Ocean Wave Energy Provides Continuous Renewable Power
Last week the Bureau of Ocean Energy Management announced that it will consider issuing a research lease for Oregon State University to study ocean wave energy off the Oregon coast. The Northwest National Marine Renewable Energy Center is seeking approval to build a test site about 5 miles off the Newport coast to study utility-scale wave energy devices. The project hopes to support up to 20 megawatts of renewable electricity production, BOEM said.
What is ocean wave energy?
The use of water power dates back thousands of years to the water wheels of Ancient Greece, which used the energy in falling water to generate power to grind wheat. Modern hydrokinetic technologies produce renewable electricity by harnessing the kinetic energy of a body of water, the energy that results from its motion. Estimates suggest that the amount of energy that could feasibly be captured from US waves, tides and river currents is enough to power over 67 million homes.
Capturing the energy contained in near and off-shore waves is thought to have the greatest energy production potential amongst hydrokinetic options. The rise and fall of ocean waves is driven by winds and influenced by oceanic geology. Ocean tides and currents, such as the Gulf Stream, are also thought to provide opportunities for power generation.
Hydrokinetic energy is attractive for its predictability – wave patterns can be predicted days in advance, and tides for centuries. Additionally, while waves and ocean currents are variable, they can provide continuous power, which is not possible from other renewables like tidal streams, wind, or solar power.
How is ocean wave energy generated?
The technologies developed to generate energy from waves and currents, called hydrokinetic energy conversion devices, are generally categorized as either wave energy converters (WECs) or rotating devices.
WECs utilize the motion of two or more bodies relative to each other. One of these bodies, called the displacer, is acted on by the waves. The second body, the reactor, moves in response to the displacer.
There are four main types of wave energy converters:
- Oscillating Water Column: Waves enter and exit a partially submerged collector from below, causing the water column inside the collector to rise and fall. The changing water level acts like a piston as it drives air that is trapped in the device above the water into a turbine, producing electricity via a coupled generator.
- Point Absorber: Utilizes wave energy from all directions at a single point by using the vertical motion of waves to act as a pump that pressurizes seawater or an internal fluid, which drives a turbine.
- Attenuator: Also known as heave-surge devices, these long, jointed floating structures are aligned parallel to the wave direction and generate electricity by riding the waves. The device, anchored at each end, utilizes passing waves to set each section into rotational motion relative to the next segment. Their relative motion, concentrated at the joints between the segments, is used to pressurize a hydraulic piston that drives fluids through a motor, which turns the coupled generator.
- Overtopping Device: A floating reservoir, in effect, is formed as waves break over the walls of the device. The reservoir creates a head of water—a water level higher than that of the surrounding ocean surface—which generates the pressure necessary to turn a hydro turbine as the water flows out the bottom of the device, back into the sea.
Rotating devices capture the kinetic energy of a flow of water, such as a tidal stream, ocean current or river, as it passes across a rotor. The rotor turns with the current, creating rotational energy that is converted into electricity by a generator. Rotational devices used in water currents are similar to the wind turbines already in widespread use today – a similarity that has helped to speed up the technological development of the water-based turbines.
Environmental Impacts of Ocean Wave Energy
While the generation of electricity by hydrokinetic devices does not produce harmful air emissions, like the greenhouse gases linked to global warming, further research is necessary to determine what other types of environmental impacts may result from tapping the energy in waves and currents. The extent of these local impacts is important to evaluate, and appropriate caution should be taken in the development of regulations surrounding hydrokinetic energy development. Scrutiny of hydrokinetic energy projects must be placed in the context of our wider energy and electricity generation mix.