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

250 m

200 m

barrage

Top view

Front view

12 m

6 m

High tide

Low tide

turbine blades

4 m

20 m

200 m

4 m

8 m

3 m

• 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.

Turning Tides into Usable Energy

Bulb TurbinesBulb 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 usedRim 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:

3t

t

2t

1P A v

2A area covered during rotation of rotor blades.

A = r

r length single blade

Power calculations for tidal currents and underwater turbine generators:

Assumptions:

3

mTidal current (v) = 0.1

secTurbine blade radius (r) = 5 m

kg Seawater density ( ) = 1027

m

Present use of Tidal EnergyTidal 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.