Wave power
From Wikipedia, the free encyclopedia
This article is about transport and capture of energy in ocean waves. For other aspects of waves in the ocean, see
Wind wave. For other uses of wave or waves, see
Wave (disambiguation).
Wave power is the transport of energy by
ocean surface waves, and the capture of that energy to do useful
work – for example,
electricity generation,
water desalination, or the
pumping of water (into reservoirs). Machinery able to exploit wave power is generally known as a
wave energy converter (WEC).
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.
[1] In 2008, the first experimental
wave farm was opened in Portugal, at the
Aguçadoura Wave Park.
[2]
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.
Photograph of the water particle orbits under a – progressive and periodic –
surface gravity wave in a
wave flume. The wave conditions are: mean water depth
d = 2.50 ft (0.76 m),
wave height H = 0.339 ft (0.103 m), wavelength λ = 6.42 ft (1.96 m),
period T = 1.12 s.
[3]
- 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.
[4]
Wave height
is determined by wind speed, the duration of time the wind has been
blowing, 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. 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 (
clapotis) near a reflecting coast, wave energy is also present as pressure oscillations at great depth, producing
microseisms.
[4] 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).
Wave power formula
In deep water where the water depth is larger than half the
wavelength, the wave
energy flux is
[A 1]
with
P the wave energy flux per unit of wave-crest length,
Hm0 the
significant wave height,
T the wave
period,
ρ the water
density and
g the
acceleration by gravity. 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
meters, and the wave period in seconds, the result is the wave power in
kilowatts (kW) per meter of
wavefront length.
[5][6][7]
Example: Consider moderate ocean swells, in deep water, a few
kilometers off a coastline, with a wave height of 3 meters and a wave
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 meter 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 energy density per unit area of
gravity waves on the water surface is proportional to the wave height squared, according to linear wave theory:
[4][8]
- [A 2][9]
where
E is the mean wave energy density per unit horizontal area (J/m
2), the sum of
kinetic and
potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy,
[4] 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:
[10][4]
with
cg 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:
[4][8]
[show]Properties of gravity waves on the surface of deep water, shallow water and at intermediate depth, according to linear wave theory |
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.
[11]
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.
[12] 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.
[13] It appears that this was the first oscillating water-column type of wave-energy device.
[14] From 1855 to 1973 there were already 340 patents filed in the UK alone.
[12]
Modern scientific pursuit of wave energy was pioneered by
Yoshio Masuda's experiments in the 1940s.
[15]
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.
[16]
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 (now merged into
Norwegian University of Science and Technology),
Michael E. McCormick from
U.S. Naval Academy,
David Evans from
Bristol University, Michael French from
University of Lancaster,
John Newman and
Chiang 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.
[17]
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.
[18]
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. Method types are point absorber or buoy; surfacing following or
attenuator
oriented parallel to the direction of wave propagation; terminator,
oriented perpendicular to the direction of 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,
[19] 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
capture systems use the rise and fall motion of waves to capture
energy.
[20] 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.
[21] The table contains descriptions of some wave power systems:
PowerBuoy |
Ocean Power Technologies |
US |
Buoy |
Offshore |
Hydroelectric turbine |
1997 |
In the United States, the Pacific Northwest Generating
Cooperative is funding construction of a commercial wave-power park at
Reedsport, Oregon that will utilize this technology which consists of
modular, ocean-going buoys.[22] The rise and fall of the waves moves a rack and pinion within the buoy and spins a generator [23].
The electricity is transmitted to shore over a submerged transmission
line. A 150 kW buoy has a diameter of 36 feet (11 m) and is 145 feet (44
m) tall, with approximately 30 feet of the unit rising above the ocean
surface. Using a 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.[24]
PB150 PowerBuoy with peak-rated power output of 150 kW.
|
Pelamis Wave Energy Converter |
Pelamis Wave Power |
UK (Scottish) |
Surface-following attenuator |
Offshore |
Hydraulic |
1998 |
The Pelamis machine consists of a series of
semi-submerged cylindrical sections linked by hinged joints. As waves
pass along the length of the machine, the sections move relative to one
another. The wave-induced motion of the sections is resisted by hydraulic cylinders which pump high pressure oil through hydraulic motors via smoothing hydraulic accumulators. The hydraulic motors drive electrical generators to produce electricity.[25] Pelamis Wave Power first tested and grid connected a Pelamis machine in 2004 at the European Marine Energy Center.[26] The first of a second generation of machines, the P2 started grid connected tests off Orkney in 2010, the machine is owned by E.ON.[27]
Pelamis prototype machine at EMEC, Scotland in 2004.
|
Wave Dragon |
Erik Friis-Madsen |
Denmark |
Surface-following attenuator |
Offshore |
Hydroelectric turbine |
2003 |
With the Wave Dragon wave energy converter large wing
reflectors focus waves up a ramp into an offshore reservoir. The water
returns to the ocean by the force of gravity via hydroelectric
generators.
Wave Dragon seen from reflector, prototype 1:4½
|
Anaconda Wave Energy Converter |
Checkmate SeaEnergy.[25] |
UK |
Surface-following attenuator |
Offshore |
Hydroelectric turbine |
2008 |
In the early stages of development, the device is a 200
metres (660 ft) long rubber tube which is tethered underwater. Passing
waves will instigate a wave inside the tube, which will then propagates
down its walls, driving a turbine at the far end.[28][29] |
AquaBuOY |
Finavera Wind Energy, later SSE Renewables Limited |
Ireland-Canada-Scotland |
Buoy |
Offshore |
xxx |
2003 |
In 2009 Finavera Renewables surrendered its wave energy
permits from FERC.[27] In July 2010 Finavera announced that it had
entered into a definitive agreement to sell all assets and intellectual
property related to the AquaBuOY wave energy technology.[30][31][32][33] |
FlanSea (Flanders Electricity from the Sea) |
FlanSea |
Belgium |
Buoy |
Offshore |
Hydroelectric turbine |
2010 |
A point absorber buoy developed for use in the southern
North Sea conditions.[31][32][33] It works by means of a cable that due
to the bobbing effect of the buoy, generates electricity.[34][35][36] |
SeaRaser |
Alvin Smith (Dartmouth Wave Energy) |
UK |
Buoy |
Nearshore |
Hydraulic ram |
2008 |
Consisting of a piston pump(s) attached to the sea
floor with a float (buoy) tethered to the piston. Waves cause the float
to rise and fall, generating pressurized water, which is piped to
resoviors onshore which then drive hydraulic generators.[37][38] |
CETO Wave Power |
Carnegie |
Australia |
Buoy |
Offshore |
Pump-to-shore |
1999 |
Currently being tested off Fremantle, Western
Australia,[35] the device consists of a single piston pump attached to
the sea floor with a float (buoy) 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 water desalination.[39][40]
CETO Buoyant Actuator during the installation process
|
Unnamed Ocean Wave-Powered Generator |
SRI International |
US |
Buoy |
Offshore |
Electroactive polymer artificial muscle |
2004 |
A type of wave buoys, built using special polymers, is being developed by Stanford Research Institute.[41][42] |
Wavebob |
Wavebob |
Ireland |
Buoy |
Offshore |
Direct Drive Power Take off |
1999 |
Wavebob have conducted some ocean trials, as well as extensive tank
tests. It is an ccean-going heaving buoy, with a submerged tank which
captures additional mass of seawater for added power and tunability, and
as a safety feature (Tank "Venting") |
|
Oyster wave energy converter |
Aquamarine Power |
UK (Scots-Irish) |
Oscillating wave surge converter |
Nearshore |
Pump-to-shore (hydro-electric turbine) |
2005 |
The wave energy device captures the energy found in
nearshore waves and converts it into electricity. The systems consists
of a hinged mechanical flap connected to the seabed at around 10m depth.
Each passing wave moves the flap which drives hydraulic pistons to
deliver high pressure water via a pipeline to an onshore turbine which
generates electricity. In November 2009, the first full-scale
demonstrator Oyster began producing power when it was launched at the
European Marine Energy Centre (EMEC) on Orkney.[43] |
OE buoy |
Ocean Energy |
Ireland |
Buoy |
Offshore |
xxx |
2006 |
In September 2009 completed a 2-year sea trial in one quarter scale form. The OE buoy has only one moving part.[44] |
Lysekil Project |
Uppsala University |
Sweden |
Buoy |
Offshore |
Linear generator |
2002 |
Direct driven linear generator placed on the seabed.
The generator is connected to a buoy at the surface via a line. The
movements of the buoy will drive the translator in the generator. The
advantage of this setup is a less complex mechanical system with
potentially a smaller need for maintenance. One drawback is a more
complicated electrical system.[45][46] |
Oceanlinx |
Oceanlinx |
Australia |
Buoy |
Offshore |
Hydroelectric turbine |
1997 |
An Australian firm is developing this deep-water
technology to generate electricity from, ostensibly, easy-to-predict
long-wavelength ocean swell oscillations. Oceanlinx recently began
installation of a third and final demonstration-scale, grid-connected
unit near Port Kembla, near Sydney, Australia, a 2.5 MWe
system that is expected to go online in early 2010, when its power will
be connected to the Australian grid. The company's much smaller
first-generation prototype unit, in operation since 2006, was since
disassembled.[47]
In May 2010, the wave energy generator snapped from its mooring lines and wrecked on Port Kembla's eastern breakwater.[48]
|
SDE Sea Waves Power Plant |
SDE Energy Ltd. |
Israel |
Buoy |
Inshore |
Hydroelectric turbine |
xxx |
A breakwater-based wave energy converter, this device
is built close to the shore and utilizes the vertical motion of buoys
for creating hydraulic pressure which in turn operates the system's
generators. In 2010 it began construction of a new 250 kWh model in the
port of Jaffa, Tel Aviv and preparing to construct its standing orders
for a 100 MWh power plants in the islands of Zanzibar and Kosrae,
Micronesia. |
WaveRoller |
AW-Energy Oy |
Finland |
Surface-following attenuator |
Offshore |
Pump-to-shore |
1994 |
The WaveRoller is a plate anchored on the sea bottom by
its lower part. The back and forth movement of surge moves the plate.
The kinetic energy transferred to this plate is collected by a piston
pump. Full-scale demonstration project built off Portugal in 2009.[49][50] |
Wave Star |
Wave Star A/S |
Denmark |
Multi-point absorber |
Offshore |
Hydroelectric turbine |
2000 |
The Wavestar machine draws energy from wave power with
floats that rise and fall with the up and down motion of waves. The
floats are attached by arms to a platform that stands on legs secured to
the sea floor. The motion of the floats is transferred via hydraulics
into the rotation of a generator, producing electricity. Wave Star have
been testing a 1:10 machine since 2005 in Nissum Bredning, Denmark, it
was taken out of duty in November 2011. A 1:2 Wave Star machine is place
in Hanstholm which has produced electricity to the grid since September 2009.[51] |
Islay LIMPET |
Islay LIMPET |
Scotland |
oscillating water column |
Onshore |
air turbine |
1991 |
Islay LIMPET is a 500 kW shoreline device uses an
oscillating water column to drive air in and out of a pressure chamber
through a Wells turbine.[52][53][54]
The chamber of the LIMPET is an inclined concrete tube with its opening
below the water level. As external wave action causes the water level
in the chamber to oscillate, the variation in water level alternately
compresses and decompresses the trapped air above, causing air to flow
backwards and forwards through a pair of contra-rotating turbines. |
R38/50 kW, R115/150 kW |
40South Energy |
UK |
Underwater attenuator |
Offshore |
Electrical conversion |
2010 |
These machines work by extracting energy from the
relative motion between one Upper Member and one Lower Member, following
an innovative method which earned the company one UKTI Research &
Development Award in 2011.[55] A first generation full scale prototype for this solution was tested offshore in 2010,[56][57][58] and a second generation full scale prototype was tested offshore during 2011.[59] In 2012 the first units were sold to clients in various countries, for delivery within the year.[60][61]
The first reduced scale prototypes were tested offshore during 2007,
but the company decided to remain in a "stealth mode" until May 2010[62] and is now recognized as one of the technological innovators in the sector.[63] The company initially considered installing at Wave Hub in 2012,[64] but that project is on hold for now. The R38/50 kW is rated at 50 kW while the R115/150 kW is rated at 150 kW. |
Potential
The realistically usable worldwide resource has been estimated to be greater than 2 TW.
[65][66]
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.
[6]
Other biophysical impacts (flora and fauna, sediment regimes and water
column structure and flows) of scaling up the technology is being
studied.
[67]
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.
[68]
Waves generate about 2,700 gigawatts of power. Of those 2,700
gigawatts, only about 500 gigawatts can be captured with the current
technology.
[20]
Wave farms
The
Aguçadoura Wave Farm was the world's first
wave farm. It was located 5 km (3 mi) offshore near
Póvoa de Varzim north of
Oporto in 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
[69] and was officially opened on the 23rd of September 2008, by the Portuguese Minister of Economy.
[70][71]
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.
[72] A second phase of the project planned to increase the installed capacity to
21 MW using a further 25 Pelamis machines
[73] is in doubt following Babcock's financial collapse.
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.
[74]
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
20 MW of capacity to be connected, with potential expansion to
40 MW. Four device manufacturers have so far expressed interest in connecting to the Wave hub.
[75][76]
The scientists have calculated that wave energy gathered at Wave Hub
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.
[77]
A
CETO
wave farm off the coast of Western Australia has been operating to
prove commercial viability and, after preliminary environmental
approval, is poised for further development.
[citation needed][78][79]
Patents
See also
Notes
- ^ The energy flux is with the group velocity, see Herbich, John B. (2000). Handbook of coastal engineering. McGraw-Hill Professional. p. A.117, Eq. (12). ISBN 978-0-07-134402-9. The group velocity is , see the collapsed table "Properties
of gravity waves on the surface of deep water, shallow water and at
intermediate depth, according to linear wave theory" in the section "Wave energy and wave energy flux" below.
- ^ For a small-amplitude sinusoidal wave with wave amplitude the wave energy density per unit horizontal area is or using the wave height for sinusoidal waves. In terms of the variance of the surface elevation the energy density is . Turning to random waves, the last formulation of the wave energy equation in terms of is also valid (Holthuijsen, 2007, p. 40), due to Parseval's theorem. Further, the significant wave height is defined as , leading to the factor 1⁄16 in the wave energy density per unit horizontal area.
- ^ For determining the group velocity the angular frequency ω is considered as a function of the wavenumber k, or equivalently, the period T as a function of the wavelength λ.
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Further reading
- Cruz, Joao (2008). Ocean Wave Energy – Current Status and Future Prospects. Springer. ISBN 3-540-74894-6., 431 pp.
- Falnes, Johannes (2002). Ocean Waves and Oscillating Systems. Cambridge University Press. ISBN 0-521-01749-1., 288 pp.
- McCormick, Michael (2007). Ocean Wave Energy Conversion. Dover. ISBN 0-486-46245-5., 256 pp.
- Twidell, John; Weir, Anthony D.; Weir, Tony (2006). Renewable Energy Resources. Taylor & Francis. ISBN 0-419-25330-0., 601 pp.
External links