Tidal Current Energy

140 R. Alcorn and T. Lewis
6.2 . Device location classification
Although only three basic classifications of devices have been shown, it is also
possible to subclassify the devices based on their designed deployment location.
6.2.1 . Onshore
These are devices that are built directly onto the shoreline or into a shoreline structure
like a sea defence wall or breakwater. The advantage is that civil works can
be land based and cable connection is made easier. The disadvantage is that the
wave energy resource at the shore is greatly reduced compared with even a short
distance offshore. Some recent schemes have justified this by designing devices
into new breakwater developments, which greatly reduces capital civil costs.
6.2.2 . Nearshore
These devices are built close to shore but out of the surf zone of breaking waves.
This would typically be in depths of water up to around 20 m. The advantage
of locating here is that gravity-based foundations can still be used and cable
runs are short. They can be fixed or floating. The disadvantage is that the wave
resource is lower than offshore and the advantage of working in limited water
depths may easily be outweighed by civil and installation costs.
6.2.3 . Offshore
These devices are floating devices moored in water from 30 m up, but a design
depth of 50 m is more typical. The advantage is that the wave resource is
undiminished. The disadvantage is that distance from the shore may be greater,
meaning cable costs may be high and O & M may be more expensive.
6.3 . Device motion classification
Often, wave energy devices are classified by the wave motion that they primarily
capture. There are six degrees of freedom possible, three rotational and three
translational. These are shown in Figure 8.12 (Plate 14).
6.4 . Capture width
The size and width of device is important in proportion to how much energy
it will capture. The incident power figures are calculated per meter width, so
the input power can be defined as the incident power times the capture width.
One interesting phenomenon about wave energy is that the capture width can
be greater than the actual width of the device.
7 .
Device Rating
The previous sections have described how the wave energy resource is quantified
and forecast. They have also shown the various methods to convert this
energy from either surface motions, subsurface motions or both. What should
be clear from these sections is that device performance is related to wave period