Wave and Tidal Energy

Wave and Tidal Energy

Water in the oceans is constantly in motion due to waves and tides, and energy can be harvested from these kinds of motions. Waves, driven by the winds, make the water oscillate in roughly circular orbits extending to a depth of one half of the wavelength of the wave (distance between peaks). Tides, related to the gravitational pull of the Moon and Sun on the oceans, are like very long-wavelength waves that can produce very strong currents in some coastal areas due to the geometry of the shoreline. In terms of power generation technologies, wave and tidal power have both similarities and differences. Both refer to the extraction of kinetic energy from the ocean to generate electricity (again, by spinning a turbine just as hydroelectric dams or wind farms do), but the locations of each and the mechanisms that they use for generating power are slightly different.

Wave energy projects extract energy from waves on the surface of the water, or from wave motion a bit deeper (a few 10s of meters) in the ocean. Surface wave energy technologies capture the kinetic energy in breaking waves – these provide periodic impulses that spin a turbine. The US Department of Energy has a nice description of different types of surface wave projects as follows:

• Oscillating Water Columns: Oscillating water columns consist of a partially submerged concrete or steel structure that has an opening to the sea below the waterline. It encloses a column of air above a column of water. As waves enter the air column, they cause the water column to rise and fall. This alternately compresses and depressurizes the air column. As the wave retreats, the air is drawn back through the turbine as a result of the reduced air pressure on the ocean side of the turbine.
• Tapchans: Tapchans, or tapered channel systems, consist of a tapered channel that feeds into a reservoir constructed on cliffs above sea level. The narrowing of the channel causes the waves to increase in height as they move toward the cliff face. The waves spill over the walls of the channel into the reservoir, and the stored water is then fed through a turbine.
• Pendular Devices: Pendular wave-power devices consist of a rectangular box that is open to the sea at one end. A flap is hinged over the opening, and the action of the waves causes the flap to swing back and forth. The motion powers a hydraulic pump and a generator.

Offshore wave energy systems are typically placed deeper in the ocean, though not too deep – perhaps a few hundred feet below the ocean’s surface. The periodic wave activity at this depth is typically used to power a pump that feeds into a turbine, generating electricity.

Tidal energy projects typically work by forcing water through a turbine or a “tidal fence” that looks like a set of subway turnstiles. The systems depend on regular tidal activity to generate power. Because this tidal activity is predictable (each coast sees at least one tidal cycle per day – high tide and low tide – and some areas actually see two tidal cycles on a daily basis), tidal energy projects have the advantage of being able to provide a fairly predictable source of electricity. The use of tidal power, globally, has been quite limited because there are only a few sites in the world that see sufficiently large variations in tides to produce enough power, as shown in the table below.