18-tidal energy

8/7/2019 18-Tidal Energy

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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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18-tidal energy

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