**3. Power take-off**

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‐

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

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

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

trical generator.

*2.2.2. Water-interface*

Wave Theories.

both shoreline and near-shore.

290 New Developments in Renewable Energy

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

head in the reservoir. The Wave Dragon is represented in Fig. 21.

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, three type of PTO is more commonly used.


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 Linear Generators and Rotational Generator.

#### **3.1. Linear generators**

Linear generator are the kind of electrical machine in which the rotor (translator) and stator are linear and translator displace in straight path inside stator. Since most of the WECs har‐ ness wave power in reciprocating directions, Linear Generators are the most appropriate machines for generating electricity by WECs. It is due to, there is not require for any other interface or other power conversion (e.g. Hydraulic) which decline the transferred power in transmission process. Linear generator is connected to the WEC via translator and displace with it. Yet, three topology of linear generators are considered in wave energy industry.

*3.1.2.1. Transverse Flux Permanent Magnet Machine (TFM)*

(see Fig. 22. Left). This machine was used in AWS.

flux concentration and stationary magnets [45].

*3.1.2.2. Vernier Hybrid Machine (VHM)*

cility of constructing are main advantages of this machine.

the translator."[45]

Transverse Flux Permanent Magnet Machines (TFM) has higher shear stresses than other machine topologies, This implies that TFM machines may be more suitable for wave energy application than the synchronous machines. There are two topology of TFM. In the first top‐ ology, the coils are in the stator and the magnets are on the translator. The translator is lon‐ ger than the stator, which means that a part of the (rather expensive) magnets is not used

Ocean's Renewable Power and Review of Technologies: Case Study Waves

http://dx.doi.org/10.5772/53806

293

**Figure 23. Left side**: TFM machine with flux concentration and moving magnets [45]. **Right side:** TFM machine with

As it is illustrated in Fig. 22 Right, the second topology, namely the double-sided movingiron TFM, is a double-sided machine in which the translator consists only of iron. "In this topology, the stator consists of coils and U-cores on both sides of the translator, which con‐ sists of two rows of magnets and flux concentrators with space for construction material in between. Both the conductors and the magnets are kept stationary. The U-cores are now simpler in shape because space for coils is no longer required, and these cores or yokes form

The Vernier Hybrid Machine is constructed from a linear toothed translator constructed from iron laminates which move inside two C-cores. The C-core has coil wounded on each pole also magnets are mounted on pole face. The translator tooth and slots width are similar in dimension to the magnet pitch, hence rapid flus reversal happens over short distance as the translator teeth move from the aligned to the unaligned position. Due to rapid flow change in a short distance, the electric frequency of the flux pulsation is greater than the translator's frequency [46, 47]. The schematic of one pole VHM is presented in Fig. 24. In spite of TFM, the VHM can be constructed from lamination which makes construction easy and small. However power factor of VHM is low, the high shear stress, small sizes and fa‐

### *3.1.1. Linear Permanent Magnet Synchronous Machine (LPMSM)*

LPMSAs are constructed in three different configuration; Single-Side, Double–Side and Tub‐ ular. In all of these structures the translator (actuator) have moving magnets and primary constitute mover. The core of the primary of a flat LPMSA is made of longitudinal lamina‐ tions with uniformly distributed slots, which house the windings. Since the windings are lo‐ cated in open slots, the effective air gap is greater than the actual air gap.

In tubular structures the laminations of the shape primary core is longitudinal or disk-shap‐ ed. Stacking of the disk laminations increases the effective air gap. The core of the secondary of a tubular LPMSA is generally made of solid magnetic steel. In LPMSM the flux density is supplied by rare earth permanent magnet. Due to ease in assembly, disk shaped laminated tubular LPMSM are preferred to flat types also slitting technique is use on disk lamination to reduce eddy current [44]. The schematic of single side LPMSM is illustrated in Fig. 22.

**Figure 22.** Schematic of single-side LPMSM [44].

#### *3.1.2. Variable Reluctance Permanent Magnet Machine (VRM)*

A family of permanent magnet machine, known as variable reluctance permanent magnet machine (VRM), has been developed with high force density. Despite its high force density, low power factor is a characteristic of VRM machines. The machine has a very high induc‐ tance, such that the current can be out of phase for almost 90 degree. The variable reluctance permanent magnet family is divided for two subcategory; Transverse Flux Permanent Mag‐ net (TFM) and Vernier Hybrid Machine (VHM).

### *3.1.2.1. Transverse Flux Permanent Magnet Machine (TFM)*

**3.1. Linear generators**

292 New Developments in Renewable Energy

Linear generator are the kind of electrical machine in which the rotor (translator) and stator are linear and translator displace in straight path inside stator. Since most of the WECs har‐ ness wave power in reciprocating directions, Linear Generators are the most appropriate machines for generating electricity by WECs. It is due to, there is not require for any other interface or other power conversion (e.g. Hydraulic) which decline the transferred power in transmission process. Linear generator is connected to the WEC via translator and displace with it. Yet, three topology of linear generators are considered in wave energy industry.

LPMSAs are constructed in three different configuration; Single-Side, Double–Side and Tub‐ ular. In all of these structures the translator (actuator) have moving magnets and primary constitute mover. The core of the primary of a flat LPMSA is made of longitudinal lamina‐ tions with uniformly distributed slots, which house the windings. Since the windings are lo‐

In tubular structures the laminations of the shape primary core is longitudinal or disk-shap‐ ed. Stacking of the disk laminations increases the effective air gap. The core of the secondary of a tubular LPMSA is generally made of solid magnetic steel. In LPMSM the flux density is supplied by rare earth permanent magnet. Due to ease in assembly, disk shaped laminated tubular LPMSM are preferred to flat types also slitting technique is use on disk lamination to reduce eddy current [44]. The schematic of single side LPMSM is illustrated in Fig. 22.

A family of permanent magnet machine, known as variable reluctance permanent magnet machine (VRM), has been developed with high force density. Despite its high force density, low power factor is a characteristic of VRM machines. The machine has a very high induc‐ tance, such that the current can be out of phase for almost 90 degree. The variable reluctance permanent magnet family is divided for two subcategory; Transverse Flux Permanent Mag‐

*3.1.1. Linear Permanent Magnet Synchronous Machine (LPMSM)*

**Figure 22.** Schematic of single-side LPMSM [44].

*3.1.2. Variable Reluctance Permanent Magnet Machine (VRM)*

net (TFM) and Vernier Hybrid Machine (VHM).

cated in open slots, the effective air gap is greater than the actual air gap.

Transverse Flux Permanent Magnet Machines (TFM) has higher shear stresses than other machine topologies, This implies that TFM machines may be more suitable for wave energy application than the synchronous machines. There are two topology of TFM. In the first top‐ ology, the coils are in the stator and the magnets are on the translator. The translator is lon‐ ger than the stator, which means that a part of the (rather expensive) magnets is not used (see Fig. 22. Left). This machine was used in AWS.

**Figure 23. Left side**: TFM machine with flux concentration and moving magnets [45]. **Right side:** TFM machine with flux concentration and stationary magnets [45].

As it is illustrated in Fig. 22 Right, the second topology, namely the double-sided movingiron TFM, is a double-sided machine in which the translator consists only of iron. "In this topology, the stator consists of coils and U-cores on both sides of the translator, which con‐ sists of two rows of magnets and flux concentrators with space for construction material in between. Both the conductors and the magnets are kept stationary. The U-cores are now simpler in shape because space for coils is no longer required, and these cores or yokes form the translator."[45]

#### *3.1.2.2. Vernier Hybrid Machine (VHM)*

The Vernier Hybrid Machine is constructed from a linear toothed translator constructed from iron laminates which move inside two C-cores. The C-core has coil wounded on each pole also magnets are mounted on pole face. The translator tooth and slots width are similar in dimension to the magnet pitch, hence rapid flus reversal happens over short distance as the translator teeth move from the aligned to the unaligned position. Due to rapid flow change in a short distance, the electric frequency of the flux pulsation is greater than the translator's frequency [46, 47]. The schematic of one pole VHM is presented in Fig. 24. In spite of TFM, the VHM can be constructed from lamination which makes construction easy and small. However power factor of VHM is low, the high shear stress, small sizes and fa‐ cility of constructing are main advantages of this machine.
