Tidal energy is electricity generated from the natural rise and fall of ocean tides. Because tides follow predictable gravitational cycles, tidal power is one of the few renewable energy sources that can be scheduled with near-perfect accuracy years in advance. It remains a small slice of global electricity production, but operational plants have proven the concept works: the world’s largest tidal facility, South Korea’s Sihwa Lake plant, produces 254 megawatts of power, enough to supply a mid-sized city.
How Tides Create Energy
Tides exist because of a gravitational tug-of-war between the Earth, the moon, and the sun. The moon’s gravity pulls ocean water toward it on the side of Earth closest to the moon, creating a bulge of higher water. On the opposite side of the planet, the moon’s pull is weaker, and the water’s own momentum pushes it outward, forming a second bulge. The result is two high tides and two low tides roughly every 24 hours as Earth rotates through these bulges.
That constant movement of billions of tons of seawater holds enormous kinetic energy. Tidal power systems capture it in two basic ways: by trapping water behind a barrier and releasing it through turbines, or by placing turbines directly in a fast-moving tidal current. Water is about 800 times denser than air, which means even a slow-moving tidal flow carries far more energy than wind moving at the same speed.
Three Types of Tidal Power Systems
There are three main designs in use or development around the world, each suited to different coastal geography.
Tidal Barrages
A barrage is essentially a low dam built across a tidal basin, which is a bay or inlet that fills with water at high tide and drains at low tide. Sluice gates let water flow in as the tide rises, then close to trap it. When the tide drops, the stored water is released through turbines to generate electricity. More advanced two-way systems generate power on both the incoming and outgoing tide. The world’s first large-scale tidal plant, France’s La Rance station, is a barrage. It was inaugurated on November 26, 1966, took six years to build, and has operated continuously for nearly six decades.
Tidal Stream Turbines
These work like underwater wind turbines. Blades mounted on the seafloor spin as tidal currents flow past, turning a rotor connected to a generator. Because water is so much denser than air, the blades can be smaller than a wind turbine’s while capturing comparable energy. The trade-off is that the equipment must be far sturdier and heavier to withstand the force of moving seawater, which makes them more expensive to build.
Tidal Fences
A tidal fence is a row of vertical-axis turbines mounted across a channel on the seabed. Water passes through the fence and spins each turbine in the line. Think of it as a middle ground between a full barrage and individual stream turbines: it doesn’t block the entire waterway but captures energy across a wider area than a single turbine would.
Where Tidal Power Works
Not every coastline is a good candidate. Tidal barrages need a basin with a large tidal range, meaning a big difference between high and low water. Sites with ranges below about 5 meters generally don’t produce enough flow to justify the cost of a barrage. Stream turbines are more flexible but still need strong, consistent currents, typically found in narrow channels, straits, or around headlands where water is forced through a constriction.
The best-known high-tidal-range locations include the Bay of Fundy in Canada (tides exceeding 16 meters), the Severn Estuary in the UK, parts of the French Atlantic coast, and the west coast of South Korea. Scotland’s Pentland Firth, between the mainland and Orkney, has some of the strongest tidal currents in Europe and hosts several turbine test sites.
The Predictability Advantage
Solar panels produce nothing at night. Wind turbines sit idle on calm days. Tidal energy doesn’t have this problem. Tides follow the orbital mechanics of the moon and sun, so their timing and strength can be calculated with precision decades ahead. Grid operators can plan for tidal generation the way they plan for a gas plant’s scheduled output. This predictability is tidal energy’s single biggest selling point compared to other renewables, because it reduces the need for backup power or battery storage to fill unexpected gaps.
Cost Compared to Other Renewables
Tidal energy is still expensive relative to mature renewables. A recent lifecycle analysis of a European tidal farm estimated a levelized cost of electricity of about 0.125 euros per kilowatt-hour (roughly $0.13 USD). That’s below the European Commission’s 2025 target of 0.15 euros per kilowatt-hour, which is encouraging, but it’s still roughly two to three times the cost of new onshore wind or solar in most markets.
The high price reflects two realities. First, the technology is young. Only a handful of commercial-scale plants exist, so manufacturers haven’t achieved the economies of scale that drove wind and solar costs down over the past two decades. Second, building and maintaining equipment underwater in saltwater is inherently harder and more expensive than mounting panels on a rooftop. Proponents argue that costs will follow the same downward curve as offshore wind once deployment scales up, but that remains to be proven.
Saltwater Is Hard on Equipment
Corrosion and biofouling (the buildup of barnacles, algae, and other marine organisms on submerged surfaces) are persistent engineering headaches. Some developers build turbines from high-grade duplex or super duplex stainless steel, which resists corrosion but raises material costs. Others use cheaper steel or aluminum coated with anticorrosion paint. Antifouling coatings are typically copper-based, which raises environmental questions about leaching into surrounding water.
Newer approaches are being tested. The U.S. Department of Energy has funded research into laser surface modification, a process that alters the texture of metal surfaces at the microscopic level to resist both corrosion and biological growth without chemical coatings. Early tests on aluminum alloys have been promising enough that researchers are expanding the work to steel. If it pans out, it could reduce both the cost and environmental footprint of keeping tidal equipment functional.
Effects on Marine Life
The most common concern about tidal turbines is whether fish and marine mammals are injured by spinning blades. Lab testing offers some reassurance. In controlled flume studies using two different turbine designs, survival rates for fish passing through the turbines exceeded 98% for one design and 99% for the other. Injury rates among fish that passed through were low and generally comparable to control fish that weren’t exposed to turbines at all.
Video observations revealed that many fish actively avoid the turbines, swimming around them even when released just 25 centimeters upstream of the blade sweep. When released farther upstream, few if any fish passed through at all. The predicted threshold for blade-strike mortality was a current velocity of about 1.7 meters per second for one turbine type and 2.5 meters per second for another, and even above those speeds, survival remained above 90% for fish under 200 millimeters in length.
These results come with caveats. Most testing involves a single turbine. A commercial array with dozens of turbines in a channel could have cumulative effects on fish movements and migration routes that single-turbine studies can’t capture. Tidal barrages raise different concerns entirely: by altering the natural flow of an estuary, they can change sediment patterns, salinity levels, and habitat availability. France’s La Rance station underwent a full 20-year ecological evaluation that documented shifts in the basin’s ecosystem, though the estuary eventually reached a new biological equilibrium.
Current Scale and Outlook
Global tidal energy capacity is tiny compared to wind or solar. South Korea’s 254-megawatt Sihwa Lake plant and France’s La Rance station (the world’s second largest) account for the bulk of it. A scattering of smaller tidal stream projects operate in the UK, Canada, and China, mostly as demonstration or pre-commercial arrays.
The technology’s path forward likely depends on whether stream turbines can follow the cost trajectory of offshore wind. Barrages work but require massive upfront investment and face significant environmental permitting hurdles. Stream turbines and fences are modular, meaning they can be deployed incrementally, but they need to prove they can survive years of saltwater operation without prohibitive maintenance costs. For coastal regions with strong tidal resources and a need for reliable, predictable clean energy, tidal power offers something no other renewable can: a generation schedule you can set your clock to.