What if wind turbines went underwater?

The most reliable renewable-energy resource may be one hidden beneath the sea. In coastal waters where tides create powerful currents, underwater turbines can capture the energy of moving water and turn it into electricity. Known as tidal-stream power, the technology draws on an unusually dependable natural cycle. The moon and sun help drive tides according to astronomical patterns that can be calculated years ahead, meaning operators can anticipate when currents will strengthen, weaken, and therefore when their turbines will generate power.

Yet tidal-stream power remains almost invisible in the global energy system. Since 2010, just 43 megawatts of tidal-stream capacity have been deployed worldwide, according to the International Renewable Energy Agency (IRENA). Solar and wind, by comparison, added more than 660 gigawatts in 2025 alone. Several tidal designs can already generate electricity at full scale, but proving the machinery works is only the beginning. Developers still need to manufacture, install, and maintain enough turbines to bring down costs in an unforgiving marine environment. 

The industry is now preparing to test that model at a much larger scale. Europe has 172 MW of tidal projects comprising around 140 devices in its pre-commercial pipeline, according to Ocean Energy Europe's Ocean Energy Market Outlook 2025. If those projects can deliver the expected savings, tidal power could begin the transition from an expensive marine technology to a viable part of the renewable-energy mix.

A turbine with a timetable

Underwater turbines borrow some familiar ideas from wind power, but engineers have developed several ways of capturing the movement of the sea. The most advanced are “horizontal-axis turbines,” which work much like underwater wind turbines, with blades turned by passing currents.

Fixed to the seabed or suspended beneath floating platforms, several have reached technology readiness levels of eight or nine, the final stages before or during commercial operation, with individual devices ranging from 100 kilowatts to 2 MW, according to the European Commission Joint Research Centre (JRC) in its Ocean Energy in the European Union 2025.

IRENA's Renewable Power Generation Costs in 2025, published in July 2026, estimates that tidal turbines can generate for at least 20 hours a day. Combined with short-duration batteries, it says, they could provide round-the-clock renewable generation. While tidal electricity is not constant because output rises and falls with the strength of the current and briefly stops when the tide changes direction, operators can know those fluctuations well in advance.

The problem beneath the water

Generating electricity underwater also means putting expensive machinery somewhere that is unusually difficult to reach.

Saltwater, strong currents, and the repeated force of moving water place components under persistent stress. Repairs can require vessels, divers, or the retrieval of entire machines. The JRC identifies expensive and difficult maintenance as a major source of operating costs and warns that evidence on device lifetimes remains limited. Estimates commonly assume tidal machines will operate for 20 to 25 years, but such lifetimes have not yet been demonstrated across a mature commercial industry.

Much of the engineering challenge therefore lies beyond the turbine blades themselves. The Offshore Renewable Energy Catapult (ORE Catapult), in its Tidal Stream Technology Roadmap 2024, identifies 10 areas of technological innovation that could lower costs, including improved anchors, subsea electrical hubs, more reliable cable monitoring, and larger rotors. The roadmap estimates that successful commercialisation of the technologies it identifies could reduce tidal-stream costs by as much as 80 percent.

Building farms rather than individual machines could bring another source of savings. Dozens of turbines can share grid connections, offshore infrastructure and maintenance crews, while producing machines in larger numbers could replace bespoke equipment with standardised components, according to IRENA.

The $358 question

While the technology is advancing, cost remains the most formidable obstacle.

IRENA estimates the current levelised cost of tidal-stream electricity at approximately $358 per megawatt-hour in 2025. The figure captures the cost of producing electricity over a project's lifetime, including the turbine, electrical infrastructure, and installation.

The comparison with established renewable technologies is stark. In 2025, global average generation costs were $33/MWh for onshore wind, $44 for solar, and $78 for offshore wind, according to IRENA. More than 90 percent of newly commissioned utility-scale renewable capacity produced electricity more cheaply than the cheapest new fossil-fuel alternative. 

Tidal power is therefore entering an energy market transformed by the extraordinary cost declines of the technologies against which it must compete.

Its developers are betting that deployment itself can narrow the gap. IRENA projects that tidal-stream costs could fall to around $140/MWh if the industry expands to about 2 GW of installed capacity, roughly 47 times the tidal-stream capacity deployed worldwide since 2010.

From machines to farms

The immediate test will come not from another prototype but from building many machines together.

Europe's planned tidal farms account for most of its ocean-energy pipeline through 2030, while China has set a 400 MW target. Projects are also advancing in Canada and the United States as the industry moves toward larger farms. But tidal power can operate economically only where currents are sufficiently strong, limiting its potential locations. Even Europe, the centre of tidal-stream development, added just 0.5 MW of ocean-energy capacity in 2024, according to the JRC.

The technology has nonetheless reached an important transition. Engineers have already demonstrated that moving seawater can generate electricity reliably. The harder task is making the machines ordinary enough to manufacture by the hundred, durable enough to remain underwater for years and cheap enough that the predictability of the tides becomes an economic advantage rather than an engineering curiosity.