RESEARCH ARTICLE

Arunachalam AMARKARTHIK, Srinivasan CHANDRASEKARAN, Karuppan SIVAKUMAR, HarenderSINHMAR

Laboratory experiment on using non-floating body togenerate electrical energy from water waves

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012

Abstract This paper describes an innovative method ofusing a non-buoyant body to harness ocean waves. All thepoint absorbers are buoyant in nature and move up due tobuoyancy and come down because of gravity. The pointabsorbers are designed to move along the waves to makethe device efficient. These devices face excessive stressduring the rough weather on account of the extreme motionof waves and cause the total device failure. The presentstudy shows that using a non-buoyant body for conven-tional point absorber principle is much efficient and saferthan any other device proposed till today. A small scalewave energy converter with non-buoyant body wasdesigned, fabricated and tested in small scale wavemaker. An electrical generator was coupled with thedevice to generate electrical energy from harnessed waves.The generator was electrically loaded and the generatedpower was measured. It was found from the experimentsthat the proposed device showed a significant improve-ment in electricity generation and safety during extremeconditions. In addition to the electricity generation, thecharacteristics of the device were also studied by usingvarious wave and device parameters.

Keywords ocean wave energy, point absorbers, heavingbody, non-floating object, heave response ratio, electricalenergy generation

1 Introduction

One of the most difficult design problems faced by the

wave energy device designers is the disparity betweenwave conditions during normal operating conditions andthose during rough weather conditions. In most of thecases, designers provide mechanical, structural or otherforms of damping to prevent the point absorbers fromhaving large heave motions. Providing damping requiresheavy structures and systems to withstand the extremestresses developed by waves during stormy conditions. Tobe economically viable, wave devices must generate usefuloutput in the wide range of operating conditions. Makingthe device more robust enables the device to efficientlyharness waves during normal wave climate. The proposednovel method addresses these issues in different perspec-tive and provides high electrical energy conversion andsafety.

2 Point absorbers in heave

Amongst the existing devices reported in the literature,heaving point absorbers are common; the proposed deviceis developed on a similar principle. Critical review of theexisting mechanisms is carried out to compare theadvantages of the newly proposed device. Most of theheaving point absorbers are designed for offshore applica-tions; they are either submerged in water or floating on thesurface [1]. As the point absorbers are small in dimensionin comparison to the average wave length, they arerestricted to heave motion only. A rotary, direct-drive waveenergy device proposes a ball screw arrangement forconverting heave motion of the float into rotation [2]; floatstores energy during its upward stroke and supplies to theinternal system during down stroke. A full scale prototypeof linear generator, mounted on sea floor is connected to afloating buoy for harnessing wave energy [3]. Spring storesenergy during upward stroke of the piston as the wave crestpasses. An improved device employed deep-draught sparand annular saucer shaped buoy that is restricted to heavemotion with respect to the spar; spring is loaded to restore

Received July 5, 2012; accepted September 7, 2012

Arunachalam AMARKARTHIK (✉), Karuppan SIVAKUMARBannari Amman Institute of Technology, Erode 638401, IndiaE-mail: a.amarkarthik@gmail.com

Srinivasan CHANDRASEKARAN, Harender SINHMARIndian Institute of Technology Madras, Chennai 600036, India

Front. Energy 2012, 6(4): 361–365DOI 10.1007/s11708-012-0210-1

the forces for down stroke [4]. A mechanical type of floatand counter weight system of wave energy uses a surfacefloating buoy that is kept hanging by a cable; counterweight is attached to the other end of the cable. The cable isrunning through a pulley, enabling it to rotate when thefloat heaves. Cables slag during upward stroke of the buoyand hence the generator gets disconnected through aratchet mechanism [5]. Archimedes wave swing (AWS) [6]is a completely submerged linear generator type waveenergy converter built at the Portuguese coast. The devicecomprises of a floor-mounted cylindrical chamberenclosed by a floater which moves relative to the cylinderwhen encountered by waves. The air inside the cylinderacts as a spring and provides restoring force to the floater.Linear generators installed inside the cylinder produceelectrical energy from the heave movement of the floater. Asimilar device was also employed on heaving body waveenergy conversion and its performance is analyzed byadding a supplementary mass [7]. A buoyant floating buoyconnected by metal cable with a linear generator mountedon sea floor is disclosed [8], a spring is used to pull the floatdown during the wave trough. In 2006 a latching control isproposed [9] to limit the heave response of buoy duringextreme wave climate. In 2009 the concept of changingnatural period of the device by adjusting float draft withupper surface immersion was presented [10].All these devices consists of a single floating buoy or

two bodies combined together working with the similarprinciple except in shape of the buoy and the design ofpower take off system. Incident wave crest pushes thefloating buoy up due to the buoyancy force and dropsbecause of gravity during the approach of wave trough.Wave energy is stored during the upward motion of float aspotential energy or spring tension and released to thepower take off during the down stroke. Further, these floatmotion is transferred to mechanical, hydraulic, pneumaticor direct power takeoff system to produce electrical energy.This action makes these devices to harness waves onlyduring upward motion and release the stored energy duringdown stroke. To maximize power capture, designers havedesigned the devices with response amplitude ratios (ratioof float displacement to wave amplitude) greater thanunity, so higher float motions will occur during operationalconditions. For economical reasons, it is desirable toensure that the float response does not exceed theallowable limits during extreme conditions; since, theeffect of extreme load is considered as failure mode. Toprevent the device from having extreme responses,structural, mechanical, hydrodynamic and other forms ofdamping are proposed by designers. The extreme nature ofwaves makes any form of damping a failure and leads tothe total device failure.The proposed device addresses these problems from a

different perspective and eliminates the issue of excessiveheave response during extreme wave climates without anyform of damping and with increased conversion rate than

any other point absorbers. The devise also provides a fewadditional advantages like simplified installation anddevice shutdown during device maintenance.

3 Non-floating body for heave response

The proposed wave energy converter is a near shore deviceto be kept on a rigid platform. The device consists of acylindrical non-floating object, an oscillating arm, aunidirectional gearbox to convert alternative rotationsinto continuous unidirectional rotation, a step-up gearboxand an electricity generating unit.Figure 1 shows an experimental setup designed,

fabricated and tested in a small scale wave maker. Themodel is designed to conduct the experiment at 2 m waveflume at Indian Institute of Technology Madras. A 0.7 mhigh, 0.3 m diameter water filled steel container is used asthe non-floating body and kept hanging from one end ofthe 2 m long steel arm. A 6 mm metal rope is used to hangthe container. Solid weights are used as counter massinstead of ballasting tank for the experimental purpose.The arm is supported by a rotatable shaft and the assemblyis mounted on a base plate. The said shaft is coupled with aunidirectional gearbox which converts alternative rota-tional input into continuous rotation. Further, an electricalgenerator is coupled with the unidirectional gearboxthrough a step-up gearbox.

3.1 Oscillating arm assembly

An oscillating arm consists of a straight frame pivoted at itsmiddle, a non-floating body kept hanging at one end viametal rope and a counter mass assembly at another end. Awater filled steel container is used as the non-floatingobject in the present case and metal plates are used ascounter mass.

Fig. 1 Experimental setup at IITM

362 Front. Energy 2012, 6(4): 361–365

3.2 Unidirectional gearbox

There are many different methods to convert both positiveand negative directional rotation into continuous unidirec-tional rotation. The proposed unidirectional gearbox is alsoone such unique technology which provides energy in theform of continuous unidirectional rotation from alterna-tively rotating arm shaft. The unidirectional gearbox has aninput gear and output gear. The output gear providescontinuous rotation for the further conversion.

3.3 Step-up gearbox

Because the frequency of ocean waves is very low, thespeed of rotation from the unidirectional gearbox will bemuch lower than what is required by any conventionalgenerator. Hence another gearbox is coupled with theunidirectional gearbox to increase the speed of the rotation.

3.4 Electrical generator

A conventional rotary type electrical generator is coupledwith the output shaft of the step-up gearbox to convertmechanical rotation into electrical energy.

4 Working of heaving non-floating bodytype wave energy converter

Figure 2 presents the schematic drawing of variouscomponents of oscillating arm.

4.1 Initial condition

The entire setup is to be mounted on a platform such thatthe hanging water container is completely immersed inwater. When the counter mass is loaded, the containerstarts to surface, as an action of balancing the additionalforce exerted by the counter mass. The cable length isadjusted such that the arm is horizontal after the exposedheight of the container reaches the required level. Once allthe initial arrangements are made, the arm will be inequilibrium due to the balancing of effective mass of thesemi immersed container (m) and counter mass (M).

4.2 Working

When incidental wave passes the semi immersed container,the effective mass (m) of the container gets reduced due tothe increase in surrounding water level and the armbecomes unbalanced between its two ends. The countermass (M) pulls the container up as an action of balancing.This action makes the arm to oscillate in one direction andwhen the wave trough approaches the container, theeffective weight of the container increases on account ofthe decrease in water level around it. This, in turn, makesthe container side heavy and pulls the counter mass sideup. This alternative balancing of forces makes the arm tocontinuously oscillate with respect to the point O.This oscillation of arm makes the input gear of

unidirectional gearbox to rotate alternatively and theunidirectional gearbox produces unidirectional rotation atits output shaft. This low speed high torque unidirectionalenergy is converted into high speed rotation by the step-upgearbox and converted into electrical energy by the rotaryelectrical generator.

4.3 Uniqueness

The following are the features which make the proposeddesign very unique and highly efficient. Oscillation of theheaving body is caused by the variation in effective massof the container during wave action rather than by thepushing action of the wave crest. The maximum oscillationcan be limited by changing the water in the ballasting tank.The heaving mass is connected via cable with the deviceand hence extreme forces will not be transferred to theinternal components of the device. The device harnessesboth the up and the down motions of the waves to produceuseful energy but all conventional systems harnesses inone stroke and uses the stored energy during the nextstroke.

5 Experimental results

An experimental setup is designed and fabricated to test inthe 2 m deep wave flume at Indian Institute of TechnologyMadras. Preliminary studies were conducted to find theheave response of water filled container with respect towide range of regular waves. It was observed from theinitial study that the device performs well between 2 to 3 speriod waves and the wave maker can generate waveamplitudes up to 30 cm. The model was kept 10 m from thewave pedal and the container was immersed 0.8 m fromeach wall. A water filled steel container of 0.4 m indiameter and 0.7 m in height was used for the experimentalpurpose. An 8 pole permanent magnet DC motor was usedas the electricity generating unit and electrical bulbs wereused as load.The experiments were conducted by varying wave

Fig. 2 Schematic view of oscillating arm

Arunachalam AMARKARTHIK et al. Electrical energy from water waves 363

heights between 10 cm and 30 cm, time period between 2and 3 s and with electrical loadings of 30 and 40 W. Thegenerated electrical energy was recorded using a poweranalyzer and device performance was monitored. Acharacteristic study was made and the variation in deviceperformance was plotted. Figure 3 illustrates the impact ofincident wave amplitude in generated power. It is foundfrom the experiment that the maximum power generated islinearly proportional to the wave amplitude in all waveperiods. Figure 4 depicts the impact of wave period inoutput shaft speed. The curve indicates that the outputspeed of the device is significantly reduced outside theband of 2.3–2.7 s. It is also observed that the device speedis under control even for higher wave amplitudes outsidethese bands of wave period. Figure 5 demonstrates theperformance of the device under 60 W loading. It is seenfrom the curve that the device performance increases to 30watts loading for higher wave amplitudes and between2.3–2.7 s wave periods.

6 Discussion

The purpose of the study was to find out the possibility ofusing a non-floating object to harness ocean waves inheave displacement. The experiments indicate that the non-floating objects provide greater energy conversion in linearwaves. The device shows significantly higher efficiency inthe range of 60% between 2.3–2.7 s wave periods andhigher wave amplitudes. With no additional modificationsor damping attachments, the energy captured significantlyreduces when the wave period goes beyond the band of2.3–2.7 s. These characteristics demonstrate that theextreme variation in the wave climate makes the deviceto capture very little energy and keeps the device safe. It isalso observed clearly from Fig. 4 that the speed of outputshaft significantly reduces even for higher wave ampli-tudes beyond the wave period band of 2.3 and 2.7 s. Theincrease in electrical loading makes the device to have ahigher energy conversion rate between 2.3–2.7 s wave

period and reduces the device performance beyond thisrange of wave periods. It was also observed that the energycapture of the device depends on the counter mass which isvaried with ease. In real sea application, the counter masstank can be de-ballasted and the container can be immersedin water to protect the device from extreme circumstances.It is also possible to drain the container and bring the entirecontainer assembly out of the water.

7 Conclusions

Offshore engineers are concerned with protecting the waveenergy converters from extreme conditions. They are moreparticular on limiting the heave response of the floatingbody during extreme conditions. It was clearly understoodthat providing mechanical stoppers or limits will not bepractically possible as the structural load will be very highin extreme conditions. The engineers also want the deviceto perform with maximum efficiency in defined range ofwave environments. In this paper, a novel method of usinga non-floating object to harness waves in heave motionwith maximum efficiency and safety was proposed. Theexperimental results showed that the energy capture of the

Fig. 3 Impact of wave amplitude on generated power under 30Wloading

Fig. 4 Impact of wave period of output shaft speed under 30 Wloading

Fig. 5 Performance of device under 60 W electrical loading

364 Front. Energy 2012, 6(4): 361–365

device is significantly higher per unit wave front comparedto other existing point absorber technologies. It was alsoproved that the device automatically damped duringextreme wave climates with no additional dampingtechniques. The unique working principle of the devicemakes all these advantages possible.

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