A tidal turbine works much like an underwater wind turbine. Tidal currents flow past its blades, the blades spin a rotor, and a generator turns that rotation into electricity, which travels to shore through a subsea cable. Because seawater is about 800 times denser than air, a tidal turbine can produce a lot of power from slow currents with a relatively small rotor. And because tides follow the Moon, its output can be predicted years ahead.
Key takeaways
- Tidal turbines capture kinetic energy from moving water. They don't need a dam.
- Power rises with the cube of current speed, so the best sites have very fast currents, typically above about 2.5 m/s at peak.
- Main designs include seabed-mounted and floating horizontal-axis turbines, plus vertical-axis, ducted and "kite" designs.
- Turbines handle reversing tides by rotating, flipping blade pitch or using blades that work in both directions.
- Output is variable through each tide cycle, but highly predictable.
Tidal stream vs. tidal barrage
There are two broad ways to make electricity from tides:
| Approach | How it works | Pros | Cons |
|---|---|---|---|
| Tidal barrage or lagoon | A dam traps water at high tide and releases it through turbines | Large output, proven | Major ecosystem effects, high cost, few suitable sites |
| Tidal stream turbines | Turbines sit in a natural current, like wind turbines underwater | Smaller footprint, no dam, modular | Harsh environment, still costly, early-stage industry |
Most new tidal development today, including in Canada's Bay of Fundy, uses tidal stream turbines. Our post on tidal energy in the Bay of Fundy explains why the region moved away from the barrage approach.
The physics: why water packs a punch
The power available in a flowing fluid depends on three things: the fluid's density, the area swept by the rotor, and the cube of the flow speed.
Power ≈ ½ × density × swept area × speed³
Plug in some numbers for a single square metre of rotor area:
- Wind at 10 m/s (about 36 km/h): roughly 0.6 kW
- Tidal current at 2 m/s: roughly 4 kW
- Tidal current at 3 m/s: roughly 14 kW
Two lessons stand out. First, density matters: slow-moving water carries far more energy than fast-moving air. Second, speed matters enormously: going from 2 to 3 m/s more than triples the available power. That's why developers seek out narrow channels and passages where coastlines squeeze the tide into fast currents.
No turbine can capture all of it. Physics caps extraction from an open flow at about 59% (the Betz limit), and real turbines capture less after blade, gearbox and generator losses.
The main turbine designs
Seabed-mounted horizontal-axis turbines
These look like wind turbines on their side, mounted on a tripod, monopile or gravity base on the seabed. The rotor faces into the current. They're out of sight and away from surface storms, but installation and maintenance require specialized vessels and short windows of slack water.
Floating turbines
Here, turbines hang below a floating hull or platform held in place by moorings. The rotors sit in the fast-flowing water near the surface, and the whole unit can be towed to port for maintenance, which can lower costs. The trade-offs are exposure to waves and weather, plus visibility on the surface.
Vertical-axis (cross-flow) turbines
Blades rotate around a vertical shaft, like an eggbeater. They accept flow from any direction and can suit shallow or wide channels, though they're generally less efficient than horizontal-axis designs.
Ducted and open-centre turbines
A shroud or duct around the rotor channels and speeds up the flow. Some designs use an open centre, which developers say helps marine life pass through.
Tidal kites and oscillating hydrofoils
A tidal "kite" is a winged turbine tethered to the seabed that flies figure-eight patterns through the current, moving faster than the water itself. Oscillating hydrofoils move up and down like a fish tail. Both aim to capture energy from slower currents.
Coping with reversing tides
Tides flow in for roughly six hours and out for about six, with brief slack water in between. Turbines handle this reversal in a few ways:
- Yawing: the whole nacelle rotates 180° to face the new flow direction.
- Pitching: blades rotate on their own axes to work in the reverse direction.
- Bidirectional blades: symmetrical blades produce power in either direction without moving.
- Floating alignment: some moored platforms swing naturally with the current.
From rotor to grid
Tidal rotors turn slowly, often in the range of 10 to 20 revolutions per minute. A gearbox or a direct-drive generator converts this into electricity. Power electronics condition the output, and subsea export cables carry it to an onshore substation that connects to the grid. Because output rises and falls with each tide, arrays are sometimes paired with batteries or spread across sites whose tides peak at different times.
Engineering and environmental challenges
- Extreme loads. Fast currents and turbulence put enormous stress on blades and foundations.
- Corrosion and biofouling. Saltwater corrodes metal, and marine growth builds up on surfaces.
- Installation and access. Work often has to fit into short slack-water windows.
- Marine life. Developers and regulators monitor how fish and marine mammals behave around rotors, using sonar, cameras and acoustic tags. Slow rotor speeds help, but long-term data is still building.
- Cost. Tidal power currently costs more per kilowatt-hour than mature renewables, though costs fall as projects scale.
Where tidal turbines fit
Tidal stream power is a small but promising part of the clean energy mix. Its biggest advantage is predictability: grid operators know exactly when output will peak, years in advance. That complements variable wind and solar power, and suits coastal regions with strong currents. In Canada, the Bay of Fundy and some B.C. channels are the main candidates. See our ocean energy hub for the full picture, including wave energy on the Pacific coast.
Frequently asked questions
Do tidal turbines harm fish?
Studies at test sites so far suggest collisions are infrequent, partly because rotors turn slowly and animals tend to avoid them. Long-term monitoring at larger arrays is still needed, which is why projects are usually deployed in stages.
How much power does one tidal turbine produce?
Commercial-scale turbines are typically rated from several hundred kilowatts to a few megawatts each. Actual output varies through each tide cycle and depends on current speed.
Are tidal turbines the same as hydroelectric dams?
No. Conventional hydro uses water stored behind a dam and released through turbines. Tidal stream turbines sit in open currents without a dam, more like underwater wind turbines.
The bottom line
Tidal turbines turn the steady, predictable movement of the tides into electricity using the same basic idea as wind turbines, in a much denser and harsher medium. The physics is compelling and the designs are maturing. The remaining work is making them durable, affordable and proven safe for marine life at scale.
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