Performance estimation of bi-directional turbines in wave energy plants

Journal of Thermal Science Vol.16, No.4 2007 346-352

Received: July 2007 V. Jayashankar : Associate Professor

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DOI:10.1007/s11630-007-0346-1 Article ID:1003-2169(2007)04-0346-07

Performance estimation of bi-directional turbines in wave energy plants

S. Anand1 V. Jayashankar1 S. Nagata2 K. Toyota2 M.Takao3 T. Setoguchi4

1. Department of Electrical Engineering, IIT Madras, India 2. IOES, Saga University, Japan 3. Department of Mechanical Engineering, Matsue National College of Technology, Japan 4. Department of Mechanical Engineering, Saga University, Japan

Oscillating water column (OWC) based wave energy plants have been designed with several types of bidirec-tional turbines for converting pneumatic power to shaft power. Impulse turbines with linked guide vanes and fixed guide vanes have been tested at the Indian Wave Energy plant. This was after initial experimentation with Well’s turbines. In contrast to the Well’s turbine which has a linear damping characteristic, impulse turbines have non-linear damping. This has an important effect in the overall energy conversion from wave to wire. Optimizing the wave energy plant requires a turbine with linear damping and good efficiency over a broad range of flow co-efficient. This work describes how such a design can be made using fixed guide vane impulse turbines. The In-dian Wave Energy plant is used as a case study.

Keywords: Bidirectional turbine, differential pressure, flow rate, impulse turbine

Introduction

The Oscillating Water Column (OWC) based wave energy plant has a three stage energy conversion process −− variations in sea surface elevations are converted to pressure in the OWC, a bidirectional turbine converts pneumatic air power into mechanical shaft power and an electrical generator coupled to the turbine provides elec-trical power that is exported to the grid. The overall plant efficiency is the product of the efficiencies of each of these power conversion stages. Near shore OWC plants operate at a finite water depth as opposed to shore based OWC plants. The plant at Sakata port [1] and the Indian Wave Energy plant [2] are the main examples of such plants with practical operational experience. In this work we mainly address such plants. The plant at Sakata port was based on a caisson with an opening of 24 m and a power module comprising twin horizontal axis 1.337 m Wells turbines coupled to a common 50 kW DC genera-

tor. The literature on the design and performance of this plant is vast [3],[4],[5], [6]. The Indian plant is unique in that several power take-off mechanisms were tested in the same caisson. Initially the plant was designed with a 2m constant chord Well’s turbine and a 110 kW squirrel cage induction generator. The next module was a twin 1m Well’s turbine coupled to a 55 kW slip – ring induc-tion generator. This was followed by a 1m linked guide vane impulse turbine using the same induction generator. Experimental results from this plant have also been pub-lished [2],[7].

One very important observation from the analysis of the Sakata port plant is the high and near constant effi-ciency of the OWC. In contrast, the OWC efficiency of the Indian plant was markedly affected by the power module. Secondly, the overall efficiency of both plants has been turbine limited rather than by the OWC. A ma-jor factor for this stems from the variability in the incident wave power. The variation is observed on both seasonal

S. Anand et al.. Performance estimation of bi-directional turbines in wave energy plants 347

Nomenclature Ci capture factor q volume flow rate (m3/s) Ct torque coefficient n number of blades Ca input coeffcient dp pressure difference across the turbine(Pa) b height of the blade Vx axial Velocity of air (m/s) l chord length(m) Tt gross torque produced by turbine in Nm. r mean radius Greek letters a annular Area(m2) φ flow coefficient, phi

and daily basis. For example, in India, the incident wave power can be as high as 30 kW/m during the monsoons and reduce to about 4 kW/m during December.

The OWC is a linear device (with respect to wave height) but with frequency dependent characteristics. The linked guide vane (LGV) impulse turbine has a non-linear pressure / flow characteristic. As the turbine serves as a damping for the caisson, the overall plant efficiency would not be necessarily be the turbine con-figuration with the highest efficiency. The problem is thus to choose a configuration that maximizes the overall energy capture from wave to wire. In this work we ini-tially predict the efficiency of the OWC based on model studies of the caisson. The turbine generator is then mod-eled. We study the behavior of the linked guide vane im-pulse, fixed guide vane (FGV) impulse and Well’s tur-bine in the same caisson and finally arrive at an optimum configuration for the plant.

Matching turbine to the OWC damping

Fig. 1 shows the prototype of the OWC based Indian Wave Energy plant as installed in 1996. The width of the OWC is 10 m and the water plane area is 150 m2. As part of the initial design effort, the OWC characteristics were established with the help of a 1: 100 model in a 300 mm narrow flume. Tests were done with regular waves for various values of damping [8]. Fig. 2 shows the capture factor of the model OWC as a function of damping and incident wave frequency. (The capture factor is the ratio of pneumatic power to the incident wave power). The optimum damping in the model (using an orifice) was found to be 4.75 Ns/m. Using Froude scaling the required damping was 475,000 Ns/m for the first prototype with a 2m Wells turbine. The vertical axis turbine based plant with a 110 kW squirrel cage generator (1000 rpm) was tested for about two years and experience with this power module showed that the no – load losses were high and it was difficult to obtain net power under low wave condi-tions. This configuration was changed to a horizontal axis twin 1m Well’s turbines coupled to a single slip ring 55kW induction generator (1500 rpm). The damping provided by this turbine was about 361,760 Ns/m. Sev-eral experiments with a high level of instrumentation for the critical stages were performed for one season in 1996. The results with this module showed turbine stalling at

moderate sea waves. It was then felt that a linked guide vane impulse turbine would be more suitable for wave energy as it shows a distinctly higher dynamic range in terms of flow coefficient [9], [10]. Hence the Indian plant was modified from 1997 to include a 1m linked guide vane impulse turbine operating at a speed of 750 rpm. The absolute value of power to the grid did improve with this turbine but there was a drop in the OWC efficiency. Some experiments were also conducted with the linked guide vane turbine modified to behave as a fixed guide one. However the absolute value of the power was lesser as the guide vanes were not at optimum angles. Subse-quent research has shown that 30o would be the optimum angle [11]. The pressure flow characteristics of the three turbines discussed so far are shown in Fig. 3. With hind-sight it is realized that although the 1m impulse turbine meant minimal changes to the structures in the caisson, its damping influence was not carefully investigated.

Fig. 1 The near shore Indian OWC plant (circa 1996)

Estimation of caisson efficiency from model tests with wave excitation

Experimental observations in the Sakata port [1] showed that the OWC efficiency is indeed high and gen-erally matched the predicted value. One reason for this is the linear damping of a Well’s turbine. Such an analy-sis has not been done for the linked guide vane impulse turbine and we propose a numerical procedure based on model tests for this purpose. This is because the damping provided by the linked guide vane is non-linear as seen from Fig. 3.

The methodology adopted is as follows: For each cycle of the incident pressure time series we

compute the frequency fi and peak value of differential

348 Journal of Thermal Science, Vol.16, No.4, 2007

Fig. 2 Model OWC Characteristics [8]

Fig. 3 Damping characteristics of turbines

pressure dpim. The dq for the cycle is determined from the turbine Ca /ϕ curve. An impedance Zi is determined as

imi

im

dpZ

dq= (1)

This is expressed in terms of the optimal turbine damping established in the previous section. The capture factor for this cycle is computed from Fig. 2. Extrapola-tion is done for intermediate values of frequencies. The incident wave power for this cycle is given by

1

(ni i

iii

p qw

C== ∑ )

(2)

Where ‘n’ is the number of points per cycle. These values are calculated for all the cycles. The

overall capture factor is determined as

( )1

N j j

jj

p q

wη

== ∑ (3)

Where ‘N’ is the number of cycles. Fig. 4 shows the computed caisson efficiency for the

plant with the 1m twin Well’s turbine. The experimental results from the plant are also shown in the form of su-perimposed data points providing validation for the model. We then predict the OWC efficiency for the 1m linked guide vane plant as shown in Fig.5. We are also able to understand from measured data the cause of lower efficiency of the caisson in spite of higher absolute

dP [P

asca

l]

Fig. 4 Predicted and measured capture factor with a twin

Well’s turbine power module

Fig. 5 Estimated capture factor of OWC plant with 1m linked

guide vane impulse turbine


power from the 1m linked guide vane turbine. Thus the objective is now to choose a turbine with the highest ef-ficiency and has more linear dp vs dq curve than the linked guide vane impulse turbine.

Fixed guide vane impulse turbines

Hyun et al. [12] have provided a performance chart for predicting the outputs from a range of 250 kW fixed guide vane turbines. Based on a study of the power and the damping characteristics of these turbines we conjec- tured that a 2 m fixed guide vane would match the Indian OWC. We also study the utility of an additional smaller turbine – a 1.2 meter turbine. Their dp/dq characteristics

0 10 20 30 40 5

0 60 70 80 900

2000

4000

6000

8000

10000

12000

14000

16000

18000

dq

dp

2m Impulse1.2m Impulse2m Wells

Fig. 6 Damping characteristics of proposed turbine

Fig. 7 A comparison of estimated capture factors with three

different turbines

are plotted in Fig. 6 along with the superimposed characteristic of the original 2 m Well’s turbine. This figure shows that the damping of these turbines is more linear and closer to the Well’s than the 1 m linked guide vane turbine. Fig. 7 shows the predicted capture factor for the OWC with these turbines as power modules as a function of incident wave power. dP

[Pas

cal]

Estimation of Turbine Performance A program has been developed for the computation of the power exported to the grid from a wave energy power module [13]. Fig 8 shows a SIMULINK model of the turbine. The power module is shown in Fig. 9. The input

m3/s

4dq

3Tt

2Pt

1p wave

y against phi

wt^2

-C-

turbine parameters

-C-to

0.8

r

phi^2

calc y

calc pneumatic power

calc dq

calc Vx

calc Tt

calc Pt

-C-

a

Ct against phi

1

|u|

Abs

2wt

1dp

Fig. 8 SIMULINK model of the turbine


to the program is the differential pressure across the tur-bine. The Ca vs ϕ and Ct vs ϕ Characteristics of turbines are from [14].

Fig. 10 shows the efficiency of the two turbine classes

(FGV, Well’s) being considered as a function of flow coefficient. The program evaluates the expression

t gd

lJ T T Tdtω

= − − (4)

Fig. 10 Efficiency of bi-directional turbines The power is estimated for the case of random excita-

tion as from waves. In this case the differential pressure is obtained from site data. Fig. 11 shows the output power as a function of pneumatic power. It can be seen that for low incident conditions, the 1.2m FGV shows a slightly improved performance as compared with the 2.0m FGV. At higher incident power the 2.0 m FGV is higher than both the 1.2m FGV and 2.0 m Well’s. Similar results using a different approach are also shown in [15].

Optimum Configuration

The overall efficiency of the plant is then the product of turbine efficiency and the OWC efficiency. This is shown in Fig. 12.

dp

wt

p wave, P1 Ttdq

turbine

total power

rotor power

pnuematic power

Tt

Tg

wt

wg

gear box

differential pressure

T torque

T power

wg

Tg

Power in rotor shaft power

total input power Machine

Pshaft

M torque

-K-

1

Gain1

-K-

pressure.mat

From File

turbine speed

Machine speed

0 100 200 300 400 5000

20

40

60

80

100

120

140

Pnuematic Power,kW

1.2m Impulse2m Impulse2m Wells

Turbine Power,kW

Turb

ine

Pow

er [k

W]

Fig. 11 Power output from three different turbines

Fig. 9 SIMULINK model of the Power Module

0 1 2 3 4 5 6 70

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

ϕ

WellsImpulse

Fig. 12 Efficiency of the configurations

We thus see that a good design of a wave energy plant for all climates will require two turbines – one for low conditions and the other for high incident conditions. Other schemes use one turbine [16]. The option for such a plant is shown in Fig. 13. Here V1 and V2 are two tur-bine valves and only one of them is open at any time. T1 is a turbine with diameter 1.2m while T2 is with diameter 2m. Here two independent turbine generators as power modules exist. This can be justified due to the fact that the overall cost of the plant is based more on the caisson cost than the power module. A single configuration with two turbines coupled to a single generator as shown in Fig.14 is also feasible.

Effic

ienc

y


Fig. 13 The wave plant with two independent turbine genera-tor power modules

Fig. 14 Optimized configuration for wave energy plant This was the configuration of the twin Wells turbine

plant. This is possible because the electrical generator generally has a fairly constant efficiency over a wide range of input.

Discussion

An analysis of operational near shore OWC plants shows that the OWC efficiency can be high and also pre-dictable. Linear damping is a necessary condition for good OWC efficiency. On the other hand floating OWCs show lesser efficiency which needs to be improved [17]. Bidirectional turbines have shown a lesser efficiency than the bottom standing OWC. It appears that im-provements are still needed in this area in order to im-

prove overall efficiency of a wave energy plant. From Fig. 12 we can see that, if a good and constant efficiency is required (for the Indian plant ), from 10 - 400 kW of input wave power, two turbines provide better perform-ance. A smaller turbine could work in the range from 10 to 70 kW and the larger one from 70 kW onwards.

G1 G2

D1 D2

v1 v2

OWC

T2 T1

In this work a comparison was made with 2m Well’s turbine and 2m fixed guide vane impulse turbine. Both machines were operated at near constant speed due to the induction generator. In practice, Well’s turbine would never be operated in fixed speed and further power elec-tronic scheme would be necessary. With fixed guide vane impulse turbine, variable speed operation permits some damping control and could be exploited in future.

Conclusion

In addition to possessing high efficiency over a broad range, turbines for wave energy plants need to be opti-mized with respect to their damping characteristics. The fixed guide vane is more adaptable to this requirement than a linked guide vane. The overall efficiency of the plant can be computed even if the damping is non-linear. Good overall efficiency over the entire incident wave power requires two turbines.

G

D1 D2

v1 v2

OWC

T2 T1

Acknowledgment

V. Jayashankar is greatly indebted to SAGA Univer-sity for providing an opportunity to be in Saga during June 2006 where this work was conceptualized. He is grateful to Prof. Raju and Prof. Ravindran who pioneered the Indian Wave Energy program. He thanks Boby George and Kishore Kumar of IIT Madras for their help.

References

[1] Takahashi S., Adachi T. et al,: Field experiments of a wave power extracting caisson breakwater data analysis of wave forces and wave power conversion, Report of the Port and Harbour Research Institute, vol.31, No.2, pp.22−54, (1992).

[2] Ravindran M., Jayashankar V., Jalihal P., Pathak A.G. : The Indian Wave Energy Program – an overview, TIDE. Teri Information Digest on Energy, Vol.7, No.3, pp. 173−188 (1997).

[3] Ojima R., Goda Y., Suzumura S. : Analysis of Efficiency of Pneumatic Type Wave Power Extractors Utilizing Caisson Break Waters – A study of development of wave power, Report of the Port and Harbour Research Insti-tute, vol.22, No.3, pp.126−158, (1983).

[4] Takahashi S., Ojima R., Suzumura S.: Air power of pneumatic type wave power excitation due to irregular


wave excitation, Report of the Port and Harbour Re-search Institute, vol.24, No.1, pp.4−41, (1985).

[5] Takahashi S., Suzumura S., Myose K., : Turbine power of pneumatic type wave power excitations utilizing caisson break waters – A study on development of wave power – 4th report, Report of the Port and Harbour Research In-stitute, vol.24, No.2, pp.206−238, (1985).

[6] Takahashi S., Adachi T., Tanaka S., : Electric power gen-eration by a large scale model of pneumatic type wave power converter - A study on development of wave power – 6th report, Report of the Port and Harbour Re-search Institute, vol.26, No3, pp.3−35, (1987).

[7] Santhakumar S., Jayashankar V., Atmanand M.A., Pathak A.G., Ravindran M., Setoguchi T., Takao M. and Kaneko K.: Performance of an impulse turbine based wave energy plant, Proceedings of ISOPE- Montreal, Canada, pp.75−80, (1998).

[8] Koola P.M. : Investigation on performance behavior of the oscillatory water column wave energy device, Doc-toral dissertation, IIT Madras, (1990).

[9] Setoguchi T., Kaneko K., Tariyama H., Maeda H., Inoue M. : Impulse turbine with self pitch controlled guide vanes for wave power conversion : guide vanes corrected by links International Journal of Offshore and Polar Energy, vol. 6, No.11, pp. 76−88, (1996).

[10] Kaneko K., Setoguchi T., Raghunathan S. : Self Rectify-ing turbines for wave energy conversion, Proceedings of ISOPE Edinburgh, UK, pp.365−392 (1991).

[11] Setoguchi T., Santhakumar S., Maeda H., Takao M., Ka-neko K.: A review of impulse turbine for wave energy conversion, Renewable energy, Vol.23, pp. 261−292, (2001).

[12] Hyun B.S., Moon J.S., Hong K., Hong S.W. : On the performance of impulse turbine system in various oper-ating conditions , Proceedings of seventh ISOPE, San Francisco, pp. 225−230, (2006).

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Appendix

The parameters used in the Simulink model for the three turbines are shown in Table A1.

Table A1 Turbine characteristics Wells Impulse 2m Impulse 1.2m

Mean radius, r 0.8m 0.85m 0.51 b 0.4m 0.3m 0.18 l 0.38m 0.36m 0.216

No. of blades 8 30 30 Hub to tip ratio 1/0.6 1/0.7 1/0.7 Operating speed 1000rpm 375rpm 375rpm

The parameters used in the Simulink model for the induction generator are shown in Table A2.

Table A2 Induction generator characteristics Synchronous speed 1000rpm

R1 0.0123 X1 0.0763 R2’ 0.00744 X2’ 0.0763 rm 63.787 xm 2.585

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Performance estimation of bi-directional turbines in wave energy plants