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Output Characteristics of Darrieus Water Turbine with Helical Blades for Tidal Current GenerationsMitsuhiro Shiono, Kdsuyuki Suzuki, Sezji KihoDepartment of Electrical Engineering, College of Science & Technology, Nihon UniversityTokyo, Japan

ABSTRACTTidal current generation uses a generator to produce energy,

changing the kinetic energy of current into a turning force bysetting a water turbine in the tidal current. Therefore, it isconsidered to be very advantageous to use a water turbine thatcan always revolve in a fixed direction without any influencefrom tidal current directions. Water turbines with thesecharacteristics are known as Darrieus water turbines. Darrieuswater turbines have a difficulty in starting, but these daysDarrieus waiter turbines have been developed with helical blades,which make it easy to get the turbines started. However, thereare very few reports regarding Darrieus water turbines withhelical blades, and therefore their characteristics are unknown.From the above points of view, this study devises andinvestigates helical blade Darrieus water turbines to clarify theircharacteristics through hydrographic experiments, and at thesame time, it compares the characteristics of helical bladeDarrieus water turbines with those of straight blade ones.

KEY WORDS: Tidal Current Generations, Darrieus WaterTurbine, Stratight Blade, Helical Blade, Solidity

INTRODUCTIONProduction ad renewable energies has been focused on due toproblems such as environmental preservation and the shortageof energy sulpplies in the future. Oceanic energies, except tidalcurrent generations, still have many difficult problems to besolved, whidh seems to require a lot of time to make their usepractical. However, there have been reports showing that ourcountry, Japa.n, has about 25 million kW of tidal current energy,approximately 219 billion kWh annually (Kiho, 2001).Moreover, the density of energy produced by seawater flow isapproximately a thousand times more than that by wind force.

Therefore, tidal current energy is considered to be a highlyavailable energy source and as yet not fully utilized. Darrieuswater turbines used for tidal current generations were originallydeveloped for wind force generations, but the authors used themfor tidal current generations. Then, straight blade Darrieuswater turbines were developed with generators in order to solveproblems of the design and strength of the turbines whenconverting from wind to water use (Kiho and Shiono 1991).Moreover, since the water turbine efficiency of straight bladeDarrieus water turbines can be altered by changes of solidity,the authors have clarified that there is an optimum solidityexisting in order to obtain maximum efficiency (Shiono, Suzukiand Kiho 1999). It is also pointed out that the present straightblade water turbines have difficulty starting, and in order tocope with this problem, helical blade water turbines weredeveloped by making the straight blades spiraled (Gorov, 1998).However, there are very few experiments regarding thecharacteristics and effectiveness of these helical blade waterturbines.Therefore, first, the authors compared the characteristics ohelical blade water turbines and straight blade ones in hydrographic experiment. The results indicated the startingeffectiveness of helical blade turbines. Moreover, like straightblade water turbines, it is presumed that helical blade turbinesalso have an optimum value of solidity to obtain maximumefficiency. Therefore, helical blade water turbines were createdusing different solidity types and examined to clarify the outputcharacteristics of the water turbines. Moreover, in order toreveal the influence of blade inclination angles on theperformance of helical blade water turbines, hydrographicexperiments were conducted using water turbines with differentheights. The results of these experiments are reported in thispaper.

Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference

Kitakyushu, Japan, May 2631, 2002

Copyright 2002 by The International Society of Offshore and Polar Engineers

ISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set)

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DARRIEUS WATER TURBINESShapes of Blades in Water Turbines

As shown in Fig.1 (a), straight blade turbines are made withblades locateid in equal intervals around a center axis. Since thistype of water turbine is a lift type turbine and revolves in a fixeddirection without any influence from the current direction, it isconsidered to be appropriate for tidal current generations, inwhich the current direction is changed in a certain period of time.

F d =- I-- --lcol rh h5 5_-(a) Straight bladeFig 1 Blade shapes of water turbines (b) Helical blade

Here, solidity cr is a value that significantly affects theperformance of water turbines, and Eq.1 uses the followingvariables: the length of a blade chord is C, the number of bladesis n and the diameter of a water turbine is d, and expressed asfollows:

nCOS-3rd 1)

On the other hand, a helical blade Darrieus water turbine, asshown in Fig.1 (b), is a spiral water turbine that is created bymaking the upper and lower surfaces of a straight blade waterturbine spiraled around its central axis. Fig.2 shows a sketch ofthe revolution surface development of the helical blade waterturbine. The inclination angle of a blade to the upper and lowersurfaces of the water turbine is defined as a blade inclinationangle , and Eq.2 is expressed as follows:

-

Fig.2 Develolped sketch of a helical blade water turbine

Tested water turbinesTable 1 shows the parameters of tested water turbines used inthe experiments. Four types of helical blade water turbines weredevised, composed of different lengths of blade chords in orderto measure and compare their different characteristics bysolidity 0. The dimension of these four types were all d=300mm and h=300mm.Table 1. Parameters of tested water turbines

Blade type 1 (5 1 C(mm) 1 (deg) 1 h(mm) H2 1 0.20 1 62.8 1 43.7 1 300

Moreover, two types of water turbines were devised using twodifferent heights, for the purpose of measuring theircharacteristics according to blade inclination angles of helicalblades.Furthermore, using NACA 633-018 for the blade shape (Abbottand Doenhoff, 1959)) which is symmetrical and straight, modelwater turbine blades were devised by making curved linesoverlapped on the circumference of the water turbine revolutionso that generated torque is not influenced due to differentlocation angles. Moreover, the number of blades was set at 3for all the turbines used.EXPERIMENTAL APPARATUS AND METHODExperimental apparatusFig.3 shows the experiment apparatus, Darrieus water turbine

used in the hydrographic experiment. Water turbines tested werepositioned so that the upper surface was located 1.5cm under thewater surface and the revolution axis was vertical. Then,electromagnetic breaks were attached through a torque detectoras loads of the water turbines.The range possible for measurement using a circular tank is3.Om in width, 1.5m in depth and 30m in length. The fluidenergy was in proportion to the cube of the current velocity, andthe current velocity was carefully measured since thedetermination of the current velocity values can be used as astandard of water turbine characteristics. Therefore, anelectromagnetic tachometer was located at the upper stream farfrom the water turbine so that the water turbine characteristicswould not be influenced by current turbulence, which is causedby probes.

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ectromagnetic break

Fig.3 Darrieus water turbine experimental apparatusStarting torque experiment 4 Revolution Angle : 6The current velocity values were determined as the followingthree types: 0.6, 1.0, and 1.4m/s. In order not to revolve, thewater turbine was fixed by inserting pins in the holes made in 5degrees intervals on the electromagnetic break disc directlyconnected to1 he water turbine axis. Then, the starting torque atthe water revolving angle was measured every 5 degrees fromthe datum location as shown in Fig.4 when looking from theupper part of the water turbine. The measurement value wasdetermined to be the average for 30 seconds.Load characteristic experimentThe current velocity values were set as follows: 0.6,0.8, 1.0, 1.2and 1.4m/s. The experimental method is as follows: After thecurrent velocity was set and the current became stable, the waterturbine was first revolved without any loads. Then, loads weregradually added to the revolution by an electromagnetic break,and the velocity of revolution and generated torque weremeasured until the water turbine was stalled and completelystopped. The measurement value was determined to be theaverage for 30 seconds.RESULTS AND DISCUSSIONStarting Torque ExperimentFig.4 shows the measurements of starting torque, Ts, at arevolution angle 8 of the water turbine with c~=O.4 and e43.7.Moreover, values of TS were converted into values per a unitlength (lm) of h so that they could be compared among differentheights of water turbines. The figure reveals that the higher thecurrent velocity became, the larger the torque was. This isbecause the torque is in proportion to a square of currentvelocity. Moreover, since the fluctuations of TS are seen inproportion to 0, Table 2 indicates the fluctuations of TS at eachcurrent velocity. Here, when the maximum torque during a

3.53.02.5

? 2.0k

2 1. 51.00.5

I I I I I

60 120 180 240 300 36B , deg)

Fig.4 Starting torque characteristics (Blade type: H4)circle of revolution was determined as Tma~ and the minimumtorque was determined as Tmin , the range of T fluctuations, Tb,can be expressed as Eq.3.Tb =T mm-T min (3)And when the average starting torque was determined as Ta, therate of pulsation, y, can be obtained by the following equation:

Tb Tmax-Tminy=z= Ta 4)

From Table 2, the average torque, Ta, and the range offluctuation, Tb, both became larger as the current velocitybecame higher. However, the ripple factor y is seen around 0.5,which confirmed no significant change.Table 2. Ripple factor y (Blade type: H4)

v6-w Ta (Nm) Tb WI Y0.6 0.45 0.29 0.641.0 1.28 0.65 0.501.4 2.46 1.33 0.54

Moreover, T was standardized after Tmax had been detected inFig.4, which is shown in Fig.5. The standardized values ofstarting torque are seen to be almost the same even usingdifferent velocities of current. Therefore, it is understood thatthe pulsation of the starting torque during a circle of revolutionis equivalent in all the cases, and the pulsation becomes larger inproportion to a square of current velocity. The samecharacteristic is seen in other awater turbines.

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0 60 120 180 240 300 3608 (deg)

Fig.5 Start&g torque characteristics (Blade type: H4)Fig.6 shows the standardized values of the starting torque at

each solidity cr of four types of helical blade water turbines(e43.7) and a straight blade water turbine when V=l.Om/s.The straight blade water turbine (Blade type: S) was used for thecomparison Iwith experimental data of ~0.18 used in document(Shiono, Suzuki,, and Kiho 1998). Fig.6 reveals that there is anobvious difference in the pulsation between the straight bladeand the helical blades, and compared with the straight blade, thehelical blades had quite a small range of fluctuation of torque inproportion to 8.

2.5

2.0

cg 1.5tk 1.0

0.5

0.0

r-,I +H2 -A-H3 -E-H4 +?-H5 XS I

0 60 120 1806 (deg)

240 300 360

Fig.6 Startin,g torque characteristics (I/=l.Om/s, ~=43.7~)Next, Table 3 shows a comparison of the numerical values ofFig.6. Compared with the straight blade, it is understood thatthe helical blades had larger Ta smaller IL and quite a smaller y.This therefore indicates that the helical blades have a favorablestarting characteristic at any 0. Moreover, a comparison amongthe helical blades used reveals that the larger B becomes, thegreater Ta is. This can be presumed from the fact that solidity cris in proportion to the area of the blade while the torque is alsowith respect to the area of the blade.Fig.7 shows starting characteristics when 0=0.4 and V=l.Om/sin order to examine the effect from $J. Seen from this, the rangeof fluctuation, Tb, does not seem to be affected by values of I .Table 4 shows Tu in proportion to $J. It is understood that

Table 3. Ripple factor y (V=l.Om/s)Blade type

H2H3

o.;o0.30

Ta (Nm) TlJ (W Y0.96 0.23 0.241.20 0.41 0.34

H4 0.40 1.28 0.65 0.51H5 0.50 1.36 1.01 0.74S 0.18 0.56 1.89 3.35

1.21.00.8

L 0.6il

-A- @ =5000.2I

j. . . . . . . . . . . .8 ~=SOO ..

0.0 II 3 I I I I0 60 120 180 240 300 360

6 kJeg)Fig.7 Starting torque characteristics (0=0.4, V=l.Om/s)Table 4. Averaging starting torque (V=l.Om/s)

Tl Tmax s almost the same at all velocities of current. Based onthis, the starting torque will not be affected by 4 but isdependant upon only crThe results above reveal that compared with the straight blade,the helical blade has a smaller rate of pulsation and a favorablestarting characteristic. Moreover, it is also understood that thestarting torque can be largely affected by solidity rather than byblade inclination angles.

oad characteristicsFig.8 shows an example of water turbine output in proportion towater turbine revolution velocity in the water turbine when e0.4 and ~=43.7. Moreover, values of Pt were converted intovalues per a unit length (lm) of h. Since water turbine output isin proportion to a cube of the current velocity V it is understoodthat the peak values at all the current velocities increase inproportion to almost a cube of the number of revolutions, N.The same characteristics were seen in the other helical bladewater turbines used in this study.

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80 I ,I i * V=1,4m/s[ + V=f.Zm/s60 . : +T V=l.Om/s: 8 V=O..Bm/si j +3 V=O.6m /sz40 . . . .k 1 I0 50 100 150 200 0 50 100 150

N rpm N b-pmFig.8 Water turbine output characteristics (Blade type: H4) Fig.10 Torque characteristics (V=l.Om/~, =43.7~)Fig.9 shows; the water turbine efficiency using different tipspeed ratios in the water turbines when -0.4. Without anyeffect from current velocities, the maximum efficiency, ll-15%,is seen around ,X=1.2-1.3, while the water turbines were stalledaround d=O.8-0.9. Therefore, the tip speed ratio, whichindicates the maximum efficiency, the non-loading conditions aswell as the stalling conditions, is understood to be almostconsistent among the turbines without any effect from thevelocity of current.

2o - I. . . . i

-* V=?.Zm/s: V=LOm/..-IS- V=O.8m/s43 V=O..Gm/s

0.5 1.0 1.5 2. 0Fig.9 Water turbine efficiency chaiacteristics (Blade type: H4)Next, the effect of solidity cr on the water turbine characteristicswas measured. Fig.10 shows the torque characteristic whenV=l.Om/s, while Fig.11 shows the water turbine efficiency. Fig.10 reveals that the larger a is, the more the peak value ofgenerated torque becomes, and then the peak value is shifted toa lower A. This is considered to be because as the area of theblades become larger, the revolution force increases, whichcauses an increase in the effect of turbulence in the current,which leads to a decrease in the velocity of current. In Fig.11,the larger 0 is, the peak of q is shifted to a smaller A. The peakvalue increa.ses around 0=0.2-0.4, and shows a maximum of14.5%, but it decrease when 0=0.5. Therefore, it is understoodthat there is an optimum value of solidity existing at the peakvalue of the water turbine efficiency.

?t 2. 0

15

10sw

5

1

0 _I0 0 0.5 1.0 1.5 2.0 2.5 3. 0R

Fig.11 Water turbine efficiency characteristics (V=l.Om/s,4F43.7)Fig.12 is the generated torque characteristics in proportion to thevelocity of revolution when V=1.2m/s. Moreover, values of Twere converted into values per a unit length (lm) of h so thatthey could be compared among different heights of waterturbines. Note that $=90.0 (presumed value) is the value using

10

8

16th4

20

........................; .

................

0 50 100 150N t- pm

Fig.12 Torque characteristics (V=1.2m/s,a=0.4)

200

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the straight blade (~0.4). In order to compare with the helicalblade, this is the presumed value when the torque characteristicsof the straight blade were covered and obtained when 0=0.366and 0.446 balsed on document (Shiono, Suzuki, and Kiho 1998).Fig.12 reveals that as the blade inclination angle becomes larger,the value of the torque also becomes greater. This is consideredto be because the current velocity, striking on the water turbineblades, makes the vertical velocity components larger againstthe front edg,e of the water turbine blade.Moreover, Fig.13 shows the water turbine efficiency based onthe tip speed ratios when V=1.2m/s. Note that $1=90.0(presumed value) is the value using the straight blade (0=0.4).Table 5 shows the maximum water turbine efficiency of all thevelocities of current used in this study.

T 20Ebl 5

1050

0.0 0.5 1.0 1.5 2.0 2.5/ I

Fig.13 Water turbine efficiency characteristics (V=l.%m/s,a =0.4)Table 5. The: maximum water turbine efficiency em (0=0.4)

Blade

Based on tire results, it is understood that the larger 4 is, thelarger ?I is, and a indicating qrn is shifted to a higher value.That is to say, as the blade inclination angle is larger, the torqueduring revolution and the water turbine efficiency both becomehigher. It can be said that the characteristics of helical bladewater turbines become closer to those of straight blade waterturbines.

CONCLUSIONIn this study, the hydrographic experiment was conducted using

seven types of helical blade water turbines, devised for thisstudy, for obtaining the starting torque and load characteristics.The results obtained are summarized as follows:1)2)

g;

Helical blades have quite a smaller rate of pulsation and more favorable starting characteristic than straight blades.In helical blades, the starting characteristic does not showany significant difference due to the use of different bladeinclination angles but is greatly affected by solidity.The highest water turbine efficiency is seen at 0.4 solidity.The larger the blade inclination angle is, the higher thetorque and the water turbine efficiency become. Thischaracteristic is close to that of straight blade water turbines,

Based on the results above, the helical blade water turbine isbetter in starting, while the straight blade water turbine is betterin energy production. Moreover, it is considered appropriate touse helical blade water turbines with unlimitedly large bladeinclination angles.REFERENCESKiho, S. and Shiono, M. (1991) . Electric power generationfrom tidal currents by Darrieus turbine at Kurushima straits,Trans. I. E. E. ofJapan, Vol.l12-D, No.6, pp.530-538.Shiono, M., Suzuki, K., and Kiho, S. (1999). Experiments onthe Characteristics of Darrieus Turbine for the Tidal PowerGeneration, Proceedings of the Ninth International offshoreand Polar Engineering Conference, pp.123-128.Alexander M.Gorov (1998).Helical Turbines for the GulfStream: Conceptual Approach to Design of a Large-ScaleFloating Power Farm, Marine Technology, Vol.35, No.3,pp.175-182.Abbott, Ira H. and von Doenhoff, Albert E. (1959). Theory ofwing section. Dover Publication Inc. p.239.Shiono,M.,Suzuki,K., and Kiho,S. (1998). The experimentalstudy on characteristics of Darrieus type turbine for the tidalpower generation, Trans. I. E. E. of Japan, Vol.l18-B, No.7/8,pp.781-787.

Therefore, according to Fig.12 and 13, among the helical bladewater turbines, the water turbines with unlimitedly large bladeinclination angles have large torque and large water turbineefficiency. In the final analysis, we came to a conclusion thatfor energy production it is more appropriate to use straight bladewater turbines than helical blade water turbines.

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