ABSTRACT
Many devices have been considered for the conversion of ocean wave motion into electricity, as reviewed by Falcão (2010). The wave resource is greatest offshore in deeper water and various rafttype devices have been developed to exploit this. The Cockerel raft in the 1980s was probably the first of this kind of system, also referred to as an attenuator, followed by Pelamis with connected cylindrical tubes (Retzler 2006) and Sea Power with box-type floats (Moore et al 2015). The M4 device extends this concept by using cylindrical (vertical axis) floats to avoid losses due to sloshing and drag and different sizes of float to give a range of natural periods to enhance power capture across a range of wave periods for an offshore site. The floats are circular in cross section with hemi-spherical or rounded bases to give minimal drag losses; the bow float is attached to a mooring buoy and the floats increase in diameter and volume from bow to stern so that drift forces also increase causing the device to head naturally in the wave direction. The initial device had three floats with distance between floats of about half a typical wavelength so that forces and adjacent float motions are predominantly in anti-phase (Stansby et al 2015a). The bow and mid floats are rigidly connected by a beam and a beam from the stern float is connected to the hinge point above the mid float for power take off (PTO). The machine has been investigated by laboratory experiments at two scales differing by a factor of 5 showing the accuracy of Froude scaling (Stansby et al 2015b). A linear frequency
2 OCEAN BASIN TESTS
The tests were undertaken in the new (2017) Lir Ocean Basin, University College Cork, Ireland. This has a curved line of 80 Edinburgh Design wave paddles with sloping beaches on two sides as shown in Figure 1.
