Tidal power

Tidal power harnesses the energy from tidal forces, and the movement of water from those forces, to generate electricity. Unlike other primary energy flows, tidal power is a predictable source of energy, as the tides occur at expected times, which gives this type of power generation an advantage over wind and solar power. The power from this type of energy generation is produced by natural forces, such as the gravitational forces of the moon and sun, and the gradual rise and fall of ocean levels, to generate electricity. Winds further contribute to wave energy, the energy which can also be used to generate electricity.

Example of a tidal power turbine.

In tidal power, the size of the tidal range, or the difference in height of sea level from high to low tide, the more power can be produced. The best known, and earliest used, tidal range technologies harness the power through large, dam-like structures that trap rising waters on one side and release it back to through turbines to generate electricity.

Further, tidal energy is considered a renewable energy, because it is carbon-free in its generation of electricity; however, many of the technologies have not been proven to be environmentally benign. The use of these technologies has raised concerns over the health of shoreline and aquatic ecosystems, which can mar the otherwise clean source of energy offered by tidal power. Some of this is explained by the large-scale tidal range systems that have dominated the development of tidal power systems. And the technology has only been around since the 1960s, while the focus and development has shifted to tidal stream technologies that have less impact on the environment but can produce almost as much energy.

As the demand for clean electricity increases, tidal power has been looked at as a possible solution. The energy from waves, tides, and ocean currents is now believed to be combined to generate enough electricity to power millions of homes. This is partially because water is denser than air, which means the energy harnessed from tidal movements offers greater energy potential than wind energy. While the predictability of tidal power offers more reliable energy generation than wind or solar energy. This has increased the intrigue surrounding tidal power, with the potential for power generation greater than other renewable or clean energy sources.

A tidal power array in the Netherlands.

The estimated total energy contained in tides worldwide is supposed to be around 3,000 gigawatts (GW), though estimates of how much energy is available for power generation by tidal barrages are between 120 and 400 GW, depending on the location and potential for conversion. By comparison, a new coal-based power generation plant produces about 550 megawatts (MW). With global energy consumption in 2016 reaching 21,000 terawatt-hours, energy experts estimate that fully built-out tidal power systems could supply much of this demand, with tidal stream power estimated to be capable of generating some 3,800 terawatt-hours per year.

Tidal power generation is still relatively new, with some of the first tidal power generation facilities built in the 1960s, and with very few commercial-sized tidal power plants operating in the world. One of the first was in La Rance, France, and the largest facility is in the Sihwa Lake Tidal Power Station in South Korea.

The ways to harness tidal energy include tidal streams, tidal barrages, tidal lagoons, and tidal fences.

Example of a tidal stream generator.

For most tidal energy generation, a turbine can be placed in a tidal stream. The tidal stream is a fast-flowing body of water created by tides, and the turbine takes that flow of energy to produce power. Given the density of water and predictability of these tidal streams, tidal generators have been shown to produce a steady, reliable stream of electricity. However, placing a turbine in tidal streams is complex, in part because the generators are large and can disrupt the tide they are trying to harness.

The environmental impact of these generators could be severe, although that depends on the size of the turbine and the site of the tidal stream. Further, turbines have been shown to be more effective in shallow water, which produces more energy but also allows ships to navigate around them. The tidal stream generators' blades turn slowly, which would help marine life and keep them from getting caught in the system.

One such tidal stream project, the PLAT-I in Grand Passage, Nova Scotia, Canada, changed the traditional design of the tidal stream generator, using a surface-based vessel with four mounted turbines on hydraulic lifts rather than being on the seabed. This increases the ease of maintenance and allows the platform to be towed to shore during severe weather. The design is also anticipated to have less impact on the marine environment than seabed-based turbines.

The PLAT-I in the Grand Passage near Wesport, Nova Scotia, Canada.

Part of the design of the PLAT-I comes from two failed tidal stream projects in Canada. The first such project was the Race Rocks Tidal Energy Project, which installed a horizontal axis turbine in 2006. The turbine generated power but was removed in 2011 after a series of mechanical and electrical failures. A similar project was developed in Cape Sharp in Nova Scotia in 2016, which was shut down days after it opened due to the operating company declaring bankruptcy. However, the bankruptcy declaration came after the turbine was noted to be damaged beyond repair.

PLAT-I's ease of maintenance and repair is intended to provide ease of maintenance and use. The hydraulic lifts can bring the four turbines, independent of each other, out of the water for maintenance, without needing to take the whole system offline. And if more expensive repairs are needed, the platform can be towed to shore. In 2021, a revision of the first PLAT-I platform was shown to be capable of generating up to 420 kW of electrical power.

The oldest structure used to capture tidal energy, the tidal barrage is similar to a dam. It is installed across an inlet of an ocean or an ocean bay to form a tidal basin. Sluice gates on the barrage control the water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through a turbine system to the outgoing ebb tide. And a two-way system can generate electricity from both the incoming and outgoing tides. This creates a rate of energy generation that can be controlled, more than just predicted.

However, the impact of a barrage system can be significant. The land in the tidal range is disrupted, the change in the water level can harm plant and animal life, and salinity in the tidal range can change, which changes which organisms can live there. Similar to dams across rivers, fish are blocked into or out of the tidal basin, and turbines move quickly, which can catch marine life in their blades.

Tidal barrage at Annapolis Royal in Nova Scotia, Canada.

The barrages are also expensive to build, especially compared with building a single tidal stream generator. Unlike those generators, barrages require a lot of heavy machinery to construct, and they require near-constant monitoring to adjust power output. There are already several tidal power barrages operating; the largest is the Sihwa Lake Tidal Power Station in South Korea, which has a generation capacity of 254 MW. The oldest and second-largest operating plant is at La Rance, France, with 240 MW of electricity generation capacity. And the third largest tidal power plant is in Annapolis Royal in Nova Scotia, Canada, with a 20 MW electricity generation capacity.

Rance River tidal barrage.

As an example of the difficulties that can be caused by a tidal barrage, the power plant at Rance River estuary in Brittany, France was built in 1966 and continues to function. The plant uses two sources of energy: tidal energy from the English Channel and river current energy from the Rance River. The barrage has led to an increased level of silt in the habitat, which has suffocated native aquatic plants and has caused a flatfish called plaice to go extinct in the area. Other organisms, such as the cuttlefish, a relative of squids, now thrive in the Rance estuaries, especially as the cuttlefish prefers cloudy, silty ecosystems.

A tidal lagoon is a power station that generates electricity from the tidal movement and works in similar ways to tidal barrages in that they capture a large volume of water behind a manufactured structure. The water is then released to drive turbines and generate electricity. However, unlike a barrage, where the structure spans an entire river estuary in a straight line, a tidal lagoon encloses an area of coastline with a high tidal range behind a breakwater, with a footprint designed to be as minimal to the local environment as possible.

In a lagoon system, as the tide comes in, the water is held back by turbine gates that control the flow through a turbine and can be closed to stop water from entering the lagoon. This creates a difference in water level between the inside of the lagoon and the sea. Once the difference between water levels is optimized, the wicket gates are opened and water rushes into the lagoon through turbines inside the breakwater wall. The process is then done in reverse as the tide goes out to provide power on both movements.

The Swansea tidal lagoon project.

As the tide has two highs and two lows every day, tidal lagoons can generate electricity over four periods each day. With the water often being held for 2.5 hours four times a day, a tidal lagoon is able to generate power for up to 14 out of every 24 hours. Often the structure of the lagoon wall is made of rubble and sand to keep it as natural as possible, also known as a rubble mound breakwater. While the size and shape of a lagoon depend on many factors, such as the tidal range, the water depth, sea conditions, and environmental impacts and navigation. Swansea Bay Tidal Lagoon, for example, has a breakwater of 9.5 kilometers, enclosing an area of 11.5 square kilometers, and includes sixteen turbines.

Set to be the first tidal lagoon power plant in the world, the Swansea Bay Tidal Lagoon, also known as the Blue Eden project, the lagoon is expected to cost $2.35 billion to develop and will include underwater turbines, floating solar power, and battery storage, intended to further generate thousands of jobs. When completed, the lagoon is expected to include sixteen hydro turbines and a 9.5-kilometer breakwater wall, and be capable of generating enough electricity for 155,000 homes for an expected 120 years.

The project will be capable of generating power on the incoming and outgoing tides, only need to keep the turbine gates shut for three hours to generate a 4-meter height difference in water levels outside and inside the lagoon. Delivery of the project is expected to take place in three phases across a period of twelve years, with the development of the tidal lagoon as the first phase. These plans estimate that the tidal lagoon will be capable of generating 320 megawatts of electrical power. The project is also intended to include floating solar arrays, a battery facility to store energy produced by the project, and an oceanic and climate change research center.

A tidal fence is another form of tidal power generation; it is considered similar to, and sometimes the same as, tidal stream technology. The tidal fence uses the flow of underwater currents for energy generation, but it also bears resemblance to a tidal barrage, as the tidal fence uses multiple turbines mounted together in a single structure that resembles a fence. The turbines in this structure are vertical, rather than horizontal, with the tidal current forced to flow through the turbine blades to cause them to rotate and generate power.

Example of a tidal fence structure.

However, tidal fences are less expensive to deploy than a tidal barrage, and they do not block the flow of tidal water nor do they make drastic environmental changes. As well, as the tidal fence uses vertical turbines; the turbines act like turnstiles, making them safer for marine life than the horizontal turbines which, even slow-moving, can cause damage to marine life. The structure of the tidal fence can still disrupt fish migration and block local navigation and shipping. However, there are ways to engineer the tidal fence to lessen these impacts.

Tidal power is considered a clean energy source because it does not emit greenhouse gases in the production of power. Further, most tidal power generation systems do not take up as much space as other types of power generation. For example, the largest such project, the Sihwa Lake Tidal Power Station in South Korea, takes up around 12.5 kilometers of coastline. While wind farms, such as the Roscoe wind farm in Texas, United States, take up 400 square kilometers of farmland. And the Tengger Desert Solar Park in China covers an area of 43 square kilometers.

Another benefit of tidal power is it offers predictable power, unless the gravitational power of celestial bodies stops. Furthermore, as high and low tide is cyclical and highly predictable, it is easier for engineers to design efficient systems. This includes the ability to predict the volume of water that will move with the flow of the tides, and this allows engineers to predict the level of power the tidal equipment will generate.

Unlike wind energy, which needs a certain wind speed in order for efficient energy generation, tidal power can produce energy when the water passing over or moving through them is moving slowly. This is largely due to the density of water, which can power a turbine even when the flow of water is moving at a snail's pace.

In contrast to other renewable energy sources, such as wind or solar, tidal power plants last a lot longer, with some estimates putting the equipment as lasting around four times longer in comparison. For example, a tidal barrage is considered to have a lifespan of as much as one hundred years, while the average lifespan of wind turbines and solar panels is around twenty to twenty-five years, making tidal power a better long-term option and reducing its cost over time.

Tidal power plants offer high power output, which is related to the density of water. This means a tidal turbine will produce substantially more energy than a comparable wind turbine. And with the turbine generating power at a low speed, it also means electricity can be generated in non-ideal conditions.

When considering tidal power systems such as tidal barrages, there are negative impacts on marine life. Tidal barrages, for example, rely on the flow of water in and out of estuaries; however, when put in place, the water forced through the turbines can disrupt the way marine life thrives, and the blades pose a risk to any marine life that tries to swim through them. And they will damage marine plants due to a change in silt deposits and the changing structure of the estuary.

The constant movement of water and saltwater can corrode machinery, which means the machinery has to be regularly maintained. And the need for corrosion-resistant materials (not that all systems use such materials) can further increase the costs of constructing tidal power systems, especially as everything from the turbine to cabling has to withstand constant exposure to water. The maintenance of the systems can be difficult as everything remains submerged underwater. However, a lot of development and research in tidal power aims to make the systems more reliable and as maintenance-free as possible.

Building the structures necessary to withstand the turbulent, corrosive nature of seawater increases the cost of building a tidal power generation system. However, despite the higher upfront costs of tidal power systems, with how long the systems are capable of lasting and generating power, they pay themselves off. This is okay for projects with a long-term outlook, but for governments concerned with short-term budgets, the initial investment of these systems are often considered a difficulty in developing the systems.

Not every seascape or shore has a suitable location for tidal power systems. They tend to require specific sets of factors to operate effectively and efficiently. One such factor is the height of the sea from high to low tide has to meet specific requirements, or the systems need specific geographic features, such as narrow inlets, to help increase the flow of water over the machines. The scarcity of such locations is one reason tidal power is not more popular.

While there is a lack of research into the true effects of tidal barrages and turbines on the marine environment, there has been some research into how barrages manipulate ocean levels, which can have similar negative effects, such as those observed with hydroelectric power. However, some effects that have been observed include the alteration of nearby estuaries, the effects on local wildlife, and the possibility of altering currents and waves. There is research into how to account for some of these, such as turbines that can turn off when larger mammals or fish approach in order to protect the wildlife. However, until there are more tidal power systems in use, the impacts of these systems cannot be fully known.

There is also a suggestion that tidal power has electromagnetic emissions, which could disrupt sensitive marine life. For example, species of sharks, skates, rays, crustaceans, whales, dolphins, bony fish, and marine turtles can be impacted by EMFs. However, the research into how much EMF tidal power emits is uncertain, although there is evidence of noise, vibrations, and electromagnetic emissions coming from tidal power systems.

Tidal energy represents an opportunity to increase renewable power generation capacity around the world. As countries develop and the global population and its reliance on energy grow, the demand for additional clean energy sources grows. Tidal energy promises to be capable of supplying a significant portion of those electricity needs, if some of the barriers, such as environmental challenges and the robustness of devices, can be improved. As well, with several ways to capture tidal energy, there are ways for countries with limited tidal potential to still develop or implement some tidal power systems that could complement other energy generation schemes.

Countries developing tidal energy projects include Canada, the United Kingdom, Russia, Japan, South Korea, China, North Korea, and the United States. The northwest coasts of North America, including the coasts of Alaska, British Columbia, and Washington have tidal energy-producing potential. While the Atlantic seaboard in Canada has been producing tidal power since 1984, tidal power could be further developed along the Maine coast. This comes as locations such as Severn River in western England is believed to be capable of supplying as much as 10 percent of the country's electricity needs; and sites such as the Bay of Fundy, Cook Inlet in Alaska, and the White Sea in Russia believed to be capable of generating even larger amounts of electricity.

In 2010, SIMEC Atlantis Energy was awarded an agreement for lease to develop the MeyGen tidal stream project of up to 398 MW at an offshore site between Scotland's northernmost coast and the island of Stroma. This 3.5 kilometer site covers some of the fastest flowing waters in the United Kingdom, and the island of Stroma creates a natural channel that further accelerates the flowing water. The project is intended to have three phases: phase 1A, 1B, and 1C.

An Andritz Hydro Hammerfest HS1500 turbine being deployed for Phase 1A.

The first phase of the project includes an operational 6 MW demonstration array. The array comprises four 1.5 MW turbines installed in a "deploy and monitor" strategy. These turbines are upstream, three-bladed, horizontal-axis turbines, fully submerged on gravity-base foundations resting on the seabed. The turbines used at this part of the project included 1 Atlantis Resources Limited AR1500, which uses a turbine developed by Lockheed Martin and is designed to withstand the environmental conditions expected to be encountered in the Pentland Firth in Scotland the Bay of Fundy in Canada. The other three turbines are Andritz Hydro Hammerfest HS1500 horizontal axis turbines, which offer a variable speed generator with a gearbox capable of handling flowing water between 1 and above 4 meters per second in depths down to 100 meters.

The MeyGen Projects AR2000 turbine on a bright yellow platform.

Also known as Project Stroma, Phase 1B includes the installation of a subsea hub, which allows multiple turbines to be connected to a single power export cable, which is intended to reduce the costs associated with grid connection for the project. This will also reduce the necessary onshore conversion equipment required for grid connection and reduce the time required for cable installation. This phase of the project also includes the installation of two additional Atlantis AR2000 turbines to the new subsea hub.

The AR2000 turbines were jointly developed by Atlantis and GE, each with a 20-meter diameter, and are expected to offer a maximum output of 2 MW at 3.05 meter-per-second flow rate and improvement on the power production of the turbines installed in Phase 1A. This will increase the project's potential power output to 10 MW.

Yet to begin, Phase 1C is intended to build out the capacity of the MeyGen project to 73.5 MW of power, with 49 underwater turbines installed. The cost of this phase of the project is expected to reach £420 million, and the build-out of this project would provide the necessary scale to justify the establishment of turbine manufacturing facilities, and further development of tidal stream projects, while further increasing the nearby grid's use of renewable energy.

With MeyGen's lease permitting up to 398 MW of tidal stream capacity to be installed, there is a potential for further phases of the project, based on the success of Phase 1. Phase 2 would include developing the site to up to 252 MW, which is what the current MeyGen grid is capable of handling. A potential Phase 3 would include the development of a further 146MW capability, subject to grid connection, turbine availability, and speed of installation.