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Proceedings of the ASME 2014 Dynamic Systems &Control Conference
DSCC 2014October 22-24, 2014, San Antonio, USA
DSCC2014/5916
DRAFT: EXPERIMENTAL STUDY OF AN OMNI-DIRECTIONAL WIND FLUTTERINGENERGY HARVESTER
Emmanuelle Arroyo ∗SUTD-MIT International Design Centre
Singapore University of Technology and Design20 Dover Drive
138682 SingaporeEmail: e [email protected]
Shaohui FoongEngineering Product Development
Singapore University of Technology and DesignEmail: [email protected]
Kristin L. WoodEngineering Product Development
SUTD-MIT International Design CentreSingapore University of Technology and Design
Email: [email protected]
Luc MarechalEngineering Product Development
Singapore University of Technology and DesignEmail: luc [email protected]
ABSTRACTWind energy harvesters based on the fluttering phenomena is
an emerging area that offers an efficient alternative approach totraditional wind turbines. These systems contain no mechanicalbearings or gears which allow longer operating life expectancyand lower manufacturing costs. This paper presents and investi-gates a novel wind-belt energy harvester that has been designedto harvest wind energy in an environment with rapidly chang-ing wind speeds and directions. An experimental study of thiswind-to-vibration converter explores and evaluates the effects ofribbon tension, length and coil position on maximum vibrationacceleration.
INTRODUCTIONThis papers presents an experimental analysis of an innova-
tive wind energy harvester based on the fluttering effect. Aeroe-lastic flutter is a self-feeding vibration of an elastic structure ina fluid flow. It occurs from the flutter limit wind speed, when
∗Address all correspondence to this author.
there is a positive feedback between the structure natural vibra-tion and the aerodynamic forces exerted by the fluid flow. Theamplitude of vibration will increase, only limited by the mechan-ical and aerodynamic damping, which can eventually result inlarge amplitude oscillations and structural failure [1]. Flutteringcan be destructive and therefore civil structures exposed to aero-dynamic forces such as wings, bridges or chimneys have to bedesigned and carefully tested in order to avoid fluttering effects.
However it has recently been demonstrated that flutter is notalways a destructive phenomenon. Energy harvesters have beenrealized, that take advantage of the self-feeding vibration of amembrane subjected to a wind flow. A piezoelectric [2, 3] or anelectromagnetic device attached to this membrane is then usedto convert the vibrations into electrical energy. Such a systemoffers a cheap, small and simple alternative to classic rotary windturbines.
The company Humdinger Wind energy [4] has first devel-oped and commercialized an energy harvester based on windfluttering. The harvested power ranges from several milli-wattfor the smallest device to 7.2 kWh for a 1 meter- length windbeltunder a wind speed of 6 ms−1, as specified by the supplier. Kim
1 Copyright © 2014 by ASME
0
2
4
6
8
Win
d s
pee
d (
m/s)
12PM 12PM 12PM 12PM 12PM 12PM 12PM 12PM 12PM 12PM
06 Ju
l 201
3
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l 201
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l 201
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(a) Wind speed over time
0 1.75 3.49 5.24 6.99 8.650
500
1000
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Tim
ec(m
in)
Windcspeedc(m/s)
Avgcspeedc=c1.33cm/s
Mostccommonlycoccurringc
speedc=c0.52cm/s
(b) Wind speed repartition
N
E
S
O
0o
30o
60o
90o
120o
150o
180o210
o
240o
270o
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4358
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Time (min)
(c) Wind direction over time
N
E
S
O
0o
30o
60o
90o
120o
150o
180o210
o
240o
270o
300o
330o
1.83
1.37
0.91
0.46
Mean wind speed (m/s)
(d) Mean wind speed over direction
FIGURE 1. WIND SPEED AND DIRECTION MEASUREMENTSOVER ONE MONTH ON SUTD ROOFTOP IN SINGAPORE.
et al. in 2009 [5] developed a micro-scale windbelt based energyharvester to target the supply of low power systems such as sen-sors networks nodes. The harvester, of volume 0.86 cm3, is madeof a polymer vibrating membrane and a simple electromagneticgenerator made of a two magnets each oscillating inside a copperwounded coil. The prototype generates a peak-to-peak voltage of81 mV at 0.53 kHz. To date, no theoretical study or modeling ofa windbelt based harvester have been performed and only oneacademic paper presents an experimental characterization of aHumdinger windbelt of 50 cm length : Pimentel et al. [6] variedthe ribbon tension from 6 to 59 N and the wind angle of attackfrom 0 to 90◦. They showed that the cut in angle is around 40-55◦ and the cut out angle around 55 -60◦. Beyond these anglesthe fluttering abruptly stops.
This paper aims at extending Pimental et al. work in orderto provide design and optimization guidelines for windbelt-typeenergy harvesters. A thin vertical membrane is subjected to awind flow inducing flutter. The influence of the ribbon length,the mass position along the ribbon and the number of masses onthe fluttering are experimentally studied.
DESIGN OF OMNI-DIRECTIONAL ENERGY HAR-VESTER
The proposed harvester is designed to be used in Singaporewhere it is primarily an urban environment with rapidly changing
Wind-to-
vibrations
converter
Mechanical
vibration
energy
Non usable
electrical
energy
Electromagnetic
transducer
Extraction
circuit
Wind
energy
Usable
electrical
energy
FIGURE 2. STRUCTURE OF THE GLOBAL WIND ENERGYHARVESTER.
fluttering ribbon
moving coil
electromagnetic
parts
FIGURE 3. PHOTO AND DRAWING OF THE PROPOSED GEN-ERATOR.
wind speeds and continuously changing directions, especiallyduring the frequent tropical storms. As an illustration, Fig. 1(a)and 1(b) show measurements of the wind speed during a typicalmonth in Singapore, at a location (1.300284, 103.780590). Themeasurements have been performed on the roof of a four-storybuilding at SUTD. The most common wind speeds are between0 and 5 ms−1 while stronger winds, up to 8.5 ms−1, occur onlyduring short periods of time generally around midday.
The frequency of wind directions measured in the same con-ditions, is plotted in Fig. 1(c) and 1(d). These two figures showthat over a month the wind is most often blowing from south-eastor south-west, but that the strongest winds come from south-westto north-west. It is thus difficult to highlight a prevailing wind di-rection.
Proposed structureThe omni-direction wind energy harvester is made of three
subsystems (Fig. 2):
- A wind-to-vibration converter, which is here a direct con-version system taking advantage of the fluttering effect on athin ribbon.- An electromagnetic transducer to convert the vibratory me-chanical energy into electricity. Rather than a piezoelectrictransducer, an electromagnetic one is chosen. This one ismore suitable in the case of large amplitude oscillations likethe ones induced by fluttering. A fixed magnetic circuit cou-
2 Copyright © 2014 by ASME
L
d
force sensor
displacement sensor
ribbon
stepper motor
mass
FIGURE 4. SCHEMA AND PHOTO OF THE EXPERIMENTALSETUP.
pled with permanent magnets induces a static electromag-netic field while the copper coil, lighter than the magneticcircuit is chosen to be mobile, attached to the ribbon. Whenthere is a relative displacement between the coil and themagnetic field, a voltage is generated across the coil endsaccording to Faraday’s law of induction.- An extraction circuit is used to rectify and regulate the gen-erated voltages.
Only the first subsystem will be studied here, that is to saythe fluttering windbelt used to convert the wind energy into vi-brations. The electromagnetic transducer and extraction circuitwill be studied and optimized in further works.
Given the unstable wind direction, an efficient windbeltbased generator should either be able to rotate around itself ac-cordingly with the wind direction, or to present the ability to flut-ter in all directions. For simplicity reasons, the second solutionis chosen here. The proposed generator is made of 4 vertical ten-sioned ribbons, as shown in Fig. 3. Two of them are alignedtowards the west, the last two are aligned in the perpendiculardirection towards the north. Aerodynamic walls are placed be-tween each two ribbons, making a 45◦ angle with them. As, ac-cording to [6], the flutter effect is only slightly altered when thewind is blowing towards the ribbon with an attack angle smallerthan around 45◦, this structure will allow the fluttering of one orseveral of the ribbon anytime when wind is blowing, whatever itsdirection.
INVESTIGATION OF THE FLUTTERING RIBBONSGiven the symmetric structure of the 4 ribbons, only one will
be studied separately to determine the optimal configuration. Inorder to complete Pimentel’s study, the length of the ribbon, theposition of the mass on it, and the effect of several masses arestudied.
TABLE 1. SUMMARY OF THE RIBBON PARAMETERS.
Valuew 13 mmL {25, 30, 35, 40, 45 } cmd {3, 12, 21, 30, 39, 48, 57, 66 } mmT {0.5, 1, 1.5, 2, 2.5, 3 } N
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Time (s)
Dis
pla
cem
ent
(mm
)
(a) Displacement of the coil before thefluttering limit
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Time (s)D
ispla
cem
ent
(mm
)
(b) Displacement of the coil under flutter-ing
FIGURE 5. RIBBON VIBRATION BEFORE AND AFTER THEFLUTTER LIMIT AT LOW TENSION.
Experimental setup
A PET ribbon of 60 µm thickness and 1.5 cm width is ver-tically tense between one force sensor on the top and a steppermotor on the bottom (Fig. 4). The force sensor retrieves the ten-sion of the ribbon and this information is used to control the step-per motor and adjust the tension to the desired value. This setupallows to get a fixed and reproducible tension of the membrane.A mass, in this case two symmetric wound coils, is fixed on theribbon close to the top edge. A laser displacement sensor point-ing towards the coil is used to measure its vibration amplitudeand frequency. The parameters names and values are detailed inTab. 1.
The device is tested under wind speeds ranging from 1 to10 ms−1. Larger wind speeds are not tested considering that theydo not correspond to practical cases occurring in Singapore atlow altitude.
3 Copyright © 2014 by ASME
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12
Fre
quen
cy (
Hz)
Wind speed (m/s)
0.5
1
1.5
2
3
T
(N)
FIGURE 6. FLUTTER FREQUENCY AS A FUNCTION OF THEWIND SPEED AND THE RIBBON TENSION
Preliminary measurements
First measurements are performed on a 45 cm length ribbonwith two 0.5 g coils attached symmetrically on top of the ribbon,in order to confirm the optimal tension. The tension is variedfrom 0.5 to 3 N, and the frequency and amplitude of vibrationsare recorded over the wind speed, when there is fluttering. As ex-pected, there is a minimal wind speed at which the flutter starts.As an example, Fig. 5(a) and 5(b) show the vibration amplituderespectively before and after the flutter limit in one random casewhen the tension is low. It is obviously much more interesting tobe beyond the fluttering limit, when the amplitude of vibration ismuch larger and stable than before.
Figure 6 shows the frequency of vibration as a function ofthe wind speed for several ribbon tensions. It appears that thefrequency of oscillation slightly increases with the wind speed.It also increases with the ribbon tension. Indeed, the tension levelinfluences the elasticity and thus the stiffness of the ribbon. An-other point is that the more tensioned the ribbon, the higher thewind speed necessary to induce flutter, and of course the higherthe frequency. As we expect the generator to operate mostly atwind speeds below 5 ms−1, the following measurements will fo-cus on a low tension of 0.5 N.
Influence of the ribbon size
The ribbon length, initially at 45 cm is decreased to 35, 30,25, and 20 cm. The coil vibration is recorded for several ribbontensions and for several wind speeds at a tension of 0.5 N. Theamplitude and frequency of the vibration for each ribbon lengthis plotted as a function of the wind speed in Fig. 7. Whateverthe length, the frequency and amplitude have a similar behaviorover the wind speed, and do not show an easily identifiable trend.However, the length influences the limit wind speed, at whichflutter starts. It’s evolution is shown in Fig. 8 for 25, 35 and45 cm. It appears that when the length is decreased the flutterlimit increases and is also less easily identifiable insofar as theflutter is less regular and remarkable. So we shall favor longerribbons in order to harvest power from low wind speeds.
Wind speed (m/s)
2 4 6 8 10
1.4
1.2
0.8
0.4
0RM
S a
mplitu
de
(mm
)
1
0.6
0.2
404535302520
L
(cm)
(a) RMS amplitude of vibration
404535302520
Wind speed (m/s)2 4 6 8 10
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50
40
30
20
10
0
Fre
quen
cy (
Hz)
L
(cm)
(b) Frequency of vibration
FIGURE 7. AMPLITUDE AND FREQUENCY OF THE VIBRA-TION FOR SEVERAL L.
1
2
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35
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0 0.5 8 1.5 2 3 3.5
Tension (N)2.5
L
(cm)
Lim
it w
ind s
pee
d (
m/s)
FIGURE 8. LIMIT WIND SPEED AS A FUNCTION OF THE RIB-BON TENSION FOR SEVERAL L
4
3
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1
0RM
S a
mplitu
de
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)
Wind speed (m/s)
2 4 6 8 10
665748393021123
d
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(a) RMS amplitude of vibration
665748393021123
Wind speed (m/s)
2 4 6 8 10
60
50
40
30
20
10
0
Fre
quen
cy (
Hz)
d
(mm)
(b) Frequency of vibration
FIGURE 9. AMPLITUDE AND FREQUENCY OF THE VIBRA-TION FOR SEVERAL d.
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30
21
12
0
2
4
6
8
10
12
0 1 2 3 4
Tension (N)
Lim
it w
ind s
pee
d (
m/s)
d
(mm)
FIGURE 10. LIMIT WIND SPEED AS A FUNCTION OF THERIBBON TENSION FOR SEVERAL d
Influence of the mass positionThe mass is constituted by two small copper coils fixed sym-
metrically on the top of the ribbon, at a distance d from the clip.Each coil weights 0.5 g. Initially at 3 mm of the clamped edge,the distance d is increased until 66 mm. Figure 9 shows the RMSamplitude and frequency of the coils vibration as a function ofthe wind speed and for each position. As the mass gets closer tothe middle of the ribbon, the torsional motion becomes strongerthan the heaving motion leading to non sinusoidal measurements.These cases are shown in dotted lines in the graphs. When themass is closer to the edge, the heaving motion becomes moreregular and a sinusoidal vibration can be recorded. The displace-ment, damped by the attach of the ribbon, is of a smaller ampli-tude. However the frequency is higher, which leads to an overallacceleration of same order of magnitude whatever the coil posi-tion.
Addition of one massOne mass is added, constituted by two similar copper coils
of 0.5 g, fixed symmetrically at the bottom of the ribbon. Thedistance between each mass and the top or bottom edge is 3 mm.The frequency and RMS amplitude of the vibration of the upperand lower masses when they are alone or when there are twomasses on the ribbon are plotted in Fig. 11. When there is onlyone mass, either at the top or at the bottom of the ribbon, itsdisplacement amplitude is larger and the frequency lower thanwhen we add another mass. Adding a mass at the other extremityof the ribbon then corresponds to adding stiffness to it.
CONCLUSIONA novel energy harvester from wind energy has been pro-
posed, that allows an omnidirectional wind to induce vibrationswhatever its direction. This paper shows first results on the wind-to-vibration part consisting of a fluttering windbelt. An experi-mental study of a windbelt has been performed in order to givesome guidelines to the design of an energy harvester based onfluttering effects. The ribbon tension, length have been studied,as well as the mass position along it and the effect of adding asecond mass at the other extremity. Targeting low wind speeds
WindLspeedL(m/s)
0 2 4 6 8 10
2
1.5
1
0.5
0RM
SLa
mplitu
deL
(mm
)
LowerLmass
UpperLmass
OneLmass
TwoLmasses
(a) RMS amplitude of vibration
OneLmass
WindLspeedL(m/s)
0 2 4 6 8 10
60
40
20
0
Fre
quen
cyL(
Hz)
LowerLmass
UpperLmass
TwoLmasses
(b) Frequency of vibration.
FIGURE 11. AMPLITUDE AND FREQUENCY OF THE VIBRA-TION FOR ONE OR TWO MASSES
TABLE 2. OPTIMAL CHOSEN PARAMETERS.
Value Namew 13 mm width of ribbonL 45 cm length of ribbond 3 mm distance coil-end of ribbonT 0.5 N ribbon tension
and strong ribbon acceleration at highest vibration frequencypossible, the optimal parameters are recapitulated in Tab. 2. Anelectromagnetic transducer has then to be added to convert themechanical energy from the vibrations into electricity. It’s di-mensions will be optimized to get the maximum power given theinput ribbon acceleration. Finally a nonlinear energy extractioncircuit will be added to rectify and regulate the coil voltage. Fur-ther works will present the optimization and test of those twosubsystems.
REFERENCES[1] Agar, T., 1989. “Aerodynamic flutter analysis of suspen-
sion bridges by a modal technique”. Engineering structures,11(2), pp. 75–82.
[2] Li, S., Xian, S., Yuan, J., and Lipson, H., 2011. “Ambientwind energy harvesting using cross-flow fluttering”. Journalof Applied Physics, 109(2), pp. 026104 – 026104–3.
5 Copyright © 2014 by ASME
[3] Kwon, S.-D., 2010. “A T-shaped piezoelectric cantilever forfluid energy harvesting”. Applied Physics Letters, 97(16),p. 164102.
[4] Frayne, S. Humdinger Wind Energy,www.humdingerwind.com.
[5] Kim, S.-H., Ji, C.-H., Galle, P., Herrault, F., Wu, X., Lee,J.-H., Choi, C.-A., and Allen, M. G., 2009. “An elec-tromagnetic energy scavenger from direct airflow”. Jour-nal of Micromechanics and Microengineering, 19(9), Sept.,p. 094010.
[6] Pimentel, D., Musilek, P., Knight, a., and Heckenbergerova,J., 2010. “Characterization of a wind flutter generator”. 20109th International Conference on Environment and ElectricalEngineering, pp. 81–84.
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