The Ocean – Harvesting The Highs And The Lows

An Introduction

Riding the Waves
Tidal Flows

Plumbing the Ocean Depths

Potential and Pitfalls

An Introduction

There is a phenomenon on Newfoundland’s East Coast Trail that is a wonder to behold. It’s called the Spout, and it’s visible for miles around, though accessible only by boat or daylong hike along the dramatic coastline. Every minute or so, a huge gasp of watery spray jets out of a hole in the ground and shoots into the air. The eruption looks like the spout of a giant whale coming to the surface to breathe. That jet of water packs quite a punch – I’ve seen it up close. In fact, I once threw a small tree down the Spout; seconds later, I watched it shoot into the air high over my head.

At the Spout, nature has conspired to create an underground cavern, shaped just right to concentrate the up-and-down action of waves over a large area into that single watery jet. It doesn’t take much – waves only a few feet high cause a spout1 100 feet or so (30 metres) above sea level. I’ve stared at the Spout for hours, wondering how on Earth it works – and whether we could duplicate that underground geometry to create a new kind of power plant.

The answer is yes – but the Scots beat me to it long ago. Their LIMPET design, on the Scottish Island of Islay, translates the up-and-down movement of waves into a stream of compressed air that spins a turbine, creating electricity. Wave power is just one way of getting energy from the ocean.

The world’s oceans are actually one massive, interconnected body of water. Covering more than 70% of the Earth’s surface, they’re the source of the world’s freshwater, as the sun evaporates water at the surface, leaving the salt behind. The five oceans (the Atlantic, Pacific, Arctic, Indian and Southern) were formed over millions of years, as the continents shifted, and they remain largely unexplored. The deepest point on the planet is the Marianas Trench, almost seven miles (11 kilometres) down. A two-man team from the US Navy reached the Marianas Trench back in 1960. Since then, only a robotic vehicle called Nereus has managed to reach it, in June 2009. No working craft remains that can take humans so deep under the sea.2

Oceans play a starring role in global climate patterns. They moderate coastal temperatures, ensuring that coastal cities are cooler in the summer and warmer in the winter than their landlocked cousins. Ocean currents like La Nina and El Nino, the Gulf Stream, and the Great Ocean Conveyor Belt3 move huge amounts of heat around the globe. The Ocean Conveyor and Gulf Stream are responsible for Europe’s moderate climate; without it, places like the UK would be as chilly as Labrador, on Canada’s east coast. Oceans also help moderate global warming, absorbing as much as one-third of our carbon emissions.

As the Earth warms, the oceans’ contribution to our climate will change in a number of ways. Instead of being a carbon sink (absorbing carbon dioxide from the air), they could flip into a carbon source. Just like an open bottle of pop left in the sun, all that absorbed carbon dioxide could bubble out, causing a huge jump in global warming. The Ocean Conveyor might slow4 or even stop, moving Europe’s climate closer to that of Canada. Hurricanes will get stronger as the oceans warm, since it’s the heat from tropical waters that fuel them. It’s already happening.

Oceans also contain energy that’s useful to us, of course, and it can be captured in three ways. First off, there are all sorts of funny-looking contraptions, with names like the Clam, the Whale and the Anaconda, that ride the waves and convert their movement into electricity. Then there are fields of underwater turbines that can be installed in tidal currents, spinning as the tides rise and fall, much like wind turbines. Finally, giant vertical tubes can plumb the ocean depths, exploiting the difference in temperature between the surface and deep water, and creating enough energy to spin a turbine.

All three forms of renewable energy work, but it’s tidal power that will likely be the easiest to exploit in a big way. There are some tricky engineering and reliability issues to solve when it comes to wave power, but it too has potential. As for those vertical tubes, they’re still more curiosity than utility. There is, however, an audacious plan for a different kind of tube – one that uses wave power to promote vast algae growth, translating wave power into carbon sequestration.

Riding the Waves

Waves are the ocean’s way of storing wind power. Wind passes over the water’s surface, causing ripples that are then amplified by more wind. By the time large swells reach the coast, they’ve often traveled thousands of miles, with the storms responsible for creating them long gone. Although waves appear to be water moving horizontally along the surface, that’s an illusion. Much like the human “wave” at sporting events, where each person stands up and sits down in turn, no single particle actually follows the wave along the water’s surface. Instead, they move up and down in a circular motion5 as the force of the wave passes by – until it breaks onshore.

Harnessing the power of waves isn’t easy but it can be done. There are almost as many ways to do it as there are people who have tried. The trick is to build a machine that translates all that motion – random and regular – into movement uniform enough to produce power. The really hard part is making sure it can stand up to the harsh ocean environment.

Onshore systems take advantage of waves’ energy as they hit the coast. One trick, an oscillating water column, works much like the Spout. Imagine an open pop bottle, with its bottom cut off, sitting in the water. As the water level goes up and down with passing waves, air moves through the opening and spins a turbine.6 The key to generating power is to perfect the geometry of the chamber to maximize the force of air for a given size of wave.

Another way to tame the waves for onshore production is to build a tapered, uphill ramp leading to a reservoir. As waves hit the ramp, they get narrower, converting their energy to height.7 It’s like pumping water up a hill using wave physics. Once the water is in the reservoir, electrical production is the same as conventional hydropower.

As for offshore production, there are dozens of tricks, with one constant: One part of the machine stays roughly still relative to the surface of the water (or what would be the surface, were it calm). Another part moves with the waves, either pumping liquid or moving air to spin a turbine.

The aptly named Clam is a large, 12-sided floating device that pushes air between 12 separate chambers. So far, it’s only a prototype. But when it’s built to full scale, the Clam will be more than 180 feet (60 metres) in diameter, with chambers big enough to hold a conventional hallway. Another prototype, the Whale, is shaped like a giant whale tail that oscillates with oncoming waves.

The only commercial-scale project to date is the Agucadoura Wave Park, off the coast of Portugal. It’s based on a Scottish-made machine called the Pelamis – a 500-foot-long (150-metre) snake-like system that bends and flexes with the waves. Unfortunately, it has been plagued with problems: A few months after it was deployed in 2009, it was hauled back to shore with technical difficulties. Then the project’s financing fell through. For now, the giant Pelamis sits in a harbor, awaiting a second chance.

The Anaconda, a similar device made by UK-based Checkmate Sea Energy, is made of rubber and fabric. Checkmate is betting that its simpler design will get past the trial stage, and the company hopes to one day produce commercial-sized Anacondas 650 feet (200 metres) long. The dream is for hundreds of these serpentine power plants to be anchored offshore, each of them delivering enough power for 1,000 homes.

Wave energy is a tricky business, not just because of the unique way waves deliver their energy, but also because the ocean is such a harsh environment. There are lots of wonderful devices and original ideas, and the wave-energy industry is probably only a decade or so away from delivering utility-scale power.

Influential thinker and scientist James Lovelock, originator of the Gaia hypothesis, 8 has proposed a simple and elegant way to suck carbon out of the air and store it in the ocean.

How does it work? Lovelock’s idea is to install a bunch of tubes, 650 feet (200 metres) in length and 65 feet (10 metres) in diameter, that go from surface to bottom, with a one-way valve on the lower end. As the tubes move up and down with the waves, the cold, nutrient-rich water close to the ocean floor rises to the surface. When the sun hits that water, algae grows and sucks carbon dioxide out of the air. When it dies, some of that algae will drop to the ocean bottom. Voila – cheap carbon capture.

Tidal Flows

Ever wish your days could last a few hours longer? Wait a few hundred million years and they will. The tides – that great sloshing-around of the oceans as they are pulled one way and then another by the moon’s9 gravity – dissipate enough of the Earth’s rotational energy to slow down the rate of spin. About 600 million years ago, one day – one spin of the Earth on its vertical axis – lasted only 22 hours, not 24.

Sir Isaac Newton laid the foundation for our understanding of gravitational forces back in 1687, when he famously explained that the moon and sun exert a force on the Earth similar to the one that pulls apples to the ground. A century later, another great mathematician, Pierre Simon Laplace, came up with differential equations10 describing how water moves horizontally given a tug upward from the moon. That horizontal movement translates into tidal currents.

In places where that moving water is forced though a channel, very strong currents result. Anyone who has battled tidal flows in a kayak knows firsthand how quickly they can arise and how powerfully they flow. The Bay of Fundy, in eastern Canada, is home to some of the most dramatic tides on the planet. The rise of the water is more than 50 feet (15 metres), and when the tidal currents are forced through the Minas Passage, the water flows at speeds of up to 8. 7 miles per hour (14 kilometres per hour). At its peak, that flow has a volume of more than all the world’s rivers and streams put together! Converting that moving water to electricity is what tidal power is all about.

The Romans were probably the first to build mills that made use of tidal currents, near what is now London, and they remained in operation throughout the Middle Ages (to see one in action, check out the reconstructed Woodbridge Tide Mill, originally elating from the 12th century, near Suffolk). The French brought tidal power into modern times in 1966, building a 240-megawatt generator enough to power 240,000 homes – in the Rance Estuary. An early Canadian effort was the much smaller 20-megawatt generator in Annapolis, Nova Scotia.

How does it work? There are two ways to do it. The first is with a tidal barrage – a kind of dam that traps water as it rises with the tide, then drains it through a turbine. In reverse, the barrage stops the water from rising before releasing it through the turbine. The process is similar to conventional hydropower, except tidal barrages build potential energy in the water by exploiting the moon’s gravitational pull. That’s how older systems, like the one in Rance, work.

A more modern method is to place turbines right in the tidal current, letting the moving water turn the blades directly. These turbines can work in both directions. Since water is about 800 times denser than air, it contains much more kinetic energy than air moving at the same speed. Water flowing at eight knots (15 kilometres per hour), for instance, contains as much kinetic energy11 as a 263- mile-per-hour (424-kilometre-per-hour) gale!

It’s not easy tapping that power. Much like offshore wind farms, these turbines need to withstand the corrosive effects of saltwater and be maintained on the open sea. To be economical, they need to work reliably for years. But tidal power is well on its way to commercial viability.

Marine Current Turbines (MCT), based in the UK, was a pioneer of the tidal-current turbine, and it’s now a world leader. Peter Fraenkel, the company’s technical director, first proved the concept in 1994 with a I5-kilowatt turbine (enough for 15 homes) in Lock Linnhe, Scotland. MCT’s latest model is the SeaGen, a giant, 1.2-megawatt generator (enough for about 1,000 homes) that costs around $14 million. Two separate turbines hang off a central tower, which is driven into the ocean floor. Both sets of blades can be hauled out of the water for maintenance. A 10-megawatt tidal farm in Wales, the Anglesey Skerries, is set to begin construction in 2011. It will grow to an industrially respectable size of 100 megawatts-enough for a good-sized town – if fully commissioned.

MCT is not the only game in town. Verdant Power, based in New York, has a 10-megawatt project in the works in the East River, and a 15-megawatt farm of turbines is planned for Canada’s St. Lawrence River (Verdant’s M.O. is a mix of tidal and river current flows). Yet another player, Clean Current Power Systems, operates a small pilot project on Canada’s west coast, which is considered to have a total tidal potential of around 4,000 megawatts, or the equivalent of four medium sized coal plants.

The biggest project currently on the table stems from a deal between the UK’s Lunar Energy and a contingent of South Korean partners. Their plan is to build a 300-megawatt tidal farm off the Korean coast, at a cost of roughly $750 million. That power is comparable to about half a medium-sized coal plant. Similar in size to the SeaGen, the consortium’s Rotech Tidal Turbine also works in both directions, but has the added twist of a “venturi” opening: Water enters a duct with a diameter of 49 feet (15 metres) and is compressed into a smaller channel before hitting the 38-foot (11.5-metre) turbine. This accelerates the water flow and straightens it, too, eliminating the need for complicated blade-pitch adjustments.

There are more plans afoot. The tiny Channel Islands of Guernsey have the potential to generate the equivalent of 25 medium-sized coal plants, and it should all start around the island of Alderney. Alderney Renewable Energy was formed in 2008 to farm the tidal power around the island, and the company could generate somewhere between one and three gigawatts of power, which will be sold into the French and UK grids.

Is this the start of something big? Time will tell, but the amount of the resource available is significant. Total worldwide capacity is estimated to be around one billion kilowatts, or two trillion to three trillion kilowatt hours of output annually. That’s about six times Canada’s electrical generation, or about the same as all American coal-based electrical production.

Plumbing the Ocean Depths

There is another way. Heat pumps, like the ones used to harness geothermal energy, can also be applied to the ocean. As the sun warms the ocean’s surface, it creates a temperature difference between that water and the water deep below. That differential can be used to drive a heat engine and generate electricity. Ocean thermal energy conversion (OTEC), as it’s called, is sort of like geothermal – but in this case, you’re diving deep to get the cold stuff, not the hot stuff. The idea behind OTEC is simple; scaling it up to utility-sized production isn’t.

How does it work? The most common method is to use the heat from the warm surface waters of southern oceans to evaporate a liquid with a low boiling point, like ammonia. (That liquid is contained in a “closed system” which means it doesn’t mix with the ocean water.) The expanding gas spins a turbine, much like a steam generator. Pipes going deep into the ocean bring up cold water, which condenses the evaporated ammonia back to liquid, and the process repeats.

So far, a few pilot plants have been built but they’ve all been far too tiny to be viable – one generated enough electricity to power the lights and computers on the barge the plant sat on. Other OTEC plans have run into major technical difficulties. In 2003, Indian engineers twice tried to lower a 2,600-footlong (800-metre) tube into the ocean as part of a one-megawatt plant, only to have the pipe drop to the ocean floor both times. Believers haven’t given up, though. Giant defence contractor Lockheed Martin is set to build a 10 to 20-megawatt plant (enough for up to 2,000 homes) in Hawaii. The plan calls for a 3,300-foot-long (one-kilometre) pipe with a diameter of 88 feet (27 metres) plumbing the ocean’s depths. Another company, OCEES International, based in Hawaii, intends to build an OTEC plant for the island of Diego Garcia, home to a US military base.

Enthusiastic proponents of OTEC claim the technology could produce limitless energy, and many dream of massive floating barges harvesting the ocean’s energy bounty. But OTEC is destined to remain a niche technology. It might take root in an isolated, highly valued property like the Diego Garcia military base, or reach small commercial scale in Hawaii, but it won’t make much of a splash anywhere else. Not only is it extremely expensive, but the structures required for utility-scale generation are huge and need to be built in an environment that’s hostile to grand engineering experiments (to say nothing of algae and barnacles).

Potential and Pitfalls

So, with OTEC relegated to the fringes, where does that leave us? Wave energy can emerge as a major energy contributor (exploiting all the technically available power for an island nation like the UK would amount to 20%12 of total electrical use), but it needs to move from prototype to commercial scale fast. Although they haven’t yet been tested on any significant scale, farms of carbon capturing, wave-powered pumps could provide some much-needed relief for our carbon-ridden atmosphere.

Tidal energy holds the most promise, and we’re well on our way to developing utility sized tidal power plants that will probably end up producing the same amount of energy as conventional geothermal does now.

Tidal power has its drawbacks, though, including big capital costs and limited geography. Plus, it’s intermittent, since tidal generators operate only when the tides flow, generating power about 10 hours a day (twice that, if the turbines are bidirectional). And since tides are dominated by the lunar cycle, which is out of sync with our 24-hour clock by about 50 minutes, the times when tidal plants generate power change day to day. Inconvenient, but not a deal-breaker. Utilities value reliable power, and the tides are as reliable as a good watch.

The bottom line is that it’s too good to pass up: Worldwide tides could deliver enough energy to match US coal-based electrical production. In countries like Canada, with its small population and lots of coastline, tidal power could be a major player.