*3.1.3. Tubular air cored permanent magnet generator (TAPM)*

The TAPM machine is proposed for wave application by Baker and Muller [48]. The principle of TAPM is shown in Fig. 25. In this topology, the magnets are magnetized in the axial direc‐ tions with altering direction of flux [37]. Flux concentrators are placed between the magnets and a varying flux wave is created outside the translator. The large constructional advantage of TAPM is elimination of normal force by removing the steel in stator. Significant support structures are needed to overcome these forces and maintain a constant air gap width. The main problem in this topology is that the magnetic reluctance in the magnetic circuit is in‐ creased considerably, since the distance the flux travels in air now is in the range of the pole pitch and not, as in steel stator machines, just over the air gap [37]. The flux of an air cored ma‐ chine is thus much smaller and the power per air gap area is considerably lower.

**Figure 26.** Common used power electronic circuit for connecting linear generator to the electrical network.

tery or capacitor) to the electric. This power electronic circuit is depicted in Fig. 26.

machine for convert low speed WEC motion is a low effective manner.

**4. Environmental consequences**

**3.2. Rotational generators**

Regardless to the type of the linear machine, a linear machine generates fluctuating voltage from reciprocating motion of WEC, which is varying in both amplitude and frequency. Thus, it is not possible to connect linear generator directly to electric grid. For solving this problem, effective power electronic instrument is implemented [49]. One of the common power elec‐ tronic circuits, implemented with linear generator, has a diode bridge rectifier or invertor rec‐ tifier in generator side, in which the generated voltage is delivered to a battery or capacitor after rectifying. Also another invertor is used in grid side to deliver power in DC-Link (bat‐

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However utilization of rotational generator in wave industry is a questionable issue, conven‐ tional generators have been used with different wave energy convertors. Pelamis, Oscillating Water Column and overtopping devices are the main WECs which use rotational generator for mechanical electrical conversion. In these cases, the Asynchronous Generator or Induction machine is a preferred machine. It is due to the capacity of this type of machine to act in varia‐ ble speed condition. Meanwhile, utilization of wound rotor induction machine makes it feasi‐ ble to control rotor current and frequency which in consequence control the velocity and extracted power by mechanical part (This use of induction machine is so called Double Fed Induction Generator (DFIG)). Amundarain et al. considered the DFIG on OWC for controlling the electrical machine in a proper manner [50]. In comparison to the linear machine, it is much more simple to connect an induction machine to the electrical grid. While, since these ma‐ chines was designed to act in higher speed, in most of the cases the utilization of rotational

Renewable sources have been considered as new source of energy to decline our depend‐ ence on fossil fuel and be a solution to the global warming and *CO*2 emission. Considering, each technology (WEC) beside its advantage might have some side effect and by taking into account the importance of environment in which a wave energy convertor acts (Ocean), finding out and preventing possible negative consequence of WEC carries a huge impor‐ tance. Thus, before any decision to build a large WEC farm this issue should be considered

**Figure 24.** One phase of Linear Vernier Hybrid Machine [46].

**Figure 25.** Tubular Air Cored Permanent Magnet Machine [37]. (**Left side:** Three dimensional view. **Right Side:** Cross section view)

**Figure 26.** Common used power electronic circuit for connecting linear generator to the electrical network.

Regardless to the type of the linear machine, a linear machine generates fluctuating voltage from reciprocating motion of WEC, which is varying in both amplitude and frequency. Thus, it is not possible to connect linear generator directly to electric grid. For solving this problem, effective power electronic instrument is implemented [49]. One of the common power elec‐ tronic circuits, implemented with linear generator, has a diode bridge rectifier or invertor rec‐ tifier in generator side, in which the generated voltage is delivered to a battery or capacitor after rectifying. Also another invertor is used in grid side to deliver power in DC-Link (bat‐ tery or capacitor) to the electric. This power electronic circuit is depicted in Fig. 26.

#### **3.2. Rotational generators**

*3.1.3. Tubular air cored permanent magnet generator (TAPM)*

294 New Developments in Renewable Energy

**Figure 24.** One phase of Linear Vernier Hybrid Machine [46].

section view)

The TAPM machine is proposed for wave application by Baker and Muller [48]. The principle of TAPM is shown in Fig. 25. In this topology, the magnets are magnetized in the axial direc‐ tions with altering direction of flux [37]. Flux concentrators are placed between the magnets and a varying flux wave is created outside the translator. The large constructional advantage of TAPM is elimination of normal force by removing the steel in stator. Significant support structures are needed to overcome these forces and maintain a constant air gap width. The main problem in this topology is that the magnetic reluctance in the magnetic circuit is in‐ creased considerably, since the distance the flux travels in air now is in the range of the pole pitch and not, as in steel stator machines, just over the air gap [37]. The flux of an air cored ma‐

**Figure 25.** Tubular Air Cored Permanent Magnet Machine [37]. (**Left side:** Three dimensional view. **Right Side:** Cross

chine is thus much smaller and the power per air gap area is considerably lower.

However utilization of rotational generator in wave industry is a questionable issue, conven‐ tional generators have been used with different wave energy convertors. Pelamis, Oscillating Water Column and overtopping devices are the main WECs which use rotational generator for mechanical electrical conversion. In these cases, the Asynchronous Generator or Induction machine is a preferred machine. It is due to the capacity of this type of machine to act in varia‐ ble speed condition. Meanwhile, utilization of wound rotor induction machine makes it feasi‐ ble to control rotor current and frequency which in consequence control the velocity and extracted power by mechanical part (This use of induction machine is so called Double Fed Induction Generator (DFIG)). Amundarain et al. considered the DFIG on OWC for controlling the electrical machine in a proper manner [50]. In comparison to the linear machine, it is much more simple to connect an induction machine to the electrical grid. While, since these ma‐ chines was designed to act in higher speed, in most of the cases the utilization of rotational machine for convert low speed WEC motion is a low effective manner.

### **4. Environmental consequences**

Renewable sources have been considered as new source of energy to decline our depend‐ ence on fossil fuel and be a solution to the global warming and *CO*2 emission. Considering, each technology (WEC) beside its advantage might have some side effect and by taking into account the importance of environment in which a wave energy convertor acts (Ocean), finding out and preventing possible negative consequence of WEC carries a huge impor‐ tance. Thus, before any decision to build a large WEC farm this issue should be considered accurately to prevent occurring of any other disaster like *CO*<sup>2</sup> emission and global warming. There are some probable problems that may afflict the ecosystem as a consequence of WEC application, some of which is as follow;

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Constructing Wave Farm in a location might cause military importance of location, avoid‐ ance of shipping and fishing which not only might change the economy of area but also cause a biological change in the area [51]. The WECs are attractive to the fish and it is due to fish use these devices as protection from predator, availability of food near them, spewing substrates and so on [52]. Hence, restriction for fishing and popularity of WECs by fish may enhance fish population in Wave Farm and this has a negative impact on local species diver‐ sity and population density [53].

WECs alter the currents and waves in implemented area and this might change sediment size distribution in area which may favour the accumulation of organic material in area [54].

It was indicated that some species of shark are sensitive to the electromagnetic field of 25-100 uT [55] and also it was shown that migrating European eels can detect the magnetic field of under‐ water cables [56]. It is worthwhile to find out possible consequence of external electromagnetic field on marine species and reduce the emitted magnetic field from under water cables.

Noises generation during site construction, maintenance and even normal working of WEC can affect dolphin, whales, seals and other fish species, which use underwater sound for communication and finding mat and so on [57, 58]. However little is known about long-term effects of noise on these species, the possible preventing technics should be developed for less interacting of WECs in ocean environment.
