Wave energy can be converted into electricity by offshore or onshore systems.
Oscillating Water Columns
These are partially submerged structures that house a column of air above a column of water. Waves are then funneled into the structure below the waterline, forcing the water column to rise and fall like a piston. This movement both pressurizes and depressurizes the air column, moving a bidirectional turbine with the resulting “push/pull” force.
The overtopping device generally is constructed on shore or on a levee. There is a collector that funnels waves over the top of the structure and into one of the device’s reservoirs positioned below the waterline. The water is then run back out to sea through one or more turbines. As the water spins the turbine rotors, electric current is generated.
The point absorber is a floating structure that captures energy from the vertical motion of the waves. This up-and-down motion of the device drives generators that create an electric current.
This style of device harnesses wave energy directly from the surging and swelling motion of waves. It uses the oscillation between a float, flap, or membrane and a fixed point. That movement creates a usable form of mechanical energy. Similar devices are also being developed that utilize pitching, heaving, and swaying motions.
This device is long and multi-segmented and floats on the surface. The attenuator is anchored in place with a mooring line and positioned perpendicularly to incoming waves. Some attenuators tap only the heave (vertical motion); others tap both heave and surge. The device captures energy as the motion of the wave causes it to flex where the segments connect. This movement then drives hydraulic pumps or generators.
Barrages or Dams
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which activate a generator. Gates and turbines are installed along the dam. When the tides produce an adequate difference in the level of water on opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity.
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as in some wind farms. The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Offshore wind turbines are being used by a number of countries to harness the energy of strong, consistent winds that are found over the oceans. Offshore winds tend to blow harder and more uniformly than on land.
The engineering and design of offshore wind facilities depends on site-specific conditions, particularly water depth, geology of the seabed, and wave loading. Today’s offshore turbines have technical modifications and substantial system upgrades for adaptation to the marine environment. These modifications include items such as: strengthening the tower to cope with loading forces from waves or ice flows, pressurizing nacelles to keep corrosive sea spray from critical electrical components, and adding brightly colored access platforms for navigation safety and maintenance access.