Methods for Harnessing Wave Energy
The idea of harnessing energy from the ocean's waves was tossed around for a couple hundred years. But it wasn't until the oil crisis of the 1970s that it started to gain some significant attention [source: CRES]. The concept resurfaces whenever oil prices rise.
So far, engineers have developed and implemented several methods for collecting wave energy. These methods can be implemented on the shoreline, near the shore or offshore. Most devices that are near or offshore are anchored to the sea floor. Here's a list of the major kinds of wave energy converters (WECs), or devices that transfer wave energy to usable electricity.
Terminator: Wave energy devices oriented perpendicular to the direction of the wave, are known as terminators. These terminators include a stationary component and a component that moves in response to the wave. The "stationary" part could be fixed to the sea floor or shore. It must remain still, in contrast to the movable part. The moving part works kind of like a piston in car -- moving up and down. This motion pressurizes air or oil to drive a turbine.
An oscillating water column (OWC), shown in the image above, is a terminator. OWCs have two openings -- one on the bottom that allows water to enter the column and one narrow passage above to let air in and out. As waves come and fill the column with water, this pressurizes the air inside, which forces the air through the opening above. The air encounters and drives a turbine. Then, as waves pull away, water rushes out, which sucks more air back down through the top, driving the turbine again.
Another terminator, an overtopping device, includes a wall that collects the water from rising waves in a reservoir. The water can escape through an opening, but while passing through, drives a turbine. The most famous kind of terminator, however, is truly the Schwarzenegger of WECs. Salter's Duck includes a bobbing, cam-shaped (tear-shaped) head that drives a turbine. Though not fully realized, theoretically, this device would be the most efficient WEC.
Attenuator: These devices are oriented parallel to the direction of the wave. One of the most well-known examples of this is the Pelamis, a series of long cylindrical floating devices connected to each other with hinges and anchored to the seabed. The cylindrical parts drive hydraulic rams in the connecting sections and those in turn drive an electric generator. The devices send the electricity through cables to the sea floor where it then travels through a cable to shore.
Point absorber: These devices aren't oriented a particular way toward the waves, but rather can "absorb" the energy from waves that come from every which way. One such device is called the Aquabuoy, developed by Finavera. In a vertical tube below the water, waves rush in and drive a piston, a buoyant disk connected to hose pumps, up and down to pressurize seawater inside. The pressurized water then drives a built-in turbine connected to an electrical generator [source: Finavera]. Many Aquabuoys can send electricity to a central point. From that point, electricity is sent down to the seafloor and then to shore via a cable.
Several WECs grouped together, such as Pelamis or Aquabuoy structures strung together, make up a wave farm.
On the next page we'll investigate some of the struggles involved in getting wave energy afloat in today's economy.