What is 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 a widely employed technology, with only a few experimental sites in existence.
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.
The fundamental challenges of wave power are:
efficiently converting wave motion into electricity… generally speaking, wave power is available in low-speed, high forces and motion is not in a single direction. Most readily-available electric generators like to operate at higher speeds, with lower input forces, and they prefer to rotate in a single direction.
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.
low total cost of electricity… wave power will only be competitive when total cost of generation (p/kWhr) is reduced. The winning team will be the one that develops the lowest-cost system (which includes the primary converter, power takeoff system, mooring system, installation & maintenance procedures)
While the industry has suffered too many failures to continue, 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 methods
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:
The AquaBuOY 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 pontoon lying in the water is driven by wave action to push or pull an electrical generator. (See Pelamis Wave Energy Converter.)
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.
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. Excess capacity (a problem common with intermittent energy sources) could be used to produce hydrogen or smelt aluminum.