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Case Study
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La Rance Tidal Power Plant
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La Rance (240 MW) has beensuccessful as the first full scale
tidal power plant (1967) , locatedin northern France on the La RanceRiver.
The dam itself is 2460 feet (750meters) long, and 43 feet (13meters) high.
The turbines used in La Rance are
Bulb Turbines (24*10 MW), weigh470tons, Diameter 17 feet.
The plant is also equipped with
pumps that allow water to be55
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Barrage was large enough to create aroad with two double lanes, saving
local citizens an eighteen mile drive.The unique nature of the power
station has also increased tourism inthe area. La Rance attracts over300,000 visitors every year.
Despite the high initial cost, thepower station has been working for
over thirty years, generating enoughelectricity for around 300,000 homes.
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Severn Barrage
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The Severn Barrage is a proposedtidal power station to be built
across the Bristol Channel (SevernEstuary). The River Severn has atidal range of 14 metres - thesecond highest in the world - making
it perfect for tidal power generation. 20 billion pound ($US30bn) Severn
Barrage would involve the
construction of a 10 mile longbarrage (dam) between LavernockPoint south of Cardiff, Wales, and
Brean Down in Somerset, England.
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The 216 tubular turbines would belocated in the central portion of the
barrage, and each would drive a 40megawatt generator, resulting in anestimated 17TWh each year (TidalFiles).
The proposed scheme has a lifetimeof at least 120 years (Taylor, 2002).Ship locks were also included in the
scheme because the Severn hasmany important ports that would belocated in the tidal basin if the
barrage was built. 1111
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Introduction
The tide induces periodical currentswhich are particularly strong in someseas, in particular along the EnglishChannel.
The kinetic energy of the currentscan be harnessed by submarine tidalturbines.
The physical phenomena involvedmust be investigated beforedesigning the suitable equipment.
The actual resource on a given site1313
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The tidal stream resource As a first approach, the power of the
water stream through a tidal turbinerotor follows a cubic law similar tothe power law of a wind turbine: W =
. r. U3 - W : power (W.m-2) - r : water
density = 1024 kg.m-3 - U :water velocity (m.s-1)
This equation shows that tidal streamenergy is attractive where the tidecreates strong currents. The suitable
zones are found where the coast1414
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Map of the tidal currents in the EnglishChannel. Maximum velocity during a
mean spring tide
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Race have strongest current about at the moment ofthe high and the low tide, with velocities exceeding 3
to 4 m.s-1 on large areas.
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The mean spring tide has a coefficientof 95, while the mean neap tide
coefficient is 45.
20
30
40
50
60
70
80
90
100
110
120
1 31 61 91 121 151 181 211 241 271 301 331 361
Day of the year
Tidalamplitude(coefficient
17Variation of the tide coefficient during the year 2001 atBrest
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Power inflow variation during a mean spring tide and a meanneap tide
0
5
10
15
20
25
-6 -4 -2 0 2 4 6
Hours relative to local high tide time
Power(kW/m)
Ouessant spring tide
Ouessant neap tide
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0
2
4
6
8
10
12
14
16
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Cumulative time per year (hours)
Power(kW/m)
Cumulative distribution of the power inflow on a site with amaximum velocity of 3 m.s-1 during mean spring tide
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0
5000
10000
15000
20000
25000
0 5 10 15 20
Nominal power input rating (kW/m)
Theoreticalenergyreso
urc
(kWh/year/m)
Relationship between the power input rating and theannual theoretical energy resource
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0
500
1000
1500
2000
2500
3000
3500
4000
2 4 6 8 10 12
Nominal input power (kW/m)
Num
berofhoursp
eryear Hours at full load
Equivalent hours of production
Relationship between the nominal power rating and thehours of production per year
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The Marenergie tidalturbine
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Designing parameter of tidalturbine
The turbine must work in asubmarine environment wheremaintenance is very difficult, so the
machinery must be made as simpleas possible
All marine operations for installation
and maintenance must take intoaccount the strong currentsprevailing in the areas of interest
A compromise must be found2323
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Solution of design of turbine
The rotor is maintained fixed in thespace and the water flowsalternatively in both directions during
flood and ebb flows.The number of moving parts exposed
to the sea water is kept to a
minimum. The blades are fixed andwelded onto the hub. The onlymoving part in sea water requiringsome attention is the seal of therotor shaft on the nacelle front face.
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choices
The rotor turns in both directionsfollowing the current direction
The blades are symmetrical: Both
ends are alternatively leading andtrailing edges.
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The peripheral velocity is kept at a relatively lowlevel (7m.s-1 ) in order to avoid cavitationphenomena on the blades.
The optimum velocity decreases when the numberof blades is increased, and a correct velocity isobtained with 6 blades.
The preliminary studies indicate the benefit of acircular belt at the rotor periphery. This enhances
the blade efficiency and eliminates most of thepotential vibrations.
It should also limit the emission of low frequencynoise at the blade tips.
The outside diameter of a 200 kW rotor is typically10 meters for a nominal current velocity of 3 m.s-1.
The actual design of the base depends on the soilnature.
The rotor may be surrounded by a duct if required.Several turbines can be arran ed in arra s2626
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the power is cancelled for a rotation speed calledfree wheeling speed, slightly higher than the
optimum speed.
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0
50
100
150
200
250300
350
400
450
0 5 10 15 20Rotor rotation speed (rpm)
Power(kW
)
1.5 m/s
2 m/s
2.5 m/s
3 m/s
3.5 m/s
ELECTRICAL OUTPUT
Typical characteristics of a Marenergie tidal turbine
Typical characteristics of a Marenergie
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Typical characteristics of a Marenergietidal turbine
It is not advisable to use the
maximum hydraulic power of theturbine when the current isparticularly strong.
The electrical generator is designedwith a nominal power of 200 kW.When the current is sufficient, thepower output is kept at this level.
The rotor is then stabilized at arotation speed corresponding to theequivalent hydraulic power. Figure
10 shows that this operation mode is2929
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Wave currentinteraction
The water is put into movement notonly by the tidal streams, but also bythe wave action. The combination of
both movements is a complexproblem.
In particular, it is known that a
current flowing against the swellincreases the wave height while thewaves are attenuated when bothphenomena are in the samedirection. 3030
Th di t b t t i t i th
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The distance between two successive crests is thewave length L which can be calculated by thefollowing implicit relation:
where g is the acceleration of gravity (9.81 m.s-2)and d is the water depth in the absence of waves.
The amplitude of the wave movement decreaseswith the depth z below the surface. The horizontalvelocity Vx induced by the wave action is given bythe formula:
In the absence of wave, the current velocity is zeroon the seabed and highest at the surface. Thevelocity profile is generally approached by the3131
)2tanh(2
2
L
dgTL
=
))//(2sin(
)2
(
))(2
(LxTt
L
dCosh
L
zdCosh
T
HVx
=
7/1
0 )/( zdVVx =
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Influence of the swell (H= 2m - T = 9s)on the velocity profiles Water depth =
30m
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-30
-25
-20
-15
-10
-5
0
5
0 1 2 3 4 5
Water velocity (m/s)
De
pthbelownorma
lsurface(m)
Crest passage
Trough passage
B h i f th tid l t bi
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Behavior of the tidal turbine:Current = 3 m.s-1 - Swell: H= 2m
T= 9s
3333
0.0
50.0
100.0
150.0
200.0
250.0
0 5 10 15 20
Time (s)
Output (kW)
Rotor velocity (rotations per minute x10)
T i l t t f i l
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Typical power output of a singleturbine with waves aligned with
the current
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0
50
100
150
200
250
0 1 2 3 4 5 6
Tidal current velocity (m/s)
Electricaloutp
ut(kW) H = 0
H = 2m - T = 9s
H = 4m - T = 10s
H = 6m - T = 12s
H = 8m - T = 13s
A t t f f
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Average output of an array ofturbines ideally spaced along the
wave length
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0
50
100
150
200
250
0 1 2 3 4 5 6
Tidal current velocity (m/s)
Electricaloutp
ut(kW)
H = 0
H = 2m - T = 9s
H = 4m - T = 10s
H = 6m - T = 12s
H = 8m - T = 13s
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Conclusions
Tidal streams offer an abundantenergy resource along the EnglishChannel. Tidal turbines can be
optimized according to the localconditions prevailing on the differentsites.
The Marenergie tidal turbine isdesigned as simple as possible.Variable speed generators arerequired, similar to the types used inmodern wind turbines. 3636
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Advantage It is very cheap
to maintainThere is no
waste orpollution
Very reliable We can predict
when tides will
be in or outThe barrage can
help to reduce
the damage of
Disadvantage It changes the
coastlinecompletely andthe estuaries areflooded so any
mud flats orhabitats thatbirds or animals
live on aredestroyed Initial building
cost is veryex ensive3737
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