Comparison of Power output by Airfoil shaped 3-Blade and 4-Blade

Vertical Axis Wind Turbine for the local wind conditions

MADHUSUDAN.M.A1,

Lecturer, A.P.S. College of Engineering, Bangalore

MANJUNATH. B2

Lecturer, A.P.S. College of Engineering, Bangalore

Abstract: With depletion of fossil fuels, there is increased dependency on non-conventional energy

sources for generation of electricity. Conversion of

wind energy to electricity is one among them. While

large wind farms are already in existence to produce

several megawatts of power, small roof-top wind

energy convertors are gaining popularity for individual

customers. This paper contains a study of two small

vertical axis wind turbine for generation of electricity.

The selected NACA 6409 airfoil as the wind turbine

blades showed that the optimized angle of attack for

the airfoil (blade) was 80 for the design maximum wind

speed of 15 m/s. Assuming the wind turbine location at Bangalore, at an elevation of 15 m from the ground

level was assumed to achieve the minimum cut in wind

speed of 5 m/s required. By using double multiple

stream-tube model through SCILAB code, the solidity,

Tip Speed Ratio and number of blades were optimized

through an iterative process to compare the power

coefficients, and the power developed by the wind

turbines were estimated. The results culminated in a

configuration which had 3-blades with a solidity of

0.45 and Tip Speed Ratio of 6.

Key Words: Airfoil, Angle of attack, coefficient of lift, coefficient of drag, Solidity, Tip Speed Ratio,

VAWT, SCILAB code.

Introduction: Wind power is an alternative to fossil

fuels, is inexhaustible, widely distributed, clean,

renewable, and produces no green house gas emissions

during operation. Exploitation of wind energy is not

new and dates back to 200 B.C when Persians used

“wind wheels” for grinding food grains. Wind mills

first appeared in Europe during the middle ages and by

the 14th Century, Dutch wind mills were used to drain

excess water from the river basins [1]*.

The effort of over a millennium of windmill development culminated in “wind turbines” for

generating electrical power. The first electricity-

generating wind turbine was a battery charging

machine installed in 1887 by James Blyth of Scotland.

A few years later, American inventor Charles Brush

built the first automatically operated wind turbine for

electricity production.

*Number within the parenthesis indicates serial

number of reference listed at the end of the report.

Today, however, new wind machines are beginning to

appear on the landscape, as windy rural areas tap a

unique opportunity to benefit from wind power.

Modern wind turbine technology now makes it possible

to generate cost-effective, clean, renewable electricity

on a scale ranging from a single wind turbine for an

individual landowner up to large, utility-scale "wind

farms." Declining costs and improving technology are

quickly making electricity generated from wind energy

competitive with all types of non-renewable fuels, like

new coal-fired regeneration.

Worldwide there are now over two hundred thousand

wind turbines operating with installed capacity of

2,82,482 MW as of end 2012. The major capacity

sharing is by the European Union (1,00,000 MW);

United States (50,000 MW) and China (50,000 MW).

India has an installed wind power capacity of January

2013 and account for 6.5% of world total wind power

capacity. Wind power accounts for 8.5% of India’s

total installed power capacity, and it generates 1.6% of

the country’s power. With the advent of technology,

interest in small wind turbines (300 to 10000 W) is increasing for roof top application either independently

or in combination with solar photovoltaic power

generators.

1. Wind Power Development in India:

The development of wind power in India began in the

1990s, and has significantly increased in the last few

years. Although a relative newcomer to the wind

industry, India has the fifth largest installed wind

power capacity in the world according to recent

survey. According to “Global Wind report 2012-13”,

wind power generation in India is 18421 MW, which

accounts for 6.5 % of total wind power developed in the world. Wind power accounts for 8.5 % of India’s

total installed power capacity, and it generates 1.6 % of

the country’s power [2].

As per recent survey the installed capacity of wind

power in India was 18421 MW, states which are the

main source of wind power in India are as listed in

Table 1.1.

Ministry of New and Renewable Energy (MNRE) has

announced a revised estimation of the potential wind

resource in India from 49,130 MW assessed at 50m

Hub heights to 102,788 MW assessed at 80m Hub height. The wind resource at higher Hub heights that

are now prevailing is possibly even more.

Table 1.1 List of India’s prominent wind power

developing states

States Installed

Capacity (MW)

Tamil Nadu 7154

Gujarat 3093

Maharashtra 2976

Karnataka 2113

Rajasthan 2355

Madhya Pradesh 386

Andhra Pradesh 435

Kerala 35.1

Orissa 2

West Bengal 1.1

Others 3.20

2. How Wind Turbines Work: The wind imposes two driving forces on the blades of a

turbine; lift and drag. A force is produced when the wind on the suction side of the airfoil must travel a

greater distance than that on the pressure side. The

wind traveling on the suction side must travel at a

greater speed than the wind traveling along the

pressure side. This difference in velocity creates a

pressure differential. On the suction side, a low-

pressure area is created, pulling the airfoil in that

direction. This is known as the Bernoulli’s Principle.

Lift and drag are the components of this force vector

perpendicular to and parallel to the apparent or relative

wind, respectively. By increasing the angle of attack, as shown in Fig. 2.1,

the distance that the suction side air travels is

increased.

Lift and drag forces can be broken down into

components that are perpendicular (thrust) and parallel

(torque) to their path of travel at any instant. The

torque is available to do useful work, while the thrust is

the force that must be supported by the turbine’s

structure.

Fig. 2.1 Forces acting on a wind turbine blade

2.1 Wind Power

Power of the wind is proportional to air density, area of

the segment of the wind being considered, and the

natural wind speed. The relationship between the above

variables is provided in equation 2.1.

3

2

1 AVPW

2.1

Where

Pw: power of wind (W)

ρ: air density (kg/m3)

A: area of a segment of the wind being

considered (m2) V∞: Undisturbed wind speed (m/s)

At standard temperature and pressure (STP = 273.15K

and 101.3 kPa), 3647.0 AVPW 2.2

A turbine cannot extract 100% of winds energy

because some of the winds energy is used in pressure

changes occurring across the turbine blades. This

pressure changes causes a decrease in velocity and

therefore usable energy. The mechanical power that

can be obtained from the wind with an ideal turbine is

given by equation 2.3.

3

2716

2

1 AVPm 2.3

Where

Pm: mechanical power (W)

Fig. 2.3 Area swept by a vertical axis wind turbine

In equation 2.3, the area, A, is referred to as the swept

area of the turbine shown in Fig. 2.3. For vertical axis

wind turbine, this area depends on both the diameter

and turbine height (blade length). For an H-rotor

vertical axis wind turbine swept area is represented by

equation 2.4.

DHA 2.4

Where

As: swept area (m2)

D: diameter of the turbine (m)

H: height of the turbine (m)

The constant 16/27 = 0.593 from equation 2.3 is

referred to as the Betz coefficient. The Betz coefficient

conveys that 59.3% of power in the wind can be

extracted in the case of an ideal turbine. However, an

ideal turbine is a theoretical case. Turbine efficiency in

the range of 35-40% is very good, and this is the reason

for low power output of the wind turbine [2].

2.2 Principle of Aerodynamics

The wind imposes two driving forces on the airfoil

shaped blades of a turbine lift (L) and drag (D). A force

is produced as the wind on the suction side of the

airfoil must travel a greater distance than that on the

pressure side. The wind travelling on the pressure side

is traveling at a lower speed than the wind travelling

along the suction side as illustrated in Fig. 2.4. This

difference in velocity creates a pressure differential. On

the suction side, a low-pressure area is created, pulling the airfoil in that direction. This is known as the

principle of aerodynamic lift. Lift and drag are the

components of this force vector perpendicular to and

parallel to the apparent or relative wind, respectively.

By increasing the angle of attack (α), the distance that

the suction side air travels is increased. This increases

the velocity of wind on the suction side and

subsequently the lift increases as the pressure

decreases. The component of resultant force of lift and

drag rotates the blade, thus torque is developed on the

blade.

Fig. 2.4 Aerodynamic lift

Torque developed at a point on wind turbine blade

depends on the force at that point on the wind turbine

blade and distance of that point on the blade from the

shaft axis which is the axis of rotation. Force at a point

on the blade is the product of pressure difference at that

point on the blade and area of the blade. Thrust on the

wind turbine blades must be supported by the shaft of the wind turbine.

Fig. 2.5 Pressure difference on the airfoil surfaces

Lift and drag forces acting on the airfoil can be

calculated using the equations 2.5 and 2.6 respectively.

Where,

CL = Coefficient of lift

CD = Coefficient of drag ρ = Density of air (kg/m3)

Aw = Wing area (m2)

V = Velocity of air (m/s)

s = span (m)

c = chord (m)

3. Airfoil selection

For the survey of the airfoil, National Advisory

Committee for Aeronautics (NACA present NASA) is

the source. For the airfoil to be selected as a blade for

vertical axis wind turbine it should have maximum

coefficient of lift and minimum coefficient of drag.

That is the airfoil should have maximum lift to drag

ratio. After a survey of several airfoils the NACA 6409 airfoil was selected. The coordinates and profile of the

NACA 6409 airfoil is as shown in Table 3.1

Fig. 3.1 shows the profile of NACA 6409 airfoil which

is generated using MS excel, the coordinates of the

airfoil has been collected from NACA official website,

www.airfoiltool.com. The coefficient of lift (CL) and

coefficient of drag (CD) for various angles of attack (α)

have been obtained from the same source for various

angles of attack, and plotted in Fig 3.2.

Table 3.1 NACA 6409 airfoil coordinate

Profile x/c y/c

Suction

side

300 0

287.28 3.852

238.941 16.134

211.824 21.459

197.13 23.833

166.289 27.906

134.52 30.456

118.77 31.08

87.945 30.258

59.864 26.622

26.94 17.667

Pressure

side

0 0

22.248 -3.24

51.771 -0.357

76.494 2.187

118.857 4.917

149.604 5.613

195.579 5.64

224.184 4.902

249.669 3.723

278.54 1.728

292.509 0.66

300 0

Fig. 3.1 Profile of NACA 6409

Fig. 3.2 shows how the ratio of CL/CD varies with

respect to angle of attack. The CL/CD ratio is almost

constant between 30 and 90.

Fig.3.2 Variation of CL and CD for NACA 6409 airfoil

with angle of attack for different Reynolds number

4. Wind conditions in Bangalore

Selection of Bangalore as the place of application, the wind conditions for Bangalore over a year was studied

[25].

Fig. 4.1 Annual wind velocities for Bangalore [25]

Fig. 4.1 shows the variation of wind velocity in

bangalore in a year, it indicates average annual wind

velocity in bangalore to be 3.2 m/s, and the wind

velocity is high in the months of june, july and august.

Table 4.1 Assumed mean wind conditions for

Bangalore

Quantity Value

Temperature 26°C

Dynamic Viscosity of Air 1.84E-5 kg/ms

Density of Air 1.184 kg/m3

Pressure 101325 Pa

Mean wind velocity at ground level 3.2 m/s

The Table 4.1 shows calculated mean temperature, dynamic viscosity, pressure, density and velocity of air

at Bangalore. For the calculation of pressure and

density of air the vertical axis wind turbine is assumed

to be placed at an elevation of 15 m above the ground

level. The viscosity of air has been calculated using the

Sutherland’s Formula, which is expressed as follows

[26]:

Where:

= is a reference temperature (K)

= is the viscosity at the reference temperature

(kg/ms)

S = is the Sutherland temperature (K)

For Air,

µref = 1.716 X 10-5kg/ms

Tref = 273.15 K

S = 110 K C1= 1.458 X 10-6 kg/msK2

Density of air can be calculated using the formula,

Where

ρ = Density, kg/m3

P = Pressure, Pa

T = Temperature, K

R = Gas constant = 287.05 J/kg-K

5. Power Calculation

Power calculation for a vertical axis straight blade

wind turbine consists of following. 1: Calculation of forces acting on the wind turbine by

numerical method.

2: Estimation of the Tip Speed Ratio (TSR), and

solidity of the wind turbine.

3: Estimation of power developed by the wind turbine.

-10 0

10 20 30 40

0 100 200 300 400

Series1

0

20

40

60

80

100

120

140

160

0 5 10 15

CL/

CD

α (degrees)

CL/CD Vs α

RE=50000

RE=100000

RE=200000

RE=500000

RE=1000000

0

1

2

3

4

1 2 3 4 5 6 7 8 9 10 11 12

Win

d V

elo

city

(m

/s)

Month

Annual wind velocity in Bangalore

wind velocity variation average wind velocity

Table 5.1 wind turbine specification

Specification Value

Rotor Diameter (D) 2 m

Height of the Blade (H) 1.5 m

Swept area of Blades (A) 2.25 m2

Density of air (ρ) 1.184 kg/m3

Number of blades (N) 3

Wind free stream velocity(V∞) 15 m/s

Table 5.1 indicates the important specifications of the

vertical axis wind turbine which have been arrived at

through numerical calculations.

5.1 Details of the Airfoil (Blade)

The blades used in the wind turbine are of the airfoil

shape. The airfoil that has been selected for the wind

turbine is NACA 6409. The specifications of the airfoil

are as shown in Table 5.2.

Table 5.2 Specifications of vertical axis wind turbine

blade

Specification Value

Chord length of the airfoil (c) 0.3 m

Length of the blade (H) 1.5 m

Surface area of the blade (AW) 0.45 m2

The vertical axis wind turbine can be controlled by stall

or pitch control. The mathematical modelling of the

vertical axis wind turbine is considered in the next

section using Blade Element Momentum and Double –

Multiple stream tube modelling.

5.2 Double-Multiple Stream Tube Model

Fig. 5.1 Double multiple stream tube model [15]

In double-multiple stream tube model, rotor area is

divided into two regions, upstream and downstream as

shown in Fig. 5.1 where air flows through two actuator

disks models in a tube. The actuator disk model is an

imaginary infinitesimal thin rotor with infinite number of blades and its only effect is to drop pressure without

changing wind speed in the rotor area. For each tube

the 1-D momentum theory is used to relate the rotor

upstream and downstream velocities by defining an

axial induction factor. For the upstream

and the downstream

, the local upstream

velocity, V, the equilibrium velocity, Ve, and the

downstream, , will differ from the free stream

velocity, V∞, by:

Fig. 5.2 Relative wind speed vectors in upstream and

downstream sections of H-rotor vertical axis wind

turbine [15]

In these relations, and are the axial induction

factors in the upstream and the downstream regions,

respectively (always ). is the equilibrium velocity in the joining region of both semi tubes. As

shown in Fig. 5.2, the local relative wind speed for the

upstream section of the rotor can be determined by:

Where

represents the local tip speed ratio.

The angle of attack is also determined by:

By combining the blade element theory and the

momentum theory for each stream tube, the induction

factor, u for the upwind section is calculated from:

in which is given by:

Where is called the solidity factor

which is measured by blade swept area divided by the

rotor area. Also CN and CT are the coefficients of

normal and tangential components of the resultant

force respectively. These forces coefficients are related to the angle of attack α, and the lift and drag

coefficients for each airfoil section as follows:

The normal force, , and tangential force, ,

component of the resultant force in the upstream

section of the rotor, as shown in Fig 5.3, are

determined by:

Where is the blade projection area, with c as the chord length and H as the height of rotor. The

rotor sweep area is defined by , with R as

the radius of the rotor.

Adding up the moment of tangential component of

the resultant force about the rotor center for each

stream tube, the upstream contribution to total torque

obtained as follows:

Where ρ is the air density. Thus the power coefficient

for the upstream section can be written as:

Fig. 5.3 Forces exerted on an airfoil section of a wind turbine blade [15]

This is repeated for downstream section with the

equilibrium velocity, given by equation 5.2, as the free

stream for the second actuator disk in the downstream

section of the stream tube. The local relative air

velocity and the angle of attack in the downstream

section are obtained by:

Similarly, by combining the blade element theory and

the momentum theory for each stream tube, one can

determine the downstream induction factor, from:

Where is given by:

In the above relation, and are the coefficients of the normal and tangential components of the

resultant force in downstream section respectively,

which are related to the angle of attack, , and the lift

and drag coefficients as follows:

The normal, and tangential, components of the

resultant force in demonstration section of the rotor are

determined by:

Adding up the moment of tangential component of the

resultant force about the center for each stream tube, the downstream contribution to total torque is obtained

as follows:

Thus power coefficient for the downstream

section can be determined by:

Adding up the power coefficient in upstream section,

and in downstream section, , the total power

coefficient is obtained for one cycle as follows:

The aim is to achieve the maximum power coefficient

at rated speed.

5.3 Numerical Calculation

The discussed double – multiple stream tube models

were programmed in SCILAB routines. In order to

calculate the induction factors, and , an iterative

method is used. In this method, the upstream induction

factor u is initially taken to be unity. Applying relations

5.4, 5.5, 5.8, 5.9 and substituting in equation 5.7

determines . A new induction factor u is then

calculated from equation 5.6. This procedure is iterated

to calculate a new induction factor until the difference

between two consecutive values of the induction

factors converges to an error band less than 0.01. Next,

the final value of induction factor in upstream section

is used as initial guess for determining the induction

factor in downstream section. The above algorithm

is repeated for downstream section until convergence is

achieved. The remaining parameters are subsequently

determined from the relations explained in the previous section.

5.4 Equations for Coefficient of Lift and Drag

The airfoil section NACA6409 is chosen due to the

relatively high lift to drag ratio of 150 at the angle of

attack of 8° for the design of this specific H-rotor

vertical axis wind turbine. Lift and drag coefficients

with respect to angle of attack for this airfoil is

obtained by curve fitting two parabolic functions, as

follows:

5.5 airfoil section

The airfoil section NACA6409 is chosen due to the relatively high lift to drag ratio of 150 at the angle of

attack of 9° for the design of this specific H-rotor

vertical axis wind turbine. Lift and drag coefficients

with respect to angle of attack for this airfoil is

obtained by curve fitting two parabolic functions, the

equation are as follows:

Fig 5.4 Coefficient of tangential Force versus

azimuthal angle for 3-blades

Fig 5.4 shows how the coefficient of tangential force

varies with respect to the azimuthal angle which has

been calculated by the numerical procedure.

5.6 Analysis of 3-blade wind turbine

5.6.1 Aerodynamic results

In the design of a straight blade vertical axis wind

turbine, one of the factors which play a very important

role in the power generation of a wind turbine is the

angle of attack (α), the airfoil profile used for the wind

turbine is NACA6409, according to the airfoil data the

maximum lift to drag ratio achieved by the airfoil is at

an angle of attack of 8°. The angle of attack for the

wind turbine blade calculated by numerical approach is

as shown in the Fig 5.5, the figure shows the variation

angle of attack of the turbine blade with respect to azimuthal angle during the upstream section.

(a)

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 100 200 300 400

Co

eff

icie

nt

of

Tan

gen

tial

Fo

rce

(C

T)

Azimuthal Angle (θ)

TSR=6,σ=0.45 for 3 blades

TSR=6,σ=0.45

0

2

4

6

8

10

12

14

16

-100 -50 0 50 100

An

gle

of

Att

ack

(α)

Azimuthal Angle (θ)

σ = 0.45

TSR=6

TSR=5

TSR=4

(b)

Fig 5.5(a),(b) angle of attack variation with azimuthal

angle for σ=0.45, and 0.6

Fig 5.6 variation of coefficient of lift by drag ratio with

respect to azimuthal angle

5.6.2 Forces acting on the wind turbine

Fig 5.7 variation of coefficient of tangential force with

the azimuthal angle for σ=0.45

Fig 5.8 variation of coefficient of tangential force with

the azimuthal angle for σ=0.6

Tangential force is the most important component of the wind turbine, it varies as the wind

turbine rotates, and the variation of coefficient of

tangential force with the azimuthal angle, has been

estimated for two solidities 0.45 and 0.6 in upstream

and downstream sections, as shown in the figures 5.7

and 5.8 respectively.

Fig 5.9 variation of coefficient of normal force with the

azimuthal angle for σ=0.45

Normal force which perpendicular with respect

to tangential force, the normal force also varies as the

turbine rotates. The variation of normal force

coefficient with the azimuthal angle has been estimated

for two solidities 0.45 and 0.6 in upstream and

downstream sections, as shown in the figures 5.9 and

5.10 respectively.

0

2

4

6

8

10

12

14

16

-100 -50 0 50 100

An

gle

of

Att

ack

(α)

Azimuthal Angle (θ)

σ = 0.6 TSR=6

TSR=5

TSR=4

0

10

20

30

40

50

60

70

-100 -50 0 50 100

CL/

CD

Azimuthal Angle (θ)

σ = 0.45 TSR = 4

TSR = 5

TSR = 6

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-100 0 100 200 300

Co

effi

cien

t o

f Ta

nge

nti

al F

orc

e

Azimuthal Angle (θ)

σ = 0.45

TSR=4

TSR=5

TSR=6

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-100 0 100 200 300

Co

eff

icie

nt

of

Tan

gen

tial

Fo

rce

Azimuthal Angle (θ)

σ = 0.6 TSR = 6

TSR = 5

TSR = 4

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-100 0 100 200 300

Co

eff

icie

nt

of

No

rmal

Fo

rce

Azimuthal Angle (θ)

σ = 0.45 TSR=4

TSR=5

TSR=6

Fig 5.10 variation of coefficient of normal force with

the azimuthal angle for σ=0.6

Variation of coefficient of tangential force for 3-blade

vertical axis wind turbine with the azimuthal angle is as

shown in the Fig 5.11. The sum and average of the coefficient of tangential force for the wind turbine has

been calculated and is as shown in the Fig 6.12, it

shows that the average coefficient of tangential force is

0.2557.

Fig 5.11 variation of coefficient of tangential force

with the azimuthal angle for σ=0.45 and TSR=6, for 3-

blades

Fig 5.12 variation of coefficient of tangential force

with the azimuthal angle for σ=0.45 and TSR=6, for 3-

blades, with sum, and average of 3-blades

5.7 Analysis of 4-blade wind turbine

5.7.1 Forces acting on the wind turbine

Fig 5.13 variation of coefficient of tangential force

with the azimuthal angle for σ=0.6 and TSR=6, for 4-

blades

Numerical analysis was conducted for a 4-blade

vertical axis wind turbine by using the similar

calculation as used in case of 3-blade wind turbine,

with assumption of specification shown in Table 6.3, the results showed that there was an increase in the

average coefficient of tangential by 30%, i.e., 0.334,

which is as shown in figure 5.14.

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-100 0 100 200 300

Co

eff

icie

nt

of

No

rmal

Fo

rce

Azimuthal Angle (θ)

σ = 0.6

TSR = 6

TSR = 5

TSR = 4

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-100 0 100 200 300

Co

effi

cien

t o

f ta

nge

nti

al fo

rce

Azimuthal angle (θ)

TSR=6, σ=0.45

blade1

blade2

blade3

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-100 0 100 200 300

Co

eff

icie

nt

of

Tan

gen

tial

fo

rce

Azimuthal angle (θ)

TSR=6, σ=0.45

blade1

blade2

blade3

sum

average

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-100 0 100 200 300

Co

eff

icie

nt

of

tan

gen

tial

fo

rce

Azimuthal angle (θ)

TSR=6,σ=0.6

blade 1

blade2

blade3

blade4

Table 5.3 Specifications for 4-blade vertical axis wind

turbine

Specification Value

Rotor Diameter (D) 2 m

Height of the Blade (H) 1.5 m

Swept area of Blades (A) 2.25 m2

Density of air (ρ) 1.184 kg/m3

Number of blades (N) 4

Wind free stream velocity(V∞) 15 m/s

Solidity (σ) 0.6

Tip Speed Ratio-TSR (λ) 6

Fig 5.14 variation of coefficient of tangential force

with the azimuthal angle for σ=0.6 and TSR=6, for 3-

blades, with sum, and average of 4-blades

Solidity of the wind turbine increases when the

analysis is conducted for the 4-blade vertical axis wind

turbine.

5.8 Power Co-efficient for 3-blade wind turbine

Fig 5.15 variation of coefficient of tangential force

with the azimuthal angle for σ=0.45 and

It is found that from Fig 5.15, the maximum power

coefficient that can be obtained from the wind turbine

is 0.1217, which is during up-stream.

6. Plan for application of the wind turbine for

a rooftop wind form

The case study was to plan a wing farm over the

mechanical department building in APS College of Engineering, Bangalore. The height of the building is

15 m, the plan is to plant the wind turbine of

specifications as shown in the paper.

According to the area available over the roof of the

building, the number of wind turbines that can be

placed was decided, and the type of arrangement of the

wind turbine was found to be staggered type of

arrangement, arrangement is such that there should not

be any obstruction for the wind flow for all the wind

turbines.

The wind turbines were planned to plant in such a way

that the first set of wind turbines were planned to be planted at an elevation of 15 m above the ground level,

and the second set of wind turbine were planned to be

planted at a elevation of 15 m above the ground level.

The specifications of the wind form over the building

are as shown in the table 6.1.

Table 6.1 Wind Form Specifications

Sl.no Specifications value

1. Area of wind form over the

building

1173.68

SMT

2. Number of wind turbines that can

be placed over the building

33 units

3. Elevation of the 1st set of wind

turbines

15 m

4. Elevation of the 2nd set of wind

turbines

17 m

5. Number of wind turbines in 1st set

16 units

6. Number of wind turbines in 2nd

set

17 units

The plan of the wind form over the building is as

shown in the Fig. 6.1. The circles with number inside it

indicates the wind turbine, it’s a total of 33 wind

turbines.

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

-100 0 100 200 300

Co

eff

icie

nt

of

tan

gen

tial

fo

rce

Azimuthal angle (θ)

TSR=6, σ=0.4

blade 1

blade2

blade3

blade4

sum

average

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

-90 -45 0 45 90 135 180 225 270

λ=6,σ=0.45

TSR=6,SIGMA=0.45

θ

CP

Fig 6.1 Wind Form Plan

6.1 Power calculation

The capacity of wind form was evaluated, depending

on the local wind speed availability over the year. The

total number of wind turbines planed to be placed over

the building is 33 units. The power capacity of the

wind form is as shown in the Fig. 6.2.

Fig. 6.2 Total Power that can be harnessed by

the wind form over the year.

Fig. 6.2, shows the mechanical wind power that can be

harnessed by the wind form over the year, there are two

curves, 16WT, 15m indicates the power that can be

harnessed by the 1st set of 16 wind turbines placed at an

elevation of 15m above ground, and 16WT, 15m

indicates the power that can be harnessed by the 2nd set

of 17 wind turbines placed at an elevation of 17m above ground. The total power that can harness over

the year by the wind turbine is 4927.248 kWh. The

power required to power the building that is as shown

in Fig. 7.1, is 30 kWh, if the wind speed remains

constant throughout the day the wind form can harness

13.5 kWh. Thus, the wind form can harness around

45% power requirement for the building. Thus, if this

plan is successfully implemented, it could be very

helpful to overcome the power crises in recent days.

10. Conclusion

An attempt has been made to establish the design

methodology for an H-Rotor vertical axis wind turbine.

As torque is provided by forces acting on profiled

turbine blades, an airfoil profile according to NACA

6409 was chosen for the blade cross section. Basing on

yearly average wind conditions for Bangalore, a flow

analysis was conducted for the turbine blades using the

CFD code COMSOL. The results indicated an optimum angle of attack of 80 for selected profile.

The number of blades, Tip Speed Ratio and blade

solidity were optimized through an iterative procedure

using the open source SCILAB code. It was observed

that for Bangalore conditions where the average ground

wind speed is about 3.2 m/s, the turbine needs to be

located at an elevation of 15 m to achieve a wind speed

of 4 m/s which is more than the required minimum cut-in wind speed of 4 m/s.

The turbine has a configuration that has 3-blades at a

Tip Speed Ratio of 6 and blade solidity of 0.45.

Application of the wind turbine to wind form of 33

units of wind turbine on the roof top of a building of

area 1173.68 SMT, it can harness 13.5 kWh per day

assuming constant wind speed though out the day and it can harness around 45% power requirement for the

building.

10.1 Scope for future work

Since availability of wind power is not assured all the

time, it is advantageous to build wind-solar hybrid

power generators so that a minimum amount of power

availability can be assured always. A solar photovoltaic

system in combination with the proposed wind turbine will be an ideal combination to meet power needs of

individual urban household and small communities.

References

1. John K. Kaldellis, D. Zafirakis. “The wind energy

(r) evolution: A short review of a long history”.

Renewable Energy Vol. 36 (2011) pp. 1887-1901.

2. Global Wind Energy Council. “GWEC Global

Wind Report Annual market updates 2012”.

Global wind 2012 Report. Retrieved 23 April

2013. 3. Det Norske Veritas and Risø National

Laboratory. “Guidelines for Design of Wind

Turbines 2002” DNV/Riso 2nd Edition.

4. John O. Dabiri, “Potential order of magnitude

Enhancement of wind form density via counter-

rotationg Vertical axis wind turbine array”.

Journal of Renewable and Sustainable Energy,

Vol. 3, Issue 4, July 19, 2011.

5. NASA official website-www.airfoiltool.com.

6. A.F.P. Ribeiro, A.M. Awruch, H.M. Gomes. “An

airfoil optimization technique for wind turbines”.

Applied Mathematical Modeling Vol. 36 (2012) pp. 4894-4907.

0

1000

2000

3000

4000

5000

6000

1 2 3 4 5 6 7 8 9 10 11 12

16WT,15m

17WT,17m

Pm (kWh)

MONTH

Power Generated

7. S. Rajakumar. “Computational fluid dynamics of

wind turbine blades at various angles of attack

and low Reynolds number”. International

Journal of Engineering Science and Technology.

Vol. 2(11), 2010, pp. 6476-6484.

8. Shengyi Wang, Derek B. Ingham, Lin Ma,

Mohamed Pourkashanian, Zhi Tao. “Numerical

investigation on dynamic stall of low Reynolds

number flow around oscillating airfoils”. Computers & Fluids Vol. 39 (2010) pp. 1529-

1514.

9. M. Saqib Hameed, S. Kamran Afaq. “Design and

analysis of straight bladed vertical axis wind

turbine blade using analytical and numerical

techniques”. Ocean Engineering Vol. 57 (2013)

pp. 248-255.

10. B. Yang, X.W. Shu. “Hydrofoil optimization and

experimental validation in helical vertical axis

wind turbine for power generation from marine

current”. Ocean Engineering Vol. 43 (2012) pp.

35-46. 11. Kazumasa Ameku, Baku M. Nagai, Jitendro

Nath Roy. “Design of a 3kW wind turbine

generator with thin airfoil blade”. Experimental

Thermal and Fluid Science Vol. 32 (2008) pp.

1723-1730.

12. H.-J. Kim, S. Lee, N. Fujisawa. “Computation of

unsteady flow and aerodynamic noise of

NACA0018 airfoil using large-eddy simulation”.

International Journal of Heat and Fluid Flow

Vol. 27 (2006) pp. 229-242.

13. Macro Raciti Castelli, Andrea Dal Monte, Marino Quaresimin, Eenesto Benini. “Numerical

evaluation of aerodynamic and inertial

contribution to darrieus wind turbine blade

deformation”. Renewable Energy Vol. 51 (2013)

pp. 101-112.

14. Vinay Thapar, Gayatri Agnihotri, Vinod Krishna

Sethi. “Critical analysis of methods for

mathematical modeling of wind turbines”.

Renewable Energy Vol. 36 (2011) pp. 3166-3177.

15. Davood Saeidi, Ahmad Sedaghat, Pourya

Alamdari, Ali Akbar Alemrajabi, “Aerodynamic design and economical evaluation of site specific

small vertical axis wind turbines”. Applied

Energy Vol. 101(2013) pp. 765-775.

16. Zhou Y., Md. Mahbub Alam, Yang H.X., Guo

H., Wood D.H. “Fluid forces on a very low

Reynolds number airfoil and their prediction”.

International Journal of heat and Fluid Flow

Vol. 32 (2011) pp. 329-339.

17. Almina El Kasmi, Christian Masson. “An

extended k-ε model for turbulent flow through

horizontal axis wind turbines”. Journal of Wind

Engineering and Industrial Aerodynamics Vol. 96 (2008) pp. 103-122.

18. P. Svacek, M. Feistauer, J. Horacek. “Numerical

simulation of flow induced airfoil vibrations with

large amplitudes”. Journal of Fluids and

Structures Vol. 23 (2007). pp 391-411.

19. Ji Yao, Jianliang Wang, Weibin Yuan, Huimin

Wang, Liang Cao. “Analysis on the influence of

turbulence model changes to aerodynamic

performance of vertical axis wind turbine”.

Procedia Engineering Vol. 31 (2012) pp. 274-

281.

20. K. Mclaren, S. Tullis, S. Ziada. “Measurement of

high solidity vertical axis wind turbine

aerodynamic loads under high vibration response

conditions”. Journal of Fluids and Structures

Vol. 32 (2012) pp.12-26. 21. Ye Li, Sander M. Calisal. “Numerical analysis of

the characteristics of vertical axis tidal current

turbines”, Renewable Energy Vol. 35 (2010) pp.

435-442.

22. J. Kjellin, F. Bulow, S. Eriksson, P. Deglaire, M.

Leijon, H. Bernhoff. “Power coefficient

measurement on a 12kW straight bladed vertical

axis wind turbine”. Renewable Energy Vol. 36

(2011) pp. 3050-3053.

23. Wilcox, D.C. “Reassessment of the scale-

determining equation for turbulence models”.

AIAA journal (1988), Vol.26, no. 11, pp. 1299-1310.

24. Satish Annigeri. “An Introduction to SCILAB”.

BVB college of Engineering and Technology,

Hubli, 2009, p. 1-2

25. www.synergyenviron.com, annual wind speed for

Bangalore.

26. Alexander J. Smits, Jean-Paul

Dussauge “Turbulent shear layers in supersonic flow”, Birkhäuser, 2006, ISBN 0-387-26140-0 p.

46 27. A.K.Chitale, R.C.Gupta. “Product Design and

Manufacturing”. PHI Publications 5th Edition,

2011.