WAVE ENERGY

                         ABSTRACT 

Due to economic social cohesion, the United Nations is promoting to improve the production of electrical energy from renewable energy sources. Sea waves have associated a form of renewable energy which can be captured by using a hydro mechanical device that in turn drives an electrical generator to produce electrical energy. After a brief description of wave formation and quantifying the power across each meter of wave front associated to the wave, the paper describes several devices used presently to extract mechanical energy from the waves and their advantages and disadvantages are presented as conclusions. In particular, the modern Pelamis system is described in some detail. Wave energy market is also discussed.

INTRODUCTION 

The oceans, large lakes and bays are huge reservoirs of various useful and renewable energy sources. World’s total estimated ocean energy reserves are about 130 x 10⁶ MW. However, only a fraction can be recovered economically.

                                  Due to rapidly depleting fossil fuel sources, the ocean energy is likely to gain a significant importance during coming decades. However, its present use is very limited.

Ocean energy sources are broadly divided into the following four categories:

• 1.     Tidal energy
• 2.     Wave energy
• 3.     Ocean thermal energy conversion(OTEC)
• 4.     Hydroelectric energy

             Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work. Sea waves are a very promising energy carrier among renewable power sources, since they are able to manifest an enormous amount of energy resources in almost all geographical regions. The global theoretical energy from waves corresponds to 8x106 TWh/year, which is about 100 times the total hydroelectricity generation of the whole planet. To produce this energy using fossil fuels it would result an emission of 2 millions of tones of CO2. This means that wave energy could contribute heavily for the attenuation of pollutant gases in the atmosphere, as defended by the Kyoto Protocol.

                        The global wave resource due to wave energy is roughly 2 TW and Europe represents about 320 GW, which is about 16% of the total resource. However, for various reasons, it is estimated that only 10 to 15% can be converted into electrical energy, which is a vast source of energy, able to feed the present all world. Eventually, wave energy could make a major contribution by yielding as much as 120 TWh/year for Europe and perhaps three times that level worldwide. The ocean is a true store of renewable energy.

Our further discussion will be concentrated over wave energy and it’s generation, development etc. 

• WHAT IS WAVE ENERGY?

    Wave energy comes from the interaction between the winds and surfaces of oceans. The energy available varies with the size and frequency of waves. It is estimated that about 50 KW of power is available for every metre width of true wave front.

                                 Ocean wave energy is due to the periodic to-and-fro, up and down motion of water particles in the form of progressive waves. The period of ocean waves is the order of a few seconds. Ocean waves are superimposed on ocean water. Ocean water surface level varies with ocean tidal cycle.

               Ocean waves possess potential energy and kinetic energy. The ocean waves originate in different parts of the ocean surface due to the surface winds. The waves travel in the direction of the wind to the shore. The waves may be due to the local winds or the planetary winds. The height of the waves depends upon the wind velocities, depth of the ocean, contour of the shore etc.

·       Ranges:

The typical ranges of ocean waves are---

       I.   Wind height = 2 x amplitude = 0.2 m  to 4 m
     II    Wave period = 4 to 12 sec.

Very dangerous and destructive waves occur during storms and gusts. They may reach heights of 10m and topple ships and damage the ocean energy plants.

• SEA WAVES FORMATION

   The combination of forces due to the gravity, sea surface tension and wind intensity are the main factors of origin of sea waves. Figure 1 illustrates the formation of sea waves by a storm. Wave size is determined by wind speed and fetch (the distance over which the wind excites the waves) and by the depth and topography of the seabed (which can focus or disperse the energy of the waves). To distances far from the fetch, sea waves have a regular shape and the phenomenon is called swell.

           The water particles excited by the wind have in each location of the ocean circular trajectories with highest diameter at the surface and diminishing exponentially with depth. The conjugation of this circular motion is responsible for the wave formation and respective propagation, as shown in figure 2.                                  

                The distance between two consecutive crests, or two consecutive troughs, defines the wavelength λ. Wave height H (crest to trough) is proportional to wind intensity and its duration. The wave period T (crest to crest) is the time in seconds needed for the wave travellers the wavelength λ and is proportional to sea depth. The frequency f = 1/T indicates the number of waves that appears in a given position. Consequently the wave speed is v = λ/T = λ/f. The ratio λ/2H is called the wave declivity and when this value is greater than 1/7 can be proved that the wave becomes unstable and vanishes. Longer period waves have relatively longer wavelengths and move faster. Generally, large waves are more powerful.

POWER ASSOCIATED TO A SEA WAVE

Ocean waves transport mechanical energy. The power associated with a wave of wavelength λ and height H and a front b is given by

                                          P= (1/2) ρgH² λ b -------------------------- (1)

Where ρ is the water specific weight and g is the gravity acceleration. The power across each meter of wave front associated to a uniform wave with height H (m) and wavelength λ (m) is then;                                          

                                        P_u =P/b= (1/2) ρgH² λ---------------------- (2)

                                                                                                          And is expressed in W/m.

            During a “tsunami”, waves far from the beach have long wavelength λ₁ a small height H₁ but great power. When the waves propagate into the beach the power is kept almost constant (neglecting friction) and the wavelength decreases to λ2. Therefore, Eq. (2) shows that the height of the wave must increases to square H₂² in order to maintain Pu constant, as illustrated in figure 3. These big waves have devastated effects on the beach!

                     For irregular waves of height H (m) and period T (s), an equation for power per unit of wave front can be derived as

                                   P_i ≅ 0.42 H² T ---------------------------------- (3)

And is expressed in kilowatts per meter (kW/m) of wave front.

                  It is significant to note that wave power varies with the square of wave height. Then, when wave height is doubled generates four times as much power [1]. Excluding waves created by major storms, the largest waves are about 15 meters high and have a period of about 15 seconds. According to the Eq. (3), such waves carry about 1700 kilowatts of potential power across each meter of wave front. A good wave power location will have an average flux much less than this, perhaps about 50 kW/m. The Atlantic waves along northwest coast have an average value of 40 kW/m.

•  FACTORS EFFECTING WIND ENERGY:

WIND ENERGY: 

With the increase in wind speed, there is an increase in wind energy.

§    The amplitude of the waves depends on wind speed.

§    During storms and gusts, big ocean waves occur, which prove highly detrimental even to ships.              

       EFFECTIVE PITCH VALUE:

 It is the uninterrupted distance on the ocean over which the wind can below before reaching the point of reference. The larger the distance, the higher the wave energy. This distance may vary from 5 km to 45 km.

  DEPTH OF OCEAN WATER:

                           

The greater the depth of ocean water the higher the wave velocity. Very large energy fluxes are available in deep ocean waves.

  v WORLD RESOURCE OF WAVE POWER

Wave energy is unevenly distributed over the globe. Figure 4 shows an Atlas of the global power density distribution of the oceans where the numbers indicate kW/m. The north and south temperature zones have the best sites for capturing wave power. The prevailing winds in these zones blow strongest in winter. Increased wave activity is found between the latitudes of 30° and 60° on both hemispheres, induced by the prevailing western winds blowing in these regions The oceanic wave climate (i.e. far offshore) offers enormous levels of energy. As waves approach the shore, energy is dissipated, leading to lower wave power levels on the shoreline. Therefore, the energy availability is sensitive to location and the distance from the shoreline.

  v TYPES OF WAVE POWER MECHANISMS

The sea wave’s motion can be converted into mechanical energy by using proper wave power mechanisms. There are currently about 40 types of mechanisms for exploiting the energy available in waves, several of which are now being constructed. These devices are generally categorized by location installed and power take-off system. Locations are shoreline, near-shore and offshore. Power take-off systems can be oscillating column of water, underwater pneumatic systems, wave dragon system and oscillating bodies system. Also these mechanisms can be lying on the bottom of the sea, on the shoreline and on sea level. Description of these systems is following presented.

      Ø SHORELINE LOCATIONS

ü  OSCILLATING WATER COLUMN

·   This system consists of a chamber built in shoreline cost with the layout shown in figure 5. The motions of ocean/sea waves push an air pocket up and down behind a breakwater. Then the air passes through an air turbine.

·                             Next, when the wave returns to the sea, an air depression will circulate through the turbine in the opposite sense. However, this turbine has been designed to continue turning the same way irrespective of the direction of the airflow .This is a rectifier Wells turbine type, designed by Professor Alan Wells of Queen's University, which drives an electric generator mounted on the same shaft, as illustrated in figure 6.

·       To control the air pressure inside the camera a valve in parallel (sometimes in series) with the turbine is used.

·       The generator delivers power into the grid with constant frequency and rms voltage.

·       Because the turbine rotates with a variable speed a synchronous machine is not appropriate.

·       A prototype of 40 kW using an asynchronous generator was installed in Pico Island, Azores, Portugal, and an optimal overall efficiency of 35 % was claimed.

·       The problem with this pneumatic system is that the rushing air can be very noisy, unless a silencer is fitted to the turbine. But the noise is not a huge problem anyway, as the waves make quite a bit of noise themselves.

   

  ü  PENDULUM SYSTEM

The Pendulum system is also installed in the shoreline and consists of a parallelepiped concrete box, which is open to the sea at one end, as shown in figure 8. A pendulum flap is hinged over this opening, so that the actions of the waves cause it to swing back and forth. This motion is then used to power a hydraulic pump and an electric generator.

      Ø NEARSHORE LOCATIONS

  ü  OFFSHORE WAVE DRAGON SYSTEM

       

 Wave Dragon System is a floating slack-moored energy converter of the overtopping type that can be displayed in a single unit or in arrays. Groups of 200 Wave Dragon units result in a wave power park with a capacity Comparable to a traditional fossil fuel based power plant.

·       The Wave Dragon system was the world’s first near shore wave energy converter producing power for the grid.

·       The basic idea of this system consists of two large "arms" that focus waves up a ramp into a reservoir. The water returns to the ocean by the force of gravity via a low head hydro turbine which drives an electric generator. Figure 9 illustrates this principle

·       Wave Dragon is a very simple construction and only the turbines are the moving parts. This is essential for any device bound for operating nearshore where the extreme forces seriously affect any moving parts.

·       In comparison with traditional hydroelectric power stations, this new technology is competitive. Figure 10 shows a photograph of the Wave Dragon system installed nearshore The Wave Dragon concept combines existing, mature nearshore and hydro turbine technology in a novel way

·       . Due to its size service, maintenance and even major repair works can be carried out at sea leading to low cost relative to others systems.

      Ø OFFSHORE LOCATIONS

ü  POWER BUOY

           This system utilizes the Power Buoy technology which consists of modular ocean-going buoys, as shown in figure 11.The rising and falling of the waves moves the buoy-like structure creating mechanical energy which is converted into electricity and transmitted to shore by means a secure, undersea transmission line.

ü  SALTER’S DUCK SYSTEM

·       One of the first methods to extract mechanical energy from the waves was invented in the 1970s by Professor Stephen Salter of the University of Edinburgh, Scotland, in response to the Oil Crisis.

·        A cross section of the Salter cam (or Duck) is shown in figure 12 and can be moored, to distances of 80 km of the cost. The cam rotates about its axis and is shaped to minimize back-water pressures. Conversion of the float movement into electrical energy is difficult because of the slow oscillations.

·       The costs and risks of a full-scale machine would be high.

  ü  PELAMIS WAVE ENERGY CONVERTER

·       The Pelamis Wave Energy Converter, a Scottish invention, consists of six articulated cylinders of 3.5 m in diameter and 30 m in length (floaters) as shown in figure 13.

·       Due to the waves, this structure up and down and side to side as a sea snake (Pelamis in Greek)                     

·       The structure is secured by flexile cables fitted to the seabed in such way that the float axis is oriented in the predominant wave direction. Figure 14 shows the Pelamis structure anchored to the seabed.

·        This long, hinged tube as the hinges bend, they pump hydraulic fluid creating pressurized oil to drive a hydraulic motor that drives an electric generator, mounted inside the 5 m floating power module, as shown in figure 15

·      
 Each of these four modules has a 250 kW electric generator giving a total power of 750 kW for each Pelamis unit. A 10 kV three phase power transformer is situated in the front floater and send the electric energy across underwater power cables to a substation in land. Figure 16 shows an association of various numbers of units constituting a wave farm [6].

·        A wave farm utilizing Pelamis technology was recently installed in Aguçadora Wave Park, about three miles off Portugal's northern coast, near Póvoa do Varzim, a total power of 2.25 MW. Other plans for wave farms include a 3MW array of four 750 kW Pelamis devices in the Orkneys, off northern Scotland, and the 20MW Wave hub development off the north coast of Cornwall, England

ü  WAVE ROLLER SYSTEM

                  The Wave Roller System is a plate lying on the bottom of the sea, whose back and forth movement caused by bottom waves is collected by a piston, as illustrated in figure 17. The piston compresses oil to power a hydraulic motor, which drives in turn an electric generator to produce electrical energy This is a typical undersea system used because the bottom waves are more continuous and predictable than surface waves. Figure 18 shows an array of these floating plates placed on seabed. Invisible from the surface, the system has a low environmental impact. Contract discussions to install the Wave Roller System are already underway in Finland.

ü  BRISTOL CYLINDER

·       The Bristol Cylinder consists in a floating cylinder that collected the wave’s movement. The cylinder is mechanically connected to the energy unit by flexible joints and rods. The rods are moving slowly with cylinder and the reciprocating motion is transferred to the axels in converter unit. This converter unit, called Escone, after his inventor Esko Raikano, is the heart of the system and converts the reciprocating motion to a rotating shaft connected direclty to a generator for generating electrical energy with high efficiency.

·       For the energy unit a suitable slow speed generator will be needed. When transferring converter movements with mechanical arms and rotation to the generator the efficiency should be kept as high as possible. The Bristol Cylinder operates under the sea level as shown in figure 21.

·       Two or more Bristol cylinders could be connected in parallel. It is also possible to make wave parks near shore or wind power units connected together like float offshore. In offshore the converter parts can be located above the sea level and the collector rotation just under the sea surface. This method of collector wave energy is in the process of pending patents in Finland.

v ENVIRONMENTAL IMPACT:

Wave energy converters (WEC’s) have effects on the adjacent coastline even though they are out at sea --------------

ü  Wave Distortion

·       WEC’s have the effect of significantly lengthening the wavelength and reducing the height of the waves, which pass through them.

·       In fact, calculations taken in force seven storm conditions showed wave heights reduced from 5.9m to 4.6m while the wavelength increased from 8.4s to 9.0s.

·       This results in coastlines being less affected by the erosion, typical of short, steep waves, and more affected by the building up of debris caused by long, shallow waves.

·       This ‘side effect’ is seen as beneficial in some areas this is because the increased amount of sediment on the beaches reduces the impact of storm waves on the shore, and the communities near the shore.

·       However this effect may not be seen as beneficial to other coastlines, and so must be considered carefully

ü  Effects on ecology:

·       As well as affecting the waves, WEC’s can alter ocean surface currents.

·       This will have no effect on benthic dwelling fish, but the many species of fish in the surface waters (e.g. salmon) which depend on currents to navigate between spawning and feeding grounds, could experience difficulties completing their life cycles which could not only affect the ecosystem but the fishing industry as well.

·       This could be minimized by ensuring that WEC’s are positioned in places in which the sea life in the ocean surface is well researched and understood, and that WEC numbers or size do not adversely contribute to the distortion of surface currents

ü  Effects on shipping and navigation:

·       The risk of collision with WEC’s is the primary hazard to shipping due to their design making them difficult to detect by both eye and by radar.

·       For this reason warning lights and clearly marked channels to ensure the safe navigation around converters are essential.

·       To a great extent all shipping would have to be excluded from the immediate vicinity of the devices.

·       This will have varying levels of interference depending on the location of a WEC.

·       These effects on shipping will have consequences for the fishing industry as boats may be excluded from prime fishing grounds.

  v ADVANTAGES AND DISADVANTAGES

      Ø ADVANTAGES:

§  It is relatively pollution free.

§  It is free and renewable energy source.

§  After removal of power, the waves are in placed state.

§  Wave power devices do not require large land masses.

§  Whenever there is a large wave activity, a string of devices have to be used. The system not only produces electricity but also protects coast lines from the destructive action of large waves, minimise erosion and help create artificial harbour.

      Ø DISADVANTAGES:

§  Lack of dependability.

§  Relative scarcity of accessible sites of large wave activity.

§  The construction of conversion devices is relatively complicated.

§  The devices have to withstand enormous power of stormy seas.

§  There are unfavourable economic factors such as large capital investment and costs of repair, replacement and maintenance.

  v PROBLEMS ASSOCIATED WITH WAVE ENERGY COLLECTION

§  The variation of frequency and amplitude makes it an unsteady source.

§  Devices, installed to collect and to transfer wave energy from far off oceans, will have to withstand adverse weather conditions.

v THE INDIAN WAVE ENERGY PROGRAMMES:

§  The Indian wave energy programme started in 1983 at the Institute of Technology, Madras and has concentrated almost exclusively on the OWC concept.

§  The initial research conducted by the Wave Energy Group, IIT Chennai focused on the choice of the wave energy device (Raju, Ravindran 1987, Raju, Ravindran 1989).

§  Based on studies on three types of devices, namely, double float system, single float vertical system, and the OWC principle, it was concluded that the OWC showed the maximum promise for India.

§  Consequently, development activities were concentrated on this device alone.

§  The research on wave energy in India as achieved a commendable status within a decade.

§  A caisson was constructed in December 1990 at Vizhinjam and two generations of power modules have been tested as of today.

§  Efforts are on to make the technology cost-effective.

§  A 150 kW prototype OWC With harbor walls was built onto the breakwater of the Vizhinjam Fisheries Harbor, near Trivandrum in India.

§  Following the successful testing of this, it is proposed to build a commercial scheme of 10 caissons, each 21 m wide, at Thangassery, on the west coast of India.

§  Each caisson will have two power modules, both with a 55 kW rating, leading to an overall rating of 1.1 MW.

  v CONCLUSIONS

                        Wave energy is not expensive to operate and maintain, no fuel is needed and no waste is produced. However, it depends on the intensity of the waves and needs a suitable site where waves are consistently strong. The infrastructure must be able to withstand very rough weather. Wave power lies not in huge plants but in a combination of on-shore generation and near-shore generation (using a different technology) focused on meeting local or regional needs. If this system prove to be economically possible, only 0.1% of the renewable energy within the world's oceans could supply more than five times the global demand for energy, The Pelamis Wave Energy Converter is a revolutionary concept resulting from many years of engineering development. It was the world’s first commercial scale machine to generate electrical energy into the grid from offshore wave energy and the first to be used in commercial wave park projects. In Portugal, Pelamis System is now proving to be successful.

  v REFERENCES

ü  NON CONVENTIONAL ENERGY SOURCES AND UTILISATION

BY Er. R.K. RAJPUT.

ü  LEÃO RODRIGUES  , NOVA UNIVERSITY OF LISBON

ü  “RENEWABLE ENERGY”, ELSEVIER ACADEMIC PRESS, 2004 EDITION

ü  B. WEEDY AND B. CORY: “ELECTRIC POWER SYSTEMS”,WILEY, FOURTH EDITION, LONDON 1998.