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Wednesday, November 26

The Oceans' Wave Power

  • For more than 200 years inventors worldwide have filed patents for wave-power technology of a dazzling variety of designs-bobbing objects ("ducks"), buoys, articulated rafts, floating bags, overspills, and many others. There has been no shortage of ideas. Many of these ideas are in fact technically feasible, so it seems a shame that this renewable energy resource has not been much used. The main challenge is building a system that is economically attractive when so many other forms of energy production (nuclear, fossil fuels) receive subsidies and already have infrastructure in place. There are some wave power systems in place around the globe: the Faroe Islands; Islay, Scotland; Oahu, Hawaii (providing power for the U.S. Marine Corps. base there); Santo, Spain; Portugal; and even the world's first commercial "wave farm" in England.
    The World Energy Council has estimated that wave power could produce as much energy in a year as 2,000 oil, gas, coal and nuclear power plants' twice the amount of electricity produced worldwide-by generating as much as 2 terawatts (that's 1 trillion watts).
    Not every place is a candidate for wave power generation. Prime locations identified are Scotland, northern Canada, southern Africa, and the Atlantic Northeast and Pacific Northwest of the United States. Experts have estimated that wave-power systems in the Pacific Northwest alone could generate up to 70 kW per meter of coastline.

Typical Designs

Wave-power systems can be located onshore or offshore, and come in a surprising range of designs. There are currently four basic "capture" methods: point absorbers (largely vertical, with a relatively small footprint on the surface); attenuators (horizontal footprint, arranged parallel to the waves to undulate with the flow); terminators (perpendicular to the waves); and overtopping (perpendicular to the waves, which break over the system). There are different power take-off systems including hydraulic ram (water hammer pumps water above the starting point); elastomeric hose pump (peristaltic, like your intestinal tract), pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator. Here are some systems already in operation, or close to it:

    1. The Salter "Duck"-In 1970 Stephen Salter ("the father of wave power"), a professor at the University of Edinburgh, designed a wave-power device that could both stop 90 percent of the wave motion and convert 90 percent of that into electricity, a standard that all other designed continue to be measured against. Ironically, the Duck itself never went into use. During the 1990s, a project based on the Duck and dubbed the OSPREY (Ocean Swell Powered Renewable Energy), commenced in the Clyde Estuary of the Scottish coast. Capable of generating 1 mW of power, the OSPREY was on its way to becoming an unqualified success until Hurricane Felix came along and sunk it (at great expense in terms of both money and confidence).
    2. The AquaBuOY wave energy device-AquaBuOYs (Finivera Renewables) really do look like navigational buoys, and this is no coincidence. Obviously, maximum output from a wave-power device should be during those times when the waves are at their highest, but if the technology can't withstand rough seas (as with the OSPREY, above), they aren't much good. Operating on the premise that since navigational buoys can survive for decades in all sorts of conditions, the AquaBuOYs were designed to ride the waves for an estimated 100 years. The vertical wave action drives a two-stroke hose pump that directs pressurized seawater into a turbine connected to a generator; the resulting power is sent via an underwater transmission line. While at least four projects are in the permitting process (including one in Makah Bay, WA), as of this writing.
    3. PowerBuoy "Like the AquaBuOY above, the PowerBuoy resembles a navigational buoy, although one with long cylinder extending far below that houses the mechanics of the system. These PowerBuoys (Ocean Power Technologies) are placed from one to five miles offshore in 100 to 200 feet of water, and can be ganged together to form a "wave-power farm" such as the one to be installed off the coast of Santo, Spain. That 1.39MW station will have one 40kW and nine 150kW PowerBuoys.

1. Pelamis - Ocean Power Delivery, Ltd., developed world's first commercial offshore wave-power facility using its Pelamis Wave Energy Converter, a string of steel cyclinders hinged to articulate. It lies half-submerged, like a 150-meter-long, bright red sea snake (pelamis is the genus for the sea snake), more or less facing into the waves. The cylinders contain hydraulic pumps activated by the wave action; the electricity comes as high-pressure oil gets pumped into generators. The first phase of the wave farm, located 5 km off the coast of Portugal, comprises three 750kW Pelamis "snakes" that combined to generate 2.25 MW; another 28 are expected to be added, bringing the total power generated to 22.5 MW'enough to provide electricity for more than 15,000 homes.

    1. Nearshore OWC-This is an offshore version of the Limpet, described below.

1. Wave Dragon-Overtopping systems work very much like hydroelectric dams, using the potential energy of water stored at an elevation higher than the turbines it drives. The Wave Dragon overtopping system funnels the waves into its own reservoir to create a head; the water is then released through channels that contain turbines. The Wave Dragon is moored 25 to 40 meters offshore in deep water, somewhat like a floating beach.

1. Limpet (Land Installed Marine Powered Energy Transformer)- This an oscillating water column (OWC) system to convert the waves' kinetic energy to electrical power. Picture a box with the open end submerged but slightly tilted toward the incoming waves, with air trapped inside the box. Now imagine there is a narrow outlet for this air, and inside this tube is a turbine. As the waves raise the level of the water inside the box, the air rushing in and out of the tube powers the turbine. A Limpet system (WaveGen) in Islay, Scotland, uses an inclined oscillating water column (OWC) system optimized for the area's anverage annial wave intensity, and feeds a pair of 250kW generators. The Limpet power station in the Faroes is very similar. WaveGen also designs a near-shore oscillating water-column system.
There are many companies designing wave-power systems using these and other designs (such as the tapered channel system, an onshore system, and the pedulor system, an offshore device), and new ones seem to come along frequently as more countries come to recognize the potential of wave power.

Wave Power Advantages
1. Wave energy is an abundant and renewable resource. 2. Even though not every country has coastline, the combined potential output of wave-power generation would meet all the electricity needs of the world. 3. Although the equipment represents a substantial investment, the "fuel" is free and not confined by geopolitical boundaries. 4. The effect on the environment is deemed to be minimal.

Wave Power Disadvantages
1. These are most effective near coastlines, of which there is a finite supply. 2. Large scale systems are still in the early stages.

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