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Congratulations to Bruce Grierson on winning a Digital Publishing Award for this article.

Edinburgh isn’t known as a hotbed of industrial espionage. But one cool and quiet spring night in the Scottish city, a high-stakes burglary was underway. Down at the old port district of Leith, thieves breached a perimeter fence and broke into the offices of a company called Pelamis Wave Power. They homed in on four laptop computers and walked right past much more expensive equipment. Pelamis, at the time (March of 2011), was riding a wave of good fortune. Company engineers had produced the first commercial-scale machine for extracting energy from waves, vaulting Pelamis to top-dog status in the marine-energy industry. Already there was interest from several European utility companies, and a Portuguese company had placed an order. So promising was the technology that just two months earlier, a delegation of 60 Chinese officials had paid a visit, with a juicy investment deal presumably in the balance. The world was getting excited about wave power. The visitors donned white hard hats and Pelamis founder and director Richard Yemm led Li Keqiang, the vice premier of China (now premier), and his charges across the factory floor during a key phase of production. Yemm was likely thinking only of the dizzying future on the other side of so much hard work, so many stillborn dreams. Protecting his company’s valuable intellectual property was not top of mind.

Yemm’s optimism was justified. At some point in 2013, the world’s energy scales tipped: for the first time, more new energy was produced by renewables than by fossil fuels. The shift is officially on. North Sea oil rigs are being dismantled. The run of coal as energy champion of Europe is over, and plans for hundreds of new coal plants across Asia have been shelved. The business case for solar is solid. Electric trains in the Netherlands run on wind energy. Google just announced that its server farms and offices will be powered entirely by renewables—mostly wind and solar—by the end of 2017.

And ocean power?

Close to 200 trillion watts of kinetic energy lurk in the seas: more than enough to power the planet, if we could somehow extract it all.

It’s there in many forms, inviting different approaches. We can exploit temperature and salt gradients, harness tides and currents, and tap waves—the method that intrigued the Chinese government enough to jet a delegation to Scotland. Of course, not all of that theoretical marine energy is practically available. The European Commission, which manages the day-to-day affairs of the European Union (EU), has set a goal to have 10 percent of Europe’s energy supplied by the sea by 2050. The EU has a big head start on the rest of the world—the United Kingdom alone has as many marine-energy projects on the go as the rest of the world combined—so a reasonable target elsewhere will no doubt be lower. But we’re still talking about a nontrivial part of the energy conversation—if the regulatory stars align for this brand-new industry. That’s a big if.

“The cure for anything is salt water—sweat, tears, or the sea,” wrote Danish author Isak Dinesen, who wasn’t thinking about knocking down carbon emissions. But she did seem to intuit that pain is the midwife of all saltwater cures. That was certainly the case for the team members at Pelamis, who, failing to secure any investment money from the Chinese delegation, were left with a nagging worry about their stolen intellectual property. And it is the case for dozens of marine-energy developers racing to produce viable, commercial-scale technology. So far, the primary thing they’ve extracted is an insight: this isn’t going to be easy.

I. Wave Energy

Neil Kermode stands atop a cliff on Mainland, one of Scotland’s Orkney Islands, watching surfable rollers pound themselves into salt mist on the craggy shore. The windswept archipelago makes more renewable energy than it will ever need. It is an ancient place that provides a glimpse of the world’s energy future.

“Wave energy is basically old wind energy,” says Kermode, head of the European Marine Energy Centre (EMEC) and director of the world’s only test site for wave-energy technology, here near the town of Stromness. “The wind starts waves moving, and the waves keep coming even after the wind stops.” The notion that waves are a kind of wind battery, retaining the energy, is the magic at the heart of wave power.

The world’s ocean waves are thought to contain around three terawatts of harvestable juice—enough to meet the world’s electricity demands at a given moment—if someone could make the technological leap to harvest it efficiently. Indeed, that was Pelamis’s goal when it launched in 1998, and EMEC’s goal when it set up the test site in 2004. For a while, the site crackled with high hopes. Entrepreneurs arrived with their comically named prototype devices, anchored them offshore, and plugged them into power cords that sent electricity to a substation on the beach. As recently as two years ago, four devices chugged away. Today there is one. In a cautious investment climate, wave-energy development has collapsed from a torrent to a trickle, forcing most of the players—including Pelamis—into administration (court-assisted relief).

To understand the ebb and flow of interest in wave energy, you need to go back to 1973, when the Arab petroleum embargo, and the sudden quadrupling of the price of oil in its wake, prompted frightened governments to start mulling alternatives.

The UK department of energy issued a Manhattan Project-level challenge to engineers: come up with a two-gigawatt power station that runs on ocean energy. It was “like somebody in 1905 asking for an Airbus A380,” says Stephen Salter, now a professor emeritus in engineering design at the University of Edinburgh.

But back then, Salter was a young engineer too green to be discouraged. He put his head down. And what he came up with was a duck.

Inspired by the float in a toilet cistern, Salter’s “duck”—imagine a tobacco pouch the size of a cottage—would bob in rough seas, generating electricity as waves bowled it off plumb. Each duck could produce, Salter reckoned, around two megawatts of energy, so he’d need a lot of them. Early tank tests showed Salter’s duck could absorb wave energy with great efficiency, leaving flat water behind it. And a mightier duck still lay in the offing, Salter forecast. If a small computer could one day ride inside the contraption, making continual adjustments to the hydraulics, the duck would bag 90 percent of the wave energy beneath it. “A bit of weather judo,” as Kermode describes the principle. “You use the wave’s energy against itself.”

The duck made Salter a little bit famous. “He was the first to capture the public imagination with the idea that waves and energy fit naturally together,” says Kermode, who remembers watching him on TV as a youth.

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But something happened on the way to a renewables renaissance.

In 1983, the government pulled the plug on wave research, and Salter’s Edinburgh duck, along with all its rivals, was dead in the water. Oil prices had come down and the need for a federally sponsored renewable-energy moonshot was over. At least that was the official explanation. Salter doesn’t believe it for a second. By his estimation, the government killed it because it was a little too promising and so represented a threat to its private crush—nuclear energy. They submitted head-scratching reports that vastly overinflated Salter’s capital costs. “They adjusted all the numbers to make it look like it was going to be terribly unreliable,” he says. A later investigation would reveal that the government had overstated the cost of Salter’s duck by at least a factor of 10.

Even today, Salter’s duck remains the most efficient wave-energy device ever designed. And that game-changing, on-board computer he dreamed of? The technology now exists. It costs about £2 (US $2.50).

Not all the expertise from that era died on the vine, however.

Salter’s doctoral student Richard Yemm went on to found Pelamis and create the promising wave-energy converter that grabbed China’s attention.

Dubbed “the sea snake,” Yemm’s device was segmented like a broken crayon, with energy ginned up at the hinge points. It was the first wave device to take the helter-skelter motion of open seawater and turn it into electrical current smooth enough for the grid. Yemm had proven that his device worked, but not that it could be profitable, and in 2014, he reluctantly declared Pelamis insolvent and bailed out into a consultancy job. In the three years following the burglary, Pelamis’s technology had failed to materialize on the renewables scene. The trail of the thieves from that March night in 2011 was cold. After Pelamis folded, Wave Energy Scotland, a newly created government research body, bought some of the company’s assets, including all intellectual property, to protect it and keep it in Britain. Just in case.

Hope for wave energy lingers in the United Kingdom. British multimillionaire Adam Norris—perhaps the closest thing marine power has to an Elon Musk—has sunk at least £50-million (US $64-million) into Wavepower, a company he launched, and last year started mustering top British engineering talent to fancy offices in Glastonbury*. But the lost years mean that wave energy lags behind every other technology in the renewables race. Meanwhile, the action in the marine-energy sector has moved to a different space: one that is deeper, darker, and more pressure packed.

II. Tidal Power

In a 45-minute window of quiet water as the tide turned one day last November, a tug towed a barge out into the Bay of Fundy off Canada’s east coast. It reached its designated berth at the Fundy Ocean Research Center for Energy (FORCE), Canada’s chief proving ground for marine-energy research. A crane lowered a 1,000-tonne turbine that looked like a jet engine, and eased it into the sea, where it settled into the rocky seabed. Then everyone waited, a little on edge.

They’d seen this movie before: humans intervening in the Bay of Fundy to generate energy. The returns had never lived up to the hype.

Capturing the tides is one of humanity’s oldest energy ideas: the ancient Romans may have used tide mills to grind grain. It is a classic barrage scheme with gates closing behind the flood tide and trapping the sea, which is then released through turbines to do work or make energy. Barrage schemes on La Rance, a river in Brittany, France, and at Annapolis Royal near Digby, Nova Scotia, have been providing a trickle of electricity to those communities since 1966 and 1980 respectively. In 2011, South Korea jumped in with the only modern tidal barrage, near Suwon. But like many dams, barrage schemes are unkind to sea animals and coastlines. Indeed, it’s environmental concerns that have held up a huge tidal-lagoon play in Swansea, Wales, for more than a year. So engineers have played around with ever-inventive designs to capture the energy of moving water without corralling the water itself: a mechanical fin that wags like a shark’s tail; a raft that floats downstream, unspooling a cable from a drum/generator onshore; a device with no moving parts at all that exploits the pressure difference in the water flow. All are being tested right now. It’s like watching evolution unfold in real time: wildly different organisms competing to see which is the fittest.

But out of the design chaos, a pattern is emerging. The consensus—though it’s not unanimous—is that the most efficient way to get electricity out of the ocean, with the least harm to the environment, is to put a windmill in the water. In other words, we’re now thinking vertical, not horizontal. Where tidal-range (barrage) schemes leverage the difference between high and low tides, tidal stream projects plug into a tidal current as it flows through a turbine stapled to the seabed, like wind moving through a pinwheel. It was a tidal-stream turbine that OpenHydro, an energy company from Dublin, Ireland, was placing in the Bay of Fundy’s Minas Passage last November. The effort was a do-over of a failed try seven years earlier at the same site, which is arguably the fiercest tidal-energy hotspot in the world.

Twice a day, 14 billion tonnes of water move through the narrows between two steep headlands, Cape Split and Cape Sharp: that’s more than the combined flow of all the rivers on Earth. On the ocean floor, enormous boulders roll with every ebb and flow tide. When the Minas Basin fills, the weight of the water causes the surrounding land to measurably dip. At full flood, the passage is a riot of gyres and standing waves. The churned silt makes the water look like your latte, if your latte moved at five meters per second.

In that first instance, OpenHydro had deployed a CAN $10-million prototype in the passage. Company engineers had tested the turbine in the stiff tides of the Orkneys and it had borne up. But in Fundy, it was as useless as windshield wipers during a car wash. Within 20 days, all 12 of the turbine’s rotor blades were damaged or destroyed. OpenHydro had made this second version much more skookum. Still, nobody could be sure it could stand up to the fearsome Fundy current.

Recent estimates put the amount of raw kinetic energy in the Minas Passage at over seven gigawatts—more than enough to meet Atlantic Canada’s energy needs if all of it was extracted (which can’t be done for reasons of physics as we’ll see in a moment). Nova Scotia has decided that tidal energy is key to future prosperity, and taxpayers are now subsidizing the bet to the tune of $36-million. The OpenHydro turbine alone is projected, as a fossil-fuel replacement, to prevent as much as 3,000 tonnes of carbon from entering the atmosphere. And the plan is to sink a second turbine nearby as early as this fall. Much of the power will likely leave Canada’s Atlantic provinces. Right now, transmission cables are being laid to the south, to ship electricity to New York and Boston.

A potential devil’s bargain looms here. Get too greedy and you put not just sea life, but coastal dwellers at risk. As you pull energy out of a water flow, you reduce that flow. It’s unwise to just cover the seabed with turbines like you’re planting corn, because the turbines start interfering with each other. And the water level rises. Coastal modeling has killed any thoughts of a vast array of turbines in the Bay of Fundy. Not long ago, scientists at Acadia University in Wolfville, Nova Scotia, calculated that of the seven gigawatts of energy coursing through the Minas Channel, turbines could skim off perhaps a third. Any more and you start messing with tides in New England. In one model, a barrage raised high tide in Boston Harbor by 23 centimeters. “Even the possibility of such an impact,” an analyst’s report noted, “was seen as sufficient to draw lawsuits from every property owner with a flooded basement from Nova Scotia to Cape Cod.”

Tidal is the new frontier of clean power. Or it’s too green to bet on.

That paradox was lost on no one at the International Tidal Energy Summit in London, England last October. At one point, Tim Cornelius, CEO of Atlantis Resources, strode to the podium with a chip on his shoulder, frustrated by pessimists thwarting progress. Atlantis owns the MeyGen AK1000, the monstrous tidal turbine that had been grabbing all the headlines. In 2010, Atlantis got clearance to build the world’s largest tidal-energy plant off Caithness, at the northern tip of Scotland. Four turbines are in place there now, with eight-meter blades pushed by some of the fastest tidal currents in the United Kingdom. The plan is to step up to an eventual 269-turbine array that would cover over 10 square kilometers, generate enough power to run 175,000 homes, and provide work for hundreds of laid-off nuclear workers from the retired Dounreay nuclear plant.

Everyone is excited about this except investors.

The promise of tidal-stream energy has failed to seduce venture capitalists: it’s too risky, too costly, too pie in the sky. A moonshot. Which is precisely what’s great about it, Cornelius suggested to the crowd of a couple hundred. “When you explain tidal power, the average punter loves the idea,” he said. He’s right. There is poetry in the idea of harnessing the moon’s gravitational pull. Because water is almost 900 times as dense as air, a tidal turbine a third the size of an offshore wind turbine can deliver the same output. And turbines are sunk out of sight, so can be set close to shore with nary a grumble from property owners (and big savings in cabling costs).

Alas, troubling practical concerns keep getting in the way of the magic. As a medium for commercial enterprise, the sea is as hostile as deep space.

“Hats off to anyone who can put something mechanical in salt water and make it work,” says Keith Collins, executive director of sustainable and renewable energy for the Nova Scotia Department of Energy. If a wind turbine breaks, you send a guy up a ladder with a toolbox. It’s quite another thing to have to dispatch divers—or worse, to have to hire a big ship to schlep your turbine to the garage.

“The vast majority of faults are very minor,” says Peter Fraenkel, inventor of the first commercially successful tidal-stream turbine, SeaGen, which was commissioned in 2008 for action off the Northern Ireland coast. “It’s usually a small electrical component or some silly little thing. If you have to replace a $20 component by spending $20,000, that’s a bit of a downer.”

And a deal-breaker for investors.

So there is another approach to tidal turbine design, one many industry insiders consider the most promising way forward: make a device that floats.

One day last October, at the EMEC tidal test site in the Scottish Orkneys, a yellow submarine bobbed on the chop, snugged to its anchor lines. This is the Scotrenewables SR2000, the world’s most powerful tidal turbine. From below the surface, with its twin rotors deployed, it looks like a Klingon Bird-of-Prey. The design lends itself to on-site repair but if it’s a more serious fix, you just pull up its twin one-megawatt turbines and tow the whole megillah to sheltered waters. The company is focusing on markets in the United Kingdom, France, and Canada, although they’re also looking into opportunities in Asia. The portability brings the whole world into play, including choice sites where the water’s far too deep to put a turbine on the seafloor.

Piggybacking on the offshore wind industry’s great leaps in engineering efficiency, companies like Scotrenewables are making the dream of a world powered solely by renewables seem less far-fetched by the day.

But that dream gives grid managers nightmares. They need to know that an electron will be available at the exact moment it’s needed—not a guarantee renewables can offer. There’s often too much or too little going on at once: too much wind or sunshine when the grid is already full; breathless calm or cloud cover when you could really use some juice. Here is where tidal has an advantage. The moon has yet to miss a shift. Twice a day it pushes water and twice a day it pulls, and we know exactly when and how much. That predictability changes the math around how affordable tidal energy actually is. The trouble is, tidal still can’t provide “always there” baseline energy—until some storage solution is perfected. Whoever cracks the nut of cheap baseline power in the renewables age will author the biggest disruption story since the internet.

But if tidal stream can’t provide that coveted baseline energy right now, there is a kind of ocean energy that can.

III. Currents from Currents

Ocean-current energy is thought to be a 100-gigawatt global resource—about the same size as tidal stream, and with the same problem of converting that power to useable energy, yet still theoretically exciting. Currents like the Gulf Stream and the Kuroshio—the Japan current that plies the western edge of the Pacific—circulate lazily around the world, swinging by many major population centers.

“The whole Asian sphere is full of moving ocean,” says Martin Edlund, CEO of a marine-energy company called Minesto, based in Gothenburg, Sweden. Not long ago, Edlund sat down with Taiwan’s energy minister to discuss how to conquer “the black current,” as the Kuroshio is sometimes called. “If we take the numbers that they themselves pull together,” he says, “we’re looking at a 50-percent contribution to the energy mix of Taiwan.”

The Zen-like steady progress of the world’s currents—no hurry, no pause—makes them potential catnip to grid operators. The hitch is the “no hurry” part. Those tens of billions of gallons of water per minute move at a speed not much quicker than the walking pace of a human late for work. Since the kinetic energy is proportional to water speed cubed, that doesn’t amount to much juice in any given spot. Which means an awful lot of turbines, or very big turbines. Or something completely different.

That’s where Edlund comes in.

“We’ve stumbled upon a unique principle,” he says. “What we do is, we fly a kite underwater.”

Think of a whole fleet of remote-controlled kites. Pushed by the current, they turn perpetual figure-eight patterns, their flight paths continually tweaked by a computer on the surface. “So in the same way that you can sail faster than the wind, you get a flow going past the wing that’s much higher than the speed the ocean is actually moving,” Edlund says.

Minesto has deeper pockets than most marine-energy companies—its principal owners are a Swedish private-equity firm and the Saudi Arabian billionaire Sheikh Mohammed Hussein Al Amoudi—which arguably gives it more leash for maverick explorations. It has tested its control systems in Sweden and floated its hardware in Northern Ireland. A commercial-scale project is getting going—the world’s first low-flow ocean-energy harvesting—in Wales.

Not everyone is sold. “The idea that there’s some clever way of taking a lot of energy out of low flows is, to my mind, misleading,” says Fraenkel. “If the energy’s not there, it’s not there to be taken.” Fraenkel remains a believer in ocean-current energy, just not in any sexy way to extract it. His former company, Marine Current Turbines, holds a patent for a six-rotor machine to operate in the Gulf Stream or the Kuroshio; you can imagine an array of them slowly grinding away, just under the surface where the water flows fastest, like nodding donkeys on the Texas plain.

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But at the International Tidal Energy Summit awards in London, Edlund received the endorsement of his peers. After apologizing for standing between the diners and their pudding, he accepted the award for most promising turbine design. In this field, that’s not a Miss Congeniality award—it’s the real deal. Because at this stage of the industry’s development, promise is still pretty much all there is, despite grand plans and sometimes juicy incentives.

IV. A Medley of Marine Solutions

Nine years ago, the government of Scotland announced the creation of the Saltire Prize—a kind of XPRIZE of the sea. The competition promised £10-million (US $12.6-million) to the first company to create a viable marine-energy system and demonstrate it in Scottish waters. (Viable meaning at least 100 gigawatt hours of power over a two-year period.) There was a lot of hype. Then-prime minister Alex Salmond hailed Scotland as “the Saudi Arabia of tidal power” and claimed it has the potential to match the wealth created by North Sea oil.

At the time, Fraenkel ran the numbers at Marine Current Turbines. “We tried to figure out if there was any way we could win it, and we decided there wasn’t,” he says. “To build the size of project you’d need to win the Saltire Prize, you’d probably have to spend £80- to £100-million. In which case £10-million is a drop in the bucket.” It’s now clear that nobody is going to win it, at least not as originally conceived. The Saltire Prize’s website now admits that “the path to commercialization is taking longer and proving more difficult than anyone initially expected.”

You could argue that there’s just not enough chicken on this bone, period. The technology is so inefficient, the costs so high, the risks so prohibitive, that marine energy just isn’t worth it.

Of the vast potential energy of the ocean, only a very small fraction is practically extractable, says Vaclav Smil, an environmental scientist at the University of Manitoba and author of the book Energy Transitions. Tidal energy, for example, is a three-terawatt resource, yet only about 60 gigawatts’ worth lies within a transmission cable’s reach of shore. That amounts to one-third of one percent of global primary energy—“hardly a notable contribution,” says Smil. “Installing triple-glazed windows and universal use of LEDs would save vastly more energy than will ever be extracted from the ocean,” he added in an email.

So if that Eeyore-ish estimate is even in the ballpark, the question is, why do this? If the sea is so reluctant to give up its treasure, why should we even bother with it?

Here’s one answer: because we have to think of energy differently now. The low-hanging fruit will soon be gone. All the other options are going to be more challenging. What will make or break the case for each of them is not so much what they are as where they are.

“Until storage gets exceedingly cheap, or social license is such that you can build wind turbines and giant hydro dams everywhere—and I don’t see that happening—you need a suite of all these different technologies,” says Bryson Robertson, a mechanical engineer at the University of Victoria-affiliated West Coast Wave Initiative in British Columbia. Blanketing the Sahara with solar panels may be the cheapest way to do renewable energy right now, but it’ll never be the answer in a temperate rainforest. Where there’s a mountain, you tap the streams spilling down it with run-of-river projects. Where there’s a pinch point in the coastal landscape, you steal energy from the tide. You buy what the Earth is selling, where it’s selling it. Indeed, to try to choose the best among renewable energy sources is as ridiculous as going all in with a single vitamin in your diet, says Stephen Salter. What’s needed is a bit of fusion cooking.

Off the coast of Argentina, a company called SeatechEnergy is making fuel from seaweed. Grown in vast farms in high-productivity zones, the seaweed is digested into natural gas, which is convertible to electricity, with no solid waste.

Off Belgium, plans are in the works for 10 to 12 manufactured protective atolls, which would guard the coast from erosion as the sea rises. The idea is that the ocean, as it sluices in and out of the lagoons, runs through tidal turbines of the same sort already built into dikes in the Netherlands. This may be marine energy’s biggest advantage over other renewables in the coming century: it naturally piggybacks on the defense barriers that every coastal community is going to need as global warming bites in.

Gunter Pauli, the “ecopreneur” who has been called the Bill Gates of sustainable energy, initiated the first idea—those manufactured islands—and is kicking the tires on seaweed power as a natural adjunct to it. This is Pauli’s so-called “blue economy”—an interdependent network of energy choices driven by carefully integrated local supply chains and meeting local needs. Cluster technologies and suddenly you have not just green solutions—that might help revive the biodiversity of coastal zones, for instance—but a solid business model. “If you do tidal plus seaweed—a strange combination to most people, because it’s not solar plus wind—you have very interesting opportunities to supply a mix of local power,” Pauli says. “That is where the future lies. It’s not, ‘Oh, we’ve got the golden egg of this new energy source.’”

Another promising turn, in a way, is suspiciously familiar. Last fall, a wave-energy converter called the Hailong (Dragon) 1 appeared at a test facility in China. It is nearly identical to the Pelamis sea snake, right down to the paint color. The Guardian newspaper pressed the Chinese government for details about the origins of Hailong 1, but received no reply. Some former Pelamis employees privately worry that Pelamis might have done an awful lot of wave-energy and development work that the Chinese are now poised to make commercially viable.

Sad for the original creators, but perhaps good for everyone?

Marine energy will never be the new coal or oil—two fossil fuels that revolutionized the world. Where it could well shine, however, is in delivering power to the 40 percent of the world that has no reliable power now. Plus, marine energy could be combined with fertilizer, feed, and food—addressing global food-security issues, Pauli notes. Even the most eccentric schemes may have value so long as they are perfectly matched to their geography and put energy decisions into local hands.

On my last day in Orkney, I woke before dawn to pack for home. As I turned on the coffee maker in the hotel room, something occurred to me. A few kilometers away at the EMEC test site, a small OpenHydro tidal turbine was quietly supplying a trickle of energy to the island.

Since the machines in Fundy and Caithness were briefly offline, and I was up before just about everybody in France, I was enjoying a staggeringly exclusive experience. With perhaps a handful of Koreans, I was one of the only people in the world drinking coffee made from the power of the sea.

It tasted quite good.

*Shortly before publication of this story, Wavepower ceased operating.

Correction: A previous version of this article said one hundred percent of Dutch trains run on wind. According to the national railway company, all electric trains are powered by wind energy.