Proceedings of the Eleventh (2001) hlternational Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright 2001 by The International Society of Offshore and Polar Engineers ISBN 1-880653-51"6 (Set); ISBN 1-880653-52-4 (Vol. I); ISSN 1098-6189 (SeO

Preliminary Study on Wave Energy Utilization in Sri Lanka

T. Watabe T-Wave Consulting Volunteer, Japan

H. Yokouchi and S.D.G.S.P. Gunawardane Muroran Institute of Technology, Japan

B.R.K. Obeyesekera and U.I. Dissanayake University of Peradeniya, Sri Lanka

ABSTRACT

Sri Lanka is rich in swell-like ocean wave energy along ~he

coasts which face the Indian Ocean. The power density is 20kW/m and the maximum wave height is below 6m, so that it is

inherently suitable for utilizing the energy for commerdal

purposes for many needs.

In order to develop such energy, we applied the latest

technology on the Pendulor studied in Japan for the generation

of electricity in a cost-effective manner. Since the wavelength is quite longer and the power stronger than the cases studied in Japan, the authors investigated modifying the ~endutor to make it suitable for Sri Lanka.

As a commercial unit, a 250kW prototype !Penduior installed

with a 21m wide caisson improved for the sites, was used in a

preliminary study.

INTRODUCTION

Sri Lanka is rich in swell-like ocean wave energy, along the coasts facing the Indian Ocean. The power density is 20kW/m and

maximum wave height is below 6m, so that it is inherently suitable for utilizing the energy for commercial purposes for many needs.

Because the Muroran Institute of Technology had developed a

New Pendulor (Watabe et al, 1999) suitable for commercial use, the Institute was looking for a site to build the Pendulor. Researchers in

Japan and Sri Lanka therefore started collaboration to build a wave power plant in Sri Lanka to harness the energy by applying the latest Pendulor technology.

Before entering into the design work, some general investigations

were studied to make the Pendulor optimal even in the various

different wave climates around the world. Since the wavelength at Sri

Lankan sites is much longer than the waves at the site where the

Pendulor was tested in Japan, the Pendulor has been modified to fit the circumstances of Sri Lanka.

As a commercial unit, a 250kW prototype Pendulor was designed

and installed with a 2 lm wide caisson improved for the sites.

WAVE CLIMATE OF SRI LANKA

Sri Lanka is an island country as shown in Fig. 1. The location is

400km south of Madras, India. The southern coasts facing the Indian

Ocean are rich with ocean wave energy.

~\~" Kankesarttura=

o~@ ~_.?"ii .... / ;q - i ').i .....

t , Oe,Fr " , -. %7"- -~.

Klinol;hc~ : " ~ ("':'

'~ ' - , , - .---, ~ / ' )' J) 7~- ~ Mu at tvu

, L " , I ~ ; : , Bay : . . . . . ~Mank flare ,.a~, 0 f =, . MANNAR ." ~ - . ; - - ' . - " ~ - ~'t ,~

T..~ . . . . .a.,._._ ~S~SVO :' ~ "" . . . . ' .... ~ / " '~ ' . . . . . . . . . . -- ~" " i . . . . . )' "'"--~ . . . . . , "> ' )~" ' / ' 2:'-, Benga l

";:i ' " : : : : - - ! . " ' .:7 '~ .' '~

t " ,- '. . . . . . : .~Trincomalee

J s w, LP~'dL" " , "-i !. : 't?" ,~ -i~

\ ' [ "} '~ ~ . . . . . . ": ~'J~ ' abarana" " " : Po qdnaruwa " " - {'Tdkkandlmadu f,~--_ ~ Pu~lam / z t - ;L, ~ ?o , " " .~

t3 -' " " ~ ' " ~- -~- ~ ', ,\( , ~ " ", - :~ , i :-"~ " '~ / { : - )' :- .... Batticaloa

'- ;i - -,,.,;~,, - ' : . r, ooog.%:;.,. ,: . . . . " . - ;{~] %f ' - '~ i , ,-- .: _ : - ] ):'-='" "" Hambanlota "==it - ........... "" - '.- :- ---. - " %" I nd ia n Ocean t.=alle ' -~#~, .&>- : ~ ~:,.. : . ; . i f Tangalla

Fig. 1 Map of Sri Lanka

596

There are two kinds of waves: Swells and Sea waves. The swell waves have traveled over large distances and are a result of the Passat

winds blowing continuously in a northerly direction in the Southern Hemisphere. A local wind field causes the sea waves. They are seen

as natural waves mixed with the swell and the sea. Fig. 2 shows an example.

Ira] 3.00

.

0.00

-3 .00 0 .0

Ira] i .5o ~ ....

~.00 -~

-o'~o ~I Ir~i - i0o: t - i .50 : m '

3.0 4 .0 6 .o 7 .0 -~ [ i t l~r l ]

. sea I

0.0

[m]

1.50

1 .00

0 .50

0 .00

-0 ,50

-1 .00

- t .50

0.0

i .0 2 .0 3 .0 4 .0 5 .0 6 .0 7 .0 ,. [m tn ]

V V.V vvv v,vv' vvv VwVV vv v-v 1.0 ~- .0 3 .0 4 .0 5 .0 6 .0 7C~Oin ]

Fig. 2 Typical wave measured

The swell has a long wavelength (over 100m). Its wave-period is nearly constant throughout the seasons (between 10-12s) and the direction changes little.

The sea waves have a short wavelength of which is function of wave-height. The stronger local wind makes the higher waves with a longer wavelength. The direction depends on the direction of the local wind.

The power density of the swell is generally greater than the power density of the sea waves. In this preliminary study, the power density Wo for the Pendulor design was taken as Wo=20kW/m, prospecting H1/3=2m, T1/3=12s (Scheffer et al. 1994). Power density w o is shown in the equation:-

w o - 0 44 x H21 (1) . /3 X ~1/3

where H~/3: significant wave height T]/3: significant wave period

DESIGN CONDIT ION OF THE PLANT

The principle of the Pendulor (Japanese pat. No. 2539742) is shown in Fig. 3. A pendulum hangs down in a water chamber, which is placed in a caisson. From the opening mouth, incident waves come

into the water chamber and alter the standing waves by superposing upon them with reflecting waves. The reciprocal flow of the standing waves at the nodal plane drives the pendulum, placing it into pendular motion. The motion is transferred through a hydrostatic power transmission to drive a generator.

In accordance with the conversion principle, the Pendulor must be made to resonate with the waves. Therefore, in this case, having two different wave periods by the swell (T1/3--10-15s) and by the sea (T1/3--4-6s), determination of the natural frequency of the Pendulor had to be done carefully. Fig. 4 shows a result of the power conversion being worsened by missing resonance with the waves.

Fortunately, the power density of the swell is over three times that of the sea waves so that it is preferable that the system fits the swell better than the sea waves.

Oil Motor

[Rotary VanePumpl

Electric Power

Generator

Pendulum

: /.

Incident Waves

. . . .

... Back Wall

Water Chamber

Caisson

Fig. 3 Principle of the Pendulor

The natural frequency of the Pendulor depends on the oscillation characterized by a combination of the pendulum and the caisson. However, khe Pendulor, which fits the swell (having a long wavelength), should use a long caisson. This fact means that the cost pressure on the caisson is strong. This problem will be studied further in a later chapter.

In order to have a Pendulor as useful as possible, the Pendulor must work for commercial purposes and must also record its operating data. The Pendulor considerations were as follows.

i The Pendulor must be reliable. 2 The Pendulor stands on commercialism. 3 The Pendulor must provide a measuring system.

597

100

8O

o

"-" 60 >, U c 40

(J -.~_

20

D : 5 m (constant) N,/N; -1

I

mean /// full tide / / / - _

. \ \ \ /TL ebbtide [ , ,' I// \ / '

. _ ,

. . . . .

0 2 z 6 8 Period T (seo)

A result by numerical simulation for Muroran Test Plant

Water chamber length: D=5m, Impedance matching condition

Water depth: h=2.75m, Tidal range: +__ 0.5m

- - - - - - - T-- i -

1

10

Fig. 4 Efficiency being worsened by missing the resonance

RELATIONSHIP BETWEEN CHARACTERISTICS

PROFILE AND

Before we touch on the Pendulor design, we give here some studies of the basic nature of the system in order to show what is the preferable profile.

In order to have a cost effective Pendulor, we should know the investment and output relationship of the system. Fig. 5 shows the relationship between output P of the system and the volume V that is occupied by the Pendulor.

P - O . 4 4 ]-] 2/ z T1/ 3 rlB (I)

V- xBh (2)

where,

x - 2lp sin 0 o

79 : Pendu!or' s efficiency B: width of the pendulum h: water depth /i,: pendulum's length 0 0: amplitude of pendular motion

0 0 is shown next in the Equation (4).

19o-- kpXp H1/3 4Yp sinh kpk

(3)

(4)

Wave number/q, is found from the dispersion relationship shown below.

co 2 - gkp tanhkph (5)

O)p -- 2n/T1/3 (6)

Xp - sinh(kph)cosh(kph) + kpk (7)

gp - kp(lp - h)sinh(kph) + cosh(kph) - i (8)

Fig. 5 shows power density: P/V of the Pendulor, which represents the output generated from the unit volume occupied by the Pendulor on the Sri Lanka coast. As the wave height becomes higher, the power density becomes greater. The 4m water depth area shows a higher power density than the 6m-depth area.

Fig. 6 shows another aspect of the Pendu!or which affects the dimension of the components. The most important parameter is the pendulum length: lp. The reason is that the manufacturing cost greatly depends on lp.

As long as the Pendulor can satisfy the optimal conditions, it will keep the plant efficiency over 40% constantly, even if {pump torqUe(m,x)/angular-displacement } ratio: (T~.xl 0 o) changes with respect to the pendulum length: lp- When the pump torque is sinusoida!, its amp!itude at the impedance match condition corresponds to Tm~x, which is as in Equation (9) (Watabe, 1997).

m~x -- 77~,3--7-7-- - - - - 2kp smh kpk

(9)

Where, p " density of water

The longer the pendulum is, the bigger the ratio (Tma j # 0) it needs for the optima! operation. This means that a Pendulor with a long pendulum, is likely to be expensive, because, the pendulum needs a big pump to optimize the system. As seen in Fig. 6, increasing pendulum length: Ip enlarges pump torque: Tm~ and decreases angular-displacement: 80-

0,-~ m

B O.z

> "~ 0,'I

0._

Atria optimal operation

h=4m: T1/3=10s, lp=7.3m

h=6m: T1/3=12s, lp=10m ~ /

_ i ! ! J_ . . . . . . .

1.o /.b" 2.0 -,'.~

Significant wave height: HII3 (m)

Fig. 5 Power density of Pendulor

From an economic view-point, it can be said that the shorter the pendulum length: lp is the better as long as t70 is kept within a reasonable range. Comparing water depth: h=4m and h=6m, the case of k=4m is more practical than h=6m, because of a higher power density with a smaller pump combination.

598

At ~e optimal operation "& 1

---- /4- tlq ........... x ~- o =bm - co

7 o

6 = x

.,.,.i

rq

C.~

6 7 9 ~ 10 1~ Pendulum length: lp (m)

Fig. 6 Basic aspects of Penduior

THE PROTOTYPE PENDULOR FOR DEMONSTRATION

Referring to the resuits investigated above, design parameters are selected as follows for the prototype Pendulor:

H1/3=2m, T1/3=12s, h=4m, B=10m, Ip=7.3m, kp=2 rc / Z =2 rc/73.7m=0.08525m -1, co p=0.5236s -1 Wo=21. lkW/m--20kW/m and w o B=200kW. In order to get a steady output, 2 pendulums are used in this case

so that the annual average input is 2 w o B=400kW. According to the studies conducted in Japan, suitable displacement

of the pump Dp is determined by the following equation (Watabe, 1997).

D - 2~/'/tTmax - (~o)

P (Pm~x + Ap)

Where, 7) t: torque efficiency of the pump, Pma~: supply pressure to motor, which corresponds to Tm~, Ap: pressure loss between pump and motor.

In actual fact, the pump torque is not sinusoidal so that Tm~ must be lessened to determine the pump displacement Dp. That is

Tact=(Tact //Tm~ ) X 7) 1Tm~=(0.65-0.75) X 0.65 Tm~ =0.42---0.49

rm~, Referring to Fig. 6, Tm~x=2,000,000 N 'm (lp=7.3m, h=4m),

therefore, T~t=910,000 N-m.

Substituting Tm.,x=910,000 N-m, r/1=0.95 and (Pmax + Ap)=25MPa into the Equation (I0), the pump displacement is found: Dp=0.217m3/rev..

The pump shown in Fig. 7 is a rotary vane-type (Japanese pat. No. 2573905), which was developed for the Pendulor (Watabe, et al. 1999). Displacement of the pump is shown below.

JTZ" 9

(1])

Where,

dl: rotor outer diameter, d2: rotor inner diameter, Wr: rotor width Parameters are selected as: dx=0.80m, d2=0.53m, Wr=0.40m.

Therefore the displacement becomes: Dp=0.2256m3/rev. This value is so huge that it is 4.9 times bigger than the old pump tested in Japan (Watabe, et al. 1999).

The caisson has a modified profile as illustrated in Fig. 8 to save on the investment of the whole system. It was considered best to have the water chamber shortened both in width and in !ength, though the ability remained the same as that of the normal chamber, which has large dimensions for Sri Lankan waves. Only the openings and the pans behind each chamber were widened to normal width. This chamber can save cost well, because not only is the caisson shortened but also the Pendulor can keep the same capacity with the shortened pendulum,. The profile improves the Pendulor to provide better survivability against storms as well (Japanese pat. Pending No. 2000- 128632).

Fig. 8 shows the general view of the 250kW Pendulor. The capacity was determined taking into consideration the unsteadiness of the wave power. Instead of the annuat average wave input being 400kW, it was assumed to correspond to an input by a wave-group having steady wave height: H=2m and period: T=12s. Therefore, design input E is written as in the Equation (12).

E- 7cpgB 1 + 2kpT sinh 2kph

(12)

When the parameters take the values indicated below, design input E becomes =598 kW. 250kW output is assumed as the plant efficiency: r~ =42%. This idea is important to have an adequate plant capable of handling an annual input of wave energy,

where, p = 1030kg/m 3, g=9.806m/s 2, B=2 x 10m,

kp=0.08525m -1, h=4m, H=2m, T=12s.

Fig. 9 shows the pendulum which receives an input of E=299kW each with the 5m wide plate. Incident wave power, which enters from the 10m wide mouth, is concentrated into the 5.4m wide (narrower) chamber. The amplitude of the pendular motion is +_ 35 under the rating load. The concept of the pendulum is like a large hand fan. At one of the ends of the shaft, the vane pump couples directly without a coupling component.

Fig. 11) shows the hydraulic circuits of the Pendulor. The two vane pumps driven by the two pendulums respectively, supply the two motors powered oil parallel and the motors drive a generator together. This system has steady output even using the motors when torque varies wavy each. This tandem drive yields a canceling effect with a phase difference method (US pat. No. 4490621). There are two automatic controllers, which control displacement of the motors to keep the system in the impedance-match condition even if the wave climate changes suddenly without warning.

The generator is an induction type that rotates at a speed synchronous with the speed of the grids. Therefore, it is not subject to speed governing but to torque governing of the driving system. The Pendulor is designed to satisfy these requirements.

599

Dp: 225.6 l/rev., No. of vanes: 2, Rotor size: ~ 800 ~ 530b400, Swing angle: -+-65 , Pressure: 25MPa, Pump torque: 0.898 MNrn, Delivery flow: 7.0 l/s (H=2m, T=12s,

B=10m, Input to the pump=193kW)

i L 1400 1320

' Rotary vane pump for Pendulor

'I

400

Fig. 7 Rotary vane pump for Pendulor

t

_2.- -~ "-

~- ;~ '2- ~ .~ T

Fig. 8 The prototype Pendulor for Sri Lanka

~ the 250kW Pendulor

600

SAFETY DES IGN AGAINST H IGH WAVES

Since the pendulum is beaten directly by waves, safety design against high waves is a very important subject especially in the case of commercial use. For this project we considered as follows (refer to F igs . 8- -9) :

1: Huge power calmot push the pendulum, because of the action of high waves. Whenever very high waves come into the chamber, the upper part of the waves passes through the opening prepared above the pendulum plate and goes behind the plate without pushing the plate.

2: When the amplitude of the pendular motion is over the straight chamber (5.4m wide), two additional passages open both sides of the plate so that an additional amount of water goes behind without pushing the plate. Passing water raises the pressure behind the plate and pushes the plate back.

Rotary vane pump , . , ._

Ifir

_ Still '~ ~--e

O

~2

Pendulum assembly

Fig. 9 Pendulum assembly

Generator 250 kW 1500 rpm Nakamura-Koki G230-60-20

l t .~ 24MPa ~ Cont~ollerra] ~ r 150_~!0 P,~ II , , ~_~ , , . . . . . . . ,

[ [ io =2@,,;evl] ........ [ Dv=22516 l/rev K. ~ '~2 / "---P~ ---/q/'-" " 3

~ ~-~~~ ]P~_~ Dm=75-250 cm/rev

~, ~\ - -~D j ] / l l -~ "-''J - Controller

250kW Pendulor circuits for Sri Lanka Project (Patent pending)

Fig. 10 Hydraulic circuits

3: The profile of the water chambers is that of an open type. Incident waves being higher than the walls, they spill over from the chambers so that the caisson loses their huge power.

4: There are stoppers provided to avoid excess movement. In order to keep the line pressure within a safety level, two relief valves are provided.

Under stormy conditions with this case, the amplitude of the motion would reach nearly __ 90 . In order to protect the system, the stopper restricts the amplitude to within _+_+ 65 because the force being generated when the pendulum impinges on the stopper, would rise to a dangerous level (Watabe. 1993).

Impinging speed: (d 0i /d r ) can be found when the impinging angle: 0 i is given. Supposing the amplitude is + 90 , the pendular angle: 0 is as follows.

O - ( rc /2 )s in cot (13)

~) - (7c /2 )c0 cos cot (14)

During a storm, co=(2 7c/7)=(2 r : /16)=0.393s -1, (dO~ /d r ) = ( 7z/2) 0.393 cos 60 = 0 .309 s-1 = 1[ 7 .7 /s

When the stopper is a linear damper, the impinging force F i is as in Equation (15).

li: the distance between the pivot center and the stopper.

When the parameters take the values betow, F~ becomes FI = 77kN. 2;I: the moment of inertia of the pendulum including added water 400 X 10Bkgm 2,

0 ~: impinging angle -- 0.96rad (55 ), z3 8 : displacement by stopper's deformation -- 0.15rad, l i -- 3.3m

F i = 77kN is at a completely safe level, because the value is smaller than the force of the pump load in a rating operation. The key parameter is the spring constant: kd of the damper.

k d = F i / z3 8 = 77kN/0.15rad = 513 kN/rad (16)

ELECTRICITY COST ANTICIPATED

Assuming the generator, the vane pump and such components are made in Japan and the caisson, the pendulum, the frame etc. in Sri Lanka, the cost of building the Pendulor was estimated. Table 1 shows the results for the 250kW Pendulor as a 1 unit manufacture.

It is assumed from the authors' investigation that the average output of the plant in the monsoon season (8 months/year) is 150kW (average input=400kW, plant eff.=40%), and 45kW (average input=120kW, plant eff.=40%) in the non-monsoon season (4 months/year). Totai electricity generated annually is 1,006,500kWh/year. Total electricity in 15 years (life expectancy of the Pendulor) becomes 15.1GWh.

601

The cost of electricity is 0.1 US$/kWh. That is:- (Total cost)/(Total electricity in 15 years) -- 0.1 US$/kWh

excluding transportation, interest, set-up and maintenance cost. The working ratio of the plant is 46%, when 15.1GWh is compared

with 32.9GWh expected by 250kW.

Table 1 Cost estimation o f250kW Pendulor

_s Components"-.... Pendulums

Pumps Frames Oil motors Accumulators Generator Control panel

Hyd. components Controllers

Caisson

Extra cost Total

Costs ( 103 Specifications

US$~0 2 sets, @7,600kg 2 sets, combination of (@4,000kg

200 pump with @3,000kg shaft) 12 2 sets 28 2 sets, Dp=280 cc/rev 40 4 sets, @ 18 l, 25MPa

170 @250kW 120 @250kVA,

including wiring 140 @25MPa,

including Piping 2 sets, system optimizers & ioad balancing devices

20

300 ! @140,000kg, r steel structure

340 1,450 @250kW

Total cost excludes transportation fee, interest, set-up cost and maintenance cost.

COMPARISON OF THE PENDULOR AND THE owe

Because of its excellent simplicity, the Oscillating Water Column (OWC) system has been tested and studied around the world. There is no difference in the conversion principle between the OWC and the Pendulor. However, the philosophy behind each mechanism has been completely different. Therefore, it must be useful to show a comparison done from an engineering viewpoint. Table 2 shows the results.

The OWC is very simple, but its simplicity has become a kind of obstacle to improving the system so that its conversion efficiency seemed to be sacrificed by it.

The Pendulor is inherently efficient, but it is supposed to be much more difficult in engineering than the OWC to get the goal (Watabe. 2001).

Table 2 Comparison between OWC and Pendulor

~ m I Terms

Principle

Power media

Rectifier

OWC

Airflow that is produced by heave motion of standing waves drives a turbine generator. Air: p -- 10kPa, Q - 10m3/s By a uni-rotational turbine, Energy loss -- 40%

Pendulor

Pitch motion of standing waves, drives a generator via pendulum & HST combination Oil: p - - 10MPa, Q- 0.0 lm3/s By a rectifying circuit with 4 check valves, Energy loss-- 5%

Caisson I With closed air chamber With open water

I chamber Output I Unsteady Quasi steady Optimal i Control of load & speed Control of displacement control of the generator of the oil motor Efficiency I I ~=10-15% 77=35-45%

vane Un-

resolved matters Cost of

I electricity Plant cost

Improve turbine eff. & reduce caisson cost

(0.30-0.50) US$/kWh (estimate) (4000--6000) US$/kWh (estimate)

Reduce cost of pump & caisson

(0.07-0.15) US$/kWh (estimate) (3000--5000) US$/kWh (estimate)

CONCLUSIONS

As explained in this paper, we can conclude as follows.

1: Sri Lanka has good wave energy characterized by a combination of the Swell and the Sea waves along the southern coasts. The power of the Swell is stronger than that of the Sea.

2: For commercial purposes, it is essential to design a wave power converter that does not resonate with the Sea but the Swell. The Pendulor studied in Japan must be modified to become suited to the Swell. The average power density of the Swell is 20kW/m in

the monsoon period. 3: At around a 4m water depth, the Pendulor can harness the Swell

most economically with a 7.3m long pendulum. For a caisson having two water chambers with an inlet width: B=10m each, a 250kW Pendulor was designed. The displacement of the pump to

fit it is about 0.22m3/rev. 4: The safety design gives the Pendulor good survivability against

storms. 5: The manufacturing cost of the 250kW Pendulor is about 1.45

Million US$. Assuming the total electricity produced is 15 GWh in 15 years at the coast, the electricity cost is estimated to be 0.1

US$/kWh. 6: Compared with the OWC system, the Pendulor needs much

higher engineering technology. Nevertheless it works inherently

efficiently.

602

The authors thank Mr. Rajaratne who drove his car such long distances along the coasts to investigate the ocean with us in winter and in summer. His sincere spirit led him to voluntarily offer co- operation for this study.

REFERENCES

Scheffer, H-J. et al (1994). "Directional wave climate study soukh- west of Sri Lanka", Report on the Wave measurements off Galle, Sri Lanka-German CCD-GTZ Project.

Watabe, T. et al (1999). "Installation of the New Pendulor for 2 nd stage sea test", Proc. of the 94 ISOPE Brest France, pp. 133-! 38.

Watabe, T. (1997). "Manual of 15kW Pendulor Design", Report on a miniature Pendulor, T-Wave C. V.

Watabe, T. et al (1999). "Improvement of a rotary vane pump for an ocean wave power converter: Pendulor", Proc. of the 44 JHPS Int. Symp. on Fluid Power Tokyo'99, pp. 73-78.

Watabe, T. (1993). ~'~Pendulor wave power converter-fifteen years study and future prospect", Proc. of ODEC, Muroran Japan 1993, pp. 41-52.

Watabe, T. (2001). ~Prospect of utilization of the ocean wave energy" Journal of Japan society for Design Engineering, Vol.36, No.l, pp.32-37. (in Japanese)

603

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