Traceability of pitch angle position for small scale wind turbine to get

a maximum Energy Extraction

Ali Musyafa’(1)

, Farid .R.M.(1)

I.M.Yulistya Negara(2)

, Imam Robandi(3)

(1,)Department of Engineering Physics, Faculty Industrial Engineering , ITS Surabaya

(2,3,)

Jurusan Teknik Elektro FTI, ITS Surabaya Kampus ITS Keputih, Sukolilo, Surabaya 60111

Telp : +6231-5947188 Fax : +6231-5923626 E-mail : musyafa@ep.its.ac.id

Abstract

Global crisis in financial, energy, together with climate and enviromental, has motivated word

community to seek for alternative energy resourses, known as the new and renewable energy source to

replace conventional widely used fossil energy. Wind energy, regarded as potential candidate of the

clean alternative source that may help to solve the crisis. The development of the world’s wind turbine

instalation in 2010 reached 30 %, with a total capacity of 160 GW. The comprise of USA=39 GW,

Germany and China = 26 GW, followed by Spain, India, Italy, etc. Indonesia as a tropical country with low

wind speeds (3-6 ms-1) is also participate in the development. In 2008 there are 1.2 MW wind turbine

instalations, and in 2011 a new plant is built in Bulukumba-South Sulawesi and Aceh, each with 0,5 MW

and 10 MW, respectively. However, the majority of the installed turbines technology come from other

countries, which are not fully suited with local conditions and the exist a great problem in dependency

of the spare parts from the producers. Therefore, it needs to develop our own local wind turbines

technology, with the key players from LAPAN,BPPT, state and private Universities in Indonesia.

This paper reports development and test of a prototype for wind turbine with blade diameter of 2

m, with three blades of airfoil NREL S83n. Crucial problem in implementation of small scale wind

turbine for low wind speed regimes ( i.e. 3-6 ms-1, like in most areas in Indonesia), is to determine the

position of pitch angle of turbine blade. The right position of blades’ pitch angle may produce maximum

mechanical power, wich means also maximum electrical energy. The proof of concept experiments were

done by considering pre-determined dimension, type and diameter of blade, as well a range of angular

velocity and power produced. The prototype of turbine was exposed to variation of wind speed flow,

and stepwise variation of blades’s pitch angle was set through PC, with angular step of 50 for range of

angle 0-90 degrees. Optimal pitch angle 0f 15± 5 degree was obtain, with a maximum value of Cp=0.54

Keyword : Pitch angle, wind turbine, mechanical energy, Electrical power.

1. INTRODUCTION

Global crisis in 2008, viz namely the financial crisis, energy crisis and climate-environmental crisis,

has motivated and encouraged the world community to seek alternative energy resources, such as new

and renewable energy to replace conventional energy. For now on, wind energy has a claim on new and

renewable energy, therefore wind energy can be proven to solve the three crises above. The world

wind turbine installation development in 2010 reached 30% with a total capacity of 196,63 GW. They

comprises of China = 44,73 GW, USA= 25,81 GW and Germany = 27,22 GW, followed by Spain, India,

Italy, etc. Indonesia as a tropical country with low wind speeds (3-6 m / s) is also not left behind. In 2008,

Indonesia has 1.4 MW wind turbine installations, and in 2011 a new plant is built in South Sulawesi and

Aceh Bulukumba consist of 0.5 MW and 10 MW respectively. However, the majority of the installed

wind turbines in Indonesia are from other countries, as a result there is a great dependency of the

components from other countries. Therefore, it is needed to develop local wind turbine either by

LAPAN, BPPT, colleges, or the general public [1-2-3].

Wind energy conversion systems (WECS) consists of wind turbines, generators, power

electronics, network systems and control systems. Small scale of WECS is wind turbines with

<10 kW capacity, this turbine is suitable for low-speed wind farms such as Indonesia. The

important part of SKEA is the wind turbine. In this part, the wind kinetic energy will be

transformed into mechanical energy and then converted into electrical energy. Fluctuation in

the wind kinetic energy value were often influenced by the wind speed changes and the type of

air. Wind turbine mechanical energy production value is parallel with the wind kinetic energy

input [4-5]. Maximum mechanical power production of wind turbines can be attained by

increasing the angle of wind turbine rotational speed. Wind turbine mechanical power production

depends on several variables of wind turbines, such as power coefficient (Cp), which depends on

the value of (λ, θ). λ is the tip speed ratio (TSR) that cost = ωR / V), the value of R constant wind

turbines, mean while the fluctuation of V value depend on the condition of the wind field, ω is

the angular velocity of the wind turbine correlated to a blade angle position. Hence, the most

influence variable of mechanical power production of wind turbines is the blade angle position

other quantities which influence over mechanical power production is the type of blade and blade

number, but its value is constant after the system was designed [6].

The wind turbine performance can be monitored, if a prototype wind turbine is outfitted with

a computer-based electronic monitoring system. Here, major scales such as V, ω and θ are likely

to be monitored. To obtain these quantities, wind turbine is outfitted with a sensing rotary

encoder for sensing turbine rotational speed. Furthermore, this rotation data is processed by

processor, and then it is displayed and recorded in real time by the PC. The variations of blade

angle position changes can be set via PC, so the blade angle position changes can be done in

every single step 5o quickly and precisely [7]. From this experiments, all data related to head

position blade angle changes, wind speed and the speed will be processed to obtain the relation

between the wind power and the turbine mechanical power output. The maximum power

production values are obtained when the blade angle is in a particular position and then this value

is used as a reference for developing the turbine and the design of control systems. This study

focuses are to find and trace the position angle blade wind turbine that can produce maximum

mechanical power.

2. WIND TURBINE SYSTEM

A wind power production by a wind turbine can be expressed through a kinetics wind

energy equation using equation (1) and (2) [13].

2wrair

2w vxAρ

2

1mv

2

1U (1)

32

2

1

2

1wrairwrair vA

dt

dxvA

dt

dUP (2)

Where U is a kinetic energy (Joule), ρair is air density (kg/m2), Ar is a sweeping blade area (m

2),

vw is a wind speed (m/s), and P is wind turbine power (watt). Kinetics energy is converted into

rotational energy or wind turbine power. When the wind passed through the blade, a wind

velocity profile can be illustrated using a contour tube as in Fig. 1. There are four points to be

considered as in Fig. 1. The velocity profile and its relation to the sweeping blade area are given

below:

1323

2vvv (3.a)

143

1vv (3.b)

1322

3AAA (3.c)

14 3AA (3.d)

Equation (4) can be used to calculate the extraction of wind power. The power can be calculated

by comparing the wind power before and after the blade:

3

11344

31141

9

8

2

1

2

1vAvAvAPPP airair (4)

3

12312

27

16

2

1

3

2

9

8

2

1vAvAP airair (5)

A substitution eq. (5) into eq. (4), and substitution the value of A, it can be obtained a

constant of 16/27 = 0.59. This value is known as a Betz coefficient [14]. This coefficient shows a

maximum efficiency of wind turbine or it is called a power coefficient (Cp). It is known that

wind power production is also effected by the Cp. The bigger Cp can produce the bigger wind

power production. The expression of extracted wind power also can be written as in eq. (6).

3

2

1wrpair vACP (6)

Cp also can be determined in a simple way based on P1 and P4 as in eq. (7)

1

14

P

PPCp

. (7)

Tip speed ratio is the ratio between the rotational speed of the blade tip to wind speed. Blade tip

speed is obtained by multiplying the rotor angular velocity (rad / s) with the radius of the rotor. This

value is then compared with the wind speed in zone 1. R is the radius of the rotor and the RPS is the

rotation per second.

V

RPSRπTSR

..2 (8)

A design of wind turbine is shown in Fig. 2. It consists of three blades, a gear blade, motor

servo, a generator and others supporting components. The blade is designed based on an air foil

contour standard of NREL S833, S835 and S834. This blade type is chosen with a consideration

of its easiness of fabrication process. The blade has a length of 100 cm and a thickness of 18 %,

15 % and 21% t/C. It is made of fiber material. A gear box is fabricated based on a specification

of the motor servo. The gear box can achieve a movement resolution of 5o. The fabricated wind

turbine parameters are shown in Table 1.

Figure. 1. Velocity profile in the wind turbine area

Figure. 2. Prototype of wind turbine

Table 1. Specification of the developed wind turbine

SPESIFIKASI

Rotor Blade Span = 85 cm Blade = 3 Airfoil = non-uniform NREL S835 (root), S833 (primary), S834 (tip) Tapered, no-twist. Blade Shaft = Stainless Steel, ø 10 mm, l = 15 cm Material = Fiberglass

Pitch Setting Motor Servo = GWS Servo S125-1T/2BB/F 360 Deg Driver = Mikrokontroler ATMega8535L Angle rotasi = 360o

Power Suplay Catu daya = 6 V Torque = 6.60 kg/cmB Gear box = Nylon, 1:1.25

Rotary Sensor Rotary Encoder = Relative, 20 slate, ø 40 mm PhotoInterrupter = Sharp GP1S53, 5 mm gap, IR (Tx) Phototransistor (Rx)

Center Plate & Hub Cover Center Plate t = 20 mm, ø 28 cm Material = Poly Vinyl Chloride (PVC) Grey Hub Cover Material = Fiberglass

Rotor Shaft Stainless Steel, ø 15 mm, t = 2 mm, l = 55 cm Shaft holder = bearing NTN 6203LU

Rotary Connector Carbon Brush Holder = Bosch dan Black Dekker Carbon Brush = Makita

The fabricated wind turbine is equipped with a monitoring software based on Visual Basic 6 and a

serial port interface for data communication. This software is used to control and to show the position

of the pitch angle of the wind turbine blade. The measurement data can be presented in real time to

indicate all of the necessary information. Monitoring system is also equipped with recording data

system based on Microsoft Office-Excel integrated with the Visual Basic. Therefore, all of measurement

process for the wind turbine can be accessed in real time and online. In this paper, wind speed data for

experiment taken for a period of 100 seconds, in a steady state for the variation of wind speed 3-8 m/s.

It indicates that the most of wind speed is in the range of 3-6 m/s. Therefore, it is necessary to design a

wind turbine with an operational wind speed within this range.

3. RESULTS AND DISCUSSION Wind turbine prototype testing is done by test of each component block and divice; which includes

hardware testing; rotating connector, rotary encoder sensors, signal conditioners, transmitters and

displays. Testing is also done on the system software. When test results have met the criteria state, then

performed the integration of hardware and software systems. WECS Testing is done by performing

experiments in the laboratory of ITS, which suport by wind power generating system (blower) that wind

speed can be set varied in the range of 3-8 m/s. At the initial stage of experiments carried out setting

changes blade pitch angle position (θ) through the software starts the smallest angle followed by the

blowing of the wind speed varied start low then increase to the highest speed. Monitoring and recording

system to read wind turbine angular velocity indicated by the PC screen. The experimental results are

used to calculate the amount of power coefficient (Cp).

Relationship varied position blade angle and wind speed varied to produce wind turbine rotational

speed is varied as well, as a whole shown in Table 2. Testing of wind turbine systems are also made to

find relationships wind speed, angular position, rotational speed, and coefficien of power and Tip

speed ratio (TSR). The result of calculation and the correlation shown in Figure [3-7]. Fig.3. show wind

speed data varied 0-8 (m/s) during the period of one year imposed on a wind turbine to see the

performance of wind turbines. From the test data obtained subsequently calculated the value of tip

speed ratio. λ values obtained from the relationship pitch angle position and rotational speed are then

used to calculate the tip speed ratio (TSR). λ and Cp values, obtained at a position angle between the

(10-20)0. Relationships of Cp, θ and λ from the calculation for the whole range shown in Fig. 6.

In the view of this experiment, there is correlation of all process variable including wind speed,

blade angle position, rotational speed, tip speed ratio, power production, and wind turbines coefficient.

The correlation of those variables can be seen in Fig. [3-7]. After V, θ, λ, ω, Cp, is found, then the input

and output mechanical power can be calculated. The two amount of those powers are shown in Fig.6. In

the line with that, the optimum wind turbine power coefficient and optimum RPM are also shown in

Fig.7-8.

0 50 100 150 200 250 300 3501

2

3

4

5

6

7

8

9

10

Time (daily rate)

Win

d S

peed

(m

/s)

Figure. 3. Wind speed versus Time

.

Figure. 4. Rotational speed versus pitch angle wind turbine for differen wind speed

Fig. 5. TSR versus Pitch angle and Wind speed

TSR achieved when the maximum wind speed 7.5 m / s, pitch angle position of 50-70 degrees. As

for the position of the pitch angle of 75 to 90 degrees, the TSR value is 0. Position that is able to produce

pitch angle of rotation is 50-70 degrees. The next 7 Cp equation is written as follows;

(9)

From equation 9, the power coefficient of wind speed at each point and a certain pitch angle can be

known. The coefficient of relationship power, wind speed and pitch angle. From these data can be

searched against the value of the maximum power coefficient of wind speed at the position of each

pitch angle. Table 3 shows the value of the maximum power coefficient of wind speed and optimum

pitch angle. From tebel known that wind turbines have a power coefficient which is good enough

considering the horizontal axis wind turbines typically have a maximum power coefficient between 0,35

- 0,40.

Fig. 6. Power coefficient versus Tip Speed Ratio for differen wind speed

Table 2. Maximum Cp in Pitch Angle

Angle (o) Maximum Cp Wind speed

5 0.523 4.8 10 0.545 7.5 15 0.545 7.5 20 0.510 7.5 25 0.398 4.1 30 0.473 7.5 35 0.435 7.5 40 0.394 7.5 45 0.435 7.5 50 0.435 7.5 55 0.352 7.5 60 0.262 7.5 65 0.308 7.5 70 0.214 7.5

Pitch Angle, the optimal pitch angle position of wind turbine is the position where the wind

turbine can produces maximum power at a certain wind speed. To obtain the optimum pitch

angle is done by two approaches. The first approach is through the relationship between power

coefficient with wind speed and pitch angle. While the second approach through the relationship

between wind speed and RPM with the pitch angle. With the first approach is known that the

pitch angle that produces a high power coefficient, is on the entire range of wind speeds at an

angle of 10, 15, and 20 degrees. In other parts for wind speed 3 m / s, wind turbines with pitch

angle positions 10, 15, and 20 have a lower coefficient of power, but> 0.3. In the second

approach RPM relationship with wind speed and pitch angle, 50-30 degrees is approached in a

position, then approximated by polynomial regression of order 3. optimum pitch angle values

obtained on the wind speed range (2.8-7.5) m / s.

Fig. 7. The optimal regime characteristics

Figure. 8. Rotational speed versus pitch angle wind turbine for differen wind speed

Table 3. Maximum Pitch Angle for difference wind speed

Wind speed (m/s) Optimal pitch angle

2.8 10.35 3.8 10.37 4.1 13.10 4.8 10.15 6.5 13.16 7.0 16.19 7.5 10.87

4. CONCLUSION

Prototipe turbin angin yang dirancang mampu menghasilkan nilai-nilai koefisien daya maksimum berada pada rentang posisi sudut pitch 10-20 derajat, ( ketika kecepatan angin 7.5 m/s dan posisi sudut blade 10 dan 15 derajat, nilai Cp= 0,545 ), sedang ( ketika kecepatan angin 7.5 m/s dan pada sudut 20 derajat nilai Cp =0,510 ), Fig.7.

The position of pitch angle that produces the maximum RPM for wind speeds from 2.8 to 7.5 m / s are : The position of blade angle 10.35 with wind speed 2.8 m / s generate RPM = 39.6 The position of blade angle 10.37 with wind speed 3.8 m / s generate RPM = 54.3 The position of blade angle 13.10 with wind speed 4.1 m / s generate RPM = 64.4 The position of blade angle 10.15 with wind speed 4.8 m / s generate RPM = 69.0 The position of blade angle 13.16 with wind speed 6.5 m / s generate RPM = 112. The position of blade angle 16.19 with wind speed 7.0 m / s generate RPM = 99.0 The position of blade angle 10.87 with wind speed 7.5 m / s generate RPM = 168.1

The results of the design pitch angle regulator capable of regulating blade angle range 0 to 90

degrees with intervals of 5 ± 0.39 degrees.

Nomenclature A rotor swept area [m2] Cp power coefficient [pu] R maximum rotor radius (m) Paero aerodynamic power [W] Trot torsional rotation at the turbine’s rotor [N.m] Ta aerodynamic torque[N.m] Tg torque generator [N.m] Taux torque auxiliary [N.m] Jg generator inertia [kg/m2] Jt rotor inertia [kg/m2] γ arodynamic damping coefficient

[N.m.s/rad] cT torsion coefficien (-) v wind speed (m/s) ωt rational speed (rad/s) To optimum torsion (N.m) vo optimum wind speed (m/s) Cop optimum power coefficient(-) Bt friction coefficien of turbine Greek symbols ωrot angular speed of rotor η efficiency (-) λ tip speed ratio [pu.] λo optimal tip speed ratio [pu.] ρ air density (kg/m3) θ pitch angle blade (o) u Hellman coefficient (-)

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