PREDICTING THE POWER OF TIDAL ENERGY

Tidal and wave energy may be the largest untapped resources of renewable energy on the planet. But will they work?

By a quirk of geography, the Orkney Islands, located off the northern tip of Scotland, are unusually well-positioned to bear witness to the ocean’s might. On the archipelago’s western shores, waves crash relentlessly against the rocks. And within its numerous channels, the tides push an enormous volume of water from the North Atlantic to the North Sea and back again, twice every day, squeezing between and around the islands of Rousay, Westray, Eday, and a myriad of other ones.

No wonder the European Marine Energy Center, one of the world’s leading agencies for developing and testing wave and tidal power technologies, chose to set up shop here; the nonprofit agency hosts both wave and tidal power testing facilities on Orkney.

EMEC’s wave-energy testing site is at Billia Croo, located on the western shore of Orkney’s largest island. On a relatively calm day last spring, Lisa MacKenzie, EMEC’s marketing and communications manager, surveyed the gray waters from the Billia Croo site. “We get an average of 2-to-3-meter wave height,” she said, or roughly 6.5 to 10 feet. “But we’ve had waves of over 20 meters” — more than 65 feet — during “really extreme conditions over the winter.”

The surrounding landscape is windswept and nearly treeless. Were one to sail directly west from this spot, “the first bit of land that you would hit is Canada,” MacKenzie said.

EMEC was founded in 2003 following a recommendation by the U.K. House of Commons Science and Technology Committee (now known as the Science, Innovation, and Technology Committee). To date, it has received about $53 million in public investment; its funders include the European Union, the U.K. government, the Scottish government, and the Orkney Islands Council. More than 20 corporate clients have used EMEC’s facilities, and more ocean energy converters have been tested at the center than at any other site in the world.

The Billia Croo facility opened in 2004 on land rented from a local farmer. An array of transformers, housed in green bins, each the size of a compact car, lines the perimeter of the site’s small parking lot. A modest stone-wall hut, which blends into the landscape, houses the facility’s control center and is filled with electronic switching equipment. The testing berths are offshore, where EMEC’s clients can test all manner of wave-energy conversion devices, with cables running along the seabed to the control hut. Any electricity produced can also be fed directly into the U.K. national grid.

Waves, like the wind that produces them, are not a constant; both are inherently variable. And they are linked: Wind imparts energy to the ocean, which then dissipates as waves over a longer time scale. As MacKenzie puts it, waves are the aftermath of wind.

Harnessing the energy of waves is one way to draw power from the oceans; another is to exploit the energy of the tides. Of the two energy sources, tidal is more constant, given the tides’ regular-as-clockwork response to the push and pull of the moon and sun.

EMEC runs a grid-connected tidal energy test facility located off the southern tip of Eday. “We get a peak tidal flow over 4 meters per second, which is about 8 knots,” MacKenzie said. “So about half a billion tons of water passes through there, every hour, at peak tide.”

As MacKenzie puts it, waves are the aftermath of wind.

And that flow is comparatively predictable — far more so than, say, wind or solar, which are stymied by calm or cloudy conditions.

“We can predict the tides 200 years into the future,” MacKenzie said. “Which means that we can predict how much power can be derived from the tides, 200 years into the future.”

There is no question that the planet’s oceans contain enormous amounts of energy. According to a 2021 study published in Proceedings of the Royal Society A, tidal stream energy alone could provide the equivalent of 11 percent of the U.K.’s annual electricity needs. Power from the oceans is “the largest untapped resource of renewable energy on the planet right now,” said Rémi Gruet, CEO of Ocean Energy Europe, the world’s largest network of ocean energy professionals.

The question is, can that energy be harnessed economically — or is the idea of pulling watts from the water doomed to be a mere sideshow in the quest for green energy? After decades of testing at tidal energy facilities like EMEC and other smaller-scale facilities around the globe, only a handful of commercial wave and tidal power facilities are online, and they contribute a minuscule amount to the world’s energy production. Even in Orkney, a leader in the quest to extract energy from the ocean, wave and tidal power account for just a fraction of the islands’ energy consumption.

“We can predict how much power can be derived from the tides, 200 years into the future.”

Notably, wave and tidal lag behind other forms of renewable energy. “It’s fair to say that we’re nowhere near a wind or solar industry at this point,” says Carrie Schmaus, a marine energy technology manager at the U.S. Department of Energy’s Water Power Technologies Office.

Still, for the technology’s supporters, the ocean is seen as a virtually limitless source of energy waiting to be tapped if only governments step up with the public investment needed to kick the industry into high gear. “There’s an energy resource there,” says Andrew Scott, CEO of Edinburgh-based Orbital Marine Power Ltd. “The question is, what are you prepared to pay to extract that energy?”

On paper, the power of the world’s oceans is indisputable: Tidal stream energy is estimated to represent a global resource of some 1,200 terawatt-hours (a terawatt is one trillion watts) per year, while wave power is even more abundant, adding up to almost 30,000 terawatt-hours per year — enough, in theory, to meet all of humanity’s energy needs 10 times over.

As promising as tidal and wave energy may seem, the list of obstacles to widespread adoption is significant: the formidable cost of scaling up the technology; bureaucratic hurdles; environmental concerns, including possible effects on fish and sea mammals; and, in the case of tidal power, geographical restrictions. There are also fears that rising sea levels could substantially alter ocean movements in a way that could impact current or planned tidal power facilities. In a 2022 paper published in the journal Renewable and Sustainable Energy Reviews, Danial Khojasteh and his co-authors noted that “long-term management decisions associated with harnessing the potential of tidal energy schemes within estuaries should be made with caution.”

The question of cost is paramount. Even though the cost of tidal and wave energy may be dropping, the cost of wind and solar are dropping even faster, said Brian Polagye, a University of Washington mechanical engineer who studies marine renewable energy. That means tidal and wave energy can be seen as succeeding and failing at the same time.

“Until your price comes down to the point where you’re competitive with other forms of generation — either because you’re directly competitive, or you’re being subsidized until you get to that point — the technologies really can’t take off,” Polagye said. Nonetheless, he added, “I do feel these are technologies that have a long-term role to play in our energy systems.”

Schmaus, at the Water Power Technologies Office, describes wave and tidal power as a nascent industry (as did others interviewed for this story). By way of comparison, she pointed out that in the early days of the wind power industry, all manner of turbine designs were tested. “And then, at some point, that technology converged,” she said. “Now we have the three-bladed turbine we all know and love. Marine energy is still in that ideation kind of area. We have not had technology convergence yet.”

For the technology’s supporters, the ocean is seen as a virtually limitless source of energy waiting to be tapped.

One of her department’s goals, she says, is to learn from small-scale demonstration projects, scale up designs, and bring down costs. This scaling-up is just what Scott’s Orbital Marine is trying to achieve in Orkney. They’re the company behind the O2 tidal stream energy generator — the world’s most powerful such device — located in the Fall of Warness, south of Eday, and connected to the grid via EMEC’s tidal energy test site. (MacKenzie described the project as “one of our biggest success stories.”) The O2 is a 240-foot-long structure shaped like a submarine (though it stays on the surface), with two submerged arms, each supporting a twin-bladed turbine. In an interview in a cavernous exhibition hall at the annual All-Energy conference in Glasgow last spring, and later by email, Scott spoke of his vision for the company and the potential of tidal stream power. He said that Orbital Marine hopes to add another six turbines to the Fall of Warness site over the next few years and, in time, perhaps another dozen.

Scott acknowledges the forbidding technical challenges — especially the difficulty of designing machinery that can withstand seawater’s salt and grime for months or years on end. And he has seen his share of unrealistic proposals over the years. At times “it was a bit of a joke,” he recalled. People saw how much traction wind energy was getting, he says and figured wind’s success could be readily duplicated beneath the waves.

“People would say, ‘Just go and ‘marinize’ it, and it will be equally successful in the tidal application,” he continued. “It was as naïve as that.”

But many of those early challenges have been overcome, Scott said. He noted that O2 is currently providing about 10 percent of Orkney’s electricity, enough to power about 2,000 homes. Because the islands are sparsely populated and rich in wind energy, Orkney actually produces more energy than is needed locally, which means the islands are already a net contributor to the U.K. grid — and some of that energy comes from O2. Scott said he foresees Orbital Marine generating about $17.5 million from electricity sales per year over the turbine array’s projected 20-year life.

“We’re effectively at that critical stage where we start to grow commercial revenues and profits,” Scott said.

SOURCE: Inverse