Principles for Capture of Energy from Ocean Waves . Phase Control and Optimum Oscillation

1. Absorption of waves means generation of waves. A body oscillating in water will produce waves. A big body and a small body may produce equally large waves provided the smaller body oscillates with larger amplitude. This may be utilised for the purpose of wave energy conversion, for instance by a small floating body heaving in response to an incident wave, in particular so if it can be arranged that the body oscillates with a larger amplitude than the wave amplitude. Generally it can be said that a good wave absorber must be a good wave-maker. 1 Hence, in order to absorb wave energy it is necessary to displace water in an oscillatory manner and with correct phase (timing). This can be obtained by an oscillating body as explained above. Alternatively, wave generation by oscillatory displacement of water can be obtained, for instance, by an oscillating water column (OWC) in a fixed chamber having an opening into the sea. It is also possible to enclose the water by blocking the chamber opening with an elastic or flexible bag, which can oscillate under wave action. In such a case sea water in the chamber is not necessary; it can simply be replaced by air in which the pressure oscillates in step with the motion of the flexible bag. Absorbing wave energy for conversion means that energy has to be removed from the waves. Hence there must be a cancellation or reduction of waves which are passing the energy-converting device or are being reflected from it. Such a cancellation or reduction of waves can be realised by the oscillating device, provided it generates waves which oppose (are in counter-phase with) the passing and/or reflected waves. In other words, the generated wave has to interfere destructively with the other waves. This explains the paradoxical, but general statement that " to destroy a wave means to create a wave ". An illustrative example, where 100 % absorption of wave energy is possible, is shown in figure 1. This corresponds to an infinite line (perpendicular to the figure) of oscillating small floating bodies, evenly interspaced a short distance (shorter than one wavelength). Complete absorption of the incident wave energy is possible also with an elongated body, of cross section as shown in figure 1, and aligned perpendicular to the plane of the figure, provided the body oscillates vertically and horizontally in an optimum manner. It …