op

uf,

Department of Mechanical Engineering, Matsue National College, Japan

a r t i c l e i n f o

similar plants. Developments in Europe cover all aspects of theenergy conversion chain including the OWC [57], the turbine[810] as well as optimization from wave to wire [1115]. A thor-ough review of the European experience (which complements nearshore efforts) is detailed in [16]. The Indian wave energy plant isa bottom standing near shore OWC with a 10 m opening width

development of the turbines in a global context and show howa new unidirectional turbine conguration would help achievehigher overall efciency.

2. Problem formulation and solution space

The average incident wave power in sea environments as ina near shore plant can be expressed as

Contents lists availab

le

els

Renewable Energy 34 (2009) 692698* Corresponding author. Tel.: 91 44 22574427; fax: 91 44 22570509.1. Introduction

A conventional OWC based wave energy plant has a three stageenergy conversion process variations in sea surface elevations areconverted to pressure in the OWC, a bidirectional turbine convertspneumatic air power into mechanical shaft power and an electricalgenerator coupled to the turbine provides electrical power that isexported to the grid. [14]. The most complete and detailedexperimental results with near shore OWC plants are possibly fromthe one at Sakata port, Japan. [2]. Experience in Europe is mostlywith shoreline devices as evidenced in the LIMPET [4] and other

(Fig. 1) and is unique in that several conceptually different powermodules were tested in the same caisson. Initially (in 1991) theplant was designed with a 2 m constant chord Wells turbine anda 110 kW squirrel cage induction generator. The next module (in1996) was a twin 1 m Wells turbine coupled to a 55 kW slip-ringinduction generator. [3] This was followed (in 1997) by a 1 m linkedguide vane impulse turbine using the same induction generator.The nal conguration (from 1998 onward to date) was a xedguide vane impulse turbine. A close analysis of the results of each ofthese congurations shows that the efciency of the power take-offmechanism needs to be improved. We initially trace the historicalArticle history:Received 14 October 2007Accepted 2 May 2008Available online 2 July 2008

Keywords:Wave energyOWCImpulse turbineTime domainFrequency domainWave to wire modelE-mail address: jshankar@ee.iitm.ac.in (V. Jayasha

0960-1481/$ see front matter 2008 Elsevier Ltd.doi:10.1016/j.renene.2008.05.028a b s t r a c t

Experimental results from near shore bottom standing OWC based wave energy plants in Japan and Indiahave now been available for about a decade. Historically the weakest link in the conversion efciency ofOWC based wave energy plants built so far has been the bidirectional turbine. This is possibly becausea single turbine has been required to deliver power when the plant is exposed to random incident waveexcitation varying by a factor of 10. A new topology that uses twin unidirectional turbines (which fea-tures a high efciency spanning a broad range) is proposed. Using the Indian Wave Energy plant as a casestudy, it is shown that the power output from such a module considerably exceeds existing optimalcongurations including those based on a xed guide vane impulse turbine, linked guide vane impulseturbine or a Wells turbine. A wave to wire efciency of the order of 50% over the incident range is shownto be feasible in a credible manner by showing the output at all stages of the conversion process. Afrequency domain technique is used to compute the OWC efciency and a time domain approach usedfor the power module with the turbine pressure being the pivotal variable.

2008 Elsevier Ltd. All rights reserved.f IOES, Saga University, JapanA twin unidirectional impulse turbine twave energy plants

V. Jayashankar a,*, S. Anand a, T. Geetha a, S. SanthakM. Ravindran c, T. Setoguchi d, M. Takao e, K. ToyotaaDepartment of Electrical Engineering, IIT Madras, IndiabDepartment of Aerospace Engineering, IIT Madras, IndiacAdvisor RUTAG, IIT Madras, IndiadDepartment of Mechanical Engineering, Saga University, Japane

Renewab

journal homepage: www.nkar).

All rights reserved.ology for OWC based

mar b, V. Jagadeesh Kumar a,S. Nagata f

le at ScienceDirect

Energy

evier .com/locate/reneneW 0:55H2s TZ kW=m (1)

compute the average OWC efciency based on frequency domaincalculations and model studies. The power module performancecan be estimated as a single unit. We then compute the overallefciency. We provide a brief review of steps involved in proposinga new power module topology.

3. The historical development of turbinesfor wave energy applications

Model studies [17] and practical measurements [2] have shownthe efciency of an OWC to be greater than 90% at an optimum

V. Jayashankar et al. / Renewable Energy 34 (2009) 692698 693where Hs is the signicant wave height and Tz is the zero crossingperiod. In India, it can vary from 4 kW/m in December to 40 kW/min June, July (during the monsoons). (It is emphasized that for anoptimal design at a given location, site specic wave data need to begathered). With an OWCwidth of 10 m it implies an incident powervarying from 40 kW to 400 kW.

The efciency of an OWC based plant is given by

h hOWChthe (2)where hOWC is the efciency of the OWC, ht is the turbine efciencyand he is the electrical generator efciency. The objective is to arriveat a conguration that can provide a high overall efciency span-ning the range of input conditions. The results need to be presentedin the form of average electrical power generated spanning therange of incident wave conditions. This is because a high efciencyof one component in the chain at one operating condition is likelyto be misleading and insufcient to characterize performance. Thedetermination of overall efciency based on Eq. (2) is involved asthe OWC efciency is dependent on the damping of the succeedingpower module and is inuenced by the incident wave spectrum.The turbine efciency is a nonlinear function of the ow coefcientwhich in turn is affected by the speed variations of the coupled

Fig. 1. The Indian near shore OWC plant (circa 1996).electrical generator. However, certain simplications such ascomputation of individual stage efciencies make the computationpossible. It is more important to cover the entire span of incidentpower rather than attempt a time domain model predicting theinstantaneous power output given a certain time series of seaelevation. Further, the sea surface elevations are typically measuredby a buoy at some distance from the plant and not at the entrance tothe OWC. From the possible range of pneumatic powers, we

Generator

Turbine

Exhaust a

Fig. 2. OWC based oating plant withdamping and when designed to match the incident excitationfrequency. At higher power levels, the efciency of an electricalgenerator can also be in excess of 90% over a wide range of inputs[2]. Thus, the component with the lowest efciency spanning theinput range is the turbine. It is this aspect that has motivated thestudy of different turbines and not surprisingly, the research inthis area has been very substantial. [1824]. The rst prototypeOWC based wave energy plant in the oating ship Kaimei is usedas an unidirectional impulse turbine in conjunction with valves[1]. Here, one set of valves are open during intake and a second setare open during exhaust of air as shown in Fig. 2(a,b). This wasnecessary in order to use a unidirectional turbine. As the waveperiod is of the order of seconds it was felt that constant openingand closing of valves might be a limiting factor for long life.Research then shifted to the development of bidirectional tur-bines. The Wells turbine was extensively tested for such use. Oneproblem with the Wells turbine is its stalling behaviour when theow coefcient exceeds a certain value. This has been observed inevery experimental plant with such a turbine [24]. When theincident excitation is random, as from waves, it is difcult to avoidstalling unless the turbine speed is made to vary over a widerange. With a simple squirrel cage generator as the electricalmachine this becomes almost impossible. A second option thatemerged was the impulse turbine and its variants. Both the linkedguide vane impulse turbine and a xed guide vane impulse tur-bine were tested at the Indian wave energy plant. A schematic ofsuch a turbine with self pitching guide vanes is shown in Fig. 3while Fig. 4 shows a recently proposed unidirectional turbine. Areview of self rectifying impulse turbines was recently discussedin [23]. Fig. 5 shows the instantaneous efciencies of severalclasses of turbines available today that can be considered for waveenergy applications. They are of three conceptual classes bi-directional turbines with stalling behaviour, bidirectional turbineswithout stalling and a class of unidirectional turbines. It doesappear from Fig. 5 that high instantaneous efciency over a broadrange is possible with a unidirectional turbine. A topology that canutilize such a turbine while avoiding valves seems the key to goodoverall performance. A logical solution is to utilize a turbine thatexhibits this feature.

Intake bvalves and unidirectional turbine.

an extreme example, one could consider no ow across one turbine

of ow. An electrical analogue of the scheme is a full wave rectier

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Flow Coefficient, phi

Effic

ie

nc

y

FGV Impulse TurbineLGV Impulse TurbineWells TurbineUni-Directional Turbine

Fig. 5. Efciency of three conceptual classes of turbines proposed for wave energyapplications [2325].

V. Jayashankar et al. / Renewable Energy 34 (2009) 6926986944. A new topology for the power module

It has been shown that a class of turbines proposed by Takao etal. [25] have the highest efciency for any unidirectional turbine[2628]. They have been specically optimized for variable owoperation and the design and motivation of this turbine are dis-cussed in [25]. Fig. 6 shows the plan view of a proposed congu-ration for an OWC plant using such a turbine as the power take-offdevice.

Here the OWC is connected to two turbines T1, T2 by a ductwhich splits into two to accommodate the turbines. Each of theturbines (T1 and T2) is a unidirectional turbine and coupled toa common electrical generator. There are no valves. During intake,the air ows from T1 into the OWC and during exhaust the air exitsthe OWC via T2. In effect, each turbine works for half a cycle. Thepressure ow characteristics of the turbines for two directions areutilized to provide full wave rectication. A brief explanation of theworking follows. Consider the pressure-ow characteristics ofvarious turbines considered in the Indian wave energy plant asshown in Fig. 7. The base case design operating point was a 2 mWells turbine with a ow rate of 35 m3/s and a pressure of 4250 Pa.For comparison, the characteristics of 2 m linked guide vaneimpulse turbine and a 2 m unidirectional turbine are also shown.Consider now the operating situation if a 1 m impulse turbine iscoupled to a 2 m impulse turbine on the same shaft. By the natureof the plant the pressure difference across the turbines must be the

Fig. 3. Bidirectional impulse turbine with guide vanes.same. The ow rates across the turbines will hence be different. Inthis case it would be about 35 m3/s across the 2 m turbine and9 m3/s across the 1 m turbine. In effect it is feasible to have twoturbines with different ow rates even in the absence of valves. As

Fig. 4. Unidirectional impulse turbine with guide vanes.with a centre tap transformer and using two diodes. The efciencyof the plant is estimated in the sequel by evaluating the efciency ofeach subsection.

5. Estimation of OWC efciency

For a xed geometry, the factors that affect the efciency of theOWC are the input power spectrum and the damping provided byand all the ow across the other turbine. If we now evaluate thesituation with two unidirectional turbines it can be seen that oneturbine would be in the proper direction for power transfer and theother would be in a choking mode. With the proper design of guidevanes it should be possible to have choking in the reverse directionFig. 6. Twin unidirectional turbine topology (plan).

the power module. As part of the initial design effort of the Indianplant, the OWC characteristics were established with the help ofa three dimensional 1:100 model in a 300 mm narrow ume. Testswere done with regular waves for various values of damping [17].Fig. 8(a) shows the capture factor of the model OWC as a function ofdamping and incident wave frequency. (The capture factor is theratio of pneumatic power to the incident wave power). A

0 10 20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2 x 104

Volume Flow Rate, dq

Differen

tial P

ressu

re, d

p, P

ascal

2m Uni-directional turbine2m FGV Impulse turbine1m LGV Impulse turbine2m Wells Turbine

Fig. 7. Damping characteristics of the turbines considered.

Cap

tu

re facto

r

0 50 100 150 200 250 300 350 400 4500.72

0.74

0.76

0.78

0.8

0.82

0.84

Wave Power, kW

2m FGV Impulse Turbine2m Wells Turbine2m Uni-Directional Turbine

V. Jayashankar et al. / Renewable Energy 34 (2009) 692698 6951.2

1.4

1.6

1.8

facto

r

1.3 Hz0.7 Hz0.8 Hz0.9 Hz

a0.8

1

0 5 10 15 20 250.2

0.4

0.6

Damping (N-s/m)

Cap

tu

re

b

Fig. 8. (a),Model OWC characteristics [17]; (b), A perspective view of capture factor asinuenced by damping and frequency.perspective view of the capture factor is shown in Fig. 8(b). Theoptimum damping in the model was found to be 4.75 Ns/m. UsingFroude scaling, the required damping for the rst prototype(commissioned in 1991) was achievedwith a 2 mWells turbine. It isknown that for a given OWC with a xed water column area, thedamping is a scaled value of the pressure ow characteristics ofdifferent turbines as long as the diameters are the same. [4]. Forcomparison, the damping provided by the optimum bidirectional2 m xed guide vane impulse turbine, the proposed 2 m unidirec-tional turbine conguration, and the original 2 mWells turbine areshown in Fig. 7. Also shown is the damping of the 1 m linked guidevane impulse turbine that was tested in the plant. Themethodologyadopted for computing the OWC efciency under random excita-tion conditions is as follows:

It is instructive to consider the differential pressure as thepivotal variable in the analysis of OWC plants. A turbine generatormodel typically accepts the pressure as an input and estimates theturbine and generator outputs. Similarly, the same time series ofpressure can be used in conjunction with model OWC tests to

Fig. 9. Computed OWC efciencies with different turbines.estimate the capture factor. The time series (8 min) for differentialpressure used in the analysis is a typical record obtained from theIndian site. Scaled values cover the entire range of wave powers

0 50 100 150 200 250 300 350 4000

20

40

60

80

100

120

140

160

180

200

Pnuematic Power, kW

Tu

rb

in

e P

ow

er, k

W

2m Uni-Directional Turbine2m FGV Impulse Turbine2m Wells Turbine

Fig. 10. Comparison of turbine powers.

from 40 kW to 400 kW available. For each cycle of the incidentpressure time series, the fundamental frequency, fi is computed andpeak value of differential pressure dpim computed in the frequencydomain. The dq (ow) for the cycle is determined from the turbineCa/4 curve. The impedance Zi (based on the fundamental frequency)is determined as

Zi dpimdqim

(3)

This is normalized and expressed in terms of the optimal turbinedamping established in the model test. The capture factor (Ci) forthis cycle is computed from Fig. 8(a). Extrapolation is done forintermediate values of frequencies.

The incident wave power for this cycle is given by

wi Xn

i1

piqiCi

(4)

where n is the number of points per cycle.These values are calculated for all the cycles. The overall capture

factor is determined as

hOWC XN

i1

Xn

j1

pjqj

wi(5)

6. Estimation of turbine performance

The estimation of the power exported to the grid from a powermodule is a comparatively well understood task. A SIMULINKmodel for this purpose has been validated and is described in [30].The input to the program is the differential pressure across theturbine as obtained from site data. The time series for pressure hasbeen scaled appropriately in order to cover pneumatic excitationpossible at site. The Ca vs 4 and Ct vs 4 characteristics of turbines arefrom [2224].

The program evaluates the expression

Jdudt

Tt Tg Tl (6)

where J is the moment of inertia of the system, Tt, Tg are the turbineand generator torques and Tl is the term accounting for losses. Thismodule has been previously validated with data from the 1 mlinked guide vane impulse turbine [29,30,]. One important point tonote is that the electrical machine is a squirrel cage machine andcan be treated as an almost xed speed machine. The details of themachine parameters used for simulation are in Appendix II.,Fig.10shows the turbine outputs as a function of pneumatic power. It isclear that the unidirectional turbine performs considerably betterthan the Wells and the xed guide vane impulse turbine.

Fig. 11 shows the ow coefcient and the corresponding turbinepower for a short span of time during a typical run of 8 min, with

corresponding waves to wire efciencies are shown in Fig. 13. The

1e, s

1

V. Jayashankar et al. / Renewable Energy 34 (2009) 692698696where N is the number of cycles. Fig. 9 shows the computed OWCefciency with the turbines discussed so far. Appendix III showsa validation of the approach based on site data with the twin 1 mWells and 1 m linked guide vane impulse turbine. It is to berecognized that the procedure described above is required for animpulse turbine as it presents a nonlinear damping to the OWC.One advantage of the Wells turbine is that it presents constantimpedance to the OWC, as long as it is operated below the stallingregion. The linear pressure drop vs ow characteristic of a Wellsturbine makes it inherently easier to optimize the turbine toa given OWC.

150 155 160 165 1700

0.5

1

1.5

Tim

150 155 160 165 170

Flo

w C

oefficien

t, p

hi

0

200

400

600

800

Tu

rb

in

e p

ow

er, kWTime, s

Fig. 11. The variation of 4 and turefciencies of the older modules tested at site (twin 1 mWells, 1 mLGV impulse) are also shown for comparison [3,31].

75 180 185 190 195 200econds

75 180 185 190 195 200the unidirectional turbine when the average turbine power was113 kW.

7. Results

The average turbine power over the range of incident waveconditions for the three classes of turbines is shown in Fig. 12. Theeconds

bine power for a short span.

that from a comparable plant with a single bidirectional turbine.In the analysis it was assumed that choking of one turbine forreverse ow was perfect. It can be appreciated that the resultswill continue to be good even if some leakage occurs in thereverse direction. One research area that directly emerges is theoptimum combination of xed stator guide vane and rotorprole for good ow in one direction with choking in the reversedirection. A further option is to use self pitching guide vanes sodesigned that the reverse ow is almost fully blocked. A nalmanner of evaluating the result is as follows: There has beensufcient experience with elastomeric valves for excess pressurerelease in the wave energy eld. Using the direction of ow asa control variable, similar powered valves can be used at the twoends open to the atmosphere in the proposed conguration.They are so controlled that at any instant of time only one valve

0 50 100 150 200 250 300 350 400 4500

20

40

60

80

100

120

140

160

180

200

Wave Power, kW

Tu

rb

in

e P

ow

er, kW

2m Uni-Directional Turbine2m FGV Impulse Turbine2m WellsTurbine

Fig. 12. Comparison of turbine powers over the range of wave power.

V. Jayashankar et al. / Renewable Energy 34 (2009) 692698 6978. Discussions

This work was mainly concerned with predictions based onthe Indian wave energy plant. Specically the time variations ofincident power are relevant to Indian conditions. The plant isnow being used for desalination [32]. The new proposal for twinunidirectional turbines can be directly traced to practical ob-servations of twin Wells turbines, linked self pitched guidevanes and xed guide vane impulse turbines. Refer www.mea-surements.in/experiments. The overall efciency calculationmethod proposed here makes full use of the fact that model datafor the OWC as well as full scale turbine performance areavailable. This makes possible the combination of frequencydomain computations for the rst stage and time domainmodeling for the power module so that predictions can coverthe span of available wave power. A purely time domain ap-proach is available in [33] while an alternative approach toturbine damping has been described in [34]. It would be veryinstructive to adopt a similar procedure for a U-OWC based plantas proposed in [35] which could ensure higher efciency overa broader range of wave power. In any case, the power produced

by a twin unidirectional turbine topology considerably exceeds

0 50 100 150 200 250 300 350 400 4500

0.1

0.2

0.3

0.4

0.5

Wave Power, kW

Efficien

cy fro

m W

ave to

W

ire

1m Twin Wells Turbine1m LGV ImpulseTurbine2m WellsTurbine2m FGV ImpulseTurbine2m Uni-Directional Turbine

Fig. 13. Wave to wire efciencies with different power modules.is open. Two unidirectional turbines are needed one for theintake and the other for the exhaust. The preceding analysis(Fig. 12) shows the excess power available in a twin turbineconguration as opposed to a bidirectional turbine. As is obvi-ous, should the power consumed by the valves be a fraction ofthe additional power available with twin turbines, the new to-pology will always be a better option. A laboratory prototype ofthe conguration is being built to conrm the predictions. Thework could form the basis of bulk power production such aswind wave systems [36].

9. Conclusion

The efciency of a near shore OWC plant can be obtained asa product of each stage of the energy conversion stage. Frequencydomain computations in conjunction with model studies can beused to compute OWC efciency in spite of varying power moduledamping. The power module performance can be estimated basedon a numerical method for solving nonlinear differential equa-tions. A new topology for an OWC based plant was proposedusing twin unidirectional impulse turbines. It was shown thatwave to wire efciency of the order of 50% is feasible. Data fromnear shore plants in India and Japan were used to arrive at thedesign.

Acknowledgment

V. Jayashankar is greatly indebted to SAGA University forproviding an opportunity to be in Saga during June 2006 whenthis work was initialised. He dedicates this to Prof. Masuda whoseNever say die attitude has been a source of inspiration. Hethanks Dr. Murugandam and Madhu Mohan of IIT Madras fortheir help.

Appendix I

NomenclatureCi: capture factor xm: shunt reactanceCt: torque coefcient Tl: loss torque N-m.Ca: input coefcient Tg: generator torqueb: height of the blade y: intermediate computation variablel: chord length(m) dq: volume ow rate (m3/s)r: mean radius n: number of bladesa: annular area(m2) dp: pressure difference across the

turbine (Pa)R1: stator resistance Vx: axial velocity of air (m/s)R2: rotor resistance referred to stator Tt: gross torque produced by turbine in

NmX1: stator reactance Greek letters

X2: rotor reactance referred to stator 4: ow coefcient, phirm: shunt resistance hOWC: capture factor of the caisson

in Fig. A3.

References

0 20 40 60 80 100 120 1400

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wave Power, kW

Cap

tu

re F

acto

r

1m Twin Wells Turbine, calculated data1m LGV Impulse Turbine, calculated data

1m Twin Wells Turbine, site data1m LGV Impulse Turbine, site data

Fig. A3. Capture factor of Indian wave energy plant with different power modules.

Wells Impulse 2 m Unidirectional 2 m

Mean radius, r (m) 0.8 0.85 0.85b (m) 0.4 0.3 0.3l (m) 0.38 0.36 0.36No. of blades 8 30 30Hub to tip ratio 1/0.6 1/0.7 1/0.7Operating speed (rpm) 1000 375 375

The parameters used in the Simulinkmodel for the induction generator are shown inTable A2. The 110 kW machine was used with the Wells turbine. With the 2 m FGVthe 110 kW machine works until 180 kW wave power and is changed to a 150 kWmachine for higher powers. The 2 m unidirectional turbine used the 150 kW ma-chine for all conditions. The Simulink model is described in Refs. [18,25].

Table A2Induction generator characteristics

Rating (kW) 110 150WSynchronous speed (rpm) 1000 1500R1 0.0123 0.00902X1 0.0763 0.0763R02 0.00744 0.0334X02 0.0763 0.0763rm 63.787 63.787xm 2.585 2.585

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[2] Ohno H, Funakoshi T, Saito K, Oikawa K, Takahashi S. Interim report on thesecond stage of eld experiments on a wave power extracting caisson inSakata Port. In: ODEC; 1993. p. 17382.Appendix III

The estimated and measured values for the capture factors ofthe Indian OWC plant with two different powermodules are shownAppendix II

The parameters used in the Simulink model for the three tur-bines are shown in Table A1.

Table A1Turbine parameters

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A twin unidirectional impulse turbine topology for OWC based wave energy plantsIntroductionProblem formulation and solution spaceThe historical development of turbines for wave energy applicationsA new topology for the power moduleEstimation of OWC efficiencyEstimation of turbine performanceResultsDiscussionsConclusionAcknowledgmentAppendix IAppendix IIAppendix IIIReferences