Ocean Wave Energy
Ocean wave energy is captured directly from surface waves or from pressure fluctuations below the surface.
Waves are caused by the wind blowing over the surface of the ocean. In many areas of the world, the wind blows with enough consistency and force to provide continuous waves along the shoreline. Ocean waves contain tremendous energy potential. Wave power devices extract energy from the surface motion of ocean waves or from pressure fluctuations below the surface.
Ocean Wave Energy Resource
Wave power varies considerably in different parts of the world. Areas of the world with abundant wave power resource include the western coasts of Scotland, northern Canada, southern Africa, Australia, and the northwestern coast of the United States, particularly Alaska.
Whereas wind resource potential is typically given in gigawatts (GW), wave and tidal resource potential is typically given in terawatt-hours/year (TWh/yr). The Electric Power Research Institute (EPRI) has completed a recent analysis of the U.S. wave energy resource potential, available here. EPRI estimates the total wave energy resource along the outer continental shelf at 2,640 TWh/yr. That is an enormous potential, considering that just 1 TWh/yr of energy will supply around 93,850 average U.S. homes with power annually. While an abundance of wave energy is available, it cannot be fully harnessed everywhere for a variety of reasons, such as other competing uses of the ocean (i.e. shipping, commercial fishing, naval operations) or environmental concerns in sensitive areas. Therefore, it is important to consider how much resource is recoverable in a given region. EPRI estimates that the total recoverable resource along the U.S. shelf edge is 1,170 TWh/yr, which is almost one third of the 4,000 TWh of electricity used in the United States each year.
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The recoverable wave energy resource for each region is estimated as:
- West Coast 250 TWh/year
- East Coast 160 TWh/year
- Gulf of Mexico 60 TWh/year
- Alaska 620 TWh/year
- Hawaii 80 TWh/year
- Puerto Rico 20 TWh/year
The National Renewable Energy Laboratory (NREL) provides a number of useful maps and tools regarding wave energy resources, including a wave energy resource atlas, available at http://www.nrel.gov/gis/mhk.html.
Ocean Wave Energy Technologies
The wave energy devices being developed and tested today are highly diverse, and a variety of technologies have been proposed to capture the energy from waves. Some of the more promising designs are undergoing demonstration testing at commercial scales.
Wave technologies have been designed to be installed in the nearshore, offshore, and far offshore locations. While wave energy technologies are intended to be installed at or near the water's surface, there can be major differences in their technical concept and design. For example, they may differ in their orientation to the waves or in the manner in which they convert energy from the waves.
Although wave power technologies are continuing to develop, there are four basic applications that may be suitable for deployment on the Outer Continental Shelf (OCS): point absorbers, attenuators, overtopping devices, and terminators.
- Terminator devices extend perpendicular to the direction of the wave and capture or reflect the power of the wave. These devices are typically onshore or nearshore; however, floating versions have been designed for offshore applications. The oscillating water column is a form of terminator in which water enters through a subsurface opening into a chamber, trapping air above. The wave action causes the captured water column to move up and down like a piston, forcing the air though an opening connected to a turbine to generate power. These devices generally have power ratings of 500 kW to 2 MW, depending on the wave climate and the device dimensions.
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| Oceanlinx Moored Oscillating Water Column Terminator off the Coast of Australia |
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Shore-Based Terminator in Islay, Scotland.
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- Attenuators are long multisegment floating structures oriented parallel to the direction of the waves. They ride the waves like a ship, extracting energy by using restraints at the bow of the device and along its length. The differing heights of waves along the length of the device causes flexing where the segments connect. The segments are connected to hydraulic pumps or other converters to generate power as the waves move across. A transformer in the nose of the unit steps up the power-to-line voltage for transmission to shore. Power is fed down an umbilical cable to a junction box in the seabed, connecting it and other machines via a common subsea cable to shore.
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| The Agucadoura Wave Farm offshore Portugal was the world's first. It was tested and operated in 2008. 3 miles offshore, using the Pelamus Wave Energy Conversion Device. |
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Artist's conception of an array of attenuator wave energy devices. |
- A point absorber is a floating structure with components that move relative to each other due to wave action (e.g., a floating buoy inside a fixed cylinder). Point absorbers often look like floating oceanographic buoys. They utilize the rise and fall of the wave height at a single point for energy conversion. The relative up and down bobbing motion caused by passing waves is used to drive electromechanical or hydraulic energy converters to generate power.
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Ocean Power Technology's (OPT) Powerbuoy wave generation system
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- Overtopping devices have reservoirs that are filled by incoming waves, causing a slight buildup of water pressure like a dam. The water is then released, and gravity causes it to flow back into the ocean. The energy of the falling water is used to turn hydro turbines to generate power. Specially built floating platforms can also create electricity by funneling waves through internal turbines and then back into the sea.
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| Wave Dragon Overtopping Device. |
Environmental Considerations
In
2007, the Bureau published the Final Programmatic Environmental Impact
Statement for Alternative Energy Development and Production and Alternate Use
of Facilities on the Outer Continental Shelf. This document examines the potential environmental
impacts related to renewable energy development on the OCS for each phase of
development (technology testing, site characterization, construction,
operation, and decommissioning). Actual proposals will be evaluated in
project-specific analyses under the National Environmental Policy Act. Additional
information regarding potential environmental impacts from wave energy
development can be found in the U.S. Department of Energy Report “Report to Congress on the Potential
Environmental Effects of Marine and Hydrokinetic Energy Technologies” and at the Ocean Energy Systems Annex IV webpage.
For More Information