**1.3. Wave measurement and prediction**

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 some of which are described as follow;

#### *1.3.1. Buoy*

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‐ ser profiles, current meter triplets, pressure transducers.

**Figure 4. Left side;** Error in measurement due to buoy movement. **Right side;** The WaveScan buoy [9].

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‐

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

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

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

**2. Wave energy convertors**

action with ocean waves.

and Interface-WECs.

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 sway acceleration [9].

#### *1.3.2. Synthetic aperture radar*

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‐ ous and global scale.

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.

**Figure 4. Left side;** Error in measurement due to buoy movement. **Right side;** The WaveScan buoy [9].
