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270 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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system. After presenting the working principles of WEC and CEC systems, this chapter will reveal
different types of generator designs, including linear and rotary systems, and types of generator
systems for different MHK designs and energy storage.

11.2 MHK TECHNOLOGIES
MHK technology is still emerging, and different types of WEC and CEC designs have been proposed
by developers. This section reviews WEC and CEC designs and their working principles.
A table to compare different MHK technologies in terms of their complexity, performance,
reliability, and cost would be useful; unfortunately, in such a detail, the MHK devices and tech-
nologies are very broad and are not ready to be disseminated to the general public. Eventually, the
winning concepts will find market and commercialization paths, and the MHK industry will reach
the level of maturity as that of the present wind industry.

11.2.1 WEC Technologies

WEC devices extract energy contained within ocean surface waves and convert it into use-
ful electrical power. To date, there are more than 100 prototypes of various WEC systems [17].
These devices are typically divided into point absorbers, terminators, attenuators, an oscillating
water column (OWC), and overtopping designs [18–20]. The first three are often categorized as
oscillating types of WEC designs and consist of one body or multiple bodies that are designed
to directly convert wave energy into electrical power from the wave-induced relative translation
motion and/or rotational motion between the body and a reference frame (e.g., seabed or another
body) through the use of a linear or rotary PTOS. The overtopping devices and OWCs generate
energy through the use of hydroturbines and air turbines, respectively. Figure 11.2 shows illus-
trations of the various types of WEC devices and illustrates the mechanisms through which the
devices extract energy [20], and Figures 11.3 through 11.5 show the prototype WECs developed
by the WEC industry.
Point absorbers are devices that are small with respect to the wavelength of incident waves.
Point absorbers typically extract energy through a heaving or pitching motion, or a combination of
both, as illustrated in Figure 11.2a. Generally, a point absorber is designed to have a system natural
frequency close to the dominant frequency of waves to maximize its power output and is an attrac-
tive WEC concept because it is theoretically capable of absorbing energy from a wave front that is
greater than the device’s diameter or width [21].
Terminators and attenuators have dimensions that are on the same order of magnitude as the
wavelength and have one dominant dimension. As shown in Figure 11.2b and c, terminators and
attenuators are oriented with their dominant dimensions parallel and perpendicular to the incoming
wave front, respectively. Theoretically, for a terminator that has a symmetrical body and is only
allowed to oscillate in one mode of motion, it can absorb 50% of the energy from a wave front of the
device’s width. It is possible to absorb 100% of incoming wave energy if a nonsymmetrical body is
implemented or the symmetrical body is allowed to oscillate in more than one degree of freedom.
An attenuator, on the other hand, captures wave energy along its length from a large wave front
length [21].
OWC and overtopping devices use turbines to generate electricity, as shown in Figure 11.2d
and e. Overtopping devices gather water in a reservoir at a height higher than the mean free surface
as waves pass over the top of the device. The resulting hydrostatic pressure difference that is created
between the reservoir and the open ocean is used to drive a turbine that is similar to those used in
conventional hydropower applications. OWC devices consist of a confined air chamber in which the
pressure varies with water height within the chamber. As the water level rises and falls, air is driven
into and out of an air turbine, which generates power.