**2. Wave energy convertors**

**1.3. Wave measurement and prediction**

278 New Developments in Renewable Energy

some of which are described as follow;

ser profiles, current meter triplets, pressure transducers.

*1.3.1. Buoy*

sway acceleration [9].

ous and global scale.

*1.3.2. Synthetic aperture radar*

Ocean waves are variable in different time scales. Not only wave elevation and wave length (wave power) but also wave propagation direction varies with time. These de‐ tailed statistics are important for designing and controlling of particular Wave Energy Convertor (WEC). In most of cases, WECs extract wave power with oscillation in special directions and these WECs absorb maximum power whereby are adjusted in the reso‐ nance frequency, a restricted frequency span that WECs oscillation frequency is equal to exciting wave frequency. Implementing upcoming characteristic parameters of ocean wave (amplitude, propagation direction and wave length or period) has a significant im‐ pact on capture width of WECs. This information can be used for generator speed con‐ trol. In all WECs, rotating or linear electric generator is utilized for generating electrical power, implementing information of upcoming wave can be used for speed control of generator and adjusting system in resonance frequency [11]. Briefly, measurement and anticipation of upcoming ocean wave parameters play a significant role in controlling of a WEC. To date, there are various instruments for measuring and predicting ocean wave

Buoys are the oldest method for measuring wave parameters. Buoys can measure wave am‐ plitude and wave period but for detecting wave propagation direction at least two buoys are needed. Different sensors have been used on buoys for measuring such as down looking la‐

However, when using buoys to measure the waves, these tend to drift with the water parti‐ cle motion and give imprecise spatial measurement (see Fig.4 left side). In order to solve these problem different methods have been offered as an example see ref. [12]. In Fig. 4 right side a measurement buoy named WaveScan is depicted, this buoy measures heaven and

It has been amply demonstrated that synthetic aperture radar (SAR) data can be used to esti‐ mate parameters of the two-dimensional (2-D) sea surface elevation field [13, 14]. Due to their high spatial resolution and all-weather and daylight capabilities, spaceborne SAR sys‐ tems are the only sensors that can provide directional ocean wave information on a continu‐

The SAR radar sends a pulse down to the ocean surface at nadir. The significant wave height is obtained from the slope of the leading edge of the return pulse, while the total backscatter gives us the wind speed. The major goals of the variable SARs were applications in ocean wave research and wave forecasting. Two-dimensional ocean wave spectra can be

derived from SAR images by inversion of the SAR imaging mechanism.

Ocean Waves has been considered as a source of energy since late 18th century and the first patent for capturing wave power was filled in France in 1799 by a father and son named Gir‐ ard [15]. After World War I when petrol became most important source of energy the inter‐ est for harnessing ocean wave power faded. In 1940s, the Japanese wave power pioneer Yoshio Masuda developed an innovative device for absorbing wave power which is known as Oscillating Water Column (OWC) [16]. Oil crisis in 1973 was a great stimulator for gov‐ ernmental funds and researches in wave power extracting but petrol price decline in early 1980s abated interest in wave power [15]. Recently, the Kyoto Protocol on reduction of *CO*<sup>2</sup> emission, has intensified the interest in this field among researchers. To date, there are vari‐ ous devices which capture power from ocean waves [16, 17]. These devices are distinct in installation location (Off-shore, Near-shore and shoreline) as well as manner of energy har‐ vesting. This section categorise Wave Energy Convertors (WECs) based on manner of inter‐ action with ocean waves.

Excising WECs harvest wave power based on two different principles, Oscillating-WECs and Interface-WECs.

Oscillating-WECs are devices that are in direct contact with ocean waves and oscillate in specif‐ ic direction related to device design and degree of freedom. J. Falnes, one of the distinguished pioneers in ocean wave power absorption, has been clarified that there are six degree of free‐ dom for a body to oscillate with ocean waves [18]. According to Fig. 5, Oscillation in direction 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 three modes for body oscillation; Surge, Have and Pitch).

**2.1. Oscillating-WECs**

Surge Oscillators.

*2.1.1.1. Buoy*

*2.1.1. Heaving oscillators*

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

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

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

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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*

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.

*Heaving Convertors* and *Submerged Heaving Convertors* [16].

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

**Figure 5.** Different oscillation mods of a Wave Energy Convertor: Surge (1), Sway (2), Heave (3), Roll (4), Pitch (5) and Yaw (6) [18].

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

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.
