Performance of Horizontal Axis Tidal Current Turbine by Blade Configuration

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Renewable Energy 42 (2012) 195e206

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Renewable Energy

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Performance of horizontal axis tidal current turbine by blade configuration

Chul hee Joa,*, Jin young Yimb, Kang hee Leeb, Yu ho Rhob

a School of Engineering, Inha University, 253, Yonghyun-Dong, Nam-Gu, Incheon 402-751, Republic of KoreabGraduate School, Department of Naval Architecture & Ocean Engineering, Inha University, 253, Yonghyun-Dong, Nam-Gu, Incheon 402-751, Republic of Korea

a r t i c l e i n f o

Article history:Received 10 March 2011Accepted 9 August 2011Available online 29 September 2011

Keywords:TCP (Tidal Current Power)Renewable energyTurbine designCWC (Circulating Water Channel)CFD (Computational Fluid Dynamics)

* Corresponding author.E-mail address: [email protected] (C. hee Jo).

0960-1481/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.renene.2011.08.017

a b s t r a c t

The west and south coastal region of Korea has very strong tidal current speeds and therefore accom-modates many suitable sites for the application of TCP (Tidal Current Power). The maximum currentspeed recorded in the south is up to 6.5 m/s. Unlike other renewable energy sources, TCP is an extremelyreliable, predictable and continuous energy source as the current pattern and speed can be predictedthroughout the year. One of the essential components in a TCP device is the rotor converting the inflowcurrent into the rotational energy. The design optimization of the rotor is very important to maximize thepower production. The performance of the rotor can be determined by various parameters including thenumber of blades, shape, sectional size, hub, diameters etc.

The blade of the rotor is one of the essential components which can convert tidal current energy intorotational energy to generate electricity. The variable blade properties determine the performance,efficiency and stability of the turbine system. This paper presents the design procedure for a 300 kW tidalcurrent turbine blade. The HAT turbine model was designed based on the wind mill turbine designprinciples together with known turbine theories. To verify the compatibility of the turbine designmethod and to analyze the properties of design factors, the 3D CFD model was applied with the ANSYSCFX program. The characteristics and performances of the blades can be applied in the design of 300 KWand larger capacity TCP rotors.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Tidal current power is now recognized as the main clean powerresource in Korea where there are strong current regions in thewest and south coasts. Many researchers have studied tidal currentpower systems. Garbuglia et al. (1993) and Paish et al. (1995)introduced a new concept of tidal stream power system withexperiments in the sea [1,2]. Shiono et al. (1999) studied theDarrieus-type device [3]. Walsum (1999) introduced the currentpower system in Fundy [4]. Jo et al. (2008) published the experi-mental results on the applications of tidal current power systems inthe cooling water weir [5]. To produce sufficient power, manydevice units are required in the power farm region. Due to thelimited areas of the concentrated current, the arrangement of thedevices is very important to maximize the efficiency and economicviability of the farm. The optimization of arrangement is essentialin the multi-arrayed formation. Jo et al. (2007) presented the

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interference rates of axial, transverse and diagonal arrangements ofrotors for changing incoming current speeds [6].

A turbine blade among components of a tidal current powergenerator is a core device that converts the flowof tidal current intoa turning force.

Technology acquisition on tidal current power turbine opti-mized design is difficult because currently, the literature on tidalcurrent power (TCP) turbine design is insufficient and evendeveloped countries avoid disclosing design technology. However,considering that a horizontal axis tidal current power turbine isbased on the design theory of a horizontal axis wind power turbineand fluid is changed into seawater in the air, it is possible to designa tidal current turbine in the sea based on the technique introducedin wind power.

Performanceevaluationmethodsona tidal currentpower turbineare experimentally performed in the real sea by producing an actualproduct and a model test that carries out an experiment in a circu-latingwaterchannel (CWC)byproducingamodel andcomputationalfluid dynamics (CFD) that simulates a CWC by using a computer.

The model test has the advantage that it can obtain highlyreliable data but it is expensive and requires a great deal of expe-rience and time.

Fig. 1. Gyroscopic imbalances.

Table 1Specification of CWC.

Main particular Measuring section

Length (m) 6.0 2.3Breadth (m) 1.0 1.0Height (m) 3.0 0.9Max.velocity (m/s) 1.2 -

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On the other hand, CFD can obtain various results at low costand is used in a variety of fields such as wind power and the fluidmachinery industry etc.

This study evaluated the performance of a designed turbine bycomparing performance analysis using CFD with the result of themodel tested in a circulating water channel.

2. Determination of turbine design elements

2.1. Number and performance of turbine blade

The factors that determine a modern blade are stability, effi-ciency, and the economic feasibility of the system.

In terms of stability, a disk is the most stable among rotatingmechanical component parts and has a predictable shape.

Compared to a 3-piece blade, for a 2-piece blade, since turbineproduction cost is low, installation is straightforward and the tipspeed ratio when maximum efficiency occurs is high, the gear costof the gear box can be lowered and the size of the generator can bereduced.

However, since theoretical efficiency is lower than that of a 3-piece blade and the tip speed ratio is higher than that of a 3-piece blade, considerable wake occurs and it therefore rangesfrom being unfavorable to complex and cavitation is highly likely tooccur.

Due to the nature of a 2-piece blade, when a blade tip crosses thetower while rotating, a strong impact is applied to the turbine dueto the tower effect and the turbine is vibrated because gyroscopicimbalances occur as shown in Fig. 1.

In order to resolve these imbalances, a separate device is neededand the turbine will become complicated structurally.

A 3-piece blade is a rotation element of a turbine and satisfiesthe number of stable minimum wings required of a disk. For a 3-piece blade, the start flow velocity is low compared to a 2-pieceblade and ranges from favorable to complex due to the lowereffect of the wake.

Fig. 2. S814 Airfoil shape.

Also, from an economic aspect, since a 3-piece blade canmaintain stable disk characteristics, it is not necessary to designblades with more than 4 pieces.

Therefore, this study designed a tidal current power turbinewith a 3-piece blade.

2.2. Blade cross-sectional shape

There are several differences between an airfoil used in aero-space engineering and an airfoil of a tidal turbine blade.

Since an airfoil for a tidal turbine blade is installed in water,when the water is polluted, the blade is not easy to maintain orrepair. Therefore, an airfoil with a shape which is less sensitive tosurface roughness is needed. In addition, because various loads arestructurally imposed on the hub part, the wing tip must havea gradually thick airfoil shape.

Also, unlike an aircraft, a wide range of data on the angle ofattack is needed.

The characteristics of a tidal current power turbine are verysimilar to those of a wind power turbine. While a tidal currentpower turbine uses a similar airfoil to that used in wind power,there is a significant difference between the viscosity and density ofair and seawater; a test and verification on the characteristics of anairfoil in seawater are therefore required.

However, since the test and verification of this airfoil are beyondthe scope of this paper, this study used the S814 airfoil (Fig. 2),previously adopted in other commonly used tidal current powerturbines.

3. Design of a turbine blade

3.1. Determination of design velocity and turbine size

Prior to the decision of the design current speed, an oceano-graphic survey on the tidal characteristics of an area where instal-lation of a tidal current generator is proposed should be carried out.

Fig. 3. Circulating Water Channel (Inha University ocean engineering lab).

Fig. 4. Distributions of chord length.

Fig. 5. Angle definition of an airfoil.

Fig. 6. Distribution

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This is because in the case of a large capacity turbine, thedistribution of tidal current according to depth of water cannot beignored and a control system that can respond to the angle inwhichcurrents flow should be considered.

Also, a tidal current generator installed in the ocean, unlikewindpower, has limitations of turbine size according to the depth ofwater of the installed generator. To design the current speed ofa tidal current turbine, a velocity is chosen that can calculate themaximum amount of power generation in the target waters byconsidering the tide range in addition to data showing the directionand speed of the tidal current such as increase in wind power.

This paper tries to compare and verify turbine performancemeasured in a circulating water channel experiment and predictedfrom CFD analysis.

Therefore, the design current speed selected for the turbine is1.0 m/s and the diameter of the turbine is 0.5 m by considering thespecifications of the circulating water channel of Table 1.

of twist angle.

Table 2TCP turbine blade design parameters.

Design parameters Values

Prated: rated power [W] 36.23Cp: Estimated power coefficient 0.4h: Estimated power coefficient 0.9Urated: Rated current velocity [m/s] 1.0r: Sea water density [kg/m3] 1025l: Tip speed ratio 5D: Turbine diameter [m] 0.5N: Blade number [EA] 3u: Angular speed [rpm] 191

Table 3TCP turbine blade design result.

No. r/R r [mm] Chord length [mm] Twist angle [degrees]

1 0.05 12.5 -2 0.10 25.0 - -3 0.15 37.5 30.000 -4 0.20 50.0 Transition Transition5 0.25 62.5 63.100 16.6826 0.30 75.0 60.363 13.9297 0.35 87.5 57.624 11.4628 0.40 100.0 54.913 9.7839 0.45 112.5 52.202 8.43210 0.50 125.0 49.490 7.32411 0.55 137.5 46.779 6.40412 0.60 150.0 44.067 5.63113 0.65 162.5 41.356 4.97414 0.70 175.0 38.645 4.41515 0.75 187.5 35.933 3.94216 0.80 200.0 33.222 3.78417 0.85 212.5 30.510 3.55018 0.90 225.0 27.799 3.24719 0.95 237.5 25.088 3.09220 1.00 250.0 22.376 3.062

Fig. 7. Framework of TCP turbine blade [ISO view].

Fig. 8. Framework of TCP turbine blade [Top view].

Fig. 9. Solid 3D model of TCP turbine [ISO view].

Fig. 10. 3D model of TCP turbine [Top view].

Fig. 11. Computational domain of turbine.

Fig. 12. Grid system of rotor.

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C.hee Jo et al. / Renewable Energy 42 (2012) 195e206 199

3.2. Decision of output and rated number of rotations

To estimate the power of a testing blade, substitute thefollowing values in Eq. (1).

When an estimated power coefficient (Cp) is 0.4, the power trainefficiency coefficient (h) is 0.9, the seawater density (r) is 1025[kg/m3], the diameter of the turbine (D) is 0.5 m, the velocity (U) is 1 m/s, and the expected output of the turbine is 36.23[W].

Pexpect ¼ hCp

rpD2U3

8

!(1)

The design tip speed ratio (TSR, l) is 5 and the rated number ofrotations is 191 rpm.

3.3. Correction of coefficient of loss of blade tip

Tip loss occurs due to the vortex of blade wing tip. In order topredict tip loss Eq. (2) presented by Ludwig Prandtl in 1919 wasapplied to the tip loss prediction model.

ftip ¼ 2pcos�1

�eððN=2Þð1�mÞ=mÞ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þðlm2=ð1�aÞ2Þ

p �(2)

Here, m is the ratio of radial direction from hub to tip and isdefined as r/R. R is the entire rotor radius of a blade and r is theblade radius of the airfoil. The axial direction velocity component offlow induced by the actuator disk is called where ‘a’ is the axial flowinduction factor and is expressed as Eq. (3).

Udisk ¼ UN � aUN (3)

Since ‘a’, the axial flow induction factor, cannot be determinedthrough repetitive calculation from the initial design stage, theideal value of 1/3 induced from the Betz limit is applied. N refers tothe number of blade wings and 3 pieces were applied.

3.4. Determination of chord length distribution

The method for determining the chord length of an airfoil isgenerally decided by Eq. (4), as presented by Schmitz as follows.

Fig. 13. Cp

C ¼ 16prCLB

sin2�13tan�1

�Rlr

��(4)

C: chord lengthB: number of blade pieces (¼3)l: Tip speed ratio (¼5)R: entire rotor radius of a blade (¼0.5 m)r: a blade radius of the airfoilFig. 4 shows the blade chord length calculated by the Schmitz

numerical formula.If the Schmitz numerical formula is used as it is, the chord length

is infinitely larger as it extends to the root part, which is wherestructural defects will occur. Therefore, chord length should bedetermined as shown in Fig. 3 by approximating linearly based ona distance of 30% from the blade tip by considering structural defectsettlement, ease of manufacture and economic feasibility.

3.5. Distribution determination of twist angle

The definition on the angle of a blade element is as shown inFig. 5. a is the angle of attack of an airfoil andF is thefluid inlet angleand is defined as the fluid inlet direction (UN(1 � a)) and relativetangential velocity of fluid by rotation of a blade Ur(1 þ a0).

The twist angle b is defined so that the sectional airfoil of a bladecan have an optimal life force coefficient depending on the radius ofthe rotor, as for the following Eq. (5).

b ¼ 23tan�1

�Rlr

�� a (5)

b: Twist anglea: Angle of attackThis twist angle is expressed as a non linear value depending on

the radius of the rotor and the calculated result is as shown in Fig. 6.

3.6. Tidal current turbine blade design result

ThefinaldesignvalueofTable3wascalculatedbasedonthedesignparameter of Table 2. Point coordinates required to visualize a 3 Dmodel were obtained and the frameworks were plotted as shown inFigs. 7 and 8 from the design information of Table 3 and modeled asshown in Figs. 9 and 10 by using CATIA, a 3Dmodeling program.

curve.

C.hee Jo et al. / Renewable Energy 42 (2012) 195e206200

4. Performance analysis using CFD

4.1. Calculation conditions

Fig. 11 shows the internal domain where a turbine rotates, theexternal domainwhere the fluid flows and the boundary condition.The external domain was then modeled into a rectangular shape

Fig. 14. Rotor pressure distri

the same as that for the Measuring Section of the circulating waterchannel for comparison with the experiment of the circulatingwater channel.

The external domain is a rectangular parallelepiped of width1 m, length 4.75 m, height 0.8 m and the internal rotation area isa cylinder of diameter 0.5 m and height 0.11 m.

butions (Pressure side).

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Normal speed condition was used as an inlet condition of theexternal domain and the incoming velocity is 1.0 m/s, which is thedesign velocity.

The external domain outlet area used an opening condition sothat it can be calculated according to the flux change due to theturbine. The walls and floors of the external domain used wallconditions that were similar to the environment of the circulatingwater channel and the upper side used the free slip condition.

Fig. 15. Rotor pressure distr

The general connect - frozen rotor condition was used as theinterface condition of the meeting part of the internal rotating areaand the external area, while the mesh connect method used the GGIcondition.

Also, the change of fluid passing through the turbine can becalculated by using the wall condition on the surface of the turbine.And the torque values were calculated by ANSYS CFX Post-processor.

ibutions (Suction side).

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4.2. Mesh and turbulence model

For the prediction of torque that occurs in the blade, a denseprism-layer was composed around the blade and the rest of theareawas composed as a tetra-prismmesh. The turbine rotating areais an unstructured mesh consisting of 2,671,523 elements and565,443 nodes and the external area surrounding the turbine is alsounstructured mesh consisting of 169,819 nodes and 947,896elements. Fig. 12 shows the composed grid system.

Fig. 16. Blade pressu

The turbulence model performed analysis by considering theunsteady flow field around an airfoil and using an SST model. SSTmodel can accurately predict size and onset of flow peel caused byan adverse pressure gradient by calculating the transport ofturbulence shearing stress and the accuracy of analysis is obtainedregardless of the yþ of the mesh because it can be easily extendedinto an automatic wall treatment.

Calculationwas performedwith a duel core CPU (3.0 GHz*2) andthe number of repetitive calculations was converged prior to the

re distributions.

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100 repetitive calculations. The condition of convergence deter-mination is a margin of error of less than 10�4 and calculationperformance on a single case required about 3 h.

4.3. CFD analysis result and review

The performance curve of a tidal current turbine on a 1.0 m/sdesign velocity is shown in Fig. 13 by calculating the torque value in6 analysis results from 2 to 7 of tip speed ratio.

Fig. 17. Blade Turbulence kine

The output coefficient showed to be more than 0.4 over a rela-tively wide range from tip speed ratio 3 to 7 and the maximumvalue of the output coefficient was calculated as about 0.51 in thesection of tip speed ratio 5.

Through CFD analysis, the pressure on the front and backsides of the turbine blade as shown in Figs. 14e19, the ambientpressure acting on the airfoil, the turbulence kinetic energy andthe flow of streamline passing through the turbine werevisualized.

tic energy distributions.

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Figs. 16e18 show the values in airfoil section according to tipspeed ratio and fluid flows from top to bottom of the picture andthe blade proceeds from right to left.

Therefore, the upper airfoil of the picture shows the pressureside and the lower airfoil shows the suction side.

The turbine designed in this paper is the lift type and uses liftcaused by the pressure difference of fluid according to the asym-metric shape of the cross section of the airfoil.

For the acting face of the turbine shown in Fig. 14, as the tipspeed ratio increases, the pressure in the region near the tip and theleading edge continues to increase and at tip speed ratio 7, negativepressure occurs at the tip part.

For the suction side of the turbine as shown in Fig. 15, negativepressure continues to increase up to tip speed ratio 5 and thengradually the increase in negative pressure slows down.

Comparing the pressure side with the suction side, the pressuredifference of about 4.0 � 104[pa] occurs.

For the pressure around the blade as shown in Fig. 16, as tipspeed ratio increases, the pressure near the leading edge of thepressure side gradually increases and negative pressure of thesuction side also gradually increases.

Passing through tip speedratio5,anegativepressureoccursonthepressure side of the maximum thickness position and the pressuredifference between the pressure side and suction side also decreases.

Fig. 18. Blade streamline an

As the pressure difference decreases, the lift force decreases andthe torque is reduced.

At tip speed ratio 2 shown in Fig. 17, the fluid inlet angle flowinginto the blade is larger because the tip speed ratio is low.

Therefore, life force dramatically decreases because the angle ofattack increases and a stall occurs.

As tip speed ratio increases and reaches 5, it becomes closer tothe angle of attack of the maximum lift to drag ratio and maximumlift force occurs.

After passing tip speed ratio 5, as it moves toward the trailingedge of the blade, turbulence occurs because the fluid does not flowalong the surface and falls off and drag caused by turbulencereduces the torque of the turbine.

Also in Fig.18, the stall in tip speed ratio 2 can be observed and astip speed ratio increases, it can be found that the fluid inlet anglereduces.

If the tipspeed ratiobecomes lower than5, theblade isplacedwitha greater angle of attack than that of themaximumLift-Drag ratio andas the angle of attack increases, a stall occurs and torque is reduced.

If the tip speed ratio becomes greater than 5, lift decreases andtorque is reduced because the angle of attack becomes smaller thanthat of maximum Lift-Drag ratio.

From Fig.19, the rotor streamline can be verified, which expandswhen passing through the turbine.

d velocity distributions.

Fig. 19. Rotor streamline distributions.

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As tip speed ratio increases, the rotor streamline expands to themaximum at tip speed ratio 5 where maximum efficiency appears.

The wake caused by the turbine develops and becomes morecomplicated as the tip speed ratio increases.

5. Conclusions

This study performed the shape design of a tidal current powerturbine from turbine design theory and a 3-D flow analysis by CFDand the following conclusions were drawn.

A horizontal axis tidal current power turbine was designed witha diameter of 0.5 m using S814 airfoil by considering tip loss basedon turbine design theory including blade element theory and thiswas embodied by using CATIA, a 3D modeling program.

An output curve and torque curve were drawn by using ANSYSCFX V11 SP1, a commercial code, and performing 3-d flow analysisof a 3D model.

At design flow 1.0 m/s, the maximum output coefficient wasabout 0.51 at tip speed ratio 5 and maximum torque was about3.65 N-m at tip speed ratio 3.2.

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The influence thatflowphenomenon (pressure, speed, streamlinedistribution) around a turbine has on the tip speed ratio was inves-tigated byanalyzing theflowaround the bladewith a Post-processor.

Astipspeedratio increases,negativepressureactingonthesuctionside increases and at a more than optimal tip speed ratio, the outputdecreases because negative pressure occurs on the pressure side.

Also, at less than optimal tip speed ratio, output decreases dueto turbulence on the suction side and over the optimal tip speedratio, the output is reduced by the turbulence of the trailing edgeand the significantly generated wake affects the flow of fluid.

Acknowledgment

This work was supported by the Human Resources Develop-ment of the Korea Institute of Energy, Technology Evaluation andPlanning (KETEP) grant funded by the Korea Government Ministryof Knowledge Economy (No. 20094020100070).

This work is the outcome of a Manpower Development Programfor Marine Energy by the Ministry of Land, Transport and MaritimeAffairs (MLTM)

References

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hydropower into the next century; 1995.[3] Shiono M. Experiments on the characteristics of Darrieus turbine for the tidal

power generation. Proceedings of the ninth international offshore and polarengineering conference; 1999.

[4] Walsum W. Offshore engineering for tidal power. Proceedings of the ninthinternational offshore and polar engineering conference; 1999.

[5] Jo CH, Lee CH, Rho YH and Yim JY. Floating tidal current power application incooling water channel. The Korean Association of Ocean Science and Tech-nology Societies conference, Jeju, 2008; pp. 2184e2187.

[6] Jo CH, Par KK and Im SW. Interaction of multi arrayed current power genera-tions. International offshore and polar engineering conference, Lisbon; 2007,pp. 302e306.