Tidal power, or tidal energy, is captured by changing the energy from tides into electricity and other useful forms of power. Although not widely used today, tidal energy has the potential to generate electricity in the future. Tides are more predictable than wind or sunlight. Compared to other renewable energy sources, tidal energy has faced challenges, such as higher costs and limited locations with strong tidal movements. These challenges have limited how much tidal power can be used. However, recent improvements in technology, such as new designs like dynamic tidal power and tidal lagoons, and better turbine systems, such as axial and cross flow turbines, suggest that tidal power may be more widely available and less expensive than previously thought.

Historically, tide mills were used in Europe and along the Atlantic coast of North America. These mills stored incoming water in large ponds, and as the tide receded, the water turned waterwheels to grind grain. The earliest tide mills date back to the Middle Ages or even Roman times. The process of using falling water and spinning turbines to create electricity began in the United States and Europe during the 19th century.

Electricity generated from marine technologies increased by about 16% in 2018 and 13% in 2019. Continued research and development supported by policies are needed to reduce costs and expand tidal energy use. The first large-scale tidal power plant was France’s Rance Tidal Power Station, which started operating in 1966. It was the largest tidal power station in terms of electricity output until South Korea’s Sihwa Lake Tidal Power Station opened in August 2011. The Sihwa station includes a sea wall with 10 turbines that generate 254 megawatts of electricity.

Principle

Tidal energy comes from the movement of ocean tides. These tides are caused by the pull of the Moon and Sun on Earth's oceans. As Earth rotates, the gravitational forces from these celestial bodies create regular changes in ocean levels. These changes are very predictable because Earth's rotation and the Moon's orbit around Earth follow a consistent pattern. The size and timing of the tides depend on the positions of the Moon and Sun relative to Earth, Earth's rotation, and the shape of the ocean floor and coastlines.

Tidal power is the only energy source that uses the movement of Earth and the Moon as they orbit each other, and to a smaller degree, Earth and the Sun. Other energy sources, like fossil fuels, wind, and solar power, get their energy from the Sun. Nuclear energy comes from special minerals in Earth, while geothermal energy uses heat from inside Earth, which is partly from Earth's formation and partly from radioactive elements.

A tidal generator turns the movement of ocean tides into electricity. Areas with large differences in tide levels and fast-moving water are better for producing tidal energy. Tidal energy is reliable, has strong energy storage, and lasts a long time.

Tidal energy is renewable because Earth's tides are caused by the Moon and Sun's gravity and Earth's rotation. Over time, the movement of tides causes energy loss in the Earth-Moon system. This happens when water moves through narrow areas near coastlines and creates friction on the ocean floor. This energy loss has slowed Earth's rotation over billions of years. In the last 620 million years, the length of a day has increased from 21.9 hours to 24 hours, and the Earth-Moon system has lost 17% of its rotational energy. While tidal power will take some energy from this system, the effect is very small and will not be noticeable in the near future.

Methods

Tidal power can be divided into four main ways to produce energy:

Tidal stream generators use the movement of water to turn turbines, much like how wind turbines use wind to create energy. Some of these generators can be built into existing structures like bridges or placed underwater, which helps reduce concerns about how they look. In places where the water moves very fast, such as narrow channels or inlets, turbines can be placed to capture this energy. These turbines can be shaped in different ways, like horizontal, vertical, open, or enclosed with ducts.

Tidal barrages use the height difference between high and low tides to create energy. These structures are like large dams built across tidal areas. When the tide rises, water is directed into a large basin behind the dam, storing energy. When the tide goes out, the stored water flows through turbines, turning them to generate electricity. Barrages are built across the entire width of a tidal estuary.

A newer tidal energy method involves building circular walls with turbines inside to capture energy from tides. These walls create reservoirs similar to those in tidal barrages, but they are man-made and do not include natural ecosystems. These reservoirs can be built in pairs or triplets, with or without pumps to help balance energy production. Extra energy from sources like wind or solar power can be used to operate the pumps. If tidal lagoons are spread out in different areas, their energy production can be spread out over time, creating a more steady supply of power. However, this method is more expensive than other options, such as using renewable energy for heating. An example of this idea was the canceled Tidal Lagoon Swansea Bay in Wales, United Kingdom, which would have been the first of its kind.

Dynamic tidal power (DTP) is a proposed method that uses both the movement of water and the height difference between tides. It suggests building very long dams (30 to 50 kilometers long) from the coast into the ocean, without enclosing an area. These dams would create differences in water levels along the coast, leading to strong tidal currents that move parallel to the shore. This idea is still theoretical and has not been tested yet.

US and Canadian studies in the 20th century

In 1924, the US Federal Power Commission conducted the first major study on large tidal power plants. The proposed power plants would have been built along the northern border of the US state of Maine and the southeastern border of the Canadian province of New Brunswick. These plants would have included dams, powerhouses, and ship locks to enclose the Bay of Fundy and Passamaquoddy Bay (see map in reference). The study was not completed, and it is unclear if Canada was informed about the study by the US Federal Power Commission.

In 1956, Nova Scotia Light and Power of Halifax asked two companies, Stone & Webster of Boston and Montreal Engineering Company of Montreal, to study the possibility of using tidal energy for electricity in the Bay of Fundy. Both studies found that the Bay of Fundy had the potential to generate millions of horsepower (equivalent to gigawatts) of energy. However, the cost of building such projects was too high to be practical.

In April 1961, an international report titled "Investigation of the International Passamaquoddy Tidal Power Project" was released by the US and Canadian governments. The report showed that the project would be beneficial for the US but not for Canada based on cost-benefit analysis.

In 1977, the governments of Canada, Nova Scotia, and New Brunswick commissioned a study called "Reassessment of Fundy Tidal Power" to evaluate the possibility of building tidal power structures in Chignecto Bay and Minas Basin, which are at the end of the Bay of Fundy estuary. The study identified three locations as financially possible: Shepody Bay (1550 MW), Cumberland Basin (1085 MW), and Cobequid Bay (3800 MW). These projects were never built, even though they were considered feasible at the time.

US studies in the 21st century

The Snohomish PUD, a government organization that provides energy services in Snohomish County, Washington, started a tidal energy project in 2007. In April 2009, the PUD chose OpenHydro, a company from Ireland, to design and build turbines and equipment for the project. The plan was to place the equipment in areas with strong tidal currents and operate it for four to five years before removing it. The project was originally expected to cost $10 million, with half of the money coming from the PUD’s own funds and the other half from federal grants. In 2009, the PUD received a $900,000 grant and used its reserves to cover about $4 million of the costs. In 2010, the estimated cost rose to $20 million, with the PUD and the federal government each expected to pay half. However, the PUD struggled to control expenses, and by October 2014, the total cost had increased to about $38 million and was expected to grow further. The PUD asked the federal government to provide an extra $10 million, referencing a gentlemen’s agreement. When the government refused, the PUD canceled the project after spending nearly $10 million from its reserves and grants. After this project ended, the PUD stopped all efforts to explore tidal energy and no longer owns or operates any tidal energy systems.

Rance tidal power plant in France

In 1966, Électricité de France built the Rance Tidal Power Station at the mouth of the Rance River in Brittany, France. It became the first tidal power station in the world. For 45 years, it was the largest tidal power station by total power it could produce. The station has 24 turbines that can generate up to 240 megawatts (MW) of power, with an average of 57 MW. This means the station produces about 24% of its maximum power on average.

Tidal power development in the UK

The world's first marine energy test facility was built in 2003 to help develop wave and tidal energy in the United Kingdom. Located in Orkney, Scotland, the European Marine Energy Centre (EMEC) has helped place more wave and tidal energy devices than any other single location globally. EMEC offers test sites in real ocean conditions. Its tidal test site connected to the power grid is at the Fall of Warness, near the island of Eday, in a narrow channel where tides are strong as they move between the Atlantic Ocean and the North Sea. This area has very powerful tides, reaching speeds of up to 4 m/s (8.9 mph; 7.8 kn; 14 km/h) during spring tides. Companies that tested tidal energy devices at the site include: Alstom (formerly Tidal Generation Ltd); ANDRITZ HYDRO Hammerfest; Atlantis Resources Corporation; Nautricity; OpenHydro; Scotrenewables Tidal Power; Voith. The energy potential in this area could be 4 terajoules per year. In other parts of the UK, 50 terawatt-hours of energy could be collected annually if 25 gigawatts of tidal energy capacity is installed with movable blades.

Current and future tidal power schemes

• The Rance tidal power plant was built over six years from 1960 to 1966 at La Rance, France. It has an installed capacity of 240 MW.
• The 254 MW Sihwa Lake Tidal Power Plant in South Korea is the largest tidal power installation in the world. Construction was completed in 2011.
• The Jiangxia Tidal Power Station, located south of Hangzhou in China, has been operational since 1985. Its current installed capacity is 3.2 MW. Additional tidal power projects are planned near the mouth of the Yalu River.
• The first in-stream tidal current generator in North America, the Race Rocks Tidal Power Demonstration Project, was installed at Race Rocks on southern Vancouver Island in September 2006. The project was shut down in 2011 after operating for five years because the cost to operate was too high to make the electricity affordable. The next phase of development is planned for Nova Scotia (Bay of Fundy).
• A small tidal power project was built by the Soviet Union at Kislaya Guba on the Barents Sea. It has an installed capacity of 0.4 MW. In 2006, it was upgraded with a 1.2 MW experimental advanced orthogonal turbine.
• The Jindo Uldolmok Tidal Power Plant in South Korea is a tidal stream generation project planned to expand to 90 MW of capacity by 2013. The first 1 MW was installed in May 2009.
• A 1.2 MW SeaGen system became operational in late 2008 on Strangford Lough in Northern Ireland. It was removed in 2016.
• A contract for an 812 MW tidal barrage near Ganghwa Island (South Korea), northwest of Incheon, was signed by Daewoo. Completion was planned for 2015, but the project was canceled in 2013.
• A 1,320 MW tidal barrage was proposed by the South Korean government in 2009 to be built around islands west of Incheon. The project was halted in 2012 due to environmental concerns.
• The Scottish Government approved plans for a 10 MW "Òran na Mara" array of tidal stream generators near Islay, Scotland. The project would cost 40 million pounds and include 10 turbines to power over 5,000 homes. The first turbine was expected to be operational by 2013, and the plan was again mentioned in 2021. As of 2023, no turbines were in use.
• The Indian state of Gujarat planned to host South Asia’s first commercial-scale tidal power station. Atlantis Resources planned to install a 50 MW tidal farm in the Gulf of Kutch on India’s west coast. Construction was planned to start in 2012 but was later canceled due to high costs.
• Ocean Renewable Power Corporation delivered tidal power to the US grid in September 2012 when its pilot TidGen system was deployed in Cobscook Bay near Eastport.
• In New York City, Verdant Power deployed and operated three tidal turbines in the East River near Roosevelt Island on a single triangular base system called a TriFrame. The Roosevelt Island Tidal Energy (RITE) Project generated over 300 MWh of electricity to the local grid, setting an American marine energy record. The system’s performance was confirmed by Scotland’s European Marine Energy Centre (EMEC) under new international standards. This was the first third-party verification of a tidal energy converter to an international standard.
• The largest tidal energy project, MeyGen (398 MW), is currently under construction in the Pentland Firth in northern Scotland. Six MW of capacity has been operational since 2018.
• Construction of a 320 MW tidal lagoon power plant outside Swansea, UK, was granted planning permission in June 2015. However, the UK government rejected the project in 2018. If completed, it would have been the world’s first tidal power plant based on a constructed lagoon.
• Mersey Tidal Power is a proposed tidal range barrage within the Mersey Estuary channel with a capacity of up to 1 GW. It is currently undergoing local consultation by the Liverpool City Region Combined Authority.
• Up to 240 MW of tidal stream generation is planned at Morlais, Anglesey, from multiple developers. The first turbines are expected to be installed in 2026. As of 2024, 38 MW of capacity has been awarded Contracts for Difference to supply power to the UK grid.
• The West Somerset Lagoon is a proposed tidal lagoon on the south side of the Bristol Channel between England and Wales. The intended power generation is 6.5 TWh per year.

Issues and challenges

Tidal power can affect marine life. The spinning blades of turbines may accidentally harm sea creatures. Some projects, like the one in Strangford, use a safety feature that stops the turbine when marine animals are nearby. However, this safety measure reduces energy production because many marine animals pass through the area. Some fish might avoid the area if they see or hear the turbines. Marine life is an important factor when choosing where to place tidal power generators, and steps are taken to protect as many animals as possible. In terms of carbon footprint, tidal power produces between 15 and 37 grams of CO2 equivalent per kilowatt-hour of energy, with an average of 23.8 grams. This is similar to wind and solar power and much better than fossil fuels. The Tethys database offers information about the environmental effects of tidal energy.

The main environmental concern with tidal energy is the risk of marine animals being hit by turbine blades or getting caught in them. Fast-moving water increases the chance of this happening. Like other offshore energy sources, tidal energy may also affect marine life through electromagnetic fields and sounds. These sounds can be louder than those from wind turbines. Depending on the sound’s frequency and strength, it may harm marine mammals, such as dolphins and whales, which use echolocation to navigate. Tidal energy projects can also change water quality and sediment patterns. These effects vary depending on the project’s size, from small changes to major impacts on nearby ecosystems.

Building a tidal barrage can change the shoreline in a bay or estuary, affecting ecosystems that rely on tidal flats. Blocking water flow may reduce the amount of saltwater and increase water cloudiness, harming fish that are important food sources for birds and mammals. Migrating fish might also be unable to reach their breeding grounds and could be injured by turbines. Tidal barrages may also produce sounds that affect marine life. While tidal projects can reduce shipping access, locks can be added to allow ships to pass. A barrage might also improve local economies by creating bridges and offering calmer waters for recreation. In 2004, a humpback whale became trapped near a tidal power station but eventually escaped.

Environmental concerns include fish being hit by turbine blades, sounds from turbines, and changes in sediment patterns. These effects are usually limited to small areas near the turbines and do not affect the entire bay or estuary.

Saltwater can cause metal parts to rust, making it hard to maintain tidal generators. Using materials like stainless steel, copper alloys, or titanium can reduce rusting. Composite materials, which do not rust, may also be used for lightweight and strong structures.

Lubricants used in tidal generators can leak into the water, harming marine life. Regular maintenance can help prevent harmful chemicals from entering the environment.

Placing structures in areas with strong tides and high marine life can lead to the growth of marine organisms on the structures.

Tidal energy has high initial costs, which may make it less popular as a renewable energy source. However, research shows people support tidal energy development. Tidal energy technology is still new, and costs may decrease in the future. The cost-effectiveness of tidal projects depends on the location. One way to measure this is the Gibrat ratio, which compares the length of a barrage to its yearly energy production.

Because tidal energy is reliable, it is possible to predict how long it will take to recover the high initial costs. A simpler turbine design, called an orthogonal turbine, can save money by reducing production time, using less metal, and improving efficiency.

A possible risk is rising sea levels from climate change, which could change local tides and reduce future energy production.

Structural health monitoring

The fact that water is about 800 times denser than air means tidal energy can produce a large amount of electricity. Tides are also more predictable and reliable than wind, making tidal energy a good choice for generating power. To use tidal energy in a way that saves money, it is important to monitor the condition of equipment regularly.