J. of Thermal Science Vol. 12, No.4
Comparison of Performances of Turbines for Wave Energy Conversion
Yoichi Kinoue Toshiaki Setoguchi Tomohiko Kuroda Kenji Kaneko Department of Mechanical Engineering, Saga University, 1, Honjo, Saga, 840-8502 Japan
Manabu Takao Dept. of Control Eng., Matsue National College of Tech., 14-4, Nishiikuma, Matsue, Shimane, 690-8518 Japan
Ajit Thakker Department of Mechanical & Aeronautical Engineering, University of Limerick, Limerick, Ireland
The Wells turbine for a wave power generator is a self-rectifying air turbine that is available for an energy conversion in an oscillating water-air column without any rectifying valve. The objective of this paper is to compare the performances of the Wells turbines in which the profile of blade are NACA0020, NACA0015, CA9 and HSIM15-262123-1576 in the small-scale model testing. The running characteristics in the steady flow, the start and running characteristics in the sinusoidal flow and the hysteretic characteristics in the sinusoidal flow were investigated for four kinds of turbine. As a conclusion, the turbine in which the profile of blade is NACA0020 has the best performances among 4 turbines for the running and starting characteristics in the small-scale model testing.
Keywords: fluid machinery, Wells turbine, airfoil, wave energy conversion, ocean energy.
Several of the wave energy devices being studied under many wave energy programs make use the principle of the oscillating water column (OWC). Potentially, the most successful device used in harnessing on wave energy has been the OWC wave energy converter. The OWC chamber, either floating or bottom standing, immersed and opened to the action of sea. A reciprocating airflow is created by the action of the free surface of the water within the chamber. The conversion of this airflow into mechanical energy may be achieved by means of a number of devices.
The representative wave energy converter using the principle of OWC is the navigational light-buoy inverted in Japan. More than one thousand navigation buoys have been produced since 1965 and marketed worldwide. Some of them have been in operation for more than 30 years. A conventional air turbine such as the Francis turbine and a system of non-return valves for rectifying the airflow have been adapted in the early navigation buoy. However, the airflow rectification system with non-return valves is complicated and difficult to maintain. In order to overcome these weak points, in recent years,
the navigation buoy with a Wells turbine which is a self-rectifying axial flow turbine for wave power conversion has been also developed instead of that with the conventional turbine and the airflow rectification system.
Since the Wells turbine installed in the navigation buoy is much smaller in comparison with that in the wave energy plant such as the LIMPET system, Islay, U.K. (that is, the Wells turbine in the navigation buoy is operated at low Reynolds number), it is important to clarify the effect of rotor geometry on the performance of a small-scale Wells turbine. There are many reports which describe the performance of the Wells turbine both at starting and running characteristics, and the following rotor blade profiles have been recommended by previous studies: NACA0020 (Saga University, Japan)Vl; CA9(University of Limerick, Ireland)E21; and HSIM 15-262123-1576 (The Technical University of Lisbon, Portugal) TM. CA9 and HSIM15-262123-1576 (it is referred as HSIM15 in this paper) were proposed as newly devised blades in order to postpone the blade stall (to extend the operation range of Wells turbine).
324 Joumal of Thermal Science, Vol.12, No.4, 2003
Table 1 Specifications of turbine
Airfoil Thickness Remarks ratio
0.15 HSIM 15- 262123 - 1576
Rotor diameter = 0.3 m Hub to tip ratio = 0.6 Tip clearance = 1 nun Chord length I = 60 mm Aspect ratio AR = 1
Number of blades z = 8 Gr = 0.64 g = 0.35 Solidity at mean radius
However the characteristics of these blade profiles under unsteady flow condition have not been compared so far. The aim of this paper is to clarify the effect of rotor geometry on the performance of the small-scale Wells turbine. In the study, four kinds of symmetrical blade profile were selected from previous studies with regard to the blade profile of the Wells turbine. The types of blade profile included in the paper are as follows: NACA0020; NACA0015; CA9; and HSIM 15-262123- 1576 (Table 1 show the specifications of four kinds of turbine). In order to determine the optimum rotor geometry of the turbine, the experimental investigations have been performed by model testing under steady flow conditions, and then the effect of blade profile on the running and starting characteristics under sinusoidal flow conditions have been investigated by a numerical simulation using a quasi-steady analysis. In addition, the hysteretic characteristics in an oscillating flow have been investigated.
Experimental Apparatus and Procedure
A schematic view of the test rig is shown in Fig.1. The test rig consists of a large piston-cylinder (1.4 m of diameter, 1.7 m of length), one end of which is followed by a setting chamber. Turbine testing is done in 300 nun diameter test section with bell-mouthed entry/exit at both ends. The piston can be driven back and forth inside the cylinder by means of three ball-screws through three nuts fixed to the piston. All three screws are driven in unison by a D.C. servo-motor/generator through chain and sprockets. A computer controls the motor, and hence the piston velocity to produce any flow velocity. The test turbine is coupled to a servo-motor/generator through a torque transducer. The motor/generator is electrically controlled such that the turbine shaft angular velocity is held constant at any set value. The overall performance was evaluated by the turbine output torque To, the air flow rate Q, the total pressure drop across the turbine AP, and the turbine angular velocity to. Tests were performed with the flow rates up to 0.320 m3/s and the turbine angular velocities up to 524 rad/s. The uncertainty of efficiency is about 1%. This uncertainty has been
obtained by taking into account the dispersions in the measurement of the physical parameters from which efficiency is obtained. The Reynolds number based on the blade chord and relative velocity at mean radius rR was approximately 2.2x105 at a peak efficiency point under steady flow condition.
/3 /Z ~,t 16
I Cylinder 9 Turbine 2 Piston 10 Torque transducer 3 Ball-screw 11 ServomoLor-generator 4 Servomotor 1t Pressure transducer 5 D/A converter 13 A/O converter 6 Micro-computer 14 Ricro-computer 7 Potentiometer 15 Test section
8 Servo-pack 16 Sett l ing chamber
Fig.1 Experimental apparatus
Experimental Results and Discussions
Turbine characteristics under steady flow conditions
The turbine performance under steady flow conditions is evaluated by turbine efficiency 0, torque coefficient Cr and input coefficient CA against flow coefficient ~. The definitions of these parameters are as follows:
o = Toto/(~a'Q) = Cr / (CA(~) (1)
Cr = To/[p(v2a + U 2 e )zblrR / 2] (2)
CA = APoQ/[p( vJ + U~ )zblva / 2] (3)
~) = Va [UR (4)
where rR, vo, (JR, b and z denote the density of air, axial flow velocity, circumferential velocity at rR, blade height and number of rotor blades, respectively.
Yoichi Kinoue et al. Comparison of Performances of Turbines for Wave Energy Conversion 325
Figs.2(a), (b) and (c) show the experimental data of Cr, CA and 7"/under steady flow condition, respectively. In the figures, the solid line shows the data of CA9, the broken line NACA0020, the one-dotted line NACA0015 and the dotted line HSIM15, respect]vely. It can be observed from Fig.2(c) that the value of the maximum efficiency for NACA0020 is the highest among four turbines. In addition, it can be observed from Fig.2(a) that the operation range for NACA0020 is the widest among four turbines (the stall point for NACA0020 is 0=0.319).
- - CA9 It ! I . . . . NACA0020
I . . . . NACA0015
~ 1 . . . . . . . . "2H2S1~3 -17576
. - - - . _ . . . ' . . . " "2
, I , I , I ~ I , 0.2 0.4 0 .6 0.8 1.0
(a) Torque coefficient for steady flow condition
8 , i , i , i , i
6 .. 'e,
5 ..0~.'" 4 . , '~
3 - - - - NACA0020
2 /~" . . . . NACA0015
1 ] " . . . . . . . . HSIM 15 / -262123-1576
i I B I B I t 0 0 2 0 .4 0 .6 0 .8 1.0
(b) Total pressure coefficient for steady flow condition
- - CA9 . . . . NACA0020 . . . . NACA0015 . . . . . . . . HSIM 15
t,,~-.- -~.-.'7 .--- ~ -_'r- --,- 0.4 0.6 0.8 1.0
(c) Efficiency for steady flow condition
Turbine characteristics under sinusoidal flow conditions
Since th~ airflow into the turbine is generated by the OWC, it is very important to demonstrate the turbine characteristics under oscillating flow conditions. The steady flow character- ristics of the turbine as shown in Fig.2 are assumed to be valid for computing performance under unsteady flow conditions.
The turbine characteristics under unsteady flow conditions are estimated in the starting and running characteristics. The starting characteristics of the turbine are evaluated by the variation in rotational speed from the rest point. The equation of motion for a rotating system of turbine is written by the following equation:
I(do9 / dt) + TL = To (5)
where L t and TL are the moment of inertia of rotor, time and loading torque.
When the turbine is in the running condition, the parameters such as To, o9, AP and Q vary periodically in a sinusoidal oscillating flow. In this case, the turbine performances should be represented by mean value such as mean efficiency. The running characteristics of the turbine under sinusoidal flow conditions are evaluated by the mean efficiency q against the flow coefficient @, which is defined as follows:
= Va I UR (7)
where T and V, denote the period of wave motion and the maximum value of the axial flow velocity, respectively.
Fig.3 shows the mean efficiencies for four turbines under sinusoidal flow conditions. The tendencies of the data of the efficiencies are almost the same as the ones in Fig.2(c). Fig.4 shows the starting characteristics for four turbines under sinusoidal flow condition without any
~ x - - CA9 / ' f '~ I . . . . NACA0020
I/ t I . . . . NACA0015 t", ! It . . . . . . . . HSIM 15
't /'1 62123,576 ',.. i
0.2 0.4 0.6 0.8 1.0
Fig.2 Fig.3 Efficiency for sinusoidal flow eondi~on
326 Journal of Thermal Science, Vol. 12, No.4, 2003
_ 1 0 | , , , , , .
o F NACA0020 ~,' tJ)C'~""""dlff ~w FX 1 = 122 ,4 ! , [ N ' '
/ \ " 7,.': # CA9 6 t- ^.?1 J: d Xs---121.f
INACAO0151 r." g [~/= 114.7 I is~.~l ~1
41- ~--~_d ~ " ! ~ 'L , "
[ , y / :~212"3--1576
1. ,~:~=~, . . .~s = t 3s ~ los ~ I XL= 01 ,
0 5 10 15 20 t
Fig.4 Starting characteristics under sinusoidal flow condition
0.5 i i ,
~Y 0.4 NACA0020 ~,
0 =0.637 ~./"/ i V=0"6 ,~/",/ i
0 .... /...-" J - - Unsteady ~ ............ Steady
-0.1 ' d l ' 01 .2 ' 01.3 ' 0.4
0 (a) Hysteretic loop of Crfor NACA0020
4 , , ,
~-~ NACA0020 (7 = 0.637 ..-'""
3 v = 0.6 , .~
1 / . . .~" - - Unsteady
~ , . - , ...... i sTadY, 0 0.1 0.2 0.3 0.4
(b) Hysteretic loop of Ca for NACA0020
load on the turbine. The result are given in the form of non-dimensional angular velocity to versus dimension- less time t*, and S, X1 and XL in the figure are the dimensionless frequency, dimensionless moment of inertia and dimensionless loading torque, respectively. It is found that all four turbines can start by themselves, but the starting characteristics for NACA0020 is superior than the others because the turbine for NACA0020 can reach asymptotic value of w* faster than the others.
Hysteresis in an oscillating flow It is known that hysteresis of the Wells turbine
occurs at angles of attack lower than the stall angle in a
0.4 , , , , ,
~'~ NACA0015 0.3 7 = 0.637 ~.
V :0:6 ~/ ' ) / i yi 0.1 "~.
0 - - Unsteady ............ Steady
?o . . . . . . .'2 -0.1 0 5" ' 0.1 0.15 0.2 0 5 ' 0.3
(a) Hysteretic loop of Cr for NACA0015
3.0 i i i i ,
~2.5 NACA0015 _ _ . " ' ' = 0.637 / 'Y (7
V = 0.6 .~. ' J 2.0
1.5 ~ /
~. . '~ - - Unsteady 0.5 . f / . / ............ Steady
f * / . I " t , i . I i I i
0.05 0.1 0.15 0.2 0.25 0.3
(b) Hysteretic loop of CA for NACA0015
08 1 081 . . . . . . 0.4 ::" i i 0.4 \
02 /(7_-0637 . / 02 / (7:0.637 \ [ i [ V- -0.6 ". l i / ~=0.319 ...... "q [ I / . J V --0.6 / / =o26 l i i - - _~te?dy ~ t I i / - - U.~t~ad:
" 0.1 0.2 0.3 0.4 0.15~ 0.3 " 0 0.05 0.1 0.2 .
(c) Hysteretic loop of 7/for NACA0020 (c) Hysteretic loop of 7/for NACA0015
Yoichi Kinoue et al. Comparison of Performances of Turbines for Wave Energy Conversion 327
reciprocating flow . The result clarifies that hysteresis occurs due to the different behavior of wakes between an increasing process and a decreasing process in the same angle of attack.
Figs.5 - 8 show the hysteretic loops for NACA0020, NACA0015, CA9 and HSIM15, respectively. In the figures, the solid lines show the unsteady (sinusoidal) data and the dotted lines show the steady data. Hysteretic loops were investigated within the operation range of turbines. It is found that all turbine have similar hysteretic loops.
I I I I I
c'--637 /:7 V =0.6 / . : / .
~ - - Unsteady ............ Steady
, i , i , 0 . i15 , I , i , 0.05 0.1 0.2 0.25 0.3
(a) Hysteretic loop of Cr for CA9
i i i i i
CA9 . . z? ..-/ 0 = 0.637 M.4 z'' V =0.6 ~..-jV
~. " / - - Unsteady ............ Steady Y,
0.05 0.1 0.15 0.2 0.25 0.3
(b) Hysterelic loop of CA for CA9
i i i i i
~ , , , : " CA9 ",,
/ / v=o6 ',,,., : / ~= 0.26
/ - - Unsteady . / . - - : " , Steady,
0.05 0.1 0.15 0.2 0.25 0.3
(c) Hysterefic loop of 77 for CA9
HSIM 15 -262123-1576
a = 0.637 v =0.6
" , , ,o. . . . . . . . . . . . . . . . . . . . . .
- - Unstead .......... Steady
0.05 0.1 0.15 0.2 0.25 0.3
(a) Hysteric loop of Cr for HSIM15
2.5 -262 t 23-1576 ~ = 0.637 v =0.6
2.0 ~ = 0.146
I I I .'.."
.,.,"" . , '""
1 . 0 ~
i ""i'"'1"" Steady i I i
0 0.05 0.1 0.15 0.2 0.25 0.3
(b) Hystew.fic loop of CA for HSIM15
HSIM '15 ' '
i i i
O" = 0 .637
v =0.6 @ =0.146
" ' .
/ ' ;f'
i~ 0.05 0.1
- - Unsteady ...... Steady
_0 .20 I , I , I J 0.15 0.2 0.25 0.3 #
(c) Hysteretic loop of 77 for HSIM15 F ig .8
(1) The running characteristics in the steady flow were investigated in the small-scale model testing and