Ocean surface waves are generated by wind passing over the ocean surface. The friction between the wind and ocean surface causes energy to be transferred from the faster moving air to the surface layer of the ocean. Wave development depends on the length of ocean, or “fetch,” over which the wind blows in a constant direction. Longer fetches with higher wind velocities will produce larger waves. Waves can travel thousands of miles with little energy loss and can combine with waves from storms and other wind-driven events to create very energetic seas. The energy of ocean waves is concentrated at the surface and decays rapidly with depth. Wave energy technologies, also known as wave energy converters (WECs), capture energy directly from the surface motion of ocean waves. WECs can be deployed at offshore, nearshore, and shore-based locations and are intended to be modular and deployed in arrays. Due to the diverse nature of waves in different regions of the world, there is a wide variety of WECs in various stages of research and development.
Point Absorber
Point absorbers extract energy through the relative motion between a body that moves in response to wave forcing and fixed or immobile structures. The moving body may be on the surface or submerged, and the 'fixed' body may be the seabed or another structure less affected by wave action. Their principal dimension is small relative to the length of waves they are absorbing energy from. Electricity may be produced using a linear or rotary generator, or a fluid may be pumped using mechanical force and motion directly.
The presence of these buoys may affect fish, marine mammals, and birds as they pose a minor collision risk or they may either attract organisms to the device or cause them to avoid the site. As with all electricity generation, there is some concern that electromagnetic fields generated by power cables and moving parts may affect animals that use Earth's natural magnetic field for orientation, navigation, and hunting. Large-scale changes in flow (from arrays) may disrupt natural physical systems to cause degradation in water quality or changes in sediment transport, potentially affecting ecosystem processes. Alternatively, devices absorbing wave energy may positively act as shoreline defense.
Photo Credit: Ocean Power Technologies
Surface Attenuator
Surface attenuators generally have multiple segments connected to one another and that are oriented parallel with incoming waves. They use the rise and fall of swells to create a flexing motion that may be converted into rotation or drive hydraulic pumps to generate electricity. Some attenuator designs consist of a single long, flexible surface expression instead of multiple segments.
Oscillating water column devices use wave action to pressurize air in a chamber, forcing it through an air turbine. As water recedes from the chamber, the resulting vacuum pulls air back through the turbine and into the chamber. They can be located onshore or in deeper waters offshore. The turbine may be coupled to a rotary generator to produce electricity.
Significant noise is produced as air is pushed through the turbines, potentially affecting birds, marine mammals, and other marine organisms within the vicinity of the device. There is also concern about marine organisms getting entrapped within the air chambers. When located offshore, concerns about collision, attraction or avoidance, electromagnetic fields, and changes in flow are similar to that of a point absorber buoy; located onshore, these concerns are no different than for a standard shoreline structure.
Overtopping Device
Overtopping devices are long structures that allow wave motion to fill a reservoir to a higher water level than the surrounding ocean. The difference in pressure between water in the reservoir and water at the surface forces fluid through a low-head turbine coupled to a generator, where electricity is produced similar to conventional hydropower. Devices can be either onshore or floating offshore.
There is some concern regarding low levels of turbine noise, marine organisms getting entrapped within the reservoir, or collision with the slow-moving turbines. When located offshore, concerns about attraction or avoidance, electromagnetic fields, and changes in flow are similar to that of a point absorber buoy; located onshore these concerns are no different than for a standard shoreline structure.
Oscillating Wave Surge Converter
Oscillating wave surge devices typically have one end fixed to a substructure or the seabed while the other end is free to move. Energy is collected from the relative motion of the body, driven by the horizontal motion of waves (surge), to the fixed point. Oscillating wave surge converters often come in the form of floats, flaps, or membranes. Rotary motion at a hinge may drive a generator to produce electricity, or the moving body may be used to pressurize a fluid.
