Table of Contents

What Is Tidal Power?

Tidal power is a form of renewable energy that converts the kinetic and potential energy of ocean tides into electricity. Tides are caused by the gravitational interaction between Earth, the moon, and the sun, making tidal power one of the few energy sources driven by astronomical forces rather than solar radiation — and the most predictable renewable energy source available.

The Moon Is an Energy Source (Sort Of)

Every 12 hours and 25 minutes, the ocean rises and falls. Billions of tons of seawater flow in and out of bays, estuaries, and coastlines with clockwork precision. This has been happening for 4.5 billion years. And for most of that time, all that energy was wasted — absorbed by friction against the seabed and shorelines, slowly (very slowly) pushing the moon away from Earth and slowing Earth’s rotation.

The idea of capturing some of that energy isn’t new. Tide mills — grain mills powered by trapped tidal water — were used along the coasts of Europe as early as the 8th century CE. One on the River Fleet in London was recorded in the Domesday Book of 1086. These weren’t fancy. Dam a tidal inlet, let the rising tide fill it, close a gate, then release the water through a mill wheel as the tide goes out. Simple, reliable, and entirely dependent on geography.

Modern tidal power takes the same basic principle and scales it up dramatically. But the geography problem hasn’t gone away. Not every coast has strong enough tides to make power generation worthwhile, which is why tidal power — despite being proven, predictable, and emissions-free — produces only about 0.02% of global electricity.

Where Tides Come From — The Physics

Understanding tidal power requires understanding tides, and tides are stranger than most people realize.

The basic mechanism: the moon’s gravity pulls harder on the side of Earth facing it than on the far side (because gravity weakens with distance). This creates a bulge of water on the near side. But there’s also a bulge on the far side — because the far side is being pulled less than the Earth’s center, so inertia carries it outward. Result: two tidal bulges on opposite sides of Earth, with troughs in between.

As Earth rotates under these bulges, coastlines experience two high tides and two low tides roughly every 24 hours and 50 minutes (the extra 50 minutes accounts for the moon’s own orbital motion).

The sun creates its own tidal bulges — about 46% as strong as the moon’s, despite being vastly more massive, because it’s so much farther away. When the sun and moon align (new and full moons), their effects combine to produce spring tides — unusually high highs and low lows. When they’re at right angles (quarter moons), the effects partially cancel, producing neap tides — modest tidal ranges.

The actual tidal range at any given location depends heavily on coastal geography. Open ocean tides are modest — about 0.5-1 meter. But funnel-shaped bays and estuaries amplify tides dramatically. The Bay of Fundy in Nova Scotia holds the record at about 16 meters (53 feet) — a five-story building’s worth of water rising and falling twice daily. The Severn Estuary in the UK sees about 14 meters. These extreme tidal ranges make the best sites for tidal power.

Three Ways to Capture Tidal Energy

Tidal Barrages — Damming the Tide

A tidal barrage is essentially a dam across a tidal estuary or inlet. Sluice gates allow water to fill the basin on the incoming tide, then close. As the tide drops, the trapped water is released through turbines, generating electricity.

The concept is proven. The La Rance Tidal Power Station in Brittany, France, has been operating since 1966 — nearly 60 years. Its 24 turbines span a 750-meter barrage across the Rance estuary, generating 240 MW of capacity and about 540 GWh per year. That’s enough to power roughly 130,000 homes.

The Sihwa Lake Tidal Power Station in South Korea, completed in 2011, generates 254 MW — slightly more capacity than La Rance. It was built into an existing seawall, reducing construction costs.

Barrages work, but they have significant drawbacks:

Cost. Building a dam across an estuary is enormously expensive. The proposed Severn Barrage in the UK — a 16-km structure across the Severn Estuary — was estimated at $30-40 billion. The project has been studied and shelved repeatedly since the 1920s.

Environmental impact. Barrages fundamentally alter estuary ecosystems. They change water flow patterns, sediment transport, salinity levels, and tidal range upstream. Fish migration is disrupted — both species that swim upstream to spawn (like salmon) and those that depend on tidal flats for feeding (many shorebirds). The La Rance barrage caused significant ecological changes, though the estuary has partially adapted over decades.

Limited sites. Very few estuaries have the right combination of high tidal range, suitable geology, and acceptable environmental trade-offs.

Tidal Stream Generators — Underwater Wind Turbines

Tidal stream generators work like underwater wind turbines — they sit on the seabed in areas of strong tidal current and spin as water flows past. No dam required.

This is the fastest-growing segment of tidal power. The technology borrows heavily from wind turbine engineering but faces different challenges — seawater is about 800 times denser than air, so a tidal turbine can generate equivalent power with much smaller blades and lower flow speeds. A tidal stream of 3 m/s (about 6 knots) carries as much energy per square meter as a wind blowing at roughly 27 m/s (60 mph).

MeyGen in the Pentland Firth, Scotland — the strait between mainland Scotland and the Orkney Islands — is the world’s largest tidal stream project. Its initial phase installed four 1.5 MW turbines on the seabed in 2016-2017, with plans to scale to 398 MW. The Pentland Firth experiences tidal currents up to 5 m/s, making it one of the best tidal energy sites in the world.

Other notable projects include the SIMEC Atlantis (now Atlantis) turbine deployments in Scotland, Sabella’s D10 turbine in France, and various demonstration projects in the Bay of Fundy, the Goto Islands (Japan), and Uldolmok Strait (South Korea).

The engineering challenges are substantial:

Corrosion and biofouling. Seawater attacks metal. Marine organisms colonize any submerged surface. Maintenance is expensive because turbines are underwater and access depends on slack tide windows (brief periods between tidal flows).

Extreme forces. A tidal turbine in a 4 m/s current experiences enormous thrust loads. Anchoring systems must hold the turbine against these forces while allowing access for maintenance.

Grid connection. Subsea cables connecting turbines to shore must withstand currents, seabed movement, and the occasional anchor drag.

Despite these challenges, tidal stream technology has several advantages over barrages: lower environmental impact (no dam), modular scalability (add turbines one at a time), and more available sites (any location with strong tidal currents, not just estuaries).

Tidal Lagoons — A Middle Ground

A tidal lagoon is an artificial enclosed area on the coast — not across an estuary but along the open shoreline. It captures the tide within a breakwater structure, then generates power as water flows in and out through turbines.

The concept combines some advantages of both barrages and stream generators. Unlike barrages, lagoons don’t block estuaries, so fish migration and sediment transport are less affected. Unlike stream generators, lagoons can store water and generate on demand (within tidal constraints), providing some flexibility in power timing.

The proposed Swansea Bay Tidal Lagoon in Wales was the highest-profile lagoon project. It would have enclosed about 11.5 km2 of Swansea Bay behind a 9.5-km breakwater, generating 320 MW — enough for about 155,000 homes. The UK government ultimately declined to fund it in 2018, citing high costs relative to other renewable options.

Tidal lagoons remain an attractive concept but unproven at scale. No full-sized tidal lagoon has been built yet, though several are in planning stages in Wales, China, and South Korea.

The Numbers — Costs, Capacity, and Comparisons

Let’s be honest about where tidal power stands economically.

Current levelized cost of energy (LCOE):

• Tidal power: approximately $0.20-0.45/kWh
• Offshore wind: approximately $0.05-0.10/kWh
• Onshore wind: approximately $0.03-0.05/kWh
• Solar PV: approximately $0.03-0.05/kWh
• Natural gas: approximately $0.04-0.08/kWh

Tidal power is currently 4-10 times more expensive than mature renewables. That’s a hard sell, and it’s the primary reason tidal power hasn’t scaled.

But context matters. Wind and solar were equally expensive 15-20 years ago. Offshore wind cost about $0.20/kWh in 2010 and has since dropped by 60-70%. Tidal power advocates argue that with similar deployment scale and learning-curve effects, costs could fall to $0.10-0.15/kWh by 2035-2040.

Installed capacity worldwide (approximate, 2024):

• Tidal power: ~530 MW
• Offshore wind: ~70,000 MW
• Onshore wind: ~1,000,000 MW
• Solar PV: ~1,400,000 MW

Tidal power is tiny — a rounding error in global renewable energy. But it’s growing, and several countries are investing seriously.

Where Tidal Power Makes Sense

Geography limits tidal power to locations with strong tidal ranges or currents. The best sites include:

UK and Ireland. The Pentland Firth, Severn Estuary, Bristol Channel, and northwest Scotland have some of the strongest tidal resources in the world. The UK has an estimated 30-50 GW of tidal resource — enough to supply a meaningful fraction of national electricity demand.

France. The English Channel coast and Brittany have strong tidal ranges. France pioneered tidal power with La Rance and continues to develop the technology.

Canada. The Bay of Fundy’s extreme tides (up to 16 meters) represent a massive energy resource — estimated at 2,500 MW of extractable capacity. Several demonstration projects are operational or planned.

South Korea. Already home to the world’s largest tidal barrage (Sihwa Lake). South Korea has identified multiple additional sites along its western coast.

China. China has the world’s third-largest tidal energy resource and several small tidal power stations. Larger projects are in planning.

Australia, Japan, the Philippines, Indonesia. All have identified significant tidal resources, particularly in narrow straits and channels.

For island nations and remote coastal communities — places where fuel must be shipped in at high cost — tidal power’s premium cost is less of a barrier. A tidal stream generator might be competitive with diesel generation on an isolated island, even at current prices.

Environmental Considerations

Tidal power is emissions-free during operation. No carbon, no particulates, no nuclear waste. Over its lifecycle (including construction), tidal power produces roughly 15-30 grams of CO2 per kWh — comparable to wind and nuclear, and far less than fossil fuels (400-1,000 g/kWh).

But the environmental story isn’t entirely clean.

Marine habitat disruption. Any structure placed on the seabed alters local habitats. Barrages transform entire estuaries. Tidal lagoons create new artificial shorelines. Stream generators occupy seabed area and create turbulence downstream.

Collision risk. Marine animals — fish, seals, dolphins, porpoises — could collide with spinning turbine blades. Studies at operational sites suggest the risk is lower than initially feared (most animals detect and avoid turbines), but monitoring is ongoing, and the data is still limited.

Noise. Underwater turbines generate noise that could affect marine mammals, which rely on sound for navigation and communication. The significance of this effect depends on species, turbine design, and background noise levels.

Sediment transport. Barrages and lagoons alter sediment flows, potentially causing erosion in some areas and deposition in others. This affects downstream beaches, mudflats, and subtidal habitats.

Positive effects. Interestingly, underwater structures often become artificial reefs. The foundations of tidal turbines attract marine organisms — algae, mussels, fish — creating new habitats. Whether this compensates for other impacts is debatable, but it’s a consistent observation at operational sites.

The Future of Tidal Power

Tidal power won’t replace solar or wind as the dominant renewable energy source. The physics of tidal resource distribution — limited to specific coastal locations — prevents that. But it could become a significant contributor in regions with strong tidal resources, particularly because its predictability addresses the intermittency problem that plagues solar and wind.

Several trends favor tidal power’s development:

Technology maturation. Turbine designs are improving. Floating tidal platforms (which are easier to maintain than seabed-mounted turbines) are being tested. New materials resist corrosion and biofouling better. Modular, standardized designs are reducing costs.

Supportive policy. The UK, France, Canada, and South Korea have established subsidies, feed-in tariffs, or contracts-for-difference to support tidal energy development. The EU has set targets for ocean energy deployment.

Climate urgency. As pressure to decarbonize electricity grids intensifies, every low-carbon source becomes more attractive. Tidal power’s predictability — you know exactly how much power you’ll get, years in advance — has real value for grid planning that intermittent sources can’t match.

Hybrid projects. Combining tidal power with other functions — coastal flood defense, harbor infrastructure, aquaculture — can spread costs and increase project value. A tidal lagoon that also protects a city from storm surges might justify its cost even if the electricity alone doesn’t.

The ocean covers 70% of Earth’s surface, and the tides will keep running for billions of years. Learning to tap that energy efficiently is worth the effort — even if the effort takes longer and costs more than the alternatives. Tides don’t stop when the wind dies down or the sun sets. That kind of reliability has a value that goes beyond the price per kilowatt-hour.

Frequently Asked Questions

How much energy can tidal power produce?

The global theoretical tidal energy potential is estimated at about 3 terawatts (TW), but only a small fraction is practically harvestable. The World Energy Council estimates that tidal power could realistically generate about 800 TWh per year — roughly 3% of global electricity demand. As of 2024, installed tidal power capacity is only about 530 MW worldwide, with the 240 MW La Rance plant in France being the largest operational facility. The potential is significant but remains largely untapped.

Is tidal power reliable?

Tidal power is the most predictable renewable energy source. Tides are driven by the gravitational pull of the moon and sun, which follow precise astronomical cycles. Tide tables can predict water levels decades in advance with high accuracy. Unlike wind and solar, tidal power output is known years ahead of time. The main limitation is that tidal output follows a roughly 12.5-hour cycle, so power generation varies throughout the day — but the variation is perfectly predictable and can be planned around.

Does tidal power harm marine life?

It can. Tidal barrages block fish migration and alter estuarine ecosystems significantly — the La Rance barrage changed the estuary's ecology, though it has partially recovered over 50+ years. Tidal stream turbines pose collision risks to marine animals, though studies suggest the risk is lower than initially feared — most fish and marine mammals seem to detect and avoid the turbines. Tidal lagoons may have less environmental impact than barrages because they don't completely block waterways. Environmental monitoring is standard for all tidal projects, and designs are increasingly optimized to minimize ecological disruption.

Why isn't tidal power more widely used?

Three main reasons: cost, geography, and engineering difficulty. Tidal power infrastructure is expensive — underwater turbines must withstand corrosion, biofouling, and extreme forces. Suitable sites require strong tidal ranges or currents, which limits locations. And the technology is still maturing. The levelized cost of tidal energy is currently about $0.20-0.45 per kWh, compared to $0.03-0.05 for onshore wind. Costs are expected to drop with scale and experience, but tidal power is decades behind wind and solar in its development curve.

Could tidal power affect the Earth's tides?

Not meaningfully. The total energy dissipated by Earth's tides is about 3.7 TW, mostly through friction along coastlines and the ocean floor. Extracting a tiny fraction of this for electricity would have negligible impact on tidal patterns. For perspective, even harvesting 1% of tidal energy (37 GW — far more than any realistic deployment) would slow the Earth's rotation by roughly one second over millions of years. The gravitational forces driving tides are so enormous that human-scale energy extraction is insignificant.

Further Reading

• U.S. Department of Energy - Marine and Hydrokinetic Technologies
• European Marine Energy Centre (EMEC)
• International Renewable Energy Agency (IRENA)
• NOAA - Tides and Water Levels