Otis Singleton
The sun's rays heat the Earth's surface unevenly, creating climate systems such as wind that blow across the ocean's surface and generate waves. These waves carry an enormous amount of kinetic energy, which can be converted into electricity. While the idea of using ocean waves to generate power is not new, the complexity of harnessing this energy has proven to be a central challenge. However, with the growing need to transition away from fossil fuels, researchers continue to explore wave power as a potential renewable energy source.
| Characteristics | Values |
|---|---|
| Wave energy potential | The National Renewable Energy Laboratory (NREL) estimated the US' potential as 1170 TWh per year or almost 1/3 of the country's electricity consumption. |
| Wave energy devices | Point Absorbers, Wave Attenuators, Oscillating Water Columns, Overtopping Devices |
| Wave energy converters (WEC) | Buoys, floats, flaps, membranes, parabolic reflectors |
| Energy source | Sun's rays heat the Earth's surface, causing wind to blow across the ocean's surface and generating waves. |
| Energy capture | Devices absorb waves' energy from all directions, either on the surface or the ocean floor, and convert the movement into electricity using a generator. |
| Energy transmission | Electricity is either generated on the spot and transmitted via undersea cables to shore, or mechanical energy is sent to land and converted to electrical energy. |
| Energy density | The concentration of energy (energy density) can be significantly increased compared to the energy source prior. |
| Challenges | Complex and dynamic nature of ocean waves, saltwater corrosion, harsh weather, extreme wave forces, mooring and anchoring systems, power take-off mechanisms, environmental concerns. |
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What You'll Learn
Wave energy converters
Ocean waves are a source of energy in motion through water, which can be harvested using wave energy converters (WECs). WECs are devices that convert the kinetic and potential energy associated with moving ocean waves into useful mechanical or electrical energy.
There are numerous types of WECs, and they tend to follow common design archetypes. One of the most common types is the point absorber, which has a small horizontal dimension compared to its vertical one and utilizes wave action. An example of a point absorber is the FO3 concept, which consists of several heaving floaters attached to a rig. The vertical motion of the floaters is converted into a rotational movement that drives a hydraulic motor, which in turn powers a generator.
Another type of WEC is the overtopping device, which operates similarly to a hydroelectric dam. The "Wave Dragon" is the best-known example of an offshore overtopping device. Its floating arms focus waves onto a slope, from which the waves overtop into a reservoir. The difference in water elevation between the reservoir and the mean sea level then drives low-head hydro turbines.
WECs can also be classified as oscillating surge converters, which exploit the surging motion of ocean waves. Examples include the Oyster, WaveRoller, and Salter Duck, all of which capture wave energy with high efficiency. While these designs have been tested in nature, there have been no large-scale operational deployments.
WECs can provide clean energy to power electrical grids and have other applications such as propulsion for ocean vehicles or pumping for seawater desalination. According to researchers from the US Department of Energy, the United States could produce an estimated Terawatt-hours per year from ocean waves, which is about 9% of the country's annual total electricity consumption. Globally, an estimated 29,500 Terawatt-hours of energy could be produced from ocean waves annually.
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Wave attenuators
The hydraulic pump method involves using the force created by the flexing motion to power a hydraulic ram at each joint of the attenuator. The hydraulic ram then drives oil through a hydraulic motor, which drives a generator to produce electricity. This method is currently being used by the Pelamis Wave Energy Converter, developed by the Pelamis Wave Power company.
Before installing a wave attenuator, developers must first identify the average wave height and length at their chosen location. They can then determine how much energy an attenuator can produce in that specific sea state. If the energy produced is not enough to meet their needs, they may need to consider a different size of attenuator or a different type of WEC altogether.
Wave energy, including that produced by wave attenuators, has certain unique characteristics that distinguish it from traditional energy sources. According to Jim McNally, a technology innovation, modeling, and assessment engineer at NREL, "There are so many variables that you need to account for in order to get a real estimate [of energy production]." These variables include the average wave height and length, as well as efficiency limitations of the device and environmental restrictions along the coastline.
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Point absorbers
These floating structures are strategically placed on the water's surface to ride the vertical motion of waves, allowing them to absorb energy from all directions and maximise their energy capture efficiency. They typically consist of a buoy or float that moves vertically with the waves, connected to a submerged reaction plate or a fixed structure. One end of the absorber is fixed or at least fixed relative to the water's surface, while the other end moves in a vertical motion as the wave crests and troughs lift and lower the device.
The relative motion between the buoy and the submerged component drives a power take-off (PTO) system, which converts the mechanical energy into electricity. Inside the point absorber, hydraulic pumps resonate with the movement, translating the mechanical undulations of the waves into electrical pulses. With each crest and trough of a wave, they generate hydraulic pressure that drives generators, creating electricity that is then sent ashore.
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Buoys
One such buoy is the "power buoy", designed and built by engineers to be 8 feet across, 10 feet wide, and 18 feet long. The power buoy uses the upward and downward motion of waves, combined with the weight of a metal plate, to move a hydraulic piston, resulting in electricity. The voltage produced by the generator on the power buoy is erratic and unpredictable, much like the waves themselves. As each wave passes, the generator first speeds up, then slows down again, generating electricity ranging from 0 to 500 volts (AC).
Another example of a buoy used to generate electricity from ocean waves is the "Wave Carpet". The Wave Carpet is a flexible membrane that runs the length of the device, undulating in response to passing waves and absorbing their energy. The membrane sends wave energy to a generator onshore. The device is designed to survive tough ocean conditions, built with corrosion-resistant materials and operating submerged to avoid surface collisions.
In addition to buoys, other technologies exist to generate electricity from ocean waves, such as overtopping wave power devices, oscillating water columns, attenuators, and oscillating wave surge converters.
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Oscillating water columns
The oscillating water column (OWC) is a prominent wave energy conversion device that has been in use since the early 19th century. It is designed to extract and convert wave energy into usable electrical power. OWCs are one of the leading technologies in development for wave energy extraction due to their simplicity, versatility, and superior management of energy conversion efficiency and peak-to-average power.
An OWC consists of a chamber that is partially filled with water and has an underwater opening that communicates with the ocean. As waves rush into the chamber through this opening, they raise the average water level, compressing the air in the upper part of the chamber. This compressed air then escapes through an orifice in the upper part of the chamber, where an air turbine is placed to capture the energy. The geometric shape of the water column, such as a J-shape or U-shape, influences the energy performance of the device.
To improve the energy performance of OWCs, researchers have proposed using hydraulic turbines instead of air turbines. Additionally, the addition of arms projecting from the side walls of the water column has been shown to benefit energy absorption. OWCs can be either fixed or floating structures, and they can be combined into arrays to share a supporting structure, offering advantages such as the pooling of resources.
While OWCs have many advantages, there are still challenges to their widespread use. Sustainability and cost-effectiveness are major hurdles, and detailed mathematical models and efficient techniques are needed to optimize their design. The performance of the associated energy-maximizing control system is also critical to improving the economic viability of OWCs.
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Frequently asked questions
Wave power captures the energy of wind waves to generate electricity. As the sun's rays strike the Earth's atmosphere, they warm it up, creating temperature differences that cause wind. When the wind passes over the ocean's surface, its kinetic energy is transferred to the water, generating waves. These waves carry the kinetic energy across the ocean's surface, and when they crash onto the shoreline, they release a large amount of energy that can be used for electricity production.
There are several methods to capture wave energy, including:
- Point Absorbers: Small vertical devices fixed to the ocean floor or tethered by a chain that absorb wave energy from all directions. Examples include floating buoys, bags, ducks, and articulated rafts.
- Wave Attenuators: Long, semi-submerged, snake-like devices oriented parallel to the waves. They use wave-induced motion to pressurise a hydraulic piston, which turns a hydraulic turbine generator to produce electricity.
- Oscillating Water Column: A partly submerged chamber fixed at the shoreline that converts wave energy into air pressure, forcing air through a turbine to create electricity.
One of the main challenges of wave power is the complexity of designing devices that can effectively capture and convert wave energy into electricity. These devices must be able to withstand the harsh ocean environment, including the corrosive effects of saltwater, harsh weather conditions, and extreme wave forces. Additionally, there are environmental concerns, such as the potential impact on marine life and the effects of underwater noise and electromagnetic fields.
