2 CHAPTER 2 OCEAN WAVE ENERGY

Transcription

1 12 2 CHAPTER 2 OCEAN WAVE ENERGY 2.1 INTRODUCTION There is a sign of increased concern over importance of utilising sustainable renewable energy sources all over the world. Though the negligible compared to other conventional energy generation systems, the scenario could be very different in a few years from now. In the mission of climate change initiatives and sustainable energy development, the evolution of renewable energy industry is emerging. The present contribution of ocean energies in the context of energy generation in India is at its early stage of development. Though the technology of generating electricity from sea is new to the entire world, India is not even close to initiating the activities of exploiting ocean renewable energy sources. Inability of the ocean energy technologies to compete with wind energy conversion technology in overall cost leads to the unattractive interest from investors. Until recently, little concern had been focussed on exploiting ocean based renewable energy sources. Since 2002, there has been a healthy sign of activities in this direction. A number of investments have been made by many countries to prove that ocean energy sources could be as attractive as sources such as solar and wind. The world has seen already a number of technologies progressed to the point of commercial installation.

2 13 The term ocean energy is used to describe various renewable energies derived from the sea, including ocean wave energy, tidal energy, ocean current energy (sometimes called marine hydrokinetic energy), offshore wind energy, and ocean thermal and salinity gradient energy. The increased technological development, emergence of new sciences, increased pressure on reducing global warming and wealth of on-going R&D leaves no consensus over one technology emerging as best technology to capture any of the ocean energy more efficiently and cheaply. A variety of new ocean power technologies initiated funded and supported by governments to address the global pressure on clean and sustainable energy development. Progresses in the area of ocean energy technologies are slow and least attractive when compared to other land and fossil fuel based energy conversion technologies due to their cheaper prize and high accessibility. 2.2 OCEAN ENERGY TECHNOLOGIES The Oceans contain vast amounts of energy. As discussed earlier, two different forms of ocean energy offer possible energy resources: thermal energy and the mechanical energy. In the proceeding section, various technologies available to harvest each of the ocean renewable energy sources are discussed in brief.

3 Tidal Energy Tidal energy concerns the natural rise and sink of ocean surface elevation, which are influenced mainly by the interaction of the gravitational fields of the Earth, moon and sun (Figure 2.1). The ocean water level gets peak twice daily, filling and emptying natural reservoirs along the shoreline. The flow of water in and out of these natural reservoirs can be used to turn turbines to produce electricity (Figure 2.2). Figure 2.1 Tidal Bugle (Clerici et al 2004) Conversion of tidal stream or marine current is one method of exploiting the tidal energy. This method uses the fast water currents, which are created by the tides and magnified by topographical features such as peninsulas, bays and channels or by the shape of the seabed when water is forced through narrow channels.

4 15 Figure 2.2 SeaGen (Courtesy of Marine Current Turbines Limited) Another method of harnessing ocean tides is similar to conventional hydroelectric power plants (Figure 2.3). The concept is called as tidal range technology. Openings and turbines are installed along a barrage or sea and storage becomes adequate, the openings are activated and water is allowed to flow across through the turbines. Rotation of turbine due to the flow of water is then connected with an electricity generating unit and electrical energy is produced. Energy can be generated by water flowing both into and out of the reservoir. As there are two high and two low tides each day, the energy can be generated in every six hours. In India, the Gulf of Cambay and the Gulf of Kutch in Gujarat on the west coast were found to be the suitable sites with a tidal variation of 11m and 8m with average tidal range of 6.77m and 5.23m respectively. An average tidal variation of 2.97m was measured in the Sundarbans of Ganges delta. The identified economic power potential is of the order of 8000 MW with about

5 MW in the Gulf of Cambay, about 1200 MW in the Gulf of Kutch in the State of Gujarat and about 100 MW in the Gangetic Delta in the Sundarbans region in the State of West Bengal. Indian government has sanctioned a project for setting up a 3.75 MW demonstration tidal power plant at Durgaduani Creek in Sunderbans, West Bengal to the West Bengal Renewable Energy Development Agency (WBREDA), Kolkata. Figure 2.3 Tidal Barrage (Courtesy Of Encyclopaedia Britannica) Ocean Currents Ocean currents are mainly driven by tidal stream, blowing wind and solar heating of the waters close to the equator, the variation in water density and salinity may also lead to flow of sea water under the surface. The

6 17 constant flow and direction of ocean currents gives them an advantage of high convertibility and reduced investment, in contrast to the tidal currents where the varying gravitational pulls of the sun and moon result in variable current intensity and direction. Ocean currents in the Gulf Stream, Florida Straits Current, and California Current are some of the best examples (Figure 2.4). Ocean currents are mainly concentrated at the surface, although a significant current continues through the deeper sea. Figure 2.4 Major Ocean Currents (Courtesy of National Oceanic and Atmospheric Administration) A similar principal to the wind turbines can be used to extract energy from ocean currents by placing a submerged turbine. The turbine rotates due to the hydrodynamic lift and drag generated by flowing current at the vicinity of rotor blades. Coupling an electricity generating unit would produce electrical energy from the rotation of current turbines. As the density of water is roughly 700 times denser than air, the energy generated through current turbines is significantly higher than wind turbines even when the

7 18 current is much slower than wind. Presently there is no identified source of ocean current in the region of seas around India Ocean Thermal Energy Conversion Ocean Thermal Energy Conversion (OTEC) is an indirect conversion of solar energy in to electric power. The OTEC exploits the thermal gradient between surface water of ocean and the deeper water to produce electrical energy. The average variation in water temperature found between surface of ocean and water in 1000 m depth are around 20 0 C. The colder. Figure 2.5 OTEC Layout - (Bedard et al 2008) The Rankine cycle is used as method of effectively harnessing the available thermal gradient (Figure 2.5). Surface water treated like heat source and deeper water works like refrigerators. A working fluid with low boiling

8 19 temperature can be vaporized using the warm water at the surface, the vaporization leads to increased pressure. The expanding high pressure vapour with can be made to flow through a turbine and make it rotate. Once the work is done by expanded vapour, deeper cold water is pumped and the vapour is made condensed back to its liquid phase. The cycle is repeated continuously and the turbine is made to power the electricity generating unit. The weather). This makes the OTEC possible of producing electricity everyday and the whole day with nearly the same energy output. Three types of OTEC systems can be used to generate electricity: 1. Closed-cycle plants circulate a working fluid in a closed system, heating it with warm seawater, flashing it to vapour, routing the vapour through a turbine, and then condensing it with cold seawater (Figure 2.6). 2. Open-cycle plants flash the warm seawater to steam and route the steam through a turbine (Figure 2.7). 3. Hybrid plants flash the warm seawater to steam and use that steam to vaporize a working fluid in a closed system. Despite the advantage of availability and density, the OTEC has a very low Carnot Efficiency, near 7.5% (Bedard et al 2008). The present technology holds a maximum efficiency of 2-3% because of energy consumed by pumping and thermal losses. The present low efficient technology prevents the OTEC from becoming a suitable alternative to the present energy demand. National Institute of Ocean Technology (NIOT), Chennai, is in the process of commissioning a 1 MW OTEC plant off Tuticorin (2010 Survey of Energy Resources, 2010). The plant is designed to generate electricity using

9 20 the Rankine cycle with ammonia as the working fluid. The OTEC plant was mounted on a barge and it is named as Sagar Shakthi (Pursuit and Promotion of Science: The Indian Experience., 2001). A 1,000m long, 1m diameter HDPE pipe which will bring cold seawater of 7º C to the plant barge is already assembled and deployed at the OTEC site, approximately 60 km south east of Tuticorin harbour, and upended on reaching the site. It will be integrated with OTEC plant barge, when it reaches the OTEC site. Figure 2.6 Closed Cycle OTEC (Clerici et al 2004)

10 21 Figure 2.7 Open cycle OTEC (Clerici et al 2004) Ocean Wave Energy Winds are caused by differential heating of earth by sun and it generates water waves as it passes over ocean surface. In the wave generation process, wind transforms some of its energy to the waves and the wave starts moving in the wind direction. Conversion of energy available in the waves is more efficient than direct collection of energy from the wind, due to the fact that waves are a more concentrated form of solar energy than wind. It has been estimated that the theoretical wave energy potential is in the order of 1 to But there are many factors affecting wave energy conversion becoming a reality. Waves are not as consistent as tides and therefore there is a specific problem regarding the match of supply and demand. This is one of the main reasons why wave power has so far been restricted to small scale schemes; no large scale commercial plant is in action. The detailed discussion on various wave energy converters and its characteristics are made in chapter 3

11 OCEAN WAVE MECHANICS Origin of Ocean Wind Waves The energy in travelling waves is indirect form of concentrated solar energy. The energy comes from the sun through the winds as they blow over the ocean surface due to the differential heating of the earth. Transfer of energy takes place during the formation of waves from wind to the water ripples. The exact energy transfer mechanism is quiet complex and studied for long by researchers. follows: To make the understanding simple the process can be briefed as The wind waves are generated due to the wind blowing over the sea surface (Figure 2.8). They develop with time and space under the repetitive action of the wind and become huge waves called ocean surface waves. Variation in shear stresses and pressure fluctuations are created by turbulent air flowing on the sea surface. Figure 2.8 Generation of Wind Waves (Courtesy Thomson Higher Education 2007)

12 23 Further wave intensifies and development happens when these oscillations and fluctuations are in phase with the waves. Wind exerted force on the upward face of the wave directly affects the generated wave height; this causes further growth of waves. At each of the above steps leads to transfer of energy from wind to waves (Figure 2.9). The amount of energy transferred and hence the size of the resulting waves is a function of the wind speed, the length of time it blows and the distance over which it blows called the fetch. As the wave continues to grow the surface facing the wind becomes higher and steeper and the wave building process becomes much more efficient. However the wave steepness is limited and no longer the waves can grow beyond a steepness value. Steepness is the ratio of the height of the wave (distance between a crest and the following trough) to its length (distance between a crest and the following one) is approximately 1:7 in deep water. At the upwind end of the fetch the waves are small but with distance they develop i.e. their period and height increase and eventually they reach maximum dimensions possible for the wind that is raising them. In the process, the waves start getting its regular shape and the phenomenon is called as swell. The wave is said to be fully developed when the water particles excited by the wind follow a circular trajectories (Figure 2.10) with highest diameter at the ocean surface and diminishing exponentially with depth Description of Waves Ocean waves travel with different speed, amplitude and direction due to its highly random nature. The waves with different magnitudes superimpose on other waves and travel in ocean. Figure 2.11 shows superposition of monochromatic propagating waves of different amplitude, frequency and directions to represent a real ocean wave. Hence, a detailed description seems impossible and it becomes necessary to make some simplifications, which makes it possible to describe the wave pattern.

13 24 Figure 2.9 Transfer of Energy from Wind to Water Waves (Courtesy The Open University) Figure 2.10 Water Particle Motion in a Wave (Courtesy Thomson Higher Education 2007)

14 25 Figure 2.11 Illustration of Superposition of Waves (Beatty 2009) Waves are classified into one of the following two classes depending on their directional spreading: Long-crested waves: 2 dimensional waves with short crest and travelling perpendicular to the coast. Short-crested waves: 3-dimensional waves with short crest and travelling in different directions. The properties of deep water waves are summarised as follows (Twidell and Weir 2006):

15 26 The real ocean waves are sets of unbroken sine waves of irregular amplitude, period and direction. The motion of any particle in a wave is circular. Any particle in a wave has no net progression, whereas the surface form of the wave shows a visible progression. An ocean wave is rising and falling water level that is transmitted along the ocean surface. Ocean waves transmit energy away from an initial disturbance without the physical transport of water. Particles on the surface always on the surface during the wave progression. The magnitude of circular motion of water particle decrease exponentially with depth. The amplitude A of the surface wave is depends on the wind velocity c or period T of the wave, however the maximum amplitude of waves rarely exceed one-tenth of the wave length. A wave breaking happens when the slope of the surface is about 1 in 7, and the energy is dissipated in the form of heat. The patterns of rising and falling water of ocean surface are complex and ever changing. However, these patterns can be described by superposition of ideal waves. No two waves are similar in its height and they move across the surface at different velocities and in different directions. Though real ocean waves can never be uniform in practice, most of the ocean structures are investigated in regular waves due to the fact that all random waves are superposition of regular waves.

16 27 Figure 2.12 Wave Nomenclature (Figure 2.12) Following are the terms used to define the parts of a typical wave Crest: The highest point of a wave Trough: The lowest point of a wave. Wave height H : Vertical distance from trough to crest. Wave amplitude a : Vertical distance of wave crest from mean sea level Wave length L : Horizontal distance between adjacent crests. Wave period T : The time in seconds for a wave crest to travel a distance equal to one wave length. Wave steepness s : the wave period. Wave number k :

17 28 Wave celerity c : The ratio of wave length and wave period. Water depth h It becomes important to understand that there is a direct relationship between wave period and wavelength but wave height is independent of either. The wave are called as deep water waves when, the waves no longer influenced by presence of sea floor. These waves are relatively much slower and steady when compared to significant variations it show at the coast line. The wave bottom surface interactions leads to significant changes in the characteristics of deep water waves. Focusing, defocusing and sheltering of waves are some of the results of this sea floor interaction. In shallow water region, waves loose considerable amounts of its energy in the form of heat and sediment transportation due to bottom friction and reduced depth. As waves approach the shore (i.e. h longer considered to be deep water waves and, as such, they can be modified in various ways: Shoaling: This is a phenomena occurs to the wave when it reaches proximity to the shore line (Figure 2.13). During this approach, the wave group velocity decreases and thus the wave height increases. When the water depth decreases further, the wave steepness increases to a limit and the wave becomes unstable and breaks. The net energy and power density of waves increases in shallow water due to shoaling.

18 29 Figure 2.13 Shoaling Process (Courtesy the COMET Program) Friction and Wave Breaking: The raise in wave height and steepness produced by shoaling are compensated by wave breaking (Figure 2.15). As waves become steeper the top layer of wave moves with constant velocity and the bottom layer slow down due to the friction developed by closing sea floor in the shallow water. Beyond a limit, the top layers of waves break and create a turbulent water motion thereby losing both height and energy. Refraction: As the waves travel into shallow waters closer to the shore, the wave fronts are bent to become more oriented to the bottom contour coast line (Figure 2.14). This phenomenon is of great importance to the wave energy devices whose capture efficiency is wave directionality dependent. Diffraction and Reflection: The above phenomenon is similar to the refraction of light. There are other behaviour of the waves similar to the light such as diffraction (waves yielding around and barriers) and reflection. The variation in seabed topography and shoreline

19 30 geometry is the primary reason for this entire phenomenon. These behaviours lead to "hot spots" where the waves are focussed and its energy is concentrated than surrounding. Figure 2.14 Refraction Process (Courtesy the COMET Program) Figure 2.15 Impact of Sea Bed Topography on Waves (Courtesy Thomson Higher Education 2007)

20 Ocean Wave Climate Enough computer and mathematical modelling tools are available at present to estimate long term wave climate of any part of any coast given enough data and analysis. The meteorological study of wave climate is a complex task involving many variables. Wave energy is strongly variable in time and space, involving a lot of uncertainty; energy levels in the waves can be estimated and predicted as accurately as weather can be. An important factor to determine is the directional climate of long fetch swell that contains most of the energy. Figure 1.4 shows the average estimated wave power density (kw/m) at various locations around the world. The areas of the world which are subjected to regular wind fluxes are those with the largest wave energy resource. South westerly winds are common in the Atlantic Ocean, and frequently pass through extensive distances, transferring energy into the water to form the huge waves which arrive off the European coastline. There are a number of places around the world with high incident wave power levels, particularly suitable for wave energy exploitation. These are generally extreme latitudes (between 40Â - 60Â latitude in the North and South) and the west coast of continents including the western seaboards of South and North America, Northern Europe and Australia. These areas blessed with annual mean wave power per unit width of wave crest of kw/m. In tropical regions annual mean power levels of kw/m are more typical. In India the research and development activity for exploring wave energy started in Primary estimates indicate that the annual wave energy potential along the Indian coast is between 5 MW to 15 MW per meter. Hence theoretical potential for a coast line of nearly 6000 Km works

21 32 out to MW approximately. However, the realistic and economical potential is likely to be considerably less and 47 kw/m is available off Bombay during Southwest monsoon period. Based on the wave statistics for the southern tip of India, a mean monthly wave power of 4-25 kw/m is estimated. The average wave potential along the Indian coast is around 5-10 kw/m. India has a coastline of approximately 6500 km. Even 10% utilization would mean are source of MW. 2.4 ENERGY FROM WAVES Ocean waves are as mentioned above essential movement of energy. Waves consist of two kinds of energy: The individual water molecules are moving steadily and rather slowly in circular way, and this energy (kinetic energy) can be utilized in different kinds of wave energy converters, either directly via some kind of propeller or indirectly by Oscillating Water Columns wave energy converters. In its circular movement the individual water molecules are elevated above the Stillwater line and thus represent a potential energy Wave Body Interaction It becomes important to understand the various modes of motion a floating body experience due to wave action. The modes of motion are also called as degrees of motion of a body. For a floating elongated body (Figure 2.16), the modes of motion are named as shown in the Table 2.1. For an axisymmetric body or vertically oriented body, the modes 1 and 2 (and 4 and 5) are interchangeable and unclear. However, the ambiguity is eliminated when there is an incident wave where the propagation direction defines the x direction. For a two dimensional case, the axis y is eliminated

22 33 and hence an axisymmetric body experiences only three modes of motion namely, surge, heave and pitch. Figure 2.16 Six Modes of Motion of an Elongated Floating Object (Falnes 2004) Table 2.1 Various modes of motion of a floating body Mode Number Mode Name 1 Surge 2 Sway 3 Heave 4 Roll 5 Pitch 6 Yaw Wave Energy Absorption by Oscillating Bodies The energy absorption by an object interacting with waves is directly related to the capability of radiating waves and creating destructive interference with the incident waves. An object oscillating in water body will create waves. A small body and big body may produce equally large waves, provided the oscillation of small body is very large than the big body. This phenomenon is utilised for the purpose of harnessing ocean waves. If a small

23 34 body is arranged such that it oscillates with larger amplitude than incident wave, the device can be as efficient as devices with larger oscillating bodies. In general a good wave absorber should be a good wave maker. To absorb wave energy, the device should displace water in an oscillatory manner in correct phase(interval). To absorb energy from waves the energy should be removed from it. Hence, the passing wave must be cancelled or reduced in its energy level. Such a cancellation or reduction of wave can occur only if the device generates waves which oppose (are in counter-phase with) the passing wave. In other sense, the generated wave should destructively interface with passing wave. The paradoxical but true statement that "Destroy a wave means to create a wave". The Figure 2.17 Interaction of Oscillating Body and Wave (Falnes 2004) illustrates the possibility of 100% energy absorption by small oscillating floating bodies infinitely long (perpendicular to the figure), evenly interspaced a small gap (smaller than incident wave length). It is possible to extract 100% of energy available at waves with an elongated body, of cross section as illustrated in figure, and aligned perpendicular to the representative image, provided the body oscillates in two linear modes (horizontal and vertical) in an optimum manner. Figure 2.17 Interaction of Oscillating Body and Wave (Falnes 2004)

24 35 In the above figure, the profile a represents an undisturbed incident wave. The curve b represents the generated wave by the oscillation of evenly spaced infinitely long array of small floating bodies in heave (up and down). This shows that only 50% of energy can be absorbed by any floating body when it oscillates only in heave. The curve c represents the anti-symmetric wave generation by oscillation of array of small floating bodies. This case shows the possibility of more than 50% of energy absorption by radiating anti-symmetric wave. The superposition (addition) of all the above curves is represented by curve d, which shows a complete absorption of the incident wave energy. However, it is possible to absorb almost all the incident wave energy in one mode of motion by a sufficiently non-symmetric body. Another illustrative example is shown in Figure 2.18, where a heaving point absorber has to radiate circular waves which destructively interface with incident regular wave to harness energy available. A point absorber is essentially a small device with horizontal dimension much less than incident wave length. It was shown by (Falnes 2004) that the maximum energy absorbed by a heaving axisymmetric body equals the wave transported by the incident wave front of width equal to the w The author named this width as absorption width of a device. Figure 2.18 Radiated Waves by Small Heaving Body in Incident Wave Field (Falnes 2004)

25 36 It becomes imperative that an optimum oscillation is necessary for an oscillating body to increase its energy capture efficiency. For incident wave sinusoidal characteristics there is an optimum amplitude and period for oscillation. This can be emphasised by the curves b and c of figure, where the radiated waves have to be exactly half of the amplitude of incident wave. This action requires the oscillating body to be having a precise value of horizontal and vertical displacement. In order to have optimum phase conditions for the two modes of oscillation, the radiated waves towards right have to have the same phase with each other. The same phase here refers to the matching of wave trough and wave crest. This also means that the symmetric and antisymmetric radiated waves destructively interface with each other towards the left. In addition to all these requirements, the phase of two oscillations should match with phase of incident wave, because the crest of both the radiated waves (b and c) must coincide with the trough of the incident wave (curve a). To be efficient any WEC should have the buoy oscillating in more than one mode of motion and work in resonance with the incident wave. The motion of oscillating body naturally becomes in phase with incident wave during the resonance condition. 2.5 SIGNIFICANCE OF OCEAN WAVE ENERGY the unexploited vast energy resource, there are a number of technical challenges that need to be addressed before. The following are the common advantages and challenges faced by a wave energy conversion technology to be commercially viable alternative.

26 Advantages The most important advantage of ocean wave energy is that it is renewable energy source with completely free energy where no fuel is needed and there is no problem with greenhouse gas emissions or other pollutions like conventional energy conversion technologies. As the waves are caused by winds it has the predictability in advance. This gives the wave energy devices an advantage of preparing for operational shutdown. Any energy conversion technology is assessed by its utility factor, which is the ratio between rated power and average energy production. The utility factor is significantly low for any renewable energy conversion technology due to the natural fluctuation of the source. The wave energy is one of a few renewable energy sources with largest utility factor. Due to the reason that the wave energy device is located off the shore and away from main land, these devices get low negative interaction with surrounding. The low visual impact from the shore gives the wave energy devices an advantage of having easy public acceptant. Ocean wave energy is top in the list of renewable energy sources with highest availability. It was reported that the waves are available for 90% of the time. Wave energy conversion technology is having a scope for high economic competitiveness when compared to other renewable energy technologies as it is not still matured and a best technology has not yet arrived.

27 38 The running cost for a wave energy device is much lower after its initial construction due to the freely available abundant wave energy resource. The only cost involve in the energy generation will be for general maintenance Challenges challenges that need to be addressed to obtain commercial competitiveness of wave power devices in the global energy market. Conversion of slow moving, random and highly intensive waves to fuel an electricity generating unit which requires a high speed uniform mechanical rotation needs a lot of engineering. Generated electricity will be accepted by any utility network only if the supplied power is with predetermined quality level. Wave energy level varies wave to wave due to variation in their height and period. Though the average wave climate can be predicted in advance, predictability of individual wave parameters are highly technology demanding and which is being researched all over the world. The wave energy devices are required to be orienting itself to the incoming wave as the waves in the deep waters are highly variable in direction, amplitude and phase. As installing a stable platform is quiet expensive, the off shore devices are mostly a floating type devices where directional control is complicated. The disparity between most occurring waves and extreme waves are so huge, that the device faces a tremendous economic challenge. To have high efficiency, the device needs to be designed for average wave climate. However, the device also has to withstand during the extreme wave conditions that occur rarely. Not only does this problem pose engineering difficulty, but it also requires high capital and running expenses.

28 39 The highly corrosive nature of the ocean environment pose a serious challenge to the life cycle of any ocean based devices. The marine growth is an another threat to the ocean surface based man made devices where the growth can be up to 10cm thick in less than a year. Access to off shore structures require boats and helicopters which is an uneconomic method of transportation. The installation and maintenance of off shore device require divers and skilled personal which leads to. Significant variation is annual average wave energy potential between different parts of the world motivates only a very few countries to be interested in exploiting waves. Even a matured technology can be economically competitive to a few countries which possess high average wave energy density. Apart from the cost of fabrication, installation and maintenance of a wave energy conversion device, the energy transportation cost significantly contributes in overall cost of the device. The energy transportation concerns on deployment of underwater cables and energy losses during the transmission. The lack of experience in world wave energy research community in designing off shore device will result in over-sizing of equipment and increased investment cost. There have been a tremendous amount of research publications made based on the hydrodynamic, mathematical and computer modelling of various wave energy devices to improve the performance. However, a detailed overall development and study is required to arrive a technology which can be made suitable for any wave climate and hence the wave energy conversion can be attractive to the entire world.