Wave power

Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation, desalination, and the pumping of water (into reservoirs). Wave power is a form of renewable energy. Though often co-mingled, wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave power generation is not currently a widely employed commercial technology.

On December 18, 2007, Pacific Gas and Electric Company announced its support for plans to build America's first commercial wave power plant off the coast of Northern California. The plant will consist of eight buoys, 2 1/2 miles offshore, each buoy generating electricity as it rises and falls with the waves. The plant is scheduled to begin operating in 2012, generating a maximum of 2 megawatts of electricity. Each megawatt can power about 750 homes.

The world's first commercial wave farm is based in Portugal, at the Aguçadora Wave Park, which consists of three 750 kW Pelamis devices. Other plans for wave farms include a 3MW array of four 750 kW Pelamis devices in the Orkneys, off northern Scotland, and the 20MW Wave hub development off the north coast of Cornwall, England.

The north and south temperate zones have the best sites for capturing wave power. The prevailing westerlies in these zones blow strongest in winter.

Physical concepts

When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory.

Motion of a particle in an ocean wave.
A = At deep water. The orbital motion of fluid particles decreases rapidly with increasing depth below the surface.
B = At shallow water (ocean floor is now at B). The elliptical movement of a fluid particle flattens with decreasing depth.
1 = Propagation direction.
2 = Wave crest.
3 = Wave trough.

See Energy, Power and Work for more information on these important physical concepts.

Waves are generated by wind passing over the sea: as long as the waves propagate slower than the wind speed just above the waves, there is an energy transfer from the wind to the most energetic waves. Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind shear stress cause the growth of the waves. The wave height increases with increasing wind speed, duration since the wind started to blow, and of the fetch (the distance of open water that the wind has blown over), see Ocean surface wave.

In general, large waves are more powerful. Specifically, wave power is determined by wave height, wave speed, wavelength, and water density.

Wave size is determined by wind speed and fetch (the distance over which the wind excites the waves) and by the depth and topography of the seafloor (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance will not produce larger waves. This limit is called a "fully developed sea."

Oscillatory motion is highest at the surface and diminishes exponentially with depth. However, for standing waves (clapotis) near a reflecting coast, wave energy is also present as pressure oscillations at great depth, producing microseisms. These pressure fluctuations at greater depth are too small to be interesting from the point of view of wave power.

The waves propagate on the ocean surface, and the wave energy is also transported horizontally with the group velocity. The mean transport rate of the wave energy through a vertical plane of unit width, parallel to a wave crest, is called the wave energy flux (or wave power, which must not be confused with the actual power generated by a wave power device). In deep water, if the water depth is larger than half the wavelength, the wave energy flux is:

$P={\frac {\rho g^{2}}{64\pi }}H_{m0}^{2}T\approx \left(0.5{\frac {\text{kW}}{{\text{m}}^{3}\cdot {\text{s}}}}\right)H_{m0}^{2}\;T,$

see below ,where:

• P the wave energy flux per unit wave crest length (kW/m),
• H_m0 is the significant wave height (meter), as measured by wave buoys and predicted by wave forecast models. By definition, H_m0 is four times the standard deviation of the water surface elevation,
• T is the wave period (second),
• ρ is the mass density of the water (kg/m³), and
• g is the acceleration by gravity (m/s²).

The above formula states that wave power is proportional to the wave period and to the square of the wave height. When the significant wave height is given in meter, and the wave period in second, the result is the wave power in kW (kilo watt) per meter wavefront length.

In major storms, the largest waves offshore are about 15 meters high and have a period of about 15 seconds. According to the above deep-water formula, such waves carry about 3.2 MW/m of power across each meter of wavefront. At moderate conditions, for a wave height of about 3 m and a wave period of 8 seconds, a wave power location will have an average flux much less than this: about 70 kW/m.

An effective wave power device captures as much as possible of the wave energy flux. As a result the waves will be of lower height in the region behind the wave power device.

Wave energy and wave energy flux

In a sea state, the energy per unit area of gravity waves on the water surface is proportional to the wave height squared, according to linear wave theory:

$E={\frac {1}{16}}\rho gH_{m0}^{2},$

where E is the mean wave energy per unit horizontal area (J/m²), the sum of kinetic energy and potential energy. The potential energy is equal to the kinetic energy, both contributing half to the wave energy E, as can be expected from the equipartition theorem.

As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the wave energy flux, through a vertical plane of unit width perpendicular to the wave propagation direction, is equal to:

$P=E\,c_{g},\,\$

with c_g the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ, or equivalently, on the wave period T. Further, the dispersion relation is a function of the water depth h. As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:

Properties of gravity waves on the surface of deep water, shallow water and at intermediate depth, according to linear wave theory
quantity	symbol	units	deep water ( h > ½ λ )	shallow water ( h < 0.05 λ )	intermediate depth ( all λ and h )
phase velocity	$\displaystyle c_{p}={\frac {\lambda }{T}}={\frac {\omega }{k}}$	m / s	${\frac {g}{2\pi }}T$	${\sqrt {gh}}$	${\sqrt {{\frac {g\lambda }{2\pi }}\tanh \left({\frac {2\pi h}{\lambda }}\right)}}$
group velocity	$\displaystyle c_{g}=c_{p}^{2}{\frac {\partial \left(\lambda /c_{p}\right)}{\partial \lambda }}={\frac {\partial \omega }{\partial k}}$	m / s	${\frac {g}{4\pi }}T$	${\sqrt {gh}}$	${\frac {1}{2}}c_{p}\left(1+{\frac {4\pi h}{\lambda }}{\frac {1}{\sinh \left(\displaystyle {\frac {4\pi h}{\lambda }}\right)}}\right)$
ratio	$\displaystyle {\frac {c_{g}}{c_{p}}}$	-	$\displaystyle {\frac {1}{2}}$	$\displaystyle 1$	${\frac {1}{2}}\left(1+{\frac {4\pi h}{\lambda }}{\frac {1}{\sinh \left(\displaystyle {\frac {4\pi h}{\lambda }}\right)}}\right)$
wavelength	$\displaystyle \lambda$	m	${\frac {g}{2\pi }}T^{2}$	$T{\sqrt {gh}}$	for given period T, the solution of:   $\displaystyle \left({\frac {2\pi }{T}}\right)^{2}={\frac {2\pi g}{\lambda }}\tanh \left({\frac {2\pi h}{\lambda }}\right)$
general
wave energy density	$\displaystyle E$	J / m2	${\frac {1}{16}}\rho gH_{m0}^{2}$
wave energy flux	$\displaystyle P$	W / m	$\displaystyle E\;c_{g}$
angular frequency	$\displaystyle \omega$	rad / s	${\frac {2\pi }{T}}$
wavenumber	$\displaystyle k$	rad / m	${\frac {2\pi }{\lambda }}$

Deep water corresponds with a water depth larger than half the wavelength, which is the common situation in the sea and ocean. In deep water, longer period waves propagate faster and transport their energy faster. The deep-water group velocity is half the phase velocity. In shallow water, for wavelengths larger than twenty times the water depth, as found quite often near the coast, the group velocity is equal to the phase velocity.

Modern Technology

Wave power devices are generally categorized by the method used to capture the energy of the waves. They can also be categorized by location and power take-off system. Method types are point absorber or buoy; surfacing following or attenuator; terminator, lining perpendicular to wave propagation; oscillating water column; and overtopping. Locations are shoreline, nearshore and offshore. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.

These are descriptions of some wave power systems:

• In the United States, the Pacific Northwest Generating Cooperative is funding the building of a commercial wave-power park at Reedsport, Oregon. The project will utilize the PowerBuoy technology which consists of modular, ocean-going buoys. The rising and falling of the waves moves the buoy-like structure creating mechanical energy which is converted into electricity and transmitted to shore over a submerged transmission line. A 40 kW buoy has a diameter of 12 feet (4 m) and is 52 feet (16 m) long, with approximately 13 feet of the unit rising above the ocean surface. Using the three-point mooring system, they are designed to be installed one to five miles (8 km) offshore in water 100 to 200 feet (60 m) deep.
• A floating near shore device called the Energen Wave Power device has floating pontoons and multiple pivot arms. [1] This device converts ocean wave energy over a large surface area and utilises each wave repetitively until it passes through the device. [2]
• An example of a surface following device is the Pelamis Wave Energy Converter. The sections of the device articulate with the movement of the waves, each resisting motion between it and the next section, creating pressurized oil to drive a hydraulic ram which drives a hydraulic motor. Two commercial projects utilizing Pelamis technology are under construction, one in Portugal the Aguçadora Wave Park near Póvoa de Varzim which will initially use three Pelamis P-750 machines generating 2.25 MW. Funding for a 3 MW wave farm in Scotland was announced on February 20, 2007 and is projected to use four Pelamis machines.
• With the Wave Dragon wave energy converter large "arms" focus waves up a ramp into an offshore reservoir. The water returns to the ocean by the force of gravity via hydroelectric generators.
• The AquaBuOY, made by Finavera Renewables Inc., wave energy device: Energy transfer takes place by converting the vertical component of wave kinetic energy into pressurized seawater by means of two-stroke hose pumps. Pressurized seawater is directed into a conversion system consisting of a turbine driving an electrical generator. The power is transmitted to shore by means of a secure, undersea transmission line. A commercial wave power production facility utilizing the AquaBuOY technology is beginning initial construction in Portugal. The company has 250 MW of projects planned or under development on the west coast of North America.
• A device called CETO, currently being tested off Fremantle, Western Australia, consists of a single piston pump attached to the sea floor, with a float tethered to the piston. Waves cause the float to rise and fall, generating pressurized water, which is piped to an onshore facility to drive hydraulic generators or run reverse osmosis desalination

• File:AeroDynamicTHUMB.PNG

  A device called Neo-AeroDynamic: It is an airfoil base design to harness kinetic power of the fluid flow via an artificial current around its center. The device differentiates from others by its capability to directly transfer wave power into rotational torque to drive a generator without moving part. As the result of its high efficiency; it's not only applicable to wind but also to the variety of hydro electric including free-flow (rivers, creeks), tidal, oceanic currents and wave via ocean wave surface currents.

• A point attenuating device called the Aegir Dynamo, currently being developed by a UK based company called Ocean Navitas uses a direct mechanical conversion technique to produce rotational energy that can be converted to electricity in a similar way to wind turbine technology, and has a mechanical efficiency of 93%.

Challenges

These are some of the challenges to deploying wave power devices:

• Efficiently converting wave motion into electricity; generally speaking, wave power is available in low-speed, high forces, and the motion of forces is not in a single direction. Most readily-available electric generators operate at higher speeds, and most readily-available turbines require a constant, steady flow.
• Constructing devices that can survive storm damage and saltwater corrosion; likely sources of failure include seized bearings, broken welds, and snapped mooring lines. Knowing this, designers may create prototypes that are so overbuilt that materials costs prohibit affordable production.
• High total cost of electricity; wave power will only be competitive when the total cost of generation is reduced. The total cost includes the primary converter, the power takeoff system, the mooring system, installation & maintenance cost, and electricity delivery costs.

Wave farms

2 of 3 P-750 machines in the harbour of Peniche/ Portugal

See also: Wave farm

Portugal has built the world's first commercial wave farm, the Aguçadora Wave Park near Póvoa de Varzim, installing three Pelamis P-750 machines generating 2.25 MW . Subject to successful operation, a further 70 million euro is likely to be invested before 2009 on a further 28 machines to generate 72.5 MW.

Funding for a wave farm in Scotland was announced on February 20, 2007 by the Scottish Executive, at a cost of over 4 million pounds, as part of a £13 million funding packages for marine power in Scotland. The farm will be the world's largest with a capacity of 3MW generated by four Pelamis machines.

Funding has also been announced for the development of a Wave hub off the north coast of Cornwall, England. The Wave hub will act as giant extension cable, allowing arrays of wave energy generating devices to be connected to the electricity grid. The Wave hub will initially allow 20MW of capacity to be connected with potential expansion to 40MW. Four device manufacturers have so far expressed interest in connecting to the Wave hub.

The scientists have calculated that wave energy gathered by this generator will be enough to power up to 7,500 households. Savings that the Cornwall wave power generator will bring are significant: about 300,000 tons of carbon dioxide in the next 25 years.

Potential

Good wave power locations have a flux of about 50 kilowatts per metre of shoreline. Capturing 20 percent of this, or 10 kilowatts per metre, is plausible. Assuming very large scale deployment of (and investment in) wave power technology, coverage of 5000 kilometres of shoreline (worldwide) is plausible. Therefore, the potential for shoreline-based wave power is about 50 gigawatts. Deep water wave power resources are truly enormous, but perhaps impractical to capture.

Discussion of Salter's Duck

While historic references to the power of waves do exist, the modern scientific pursuit of wave energy was begun in the 1970s by Professor Stephen Salter of the University of Edinburgh, Scotland in response to the Oil Crisis.

His invention, Salter's Edinburgh Duck, continues to be the machine against which all others are measured. In small scale controlled tests, the Duck's curved cam-like body can stop 90% of wave motion and can convert 90% of that to electricity. While it continues to represent the most efficient use of available material and wave resources, the machine has never gone to sea, primarily because its complex hydraulic system is not well suited to incremental implementation, and the costs and risks of a full-scale test would be high. Most of the designs being tested currently absorb far less of the available wave power, and as a result their Mass to Power Ratios remain far away from the theoretical maximum.

According to sworn testimony before the House of Parliament, The UK Wave Energy program was shut down on March 19, 1982, in a closed meeting, the details of which remain secret. The members of the meeting were recruited largely from the nuclear and fossil fuels industries, and the wave programme manager, Clive Grove-Palmer, was excluded.

An analysis of Salter's Duck resulted in a miscalculation of the estimated cost of energy production by a factor of 10, an error which was only recently identified. Some wave power advocates believe that this error, combined with a general lack of enthusiasm for renewable energy in the 1980s (after oil prices fell), hindered the advancement of wave power technology.

See also

Renewable energy

Renewable energy effectively uses natural resources such as sunlight, wind, rain, tides and geothermal heat, which are naturally replenished. Renewable energy technologies range from solar power, wind power, hydroelectricity/micro hydro, biomass and biofuels for transportation.

Main article: Renewable energy

Ocean energy

• Pelamis wave energy converter
• Wave farm
• Tidal power
• Ocean energy
• Ocean thermal energy conversion

Other renewable energy

• Wind power
• Solar power
• Hydroelectricity
• Geothermal power
• Biofuels
• Biomass

Other

• World energy resources and consumption

Patents

• U.S. patent 3,928,967 -- Apparatus and method of extracting wave energy - The original "Salter's Duck" patent
• U.S. patent 4,134,023 -- Apparatus for use in the extraction of energy from waves on water - Salter's method for improving "duck" efficiency
• U.S. patent 6,194,815 -- Piezoelectric rotary electrical energy generator - Assignee: Ocean Power Technologies
• US application 20,040,217,597  -- Wave energy converters utilizing pressure differences - PowerBuoy Video

Sources and external articles

Institutional links

• Ocean Renewable Energy Coalition
• European Ocean Energy Association
• The European Marine Energy Centre Ltd.
• MIT Underwater Energy Research Group
• Danish Wave Energy Society
• IEA - Ocean Energy Systems

News articles and compilations

• "Is wave power commercially viable?"
• "A Swedish Wave Power project that has come very far"
• Forskningsprojekt
• "The untimely death of Salter's Duck"
• "Ocean Power Fights Current Thinking"
• "Wave energy in New Zealand"
• "How it works: Wave power station"
• An Ocean Wave Energy Converter Using Rotors
• Oceanography: Waves
• "Waves"
• Corvallis Gazette Times
• List of most wave energy devices
• EU Wavetrain project — A series of full-text, on-line scientific publications on physical concepts.

Wave climate and forecasts

• Information on the long-term statistics of ocean waves can be found at the KNMI wave atlas.
• The US National Oceanic and Atmospheric Administration provides imagery and forecasts of wave height on a global scale. On-line NOAA animation of a user-specified region-of-interest can be set to either wave height or wave period forecasts.

Demonstration of physical concepts

• The interplay between kinetic and potential energy in pendula and waves is demonstrated by this apparatus at Arizona State University.

Company and institutional links with technology descriptions

• SeaStar Wave Power, Inc. -- Sustainable energy from Ocean and Lake waves
• Indian wave energy device --Near shore floating device facing incoming waves.The rise and fall of the waves is converted to mechanical motion by heavy buoyant piston driving an overhead crankshaft that is in turn connected to gearbox and generator. see animation in webpage.
• AW-Energy -- The company has made a near shore machine called "WaveRoller" which operates on utilising the ‘bottom wave’ phenomenon. An underwater wing is attached to a hydraulic arm and uses the backwards and forwards underwater pressures produced by surface waves as they enter shallower waters.
• AWS Ocean Energy -- Submerged (about 50 meters underwater) free-floating buoys are filled with gas and are partly open at the bottom. Each free-floating buoy fits (like a sleeve) over a stationary buoy, and it rises and falls due to pressure changes from waves passing overhead. Power take-off is linear magnetic generator.
• Finavera Renewables Inc.(AquaBuOYs) -- A buoy is attached to a long piston, which pumps water to a common (shared by a number of buoys) hydroelectric generator on the seabed. Electricity is transmitted ashore.
• BioPower Systems -- Oscillating flexible arms, based on kelp fronds, drive an electrical generator via a proprietary gear system.
• Brooke Ocean Technology Ltd (SeaHorse -- Wave-Powered Moored Ocean Profiler) -- (This device is not suitable for electricity generation.) A suitcase-sized ocean sensor is attached to a rope between a buoy and a seabed anchor. It uses the motion of waves to power a ratchet mechanism. This mechanism drives the device up and down the rope to programmed depths. Water density, temperature, and turpidity data is gathered.
• C-Wave Ltd -- Two or more vertical plates sit underwater and normal to the direction of wave propagation. Wave forces cause the plates to be alternately drawn together and forced apart. Hydraulic pistons utilize these forces.
• Oceanlinx (formerly Energetec) -- A parabolic face focuses waves into an inverted basin, and the rising and falling of the water moves an air column. The air column drives a special air turbine generator, one whose vanes rotate to maintain generator direction when the air column reverses.
• Gyro-Gen, developed by Aaron Goldin -- The device includes a spinning gyroscope and a power generator inside a buoy. As the buoy travels over a wave, it tilts, first one way and then the other, and this motion causes the gyro to undergo precession. The gyro resists the rocking motion, not by tilting in the opposite direction, but by turning on the axis of the tilting force. This action is harnessed to move a crank that turns a generator.
• Ing Arvid Nesheim (Oscillating device) -- A floating column is fitted into a sleeve (to enable sliding) and through a large hole in the center of a buoy. The sleeve is attached to the buoy by means of a universal joint, which enables more active (adaptive) up-and-down movement of the buoy. The movement powers an hydraulic electrical generator. (The column has a sea anchor attached to its bottom to reduce vertical movement.)
• Independent Natural Resources Inc (SEADOG Pump) -- A buoyancy block moves up and down in a buoyancy chamber, which rests on a water tank on the seabed. Movement of the buoyancy block drives a piston, which pumps pressurized water into the tank and from there to a reservoir onshore. Water from the reservoir runs through hydroelectric turbines and back into the sea.
• Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Mighty Whale) -- A large steel raft has a work deck aft and a vertical grill that faces the waves. The device uses an oscillating water column to move air in each of three pneumatic chambers. The turbines that convert the pneumatic energy to electrical energy are self-reciprocating. Specifically, the vanes are fixed pitch and have reflective symmetry normal to the direction of airflow, creating bidirectional equivalent lift and drag. (See image of "Wells Turbine".)
• Neo-AeroDynamic: A rotating turbine made of airfoils harnesses kinetic energy of the wave surface current.
• Ocean Power Technologies (PowerBuoy) -- A mostly-submerged buoy connects to a generator on the sea floor.
• Kneider's Sea Wave Energy Propulsion Technology -- (This device is not suitable for electricity generation.) Wave action on flexible flippers forces a boat through the water.
• Ocean Motion International -- Buoys are suspended from a platform (like a fixed oil platform) and are able to move up and down. The buoys are quite heavy (even though buoyant), and they work (pumping water) as they descend into wave troughs. The pressurized water is intended for hydroelectric use or water purification.
• Ocean Navitas (Aegir Dynamo Wave Energy Converter) -- The point attenuating device converts the linear (rise and fall) motion of ocean waves and swells into rotation energy in one phase with an efficiency of 93%. This rotational energy is passed through standard permanent magnet alternators (as used in modern wind turbines)to create grid compliant electrical energy. The efficiency of the mechanical dynamo enables the technology to be scalable and can be deployed in various scenarios.
• Ocean Renewable Energy Group (OREG) -- This Canadian association studies wave and tidal energy development and maintains an extensive online library of ocean energy information.
• OWECO Ocean Wave Energy Company -- The Ocean Wave Energy Converter (OWEC) is a system of quick-connectable modules that form neutrally-buoyant arrays stabilized and sea-anchored by damper sheets. The system may be slack-moored. Large wave-following buoys convert reciprocal motion to counter-rotating, direct-drive electrical generators located in submerged chambers. Sensors control ballast volume and generator resistance. Electricity from multiple modules is combined through linking tubes to output terminals. Major components are shaped to permit volume manufacturing, shipping, and deployment. The electricity produced can be used to desalt water or produce hydrogen.
• Ocean Wave Energy Conversion System (SARA) -- A surfboard-shaped buoy is attached to a long rod. The rod is embedded with magnets, and it moves up and down within a linear generator housing, which is stabilized by an anchored damping plate. A ballast is connected to the bottom of the rod, to pull the rod down after each wave.
• Pelamis Wave Power (Pelamis Wave Energy Converter) -- The machine is long and narrow (snake-like) and points into the waves; it attenuates the waves, gathering more energy than its narrow profile suggests. Its articulating sections drive internal hydraulic generators (through the use of pumps and accumulators).
• Renewable Energy Holdings Plc (CETO) -- A gas-filled tank has rigid sides and base and a flexible (bellows-like) top. The center of the top, which is attached to a lever, rises and falls in response to pressure changes from the waves passing (about 10 m) overhead. The lever drives pistons, which pump pressurized water ashore, for hydroelectricity or reverse osmosis.
• Sea Electrical Generators Ltd -- A wave power device is made of polyethylene tubes. Details are not specified.
• S.D.E. (Sea Wave Power Plant) -- A buoyant metal plate is attached at one side to a concrete seawall. Waves press the plate up (in a cantilever action) and drive an hydraulic ram. The hydraulic system is connected to a hydroelectric system.
• Seabased AB -- A buoy pulls on a rope attached to a linear electromagnetic generator on the seabed. Permanent magnets (NdFeB) are used. The device is claimed ideal for calmer seas. The mechanism for adjusting the generator housing in sympathy with tidal sea levels is not specified.
• Sperboy (Embley Energy) -- A large cylinder contains an oscillating water column. The cylinder is kept in place by buoyancy and ballasts tanks and by about 12 vertical anchor lines. The water column drives air in and out of 4 horizontal ducts that radiate out from the top of the main cylinder. The ducts contain self-reciprocating turbines that convert the pneumatic energy to electrical energy.
• SyncWave Energy -- Two buoys of different buoyancy are connected by a mechanical power take-off. Electronics control the mechanical resistance of this connection.
• Vortex Oscillation Technology -- Claims involve discussion of theoretical hydrodynamic concepts. Details are not specified.
• Wavebob The device is a point absorber that is designed for rough, winter conditions. The top of the unit rests at or just below the surface. The incorporated linear generator uses adaptive electronics to match the wave conditions.
• Wave Dragon -- A parabolic face focuses waves onto a ramp. Waves overtop the ramp and spill into a low dam. Water from the low dam flows through hydroelectric turbines into the sea beneath the floating structure. See also Wave Dragon.
• WAVEenergy AS (Seawave Slot-Cone Generator) -- Waves wash up a slotted ramp (over swept-back louvers) into tiered basins, which drain into a multi-stage hydroelectric system.
• Wavegen (Limpet) — A shore-side inverted basin contains an oscillating water column, which moves an air column. The turbines that convert the pneumatic energy to electrical energy are self-reciprocating. Specifically, the vanes are fixed pitch and have reflective symmetry normal to the direction of airflow, creating bidirectional equivalent lift and drag. (See image of "Wells Turbine".)
• Wave Star Energy -- A long truss is mounted on steel piles. Articulating arms are attached to the truss, and buoys are attached to the ends of the arms. Movement of the arms forces fluid into a central hydraulic accumulator and through a generator turbine.
• Waveberg -- A central float is connected to 3 bent lattice arms, each of which has another float on its outer end. Vertical movement of the outer floats drives hydraulic rams, which pump high-pressure water to shore. This high-pressure water can then be used for hydroelectric generation.