7.3.1 Fixed devices
Fixed seabed and shore-mounted devices are usually terminators. These have been the most common types of wave energy converter tested as prototypes at sea.
What advantages and disadvantages do you think apply to fixed and shore mounted devices over floating ones?
Table 2 Advantages and disadvantages of fixed and shore mounted devices over floating ones
|Have a fixed frame of reference||Generally operate in shallow water and hence at lower wave power levels|
|Are closer to a grid||Geographical location – only a limited number of sites are suitable for deployment|
|Good access for maintenance purposes||To optimize output, they need to be positioned in an area of small tidal range, otherwise their performance may be adversely affected|
|The seabed attenuates storm waves that could otherwise destroy the device and turbine|
It is also worth noting that mass production techniques are unlikely to be totally applicable to shore-mounted schemes, as site-specific requirements will demand a tailored design for each device, adding to production costs.
The majority of fixed devices tested and planned are of the oscillating water column (OWC) type. In these, an air chamber pierces the surface of the water and the contained air is forced out of, and then into, the chamber by the approaching wave crests and troughs. On its passage from and to the chamber, the air passes through an air turbine – usually of the Wells type shown in Box 3.
Box 3: Wells Turbine
A Wells turbine is an axial flow device that drives a generator and so produces electricity.
It rotates in the same direction irrespective of the direction of airflow, and has aerodynamic characteristics particularly suitable for wave applications.
A very attractive feature of its use in OWCs is that the cross-sectional area of the air turbine can be much smaller than the area of the moving water surface. This acts like gearing to increase the air velocity through the turbine so that it can operate at high speed.
In the next section we’ll look at some examples of OWC devices.