Wave power and global warming
Wave power refers to the extraction of mechanical or electric energy from ocean waves, utilizing devices designed to capture the oscillatory motion of water. This renewable energy source is based on the predictable nature of waves, which are typically driven by wind. In theory, waves have the potential to generate significant energy, with estimates indicating that coastal wave power could range from 20 to 70 megawatts per kilometer. However, the installation and maintenance of wave power plants present challenges due to high costs, extreme sea conditions, and the corrosive nature of saltwater.
Despite these challenges, wave power is considered a clean and carbon-neutral energy source, which could play a significant role in mitigating global warming. It has been estimated that waves off the U.S. coast alone could generate over 2.6 trillion kilowatt-hours of electricity annually, representing a substantial portion of the country’s electricity needs. The development of wave energy technologies, including various designs like oscillating water columns and point absorbers, continues to progress, with some onshore projects being explored. Overall, wave power presents a promising avenue for renewable energy, contributing to efforts to reduce greenhouse gas emissions and combat climate change.
Subject Terms
Wave power and global warming
Definition
Wave-power devices extract mechanical or electric power from the oscillatory motion of water, or from pressure fluctuations below the surface that are due to surface waves. Wave-power plants are set up either on the coast or anchored farther out in the ocean. Schemes for extracting steady power from waves are based on the predictable, repetitive nature of waves. However, they must be able to withstand the immense forces of extreme sea states and the corrosiveness of saltwater. Installation costs exceed $15 per watt, too high to compete with other power sources.
Waves transmit energy through a medium. Ocean surface waves are usually driven by winds. While this kinetic energy itself is transmitted rapidly, the water propagating the wave moves little. The movement of the water as a wave passes follows an elliptical path around its mean position. The resulting waves are called surface gravity waves, because water rises and falls as a wave passes in an exchange between hydrodynamic pressure and gravity. In deep seawater, defined as deeper than half the wavelength between successive wave crests, the theoretical value of wave power in kilowatts per meter length along a wave crest is 0.489 times the product of the square of the wave height in meters and the time between waves in seconds, or
0.489 TH2
where T is the time between waves and H is the height of the wave. Thus, a wave height of 2 meters, repeated every 15 seconds, generates roughly 29 kilowatts per meter. Estimates of coastal wave power range from 20 to 70 megawatts per kilometer. Waves lose energy quickly as they come into shallow water, but the wave height grows.
The most efficient capture of wave power is in a resonant device, whose natural frequency matches that of the waves. The Salter Duck concept claims to extract over 94 percent of the power in ocean waves using actively controlled cam and float devices. Its proponents claim that the maximum stresses in the device never exceed those associated with power extraction, and that the device can survive the largest and sharpest breakers without capsizing or breaking. Full-scale devices are large and complex and require substantial initial costs to construct and deploy. The Energren system comprises several floats and lever arms and extracts energy over a large surface area, repeatedly using each wave. The Pelamis wave energy converter uses hydraulic cylinders that are pressurized using lever arms attached to floats.
Terminator devices extend perpendicular to the direction of wave motion and capture or reflect the power. One version is an oscillating water column, in which seawater passes through submerged inlets, compressing and expanding a trapped air column above it. The alternatively compressed and expanded air is used to drive turbines.
Point absorbers are floating structures with linkages that extract work. The buoyancy of water pushes long vertical cylinders up and down as a wave goes by. An example is the 40 kilowatt, 4 meter diameter, 16 meter long Power Buoy used in a commercial facility at Reedsport, Oregon. The Aqua Buoy tested in Portugal uses the vertical motion due to waves to run a two-stroke pump driving seawater through a continuous turbine. Some large seagoing vessels also direct wave-driven water through turbines and extract electric power.
Overtopping devices use tapered channels to increase wave height and pour water into reservoirs above the mean sea level. When the water is released to fall through turbines, energy is extracted. Attenuator devices are long, floating structures oriented parallel to the direction of the waves, with multiple sections. Height differences of waves along the length of the device, attributed to phase differences between the waves at different points, cause flexing and drive hydraulic pumps. A Pendulor device is a rectangular box open to the sea at one end. A hinged flap swings back and forth and drives a hydraulic pump and generator.
Wave power is also used to run by reverse osmosis. A piston pump anchored to the sea bottom rises and falls, driven by wave action, driving seawater through the membrane. Many wave energy projects had been attempted by the mid-2020s but constructing them in the ocean was challenging and expensive. By 2024, progress was being made in developing wave energy projects onshore. According to Planetizen, a pilot program for the first onshore wave energy project was planned in Los Angeles. The project would mount a number of floaters on coastal infrastructure that would rise and fall to the motion of the waves.
Significance for Climate Change
Wave power is clean and renewable, and according to CNBC in 2022, waves off the coast of the United States could generate 2.6 trillion kilowatt-hours of electricity each year. This was more than 64 percent of the electricity generated in 2021. Wave power projects were difficult to construct, however, because of powerful waves and currents and the corrosiveness of the ocean. Despite this, as a renewable energy source with no emissions to speak of, wave power, to the extent that it is viable, represented a carbon-neutral source of energy and a means to reduce contributions to global warming.
Bibliography
Brigham, Katie. "How Waves Could Power a Clean Energy Future." CNBC, 7 Sept. 2022, www.cnbc.com/2022/09/07/why-wave-power-could-complement-solar-and-wind.html. Accessed 17 Dec. 2024.
Craddock, David. Renewable Energy Made Easy: Free Energy from Solar, Wind, Hydropower, and Other Alternative Energy Sources. Ocala, Fla.: Atlantic, 2008.
Ionescu, Diana. "First US 'On-Shore' Wave Energy Project Planned for Port of Los Angeles." Planetizen, 21 Nov. 2024, www.planetizen.com/news/2024/11/132785-first-us-shore-wave-energy-project-planned-port-los-angeles. Accessed 17 Dec. 2024.
Mei, C. C. “Basic Gravity Wave Theory.” Chapter 2 in Handbook of Coastal and Ocean Engineering, edited by John Herbich. Houston: Gulf, 1990.
Minerals Management Service. “Technology White Paper on Wave Energy Potential on the U.S. Outer Continental Shelf.” Washington, D.C.: U.S. Department of the Interior, 2006.