Broadly, there are three ways of classifying a wave energy converter in terms of its position. Shoreline devices are mounted rigidly to the shore, thus ensuring simple maintenance and grid connection. The cost of land intrusion and the possibility of naturally advantageous sites being environmentally sensitive has to be considered in these schemes. Secondly there are nearshore devices, typically situated in 10-25 m depth of water. The device may be tight moored to the sea bed, either by way of pillars or tensioned cable, thus providing both stability and a datum to react force against.
Deep water or slack moored devices have no such direct contact with the seabed, and any moorings are to maintain geographical position only. It is this third category which is to be strived for. Although it presents the most technical problems it represents the largest energy resource (Table 1-2).
It is possible to group WECs according to their geometry as terminator, attenuator or point absorber, the latter of which has already been introduced. A terminator has its principal axis parallel to the incident wave crests and as such the waves are stopped as they reach the device. These devices hence tend to be quite wide, so that a large amount of the wave front will affect the device at one time. The classic example of this type of WEC and arguably the most famous of all WECs is the Edinburgh or Salter Duck . This is a cam shaped floating device which is allowed to rotate about a spine. Long lines of these devices are capable of removing 100 % of the energy contained in incident waves. As a wave approaches the device the cam rocks about its axis, which is assumed to be relatively stationary. The design is such that the circular rear section does not transmit waves downstream of the device, leaving a theoretically flat water surface.
Attenuator devices have their principal axis perpendicular to the wave crests and so face the waves 'head on'. The energy is converted by relative movement of parts of the device as a wave passes underneath it. A device of this type, known as the Pelamis [16,17,19], is currently being developed for deployment off the Isle of Islay, Scotland. It is a snake like structure consisting of a number of floating segments hinged together. Power take off is in the form of hydraulic rams driven by the relative movement of the segments as a wave passes underneath.
Many other WECs have been proposed, e.g. [18, 19, 20, 21], some of which lend themselves more favourably to direct drive e.g. . Good summaries covering many concepts to date are available e.g. [2, 23, 24]. Only three are presented here, the first because it is conceptually the simplest, and has been the subject of much enlightening mathematical analysis , the second because it has direct drive power take off and has an active research programme, and the third because its structure naturally lends itself to direct drive operation.
A heaving buoy consists of a floating body, typically cylindrical or spherical, following the water surface in the vertical plane and reacting either against the seabed or a submerged drag plate. It may hence be a nearshore or deepwater device. As it only has one degree of freedom, it is limited to capturing a maximum of 50% of the energy available in its absorption width.
There have been many methods proposed for the power take off of this device, for example an elastomeric hose whose change in cross-sectional area with flexing is used to pump water . Figure 1.5 shows the direct drive proposal, whereby the rotor of a linear generator is coupled directly to the buoy, and the stator is mounted in a submerged drag plate. Mathematical descriptions of similar two body proposals have been presented [27, 28], along with strategies for its efficient control . Some characteristics of this device are explored more fully in Chapter 2.
18.104.22.168 Archimedes Wave Swing (AWS)
The AWS, as shown in Figure 1.6, is a nearshore device mounted on the seabed. It consists of an air filled chamber, which has the freedom to move in a vertical plane relative to its base. As a wave passes over one of these devices, the added depth of the water causes an increase in water pressure surrounding the device. The volume of air within the device is hence compressed allowing the entire hood of the device to fall. The device will rise again when a trough passes over the device, and the net result is hence slow speed reciprocating motion. A 2 MW device was due to be commissioned in September 2001 which has a 3-phase permanent magnet linear synchronous machine as the power take off mechanism . Various technical problems have prevented the successful deployment of the device . Details of a prototype of this device are given in Chapter 8.
22.214.171.124 Interproject Sweden (IPS) Buoy
This deep water device, also known as the mace, was first developed in Sweden in the 1980's , and a sloped variation has been the subject of more recent research [32, 33]. Current proposals for its development have the power take off device either as a sea water pump , or high pressure oil rams .
Figure 1.7 shows a schematic diagram of the possible direct drive device. It shows a semi submerged float coupled to a totally submerged hollow tube, open to the sea at both ends. Part of the tube forms a cylinder enclosing a piston connected to a rod, it is relative motion between the rod and the float which forms the basis for power take off.
The enclosing cylinder prevents the water from simply slipping around the piston, effectively coupling it to the weight of the encapsulated water. The entire tube, float and cylinder will hence follow the water surface whilst the piston itself is held relatively still. If the amplitude of oscillation becomes too large, the piston will move out of the cylinder and become de-coupled from the mass of the surrounding water, thus allowing it to follow the oscillation of the tube. This feature provides built in protection against the power take off device being damaged, a function normally requiring end stops. A further advantage of this device is the possibility of running it inclined, which alters the buoyancy force and hence frequency response. It has been shown that the net effect of this is to widen the bandwidth of resonance and so shift the burden of control from the power take off mechanism to a buoyancy controller .
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