**2.1. Oscillating-WECs**

of 1st, 2ed and 3rd arrows are known as surge, sway and heave respectively. Also rotation around x-axis (mode 4), y-axis(mode 5) and z-axis (mode 6) are named respectively roll, pitch and yaw. Oscillating-WECs are displaced in only these six modes, most of these WECs have os‐ cillation in only one of these modes which are known as single-mode oscillators and some oth‐ ers have oscillation in more than one mode that are regarded as multi-mode oscillators. (Note that in most of Oscillating-WECs there is no variation in y direction, in this case there are only

**Figure 5.** Different oscillation mods of a Wave Energy Convertor: Surge (1), Sway (2), Heave (3), Roll (4), Pitch (5) and

Interface-WECs are special forms of WECs that are not oscillating in aforementioned modes. In fact these WECs are devices or structures that are fixed in a location and have not interaction with ocean waves. Interface-WECs are used to deliver wave power by an interface (water or air) to the PTO (power take-off). The scheme of classification of WECs is illustrated in Fig. 6.

three modes for body oscillation; Surge, Have and Pitch).

280 New Developments in Renewable Energy

Yaw (6) [18].

**Figure 6.** Categories of Wave Energy Convertors.

In this subsection, different WECs, which have one or two degrees of freedom, are investi‐ gated. Indeed, these WECs are simple in motion due to their restricted motion modes and they can be divided to three distinct categories; Heave Oscillators, Pitch Oscillators and Surge Oscillators.
