The energy that is present in ocean waves can be used to generate electricity that is renewable and free of harmful emissions. Wave power derives from the combination of kinetic energy in the wave (water particle motion) and potential energy (particle elevation). A propagating wave causes water particles to move in an elliptical motion (Figure 1), and this motion can be converted to energy using a variety of mechanical devices. The amount of energy that can theoretically be generated is a function of wave height (elevation difference between crest and trough), wave period (time for the wave to propagate the distance between two crests), and water density.
For an ocean wave height of 3 m (10 ft) wave energy density would be around 36 kW/m (11 kW/ft) of wave crest. Although wave power is less intermittent than wind or solar power, it is still variable. It varies from year to year and from season to season, with energy density in winter a factor of 2 or more times that in the summer (in the northern hemisphere). This provides an interesting synergy for power generation between wave power that peaks in the winter and solar power that peaks in the summer.
Because of this variability, one typically refers to the average annual power density of waves at a particular location. A (2005 study by EPRI
) (Electric Power Research Institute, Inc) determined that the average annual power density in six states studied ranged from 4.9 kW/m (Maine) to 26.5 KW/m (Washington). Extrapolating to the rest of the country, EPRI estimated the total available energy from US coastal waters to be approximately 2,100 TWh/yr. This compares to approximately 3,800 TWh of total electricity consumed in the US in 2017. If industry could harness 25% of that potential at a 50% conversion efficiency, the electricity provided by wave power would be comparable to that generated by hydroelectric power.
There are five main types of mechanical devices that can capture wave energy:
- Absorbers - An absorber is a floating structure which absorbs energy from wave motion through its movements at the water surface. It converts the motion of the buoyant top relative to the base into electrical power. The PowerBuoyTM developed by Ocean Powers Technology (Figure 2) is an example of absorber technology that can be used for a variety of offshore applications, including defense, oil and gas exploration, ocean observing, and communications. The system integrates patented technologies in hydrodynamics, electronics, energy conversion, and computer control systems to provide an autonomous system to maximize the extraction and conversion of the natural energy in ocean waves. The system consists of a float, spar, and heave plate as shown in the schematic here. The float moves up and down the spar in response to the motion of the waves. The heave plate maintains the spar in a relatively stationary position. The relative motion of the float with respect to the spar drives a mechanical system contained in the spar that converts the linear motion of the float into a rotary motion. The rotary motion drives electrical generators that produce electricity for onboard use or for export to nearby marine applications using a submarine electrical cable. This wave energy conversion system provides continuous power in wave environments between 1 - 5 m (3 - 16 ft) seas. The spar also contains space for additional battery capacity if required to ensure power is available under extended no wave conditions. At this time the PB3 model (3 kW rated capacity) is commercial ready with the PB15 (15 kW capacity) under development.
- Attenuators - Attenuators capture energy by being placed perpendicular to the crest of the wave. The traveling crests and troughs of the waves causes the attenuator to continuously flex at the joints where segments are connected. This motion drives hydraulic rams which pump high-pressure oil through hydraulic motors to generate electricity. The Pelamis Wave Energy Converter (see Figure 3), developed by Pelamis Wave Power, was used for the world's first commercial scale offshore wave farm. The Islay Agucadoura Wave Farm, located 5 km off the Portuguese coast, started delivering 2.25 MW produced by three Pelamis generators in September 2008. Unfortunately, the generators had to be towed back to the port after four months of commissioning because of technical problems. The global financial crunch of 2008 made financing of the re-installation of the generators even more difficult. The wave farm has since then been shut down.
- Oscillating Water Columns - An Oscillating Water Column (OWC) is a partially submerged enclosed structure. The upper part of the structure, above the water, is filled with air, and incoming waves are funneled into the bottom part of the structure. The water column in the device then rises and falls with the wave which causes the air in the top structure to pressurize and depressurize. This is similar to a naturally occurring blowhole which expels the air and foamy water into the atmosphere. In the OWC the air is pushed and pulled through an air turbine at the top of the structure, converting the energy to electricity. (See Figure 4.) The first commercial wave energy system of any type, the Islay LIMPET (Land Installed Marine Power Energy Transmitter), was a 250 KW capacity OWC installed in 2000 on the Scottish island of Islay. It was decommissioned in 2012. More recently, the company Wave Swell Energy completed a one-year test of its UniWave 200 (see video) off Australia in 2022 and achieved approximately 50% efficiency and 80% availability during its trial run.
- Overtopping - An overtopping device captures sea water from waves that overtop a barrier and fill a reservoir situated above sea level (Figure 5). The water is then released back to sea through low-head turbines. This technology is very similar to a conventional hydropower dam.
- Oscillating Wave Surge Converter - An oscillating wave surge converter, or more simply an inverted-pendulum device, uses the surge motion of waves to move a hinged paddle back and forth. The motion of the paddle drives hydraulic pumps which drive electrical generators.
The primary disadvantage to wave energy is cost. Although the EPRI report painted a positive picture for the resource in 2005, there has been relatively little technology development (compared to wind and solar power) since then. The best currently operating wave power generation facility is producing electricity for about 7.5 cents/KWh per the Ocean Energy Council
while new onshore wind averages 4.8 cents/KWh and solar photovoltaic averages 5.9 cents/KWh per the US Energy Information Association (EIA)
Although there has been concern expressed about aesthetics, impact to marine life and ocean use, EPRI concluded that, given proper care in site planning and early dialogue with local stakeholders, offshore wave power promises to be one of the most environmentally benign electrical generation technologies.