Underwater turbines extract energy from moving water, whether driven by tides, ocean currents, or river flow, and convert it into electricity using submerged rotors that work on the same basic principle as a wind turbine. The technology is less mature than offshore wind, but it carries a distinct advantage: tidal flows are predictable decades in advance, which makes power output far easier to forecast than wind or solar. Several commercial-scale projects are already operating, and costs have begun dropping toward levels that could compete with other renewables. The engineering challenges, though, are real and range from barnacles on blades to seals avoiding the noise.
How Underwater Turbines Work
The core idea is simple. Water flows past a rotor, the rotor spins, and a generator converts that rotation into electricity. Because water is roughly 800 times denser than air, a turbine sitting in a tidal channel can harvest far more energy per swept area than a wind turbine of the same size. That density advantage means underwater turbines can be compact, typically with rotor diameters between about 10 and 25 meters, yet still produce meaningful power.
Designs split broadly into horizontal-axis and vertical-axis types. Horizontal-axis turbines look like underwater windmills, with blades spinning around a shaft that points into the current. Vertical-axis machines, especially straight-bladed Darrieus designs, spin around an upright shaft and can accept flow from any direction without needing to yaw. Both families remain active areas of research and development, and many engineering teams are pursuing designs that could serve in both tidal and river environments rather than being locked into one setting.
Tidal-stream turbines and ocean-current turbines share operational principles but differ in application. Tidal devices sit in channels or straits where the tide accelerates between land masses, producing strong, reversing currents. Ocean-current devices would tap into steadier, slower-moving flows like the Gulf Stream. The tidal side of the industry is much further along commercially, partly because those high-velocity sites pack more energy into a smaller footprint.
What Biofouling Does to Performance
Any structure placed in the sea becomes a surface for marine organisms to colonize. Barnacles, mussels, algae, and biofilms all accumulate on turbine blades, and this fouling changes the blade’s hydrodynamic profile in ways that matter. A study modeling the effects of surface fouling on tidal turbine blades found that it reduced the power coefficient by up to 13% at the designed operating condition, with even steeper losses at higher tip-speed ratios and a narrower usable range of operating speeds.1Applied Energy. Prediction of the fouling penalty on the tidal turbine performance and development of its mitigation measures The researchers noted that adjusting the turbine to run at a slightly lower speed between maintenance visits could minimize those efficiency losses.
Blade roughness, which can come from fouling residue or from erosion by sediment-laden water, has its own distinct effects. In experimental tests, artificially roughened blades saw a significant drop in peak performance, with the maximum power coefficient falling from 0.42 for clean blades to 0.34 for the roughened set.2Renewable Energy. Experimental and numerical studies of blade roughness and fouling on marine current turbine performance Interestingly, the same study found that when actual biological fouling organisms were attached to the blades, most of them sheared off under operating flow speeds. The bits that stayed behind actually delayed stall to higher angles of attack, momentarily helping performance. That quirk does not translate into a practical strategy, but it illustrates how differently fouling behaves depending on whether the problem is living organisms or the rough texture they leave behind.
Arranging Turbines in Arrays
A single turbine is a proof of concept. Commercial power comes from arrays of many devices, and how you arrange them matters a great deal. Each turbine creates a wake of slower, more turbulent water behind it, and the downstream machines have to contend with that degraded flow. Experimental studies of tidal turbine arrays have shown that the lateral spacing between devices, meaning how far apart they are across the channel, has a pronounced effect on how quickly the wake recovers. Well-chosen lateral distances can actually accelerate wake recovery through beneficial flow interactions, but packing turbines too tightly across the channel leads to significantly reduced velocity recovery downstream.3Renewable Energy. Experimental study of wake characteristics in tidal turbine arrays
Longitudinal spacing, the distance between rows in the direction of flow, turned out to be less influential than you might expect, especially when the turbines were already spread wide laterally. Differences in wake velocity deficit of up to 10% were observed across various configurations, with the front rows of a staggered array being most sensitive to lateral arrangement.4Renewable Energy. Experimental study of wake characteristics in tidal turbine arrays The takeaway for developers is that the geometry of an array is not just about fitting as many turbines as possible into a site. It is an optimization problem where marginal gains in spacing translate into meaningfully different energy yields.
Effects on Marine Mammals
One of the most scrutinized questions about underwater turbines is whether they pose a collision risk to marine mammals. The evidence so far is more reassuring than early fears suggested, largely because animals appear to avoid the devices. A study tracking seal movements around an operating tidal turbine array found that seals showed overt avoidance during turbine operations, with predicted seal abundance dropping by roughly 11% to 49% within about two kilometers of the array.5Renewable Energy. Quantifying the effects of tidal turbine array operations on the distribution of marine mammals: Implications for collision risk Collision risk models had traditionally assumed some arbitrary degree of avoidance, or none at all. The measured avoidance response suggests the actual annual collision-related mortality could be reduced to a fraction of a percent to around 2% of the local seal population, far below worst-case projections.
A separate experiment tested whether the avoidance was driven specifically by the sound turbines produce. Researchers played back recorded turbine noise underwater near harbour seals and tracked their response. The overall number of seals in the broader channel did not change during playback, but seals significantly avoided the area around the sound source, with usage declining by 11% to 41% at the playback location and measurable avoidance extending out to 500 meters.6Journal of Applied Ecology. Harbour seals avoid tidal turbine noise: Implications for collision risk The implication is that turbine sound acts as an acoustic warning, giving animals a cue to steer clear before they get close to the rotor. Whether this avoidance behavior holds for all marine mammal species, or whether animals habituate over time, remains an open question, but the early data points toward a built-in safety mechanism that regulators had not counted on.
Underwater Noise and Its Reach
Turbine noise matters beyond just its role as a collision deterrent. Continuous low-frequency sound can mask the natural acoustic signals that marine animals depend on for communication, navigation, and foraging. Assessments of a single operating tidal turbine found that physiological injury to the hearing systems of fish, invertebrates, and marine mammals was improbable within the affected area. Behavioral disturbance from a single device was considered possible for harbour porpoises only, and only within about one kilometer.7PubMed. Underwater operational noise level emitted by a tidal current turbine and its potential impact on marine fauna For a solitary turbine, that radius of influence was deemed a minor concern.
The picture gets more complex when you think about arrays of many turbines or about masking effects rather than outright hearing damage. Modeling of how turbine noise reduces the “listening space” of nearby animals, meaning the distance over which they can detect natural sounds, showed that harbour seals experienced the greater impact. Average listening-space reductions exceeded 90% within about 62 meters of a turbine for seals, compared to a maximum average reduction of 71% within 10 meters for harbour porpoises.8Renewable and Sustainable Energy Reviews. Providing ecological context to anthropogenic subsea noise: Assessing listening space reductions of marine mammals from tidal energy devices The masking effect also varied by season and ambient sound conditions. In a noisy environment with waves and shipping traffic, the turbine’s additional contribution to masking is proportionally smaller than in a quiet area. Array-scale noise assessments, where dozens of turbines contribute simultaneously, are still limited, and this gap is one of the bigger uncertainties in environmental permitting.
Sediment and Seabed Changes
Extracting kinetic energy from a current slows the flow, which can alter how sediment moves along the seabed. This is a legitimate concern in areas with mobile sand waves or near estuaries where sediment transport matters for coastal morphology. Modeling of tidal turbine farms in the Pentland Firth, one of the world’s premier tidal energy sites, found that the changes in bed level caused by four operating farms were small: less than 0.2 meters over a modeled lunar month, with root mean square differences of 0.18 meters in the sand wave area and 0.03 meters at another nearby feature.9Renewable Energy. The cumulative impact of tidal stream turbine arrays on sediment transport in the Pentland Firth Compared to natural bed level changes of up to 5 meters in the same area, the turbine-induced shifts were considered insignificant.
Other modeling work paints a more nuanced picture depending on the site. Simulations of a tidal farm in the Alderney Race between France and the Channel Islands confirmed that energy extraction tends to reduce the bedload transport rate and deflect sediment pathways. In that case, the location of the farm within the channel substantially changed where suspended sediment eventually settled, shifting the balance between deposition in the eastern versus western English Channel.10Renewable Energy. Numerical modeling of the effect of tidal stream turbines on the hydrodynamics and the sediment transport – Application to the Alderney Race Raz Blanchard, France A broader study of tidal systems found that energy extracted from regions with strong tidal asymmetry, where flood and ebb currents differ significantly, led to a 20% increase in the magnitude of average bed level change across an entire estuary compared to extraction from more symmetrical sites. Regardless of location, though, energy extraction generally reduced overall bed level changes relative to no-extraction scenarios.11Renewable Energy. The impact of tidal stream turbines on large-scale sediment dynamics
The lesson is that sediment impacts are site-specific. A farm in a high-energy channel with mostly rocky seabed may have negligible sediment effects, while a farm near a sandy estuary needs careful modeling before construction.
River Turbines and Their Own Set of Problems
Most of the commercial momentum in underwater turbines involves tidal sites, but there is a parallel effort to develop hydrokinetic turbines for rivers. These devices are especially appealing in remote or developing regions where dam construction is impractical or environmentally undesirable. A river turbine can sit in the flow without any impoundment, generating power from the current alone.
The practical challenges differ from tidal settings. River sites with the fastest water, which is what you want for energy density, tend to be shallow, making it hard to fit a turbine with enough swept area to be worthwhile. High-velocity zones also tend to occur on the outside of bends where the flow is highly non-uniform and positioning a device is difficult. And then there is debris. Floating and submerged logs, branches, and vegetation are a constant threat that can damage blades, foul rotors, or block intake areas entirely.12Energy for Sustainable Development. Towards more cost-effective river hydrokinetic turbines Designing a turbine that can handle the occasional collision with a waterlogged tree trunk is a very different engineering problem from surviving the clean, fast water of a tidal strait.
Power Fluctuations from Waves
Tidal turbines sit in currents, but they also experience surface waves, and the interaction between wave motion and current flow creates oscillating loads on the rotor. These fluctuations affect both the structural fatigue life of the turbine and the quality of the electrical output. Experimental work on horizontal-axis tidal turbines operating in combined wave-current environments has identified dominant power fluctuation frequencies tied to the wave period, the rotor rotation rate, and harmonics generated by the interaction of the two.13Physics of Fluids. Experimental investigation of wake dynamics and power fluctuations of a horizontal-axis tidal stream turbine under co- and counter-directional wave-current environment Waves arriving from the same direction as the current affect the turbine differently than waves running against the current, and both cases produce different load signatures than current alone. Understanding these patterns is essential for designing turbine blades that survive 20 or more years in service and for smoothing the electrical signal before it reaches the grid.
What Electricity from Underwater Turbines Costs
Cost is the central question for any energy technology, and underwater turbines have historically been expensive relative to offshore wind or onshore solar. The levelized cost of electricity, which accounts for capital spending, maintenance, and total energy produced over a project’s lifetime, varies widely depending on the device, the site, and the scale of the array. A lifecycle and economic assessment of a second-generation tidal device found an LCOE of 0.125 euros per kilowatt-hour, a figure consistent with European Commission projections for the technology.14Heliyon. Life cycle and economic assessment of tidal energy farms in early design phases: Application to a second-generation tidal device That sits above the cost of mature offshore wind but is approaching a range where it could be competitive in specific grid contexts, especially islands or coastal regions with limited alternatives and strong tidal resources.
One route to bringing costs down is to share infrastructure with offshore wind. Analysis of co-locating wind turbines at tidal array sites, specifically the MeyGen project in the Pentland Firth, showed that adding wind capacity reduced the overall cost of electricity compared to operating tidal turbines alone, by increasing total energy yield while splitting the expense of grid connection and support structures.15Renewable and Sustainable Energy Reviews. Co-located deployment of offshore wind turbines with tidal stream turbine arrays for improved cost of electricity generation The complementarity is not just financial: tidal power fills in during calm periods when wind output drops, smoothing the combined generation profile. Researchers have also explored fully integrated floating platforms that mount both a wind turbine above the waterline and tidal turbines below it, though that concept introduces new stability challenges. Simulations of one such system found that when one of the submerged turbines experienced a controller fault, platform yaw increased by up to about 163% relative to normal operation, raising concerns about structural loading on moorings even though overall mooring tensions remained within safe margins.16Energy. Stability and power performance of a floating wind-current energy system under tidal turbine operating mode variations
Biomimetic Alternatives to Spinning Blades
Not every underwater energy harvester needs to be a spinning rotor. A growing body of research explores flapping-foil devices that mimic the oscillating motion of a fish tail or a bird wing to extract energy from flowing water. These flapping-foil energy harvesters use a hydrofoil that pitches and heaves in the current, and the periodic motion drives a generator. A comprehensive review synthesizing four decades of work on these devices reports peak hydrodynamic efficiencies of around 40% for systems with prescribed motion, rising above 50% for flexible or actively cambered foils.17Applied Ocean Research. Flapping-foil energy harvesters: Principles, performance, and prospects
Why bother with flapping foils when conventional rotors work? A few potential advantages stand out. Flapping foils sweep a rectangular cross-section of the flow rather than a circle, which means they can operate efficiently in shallow water where a large-diameter rotor would not fit. They also move at lower tip speeds than rotary blades, which could reduce the risk to marine life and lessen cavitation problems. Computational studies have found that adding structural flexibility to the foil, inspired by the compliant fins of fish, boosts efficiency further. At certain operating conditions, a flexible wing generated nearly 8% higher efficiency than a rigid one, and at very low angles of attack the flexible version extracted roughly six times more energy.18Bioinspiration & Biomimetics. A bio-inspired study on tidal energy extraction with flexible flapping wings These devices remain largely at the laboratory and prototype stage, but they hint at a future where underwater energy harvesting looks less like an industrial turbine farm and more like a field of slowly waving fins.
Sharing the Sea with Fishers and Wildlife
Finding a good site for tidal turbines is only part of the challenge. Strong tidal channels are also productive fishing grounds, shipping lanes, recreation areas, and wildlife habitats, and every stakeholder has a claim. Marine spatial planning workshops held around potential tidal sites in Scotland revealed the tensions plainly. Fishing representatives pointed out that negotiations seemed to assume tidal energy would take priority over their livelihoods, while tourism stakeholders observed that the mere threat of losing access caused everyone to exaggerate how much they relied on the area.19PLoS ONE. Interactive Marine Spatial Planning: Siting Tidal Energy Arrays around the Mull of Kintyre Suggestions to move devices further offshore to reduce conflict ran into the practical objection that deeper water raises installation costs.
Systematic spatial planning approaches that account for multiple uses simultaneously have shown some promising paths forward. Modeling that incorporated goals for biodiversity conservation, two types of renewable energy, and three types of fishing found non-linear trade-offs between industries. Allowing certain types of fishing to continue within renewable energy zones, a co-location strategy, significantly reduced costs to the fishing industry, including for fisheries that were not directly co-located. Co-location also changed the optimal placement of the energy zones themselves, meaning that planning in isolation produces different and worse outcomes for everyone.20PubMed. Ocean zoning for conservation, fisheries and marine renewable energy: assessing trade-offs and co-location opportunities The research supports what seems obvious in retrospect: you get better results when you plan the whole ocean together rather than allocating space to one use at a time.
Cavitation and the Limits of Speed
When a turbine blade moves fast enough through water, the local pressure on the blade surface can drop below the vapor pressure of seawater, causing tiny bubbles to form and then violently collapse. This is cavitation, and it erodes blade surfaces, generates noise, and reduces efficiency. For underwater turbines, cavitation becomes a concern at high tip-speed ratios, particularly near blade tips where velocity is greatest. Experimental visualization of cavitation patterns on horizontal-axis tidal turbines has linked specific types of cavitation to drops in performance and increases in radiated noise.21Journal of Ocean Engineering and Marine Energy. Impact of cavitation and inflow perturbation on the performance of a horizontal-axis tidal turbine Blade designers manage this by controlling the rotor speed relative to the inflow velocity and by shaping blade profiles to keep pressure distributions above the cavitation threshold across the operating range. It is one more constraint that separates underwater turbine engineering from the superficially similar problem of building a wind turbine.