Tidal Energy

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Energy from Tides and
Waves
Tidal action caused by gravitational effects of moon and sun on earth’s
oceans.
Energy from the moon
• Tides generated by the combination
of the moon and sun’s gravitational
forces
• Greatest affect in spring when moon
and sun combine forces
• Bays and inlets amplify the height of
the tide
• In order to be practical for energy
production, the height difference
needs to be at least 5 meters
• Only 40 sites around the world of this
magnitude
• Overall potential of 3000 gigawatts
from movement of tides
200 m
Top view
250 m
barrage
Front view
200 m
High tide
6m
3m
Low tide
8m
20 m
12 m
4m
4m
turbine blades
Turning Tides into Usable Energy
• Ebb generating system
• A dam (barrage) is built
across the mouth of an
estuary.
• Sluice gates allow
incoming tides to fill the
basin.
• As the tide ebbs, the
water is forced through a
turbine system to
generate electricity.
Bulb Turbines
Bulb or Tubular turbines are designed into the water delivery tube. A
large bulb is centered in the water pipe which holds the generator, wicket
gate and runner. Tubular turbines are a fully axial design, whereas
Kaplan turbines have a radial wicket gate. Bulb turbine used at La Rance
tidal plant on the Brittany coast in France is shown below
Other Turbines used
Rim turbine used at Annapolis Royal in Nova Scotia
Tubular turbine proposed for use in the Severn tidal project in Great Britain
How it works
• First generation, barrage-style tidal power plants
• Works by building Barrage to contain water after
high tide, then water has to pass through a
turbine to return to low tide
• Sites in France (La Rance), Canada (Annapolis),
and Russia
• Future sites possibly on Severn River in England,
San Francisco bay, Passamaquoddy
Second-generation tidal power plants
• Barrage not need, limiting total costs
• Two types- vertical axis and horizontal axis
• Davis Hydro turbine….. Successfully tested in
St. Lawrence Seaway
• Harness the energy of tidal streams
• More efficient because they allow for energy
production on both the ebbing and surging tides
• One site has potential to equal the generating
power of 3 nuclear power plants
• Tidal Turbines
• Only been feasible for
about 5 years
• Similar to wind
turbines, they use tidal
currents to turn
propellers mounted on
the seabed to generate
power.
Other Possibilities
• Tidal Fences
• Completely blocks a
channel so as the tide
rises, water is forced
through the styles to
turn them.
• Can be used between
islands or between a
mainland and an island
as opposed to only
across the mouth of a
confined bay.
Turbine power equation (same as for wind
turbine) for each turbine generator is:
1
P   At    v3
2
A t  area covered during rotation of rotor blades.
At =   r 2
r  length single blade
Power calculations for tidal currents
and underwater turbine generators:
Assumptions:
m
Tidal current (v) = 0.1
sec
Turbine blade radius (r) = 5 m
kg
 Seawater density () = 1027 3
m
Present use of Tidal Energy
Tidal power has on a small scale been used through out the
history of mankind. It was not until the twentieth century that
large scale tidal projects were considered. Today, sites suitable
for the utilization of tidal power exist in many places around
the world.
–
–
–
–
–
France
United Kingdom
Former Soviet Union
Canada
United States
The extraction of large quantities of tidal energy is possible
however, large scale tidal power operations are not
technologically or economically feasible at the present time.
Tidal sites are therefore limited to more modest
developments.
h
Disadvantages
• Presently costly
– Expensive to build and maintain
– A 1085MW facility could cost as much as 1.2 billion dollars to
construct and run
• Connection to the grid
• Technology is not fully developed
• Barrage style only produces energy for about 10 hours
out of the day
• Barrage style has environmental affects
– Such as fish and plant migration
– Silt deposits
– Local tides change- affects still under study
Advantages
• No pollution
• Renewable resource
• More efficient than wind because of the density of
water
• Predictable source of energy vs. wind and solar
• Second generation has very few disadvantages
– Does not affect wildlife
– Does not affect silt deposits
– Less costly – both in building and maintenance
Turning Waves into Usable Energy
• Oscillating water column
• Incoming waves force air up
column to turn the turbine
• Outgoing waves suck air down
column to turn the turbine
Tapered Channel System (TAPCHAN)
• Waves feed through
tapered channel into
reservoir and are then fed
through a turbine
• Kinetic energy of the
moving wave is changed to
potential energy as water is
collected in the reservoir
• Concept is similar to that of
traditional hydroelectric
devices
Floating Devices
•
•
•
The Salter Duck, Clam, Archimedes wave swing, and
other floating wave energy devices generate
electricity through the harmonic motion of the
floating part of the device. In these systems, the
devices rise and fall according to the motion of the
wave and electricity is generated through their
motion.
The Salter Duck is able to produce energy very
efficiently, however its development was stalled
during the 1980s due to a miscalculation in the cost
of energy production by a factor of 10 and it has
only been in recent years when the technology was
reassessed and the error identified.
Anaconda Wave Energy Converter
Floating Devices
(Salter Duck, Clam, Archimedes)
• Salter Duck-Electricity is
generated through the
movement of the device
on the wave (bobbing up
and down)
Salter “Ducks”
• Scottish physicist Prof. Stephen Salter
invented “Nodding Duck” energy
converter in 1970
• Salter “ducks” rock up and down as the
wave passes beneath it. This oscillating
mechanical energy is converted to
electrical energy
• Destroyed by storm
• A floating two-tank version drives
hydraulic rams that send pressurized
oil to a hydraulic motor that drives a
generator, and a cable conducts
electricity to shore
• Advantages
•
•
•
•
The energy is free - no fuel needed, no waste produced.
Most designs are inexpensive to operate and maintain.
Waves can produce a great deal of energy.
There are minimal environmental impacts.
• Disadvantages
• Depends on the waves - sometimes you'll get loads of energy, sometimes
nothing.
• Needs a suitable site, where waves are consistently strong.
• Must be able to withstand very rough weather.
• Disturbance or destruction of marine life
• Possible threat to navigation from collisions because the wave energy
devices rise only a few feet above the water.
• Degradation of scenic ocean front views from wave energy devices located
near or on the shore, and from onshore overhead electric transmission
lines.
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