to do useful work – for example, electricity generation, water desalination, or the pumping

of water. Machinery able to exploit wave power is generally known as a wave energy converter.

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, although

there have been attempts to use it since at least 1890. In 2008, the first experimental

wave farm was opened in Portugal, at the Aguçadoura Wave Park. The major competitor of wave power

is offshore wind power.

Physical concepts

See energy, power and work for more information on these important physical concepts. see

wind wave for more information on ocean waves. Waves are generated by wind passing over the

surface of 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 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, making the water to go into the shear stress causes the growth of the waves.

Wave height is determined by wind speed, the duration of time the wind has been blowing,

fetch and by the depth and topography of the seafloor. A given wind speed has a matching

practical limit over which time or distance will not produce larger waves. When this limit

has been reached the sea is said to be "fully developed".

In general, larger waves are more powerful but wave power is also determined by wave

speed, wavelength, and water density. Oscillatory motion is highest at the surface

and diminishes exponentially with depth. However, for standing waves 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. Wave power formula

In deep water where the water depth is larger than half the wavelength, the wave energy

flux is

with P the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, Te

the wave energy period, ρ the water density and g the acceleration by gravity. The above

formula states that wave power is proportional to the wave energy period and to the square

of the wave height. When the significant wave height is given in metres, and the wave period

in seconds, the result is the wave power in kilowatts per metre of wavefront length.

Example: Consider moderate ocean swells, in deep water, a few km off a coastline, with

a wave height of 3 m and a wave energy period of 8 seconds. Using the formula to solve for

power, we get

meaning there are 36 kilowatts of power potential per meter of wave crest.

In major storms, the largest waves offshore are about 15 meters high and have a period

of about 15 seconds. According to the above formula, such waves carry about 1.7 MW of

power across each metre of wavefront. 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 average(mean) energy density per unit area of gravity waves on the water

surface is proportional to the wave height squared, according to linear wave theory:

where E is the mean wave energy density per unit horizontal area, the sum of kinetic and

potential energy density per unit horizontal area. The potential energy density is equal

to the kinetic energy, both contributing half to the wave energy density E, as can be expected

from the equipartition theorem. In ocean waves, surface tension effects are negligible for

wavelengths above a few decimetres. 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:

with cg the group velocity. 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: Deep-water characteristics and opportunities

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 about twenty times the water depth,

as found quite often near the coast, the group velocity is equal to the phase velocity.

History The first known patent to use energy from

ocean waves dates back to 1799, and was filed in Paris by Girard and his son. An early application

of wave power was a device constructed around 1910 by Bochaux-Praceique to light and power

his house at Royan, near Bordeaux in France. It appears that this was the first oscillating

water-column type of wave-energy device. From 1855 to 1973 there were already 340 patents

filed in the UK alone. Modern scientific pursuit of wave energy was

pioneered by Yoshio Masuda's experiments in the 1940s. He has tested various concepts

of wave-energy devices at sea, with several hundred units used to power navigation lights.

Among these was the concept of extracting power from the angular motion at the joints

of an articulated raft, which was proposed in the 1950s by Masuda.

A renewed interest in wave energy was motivated by the oil crisis in 1973. A number of university

researchers re-examined the potential to generate energy from ocean waves, among whom notably

were Stephen Salter from the University of Edinburgh, Kjell Budal and Johannes Falnes

from Norwegian Institute of Technology, Michael E. McCormick from U.S. Naval Academy, David

Evans from Bristol University, Michael French from University of Lancaster, Nick Newman

and C. C. Mei from MIT. Stephen Salter's 1974 invention became known

as Salter's duck or nodding duck, although it was officially referred to as the Edinburgh

Duck. 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 giving 81% efficiency.

In the 1980s, as the oil price went down, wave-energy funding was drastically reduced.

Nevertheless, a few first-generation prototypes were tested at sea. More recently, following

the issue of climate change, there is again a growing interest worldwide for renewable

energy, including wave energy. The world's first marine energy test facility

was established in 2003 to kick start the development of a wave and tidal energy industry

in the UK. Based in Orkney, Scotland, the European Marine Energy Centre has supported

the deployment of more wave and tidal energy devices than at any other single site in the

world. EMEC provides a variety of test sites in real sea conditions. It's grid connected

wave test site is situated at Billia Croo, on the western edge of the Orkney mainland,

and is subject to the full force of the Atlantic Ocean with seas as high as 19 metres recorded

at the site. Wave energy developers currently testing at the centre include Aquamarine Power,

Pelamis Wave Power, ScottishPower Renewables and Wello.

Modern technology Wave power devices are generally categorized

by the method used to capture the energy of the waves, by location and by the power take-off

system. 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. When evaluating wave energy as a technology type, it is important

to distinguish between the four most common approaches: point absorber buoys, surface

attenuators, oscillating water columns, and overtopping devices.

Point Absorber Buoy This device floats on the surface of the water,

held in place by cables connected to the seabed. Buoys use the rise and fall of swells to drive

hydraulic pumps and generate electricity. EMF generated by electrical transmission cables

and acoustic of these devices may be a concern for marine organisms. The presence of the

buoys may affect fish, marine mammals, and birds as potential minor collision risk and

roosting sites. Potential also exists for entanglement in mooring lines. Energy removed

from the waves may also affect the shoreline, resulting in a recommendation that sites remain

a considerable distance from the shore. Surface Attenuator

These devices act similarly to point absorber buoys, with multiple floating segments connected

to one another and are oriented perpendicular to incoming waves. A flexing motion is created

by swells that drive hydraulic pumps to generate electricity. Environmental effects are similar

to those of point absorber buoys, with an additional concern that organisms could be

pinched in the joints. Oscillating Water Column

Oscillating water column devices can be located on shore or in deeper waters offshore. With

an air chamber integrated into the device, swells compress air in the chambers forcing

air through an air turbine to create electricity. Significant noise is produced as air is pushed

through the turbines, potentially affecting birds and other marine organisms within the

vicinity of the device. There is also concern about marine organisms getting trapped or

entangled within the air chambers. Overtopping Device

Overtopping devices are long structures that use wave velocity to fill a reservoir to a

greater water level than the surrounding ocean. The potential energy in the reservoir height

is then captured with low-head turbines. Devices can be either on shore or floating offshore.

Floating devices will have environmental concerns about the mooring system affecting benthic

organisms, organisms becoming entangled, or EMF effects produced from subsea cables. There

is also some concern regarding low levels of turbine noise and wave energy removal affecting

the nearfield habitat. Oscillating Wave Surge Converter

These devices typically have one end fixed to a structure or the seabed while the other

end is free to move. Energy is collected from the relative motion of the body compared to

the fixed point. Oscillating wave surge converters often come in the form of floats, flaps, or

membranes. Environmental concerns include minor risk of collision, artificial reefing

near the fixed point, EMF effects from subsea cables, and energy removal effecting sediment

transport. Some of these designs incorporate parabolic reflectors as a means of increasing

the wave energy at the point of capture. These capture systems use the rise and fall motion

of waves to capture energy. Once the wave energy is captured at a wave source, power

must be carried to the point of use or to a connection to the electrical grid by transmission

power cables. The table contains descriptions of some wave

power systems: A more complete list of wave energy developers

is maintained here: Wave energy developers Environmental Effects

Common environmental concerns associated with marine energy developments include:

The risk of marine mammals and fish being struck by tidal turbine blades;

The effects of EMF and underwater noise emitted from operating marine energy devices;

The physical presence of marine energy projects and their potential to alter the behavior

of marine mammals, fish, and seabirds with attraction or avoidance;

The potential effect on nearfield and farfield marine environment and processes such as sediment

transport and water quality. The Tethys database is an online knowledge

management system that provides the marine energy community with access to information

and scientific literature on environmental effects of marine energy developments.

Potential The worldwide resource of wave energy has

been estimated to be greater than 2 TW. Locations with the most potential for wave power include

the western seaboard of Europe, the northern coast of the UK, and the Pacific coastlines

of North and South America, Southern Africa, Australia, and New Zealand. The north and

south temperate zones have the best sites for capturing wave power. The prevailing westerlies

in these zones blow strongest in winter.

Challenges There is a potential impact on the marine

environment. Noise pollution, for example, could have negative impact if not monitored,

although the noise and visible impact of each design varies greatly. Other biophysical impacts

of scaling up the technology is being studied. In terms of socio-economic challenges, wave

farms can result in the displacement of commercial and recreational fishermen from productive

fishing grounds, can change the pattern of beach sand nourishment, and may represent

hazards to safe navigation. Waves generate about 2,700 gigawatts of power. Of those 2,700

gigawatts, only about 500 gigawatts can be captured with the current technology.

Wave farms

Portugal The Aguçadoura Wave Farm was the world's

first wave farm. It was located 5 km offshore near Póvoa de Varzim, north of Porto, Portugal.

The farm was designed to use three Pelamis wave energy converters to convert the motion

of the ocean surface waves into electricity, totalling to 2.25 MW in total installed capacity.

The farm first generated electricity in July 2008 and was officially opened on September

23, 2008, by the Portuguese Minister of Economy. The wave farm was shut down two months after

the official opening in November 2008 as a result of the financial collapse of Babcock

& Brown due to the global economic crisis. The machines were off-site at this time due

to technical problems, and although resolved have not returned to site and were subsequently

scrapped in 2011 as the technology had moved on to the P2 variant as supplied to Eon and

Scottish Power Renewables. A second phase of the project planned to increase the installed

capacity to 21 MW using a further 25 Pelamis machines is in doubt following Babcock's financial

collapse. United Kingdom

Funding for a 3 MW 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 package

for marine power in Scotland. The first of 66 machines was launched in May 2010.

A facility known as Wave hub has been constructed off the north coast of Cornwall, England,

to facilitate wave energy development. 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 20 MW of capacity to be connected, with potential