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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.