*2.2.1. Air-interface*

Oscillating water column (OWC) WEC is an Air-interface WEC in which wave power is con‐ verted to the electrical power without direct oscillation of WEC body by water particle. OWC is consists of a floating or bottom fixed structure whose upper part encloses a column of air and whose immersed part is open to the wave action [36]. The scheme of OWC is pre‐ sented in Fig. 20.

When waves attack the OWC, the water pressure below the chamber compresses the trap‐ ped air in the chamber and the air is guided to a turbine hence the flow of air to outside of chamber turns the turbine and the coupled generator. Meanwhile, by decline of water pressure below the chamber, the pressure of trapped air reduces and consequently air flows into chamber and drives the turbine again. To date, two type of self-rectifying tur‐ bine has been used in OWC; the Well Turbine and Impulse Turbine, both of which have the capacity to rectify air flow that makes it feasible to have unidirectional motion in elec‐ trical generator.

The early inventor and developer of OWC is Yoshio Masuda who had commercialized the floating OWC in Japan since 1965 [16]. The OWC has been used for energy converting in both shoreline and near-shore.

**Figure 21.** Wave Dragon. (**Above:** Scheme [39] **Below:** Prototype [39])

three type of PTO is more commonly used.

effective method for energy storing in this PTO.

Linear Generators and Rotational Generator.

In process of wave energy convertion, after power extraction in WEC, another energy con‐ version happens in Power Take-Off (PTO). Power take-Off is (mostly mechanical or hy‐ draulic) device in which the absorbed power is transferred to an electric generator. To date,

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**•** *Air/Water Turbine* forms part of an integral system that consists of a capture device, which also includes an electric generator. This PTO system is used in Interface-WECs (Oscillat‐ ing wave columns and Overtopping devices). The air turbines which are used in OWC are mostly self-rectifying turbines which convert reciprocating air flow to unidirectional torque. The two implemented turbine in OWC are Well turbine [41] and Impulse turbine [42]. Probably one of the advantages of turbines in PTO is facility of using flywheel for

**•** *Hydraulics PTO* system is consisting of a hydraulic circuit which transfers absorbed power of wave to a hydraulic motor which drives an electrical generator [43]. This kind of PTO is used in Pelamis and bottom hinged pitching flap. Implementation of accumulator is an

**•** *Direct Drive* is consisting of a moving part named translator on which linear genera‐ tors is mounted. Direct drive is the simplest and probably the most efficient Power Take-Off system. PTO system of heaving oscillators and submerge heaving oscillators

Regard less to the type of PTO, all of the PTOs are connected to an electrical generator which generates useful energy from waves (except especial uses of WEC that is not intended to generate electricity [32]). The two type of electrical generators implemented in WECs are

**3. Power take-off**

energy storage.

are from this kind.

**Figure 20. Left side:** Schematic of OWC. **Right side:** Back view of an OWC [37].

#### *2.2.2. Water-interface*

Overtopping wave energy convertors are the main members of water-interface WEC. To date, three overtopping wave energy convertor has been developed; The Tapchan (Tapered Channel Wave Power Device) [38], Wave Dragon [39] and SSG (Seawave Slot-Cone Genera‐ tor) [40]. One of the well-known overtopping WECs is Wave Dragon. The Wave Dragon is an off-shore WEC, installed in location with depth of 25-40m, which is moored like a ship and consists of three main sections. The main platform, a reservoir with a double curved ramp, is float on the ocean surface and other parts are mounted on it. Two wave reflectors are installed on both sides of platform and intensify wave amplitude approaching to the platform. Finally a Hydro turbine, a set of low head Kaplan turbine, converts the hydraulic head in the reservoir. The Wave Dragon is represented in Fig. 21.

When waves approach to Wave Dragon, the reflectors focus them and guide the water to‐ ward the ramp, water overtop the ramp and fill in the reservoir then the turbine generates electricity from water motion inside the device. In Wave Dragon the wave energy convertor has not oscillation with wave, indeed wave power is transferred to PTO by water. In spite of air-interface WECs, water-interface WECs are not capable to be analyzed by Linearized Wave Theories.

**Figure 21.** Wave Dragon. (**Above:** Scheme [39] **Below:** Prototype [39])
