OWC is the abbreviation for Oscillating Water Column and describes a design which makes use of the wave motion in rising and falling water columns. Figure 5a shows how a typical OWC functions. It consists of a chamber with two openings. One of these is on the side towards the incoming waves and lies beneath the water level. The water can enter the chamber through this opening, driven by the wave energy. The second opening allows the pressure to equalize with the surrounding air. The water column in the chamber moves up and down at the wave frequency, and thus “breathes“ air in and out through the second opening. This “breath” drives an air turbine. It is constructed in such a way that it converts the oscillating motion of the air column into continuous rotational motion (see the infobox “Operating Principles of OWC Turbines“ on p. 89).
In principle, an OWC represents a simple motion-conversion device (Figure 5b): In order to drive the generator, it converts the strong force at low velocity of the wave motion into a motion of the air column with a weak force but a high velocity. The essential aspect is that the low specific mass of the air permits a high acceleration.
OWC systems have been in use for decades for the energy supply of beacon buoys (Figure 6). They were invented by the Japanese Yoshio Masuda. In an OWC buoy, a vertical pipe assumes the function of the chamber. It reaches down into the calmer water layers below the buoy. Therefore, the water column in the pipe is at rest relative to the waves outside – but it moves relative to the buoy, since the latter is raised and lowered by the wave motion. Like standard OWCs, most buoys employ an air turbine. Such buoys have rather quickly become standard for applications that require limited power outputs. Some of them have survived more than twenty years of operation at sea.
FIG. 5 THE OWC PRINCIPLE
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