4.2 Classification of Wave Energy Converters

Wave energy converters come in various shapes and sizes, each designed to harness ocean power differently. From floating buoys to submerged pressure systems, these devices capture wave motion and convert it into electricity.

The location of wave energy converters also plays a crucial role in their effectiveness. Offshore devices tap into powerful waves, while nearshore and onshore options offer easier maintenance. Each type has its own pros and cons in the quest for clean energy.

Device Types

Oscillating Water Column (OWC) and Point Absorbers

• Oscillating water column (OWC) devices capture wave energy using a partially submerged structure with an opening below the water surface
  • As waves enter the structure, the water column inside rises and falls, compressing and decompressing the air above it
  • This air flows through a turbine, generating electricity (Wells turbine, impulse turbine)
• Point absorbers are floating or submerged structures that absorb wave energy from all directions at a single point
  • They typically have a small horizontal dimension compared to the wavelength of the incoming waves
  • Point absorbers often use a buoy or float that moves relative to a fixed reference (seabed, bottom-mounted structure)
  • The relative motion between the buoy and the reference is used to drive a power take-off system (linear generator, hydraulic system)

Attenuators, Terminators, and Overtopping Devices

• Attenuators are long, slender floating devices oriented parallel to the direction of wave propagation
  • They ride the waves like a ship, with the wave-induced motion of the device segments relative to each other used to generate electricity
  • Attenuators are typically composed of multiple connected sections that flex and bend as waves pass (Pelamis wave energy converter)
• Terminators are devices oriented perpendicular to the direction of wave propagation, intercepting the waves
  • They use the wave's kinetic energy to drive an oscillating flap or pendulum, which is connected to a power take-off system
  • Terminators are often bottom-mounted or fixed to a structure (oscillating wave surge converter, pendulum device)
• Overtopping devices capture water from incoming waves into a reservoir above the sea level
  • The potential energy of the captured water is then converted into electricity as it is released back to the sea through a conventional low-head turbine
  • Overtopping devices can be either onshore or floating structures (Wave Dragon, Sea Slot-cone Generator)

Submerged Pressure Differential, Bulge Wave, and Rotating Mass Devices

• Submerged pressure differential devices are located below the water surface and use the pressure difference created by passing waves to generate electricity
  • They typically consist of a fixed structure and a movable part that oscillates due to the pressure differential
  • The motion of the movable part is used to drive a power take-off system (Archimedes Wave Swing)
• Bulge wave devices are long, slender tubes filled with water and moored to the seabed
  • As waves pass over the device, they create a bulge that travels along the length of the tube
  • The bulge motion is used to drive a power take-off system, such as a hydraulic motor or turbine (Anaconda wave energy converter)
• Rotating mass devices use the motion of waves to rotate a mass inside the device
  • The rotation of the mass is then converted into electricity using a generator
  • Rotating mass devices can be designed to capture energy from the heave, surge, or pitch motion of waves (Wello Penguin, SEAREV)

Location Classification

Offshore, Nearshore, and Onshore Devices

• Wave energy converters can be classified based on their location relative to the shoreline: offshore, nearshore, and onshore
• Offshore devices are located in deep water (typically >40m depth) and are exposed to more powerful, unobstructed waves
  • They require more robust mooring and anchoring systems to withstand the harsher environmental conditions
  • Offshore devices have higher transmission costs due to the longer distance to shore but benefit from better wave resources (Pelamis, Wave Dragon)
• Nearshore devices are located in intermediate water depths (10-40m) and are often closer to the shore than offshore devices
  • They experience less powerful waves compared to offshore devices but still benefit from good wave resources
  • Nearshore devices may have lower transmission costs and easier access for maintenance (Oyster, CETO)
• Onshore devices are located at or integrated into the shoreline, such as breakwaters or cliffs
  • They have the advantage of easier access for installation and maintenance, as well as lower transmission costs
  • However, onshore devices are exposed to less powerful waves and may have a greater visual impact on the coastline (Limpet, Pico OWC)