induction generator, foundations and subsea/onshore connection. Not current product specifications or evidence that our original model is a replica. Marketing/environmental absolutes and unsuitable tide-period wording are not adopted.
- EMEC. Tidal devices.
Actual introduction/horizontal-axis paragraph checked; publication date not stated. Accessed 3 October 2026.
- M. C. Sousounis, J. K. H. Shek & M. A. Mueller. Modelling and Control of Tidal Energy Conversion Systems with Long Distance Converters. PEMD2014. DOI 10.1049/cp.2014.0367.
Actual §2 p.2 and introduction checked for SCIG, three-phase cable, onshore adaptation/filter/VSC and grid interface; long cables can create harmonic and overvoltage problems. This steady-state lesson does not reproduce its time-domain control simulation.
- James L. Kirtley Jr. Analytic Design Evaluation of Induction Machines. MIT6.061 Class Notes Chapter10, ©2003, course2011.
Actual p.4 Equation12, p.6 Equations33–34 and p.8 §3 checked for frequency/slip relationships and generating operation. Fixed slip and total pole count in our example remain illustrative assumptions.
Download the editable tidal Blender scene · Compare geared wind conversion · Worked three-phase fundamentals · Practise reading single-line diagrams.
Inside the engineering
Follow the machinery, the working fluids and the electrical connection.
Mechanical
Three hydrofoil blades drive a low-speed carrier; visible 12/36/84 fixed-ring planetary gearing drives the sun/generator eight times faster with the same local sign. Fixed bearings, seals and stator remain supported inside a yawing pressure hull. A turret/tripod load path reaches gravity pads; a finite-yaw service loop, fixed J-tube and seabed cable preserve electrical continuity. Clearance checks do not establish marine load, pressure, bend-radius approval or fatigue capacity.
Thermal & chemical
The resource is moving water, not combustion or barrage head. Seawater corrosion, coatings, anodes, shaft seals and ingress remain maintenance concerns. A closed cooling concept rejects losses without admitting seawater to the selected dry air gap; temperatures and cooling capacity are not calculated. Real site current includes non-tidal contributions and need not be in phase with sea level.
Electrical
Variable-frequency three-phase SCIG output travels ashore to voltage adaptation/filtering, a generator-side VSC, paired capacitor-supported DC link, grid-side VSC/filter and transformer/switchgear/grid. An illustrative 16-total-pole, −2% slip assumption sets generator frequency; the grid is selected 50 Hz. One bounded power balance deducts six losses and caps shore generation at 1 MW; both aligned directions export nonnegative power. Slack/yaw interlock parks generation. Standby imports, switching/harmonics, cable ampacity/resonance, AC network and protection are uncalculated.
Main types and variations
The animation represents a seabed-mounted horizontal-axis tidal-stream turbine. Mounting method, rotor orientation and tidal-range generation are different design choices; the examples below explain how each changes the route from moving water to electricity.
Seabed-mounted tidal-stream turbine
A support structure on the seabed holds a submerged rotor in the current. The foundation transfers the rotor’s thrust into the seabed, while the nacelle houses the drivetrain. Pitch and, in some designs, yaw control help the turbine work through changing flow directions and speeds.
What changes in practice
A fixed foundation keeps the rotor positioned, but installation, retrieval and cable routing must all be planned around the underwater site and tidal conditions.
Floating tidal-stream turbine
A moored floating structure supports submerged rotors in the passing tidal current. On Orbital’s arrangement, hinged legs can lift the turbine assemblies to the surface for access. Moorings hold the platform on station, and an export cable connects the moving structure to the seabed connection.
What changes in practice
Surface access can simplify some maintenance tasks. The design must also accommodate platform movement, mooring loads and the flexible electrical connection.
Vertical-axis tidal-stream turbine
Blades rotate around a vertical shaft as tidal flow passes across the rotor. This changes the blade motion and drivetrain arrangement from the propeller-like horizontal-axis machine. It remains a current-energy device: no basin or dam is needed to create a difference in water level.
What changes in practice
Rotor orientation is a separate choice from whether the supporting structure floats or rests on the seabed. “Tidal turbine” does not identify one universal geometry.
Tidal barrage
A barrier across a tidal basin uses gates to control filling and emptying. As the sea level changes, a level difference develops across the barrier. Water then passes through turbines. Depending on the equipment and operating strategy, generation can use the outgoing tide, the incoming tide or both.
What changes in practice
The resource is a changing water-level difference, not just a fast current. Altered water levels, sediment and estuary habitats are central design considerations.
Tidal lagoon
A seawall encloses an area of water rather than necessarily spanning an entire estuary. Gates and turbines control exchange with the sea as the tide rises and falls. A difference between the enclosed and external water levels supplies the hydraulic head for generation.
What changes in practice
A lagoon is another tidal-range layout. Its output remains tied to tidal cycles, and the enclosing wall and local coastal effects require project-specific assessment.
Worth knowing
- Astronomical tidal forcing is predictable, but current, sea level and generated output are different quantities. Site measurements are needed; this signed-current scenario is not a forecast.
- Tidal barrages, like La Rance in France, instead trap water behind a dam.
- Wave energy converters capture the motion of surface waves rather than currents.
See real examples
The atlas includes 10 wave and tidal records worldwide.
Show on the mapRelated answers
Textbook references
Suggested technical background for this topic. These books are further reading, not proof that every explanation was checked against every edition. See the full bibliography and review scope.
- Ocean Energy: Tide and Tidal PowerTidal range and tidal stream technologies, history and projects.
- Sustainable Energy — Without the Hot AirHow much energy tides can realistically provide (free to read online).
Sources
- EIA: Tidal power ↗
- ANDRITZ Hydro Hammerfest: Tidal turbine drivetrain and control principles ↗
- Charlier & Finkl: actual chapter 8 tidal-current preview ↗
- Wani, Dong & Polinder: actual technical book chapter on tidal generators ↗
- University of Edinburgh: onshore long-distance tidal converter architecture ↗
- MIT: actual induction-machine frequency, slip and generation notes ↗
Cutaways reveal enclosed equipment. Layouts, dimensions, materials and animation speeds are illustrative; this is an educational model, not a plant design or a thermodynamic simulation.