The ocean waves are an important renewable energy resource that, if extensively exploited, may contribute significantly to the electrical energy supply of countries with coasts facing the sea. A wide variety of technologies has been proposed, studied, and in some cases tested at full size in real ocean conditions. Oscillating-water-column (OWC) devices, of fixed structure or floating, are an important class of wave energy devices.
A large part of wave energy converter prototypes deployed so far into the sea are of OWC type. In an OWC, there is a fixed or floating hollow structure, open to the sea below the water surface, that traps air above the inner free-surface. Wave action alternately compresses and decompresses the trapped air which is forced to flow through a turbine coupled to a generator. The paper presents a comprehensive review of OWC technologies and air turbines.
This is followed by a survey of theoretical, numerical and experimental modelling techniques of OWC converters. Reactive phase control and phase control by latching are important issues that are addressed, together with turbine rotational speed control.
The ocean waves are an important renewable energy resource that, if extensively exploited, may contribute significantly to the electrical energy supply of countries with coasts facing the ocean . A wide variety of technologies has been proposed, studied, and in some cases tested at full size in real ocean conditions , , , . The mechanical process of energy absorption from the waves requires a moving interface, involving (i) a partly or totally submerged moving body and/or (ii) a moving air–water interface subject to a time-varying pressure.
In the latter case, there is a fixed or oscillating hollow structure, open to the sea below the water surface, that traps air above the inner free-surface; wave action alternately compresses and decompresses the trapped air which forces air to flow through a turbine coupled to a generator. Such a device is named oscillating-water-column (OWC). Although the concept was already known in the 1940s, this designation seems to have appeared for the first time in published paper form in 1978  and has been widely used ever since, even if the moving water inside the structure is far from shaped like a column. Before that, this type of wave energy converter (WEC) was sometimes known as the Masuda device.
The main advantage of the OWC versus most other WECs is its simplicity: the only moving part of the energy conversion mechanism is the rotor of a turbine, located above water level, rotating at a relatively high velocity and directly driving a conventional electrical generator. OWCs are a major class of wave energy converters, possibly the class that has been most extensively studied and with the largest number of prototypes so far deployed into the sea.
In almost all OWCs, the air alternately flows from the chamber to the atmosphere and back, although in some concepts the flow is in closed circuit. Unless rectifying valves are used, which is widely regarded as unpractical except possibly in small devices like navigation buoys, the turbines are self-rectifying, i.e. their rotational direction remains unchanged regardless of the direction of the air flow. Several types of such special turbines have been developed. The axial-flow Wells turbine, invented in the mid-1970s, is the most popular self-rectifying turbine, but other types, namely self-rectifying impulse turbines, have also been proposed, studied and used.
Apart from reviews on WECs in general , , , , more specific reviews on OWCs can be found in Refs. , . Reviews on air turbines for OWCs were published in Refs. , , , , . A detailed historical description, until about 1995, of the development of wave energy conversion in general, and OWCs in particular, can be found in Ref. , a book written from a non-technical point of view by a freelance journalist.
The present review paper concentrates on what is specific of OWC wave energy converters. Issues like moorings, electrical equipment and environmental impact (except air turbine noise) that are common to other wave energy technologies are left out. A review of OWC technologies is presented in Section 2. This is followed, in Section 3, by a review of air turbines for OWC applications, especially self-rectifying turbines. Section 4 is devoted to theoretical, numerical and experimental modelling techniques of OWC converters. Phase control and rotational speed control are dealt with in Section 5. Conclusions are presented in Section 6.
Early developments until 1990
Yoshio Masuda (1925–2009) (Fig. 1), a Japanese navy officer, may be regarded as the father of modern wave energy technology, with studies in Japan since the second half of the 1940s. He developed a navigation buoy powered by wave energy, equipped with an air turbine (Fig. 2), which was in fact what was later named as a (floating) OWC , , . Such buoys were commercialized in large numbers in Japan since 1965 (and later in USA), and were the first wave energy devices successfully
Theoretical hydrodynamic modelling
Not surprisingly, OWCs were among the first wave energy converters to be theoretically modelled. This was particularly the case of the OWC spar-buoy which was analysed by Michael McCormick, one of the wave energy pioneers, in what were two of the first theoretical papers on wave energy converters to have been published in journal form , . These were followed, in 1978, by a paper by Evans  with analytical solutions for simple geometries: a vertical thin-walled tube-shaped OWC and a
Reactive phase control
In WECs of oscillating-body and OWC types, the highest efficiency of wave energy absorption from regular waves is attained under conditions close to resonance. Especially in the case of relatively small devices (the so-called point absorbers), it is well known that the resonance bandwidth is relatively narrow, which implies that their performance in irregular waves is relatively poor. Besides, for many point absorbers, the natural frequency of resonance is higher than the typical frequency of
The OWC was the first concept for wave energy conversion to be developed, and is still the favourite technology among a large part of the wave energy conversion community. It can be employed in isolated shoreline or nearshore situations, integrated into a breakwater, or in single- and multi-OWC floating plants. From the mechanical viewpoint, the PTO is particularly simple and reliable: the only moving part is the rotor of an air turbine, located above sea water, directly driving a conventional