**1. Introduction**

In last decades, in effect of high price of fossil fuel, environmental pollution due to fossil fuel utilization and greenhouse effect, renewable energy resources are considered as an alterna‐ tive energy resource to the World's excessive energy demand. Nowadays, different technol‐ ogies are utilized to energy generation from hydro power, fuel cell and hydrogen, biomass, geothermal, solar thermal, photovoltaic and wind, while the technology for converting ocean powers are still in infancy. The aim of this chapter is to introduce potential renewable power sources of ocean, mostly ocean wave power, as well as available technologies for ex‐ tracting wave power. Due to high energy amount available in ocean, the issue has a strong importance to investigate. Furthermore there are variety of technologies that are developed for harnessing wave power each of which has an individual mechanism. Harvesting ocean wave power and converting to electrical power is a challenge for marine, mechanical, elec‐ trical and control engineers and we hope to give essential information about ocean wave, methods of energy extracting from wave and related electrical equipment.

### **1.1. Ocean**

The oceans contain 97.2% of total world water which are covering 71% of Earth's surface [1]. Also the oceans intrinsically are couple with atmosphere via air-water interface and they ex‐ change heat, moisture, momentum and trace constituents by means of air-water interface [2]. The fundamental processes that transfer energy from atmosphere to ocean are energy in‐ put to ocean by wind and net surface heat flux [3]. Furthermore, ocean absorbs heat of geo‐ thermal energy via geothermal vent in ocean bed. So that, oceans are vigorous and ubiquitous sources of renewable energy which contain 93100 TWh of energy annually [4]. Energy in oceans comes in various forms such as tides, surface wave, thermal gradient,

© 2013 Enferad and Nazarpour; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Enferad and Nazarpour; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ocean circulation and salinity gradients. It is apparent that ocean with its high amount of en‐ ergy and global realm on Earth surface can be appropriately utilized for generating electric power. To date, diverse technologies has been developed for extracting different energy forms of ocean most of which are in infancy stage and there is a challenging road before sci‐ entists and engineers to generates electricity from ocean in a cost-effective manner.

cording to the Eq. (1), by approaching to the sea bed *Z*0→ −*h* so that sinh(*Z*0 + *h* ) and *Z*1

Another important characteristic of gravity waves is propagation velocity of ocean waves. The importance is due to intrinsic relation of ocean wave power with group propagation ve‐ locity. Angular velocity for different harmonics of ocean waves is calculated according to

= *gk kh* tanh( ) (2)

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275

= = (3)

*p v gh* = (4)

= (5)

*g*

æ ö =+ = ç ÷ è ø (6)

 r

the Eq. (2), so called dispersion relation, and phase velocity is achieved from Eq.(3);

2 tan( ) 2 *p g h v k* w

p

l

 p

 l

In above equation, *vp* is phase velocity of ocean wave harmonics and λ is wavelength of ocean wave. As Eq. (3), the phase velocity depends on ocean depth so that a distinct wave propagates in different depth of ocean with different velocities. There are two possible val‐ ues for phase velocity. In shallow water, where ocean depth is significantly less than wave‐

And phase velocity in deep water, where ocean depth is more than wave length (*h* ≫*λ*), is

2 *p g v* l

p

With respect to Eq. (4), in shallow water, wave is not dispersive and various harmonics of wave propagates with same velocity, while in deep water, according to Eq. (5), waves are dispersive. It means wave with different wavelength propagate with different velocities pro‐

Total power that is carried by one harmonic of wave in unit length of wave crest, called

While, *ρ*is density of ocean water and *vg* is group velocity of wave that is equal to phase velocity in shallow water*vg* =*vp* = *gh* and in deep water is equal to half of phase velocity

2 2 2 2 1 tan( ) 4 sinh(2 ) 2 *I I*

*g A kh gA J kh v kh*

2 w

length (*h* ≪*λ*), the phase velocity is;

portional to their wavelength.

wave energy transport, is as below equation;

r

w

as follow;

tend to zero and water particle displacement abates in vertical direction.

#### **1.2. Ocean waves**

The most potent form of ocean energy is ocean wave. According to the International Energy Agency report, It is estimated that ocean waves have approximately 10000-15000 TWh of en‐ ergy annually [4], also last researches clarified that 2.11±0.05TW (to 95% confidence) of ocean waves power facing total coastlines of the world [5].

Ocean wave is created by wind, as a byproduct of the solar energy. As solar energy is con‐ verted to wind energy, the time-averaged power flow is spatially concentrated, from an in‐ tensity of typically 0.1–0.3 kW/ m2 horizontal surface of the earth to 0.5 kW / m2 envisaged area perpendicular to wind direction. As wind energy is converted to wave energy, even more spatial concentration takes place. Just below the ocean surface, average power flow in‐ tensity is typically 2–3 kW / m2 of envisaged area perpendicular to direction of wave propa‐ gation [6]. Because of increase in power intensity, the wave energy is more persistent than wind energy or solar energy. In addition to wind, passing ships and subsea earthquakes generate waves while their contribution in total generated ocean wave power is negligible in comparison to wind.

When wind generates disturbance on ocean surface, gravity or surface tension act as restor‐ ing forces that tend to drive water toward its equilibrium state consequently ocean waves manifest themselves. Ocean waves can travel thousands of kilometers with little energy loss. When a wave is propagating, the water particles are not traveling, in fact they move clock‐ wise around a small ellipse with the same period as the progressive wave which drives the motion [7]. The ellipse has its axes vertical and horizontal. By approaching to the seabed, the ellipses become progressively thinner in the vertical direction. At the sea bed, the water par‐ ticles slip back and forth horizontally. Since the entire particle paths are closed loops, there is no net mass transport by the wave. Equation of water particles' pathway while taking part in a gravity wave motion is as follow;

$$\left(\frac{X\_1}{\cosh(Z\_0 + h)} - \frac{A\_1 \text{gk}}{a^2 \cosh(kh)} \sin(kX\_0)\right)^2 + \left(\frac{Z\_1}{\sinh(Z\_0 + h)} + \frac{A\_1 \text{gk}}{a^2 \cosh(kh)} \cos(kX\_0)\right)^2 = \left(\frac{A\_1 \text{gk}}{a^2 \cosh(kh)}\right)^2\tag{1}$$

Where *AI* is amplitude of wave, *X*0 and *Z*0 are the initial position of water particle in x (wave propagation direction) and z (gravity acceleration direction) directions in Cartesian coordinates respectively, h is ocean depth, ω is angular velocity of ocean wave harmonics, g is gravity acceleration and k is wave number which is achieved from dispersion relation. Ac‐ cording to the Eq. (1), by approaching to the sea bed *Z*0→ −*h* so that sinh(*Z*0 + *h* ) and *Z*1 tend to zero and water particle displacement abates in vertical direction.

ocean circulation and salinity gradients. It is apparent that ocean with its high amount of en‐ ergy and global realm on Earth surface can be appropriately utilized for generating electric power. To date, diverse technologies has been developed for extracting different energy forms of ocean most of which are in infancy stage and there is a challenging road before sci‐

The most potent form of ocean energy is ocean wave. According to the International Energy Agency report, It is estimated that ocean waves have approximately 10000-15000 TWh of en‐ ergy annually [4], also last researches clarified that 2.11±0.05TW (to 95% confidence) of

Ocean wave is created by wind, as a byproduct of the solar energy. As solar energy is con‐ verted to wind energy, the time-averaged power flow is spatially concentrated, from an in‐

area perpendicular to wind direction. As wind energy is converted to wave energy, even more spatial concentration takes place. Just below the ocean surface, average power flow in‐

gation [6]. Because of increase in power intensity, the wave energy is more persistent than wind energy or solar energy. In addition to wind, passing ships and subsea earthquakes generate waves while their contribution in total generated ocean wave power is negligible in

When wind generates disturbance on ocean surface, gravity or surface tension act as restor‐ ing forces that tend to drive water toward its equilibrium state consequently ocean waves manifest themselves. Ocean waves can travel thousands of kilometers with little energy loss. When a wave is propagating, the water particles are not traveling, in fact they move clock‐ wise around a small ellipse with the same period as the progressive wave which drives the motion [7]. The ellipse has its axes vertical and horizontal. By approaching to the seabed, the ellipses become progressively thinner in the vertical direction. At the sea bed, the water par‐ ticles slip back and forth horizontally. Since the entire particle paths are closed loops, there is no net mass transport by the wave. Equation of water particles' pathway while taking part

2 0 0 2 2 0 0 sin( ) cos( ) cosh( ) cosh( ) sinh( ) cosh( ) cosh( ) *X A gk I Z A gk I I A gk kX kX*

æ ö æ öæ ö ç - ++ = ÷ ç ÷ç ÷ ç ÷ ç + + è ø è øè ø

w

Where *AI* is amplitude of wave, *X*0 and *Z*0 are the initial position of water particle in x (wave propagation direction) and z (gravity acceleration direction) directions in Cartesian coordinates respectively, h is ocean depth, ω is angular velocity of ocean wave harmonics, g is gravity acceleration and k is wave number which is achieved from dispersion relation. Ac‐

horizontal surface of the earth to 0.5 kW / m2 envisaged

of envisaged area perpendicular to direction of wave propa‐

2 2 2

 w*kh kh* (1)

entists and engineers to generates electricity from ocean in a cost-effective manner.

ocean waves power facing total coastlines of the world [5].

tensity of typically 0.1–0.3 kW/ m2

tensity is typically 2–3 kW / m2

in a gravity wave motion is as follow;

w

*Z h*

1 1

*kh Z h*

comparison to wind.

**1.2. Ocean waves**

274 New Developments in Renewable Energy

Another important characteristic of gravity waves is propagation velocity of ocean waves. The importance is due to intrinsic relation of ocean wave power with group propagation ve‐ locity. Angular velocity for different harmonics of ocean waves is calculated according to the Eq. (2), so called dispersion relation, and phase velocity is achieved from Eq.(3);

$$
\rho u^2 = gk \tanh(kh) \tag{2}
$$

$$\psi\_p = \frac{\alpha \nu}{k} = \sqrt{\frac{g\lambda}{2\pi} \tan(\frac{2\pi h}{\lambda})}\tag{3}$$

In above equation, *vp* is phase velocity of ocean wave harmonics and λ is wavelength of ocean wave. As Eq. (3), the phase velocity depends on ocean depth so that a distinct wave propagates in different depth of ocean with different velocities. There are two possible val‐ ues for phase velocity. In shallow water, where ocean depth is significantly less than wave‐ length (*h* ≪*λ*), the phase velocity is;

$$
v\_p = \sqrt{gh} \tag{4}$$

And phase velocity in deep water, where ocean depth is more than wave length (*h* ≫*λ*), is as follow;

$$\upsilon\_{\boldsymbol{\nu}} = \sqrt{\frac{g\lambda}{2\pi}}\tag{5}$$

With respect to Eq. (4), in shallow water, wave is not dispersive and various harmonics of wave propagates with same velocity, while in deep water, according to Eq. (5), waves are dispersive. It means wave with different wavelength propagate with different velocities pro‐ portional to their wavelength.

Total power that is carried by one harmonic of wave in unit length of wave crest, called wave energy transport, is as below equation;

$$J = \frac{\rho g^2 A\_I^2}{4\alpha \nu} \left( 1 + \frac{2kh}{\sinh(2kh)} \right) \tan(kh) = \frac{\rho g A\_I^2}{2} \upsilon\_\chi \tag{6}$$

While, *ρ*is density of ocean water and *vg* is group velocity of wave that is equal to phase velocity in shallow water*vg* =*vp* = *gh* and in deep water is equal to half of phase velocity 2*vg* =*vp* = *gλ* / 2*π*. According to different wave propagation velocity in deep and shallow waters and related energy transport by wave in these environments and by considering var‐ ious methods of extracting wave power by different devices and related commercial, instal‐ lation and maintaining issues, ocean waves study is divided to three different areas called; shoreline, near-shore and off-shore.

ocean wave power density to latitude as Fig. 2. It is shown that ocean wave power is mostly

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277

Ocean waves are variable in different time scales. The average wave energy for a winter month can be 5–10 times the mean value for a summer month. The wave energy can vary 10 times from one week to the next. The wave energy during one storm can be five times higher than the mean value for the week the storm occurs. Wave energy in a wave group can be up to 50 times the wave energy between wave groups [10]. In Fig. 3, Monthly mean ocean wave power is plotted for different months of year. According to this figure ocean wave power in both Hemispheres is significantly higher in winter season in comparison to summer. Also seasonal variation of

travelling between 40º-60º latitudes [9].

**Figure 2.** Global annual mean wave power density with respect to latitude [9].

wave power in southern Hemisphere is lower than northern one.

**Figure 3.** Monthly mean wave power for Northern and southern Hemispheres [5].

**Off-shore** is a location of ocean in where depth is more than 40 meters. In this location ocean waves have the most power.

**Near-shore** is location with ocean depth of 10-30 meters and typically has a distance of 0.5-2Km from coastline. In near-shore, seabed fraction is the major source of incident wave power reduction. For instance, in location with ocean depth of 10 meters, different harmon‐ ics of ocean waves losses 2-10% of their total power [8].

**Shoreline** is the location of ocean where depth is less than 10 m. In this location most of the wave power is declined due to seabed fraction and wave breaking.

Not only amount of wave power is various in off-shore and in-shore (shoreline and nearshore) but also ocean wave power is not uniformly separate in all oceans. Fig. 1 illustrates global distribution of wave power density [5]. The arrows on the plot show the mean best wave propagation direction. This figure represents that most of wave power is concentrated in western part of continents which is due to west to east winds. The highest levels in the Northern Hemisphere are off the west coast of the British Isles, Iceland and Greenland, with somewhat lower energy levels in the Pacific off the western seaboard of the US and Canada in Southern Hemisphere Chile, South Africa and the entire south and south west coasts of Australia and New Zealand.

**Figure 1.** Global annual mean wave power density and annual mean best direction (arrow) [5].

In other point of view, ocean wave power is denser between 40º-60º latitudes in both North‐ ern and Southern hemisphere. Stephen Barstow et al. represented relevance of annual mean ocean wave power density to latitude as Fig. 2. It is shown that ocean wave power is mostly travelling between 40º-60º latitudes [9].

**Figure 2.** Global annual mean wave power density with respect to latitude [9].

2*vg* =*vp* = *gλ* / 2*π*. According to different wave propagation velocity in deep and shallow waters and related energy transport by wave in these environments and by considering var‐ ious methods of extracting wave power by different devices and related commercial, instal‐ lation and maintaining issues, ocean waves study is divided to three different areas called;

**Off-shore** is a location of ocean in where depth is more than 40 meters. In this location

**Near-shore** is location with ocean depth of 10-30 meters and typically has a distance of 0.5-2Km from coastline. In near-shore, seabed fraction is the major source of incident wave power reduction. For instance, in location with ocean depth of 10 meters, different harmon‐

**Shoreline** is the location of ocean where depth is less than 10 m. In this location most of the

Not only amount of wave power is various in off-shore and in-shore (shoreline and nearshore) but also ocean wave power is not uniformly separate in all oceans. Fig. 1 illustrates global distribution of wave power density [5]. The arrows on the plot show the mean best wave propagation direction. This figure represents that most of wave power is concentrated in western part of continents which is due to west to east winds. The highest levels in the Northern Hemisphere are off the west coast of the British Isles, Iceland and Greenland, with somewhat lower energy levels in the Pacific off the western seaboard of the US and Canada in Southern Hemisphere Chile, South Africa and the entire south and south west coasts of

shoreline, near-shore and off-shore.

276 New Developments in Renewable Energy

ocean waves have the most power.

Australia and New Zealand.

ics of ocean waves losses 2-10% of their total power [8].

wave power is declined due to seabed fraction and wave breaking.

**Figure 1.** Global annual mean wave power density and annual mean best direction (arrow) [5].

In other point of view, ocean wave power is denser between 40º-60º latitudes in both North‐ ern and Southern hemisphere. Stephen Barstow et al. represented relevance of annual mean Ocean waves are variable in different time scales. The average wave energy for a winter month can be 5–10 times the mean value for a summer month. The wave energy can vary 10 times from one week to the next. The wave energy during one storm can be five times higher than the mean value for the week the storm occurs. Wave energy in a wave group can be up to 50 times the wave energy between wave groups [10]. In Fig. 3, Monthly mean ocean wave power is plotted for different months of year. According to this figure ocean wave power in both Hemispheres is significantly higher in winter season in comparison to summer. Also seasonal variation of wave power in southern Hemisphere is lower than northern one.

**Figure 3.** Monthly mean wave power for Northern and southern Hemispheres [5].