2. Reviewed Mech final

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN 2249-6890 Vol. 2 Issue 4 Dec - 2012 11-18 Š TJPRC Pvt. Ltd.,

PERFORMANCE ANALYSIS & IMPACT OF PERTURBATION FACTOR ON WIND POWER ESTIMATION 1

PRASHANT BAREDAR, 2HITESH KHARE & 3MUKESH PANDEY 1

Maulana Azad National Institute of Technology Bhopal, India 2

3

Terchnocrat Institute of Technology Bhopal, India

University Institute of Technology, Rajeev Gandhi Technological University, Bhopal, M.P., India

ABSTRACT This research paper is prepared as an account of work sponsored by an

agency of India, All India Council of

Technical Education (AICTE) under Research Promotion Scheme (RPS). The power efficiency of wind energy systems has a high influence in the economic analysis of wind energy systems. The power efficiency in these systems depends on many elements i.e. blades, gearbox, electric generator and different control systems. Some factors that are involved in blade efficiency are the wind features, e.g. its probabilistic distribution, the mechanical interaction of blade with the electric generator, and the strategies dealing with pitch and rotational speed control. This paper focuses the effect of change in perturbation factor on axial thrust and the torque development by the turbine which is much more significant to take up the issue.

KEYWORDS: Wind Energy, Induction Factor, Betz Criteria INTRODUCTION Wind turbines harness the energy from the wind by extracting useful work. It is estimated that wind power in many countries is already competitive with fossil fuel and nuclear power if social/environmental costs are considered. [1] The rotor extracts power in the wind. It consists of number of blades which are attached to a hub. The mechanical energy is extracted by the rotor in the form of rotational speed and torque. Then, this mechanical energy is converted to electrical energy which has the rotor that runs an electric generator. [2] The total installed wind generation capacity in India is 3000 MW. According to Indian government officials, the growth of Indian economy is highly dependent on the growth on its energy consumption [3].

The power produced by wind turbine depends on number of factors such as wind speed, height of the wind turbine, air density, geographical location of the wind turbine, texture of the land over which wind turbine is installed, and number of other factors. [4] Among these, one of the important factors, which are based on analytical evaluation, is the perturbation factor. It has remarkable effect on the percentage of wind power availability, It is also known as induction factor or interference factor. Perturbation factor is nothing but the fractional wind speed decrease at the turbine.

Perturbation factor has its impact over power extraction, torque developed & Axial thrust on the turbine.

Through the readings that we have taken regarding the effect of perturbation factor on the power extraction from the wind, we find that for values of zero perturbation factors there was no power generation and similarly for a perturbation factor of unity stall condition was achieved. Maximum value was achieved at 1/3 of the value. This wide variation in power extraction on account of a change in perturbation factor was what prompted us to take up the research. Similarly the effect of change in perturbation factor on axial thrust and the torque development by the turbine was much more significant

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Prashant Baredar, Hitesh Khare & Mukesh Pandey

to take up the issue in the 1930’s Glauert applied classical aerodynamic methods to airplane propeller designs in an effort to optimize performance of the horizontal axis machine for propulsion. [5] We have tried to show in our paper that controlling the range of perturbation factor can greatly affect the way in which we harness the wind power. If implemented we are sure that this important discovery has the potential to change the way we look at the wind power generation. Experimental data, consisting of hourly records over a specified period, is measured in the Energy Park of UIT-RGPV Bhopal. It can be expected that possible applications of wind energy systems could be considered for the efficient utilization of these resources.

WIND TURBINE AERODYNAMICS ASPECTS Proper modeling of the aerodynamic aspects of wind energy systems is very important for successful design and analysis of wind turbines. Wind energy conversion system aerodynamic models are used to obtain wind inflow conditions from load which is applied on the turbine.[6]The field of wind turbine aerodynamics analysis is having a great complexity; hence after lots of investigation and trial, researchers have realized that the typical atmospheric conditions are very complex under which a wind turbine operates. Surface imperfections, squashed bugs, surface-water droplets, or ice buildup are responsible for the instability of the flow over the rotor blades.

The boundary layer flow which passes through a wind turbine is inhomogeneous, unsteady, and turbulent, which results in rapid velocity and pressure fluctuations. [7]Turbulence occurs due to inertial effects overwhelm viscous stresses which are inherent in the flow stream as the flow speed increases. With the slight flow perturbation, it results in unstable flow that can become turbulent. The turbulence increases at the lowest wind speeds due to irregular terrain, such as mountains, valleys, houses, and trees. The situation becomes more complicated when wind directions change or the turbine becomes yawed. If a rotor is not aligned with the wind, it experiences cross-flow whose magnitude and direction relative to the rotor changes as the blades rotates. In such cases, flow separation and reattachment, span wise flow, and threedimensional effects occurs.

ENERGY AVAILABLE IN WIND Aero turbines convert the wind energy to rotary mechanical energy. A mechanical Interface consisting of a step up gear and a coupling transmits the rotary mechanical energy to an electrical generator. [8] The output of generator is connected either to load or power grid. The purpose of controller is to sense wind speed, wind direction, shafts speeds and torques, output power and generator temperature. A weather monitoring station is installed in Energy Center, University Institute of Technology Rajeev Gandhi Technological University Bhopal. This gives various data such as wind speed, wind direction, temp, rainfall condition etc.

If uo is the speed of free wind in unperturbed state, A1u0 gives the volume of air column passing through an area A1 per unit time. If ρ is the density of air, the air mass flow rate, m through area A1 is given by m = ρA1u0 kg /s.

Power available in wind, P0 is equal to kinetic energy associated with the mass of moving air, i.e.

P0= (1/2) (ρA1u0) u02 W Or

P0 = (1/2) ρA1 u03 W

Power available in wind per unit area:

Performance Analysis & Impact of Perturbation factor on wind Power Estimation

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P0/A1= (1/2) ρ u03W/m2 This indicates that power available in wind is proportional to the cube of wind speed. Assuming a typical value of wind density, ρ at 15°C and at sea level to be 1.2 kg /m3, power available in moderate wind of 10 m/s is 600 W/ m2.

Wind turbine is used to produce useful mechanical power from wind. The rotor of the turbine collects energy from the whole area swept by the rotor. The variation in wind speed at a location is often described by the Weibull probability distribution function. [9] For the purpose of simple analysis a smooth laminar flow with no molly used, is considered. The rotor may be considered as an actuator disk across which there is reduction of pressure as energy is extracted. Air mass decrease as air expands. The stream tube model, also known as Betz model of expanding air stream tube is shown in Fig.2. The stream tube area of constant air mass is A0 upstream, which expands to A1 while passing though rotor, and become, A2 downstream. [10] Fig.1 Wind speed and wind gust potential at UIT RGTU Bhopal

PERTURBATION ANALYSIS OF WIND POWER RESOURCE AT UIT RGTU BHOPAL Fig. 2 Wind stream tube in presence of turbine If u0 and u2 are wind speeds upstream and downstream respectively, the force or thrust on the rotor is equal to the reduction in momentum per unit time from the air mass flow rate m: F=mu0-mu2 This force is applied by the air at uniform airflow speed of u1, passing through actuator disk (turbine), the power extracted by the turbine is: PT = Fu1=m (u0-u2) u1 The power extracted from wind is also equal to loss in K.E per unit time, thus: Pw = (1/2) m (u02-u22) u1 = (u0+u2)/2 As an extreme case, considering u2 to be zero (which is not practical as downstream air must have some Kinetic energy to leave the turbine region), u1= u0 /2.Thus, according to this linear momentum theory, the air speed through the actuator disk cannot be less than half the speed of upstream air. [11] An interference factor, ‘a’ is fractional wind speed decreases at the turbine thus: a = (u0-u1)/u0 u1 = (1-a )/ u0 a = (u0-u2 )/2 u0 ‘a’ is induction or perturbation factor.

Using above equation power extracted by the turbine may be written as : PT=4a (1-a) 2[(1/2)ρA1u03)

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Prashant Baredar, Hitesh Khare & Mukesh Pandey

Comparing with Eqn. PT= Cp. P0 Where Cp is the fraction of available power in the wind that can be extracted and is known as power coefficient. Cp is given as: Cp =4a(1-a) 2 Fig. 3 Power coefficient variations with velocity at the turbine The power coefficient Cp is the fraction of available power in the wind that is extracted .The variation of power coefficient Cp at various velocities at the turbine u1 is shown in the graph .The reading shows that at 3 m/s the value of power coefficient Cp is 0.58 i.e. closer to Betz limit.

Fig. 4 Power coefficient variations with downstream velocity at the turbine The power coefficient Cp fraction of available power in the wind that is extracted and it’s variation at various downstream velocities of the turbine u2 is shown in the graph. At 0.5 power coefficient Cp the value of downstream velocity is almost ‘0’ i.e. closer to Betz limit.

Fig. 5 Perturbation factor variations with downstream velocity at the turbine When u2 =0,’a’=0.5, the simple model breaks down as no wind is predicted to be leaving downstream. In practice, this is equivalent to onset of turbulence downstream. Power extraction decreases due to mismatch of rotational frequency and wind speed and partial stalling begins .The turbine blades will still be turning, causing extensive turbulence in the air stream, leading to more losses.

Fig. 6 Perturbation factor variations with velocity at the turbine When the wind speed at the turbine is reduced to zero (i.e. u1 =0), ‘a’ the perturbation factor becomes unity and no power is extracted. This state is nothing but the (complete) Stall State of blades.

Value of Cp for different values of ‘a’ can be tabulated as shown below. The maximum possible efficiency for an idealized wind turbine is roughly 59.3%. In practice, three effects prohibit a real wind turbine from achieving this efficiency: Fig. 7 Perturbation factor variations with power coefficient of wind turbine The variation of fraction of available power in the wind that is extracted with fractional wind speed decrease at the turbine is very well described by the graph of perturbation factor ‘a’ versus power coefficient ‘cp’ .At the value of 0.33 perturbation factor, power coefficient is obtained ‘0.58’ i.e. almost closer to the Betz limit.

WIND RESOURCE POTENTIAL AT RGTU BHOPAL ‘WRPLOT’ Windows software program is used to generate wind rose statistics and plots for weather monitoring stations located at University Institute of Technology Bhopal for the year 2007-2008.

Fig. 8 Wind Rose construction of Site RGTU, Bhopal Here the Wind Rose graphically displaying the frequency distribution of occurrences of winds in each of the defined direction sectors and wind speed classes for the month Jan 2008. The resultant vector of the Wind Rose plotted above is calculated by computing the vector resultant or vector sum of the unit vectors that represent the various directions in the data. Here the direction of resultant vector is 222 0representing the mean resultant vector 58%.

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Performance Analysis & Impact of Perturbation factor on wind Power Estimation

Fig. 9 Wind class frequency distribution in the WRPLOT view Here the Frequency Distribution plot displaying the normalized frequency of occurrences of winds in each direction sector and each wind speed class. The sub-totals for each column and row (total occurrence of wind class and wind direction respectively) are displayed; also the number of calms and total wind data are shown.

CONCLUSIONS Thus from the above observation & calculation we have shown how important the effect of perturbation factor has in the harnessing of wind power. Torque available at the blades is also very high. When the air just passes through the turbine blades in this case without any reduction in velocity and thus the turbine taps no wind power. It was found that maximum axial thrust occurs at a perturbation factor of 0.5. This is the condition for maximum power extraction, it related to the ideal case that is nothing but the Betz criterion. For a commercial turbine the value of power extraction is always less than this. When a perturbation factor is equals to 0.5. In this case the simple model breaks down as no wind is predicted to leaving downstream. This is equivalent to onset of turbulence downstream. Power extraction decreases due to frequency mismatch and partial stalling begins. When a perturbation factor is equals to 1, there is no power extraction in this case.

This state is nothing but the stall state of the blades. When no load is coupled to the turbine the blades just freewheel. There is no reduction of wind speed at the turbine, therefore, u1 =u0 and the value of ‘a’ is zero. The air just passes through turbine without any reduction of speed.

Now as load is applied, power is extracted, so, Cp increases as u1 decreases. Maximum value of Cp (i.e. Cp max =16/27=0.593) occurs at ‘a’ =1/3. At this condition: u1 = (2/3)u0 and, u2 = (1/3)u0 That means, at maximum power extraction condition the upstream wind speed is reduced to two third at the turbine and further reduced to one-third downstream. The criterion for maximum power extraction, i.e., Cp max =(16/27) is a Betz criterion. This applies to an ideal case. For a commercial wind turbine, however, maximum power coefficient is less than ideal value.

REFERENCES 1.

Beurskens, J., Jensen, P.H. 2001. Economics of wind energy prospects

and directions. Renewable Energy

World 4 (4), 103-121.

2.

Barnard, J.C. and Wendell, L.L. 1997. A simple method of estimating wind turbine blade fatigue at potential wind turbine sites. ASME Journal of Solar Energy Engineering, 119(2): 174–179.

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Ghosh, S. 2006. Future demand of petroleum products in India. Energy Policy, 34 (15), 2032-2037.

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Ozgener O. Hepbasli A. (2002) 'Current status and future directions of wind energy application in Turkey' Energy sources. Vol.24 PP 1117-1129

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Glauert, H., 1935, “Airplane Propellers,” Aerodynamic Theory, ed, W. F. Durand, Julius Springer, Berlin, pp.

169-360.

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White D.L. and Musial W.D. 2004 ‘The Effect of Load Phase Angle on Wind Turbine Damage’‘42nd AIAA Aerospace Sciences Meeting and Exhibit Reno, Nevada January 5–8, 10

Blade Fatigue

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Dr. Pecen Recayi, Dr. Salim MD, & Mr. Zora Ayhan (2004) ‘ A lab view based instrumentation system for a wind- solar hybrid power station ’in journal of industrial technology. Vol. 20 No. 3 PP 2-7.

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Akhlaque Ahmad M., Firoz Ahmad, and M. Wasim Akhtar, 2006, Assessment of Wind Power Potential for Coastal Areas of Pakistan Turk Journal of Phys30 pp 1 -9

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Gupta, B.K. 1986. Weibull parameters for annual and monthly wind speed distributions for five locations in India. Solar Energy 37 (6), 469-471.

10. Jonkman J.M. 2003 Modeling of the UAE Wind Turbine for Refinement of FAST_AD NREL/TP-500-34755 11. Chen Y. M., Hung S. C, Cheng, and Y.C. Liu2005, Multi-Input Inverter for Grid-Connected Hybrid PV/Wind Power System IEEE. Pp.850-856

U0

U0 A1

A) Unperturbed Wind Stream Tube in Absence of Turbine

U1 Upstream P0,ρ 0 A0

A1

Downstream P2,ρ2 A2

Velocity at the Turbine u1(m/s)

Fig. 1: B) Wind Stream Tube in Presence of Turbine

4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0.324 0.433 0.512 0.562 0.588 0.592 0.576

0.5

0.384

Power Coefficient 'Cp'

Fig. 2: Power Coefficient Variations with Velocity at the Turbine

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Performance Analysis & Impact of Perturbation factor on wind Power Estimation

Downstream Velocity 'u2'(m/s)

4 3 2 1 0 0.324 0.433 0.512 0.562 0.588 0.592 0.576

-1

0.5

0.384

-2 Power Coefficient 'Cp'

Downstream Velocity 'u2'(m/s)

Fig. 3: Power Coefficient Variations with Downstream Velocity at the Turbine

4 3.5 3 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5

0.1

0.15

0.2

0.25

0.3

0.33

0.4

0.5

0.6

Perturbation Factor'a'

Velocity at The Turbine 'u1'(m/s)

Fig. 4: Perturbation Factor Variations with Downstream Velocity at the Turbine

4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0.1

0.15

0.2

0.25

0.3

0.33

0.4

0.5

0.6

Perturbation Factor'a'

Power Coefficient 'Cp'

Fig. 5: Perturbation Factor Variations with Velocity at the Turbine

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.1

0.15

0.2

0.25

0.3

0.33

0.4

0.5

0.6

Perturbation Factor 'a'

Fig. 6: Perturbation Factor Variations with Power Coefficient of wind Turbine

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Prashant Baredar, Hitesh Khare & Mukesh Pandey

Fig. 7: Wind Rose Construction of Site RGTU, Bhopal

Fig. 8: Wind Class Frequency Distribution in the WRPLOT View