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Tidal power
Tidal power, sometimes called tidal energy, is a form of hydropower that converts the
energy of tides into electricity or other useful forms of power.
Although not yet widely used, tidal power has potential for future electricity generation.
Tides are more predictable than wind energy and solar power. Historically, tide mills
have been used, both in Europe and on the Atlantic coast of the USA. The earliest
occurrences date from the Middle Ages, or even from Roman times.[1][2]
Generation of tidal energy
Main articles: Tide and Tidal acceleration
Tidal power is the only form of energy which derives directly from the relative motions
of the Earth-Moon system, and to a lesser extent from the Earth-Sun system. The tidalforces produced by the Moon and Sun, in combination with Earth's rotation, are
responsible for the generation of the tides. Other
sources of energy originate directly or indirectly from
the Sun, including fossil fuels, conventional
hydroelectric, wind, biofuels, wave power and solar .
Nuclear is derived using radioactive material from the
Earth, geothermal power uses the heat of magma below
the Earth's crust, which comes from radioactive decay.
Tidal energy is generated by the relative motion of the
Earth, Sun and the Moon, which interact via
gravitational forces. Periodic changes of water levels,
and associated tidal currents, are due to the gravitational attraction by the Sun and
Moon. The magnitude of the tide at a location is the result of the changing positions of
the Moon and Sun relative to the Earth, the effects of Earth rotation, and the local shape
of the sea floor and coastlines.
A tidal energy generator uses this phenomenon to generate energy. The stronger the
tide, either in water level height or tidal current velocities, the greater the potential for tidal energy generation.
Tidal movement causes a continual loss of mechanical energy in the Earth-Moon system
due to pumping of water through the natural restrictions around coastlines, and due to
viscous dissipation at the seabed and in turbulence. This loss of energy has caused the
rotation of the Earth to slow in the 4.5 billion years since formation. During the last 620
million years the period of rotation has increased from 21.9 hours to the 24 hours [3] we
see now; in this period the Earth has lost 17% of its rotational energy. Tidal power may
take additional energy from the system, increasing the rate of slowing over the next
millions of years.
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Categorías de mareas
Energía de las mareas se pueden clasificar en dos tipos principales:
• Sistemas de flujo de las mareas hacer uso de la energía cinética del agua para
mover las turbinas de poder, de manera similar a utilizar los molinos de vientoque se desplazan el aire. Este método está ganando popularidad debido a la
menor coste y menor impacto ecológico en comparación con presas.
• Presas hacer uso de la energía potencial en la diferencia de altura (o cabeza)
entre altas y bajas mareas. Presas sufren de muy alto los costos de la
infraestructura civil, una escasez mundial de sitios viables, y las cuestiones
ambientales.
Modernos avances en tecnología de turbinas pueden llegar a ver grandes cantidades de
energía generada de los océanos, especialmente las corrientes de marea utilizando el
flujo de las mareas diseños. Flujo de turbinas de mareas pueden ser organizados en alta
velocidad natural de las zonas donde las corrientes de marea actual se concentran, como
el oeste y este de Canadá, el Estrecho de Gibraltar, el Bósforo, y numerosos sitios en el
sudeste de Asia y Australia. Esas corrientes se producen casi en cualquier parte donde
hay entradas a bahías y ríos, o entre masas de tierra, donde las corrientes de agua se
concentran.
Shrouded tidal energy turbines
An emerging tidal stream technology is the shrouded tidal turbine enclosed in a Venturi
shaped shroud or duct producing a sub atmosphere of low pressure behind the turbine,allowing the turbine to operate at higher efficiency (than the Betz limit [22] of 59.3%) in
one case nearly 4 times higher power output [23] than the same minus the shroud.
The Race Rocks Tidal Current Generator before installation.
This working example of a shrouded turbine in the photo was deployed by Clean
Current Power at Race Rocks in southern British Columbia in 2006. It operates bi-
directionally and has proven to be efficient in contributing to the integrated power
system of Race Rocks. The turbine was removed in May 2007 so that the bearing
system could be redesigned.
Considerable commercial interest has been shown in shrouded tidal stream turbines due
to the increased power output. They can operate in shallower slower moving water with
a smaller turbine at sites where large turbines are restricted. Arrayed across a seaway or
in fast flowing rivers, shrouded turbines are cabled to shore for connection to a grid or a
community. Alternatively the property of the shroud that produces an accelerated flow
velocity across the turbine allows tidal flows formerly too slow for commercial use to
be used for energy production.
While the shroud may not be practical in wind, as the next generation of tidal stream
turbine design it is gaining more popularity and commercial use. Tidal Energy PtyLtd[25] in Australia make use of the design and Lunar Energy
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(http://www.lunarenergy.co.uk/duct.htm) use a double ended shroud. The Tidal Energy
Pty Ltd tidal turbine is multi directional able to face up-stream in any direction and the
Lunar Energy turbine bi directional. All tidal stream turbines constantly need to face at
the correct angle to the water stream in order to operate. The Tidal Energy Pty Ltd is a
unique case with a pivoting base. Lunar Energy use a wide angle diffuser to capture
incoming flow that may not be inline with the long axis of the turbine. A shroud canalso be built into a tidal fence or barrage increasing the performance of turbines.
Types of shroud
Not all shrouded turbines are the same - the
performance of a shrouded turbine varies with the
design of the shroud. Not all shrouded turbines
have undergone independent scrutiny of claimed
performances, as companies closely guard their
respective technologies, so quoted performance
figures need to be closely scrutinised. Claims varyfrom a 15%-25% to a 384% improvement over
the same turbine without the shroud. Shrouded
turbines do not operate at maximum efficiency
when the shroud does not intercept the current
flow at the correct angle, which can occur as
currents eddy and swirl, resulting in reduced
operational efficiency. At lower turbine
efficiencies the extra cost of the shroud must be
justified, while at higher efficiencies the extra cost
of the shroud has less impact on commercial returns. Similarly the added cost of the
supporting structure for the shroud has to be balanced against the performance gained.
Yawing (pivoting) the shroud and turbine at the correct angle, so it always faces
upstream like a wind sock, can increase turbine performance but may need expensive
active devices to turn the shroud into the flow. Passive designs can be incorporated,
such as floating the shrouded turbine under a pontoon on a swing mooring, or flying the
turbine like a kite under water. One design yaws the shrouded turbine using a turntable
Advantages
• A shroud of suitable geometry can increase the flow velocity across the turbine
by 3 to 4 times the open or free stream velocity allowing the turbine to produce
3 to 4 times the power than the same turbine without the shroud.• More power generated means greater returns on investment.
• The number of suitable sites is increased as sites formerly too slow for
commercial development become viable.
• Where large cumbersome turbines are not suitable, smaller shrouded turbines
can be sea-bed-mounted in shallow rivers and estuaries allowing safe navigation
of the water ways.
• Hidden in a shroud, a turbine is less likely to be damaged by floating debris.
• Bio-fouling is also reduced as the turbine is shaded from natural light in shallow
water.
•
The increased velocities through the turbine effectively water-blast the shroudthroat and turbine clean as organisms are unable to attached at increased
velocities.
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• Described as 'eco-benign', the slow r.p.m. of tidal stream turbines does not
interfere with marine life or the environment and has little or no visual amenity
impact.
Disadvantages
• Most shrouded turbines are directional, although one exception is the version off
Southern Vancouver Island in British Columbia. One-direction fixed shrouds
may not capture flow efficiently - in order for the shroud to produce maximum
efficiency to use both flood and ebb tide they need to be yawed like a windmill
on a pivot or turntable, or suspended under a pontoon on a marine swing
mooring allowing the turbine to always face upstream like a wind sock.
• Shrouded turbines need to be below the mean low water level.
• Shrouded turbine loads are 3 to 4 times those of the open or free stream turbine,
so a robust mounting system is necessary. However, this mounting system needs
to be designed in such a way as to prevent turbulence being spilled onto the
turbine or high-pressure waves occurring near the turbine and detuning performance. Streamlining the mounts and or including structural mounts in the
shroud geometry performs two functions, that of supporting the turbine and
providing a net benefit of 3 to 4 times the power output.
• Shrouded turbines may be hazardous to marine life, as fish or marine mammals
can get sucked into the turbine blades, through the venturi.