Most of the energy available over the year is in the form of waves that are small compared with those that occur during storms. A good wave energy machine must therefore be able to cost effectively maximise absorption in calm conditions while remaining survivable in larger seas.
The Pelamis makes use of the phenomenon of resonance to increase power capture in small seas. The response of the machine is matched to the incoming waves, like a playground swing pushed in time with its motion to maximise response.
The joints of the machine can be actively controlled to induce a cross-coupled resonant response. Importantly, the default or natural condition of the machine is benign and non-resonant, and so inherently capable of dealing with extreme conditions.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_single_image image=”274″ img_size=”full”][/vc_column][vc_column width=”1/2″][vc_column_text]1. Pelamis in non-resonant condition – restraint equal about the Pitch and Yaw axis. Response vertical and non-resonant.
2. Pelamis in resonant condition – restraint much greater about Pitch axis than Yaw. Response inclined and resonant.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]
Line-absorbers are around 2.5 times more effective than surging absorbers and 5 times more effective than heaving buoy systems.
All wave energy converters have theoretical limits on the amount of power they can absorb. This is similar to the Betz limit for a wind turbine or the Carnot limit for a thermal engine.
A unique and often confusing aspect of wave energy is that a wave energy machine can absorb more energy from the waves around it than is contained within its frontal width facing the seas. This is because the machine is interacting with the entire wave field around it not just with the waves that pass directly under the system. The energy capture is often expressed as ‘capture width’; this is the width of wave front from which all the energy is extracted.
The theoretical limit can be worked out by considering each wave machine type to be a wave-maker, with their motion radiating waves (like a stone cast into a pond radiates waves from its impact) that cancel out the incident ocean waves as they pass. By analysing each pattern the ultimate capture width limit can be determined for each differing design.
In this way, it can be shown that the Pelamis’s method of absorption (often termed ‘a line absorber’) has the highest theoretical limit for a given volume of machine of any wave converter. This is a fundamental competitive advantage – much like the advantage enjoyed by all modern wind turbines which use lift to drive the rotor, rather than earlier designs that used drag.[/vc_column_text][vc_single_image image=”275″ img_size=”full”][vc_column_text]Diagram: A comparison of radiation patterns. The top-left picture shows the uniform wave radiation pattern due to a single heaving body in calm water and the bottom-left picture shows the focused wave radiation pattern due to the heaving snake-like motion of a line absorber. The right-hand figures show the same wave radiation patterns interacting with an incident wave travelling from left to right. The reduced wave amplitude down-wave of the body arises from the cancellation of the incident wave by the radiated wave and relates to the power absorbed by the body. The theoretical limit of maximum absorbed power is the optimal balance between the down-wave absorption and the radiation losses in all other directions. The line absorber attains a higher theoretical limit because of the relatively small radiation losses up-wave and to the sides.[/vc_column_text][vc_column_text]
Capture width limits of different types of wave energy converter
|Mode of absorption||Theoretical ultimate capture width (percentage of wavelength)|
|Offshore heaving body||16%|
|Offshore surging or pitching||32%|
|Offshore heaving-and-surging or heaving-and-pitching||48%|
|Line absorber of length = wave length (current Pelamis)||50%|
|Line absorber of length = twice the wave length (ultimate Pelamis)||75%|