Wave Energy

Question 1

What is the estimated potential global power output from wave energy?

Answer 1

2TW (2x10^12 W)

Question 2

What are the four basic sources of wave formation?

Answer 2

1. Wind
2. Displacements (e.g. earthquake, landslide)
3. Change in atmospheric pressure
4. Gravitational pull (e.g. tides)

Question 3

Describe the process of wind-driven wave generation for a steady state “fully developed” wave system.

Answer 3

1. A steady wind blowing over an open stretch of calm water will create ripples which travel across the surface in the same direction as the wind.
2. If the wind continues to blow for long enough and sufficient length of water (‘fetch’) is available, the ripples will advance, growing in length and height until they can more properly be called waves.
3. At the same time, the wind generates new ripples on the surface of the growing waves and these ripples will eventually grow into waves themselves.
4. If the wind continues to blow at a constant velocity for long enough and sufficient fetch is available, the rate at which energy is absorbed by the waves will eventually be exactly balanced by the rate of energy dissipation and a steady state “fully developed” wave system will be achieved.

Question 4

What are the two principal wave decay mechanisms?

Answer 4

Wave breaking and viscosity

Question 5

Describe the two principal wave decay mechanisms

Answer 5

If the wind ceases to blow, the wave system it has generated will gradually decay. Since wave breaking is a relatively powerful decay mechanism, the short steep waves, which are more likely to break, decay first. This leaves the longer waves to be dissipated by the relatively weak forces of viscosity. This decay process may last several days, during which these fast moving long waves may travel several thousand kilometres and be recognised at some distant location as well.

Question 6

Why do waves increase in height as they approach the beach?

Answer 6

• It is because water depth affects wave height.
• As the wave approaches the shore, there comes a point where the water depth is half of the wavelength and the wave starts to “feel the bottom”.
• This means that the circular energy of the wave starts dragging water against the seafloor, changing the circular motion of the water particles and making them follow an oval path which makes the wave higher and narrower.
• As it moves across progressively shallower water, the wave drags more and more against the bottom.
• This, combined with the forward push of wave energy, produces “breakers” that dissipate their energy in the surf line by violently stirring the water into a frenzy of turbulence.
• These releases of wave energy are releases of accumulated wind energy that were stored in the waves.
• When they are strong enough, they may eventually change the shape of a beach.

Question 7

What is the definition of the group velocity of a wave?

Answer 7

The velocity with which the overall shape of the waves’ amplitudes (known as the modulation or envelope of the wave) propagates through open space.

Question 8

Why is the calculation of group velocity important in the context of wave energy?

Answer 8

Wave energy depends on group velocity.

Question 9

How is group velocity calculated?

Answer 9

• C = [sqrt(gλ/2π)tanh⁡(2πd/λ)] where:
• In shallow water, the group velocity is equal to the shallow water phase velocity (Cg = C).
• In deep water, the group velocity is equal to half the phase velocity (Cg = 0.5C).

Question 10

What is the equation for the energy density (energy per unit plan area) of a wave?

Answer 10

E = (ρgH^2)/8 (half = potential, half = kinetic energy)

Question 11

What is the equation for power transferred per unit width as a wave passes?

Answer 11

P = ECg

Question 12

Name some examples of wave energy capture facilities

Answer 12

• WEC Project
• Aquabuoy
• Limpet
• WaveNET
• Pitch-Heave: PS-Frog
• Wave Carpet
• Wave Dragon

Question 13

What are the key technical challenges and risks that affect the design of wave energy converters?

Answer 13

They operate in extreme offshore environments, thus must withstand:

• Exposure of the structure to the extreme action of waves and wind.
• Fouling that seriously influences any structure and obstructs moving parts.
• Marine debris e.g. fishing nets, plastics, containers, oil etc.

Question 14

What is the theoretical maximum power output from a point absorber?

Answer 14

P = ((λ/2π)CgρgH^2)/8

Question 15

Describe how the Aquabuoy works

Answer 15

• A vertical “acceleration tube” is fixed to a buoy and open to water.
• Waves rush in, driving a piston.
• Acceleration tube moves up and down, but inertia of water mass prevents piston from moving with acceleration tube.
• There are two hose pumps within; one is stretched, the other is compressed, pressurising the seawater.
• The pressurised water drives a turbine connected to a generator at a rate of two times per wave period, before returning to the sea.

Question 16

Describe how a Limpet works

Answer 16

• A Limpet uses an oscillating water column to drive an air turbine.
• The oscillating airflow can be arranged to always flow in the same direction over the turbine, or a Wells turbine can be used which operates in the same rotational direction, whichever the direction of the incident flow.
• It is easy to build, install and maintain.

Question 17

Describe how a Wave Dragon works

Answer 17

• The Wave Dragon uses well-known and well-proven principles from traditional hydro power plants in an offshore floating platform.
• It is very simple and like a dam: the Wave Dragon overtopping device elevates ocean water to a reservoir above sea level, where water is let out through a number of turbines and in this way turned into electricity.
• It is therefore a three-step energy conversion process: overtopping (absorption) > storage (reservoir) > power take-off (low-head hydro turbines).
• Wave Dragon is the only wave energy converter technology under development which can be freely up-scaled.