Wave power

When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory.
When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory.
Waves overtopping a test wave generator of the Wave Dragon in Nissum-Bedding, Denmark. Image courtesy of Earth Vision.
Waves overtopping a test wave generator of the Wave Dragon in Nissum-Bedding, Denmark. Image courtesy of Earth Vision.
Pelamis machine pointing into the waves: it attenuates the waves, gathering more energy than its narrow profile suggests. See Pelamis Wave Energy Converter
Pelamis machine pointing into the waves: it attenuates the waves, gathering more energy than its narrow profile suggests. See Pelamis Wave Energy Converter

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 a widely employed technology, with only a few experimental sites in existence.

The waves

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."

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

Wave motion is highest at the surface and diminishes exponentially with depth; however, wave energy is also present as pressure waves in deeper water.

The potential energy of a set of waves is proportional to wave height squared times wave period (the time between wave crests). Longer period waves have relatively longer wavelengths and move faster. The potential energy is equal to the kinetic energy (that can be expended). Wave power is expressed in kilowatts per meter (at a location such as a shoreline).

The formula below shows how wave power can be calculated. Excluding waves created by major storms, the largest waves are about 15 meters high and have a period of about 15 seconds. According to the formula, such waves carry about 1700 kilowatts of potential power across each meter of wavefront. A good wave power location will have an average flux much less than this: perhaps about 50 kw/m.

  • Formula: Power (in kw/m) = k H2 T ~ 0.5 H2 T,

where k = constant, H = wave height (crest to trough) in meters, and T = wave period (crest to crest) in seconds.

  • Formula links: 1 2

Challenges

The fundamental challenges of wave power are efficiently converting wave motion into electricity and 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. While the industry has suffered many failures, it has benefited in recent years from increases in support from governments, universities, and angel investors. Several promising prototypes are now in operation.

State of the art

Existing wave power devices are categorized by the method used to capture the energy of the waves, by the intended location, and by the power take-off. Method types are wave power point absorber, occupying a small area; wave power attenuator, occupying a line parallel to wave propagation; and wave power terminator, occupying a line perpendicular to wave propagation. Locations are shoreline, offshore, and deep water. Types of power take-off include these: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator.

Systems include oscillating water column, articulated pontoon, wave pump, anchored buoy, fixed buoy, and overtopping reservoir. Several 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:

  • A pontoon lying in the water is driven by wave action to push or pull a generator.
  • Wave action compresses air in a tunnel which drives the vanes of a generator.
  • A device called CETO, currently being tested off Fremantle, Western Australia, has a seafloor pressure transducer coupled to a high-pressure hydraulic pump, which pumps water to shore for driving hydraulic generators or running reverse osmosis desalination.
  • Waves overtop the side of a reservoir, and the water in the reservoir runs hydroelectric generators.

Potential

Wave power could yield much more energy than tidal power. Tidal dissipation (friction, measured by the slowing of the lunar orbit) is 2.5 terawatts. The energy potential of waves is certainly greater, and wave power can be exploited in many more locations. Countries with large coastlines and strong prevailing winds (notably, Ireland and the UK) could produce five percent or more of their electricity from wave power. And excess capacity (a problem common with intermittent energy sources) could be used to produce hydrogen or smelt aluminum.

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 1970's by Professor Steven 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 have for this reason much higher Mass to Power Ratio than is theoretically possible.

According to sworn testimony before the House of Parliament, The UK Wave Energy program was shuttered 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. See article.

A past analysis of Salter's Duck resulted in a miscalculation of the estimated cost of energy production by a factor of 10, an error 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), acted to hold back the advancement of wave power technology. See article.

Company and institutional links (with technology descriptions)

  • Aerovironment -- A buoy is attached to a cable, and the buoy sits several meters underwater. The buoy rises and falls in response to pressure changes from waves passing overhead. (Although the buoy's design is unclear, it probably includes a flexible, gas-filled bladder.) The bottom of the cable is connected (through an unspecified mechanism) to a generator on the seabed.
  • AquaEnergy Group Ltd (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.
  • 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.
  • Energetech -- 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. (See article on "Tom's Turbine".)
  • 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".)
  • 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 Power Delivery (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).
  • 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 bouy 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.
  • 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 meters) 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 Energy 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.
  • Seavolt (Wave Rider) -- A cam shaped buoy rolls with the passing of waves. The rolling action drives hydraulics, which run a hydroelectric generator.
  • Vortex Oscillation Technology -- Claims involve discussion of theoretical hydrodynamic concepts. Details are not specified.
  • 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.
  • 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".)
  • Wavemill Energy Corp -- Water flows up a ramp, which is on a modular concrete structure. A system of valves captures the water and uses it hydroelectrically.
  • WavePlane Production A/S -- A raft shaped like an obtuse angle is anchored (by chain or cable) in the middle. The point of the angle is designed to self-orient into the waves. Horizontal damping plates reduce vertical movement. Waves spill into guide vanes ("funnels"), which direct water towards a cylindrical tube. The water enters the tube tangential to the curved surface, creating a spinning cylinder of water ("fly wheel tube"). The type of turbine appropriate for utilizing that spinning energy is not specified.
  • A.W.S. BV (Wave Swing) -- A large buoyant cylinder is open at the bottom. The cylinder surrounds and slides up and down on a cylindrical framework, which is attached to a platform on the seabed. The cylinder is mostly full of gas, and it rises and falls as the gas pressure equalizes with the sea pressure, which changes as the wave peaks and troughs pass overhead. The whole assembly is a linear electrical generator.
  • 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.