FAQ: Energy on the high seas

Harnessing the power of the ocean may be the next big opportunity in energy. Here's what you need to know. Photos: Tapping wave power

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It sounds like a can't-miss proposition: harness the power of the ocean to generate clean, affordable, renewable electricity.

Then there is the reality: staggering construction costs, unpredictable weather conditions, environmental dangers, uncertain outcomes and omnipresent skepticism. Still, several researchers and start-up companies say they have devised systems that will be capable of with waves and tides, or other means related to the ocean. Here is rundown of some of the facts, figures and ideas behind sea power.

Why the sea?
Water is more than 800 times denser than air at sea level. Thus, even slow-moving waves or tides can generate far more electricity than wind turbines could even if the wind blew at 110 miles per hour. Facilities thus require less real estate. Ocean power also remains far more predictable than other alternative energy sources. Solar and wind power vary with the weather. Waves are essentially a form of solar power too and thus will also vary: the sun causes wind, and the wind generates waves. Waves, however, can be tracked from far offshore, allowing computer models to predict electrical output several days in advance.

Tidal power is even more predictable because tides are created by the gravitational pull of the moon.

"With a computer you can produce a timetable for decades. Unlike wind or solar, you can figure out how many megawatts you can sell," said Peter Fraenkel, technical director of Marine Current Turbines. "It is not something that will change the world, but it could save a lot of fossil fuel."

Where is the best place to situate wave or tidal power plants?
The U.S. Pacific Coast, the Chilean coast and Atlantic Europe are good locations, but so are Alaska, Hawaii and the equator. Tidal power is more site-specific, but could work in most of the same areas. A start-up, Verdant Power, this year inserted the first of six prototype turbines in New York's East River. It's a cost/benefit trade-off, but you'll likely be able to see a lot of these facilities from shore. Waves begin to dissipate energy when the water gets less than 200 meters deep. At 20 meters in depth, a wave might have only one third of the energy it had in deep water, according to a 2006 report from Michael Robinson of the National Renewable Energy Laboratory (NREL). Putting wave harvesting systems farther offshore, however, means that you need a longer cable to connect the harvesting system to the power grid.

How much potential power is out there?
Ten years from now, the U.S. could produce 10 gigawatts of wave power and 3 gigawatts of tidal power, says Roger Bedard, ocean energy program leader for the Electric Power Research Institute and an admitted optimist on the subject. That's enough for 4.3 million homes (assuming 3 kilowatts a home). Bedard further estimated that there is a potential 2,100 terawatt-hours worth of wave energy off the shores of the U.S. and 250 terawatt-hours of it could be harvested economically. That's about 6 percent of U.S. electrical demand. Tidal, river and stream power could replace another 3 percent. Bedard said he doesn't know which of these ideas will succeed. "There is no magic bullet," he said. "We as a country ought to look at harnessing power from waves and tides. We need all of the alternatives we can get."

OK, sounds good. So what's the cost?
A lot. Estimates range from $4,000 to $15,000 a kilowatt, before rebates, according to NREL, or 9 cents to 11 cents per kilowatt-hour with rebates and incentives. Putting solar panels on a home costs roughly $10,000 a kilowatt before benefits ($30,000 divided by 3). Solar panels, however, are almost risk-free and require almost no maintenance.

What are some of the main approaches?
Buoys: Finavera Renewables and AWS Ocean Energy have created wave power systems that rely on buoys that act as hydraulic pumps. Waves push the buoys down, which drives a turbine. When the wave passes, the buoy returns to its normal spot, only to be pushed again by the next wave.

Finavera's buoys will stick more than 6 feet out of the water and descend more than 70 feet below the surface. AWS' are completely submerged. The hydraulic fluid inside Finavera's buoy is seawater while AWS' Archimedes Water Swing relies on air. A full-scale buoy from Finavera will be capable of generating 250 kilowatts, enough for 80 homes. A 100-megawatt array of them could be squeezed into two to three square miles, said Myke Clark, vice president of policy for Finavera. "It is a lot smaller footprint than offshore wind (turbines)," Clark said. "Our long-term goal is to get to 5 cents a kilowatt-hour, but a whole bunch of things have to happen to get to that point."

The company is almost done installing a half-size prototype off the coast of Oregon and hopes to erect four of the 250-kilowatt devices off the Washington coast by 2009. "Alaska is very interested," Clark added. AWS, meanwhile, will install a 250-kilowatt prototype off the Orkneys in Scotland in 2008 and build a field with 500-kilowatt devices in the U.K. by the third quarter of 2009. By 2013, it hopes to have a 100-device field. The design of the coming devices from AWS were influenced by a pilot study the company kicked off in Portugal in 2004.

Sea snake: Ocean Power Delivery is testing the Pelamis, a device 120 meters (about 395 feet) that looks like a segmented snake. When the segments bob up and down, buoys attached at their joints generate hydraulic pressure. The company has built a 2.25MW system off Portugal consisting of three 750-kilowatt Pelamis wave-energy converters and is aiming to built 5MW and 3MW systems off the coasts of England and Scotland in the next few years.

The water column: Wavegen, a division of Voith Siemens Hydro Power Generation, is experimenting with the Limpet, an oscillating water column. Think of a large cement tube submerged in the ocean, but not attached to the bottom. Waves come in; water rushes into the tube from below and cranks a turbine. The company last month won a contract to install a Limpet in Mutriku in northern Spain that will produce 250 to 300 kilowatts when opened in late 2008 or 2009. Wavegen has had a prototype running off Scotland since 2000.

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