8. Ocean power

Key facts

• Energy in ocean currents, tides, and waves could provide twice the world’s current energy use.

• Wave energy alone, using current technology, could provide 15% of the world’s electricity.

• If the United States could harness 40% of the nearshore wave energy, it would capture as much energy as is generated by all freshwater hydropower now available in the U.S.

• The oldest use of ocean energy is from the tides. During their occupation of Great Britain, the Ancient Romans built a dam that captured tidal water and let it flow out through a waterwheel. In Medieval England, use of tidal power was not uncommon.

• The most successful modern tidal power plant is off the coast of Brittany, France, producing 10 million watts of electricity a year.

• Currently proposed is the world’s largest tidal power plant, in Great Britain’s Severn River estuary. It would have a generating capacity of 2 billion watts.

The wave of the future?

In March 2008, the Suntory Mermaid II, a new kind of boat, left Honolulu with a plan to travel more than 3,700 nautical miles (Figure 8.1). The trip had been done before, of course, but this boat’s propulsion system was new—the Suntory Mermaid II has two horizontal fins that move up and down with the waves and generate the power to push the boat forward. Solar energy provides electricity. Dr. Yutaka Terao of the Department of Naval Architecture and Ocean Engineering at the Tokai University School of Marine Science and Technology invented the system that powers the boat.^{1, 2}

Figure 8.1 The Suntory Mermaid II—how to ride waves. (Illustration by Kevin Hand, www.kevinhand.com)

Will this be the wave of the future, or just another futile attempt to harness the vast energy of ocean waves and currents and make it an important, practical alternative source of energy for us?

In Chapter 4, “Water Power,” we talked only about power generated by freshwater—rivers, streams, hydroelectric dams. We are giving the biggie—the world’s saltwater—a chapter all its own, not just because oceans are bigger but also because harnessing their energy is a whole different ballgame. The oceans are a vast renewable resource that, constantly in motion, could be a huge source of energy. The problem is that except in a few cases, their storms, currents, waves, and tides have been too powerful for our energy-converting machines.

Think of the ocean as a giant solar-energy collector, covering 70% of the Earth’s surface. As the National Energy Research Laboratory explains it: “In an average day, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil. If less than one-tenth of one percent of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.”³

The ocean holds two kinds of energy: one from its moving currents, waves, and tides, which we might informally call mechanical energy, and the other from the temperature difference between surface water and deep water, which we might informally call thermal energy. The World Energy Council writes that the ocean could provide the equivalent of “twice the world’s electricity production,”⁴ and that wave energy alone, with current technologies, could provide 15% of the world’s electric energy.

These are rough estimates, to some extent limited to the efficiencies of existing technologies. It’s hard to forecast improvements in efficiency and advances in the kinds of environments where ocean energy could be tapped, and therefore it is difficult to figure out how this energy could be captured and turned into electricity. Engineers generally try to take limitations into account. For example, another estimate assumes that only 20% of America’s offshore wave energy would be harnessed, and that would be at 50% efficiency (meaning that half of the energy in the waves would end up as usable electricity). Even given these limitations, wave energy could still provide an amount of energy equal to all U.S. hydropower in 2003.

In other words, there’s a hell of a lot of energy out there (Figure 8.2). The big trick is figuring out a way to turn that energy into reliable electricity with technologies that won’t be quickly destroyed by ocean storms and the powerful eroding ability of seawater. There are a few successes, but big advances still lie in the future. The Electric Power Research Institute, a nonprofit backed by major U.S. power corporations, is sponsoring several projects to test technology for harnessing ocean energy.

Figure 8.2 Some regions believed to have great ocean energy potential. (Electric Power Research Institute)

Ocean motion

It is helpful to think of the mechanical energy in the ocean’s moving waters as being of two kinds: tidal power along the shore and in river estuaries; and the power of offshore waves and currents.

Tidal power

Given the difficulty of capturing ocean energy, it’s surprising that the use of tidal power traces back at least to the Roman occupation of Britain. Archaeological excavations show that dams were built then to store water from high tides, and this water was released to run mills that ground grain.⁵ In the Middle Ages, tidal power remained in use. The famous Doomsday Book of AD 1086 mentions tidal mills—more or less conventional water mills built to run off the tide as it flowed in and out.^{6, 7} The Eling Mill in England, still in operation, is believed to date back to those times.

La Rance: proving that the tides can give us electricity for decades

One of the most successful modern tidal power plants is La Rance, along the coast of Brittany, France, which for years was the only full-scale tidal power station in the world. It was built in 1967 and produces 10 MW—enough to provide electricity for 300,000 homes, or for 4% of the population of Brittany—from 24 turbines, which produce 0.5 billion kilowatt-hours of power a year (Figure 8.3).⁸ Wisely, this facility was built along a part of the Brittany coast that has one of the greatest tidal ranges in the world, about 40 feet between high and low tides. This maximizes the energy that can be obtained. Also impressive is that this power plant has never suffered serious damage or mechanical breakdown and, in contrast to many major energy sources that are eyesores, has become a tourist attraction.⁹

Figure 8.3 La Rance Tidal Power Plant, said to be the only full-scale operational tidal plant of its kind in the world. It was built in 1967 and has continued to operate without suffering major damage from corrosive saltwater.¹¹ (Dani 7C3/Wikimedia Commons)

We can’t expect La Rance imitators to be set up and work everywhere, because only a few places have such favorable topography. Another famous one is the Bay of Fundy in Canada, whose maximum tidal range is even a bit larger, at about 49 feet. There are also good sites along the northeastern United States. Present conventional technology requires a tidal range of at least 26 feet.

One disadvantage of a tidal-power dam is that it could restrict fish traveling up and down the river. There could also be other local damage to wildlife and fish habitats.¹⁰

Great Britain is currently proposing to build the world’s largest tidal power plant in the Severn River estuary, the location of the world’s second-largest tidal range, more than 45 feet. If built, it would produce 17 billion kilowatt-hours a year.¹² This is equivalent to a 2-million-kilowatt conventional power station. The British government proposes to spend $29 billion to build the facility, which means that the installation cost would be $0.07 per watt, cheaper than any other commercial form of energy. The tidal power plant is expected to run for 120 years.^13,14

Environmental groups are opposing its construction, however. They say that it would threaten 70,000 acres of protected wetlands where almost 70,000 birds winter and also disrupt salmon, shad, lamprey, and sea trout migrations. They argue in addition that the project is too expensive and that there are other, much cheaper alternatives.

Proponents point out that if global warming raises the sea level as some climatologists forecast, the Severn estuary will be flooded anyway. And as for the cost, is it really so expensive?

Experimenting with ocean waves and currents

With all the energy potentially available from the ocean, a lot of imagination is going into designing machines to turn the energy in ocean currents and waves into electricity. The idea is to get away from building dams and other fixed structures, which fight the very motion that they try to use, and instead seek devices that would be immersed in the ocean and convert the motion of currents into electricity in a more forgiving way.

An Australian company has taken a bioengineering approach, using designs that occur in nature. One of their devices looks like a frond of giant kelp, the brown seaweed that forms underwater “forests.” Like the kelp, the new device has holdfasts that anchor it to the bottom of the ocean and a flexible arm that waves back and forth with the motion of the water like the kelp’s frond. This motion turns a generator that makes electricity.

Another device, shaped like a shark’s fin, rolls with the moving water and lets extremely large waves pass by (Figure 8.4). Still another, called the Pelamis, is being developed by Ocean Power Delivery in Edinburgh, Scotland. The New York Times describes it as “a snakelike wave energy machine the size of a passenger train, which generates energy by absorbing waves as they undulate on the ocean surface.”¹⁵

Figure 8.4 One of the new approaches to harnessing the ocean’s energy is this device, shaped like a fish’s fin so it can “swim” with the ocean currents and thus is less likely to be damaged. (bioSTREAM™, Biopower Systems, ©www.biopowersystems.com)¹⁶

Whether these turn out to be practical, efficient, and durable, only time will tell, but the potential energy is so great that this seems a worthwhile area to explore. In addition to the obvious benefits of not producing greenhouse gases, the devices are within the ocean, so they are not unsightly. One caveat: The extent to which they might affect ocean habitat, if installed on a large scale, remains unknown.

In addition to small start-up companies, some of the large electrical generator corporations are getting interested in this kind of ocean technology, including General Electric, the Norwegian company Norsk Hydro, and Eon Corporation of Germany.¹⁷

Thermal energy: using the ocean’s temperature differences

The engines we are most familiar with today—gasoline and diesel internal combustion engines, steam turbines, and jet engines—are heat engines: They rely on the temperature difference between a hot and cold gas or liquid in different reservoirs to produce usable mechanical and electrical energy. Usually, the cool reservoir is the same temperature as the outside air or water, whereas the hot gas or liquid is heated by burning a fuel.

The idea of an ocean heat engine is not new. This classic kind of energy device was the focus of (and the motivation for) development of the science of thermodynamics in the 19th century. Jacques Arsene d’Arsonval, a French physicist, first proposed using the heat energy of the ocean in 1881. No one tried it, however, until 50 years later, when a student of his, Georges Claude, built an experimental system at Matanzas Bay, Cuba. It worked, producing 22 kilowatts—enough to light 220 100-watt incandescent lightbulbs. Five years later he built a bigger plant housed on a cargo ship off the coast of Brazil. The Cuban and Brazilian systems worked but were destroyed by storms.¹⁸ More recently, in 1993, a system was installed by the Electric Power Research Institute at Keahole Point, Hawaii, and ran for a while as an experiment, during which it produced 50 kilowatts of electricity. These systems are still considered experimental and not ready for large-scale electrical generation.

In theory, you can produce mechanical and electrical energy using fluids of any two different temperatures, even if the temperatures differ by only a few degrees. But the closer the temperatures of the hot and cool fluids are to each other, the smaller the amount of useful energy yielded and the larger the machine must be to produce a practical amount. An automobile’s internal combustion engine can be relatively compact because the burning gasoline within the cylinder is much hotter than the outside air. The pistons of an old-fashioned train’s steam engine were also relatively small because the steam was so much hotter than the outside air.

In most places most of the time, surface ocean waters are somewhat warmer than deep ocean waters, and it was long ago realized that, at least in theory, you could operate a huge piston by pumping cold water from the depths and using it with the slightly warmer surface waters. However, one of the problems with developing a practical ocean-heat engine is that because there’s only a few degrees’ difference between deep and surface ocean waters, the engine would be huge and unwieldy, not likely to do well in storms.

The current focus is therefore much more on the technologies we discussed earlier that make use of “ocean motion.” Still, given the huge energy potential of an ocean-heat engine, attempts to develop a practical one continue. According to the Electric Power Research Institute, “As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC [Ocean Thermal Energy Conversion] system can produce a significant amount of power. The oceans are thus a vast renewable resource.” Such a system would provide about 10 billion KW, or two and a half times the amount of electricity available from present generating systems around the world.¹⁹

The bottom line

• The ocean offers huge amounts of energy from its currents, tides, and waves, but this is little developed, largely because of the problems posed by storms and the corrosive power of seawater. Progress seems possible, however, and this may be the best source of waterpower in the future.

• Tidal energy has been captured and used for several thousand years and is used today where conditions are best—a large tide and not much chance of severe storms. Considerable potential exists to expand tidal power in such specific locations.

• A proposed tidal power plant on the Severn River estuary in Great Britain would be the world’s largest, providing 5% of that nation’s electricity. It is, however, controversial among environmentalists because it could affect bird and fish habitats.

• Wave energy is obviously widespread, and rapid invention and technological development are under way. Whether the new devices will make tapping wave power practical enough to provide a significant amount of the world’s energy is unknown, but it is promising enough to warrant research and development.

• The Electric Power Research Institute, a nonprofit backed by major U.S. power corporations, is sponsoring several projects to test technology for harnessing ocean energy. However, few large corporations and government agencies around the world have been set up to use ocean energy.

• Because ocean energy is still in the developmental stage, it is probably a place for venture capital and small start-up companies. But in a truly enlightened world devoted to renewable, sustainable, nonpolluting energy sources, funds now going to nuclear energy and the quest for more fossil fuels would be diverted to ocean energy technologies.