In the Pentland Firth off of Scotland’s coast, an array of underwater turbines churn 24/7, spun by powerful ocean tides. Operational since 2018, this tidal stream project reached a milestone last year, generating 51 gigawatt hours of power — a first anywhere in the world for tidal stream technology.

Though limited to only four turbines, this long-running demonstration (known as the MeyGen project, and operated by the SAE Renewables company) is one of several tidal stream trials that experts say offers proof of concept and shows off the possibilities of harnessing the world’s tides as an alternative energy source. Its annual output is enough to power roughly 6,000 homes.

The Pentland Firth array could eventually scale up to 269 turbines and produce up to 398 megawatts of clean power — enough to power around 175,000 homes. “MeyGen has demonstrated now beyond doubt that three-bladed, horizontal axis, seabed-mounted tidal turbines work,” says Fraser Johnson, the project’s operations and maintenance manager.

But to become an alternative energy player in the coming years, he adds, the nascent tidal stream industry needs to get more turbines in the water and demonstrate ongoing proof of capabilities, while driving down costs.

For now, the United Kingdom stands at the forefront of tidal stream development, with MeyGen and around half a dozen other U.K. projects currently generating enough electricity to power thousands of homes. If planned developments go ahead, around 130 megawatts of capacity will be operational in the country’s surrounding waters by 2029, though this figure is far from the U.K.’s theoretical tidal harvest potential of 11 gigawatts, which would provide roughly 11% of electricity demand (enough to power around 10 million homes).

Building out the U.K.’s planned capacity would represent a “massive increase” for the tidal industry, according to Danny Coles, a senior research fellow at Plymouth University, whose work focuses on tidal power. But he credits advances to date as “an important step that’s accelerating the sector forward.” On the down side, tidal stream project costs remain high, and experts recognize challenges ahead, ranging from reducing supply chain production costs to protecting against environmental impacts.

Tidal power and wave power coming of age

The awesome power of the ocean’s daily shifting tides has long inspired dreams of limitless cheap energy. Tidal mills — submerged water wheels that ran machinery in tidal rivers — existed as far back as the 18^th century on the U.S. Maine coast, with a failed U.S. federally funded attempt there in the 1930s.

But the scale of such operations was small. And turning the tremendous power of the sea into large amounts of electricity, while tantalizing, has yet to be fully realized. Tidal stream generators — utilizing underwater or surface turbines to extract energy from run of river or tidal estuaries — may offer the next best hope.

A 2023 alternative energy road map outlines the potential: Published by the International Energy Agency’s Ocean Energy Systems program, it states that planetwide by 2050, tidal and wave technology combined could offer as much as 300 gigawatts of power and prevent 500 million tons of carbon emissions. A separate report states that delivering on 100 gigawatts of capacity in Europe alone would generate around 10% of the region’s electricity consumption today.

But since the first U.K. tidal stream test in the mid-2000s, the industry has been buffeted by repeated waves of hype, hope and failure as companies ventured into the space and tested innovative devices, only to see their firms founder due to technical difficulties or rocketing research and development costs. The most recent of those failures came this year when a firm attempting to harness the immense tides of Canada’s Bay of Fundy folded, with investors backing out of the project.

Despite this checkered and costly past, U.K. tidal project developers are now buoyed with optimism. Described as a “pre-commercial” technology in 2021, many experts now say tidal stream technology is on the cusp of breaking through, thanks to successful demonstrations in Scotland that are now expanding.

But tidal power’s development still trails about 10 years behind wind and solar, says Seumas Mackenzie, chief operating officer for tidal developer Nova Innovation, based in Edinburgh, Scotland. His company ran a successful trial of six tidal devices in Shetland and now plans to deploy a 16-device array near the Orkney Islands, with funding from the European Union. “We are at the point where we’re demonstrating the technology reliably and we’re scaling it up to commercial utility scale,” he says.

But at this stage, continued government subsidies remain vital to keeping these efforts afloat and companies solvent (which was also true in the early days of private wind and solar startups). This state funding continues to come in: A recent U.K. government renewable energy contract supported a further six tidal projects; a “major step forward” for the industry, according to analysts.

“If those market support mechanisms were to go away, it wouldn’t make economic sense to develop those technologies,” says Brian Polagye, a professor in the Department of Mechanical Engineering at the University of Washington, in the U.S., who researches ocean energy. Other industry experts note that further technological advances are still needed to ensure reliability and reduce operating costs.

Some, such as Richard Parkinson, CEO of Inyanga, a U.K. tidal developer, say the industry may have aimed too high too fast in the early years, which led to a drop-off in investment and support.

That’s partly because deploying tidal power and keeping it running in harsh ocean conditions — where it’s exposed to corrosion and the daily strain of rushing tides — is challenging and requires a “realistic” fiscal approach, he says. “What we’ve got to do is look at relatively small projects, just to demonstrate that the technology is reliable, overcoming environmental hurdles, and also build momentum and the scale we need.”

Although the U.K. remains at the forefront of tidal stream development for now, countries across the globe are pursuing tidal and other ocean energy technologies. The U.S. has channeled millions of dollars into a suite of ocean power pilots. China has also invested heavily in ocean energy, pushing forward its own innovations with some success, particularly in wave technology. Similarly, the European Union is pumping money in, envisioning 1 gigawatt of ocean derived-output by 2030, and 40 gigawatts by 2040.

Like tidal stream technology, wave energy has faced years of frustrating development but is now moving toward commercialization, according to Ocean Energy Europe, an industry group based in Brussels. Wave power converts the energy of windblown waves into electricity.

Companies are now increasingly deploying small-scale wave energy tests, and that technology’s global potential could ultimately far surpass tidal as a power source. But it still faces monumental technical hurdles and is further down the technological readiness ladder than tidal, according to some.

“Wave energy devices need to be designed to withstand and survive storm conditions,” says Conchúr Ó Brádaigh, vice president and head of the Faculty of Engineering at the University of Sheffield, and that could be a major concern in a warming world where storms are increasingly more extreme. “There’s a lot of wave energy out there, but it’s certainly more expensive and technically difficult to exploit.”

Balancing grids, decarbonizing islands

While ocean energy will never likely become a dominant form of power, it could play an integral role in a balanced energy grid, the experts Mongabay spoke to say. It holds some advantages over unpredictable terrestrial and offshore wind projects because tides and waves are highly predictable. These technologies may be especially well suited to support a renewables transition for small island nations that lack the space for wind and solar installations and that depend on imports of expensive and polluting fossil fuels.

U.K.-based tidal energy company Inyanga is developing a project in the Philippines to provide energy for the island of Capul with the company’s 1 megawatt modular seabed-mounted HydroWing.

Inyanga’s CEO Parkinson hopes a successful demonstration there will pave the way for other deployments on small Philippines islands. “There’s always been a market for tidal power in remote island communities because we’re competing against diesel generation, which is very expensive,” he says. “There’s a ready market, and I think the sector’s been too slow to try and exploit that.”

Research also shows potential for widescale deployment of tidal power in Indonesia. Several companies have signed agreements to scope out projects serving island communities. Nova Innovation’s Mackenzie says the potential for tidal in Indonesia is “huge,” based on a study carried out by his company.

Ocean energy is “a promising solution” for Southeast Asian countries on the path to energy security and decarbonization, says Mary Ann Franco, senior consultant & Southeast Asia regional coordinator with Aquatera, an environmental services consultancy. “One of the advantages of having wave and tidal is it can be small-scale, modular or come in arrays,” Franco says. “That’s something that could be quite beneficial for communities … especially if it helps in powering their sustainable livelihoods.”

But she underlines that proposed projects must take local context into account. “By conducting larger-scale demonstrations that consider the socioeconomic and environmental factors specific to this region, we can maximize the benefits of tidal power while minimizing its potential impacts,” she says.

An ocean ecosystem footprint

Industry proponents concede that tidal stream projects come with environmental hazards, including the potential for collisions between fast-moving turbines and marine mammals, fish and other wildlife, not unlike the impacts on birds and bats of onshore and offshore wind installations.

Gordon Hastie, with the Wildlife and Tidal Energy Research Group at the University of St Andrews, says that though monitoring at the U.K.’s tidal trials indicates low collision risks, those collisions could still be fatal. That’s a concern in Scotland, where tidal stream projects are planned where harbor seal populations are already in decline. “Even losing a few animals [there] could potentially have significant impacts,” he says.

Beyond collisions, turbines can disturb estuarine wildlife movement patterns, causing marine mammals to avoid areas they once occupied. Hastie and his team at St Andrews have developed tools to monitor these effects but more research is needed, he says. “I think the next big question is how do we monitor as we scale up?”

If and when large numbers of arrays are deployed, that will likely increase marine ecosystem risks. Impacts may also vary depending on type of device deployed; some tidal devices are seabed-mounted, others float on the surface, while one Swedish company is pursuing a “kite” model that moves around the ocean.

Likewise, wave energy’s environmental footprint may be lower than conventional energy sources, but large-scale deployments could disrupt marine ecosystems by changing migratory paths, animal behavior and affect wave dynamics and coastal erosion. Careful site selection and array designs could help minimize such impacts, but more research is needed as test devices are deployed.

Questions also arise as to how transferable the monitoring data collected from tidal devices in Scotland’s waters may be to other locations and species. Deploying tidal stream devices in Southeast Asia or North America, for example, may alter species risk profiles, Hastie says. Environmental groups raised alarm about planned trials in Wales because they feared impacts on dolphin populations. Developers, meanwhile, stress that environmental monitoring and impact studies remain part of their project process.

Beyond species-specific impacts, deployment of large tidal arrays in the world’s oceans could have unforeseen “cascading effects” on ecosystems, says Michela De Dominicis, a physical oceanographer with the National Oceanography Centre in the U.K.

“If you have one tidal turbine, even if there are impacts, they are going to be limited,” she says. “If we scale up to thousands of turbines, then we are perturbing the environment” in potentially new, more profound ways.

Her research has focused on theoretical large-scale deployment of tidal power in the North Sea, where her modelling indicated that deploying devices en masse could lead to an increase in water column stratification, with adverse implications for nutrient cycling and oxygen flows. But she adds that this impact will likely be dwarfed by the predicted effects of climate change by 2050.

“These small changes [in stratification due to large tidal projects] can have cascading impacts on animal populations,” she says. The concern is that fewer nutrients or less oxygen could have implications for plankton and thus on up the marine food chain. She adds, “These are only small changes compared to climate change, but we don’t know if these small changes can amplify and impact animal populations.”

Hastie agrees that efforts must be made to mitigate wildlife harm during tidal project design, implementation and operation, and that climate change remains the “elephant in the room.”

“Trading the two off against each other, there’s no doubt that, for some populations, there may be short-term significant impacts due to the introduction of renewable energy,” he says. But on the other hand, “the long-term picture for these species of not going to a low-carbon society is pretty dire.”

The need to scale up

On its current trajectory, tidal stream’s future appears fairly bright, experts say. The sector is now building on years of demonstration projects, with more in the pipeline. Of course, it’s unlikely to ever compete with either offshore or onshore wind, or even wave energy (if successfully proven at scale), but the constancy and predictability of tidal stream means that it could have a niche in a balanced alternative energy mix.

Polagye says that set against other renewables, such as wind and solar, it is clear the global potential of tidal is smaller. “Can it make regionally and nationally significant contributions?” he asks, answering, “yes, absolutely,” to his own question.

But achieving the lofty potential indicated by modeling will remain dependent on continued government subsidies, future private investment, success in bringing down costs and continued demonstrations of both energy and environmental performance.

“I think the sweet spot would be to put in 30 megawatt-plus projects because I think that’s the point where the big investors will start taking an interest in tidal,” Parkinson says. “In the next five years, I think we’ll be going through a lot of these demonstration projects. As long as these can get delivered and get the investment, I think tidal will be in a very strong place.”

Banner image: Waves off Scotland’s coast. Harnessing the ocean’s power offers the possibility of a reliable source of renewable energy. After surviving a promotional hype bubble and a funding bust in recent decades, small-scale tidal stream technology projects are now increasing across the globe. Image by Thomas Horig / Ocean Image Bank.

Citations:

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Pennock, S., Vanegas Cantarero, M., Bloise Thomaz, T., Jeffrey, H., & Dickson, M. J. (2021). Life cycle assessment of a point-absorber wave energy array. SSRN Electronic Journal. doi:10.2139/ssrn.3870980

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