Tidal Current Energy

138 R. Alcorn and T. Lewis
Wave direction
Figure 8.9 . Direct mechanical device.
Mechanical device
Seabed
can be based on many parameters, but a simple way to think of them is how the
device and the power take-off interact with the wave, regardless of whether they
are surface or subsurface devices. Using this definition there are three basic types.
6.1 . Device type classification
6.1.1 . Direct mechanical device
In this type of device, the structure interacts directly with the waves in order
to provide power take-off. For example, a heaving buoy with hydraulic power
take-off would fall into this category, as would a shoreline flap device. An example
is shown in Figure 8.9 . In general, this type of device has the potential for
high power-to-weight ratio, since the structure is directly driving the power
take-off. This type of device can be floating, or fixed either to the seabed or to a
breakwater. The disadvantage is that there is no isolation between the structure
and the power take-off, and hence large forces on the structure can be transferred
to the power take-off. Power take-off systems in these devices consist of
oil hydraulics, high-pressure water hydraulics or linear electrical generators.
There is potential for short-term storage in these devices in the form of hydraulic
accumulator pressure.
6.1.2 . Indirect pneumatic device
In this type of device the structure acts a gearbox and buffer. In an oscillating
water column (OWC)-type device, the waves cause a reciprocating air flow
through a pneumatic power take-off, as shown in Figure 8.10 . The large area
of the water surface in comparison with the annular area of a turbine produces
the gearing-up of velocities, from low water surface velocity to much faster air
velocities suitable for driving an air turbine. The advantage of this type of system
is that there is no structural link to the power take-off and hence large wave
forces cannot be transferred to the power take-off. The disadvantage is of course
that a larger structure is needed to enclose the air volume required and hence