Oceans of Electricity: Science News Online, Apr. 14, 2001
Food for Thought
Vitamin E targets dangerous inflammation
Caving in Comfort
70 Years Ago in Science News
Oceans of Electricity
New technologies convert the motion of waves into watts
By Peter Weiss
When the steel shell called Osprey confronted the sea, it was David facing Goliath. This time, Goliath won.
Osprey was an experiment in renewable energy production that challenged the power of the sea in 1995. A 750-metric ton structure the size of a small apartment house, it was designed to squat on the seabed, half submerged in 14 meters of water. Placed 100 m from shore at Dounreay, Scotland, it was to convert the energy in the waves striking it into electricity. This David was supposed to shrug off the worst that the oceanic Goliath could throw at it and keep powering the lights of Dounreay with a steady stream of environmentally "green" electricity.
Instead, Goliath caught David off guard before the challenger could even take a stand. One day in early August, before the construction crew had finished installing Osprey, huge waves rolled in and smashed open the structure's giant ballast tanks. Although Osprey's builders had time to rescue valuable equipment, the sea eventually tore the device apart.
"It was not the best day I've had," recalls Allan Thomson, head of the company that built Osprey. "But we learned a lot from it," he maintains.
For instance, they learned that they had better go ashore to build their next wave-power plantwhich they did. Now Thomson and his colleagues at Wavegen, a company in Inverness, Scotland, are celebrating. In November, they commissioned as the world's first commercial wave-power station a device they built into the rocky west coast of the Scottish island Islay. It generates a peak power of 500 kilowatts (kW), enough to run about 400 island homes.
The opening of the plant, named Limpet, stems from a dream that dates back at least 2 centuries. That's when two French inventors filed the first known patent for a scheme to harness ocean waves to run a machine. Even Thomson and others at Wavegen admit, however, that the small power plant they've built is far from the ultimate realization of that dream.
Indeed, wave-power developers worldwide are devising many potentially more effective technologies that they anticipate could compete with other power sources and contribute to more electric grids within the next few years. Ironically, these innovations in renewable energy build upon the achievements of a decidedly nonrenewable branch of the energy business: offshore oil and gas drilling.
If the spin-offs from that industry had been available to wave-power researchers in the 1970s, ocean power plants might already be common. Without such a head start, however, Stephen Salter of the University of Edinburgh and other researchers, mainly in the United Kingdom, devoted about a decade to the goal of building large-scale, 2,000-megawatt wave-energy plants. The collapse of that programwhether because of inadequate support or overly ambitious goalsleft wave energy with a credibility problem and scared off investors. Now, wave energy is riding a new surge.
Waves are ultimately a form of solar energy. The sun heats up Earth's surface, causing winds that, in turn, drive waves. The best wave-energy regions tend to be on seacoasts at the receiving end of waves driven by the wind across long fetches of water. As the waves travelsay across the North Atlantic to the west coasts of Europethe winds continually pump energy into them. By the time the waves hit the coast, they're brimming with power.
Wave-energy specialists have estimated the power content of waves off coasts all over the world. They rank the areas in terms of their waves' average rate of energy production, or power, in kilowatts per meter (kW/m) of shoreline. The ratings of the most power-rich areas, such as the west coasts of Scotland, northern Canada, the U.S. northwest and northeast seaboards, southern Africa, and Australia, range from about 40 to 70 kW/m. A typical American home, without electric heat, draws around 1 kilowatt on average and 3 to 4 kW during peak summertime use, says Steve Rosenstock of the Edison Electric Institute in Washington, D.C.
Wave-power-poor areas include the coasts of the southeastern United States, northeastern South America, and southern Japan, where waves deliver only 10 to 20 kW/m. Power ratings vary seasonally. The most potent winds and waves appear in winter, when greater atmospheric temperature differencesand therefore greater atmospheric pressure differencesgive rise to stronger winds.
Although variable from place to place and season to season, ocean waves stack up globally as a vast energy reserve. Renewable-energy-markets analyst Thomas W. Thorpe of AEA Technology in Harwell, England, has calculated wave power's potential worldwide contribution. If the technologies being developed today become widely used, wave energy could amount to nearly 16 percent of the world's current total electricity output, says Thorpe.
That would be nearly 2,000 terawatt-hours (TWh) annually, or as much as the world's large-scale hydroelectric plants produce, Thorpe reported last December at the Fourth European Wave Energy Conference in Aalborg, Denmark.
Helping spur the technology toward those goals, Europe's central government, the European Union, is aiming to double to 12 percent by 2010 the contribution of renewable sources, including waves, to the region's energy supply. Meanwhile, the United Kingdom recently passed legislation covering the same time period. It will require power companies to boost the renewable portion of their total power output to 10 percent.
On this side of the Atlantic, however, the U.S. government has made little effort to develop wave energy, despite what some researchers say is great potential for the technology along some U.S. coasts. For instance, the Department of Energy sponsors no wave-power research, according to spokesman Christopher Powers of the agency's National Renewable Energy Laboratory in Golden, Colo.
To convert wave action into useful energy, a power plant must provide a way for the waves to drive somethingsuch as turbine blades or pistons. The apparatus might briefly store the waves' energy, or it might apply the waves' momentum immediately to some mechanism.
The first wave-power patent was for a 1799 proposal by a Parisian named Monsieur Girard and his son to use direct mechanical action to drive pumps, saws, mills, or other heavy machinery. These French inventors envisioned attaching heavy wooden beams to docked battleships and taking advantage of their vessels' bobbing to operate the beams as levers against fulcrums on shore. However, there is no evidence that the men ever carried out their plan.
Nowadays, a wave harnesser's objective is typically electricity from a generator. Within such a device, the converted wave momentum spins coils of wire inside ring-shaped magnets to produce a current.
Wavegen's Limpet plant, as well as prototypes built in Scotland, Australia, India, China, and elsewhere during the past 15 years, use what's known as an oscillating water column to turn wave energy into electricity. Engineers are preparing to power up soon, another 400-kW oscillating-water-column plant sponsored mainly by the European Union on Pico Island in Portugal's Azores. It is expected to supply some 10 percent of the electricity for the island's 15,000 inhabitants.
These onshore systems trap waves in a partially submerged, artificial cavern with a hole in one wall above the water line. The hole leads to an air-driven turbine. As the crest of a wave enters the cavern, it raises the water level quickly, pushing the air above the wave through the hole and spinning the turbine's blades. The turbines are designed to turn the same way no matter which way the air flows through them, so the machine also runs as the retreating wave sucks air back into the chamber. When the waves become too rough, a valve closes to protect the turbine.
Although Limpet now supplies power to the local electric grid, it can't yet beat the prices of other renewable or conventional energy sources. Wavegen expects to generate a kilowatt-hour of electricity for 7 to 8 cents, whereas fossil fuel and nuclear plants yield the same energy for about 5 cents. Nonetheless, wave-energy developers take heart that their projected generation cost has already dropped to about half of what it was for wind energy at an equivalent developmental stage.
For now, being onshore is an advantage for wave-power generators. The relative simplicity of onshore plants, compared with devices meant for deep water, is helping researchers and builders get the Limpet and the Azores plants up and running. However, such shoreline plants also face an insurmountable obstacle that will soon end their dominance within the field, wave-power developers say.
No matter how potent the waves off a particular stretch of coast may be, "in shallower water, the wave energy is absorbed by the seabed," notes George W. Taylor of Ocean Power Technologies in Pennington, N.J. "By the time a wave is breaking on a reef or sand bar, it has lost most of its energy."
Taylor is cofounder of a company that is creating buoys that generate 20 kW of electricity each for recharging U.S. Navy robot submarines. The devices, developed with funding from the Office of Naval Research, could also supply electricity to offshore desalination plants or, in arrays, produce municipal powerthe company's main goal, he says. The buoys generate additional power from streamers of an innovative piezoelectric plastic (SN: 11/18/89, p. 328). A piezoelectric substance creates electricity when deformed by an outside force. The streamers hang below the waves, and their flexing captures energy from the passing tidal currents, says Taylor.
The optimal location for harvesting wave energy is in water about 50 to 100 m deep, says wave-power developer Richard Yemm of Ocean Power Delivery Systems in Edinburgh. There, waves retain nearly all the power they've gathered while crossing the ocean, but the sea bottom is near enough that anchoring wave-power equipment is easier and cheaper than in deeper waters.
Yemm's company is developing a sinuous device made up of interconnected floating tanks meant to wriggle along the wave tops. Named Pelamis after a genus of sea snakes, the 120-m-long sea serpent pushes pistons with its flexing motion. That action pressurizes oil, which then runs electric generators.
As a former wind-power developer, Yemm argues that placing a wave-power plant onshore is akin to "building a wind turbine behind a tall building."
The names of many of the offshore devices now being developed sound as jolly as carnival rides. There are the Mighty Whale, Wave Dragon, Archimedes Wave Swing, WavePlane, Pendulor, and Salter's Nodding Duck, to name just a few.
In addition to Pelamis and Taylor's buoy, the menagerie includes a bewildering variety of designs using floating tankssingly or in groups. The devices bob, flex, or wiggle on or under the water's surface.
However, as the fate of the Osprey illustrates, even wading a little way out to sea may prove to be anything but fun. Offshore wave-power plants are operating in "a very hostile environment," says Antonio F. de O. Falcão of the Lisbon Institute of Technology in Portugal.
Although offshore is the way to go, he says, "you have problems of getting electricity to land and also of anchoring. In stormy weather, you have very large forces, and you have the problem of maintenance if you are very far from the coast, especially in winter." There's also the tendency of seawater to short-circuit and corrode equipment. Falcão is the leader of the Pico Island project.
Offshore-wave-power developers are moving ahead with some trepidation. Yemm, for instance, is concerned that one of the new offshore devices might pull loose of its moorings and drift into a ship's path or wash up on a beach. The demise of the Osprey didn't help wave-power public relations. "We can't afford a screw-up," says Salter.
Fortunately, wave-power developers say, many solutions to the problems Falcão notes already exist, thanks to the offshore oil and gas industry. The fossil-fuel industry has developed better ways to anchor equipment, more durable and corrosion-resistant materials, and improved cables for carrying electric current underwater. For instance, electrical connectors that are easily mated and unmated underwater are proving vital to modular wave-energy designs. Because of the know-how and technology of oil-rig builders, says Yemm, "wave energy is now [becoming] not only practical but inevitable."
From Science News, Vol. 159, No. 15, Apr. 14, 2001, p. 234.