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Periodic movement of the ocean water, cause by the moon and the sun
Tidal Movement: Tide
- rises and falls
- period of 12.5 hours
Tidal Movement: Tidal Current
- flows and ebbs
- periodic horizontal movement of the water
bring the tide producing forces
- caused by surface winds
- period of about 6 seconds
Spring tide cycles
- half-day cycle -> due to rotation of earth
- 14 day cycle -> due to moon and sun gravitational fields
Structure placed across a flowing body of water to stop the flow
Barrage with turbine (diagram)
Tidal power calculation explaination
basic concept of tidal power is derived from the tidal range
Gravitation energy per tide (eqn)
Average potential power per 1 tidal period (eqn)
Differences between tidal and wind energy (2)
- 1. water is 800 times denser that air and has a much slower flow rate
- -> The turbine experiences much bigger forces and moments
2. Water turbines have smaller diameters
Tidal Energy Advantages (6)
- 1. Renewable source
- 2. Once dam is constructed, power is free
- 3. It needs no fuel
- 4. No greenhouse gases or other wastes
- 5. Not expensive to maintain
- 6. Tides are predictable
Disadvantages of tidal energy (4)
- 1. Expensive to build plant
- 2. Sea water is corrosive
- 3. Destruction to ecosystem
- 4. Appropriate waves and tides are location dependent
How to prevent sea water corrosion
use stainless steel with a high chromium content and small content of molybdenum
use aluminum bronzes
How tidal energy destructs ecosystem
- disrupts fish migration
- kills fish passing through the turbines
Turbines used in barrier tidal power stations (3)
- 1. bulb turbine
- 2. Straflo turbine
- 3. Tubular turbines
Bulb turbine advantage and disadvantage
- very efficient in terms of space and size (no need for separate housing unit)
- cannot be maintained without stopping the flow
Rim turbine advantage and disadvantage
- the generator can be maintained easier
- -sealing between the runner blades and radial generator causes problems
- -more delicate
- -> harder to pump water through and regulate power produced
Tubular turbine advantages
- blades can be adjusted (to meet energy demand)
- -> allow turbine to move more efficiently, no excessive wear
How tidal streams work
harness the horizontal flow of the tidal currents and streams (KE of the tides)
Configurations for tidal streams (4)
- tidal fences
- tidal stream turbines
- tidal lagoons
- prototype tidal generator designs
tidal fences advantages
Unlike tidal barrage power stations, tidal fences can be used unconfined basins.
They do not require flooding of the basin and are cheaper to install.
Less environmental effect than equivalent-scale barrage. (Salinity does not change)
Tidal current turbines
- Resembles horizontal axis wind turbine.
- Utilize tidal currents that are moving velocities 2 and 3 (m/s) to generate 4-13 kW/m2.
use a rubble mound impoundment structure and low-head hydroelectric generation equipment situated up to a mile or more offshore in a high tidal range area.
tidal lagoon vs barrage
Tidal Lagoons are similar to barrages but do not take up as much room as they do. Also they have a lower cost and impact on the environment. This means that they can generate continuously.
Prototype tidal generator designs (3)
- Gorlov Helical Rotor
- Verturi Tidal Turbine
Consists of a hydroplane that has an attack angle relative to the approaching water stream varied by a simple mechanism.
This causes the supporting arm to oscillate, which in turn forces hydraulic cylinders to extend and detract a cylinder, which produces high-pressure oil that drives a hydraulic motor that in turn drives an electric generator.
Gorlov Helical Rotor
Gorlov's helical turbine is based on the Darrieus turbine. It could also work underwater to produce energy from currents.
Gorlov’s helical turbine has twisting blades in the shape of a helix, which is very efficient.
- Fast currents
- natural flows are concentrated between obstructions