Output Characteristics of Darrieus Water Turbine with Helical Blades for Tidal Current Generations PDF

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Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference Kitakyushu, Japan, May 26 –31, 2002 Copyright © 2002 by The International Society of Offshore and Polar Engineers ISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set) Output Characteristics of Darrieus Water Turbine ...

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Output Characteristics of Darrieus Water Turbine with Helical Blades for Tidal Current Generations Oscar Reyes Rodriguez

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Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference Kitakyushu, Japan, May 26 –31, 2002 Copyright © 2002 by The International Society of Offshore and Polar Engineers ISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set)

Output Characteristics

of Darrieus Water Turbine with Helical Blades for Tidal Current Generations

Mitsuhiro Shiono, Kdsuyuki Suzuki, Sezji Kiho Department of Electrical Engineering, College of Science & Technology, Nihon University Tokyo, Japan

Therefore, tidal current energy is considered to be a highly available energy source and as yet not fully utilized. Darrieus water turbines used for tidal current generations were originally developed for wind force generations, but the authors used them for tidal current generations. Then, straight blade Darrieus water turbines were developed with generators in order to solve problems of the design and strength of the turbines when converting from wind to water use (Kiho and Shiono 1991). Moreover, since the water turbine efficiency of straight blade Darrieus water turbines can be altered by changes of solidity, the authors have clarified that there is an optimum solidity existing in order to obtain maximum efficiency (Shiono, Suzuki and Kiho 1999). It is also pointed out that the present straight blade water turbines have difficulty starting, and in order to cope with this problem, helical blade water turbines were developed by making the straight blades spiraled (Gorov, 1998). However, there are very few experiments regarding the characteristics and effectiveness of these helical blade water turbines.

ABSTRACT Tidal current generation uses a generator to produce energy, changing the kinetic energy of current into a turning force by setting a water turbine in the tidal current. Therefore, it is considered to be very advantageous to use a water turbine that can always revolve in a fixed direction without any influence Water turbines with these from tidal current directions. Darrieus characteristics are known as Darrieus water turbines. water turbines have a difficulty in starting, but these days Darrieus waiter turbines have been developed with helical blades, which make it easy to get the turbines started. However, there are very few reports regarding Darrieus water turbines with helical blades, and therefore their characteristics are unknown. From the above points of view, this study devises and investigates helical blade Darrieus water turbines to clarify their characteristics through hydrographic experiments, and at the same time, it compares the characteristics of helical blade Darrieus water turbines with those of straight blade ones.

KEY WORDS: Tidal Current Generations, Turbine, Stratight Blade, Helical Blade, Solidity

Darrieus

Therefore, first, the authors compared the characteristics of helical blade water turbines and straight blade ones in a hydrographic experiment. The results indicated the starting effectiveness of helical blade turbines. Moreover, like straight blade water turbines, it is presumed that helical blade turbines also have an optimum value of solidity to obtain maximum efficiency. Therefore, helical blade water turbines were created using different solidity types and examined to clarify the output characteristics of the water turbines. Moreover, in order to reveal the influence of blade inclination angles on the performance of helical blade water turbines, hydrographic experiments were conducted using water turbines with different heights. The results of these experiments are reported in this paper.

Water

INTRODUCTION Production ad renewable energies has been focused on due to problems such as environmental preservation and the shortage of energy sulpplies in the future. Oceanic energies, except tidal current generations, still have many difficult problems to be solved, whidh seems to require a lot of time to make their use However, there have been reports showing that our practical. country, 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 is approximately a thousand times more than that by wind force.

859

DARRIEUS

WATER TURBINES

Shapes of Blades in Water Turbines

zyxwv Tested water turbines

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

-h lcol5

F

--

Table 1 shows the parameters of tested water turbines used in the experiments. Four types of helical blade water turbines were devised, composed of different lengths of blade chords in order to measure and compare their different characteristics by solidity 0. The dimension of these four types were all d= 300mm and h=300mm.

d

=-I

Table 1. Parameters

h r& 5 _-

Blade type H2

1

1

of tested water turbines

(5 0.20

1 C(mm) 1 62.8

1 1

#(deg) 43.7

1 h(mm) 1 300

1

zyxwvutsrqp zyxwvutsrqp (a) Straight blade

(b) Helical blade

Moreover, two types of water turbines were devised using two different heights, for the purpose of measuring their characteristics according to blade inclination angles of helical blades.

Fig.1 Blade shapes of water turbines

Here, solidity cr is a value that significantly affects the performance of water turbines, and Eq.1 uses the following variables: the length of a blade chord is C, the number of blades is n and the diameter of a water turbine is d, and expressed as follows:

OS-

nC 3rd

Furthermore,

using NACA 633-018 for the blade shape

(Abbott

and Doenhoff, 1959)) which is symmetrical and straight, model water turbine blades were devised by making curved lines overlapped on the circumference of the water turbine revolution so that generated torque is not influenced due to different location angles. Moreover, the number of blades was set at 3 for all the turbines used.

(1)

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

EXPERIMENTAL Experimental

APPARATUS

AND METHOD

apparatus

Fig.3 shows the experiment apparatus, Darrieus water turbine used in the hydrographic experiment. Water turbines tested were positioned so that the upper surface was located 1.5cm under the water surface and the revolution axis was vertical. Then, electromagnetic breaks were attached through a torque detector as loads of the water turbines. The range possible for measurement using a circular tank is 3.Om in width, 1.5m in depth and 30m in length. The fluid energy was in proportion to the cube of the current velocity, and the current velocity was carefully measured since the determination of the current velocity values can be used as a standard of water turbine characteristics. Therefore, an electromagnetic tachometer was located at the upper stream far from the water turbine so that the water turbine characteristics would not be influenced by current turbulence, which is caused by probes.

-,

Fig.2 Develolped sketch of a helical blade water turbine

860

3.5

ectromagnetic

break

3.0 2.5 ? k

2.0

(2 1.5 1.0 0.5 I

I

0

I

I

60

120

180

240

I

300

360

B ,(deg)

Fig.3 Darrieus water turbine experimental

apparatus

4

Starting torque experiment The current velocity values were determined as the following three types: 0.6, 1.0, and 1.4m/s. In order not to revolve, the water turbine was fixed by inserting pins in the holes made in 5 degrees intervals on the electromagnetic break disc directly connected to1the water turbine axis. Then, the starting torque at the water revolving angle was measured every 5 degrees from the datum location as shown in Fig.4 when looking from the upper part of the water turbine. The measurement value was determined to be the average for 30 seconds. Load characteristic

Fig.4 Starting torque characteristics

Revolution

A ngle : 6

zyx

(Blade type: H4)

circle of revolution was determined as Tma~ and the minimum torque was determined as Tmin, the range of TS fluctuations, Tb, can be expressed as Eq.3. Tb =T mm-T

min

(3)

And when the average starting torque was determined as Ta, the rate of pulsation, y, can be obtained by the following equation:

experiment

The current velocity values were set as follows: 0.6,0.8, 1.0, 1.2 and 1.4m/s. The experimental method is as follows: After the current velocity was set and the current became stable, the water turbine was first revolved without any loads. Then, loads were gradually added to the revolution by an electromagnetic break, and the velocity of revolution and generated torque were measured until the water turbine was stalled and completely stopped. The measurement value was determined to be the average for 30 seconds.

Tb

Tmax-Tmin

y=z=

(4)

Ta

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

RESULTS AND DISCUSSION

zyxwvutsr zyxwvut v6-w

Starting Torque Experiment

0.6 1.0 1.4

Fig.4 shows the measurements of starting torque, Ts, at a revolution angle 8 of the water turbine with c~=O.4 and e43.7”. Moreover, values of TS were converted into values per a unit length (lm) of h so that they could be compared among different heights of water turbines. The figure reveals that the higher the current velocity became, the larger the torque was. This is because the torque is in proportion to a square of current velocity. Moreover, since the fluctuations of TS are seen in proportion to 0, Table 2 indicates the fluctuations of TS at each current velocity. Here, when the maximum torque during a

Ta

(Nm)

0.45 1.28 2.46

Tb WI 0.29 0.65 1.33

Y 0.64 0.50 0.54

Moreover, TS was standardized after Tmax had been detected in Fig.4, which is shown in Fig.5. The standardized values of starting torque are seen to be almost the same even using different velocities of current. Therefore, it is understood that the pulsation of the starting torque during a circle of revolution is equivalent in all the cases, and the pulsation becomes larger in proportion to a square of current velocity. The same characteristic is seen in other awater turbines.

861

Table 3. Ripple factor y (V=l.Om/s) Ta (Nm) 0.96 1.20 1.28 1.36 0.56

Blade type o.;o 0.30 0.40 0.50 0.18

H2 H3 H4 H5 S

TlJ (W 0.23 0.41 0.65 1.01 1.89

Y 0.24 0.34 0.51 0.74 3.35

zyxwvutsrqpon 1.2

0

60

120

240

180

8

300

360

1.0

(deg)

Fig.5 Start&g torque characteristics

0.8

(Blade type: H4)

L$ 0.6

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 the comparison Iwith experimental data of ~0.18 used in document (Shiono, Suzuki,, and Kiho 1998). Fig.6 reveals that there is an obvious difference in the pulsation between the straight blade and the helical blades, and compared with the straight blade, the helical blades had quite a small range of fluctuation of torque in proportion to 8. 2.5

2.0

il

I

0.0

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

j.

0.2

’ 0

-A- @ =500” 8

~=SOO”

I

3

I

I

I

60

120

180

240

300

6

I

I

360

kJeg)

Fig.7 Starting torque characteristics

(0=0.4, V=l.Om/s)

Table 4. Averaging

(V=l.Om/s)

starting torque

.‘.

r-,

I

+H2

-A-H3

-E-H4

+?-H5

XS

I

cg 1.5 t k

Tl Tmax is almost the same at all velocities of current. Based on this, the starting torque will not be affected by 4 but is dependant upon only cr

1.0

0.5

0.0 0

60

180

120 6

Fig.6 Startin,g torque characteristics

240

300

360

(deg)

(I/=l.Om/s,#~=43.7~)

Next, Table 3 shows a comparison of the numerical values of Fig.6. Compared with the straight blade, it is understood that the helical blades had larger Ta, smaller IL and quite a smaller y. This therefore indicates that the helical blades have a favorable starting characteristic at any 0. Moreover, a comparison among the helical blades used reveals that the larger B becomes, the greater Ta is. This can be presumed from the fact that solidity cr is in proportion to the area of the blade while the torque is also with respect to the area of the blade. Fig.7 shows starting characteristics when 0=0.4 and V=l.Om/s in order to examine the effect from $J. Seen from this, the range of fluctuation, Tb, does not seem to be affected by values of I#. Table 4 shows Tu in proportion to $J. It is understood that

zyxwv

The results above reveal that compared with the straight blade, the helical blade has a smaller rate of pulsation and a favorable starting characteristic. Moreover, it is also understood that the starting torque can be largely affected by solidity rather than by blade inclination angles. Load characteristics

Fig.8 shows an example of water turbine output in proportion to water turbine revolution velocity in the water turbine when e 0.4 and #~=43.7”. Moreover, values of Pt were converted into values per a unit length (lm) of h. Since water turbine output is in proportion to a cube of the current velocity V, it is understood that the peak values at all the current velocities increase in proportion to almost a cube of the number of revolutions, N. The same characteristics were seen in the other helical blade water turbines used in this study.

862

80

,

I

‘i‘I 0

50

[

+

V=1,4m/s V=f.Zm/s V=l.Om/s

:

8

V=O..Bm/s

j

+3

V=O.6m/s

1

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

z40 k

*

.:. +T

........ ................i

60

i

100

150

?

I

.

t 2.0 h

50

0

200

100

Fig.8 Water turbine output characteristics

(Blade type: H4)

Fig.10 Torque characteristics

Fig.9 shows; the water turbine efficiency using different tip speed ratios in the water turbines when -0.4. Without any effect from current velocities, the maximum efficiency, ll-15%, is seen around ,X=1.2-1.3, while the water turbines were stalled around d=O.8-0.9. Therefore, the tip speed ratio, which indicates the maximum efficiency, the non-loading conditions as well as the stalling conditions, is understood to be almost consistent among the turbines without any effect from the velocity of current.

(V=l.Om/~,#=43.7~)

15

1

10 s w 5

I

2or-

_I

0

0.0

1.0

0.5

1.5

. .. . .. i

V=?.Zm/s

:&

V=LOm/..

-IS- V=O.8m/s

Fig.9 Water turbine efficiency

3.0

1.0

chaiacteristics

1.5

turbine

efficiency

characteristics

(V=l.Om/s,

Fig.12 is the generated torque characteristics in proportion to the velocity of revolution when V=1.2m/s. Moreover, values of T were converted into values per a unit length (lm) of h so that they could be compared among different heights of water turbines. Note that $=90.0“ (presumed value) is the value using

43 V=O..Gm/s 0.5

2.5

2.0

R

Fig.11 Water 4F43.7”) -*

150

N b-pm)

N (rpm)

2.0

(Blade type: H4) 10

Next, the effect of solidity cr on the water turbine characteristics was measured. Fig.10 shows the torque characteristic when V=l.Om/s, while Fig.11 shows the water turbine efficiency. Fig. 10 reveals that the larger a is, the more the peak value of generated torque becomes, and then the peak value is shifted to a lower A. This is considered to be because as the area of the blades become larger, the revolution force increases, which causes 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 peak value increa.ses around 0=0.2-0.4, and shows a maximum of 14.5%, but it decrease when 0=0.5. Therefore, it is understood that there is an optimum value of solidity existing at the peak value of the ‘water turbine efficiency.

8

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

16 t h4

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

2 0 0

50

100 N (t-pm)

Fig.12 Torque characteristics

863

...