Tidal action caused by gravitational effects of moon and sun on earth’s oceans.

Common tidal energy system is a barrage system as shown in the following slide.

200 m Top view 250 m barrage

High tide Low tide Front view 200 m 6 m 3 m 8 m 12 m 4 m turbine blades 4 m 20 m 200 m 3 m 8 m 4 m

High tide depth (h) = 6 m Width = 200 m Length = 250 m Depth of water at low tide = 12 m Seawater r = 1027 kg/m3 Tidal period = 12 hrs Area behind barrage (A) = 200 m x 250 m = 50,000 m2 Estimate of total energy stored at high tide (assuming all mass is located at mid- high tide depth (11 m):

Assume power generated only during ebb tide (flow out) Assume power generated only during ebb tide (flow out). Time of flow out = 6 hrs. Assume all water stored during flow in is discharged during ebb tide Volume of water stored during inflow: V = 6 m (250 m x 200 m) = 3 x 105 m3

Assume average flow rate out of barrage is given by: Assume all of this flow passes through the barrage opening of 20m by 4 m. Let a = 20m x 4 m = 80 m2

Average fluid velocity through this opening is given by: v = F/a = (5 x 104 m3/hr)/80 m2 = 625 m/hr In this opening there will be 5 turbine generators with blade radius equal to 2 ft.

Turbine power equation (same as for wind turbine) for each turbine generator is:

Power calculations for tidal currents and underwater turbine generators: Assumptions:

Fairly low power output, but can run 24 hrs a day in all weather (tidal currents are reasonably constant)

For comparison a person riding a bicycle with the rear wheel linked to a generator can produce about 75 watts of electricity. The problem is that most people can’t keep this up for more than an hour. (A possibility for Appledore is to open a recreation/fitness center with 10 or 15 bikes connected to generators). By contrast Lance Armstrong can generate 500 watts --- but only for 20 minutes

In all of these options energy is lost in the transfer from mechanical to electrical energy. Typical efficiency of the overall processes is 40 – 50%.

Other “ocean – power” options include capturing wave energy Other “ocean – power” options include capturing wave energy. Some of these options are shown in the following slides. There are only a few of these schemes in actual operation so there is not sufficient data to assess their effectiveness – plus Appledore Island generally doesn’t experience significant wave action.

Ocean Shore