A tidal turbine converts the kinetic energy of moving ocean water into electricity, using a process nearly identical to how a wind turbine captures energy from air. Water flows over the turbine’s blades, spinning them. That rotation drives a shaft connected to a generator, which produces electrical current. The key advantage over wind is that water is roughly 800 times denser than air, so a tidal turbine can generate significant power from a relatively small rotor at much lower flow speeds.
From Moving Water to Electricity
The conversion happens in three stages. First, tidal currents push against angled blades, creating lift and drag forces that cause the rotor to spin. This converts kinetic energy (the movement of water) into mechanical energy (the rotation of a shaft). Second, that spinning shaft feeds into a gearbox, which increases the rotational speed to a range suitable for the generator. Third, the generator converts the mechanical rotation into electrical energy, the same basic principle behind nearly every power plant on Earth.
Some newer designs skip the gearbox entirely, using direct-drive generators that can operate at the slower speeds a tidal rotor produces. Eliminating the gearbox removes a major point of mechanical wear, which matters a great deal when your equipment sits underwater and is expensive to access.
Horizontal vs. Vertical Axis Designs
Most tidal turbines in development or deployment use a horizontal axis, meaning the rotor shaft runs parallel to the direction of water flow, like a traditional wind turbine laid on its side. These are the more efficient option, extracting more energy per unit of rotor area.
Vertical axis turbines spin around an upright shaft, like a revolving door. Their main advantages are simplicity and omnidirectionality: they capture water flowing from any direction without needing to reorient themselves. That matters in locations where tidal currents don’t flow in a perfectly straight line. They’re also cheaper and easier to build. The trade-off is lower efficiency and difficulty self-starting, meaning they sometimes need an external push to begin spinning. Blade-pitching systems can help with both problems, but they add mechanical complexity.
Vertical axis designs also pair well with floating platforms, because the generator and gearbox can sit above the waterline where technicians can actually reach them. Vertical axis turbines perform reasonably well even in skewed or angled flows, which makes them forgiving of the motion a floating platform introduces.
How They’re Anchored to the Seabed
Tidal turbines need a stable base in an environment with powerful, constantly reversing currents. The foundation type depends mostly on water depth and seabed conditions.
- Gravity bases are heavy concrete or steel structures that sit on the seabed and hold the turbine in place through sheer weight. They’re straightforward to install but massive, and the seabed needs to be prepared (leveled) beforehand.
- Monopiles are single steel tubes driven into the seabed, commonly used in waters up to about 30 meters deep. They’re simple and relatively inexpensive but require heavy piling equipment and suitable geology.
- Jacket foundations use a lattice steel frame, offering good stability in deeper water (up to around 60 meters) at higher cost and installation complexity.
- Floating platforms are moored to the seabed with cables and anchors, suited for depths beyond 50 meters. They allow access to stronger currents found further from shore, though they introduce more movement and are more expensive.
Efficiency and the Betz Limit
No turbine, whether in water or air, can capture all the kinetic energy passing through it. If it did, the water would stop completely behind the blades and block incoming flow. The theoretical ceiling, known as the Betz limit, caps the power extraction ratio at about 59.3% (16/27) of the energy in the flow. The actual conversion efficiency of individual turbines maxes out at around two-thirds of the incoming energy.
Interestingly, tidal turbines can actually exceed this limit under specific conditions. When turbines are packed closely together in a channel, they partially block the water’s path, forcing it to accelerate through and around them. If turbines fill a large enough fraction of the channel’s cross-section (above about 75%), the effective conversion efficiency can approach nearly 100% of what the flow delivers to the rotor. Array designers exploit this by staggering turbines in rows, using the accelerated flow between adjacent turbines to boost overall output.
Getting Power to Shore
A single tidal turbine generates electricity underwater, but that power needs to reach the grid onshore. For arrays of multiple turbines, engineers use one of two main wiring layouts.
In a hub-and-spoke arrangement, each turbine connects via a short cable to a central hub nearby. That hub aggregates output from several devices and sends it to shore through a single export cable. In a daisy-chain arrangement, turbines connect to each other in series, like Christmas lights, with the combined output leaving through one cable at the end of the chain. This mirrors how offshore wind farms are typically wired.
A collector substation, either a fixed platform or a subsea structure, handles voltage conversion. A single substation might connect up to 32 individual turbines. Transformers step the voltage up for efficient long-distance transmission through a subsea cable to shore. Some experimental designs use medium-voltage direct current for the underwater transmission, then convert to alternating current at the substation before connecting to the grid.
Surviving Saltwater
Anything submerged in the ocean faces two relentless enemies: corrosion and biofouling (the buildup of barnacles, algae, and other marine organisms on surfaces). Biofouling on turbine blades changes their shape and roughness, reducing efficiency. On mechanical seals and joints, it can cause failures.
Prevention strategies range from traditional antifouling coatings (similar to what’s used on ship hulls) to newer biomimetic approaches that mimic natural surfaces hostile to biological attachment. Some coatings use micro-scale textures inspired by shark skin or lotus leaves, creating physical barriers that make it difficult for organisms to grip. The industry is also moving toward degradable and eco-friendly antifouling materials, since the older chemical-based coatings can leach toxins into the water.
Corrosion protection typically involves cathodic protection systems (sacrificial metal anodes that corrode instead of the turbine structure) combined with marine-grade coatings and stainless or duplex steel alloys for critical components.
Effects on Marine Life
The primary concern is whether fish and marine mammals collide with spinning blades. Collision risk depends on how many animals naturally use the tidal site, how they dive and move through the water column, and whether they detect and avoid the turbines.
Research on harbour seals exposed to simulated tidal turbine sound found that seals exhibited significant spatial avoidance, reducing their use of the area around the sound source by 11% to 41% at the playback location. This avoidance effect was still measurable up to 500 meters away, where usage dropped by 1% to 9%. The overall number of seals in the broader channel didn’t change, suggesting the animals rerouted rather than left the area entirely. These findings indicate that at least some marine mammals hear the turbines and steer clear, which reduces (but doesn’t eliminate) collision risk.
The noise itself is another consideration. Turbines produce low-frequency sound from blade rotation and mechanical vibration. While the seal study showed avoidance rather than distress, long-term displacement from important habitat could be a concern if turbine arrays grow large enough to occupy significant portions of a channel.