290 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
operation costs, establishment difﬁculties, and power rating. Some of the mainly used
wave energy harvesting technologies are as follows: OWC, Overtopping devices, Pelamis,
Wave Dragon, Archimedes wave swing (AWS) device, Wave Star Energy (WSE), and
magnetohydrodynamics (MHD) generator.
OWC systems consist of a partially submerged structure, which forms an air chamber, with
an underwater opening that allows the seawater to ﬂow into the chamber. The volume of air
inside the chamber is compressed as the water rises inside the chamber, driving air through
a turbine. As the water level in the chamber subsides, the air is drawn back through the
turbine. Bidirectional and self-rectifying air turbines have been developed. The axial-ﬂow
Wells turbine is the best-known turbine for this kind of application and has the advantage
of not requiring rectifying air valves .
Overtopping devices: Overtopping devices guide incoming waves into a reservoir raised
slightly above sea level, through a ramp. The water trapped in the reservoir ﬂows back to
the sea through a conventional low-head hydroelectric generator.
Float systems: Their common feature is a buoy that sits on the ocean’s surface. The motion
of this buoy is converted into electricity typically by a hydraulic PTO such as a hydraulic
ram. These ﬂoat systems have various shapes and forms.
Hinged contour devices: Hinged contour devices contain different ﬂoating sections, which
are hinged together. As the wave passes, the sections move relative to each other and the
hinges produce power. The power conversion uses hydraulic elements.
4.4.1 Oscillating Water Column
The OWC is one of the most common and most maturated WEC devices . The conceptual
studies on OWC started in the 1970s. Up to date, a number of plants were built in different
countries such as Osprey, Scotland, and in Japan, which is called Mighty Whale.
The upcoming waves enter the water column, which is placed in the lower part of the
chamber. This wave action on the water column occurs through the submerged entrance
located on the device’s front side .
As the water level increases, the air pressure in the upper part of the chamber increases.
In other words, the oscillatory motion of the water causes a difference in air pressure
within the chamber. The turbine is located in a channel, which links the air chamber to
the outside. The axial airﬂow direction reverses when the wave level and the air pressure
decrease . The turbine should be specially designed in order to rotate in the same direc-
tion as air, since the airﬂow direction is bidirectional. Wells turbines are employed within
the most full size OWC applications. These turbines are generally ranging from 500 kW
up to 1 MW depending on the diameter sizes of 2 and 3.5 m. When employed toge