Week 9

Question 1

What are some examples of types of energy disturbances that are required for waves to be formed?

Answer 1

Wind
Displacements (Earthquakes, landslides)
Changes in atmospheric pressure
Gravitational pull (tides)

Question 2

Indicate the rough potential period lengths of waves caused by: wind, earthquakes/storms and the sun/moon respectively.

Answer 2

Wind- 0-5 minutes
Earthquakes/Storms - 2/3 minutes - 24 hours
Sun/Moon- 12 hours upwards

Question 3

In comparison to other causes of waves, describe the effect of wind.

Answer 3

Waves from wind are more powerful and reasonably predictable

Question 4

Discuss the process of wind action in wave creation.

Answer 4

Wind forms small ripples on water surface
As waves develop, they offer more surface area for the wind to press against
Size of the wave increases dramatically to a maximum

Question 5

How is the energy absorption from wind to wave energy countered?

Answer 5

By two principal decay mechanisms:

Wave breaking and viscosity

Question 6

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

Answer 6

It is because water depth also affects wave height. (Diagram and more detailed notes on p17)

Question 7

What is group velocity?

Answer 7

The group velocity of a wave is the velocity with which the overall shape of the waves’ amplitudes propagate through space

Question 8

What is wave celerity?

Answer 8

The speed at which the wave appears to move

Question 9

How does celerity/phase velocity relate to group velocity?

Answer 9

In shallow water: Phase velocity = group velocity)

In deep water: 0.5*Phase velocity = group velocity

Question 10

What is the equation for wave energy density? Define the units of this value and what it is made up of.

Answer 10

E = (rho)(g)(H^2)/8

J/m^2

Half of E is potential energy whilst the other half is made up of kinetic energy

Question 11

What is the equation for wave energy flux? Define the units of this value

Answer 11

This value represents power transferred per unit width as a wave passes

P = E C(subG)

W/m

Question 12

How should the maximum power of a point absorber be calculated?

Answer 12

The power that can be absorbed from the sea by a heaving buoy at a point is equal to that corresponding to the power available over a width of sea = λ/2π

Mac Theoretical Power Output = (λ/2π)EGroup velocity

Question 13

How can additional power be obtained from a point absorber?

Answer 13

Oscillating horizontally and by spreading the device across the waves (e.g. Salters Duck and Wave Dragon) or along the waves (e.g. Pelamis)

Question 14

What are the advantages of a Limpet owc?

Answer 14

Easy to build and install

Easy maintenence

Question 15

What are the benefits of WaveNET arrays?

Answer 15

They are modular and scalable

They operate at a high level of efficiency

Question 16

Describe a device which has been created to extract power from pitching and surging.

Answer 16

Pitch-Heave: PS Frog

Developed from a heavy buoy to: reduce amplitude; provide better control of the reaction mass and produce more power

Question 17

What are some advantages ti the use of the wave dragon?

Answer 17

1) The Wave Dragon concept combines existing, mature offshore and hydro turbine technology in a novel way
2) Wave Dragon is the only wave energy converter technology under development that can be freely up-scaled
3) Due to its size service, maintenance and even major repair works can be carried out at sea leading to low O&M cost relatively to other concepts

Question 18

The surrounding environment to wave energy converter devices have a significant impact on the function of the device. What type of things should be considered within their environment?

Answer 18

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