FAQ: Energy on the high seas

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 generating power 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.

The skate ramp: With the Wave Dragon, wave reflectors more than 100 meters long guide waves up a ramp, where the water dumps into a reservoir. The added pressure forces a turbine at the bottom to turn. The device's developer, a company that is also called Wave Dragon, is building a 7MW prototype off of Wales in 2008 and wants to do a 77MW project in the Celtic Sea by 2010. The Wave Dragon is slack moored, so that it can flow with the power of the ocean. A 20-kilowatt prototype running off Denmark's shore since 2003 has helped iron out the technical kinks, wrote Wave Dragon founder Erik Friis-Madsen in an e-mail. "The device can be up scaled to whatever size is needed, and the efficiency of the device grows in line with the size," he wrote. "The biggest difficulty seems to be to secure the funding for the first years of commercial development."

Tidal power, wave power: What's the difference?
Tidal advocates want to put turbines, similar to wind tunnels, where tides snake in the ocean. The tides would turn a turbine and generate power. Marine Current Turbines has had a prototype for four years in southern England generating 300 kilowatts of power. It will start installing a 1.2MW system off Northern Ireland this fall. ="" href="http://www.elingtidemill.wanadoo.co.uk/sitem.html">Eling Tide Mill in England is more than 900 years old.

Why are none of these past the prototype stage?
Cost and unknowns. The 1.2MW system from Marine Current, for instance, will run 7.5 million pounds, or nearly $15 million. It has been delayed several times because of corporate management changes, a lack of funds, and other projects getting priority on construction equipment. "It is like wind was in 1980," said Marine Current Turbines' Fraenkel. At the moment, investors aren't gushing. Few venture capital firms have paid much attention to sea power. In April, Ocean Power Technologies, which makes a buoy system, held a U.S. initial public offering, selling its shares for $20. At the end of trading on Tuesday, it was down to $12.39.

What are some of the environmental risks?
Animals, plants and birds are the main concerns. Ocean-power advocates, however, point to the relatively minimal impact wind turbines have had on birds over the past 20 years. Other dangers include debris and escaping oils, but these can be minimized.

Overbuilding, conceivably, could also present problems by attenuating the force of tides or waves. A large, dense wave facility could reduce wave power by 10 percent to 15 percent in its vicinity, NREL projected, although the impact will be minimal a few kilometers away. The impact on the biological community is unknown. Fraenkel argues that high capital costs will help minimize that problem.

Can you leverage the sea in other ways?
European utilities have already erected offshore wind farms in the United Kingdom and Denmark. These farms--which sport turbines with blades that can measure more than 100 meters long--sit in about 30 meters of water. Offshore winds can be steadier, thus generating more power. Putting the turbines offshore also eliminates some of the "not in my backyard" problems.

RePower Systems of Germany has created a 5MW offshore turbine. Right now, it is creating a 300MW field in water 82 feet deep and 18 miles off the coast of Belgium. Combined, the six turbines in Belgium will be only 54 megawatts smaller than the largest solar thermal plant in the world, located in California.

A significant portion of the U.S. offshore wind potential is located in deep-water areas, but that would require building more robust turbines that can withstand harsher winds, waves and tides. Instead of being anchored directly into bedrock, deep-water turbines might have to be anchored to floating platforms, which in turn are anchored into earth. Corrosion and maintenance are issues, as are environmental concerns. The turbines also need to be connected to the grid via electrical transmission cables. Luckily, a large portion of the U.S. population lives near the sea. The same would go for solar power farms at sea, which could convert sunlight or heat into electricity. (Such facilities do not exist, however.)

Hydrogen on the high seas?
Put this in the "Great idea, but I might be on Social Security then" category. Under this scenario, offshore platforms would harvest power from deep-sea waves and then exploit that power to split water molecules into hydrogen and oxygen. The hydrogen could then be pumped into a carrier and then shipped to shore. (Transporting hydrogen through pipelines or liquefying it presents whopping technical and economic problems).

But right now, demand for hydrogen is low and converting wave power to hydrogen and then to electricity is not cost-effective. Some have proposed delivering power generated at something like the deep sea platform described above over a submarine cable. This would eliminate the conversion inefficiencies, but then there's the question of how far offshore you can go.

What is ocean thermal technology?
Another far-off experimental technology. Electricity can be generated from differentials in temperature. Some companies are already trying to employ temperature differentials to power small sensors. Because the surface of the ocean is warmer than water deeper down, it could be possible to harvest the difference for electricity, but the colder water would have to be made to come in contact with the warmer water.