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13th World Congress in Mechanism and Machine Science, Guanajuato, México, 19-25 June, 2011 A29_579

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Mechanism to Control Power in Small Wind Turbines

G. Munoz-Hernandez* M. Matinez-Jimenez† CIATEQ A.C.

Queretaro, Mexico.

Abstract — Power control of small wind turbines was

studied by using the active stall method. Pitch angle of

the blade was changed in order to reduce the power of

the turbine rotor for high wind speed. The measurement

of pitch moment of the blade was the first stage of the

study. The results of pitch moment were measured as a

function of angular speed of wind turbine rotor. The next

stage was the evaluation of the control options. Pitch

control to feather and active stall was evaluated. Active

stall showed advantages and it was used in a rotor with a

diameter of 1.6 m.

Keywords: Small Wind Turbine, Pitch Control, Active Stall, Wind

energy.

I. Introduction

Small and large wind turbines can increase the use ofrenewable energy sources. The generation of electricityfrom small wind turbines has shown growth of around

78% for the year 2008 in the United States of America[1]. International standard IEC 61400-2 [2] classifiedwind turbines as small for rotor swept areas of less than200 m2.

The development of small wind turbines (SWT) includesseveral challenges; one of them is the operation controlor protection system for high wind speed. Wind speedhigher than design speed may cause failure of the turbinesystems from mechanical or electrical overload. Thereare several ways to protect the turbines at high windspeed. The most logical way is to avoid capture powerfrom the high speed wind; that is, from the rotor [3].

There are three methods to limit aerodynamic power. Thefirst method is to tilt the rotor relative to the wind flow.Wind speed is reduced through the rotor, and then the

power captured is also reduced (Figure 1). This system iscalled furling, but it has a disadvantage. Wind gusts orsudden movements of the rotor produce impacts on thecomponents or vibration with significant fatigue loads[3].*[email protected]† [email protected]

Fig. 1. Furling method

The second method is called stall. The blade is designedto produce stall in high winds. Stall increases the dragforce in the rotor blades and reduces the lift force. This

phenomenon produces turbulence and low efficiency.The design of the system is very complex and involvesthe use of auxiliary control systems. Additionally, it

presents vibrations [4].

The third method is pitch control or the change of blades pitch angle. The pitch angle (θ) is determined by theaerofoil chord and rotation plane of the rotor (Figure 2).One option of this method is to increase the pitch angleof the blade to the feather position. This will reduce theangle of attack, α and the force known as lift L is alsoreduced. Therefore, the efficiency of the rotor is alsodecreasing. Another option is to reduce the pitch (toincrease attack angle) to increase the drag force (D) andthe blades induce stall. The latter option is called activestall, which also reduces the efficiency of the rotor bychanging the pitch angle. This method is used in largewind turbines through electromechanical control systemsand hydraulics. SWT designs are still looking for optionsthat allow reliability and low cost. Different controlsystems have been patented in SWT by using centrifugalforce [5] [6] [7]. For example the patent cited [8] showsmasses located on the blades, the centrifugal force on themass makes the blade rotates go to a feathered state.

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Fig. 2. Parameters of blade. M is the pitch moment. V ∞ is therelative wind speed, c is the chord of the blade, φφφφ is the relative angle, αααα is the attack angle and F is the resultant force on the blade, where L and D are components.

The pitch angle is one of the most used methods tocontrol large wind turbines [4] and is the subject of studyin this work. Thus, the following objectives were defined:

a) To measure the torque M to allow rotation of the blade

b) To propose a pitch control system that allows controlof the blade in high wind speed.

II. Methodology

A. Measurement of torque M

The experimental unit was a 3-blade rotor with sweptarea of 2 m2, a diameter of 1.6 m and a design power of

250 W (Figure 3). This rotor was designed by using themethod of blade element momentum (BEM) [9] [3] [10][11] and a blade profile designed by NREL and specificapplication for SWT [12]. The rotor hub supports therotor blades on two bearings. The bearings allow freerotation of the blades. The base of the blade wasmanufactured of aluminum alloy 6061 T6. The aluminum

base is embedded in the fiberglass blade. The rotor wasmounted on a shaft of a Harbart alternator of 500 W(Figure 3). To induce a wind current in the rotor, a 7.5 hpcentrifugal fan was used. It generated wind speeds in theswept area as illustrated in Figure 4. Wind speeds weremeasured with a digital anemometer Xtech 451126 CFM

model. The rotor speed was determined with a digitaltachometer DT-2234C of Travers Tool Co.

Fig. 3. Three-bladed rotor designed by BEM method

The measurement of the torque M (Figure 2) is particularly important, when the turbine has reached itsrate power or design power. Due to the rotation of the

blades during operation of the rotor, it was not possible touse load cells or strain gauges to measure the moment M .The electrical wires and limited space for devices makesinstallation complex. A wireless transducer was not afeasible option, especially if batteries were required.

Fig. 4. Wind speed distribution in the rotor swept area of the SWT

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One interesting option to measure M is the use of sewingwire previously calibrated. The tensile strength of eachwire can estimate the force Fae (Figure 5).

Fig. 5. Pre-loaded wires to determine Fae force

The tensile strength times the radial distance from the bearings center to the wire, provides the value of torque M . Torque M can be also estimated from the wind tunnel by using the equation (1) [9]. C M is the coefficient of pitch moment and ρ is the wind density.

M C cV M 22

2

1∞= ρ

(1)

Five tests were conducted to determine the tensilestrength of each wire. Average tensile strength was 1.22kg-f (11.9 N) with a standard deviation of 0.012 kg-f(0.117 N). The measurement procedure was as follows:

i. Calibration of each wire to determine the tensilestrength of failure. This was done by using a springscale and a pre-loaded Ohaus Model BrandEP2102C scale with 10 mg precision.

ii. Progressive placement of wires on the blade. The blade was placed in working position with an initial

pitch angle of 0°. The first wire was used. The fanwas connected to provide a wind speed as shown in

Figure 4. When the rotor reached 200 rpm, the fanwas disconnected. At 200 rpm the wire failure hadnot occurred. Next run was on 367 rpm and wascontrolled by turning off the fun. At angular speedof 367 rpm, the wire failure occurred. The angularrotor speed was recorded on the digital tachometer.

iii. Now two wires were used and the rotor worked on468 rev / min. The two wires failed.

iv. Three wires were used on 633 rpm. They did notfail.

v. These tests were progressive for the speed at whichfailure occurred up to 5 wires for 767 rpm, which isclose to design speed.

By recording angular speed of the rotor and the number

failed wires, it is possible to obtain Fae. The torque M isthe product of Fae and the radial distance of 0.033 m.

Now different drives can be used to control the pitch.

B. Pitch control options

Two options for pitch control were proposed. The firstone consisted of a centrifugal force driven by threemasses. They were placed into radial pipes. Thecentrifugal forces of the masses pull to rotate blades atdesign speed (Figure 6). If the centrifugal load is lowerthan Fae, then the blade does not rotate. It means a

bigger mass has to be considered.

Fig. 6. Drawing of the masses fit on steel pipes to control the pitch ofthe blade.

The second option was to avoid masses and drive the blade to stall by using the torque M . In angular speedlower than design speed (700 rpm), the position of the

blade is kept on pitch 0°, just by using a pre-calibratedtorsion spring. It means the blade works at optimal attackangle at all times. If the angular speed is higher thandesign speed, the torque M is higher than the springtorque. This option is called “active stall” and the poweris decreased by increasing the drag force. Figure 7 showsan example of active stall control for a 1.5 MW turbine.It can be observed that pitch angle is lower than 5° to

reach power control at nominal power [13].

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Figure 7. Active stall to control power in a 1.5 MW wind turbine [13]

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III. Implementation and results

Table 1 shows the results of measured torque M fordifferent angular speeds of the rotor.

Table 1. Torque M versus the angular speed

The results showed that the torque M varies dependingon the angular speed of the rotor and therefore therelative wind speed V

∞. The torque at 700 rpm was

estimated at 4.5 Nm. The torque required by each bladein the design speed was 1.9 Nm, i.e. 42% of design rotortorque. According to equation (1), the C M coefficient can

be estimated for any rotor. By using a torsion spring, it isnow possible to calibrate the system to reach a specificnominal power. A wind tunnel is a way to calibrate thesystem.

The first option of control with three masses wascomplex. Some disadvantages can be listed:

i. Torque M requires a big mass for the rotor. Itmeans a big load or large arm torque to reachrotation of the blade

ii. Due to the big magnitude of the masses, auniform rotation mechanism of the three

blades has to be implemented because theindependent rotation and large masses implydynamic rotor unbalance.

iii. The use of cables and bending arms is required

to transmit the centrifugal forces and impliesthe failure of the cables due to fatigue. Some proprietary commercial systems involve amechanism without the use of cables, but theyshow a complex design.

Active stall was tested in the 1.6 m rotor by using the fanand the wind speed shown in Figure 4. The rotor operatedwithout active stall worked at speeds over 1300 rpm,while with the use of active stall control with 7.2°, theangular speed was obtained at very close to design speed,743 rpm (Figure 8).

Fig. 8. Active stall test method

The active stall method showed the followingadvantages:

i. Lower angles are needed to control speed.Spruce [13] and Burton et al [14] showed that5° of pitch was enough to control power atvalues close to nominal power in big windturbines.

ii. Aerodynamic torque is generated using thesame blade or M .

iii. Simple mechanism did not use cables, mass orfriction parts.

iv. The spring mechanism is patented. It creates a precise torque without changing the spring.Thus the nominal power can be selecteddepending on the design.

IV. Conclusions

The measurement of torque M is an essential activity inthe design of a pitch control system. Measuring bysewing wires was an interesting method, but smalldeformations involving the wire can result in anglesgreater than 2° in the pitch and can lead to differentvalues in M . The ongoing review of options has shownthat pressure sensing films are an alternative that can

prevent distortion of the wires.

The rotor speed depends on wind speed through the

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relative velocity. Therefore, the power control of theturbine can be achieved through the rotor speed. Thesignificant reduction in operating speed to 743 rpm andits stability showed that the “active stall” method works.

One of the disadvantages set out in the systems that usestall is the presence of vibrations due to the phenomenonof stall on the blades of large turbines. Even when thevibrations were not detectable at the small turbine, it issuggested to avoid large pitch angles and makesexperiments with values lower than 5°, according to the

performance of the turbine.

References

[1] Spruce, C.J. Power performance of active stall wind turbines with

blade contamination. 2006. EWEC 2006. Disponible en:

www.ewec2006proceedings.info/.../0672_Ewec2006fullpaper.pdf.

[2]. Burton, Tony. WIND ENERGY HandBook. Baffins Lane,Chichester West Sussex, PO19 1UD, England : JOHN WILEY &SONS, LTD, 2001. ISBN 0 471 48997 2.

[3] IEC. Design Requirements for Small Wind Turbines. 2a-2006-03 Design Requirements for Small Wind Turbines. 2006. IEC 61400-2.

[4] Piggott H. Windpower Workshop. Machynlleth, Powys, SY20 9AZ,UK. : Centre for Alternative Technology Publications, 2000.

[5] DWIA. [En línea] June de 2003.http://www.talentfactory.dk/en/tour/wtrb/powerreg.htm.

[6] Zahorecz Z. P. Variables picth impeller. US 4178127 1979. US.

[7] Bertoia V. O. Fluid-driven turbine with speed regulation. 4316698

United States of America, 1982. US.

[8] Wastling M. A.. Passive speed and power regulation of a wind

turbine. 7172392 United States of America, 2007. US.

[9] Shi M. Alibaba.com. Qingdao anhua New Energy Equipment Co ,2010. http://www.alibaba.com/product-gs/205151958/AH_10kw_Pitch_controlled_wind_turbine/showimage.html.

[10] Ingram G. Wind Turbine Blade Analysis using the Blade Element

Moment Method. School of Engineering, Durham University. UK :

s.n., 2005. Available at:http://www.dur.ac.uk/g.l.ingram/download/wind_turbine_design.pdf.

[11] Duran S. Computer-Aided Design of Horizontal-Axis Wind

Turbine Blades. Thesis of Mechanical Enegineering. METU

Library, İnönü Blv.06531 Ankara/TÜRK İYE : The Gradute Schoolof Natural and Applied Sciences of Middle East TechnicalUniversity., 2005. Disponible en

http://hitit.lib.metu.edu.tr/oai/viewrecord.php?id=10289.

[12] Somers D. M. The S833, S834, and S835 Airfoils. NREL. USA : NREL/SR-500-36340, 2005.

http://wind.nrel.gov/designcodes/papers/NREL%20Airfoil%20Families%20for%20HAWTs.pdf.

[13] Hansen M. O.L. Aerodynamics of Wind Turbines. Sterling VAUSA : Editorial Earthsacan. Second Edition, 2008.

[14] AWEA. American Wind Energy Association, 2010.http://www.awea.org/smallwind/pdf/09_AWEA_Small_Wind_Glo

bal_Market_Study.pdf.

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