*2.1.1. Heaving oscillators*

Heave Oscillators are the simplest oscillators for absorbing wave power. These devices, ex‐ tract wave power with motion in perpendicular direction to the Sea Water Level and accord‐ ing to the mooring and working principle are divided to three groups; *Buoys*, *Two Body Heaving Convertors* and *Submerged Heaving Convertors* [16].

#### *2.1.1.1. Buoy*

Point absorbers or buoys are convertors with one floating body in sea level which is con‐ nected to the PTO (Power Take-Off) via a steel structure or a cable (translator). The body fluctuates with ocean waves in earth gravity direction (heave) which cause the steel struc‐ ture or cable to oscillate with it hence the bidirectional movement of buoy is transferred to the PTO in the other point of translator and PTO generates electricity. To date various buoys have been designed for wave energy harvesting which are same in principal but different in detail. One of these is a buoy that was designed by Budal et al. in Norway [19]. The device was linked to anchor on the sea bed via universal joint (see Fig. 7). An air turbine was imple‐ mented on the device for energy converting and it was controlled by latching control.

**Figure 7.** Norwegian buoy (courtesy of J. Falnes) [16].

**Figure 8. Left side;** Taut moored heaving buoy with linear electrical machine PTO [16]. **Right side;** Component of tout moored WEC, Sweden [22].

**Figure 10.** Wave Star WEC, Denmark [22].

**Figure 11.** Power capture performance of Wave Star WEC [22].

*2.1.1.2. Two body heaving convertors*

WEC was investigated by Babarit et el. [22] and the result is as Fig. 11.

In the case of heaven buoys, Wave Star Energy company has developed an innovative WEC namely Wave Star WEC. In this device a jack up structure stands on sea bed and provides a ref‐ erence to the buoys (see Fig. 10). A full sized system is consisting of different buoys in which a hinging arm is utilized to transfer each buoy's motion to the PTO. The buoys have ability to be polled up (survival mode) in case harass ocean condition. Furthermore, hydraulic rams are em‐ ployed to convert hinging motion of arms. Power capture preformation of 20 buoys Wave Star

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Two body heaven convertors are multi-body convertors in which ocean power is extracted from the relevant motion between two bodies. One of these bodies is float on ocean surface and anoth‐ er is completely submerged. Ocean wave interaction with this WEC causes the float body ele‐ vates in heave mode as well as it pushes submerged body to the sea bottom due to increase in sea

Another buoy type is floating body connected to bottom fixed structure via a cable (taut moored). Due to cable flexibility, this device is not restricted in heave mode and it also has oscillation in surge direction. PTO of device is fixed in ocean bed and cable transfer WEC motion to PTO. There are two different PTO that has been coupled with this type of WEC. Hydraulic PTO was implemented by this WEC in Denmark in 1990s which by piston pump supplies high pressure water to a hydraulic turbine [20] and another is linear electric genera‐ tor that is housed in inside a steel hull mounted on a concrete ballast structure and converts linear motion of cable (translator) to electric power [21]. This system is developed in Sweden and the scheme of system is represented in Fig.8. Barbarit et al. have investigated different power capture preformation of various WECs by numerical method [22] and capture width of taut-moored WEC is as Fig.9.

**Figure 9.** Power capture performance of taut moored WEC for various waves (the vertical axis is wave ampli‐ tudes in meter and horizontal axis is wave period in second and the annual men absorbed power amount on the table are in kW) [22].

**Figure 10.** Wave Star WEC, Denmark [22].

**Figure 8. Left side;** Taut moored heaving buoy with linear electrical machine PTO [16]. **Right side;** Component of

Another buoy type is floating body connected to bottom fixed structure via a cable (taut moored). Due to cable flexibility, this device is not restricted in heave mode and it also has oscillation in surge direction. PTO of device is fixed in ocean bed and cable transfer WEC motion to PTO. There are two different PTO that has been coupled with this type of WEC. Hydraulic PTO was implemented by this WEC in Denmark in 1990s which by piston pump supplies high pressure water to a hydraulic turbine [20] and another is linear electric genera‐ tor that is housed in inside a steel hull mounted on a concrete ballast structure and converts linear motion of cable (translator) to electric power [21]. This system is developed in Sweden and the scheme of system is represented in Fig.8. Barbarit et al. have investigated different power capture preformation of various WECs by numerical method [22] and capture width

**Figure 9.** Power capture performance of taut moored WEC for various waves (the vertical axis is wave ampli‐ tudes in meter and horizontal axis is wave period in second and the annual men absorbed power amount on

tout moored WEC, Sweden [22].

282 New Developments in Renewable Energy

of taut-moored WEC is as Fig.9.

the table are in kW) [22].

**Figure 11.** Power capture performance of Wave Star WEC [22].

In the case of heaven buoys, Wave Star Energy company has developed an innovative WEC namely Wave Star WEC. In this device a jack up structure stands on sea bed and provides a ref‐ erence to the buoys (see Fig. 10). A full sized system is consisting of different buoys in which a hinging arm is utilized to transfer each buoy's motion to the PTO. The buoys have ability to be polled up (survival mode) in case harass ocean condition. Furthermore, hydraulic rams are em‐ ployed to convert hinging motion of arms. Power capture preformation of 20 buoys Wave Star WEC was investigated by Babarit et el. [22] and the result is as Fig. 11.

#### *2.1.1.2. Two body heaving convertors*

Two body heaven convertors are multi-body convertors in which ocean power is extracted from the relevant motion between two bodies. One of these bodies is float on ocean surface and anoth‐ er is completely submerged. Ocean wave interaction with this WEC causes the float body ele‐ vates in heave mode as well as it pushes submerged body to the sea bottom due to increase in sea water inertia. One of the important advantages of this WEC is facility in its installation. Because device does not require any sea-bed connection, it can easily be installed in off-shore. Yet, differ‐ ent devices are developed in this principal one of which is ISP buoy developed in Sweden [23], Irish Wavebob [24] and PowerBuoy inspired by Ocean Power Technologies company.

The schematic of Wavebob is depicted in Fig. 12 left side. According this picture wavebob is consist of an inner buoy, submerged body (body2), and floating buoy (body 1) which are ax‐ ially connected to each other. Additionally a high Pressure oil hydraulic system is imple‐ mented to deliver extracted power to electric generator. Fig. 13 illustrates power capture matrix of Wavebob WEC [22].

**Figure 14.** Archimedes Wave Swing (**Left side:** Scheme [16]. **Right side:** Prototype [37]).

chine. The AWS was successfully tested in 2004 [25].

problem that heaving oscillators face with.

*vertors* and *Submerged Pitching Convertors*.

*2.1.2. Pitching oscillators*

The Archimedes Wave Swing (AWS) is a fully submerges off-shore WEC. It is construct‐ ed from two main parts, Silo or Basement is an air filled cylindrical chamber which is moored on seabed and floater is oscillating upper part as is illustrated in Fig. 14. Floater oscillates by water pressure variation. By crossing wave crest upon AWS the floater moves down compressing the air inside the chamber and by passage of wave trough upon AWS, the air inside the chamber expands and consequently push the floater up. Beside its unique design, AWS is the first WEC that a linear electric machine is imple‐ mented as PTO. One side of linear machine is fixed to the basement and the other side is connected to the floater via a translator, hence oscillation of floater excites linear ma‐

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Due to their mooring structure and the environment they work in (off-shore), almost all of the heaving oscillators requires highly maintenance. Meanwhile delivering generated elec‐ tric power to the electric network or consumer is another problem intrinsically related to these off-shore WECs. High cost and Long distance underwater cabling is a commercial

Pitching oscillators are WECs which extract wave power by hinging motion in wave propa‐ gation direction. As it is demonstrated in Fig. 5, the hinging motion of pitching oscillator oc‐ curs in the axis in which WEC is installed on. According to device mooring and body design theses WECs are categorized to three types; *Float Pitching Convertors*, *Two Body Pitching Con‐*

*2.1.1.3. Submerged heaving convertors*

**Figure 12. Left side;** Principle of Wavebob WEC [16]. **Right side;** ¼ Scale Wavebob WEC, Ireland [22].

**Figure 13.** Power capture performance of Wavebob WEC [22].

**Figure 14.** Archimedes Wave Swing (**Left side:** Scheme [16]. **Right side:** Prototype [37]).

#### *2.1.1.3. Submerged heaving convertors*

water inertia. One of the important advantages of this WEC is facility in its installation. Because device does not require any sea-bed connection, it can easily be installed in off-shore. Yet, differ‐ ent devices are developed in this principal one of which is ISP buoy developed in Sweden [23],

The schematic of Wavebob is depicted in Fig. 12 left side. According this picture wavebob is consist of an inner buoy, submerged body (body2), and floating buoy (body 1) which are ax‐ ially connected to each other. Additionally a high Pressure oil hydraulic system is imple‐ mented to deliver extracted power to electric generator. Fig. 13 illustrates power capture

Irish Wavebob [24] and PowerBuoy inspired by Ocean Power Technologies company.

**Figure 12. Left side;** Principle of Wavebob WEC [16]. **Right side;** ¼ Scale Wavebob WEC, Ireland [22].

**Figure 13.** Power capture performance of Wavebob WEC [22].

matrix of Wavebob WEC [22].

284 New Developments in Renewable Energy

The Archimedes Wave Swing (AWS) is a fully submerges off-shore WEC. It is construct‐ ed from two main parts, Silo or Basement is an air filled cylindrical chamber which is moored on seabed and floater is oscillating upper part as is illustrated in Fig. 14. Floater oscillates by water pressure variation. By crossing wave crest upon AWS the floater moves down compressing the air inside the chamber and by passage of wave trough upon AWS, the air inside the chamber expands and consequently push the floater up. Beside its unique design, AWS is the first WEC that a linear electric machine is imple‐ mented as PTO. One side of linear machine is fixed to the basement and the other side is connected to the floater via a translator, hence oscillation of floater excites linear ma‐ chine. The AWS was successfully tested in 2004 [25].

Due to their mooring structure and the environment they work in (off-shore), almost all of the heaving oscillators requires highly maintenance. Meanwhile delivering generated elec‐ tric power to the electric network or consumer is another problem intrinsically related to these off-shore WECs. High cost and Long distance underwater cabling is a commercial problem that heaving oscillators face with.

#### *2.1.2. Pitching oscillators*

Pitching oscillators are WECs which extract wave power by hinging motion in wave propa‐ gation direction. As it is demonstrated in Fig. 5, the hinging motion of pitching oscillator oc‐ curs in the axis in which WEC is installed on. According to device mooring and body design theses WECs are categorized to three types; *Float Pitching Convertors*, *Two Body Pitching Con‐ vertors* and *Submerged Pitching Convertors*.
