The DOE and NOAA have launched the next phase of the $2.4M Ocean Observing Prize focused on hurricane monitoring using self-charging, wave-powered, AUVs.
A technology brief on wave energy converters (WECs) that harvest energy from ocean waves
An ocean wave is energy in motion through water. This energy, in the form of kinetic or potential energy, can be harvested using wave energy converters, or WECs.
According to researchers from the US Department of Energy, in the United States there is an estimated Terawatt-hours per year technically available from ocean waves around the United States. This equates to about 9% of annual total electricity consumption in the US. You can explore this data at the Marine Energy Atlas, hosted by the National Renewable Energy Laboratory
Globally, an estimated 29,500 Terawatt-hours of energy could be produced from ocean waves annually according to the The Intergovernmental Panel on Climate Change (IPCC).
Ocean waves can be produced from several phenomena, such as boats, seismic disturbances, the moon and sun gravitational pull, and wind. It is this last one that produces waves with the most energy density and which are usually used for wave energy harvesting.
Therefore the best waves for wave energy conversion are usually found in areas with strong, consistent winds. The map here shows the global hot spots for wave energy, expressed in units of kilowatts per meter of wave front. Notice that most high-power waves are found near the poles. The Southern Ocean, with it’s enormous wind fetch, has tremendous energy. The Marine Energy Atlas shows area of wave energy around the United States.
Wave energy converters (WECs) are devices that convert the kinetic and potential energy associated with a moving ocean wave into useful mechanical or electrical energy. Wave energy converters can provide clean energy to power the electrical grid as well as many other applications such as propulsion for ocean vehicles or pumping for seawater desalination.
While there are many unique wave energy converters, they tend to follow common designs, or design archetypes. The seven most common forms of wave energy converters are shown below:
Below is a snapshot of the archetype landscape of wave energy converters globally as of 2018. The most common type of wave energy converter is the point absorber, but there are many variants on this design archetype. You can explore over a hundred different wave energy converters in the data table below.
Before we dive-in and look at the different archetypes of wave energy converters, it’s useful to know the six degrees of freedom that floating bodies experience. Take a look at the image below which shows each: heave, yaw, surge, roll, sway, and pitch. These motions can be used by a WEC to extract energy. Reference this image as you read about different wave energy converters archetypes in the sections below.
There are numerous types of wave energy converters; in fact, that’s part of the reason that wave energy hasn’t grown as rapidly as wind or solar, there is still not convergence on a single design archetype. Often, one device might work well in one wave environment, but very inefficiency in another. Each design is unique and R&D efforts are spread across many different archetypes which makes for slow progress. The devices below are the most common types of wave energy converters. Click on the different design archetype names to see examples and learn more about each.
Point absorber wave energy converters are floating structures that have a small horizontal dimension compared with their vertical dimension and utilize the wave action at a single point. Most designs for point absorbers resemble a run-of-the-mill buoy, at least from the surface. In a the typical point absorber design one end of the absorber is fixed (or at least fixed relative to the water’s surface) while the other end moves in a vertical motion as the wave crests and troughs lift and lower the device. The resulting reciprocating action is used to pump a fluid or drive a linear generator, which in turn can provide usable power. Referencing our six degrees of freedom above, this device takes advantage of the heaving motion from the up-and-down of ocean waves. Point absorbers are one of the most prevalent design archetypes in the marine energy sector today.
Overtopping wave energy converters, sometimes called terminators, utilize a difference in potential energy by raising a volume of water to a height above the ocean’s surface. These devices recreate a similar wave action that you might find naturally on a beach. Floating outstretched arms focus waves so that they build in height as they approach an artificial ‘beach’ at the center of the device. As the waves hit the artificial beach they run-up a ramp and into a storage reservoir which is at an elevation higher than the surrounding sea level. From here the fluid is allowed to drain back down, courtesy of gravity, and the flow is used to power a turbine. In some respects, these designs are very similar to the methods use in hydroelectric dams. Note that these types of wave energy converters must adjust, or tune, themselves to the height of the oncoming waves for best efficiency. These devices are more likely to be found near shore, but some have been converted for offshore use, such as the Wave Dragon for example.
Oscillating Water Columns (OWC) often resemble an ‘L’ shape at the water’s surface. In this design air is trapped in a chamber between the water’s surface and a bi-directional air turbine mounted on the top of the platform. As waves pass-by underneath the machine, their reciprocating motion acts like a piston on the air in the chamber, raising and lowering the pressure in the chamber. As the water level rises with the crest of the wave, it squeezes the air and raises the air pressure in the chamber which forces it through the turbine causing it to spin. When the the water level drops from the trough of the wave, it creates a slight vacuum within the chamber and air is drawn from the outside through the turbine, again causing it to spin. Although the turbine blades on either end of the bi-directional turbine may spin in opposite directions, the turbine’s main shaft spins in one direction, which allows it to drive a turbine generator and produce electricity.
Attenuators are wave energy converters that are oriented parallel to the direction of wave travel. They are usually (but not always) modular in design and rely on the flexing of joints to generate power. Think of two barges linked together like wings. Referencing our six degrees-of-freedom diagram, these devices try to capitalize on several different translations of motion; such as surge, sway, and heave as examples. The dominant wave length is an important consideration for these designs so that maximum power can be extracted from a given wave climate. An example of this type of WEC is Pelamis, manufactured by Pelamis Wave Power. Pelamis is a semi-submerged, articulated structure composed of cylindrical sections connected with hinged joints. The wave-induced motion of the joints is resisted by hydraulic rams that pump high-pressure fluid to drive hydraulic motors, which in turn power electrical generators to produce electricity. Pelamis is unfortunately no longer with us, but this archetype lives on in other companies’ designs.
This design archetype of wave energy converters uses the surge motion of ocean waves to flap back-and-forth. These devices are almost always completely submerged, generally resting on the seafloor in shallow water, sometimes even in the more dangerous breakwater areas. The most simple design is a pendulum arm flap that pivots on a hinged joint as wave motion acts on it, causing it to oscillate back and forth. The movement of the flap acts like a large lever arm which can be mechanically linked to a generator to produce electricity or a pump to pressurize a fluid.
This class of wave energy converter comes in two different flavors. In one type, the device rests on or near the seafloor and relies on pressure fluctuations as a wave passes overhead to flex a pliable material such as a air bladder and squeeze a fluid to drive a turbine or some other power take-off unit. The other type is similar to a point absorber, but submerged. In this type, a buoyant, submerged float is actuated by passing waves and this reciprocating motion is converted to energy with a linear generator. Why would you want to submerge a point absorber? One reason might be to help with storm survivability – the energy in a wave decreases with depth, so while there may be less energy to extract it also might be a less dangerous area to operate.
Rotating mass wave energy converters are generally surface riders that use an internal weight rotating about a fixed point to drive a rotational alternator. The rocking motion of ocean waves cause the hull’s center of buoyancy and center of gravity to shift, and the rotating mass thus rotates about its axis to find its new center point as the vessel’s trim and pitch flucuate. Since waves keep the device constantly rolling and swaying, the mass is constantly rotating trying to reach equilibrium while also generating power. This device takes advantage of multiple translations of motion, most notably rolling and pitching caused by waves.
Explore over a hundred different wave energy converters in the interactive data table below. To read a description of each technology, click on the row of interest. To visit the company’s webpage, click on the colored squares in each row. The visualization is best viewed on a laptop or PC.
Wave energy is a challenging form of renewable energy to commercialize for several reasons, including:
The DOE and NOAA have launched the next phase of the $2.4M Ocean Observing Prize focused on hurricane monitoring using self-charging, wave-powered, AUVs.
Wave energy is capable of much more than powering the electrical grid. Here are eight examples where its providing energy for non-grid applications.
This is the second of a two-part series on technologies used to provide power in the deep sea. In this article I highlight some of
The European Marine Energy Center (EMEC)
International Energy Agency – Ocean Energy Systems (IEA-OES)
U.S. Department of the Interior, Bureau of Ocean Energy Management (BOEM)
U.S. Department of Energy, Water Power Technologies Office (WPTO)
The Portal and Repository for Information on Marine Renewable Energy (PRIMRE)
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