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Tidal action caused by gravitational effectsof moon and sun on earths oceans.
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Common tidal energy system is a barragesystem as shown in the following slide.
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250 m
200 m
barrage
Top view
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F ront view
12 m
6 m
High tide
Low tide
turbine blades
4 m
20 m
200 m
4 m
8 m
3 m
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High tide depth (h) = 6 m Width = 200 m Length = 250 m
Depth of water at low tide = 12
mSeawater V !1027 kg/m 3Tidal period = 1 2 hrsArea behind barrage (A) = 200 m x 250 m = 50,000 m2
Estimate of total energy stored at high tide (assuming allmass is located at mid- high tide depth (11 m):
2 8 2 23 2
9
h 1E = mass water g A h g h
2 21 1 kg m
A g h 1 0 m 1 027 9 .81 (3 m)2 2 m sec
E 2 .27 x10 joules = 6 29 .7 kWhr.
! ! !
!
V
V
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Assume power generated only during ebb tide(flow out). Time of flow out = 6 hrs.
Assume all water stored during flow in isdischarged during ebb tide
Volume of water stored during inflow:
V = 6 m ( 250 m x 200 m) = 3 x 1 05 m3
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Assume average flow rate out of barrage is given by:
5V mF 5 x 1 0 6 hrs hr
! !
Assume all of this flow passes through the barrageopening of 20 m by 4 m. Let a = 20 m x 4 m = 8 0 m2
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Average fluid velocity through this opening is given by:
v = F /a = ( 5 x 1 04 m3/hr)/8 0 m2 = 625 m/hr
In this opening there will be 5 turbine generators with bladeradius equal to 2 ft.
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Turbine power equation (same as for wind
turbine) for each turbine generator is:
3t
t
2t
1P A v
2A area covered during rotation of rotor blades.
A = r
r length single blade
! !
!
V
T
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32
3
total
total
1 kg mP ( 2 m) 1 027 625 34w
2 m hr
F or 5 turbines , P 5 P 16 9w
Total Energy for one tidal cycle
= P 6 hr 0 .20 3 kWh
! T !
! !
!
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Power calculations for tidal currents andunderwater turbine generators:
Assumptions:
3
mTidal current (v) 0 .1sec
Turbine blade radius (r) 5 m
kgeawater density ( ) 1 027 m
y
y
y V
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2 3 2 33
1 1 kg mP r v ( 5 m) 1 027 (0 .1
P 4 0
)2 2 m sec
(per turbine genew rator)
! T V ! T
!
F airly low power output , but can run 24 hrs aday in all weather (tidal currents are reasonablyconstant)
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F or comparison a person riding a bicyclewith the rear wheel linked to a generator canproduce about 75 watts of electricity. Theproblem is that most people cant keep thisup for more than an hour.
(A possibility for Appledore is to open arecreation/fitness center with 1 0 or 1 5 bikes
connected to generators).
By contrast Lance Armstrong can generate500 watts --- but only for 20 minutes
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In all of these options energy is lost in thetransfer from mechanical to electricalenergy. Typical efficiency of the overall
processes is 4 0 50% .
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O ther ocean power options includecapturing wave energy. Some of theseoptions are shown in the following slides.
There are only a few of these schemes inactual operation so there is not sufficientdata to assess their effectiveness plusAppledore Island generally doesntexperience significant wave action.
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ShoreO cean
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