Wave Power Overview
- Energy Source: Ocean waves
- Energy Type Converted: Varied, mostly kinetic and potential
- U.S. Theoretical Energy Potential: 1,500 TWh/year (varies with water depth)
- Power Density: 2 – 100 W/m2 (est.) (varies with water depth and wave height)
- Device Types: Point Absorbers, Terminators, Attenuators, and Oscillating Wave Surge Converters
- LCOE: > $0.80 / kWh
A wave energy converter (WEC) is defined as a device that converts the kinetic and potential energy associated with a moving wave into useful mechanical or electrical energy. There are a bunch of Wave Energy Converter (WEC) devices out there; a quick search on Google Patents returns over 7,400 results. I know you’d love for me to provide a SWOT analysis on each and every single one, but let’s stick to the basics. This page will hopefully introduce you to general ideas and principals for how most WECs operate.
WECs can be classified into one of four different categories: point absorbers, terminators, attenuators, and oscillating wave surge. As we’ll learn there isn’t a tried and true method like the three blade horizontal-axis wind turbine that you see everywhere, at least not yet.
Before we dive-in, take a look at this image that depicts the six degrees of freedom that a floating body can experience, we’ll reference this occasionally throughout.
Point absorbers 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 typical buoy. Generally one end of the absorber is fixed while the other moves in a vertical motion, 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. The PowerBuoy and the AquaBuOY are two examples of this technology type of WEC.
Let’s be clear, these devices have nothing to do with Skynet. Terminators are WECs that are situated perpendicular to the direction of wave travel. There are two sub-groups within this category that are most common: overtopping devices and oscillating water columns (OWCs)
Overtopping devices utilize a difference in potential energy to generate useful energy. These devices recreate a similar wave action that you’ll find on the beach. Floating outstretched arms focus waves so that they build in height. The waves are then directed towards an artificial beach where the water will run up a ramp and into a storage reservoir which is at a higher elevation than the surrounding sea level. From here the fluid is drained from the higher elevation and the flow is used to power a turbine. 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. This type of WEC must adjust itself to the height of the oncoming waves for best efficiency, so a pressurized air system is used to adjust the floating height.
Oscillating Water Column (OWC)
In an OWC the incoming wave is used to generate a reciprocating motion of a fluid, which can then drive a turbine. The device works buy first trapping a chamber of air above the water level, kind of like being under an upturned boat. On top of the air chamber is an air duct that allows flow through a bi-directional turbine. When a wave passes across the device the water level will naturally rise and fall. As the water level drops, a slight vacuum is created, drawing in air from the outside. When the water level rises, the air is forced back out. This back and forth motion is used to drive a turbine generator to produce electricity. OceanLinx is well known for this type of device.
Attenuators are devices that ride the waves so to speak, or in other words they 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. 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 most popular 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.
Oscillating Wave Surge Converters
This class of device is fairly self-explanatory: they use the surge motion of ocean waves. These devices are usually submerged or on the seafloor in shallow water. The most simple design is a pendulum arm with a flap on the end that pivots on a hinged joint as wave motion acts on it. The hinged joint can be used to pressurize a fluid for driving hydraulic motors and generators.
As we can see there are numerous methods for harnessing wave power. Some devices fall into multiple categories of those discussed above. In fact many experts believe that if a robust WEC is to be developed it will need to be able to convert wave energy using as many different methods as possible to improve its efficiency in all sea states.