Aerodynamic,rotor design and rotor performance of horizontal axis wind turbine(HAWT)

-Aerodynamics of wind
turbines2-Aerodynamic theories
-Axial momentum theory
-Blade element theory
- Rotor design
1-Airfoil
-Rotor performance

-Aerodynamics of wind turbines
Aerodynamics deals with the motion of air or other gaseous fluids and the forces
acting on bodies moving through them. During the course of wind turbine
development, several efforts were made to understand and interpret the aerodynamic
principles governing wind energy conversion and to apply them in the successful
system design. Earlier initiatives in this direction relied more on the aviation
industry. Aerodynamic theories developed for airplanes and helicopters were
adopted for defining the performance of wind turbines. However, now, theories are
specifically formulated for wind turbines which are further refined and reinforced
with the help of experimental techniques. Let us discuss some of these basic
principles in the following sections.
1-Airfoil
For the efficient energy extraction, blades of modern wind turbine are made with
airfoil sections. Major features of such an airfoil are shown in Fig. 2.11. The airfoils
used for the earlier day’s wind turbines were the aviation air foils under the NACA
(National Advisory Committee for Aeronautics) series. NACA specifies the features
of the airfoil by numbers.

For example, in a four digit specification, the first number denotes the maximum camber of the
airfoil at the chord line (in per cent of chord), the second number gives the location of the point of
maximum camber from the leading edge (in tenth of the chord) and the third and fourth numbers
indicate the maximum thickness (in per cent of the chord). Thus a NACA 2415 air foil have maximum
camber of 2 per cent, located at 0.4 times the chord length from the leading edge and the maximum
thickness is 15 per cent of the chord. The five digit specification of airfoil is similar, but information
on the lift characteristics are also included. In a five-numbered NACA airfoil, the design lift
coefficient in tenth is given by 1.5 times of the first digit. One-half of the distance from the leading
edge to the location of maximum camber (in per cent of the chord) is represented by the second and
third digits. The fourth and fifth digits are the thickness of the airfoil (in per cent of the chord). Thus a
NACA 23012 series airfoil will have a design lift coefficient of 0.3. The maximum camber of the
airfoil is 0.15 times the chord and its thickness is 12 per cent. Blades with higher
digit NACA numbers are also available.

As far as the requirements of a wind turbine are concerned, there is disadvantages for
NACA specified airfoils :-
 Poor in their stall characteristics.
 They are insensitive to wide variations in Reynolds number under which a wind
machine is expected to work.
 They have low structural efficiency at the root region.
These limitations called for the development of air foils tailored for wind turbines.
Several research organizations like the Delft University of Technology National
Renewable Energy laboratory ,and Riso National Laboratory are engaged in research and
development of airfoils exclusively for wind energy systems.
When an airfoil is placed in a wind stream, air passes through both upper and
lower surfaces of the blade. Due to the typical curvature of the blade, air passing
over the upper side has to travel more distance per unit time than that passing
through the lower side. Thus the air particles at the upper layer move faster. According to
the Bernoulli’s theorem, this should create a low-pressure region at the top of the airfoil.
This pressure difference between the upper and lower surfaces of the airfoil will result in
a force F. The component of this force perpendicular to the direction of the undisturbed
flow is called the lift force L (Fig. 2.12). The force in the direction of the undisturbed
flow is called the drag force D.

2-Aerodynamic theories
Different theories are proposed to analyze the aerodynamics of wind turbines.
These theories give an insight to the behavior of the rotor under varying operating
conditions. Let us discuss some of the fundamental theories among them, applicable
to HAWT.
 Axial momentum theory
The conventional analysis of HAWT originates from the axial momentum concept introduced by
Rankine, which was further improved by Froudes for marine propellers.
Assumption:-
 The flow is assumed to be incompressible and homogeneous.
 The rotor is considered to be made up of infinite number of blades.
 Static pressures far in front and behind the rotor are considered to be
equal to the atmospheric pressure.
 Frictional drag over the blades and wake behind the rotor are
neglected.

Consider a wind turbine with rotor of area AT, placed in a wind stream as shown in Fig. 2.15. Let
A and A’ be the areas of the sections 1-1, and 2-2 and V and V’ are the respective wind velocities
at these sections.
VT is the velocity at the turbine section. According to the law of conservation of mass, the mass of
air flowing through these sections is equal. Thus:

The thrust force experienced by the rotor is due to the difference in momentum of the
incoming and outgoing wind, which is given by
The thrust can also be represented as the pressure difference in the upstream and down stream
sides of the rotor. Let PU and PD be the pressure at the upstream and down stream side of the
rotor respectively. Hence:
Applying the Bernoulli’s equation at the sections and considering the assumption that the static
pressures at sections 1-1 and 2-2 are equal to the atmospheric pressure P, we get

and
From Eqs. (2.22) and (2.23),
Substituting the above expression for (pU –pD) in Eq. (2.21) 𝐹 = (pU –pD) AT (2.21)
Comparing Eqs. (2.25) and (2.20)
we get
F = ρa AT VT (V-V’) (2.20)

Thus the velocity of the wind stream at the rotor section is the average of the velocities at its
upstream and downstream sides.
At this stage, we introduce a parameter, termed as the axial induction factor into
our analysis. The axial induction factor a : indicates the degree with which the wind
velocity at the upstream of the rotor is slowed down by the turbine. Thus
From Eqs. (2.26) and (2.27),
As we have seen earlier, the power imparted to the wind turbine is due to the transfer of kinetic
energy from the air to the rotor. The mass flow through the rotor over a unit time is

Hence the power developed by the turbine due to this transfer of kinetic energy is
Substituting for VT and V´ from Eqs. (2.28) and (2.29), we get
Comparing Eq. (2.32) with the expression for power coefficient in Eq. (2.8),
where Eq. (2.8) is ( Cp =
2 𝑃𝑡
𝜌𝑎 𝐴𝑡 𝑣^3
),
we can see that

Thus differentiating Eq. (2.33), equating it to zero and solving, we get a=1/3.
Substituting for a in Eq. (2.33), the maximum theoretical power coefficient of a horizontal axis
wind turbine is 16/27 and the maximum power produced is
It should be noted that, several assumptions are involved in this analysis. Some of these may be
questionable when we consider the real flow conditions around a wind turbine. For example,
the practical rotor has finite number of blades and the aerodynamic drag and tip losses cannot
be neglected. Further, the flow ahead and behind the rotor is not completely axial as assumed
under the ideal condition.
When the fluid applies torque to the rotor, as a reaction, rotational wake is generated behind the
rotor as shown in Fig. 2.16. This will cause energy loss and reduce the peak power coefficient.

Considering the tangential flow behind the rotor, we introduce another factor termed as the
tangential induction factor a’in the analysis such that:
Here ω is the induced tangential angular velocity of flow and Ω is the angular velocity of the
rotor. Consider an annular stream tube of thickness dr at a distance r from the root of the blade
as in the figure. Area of this annular element is
Hence, the thrust force experienced by the annular element may be expressed as

The power developed by the rotor is the product of this annulus torque and angular velocity,
integrated over the total blade span. Thus the power is given by

 Blade element theory
Blade element theory was initially proposed by Froude and Taylor.
In this approach it is considered:-
 The blades are made up of a number of strips arranged
in the span wise direction.
 The strips have infinitesimal thickness.
 These strips are aerodynamically independent and do not have any interference between
them.
Under this analysis,
the lift and drag forces acting over the strip are estimated and integrated over the total blade span
incorporating the velocity terms, to obtain the torque and power developed by the blade. This is further
multiplied by the number of blades to get the total rotor torque and power. The blade element theory
gives us more understanding on the relationship between the airfoil properties, thrust
experienced by the rotor and the power produced by it.

The velocities and forces acting on an infinitesimal blade element are shown in Fig. 2.17. The
undisturbed wind velocity V is slowed down to (1-a)V as it reaches the rotor.
A velocity of Ω r (1+a’) is experienced by the element due to the rotation of the blades and the
wake behind the rotor, where a’is the tangential induction factor. W represents the resultant of
these two velocities. α is the angle of attack, ɸ is the flow angle and β is the blade setting angle.

Lift forces acting on the infinitesimal airfoil section is given by:
where C is the airfoil chord, dr is the thickness of the section considered and CLis the lift
coefficient. Similarly, the elemental drag is given by
where CD is the drag coefficient.
We can see that the thrust (dF) and torque (dT) acting on the blade element are
From Eqs. (2.42) to (2.45), and considering the total number of blades B

From the geometry of the figure
we see that,

Similarly, W can be represented as the resultant of the velocities such that
The total torque developed by the rotor can be computed by integrating the elemental torque
from the root to the tip of the blade. Rotor power is estimated by multiplying this torque by
its angular velocity.

-Rotor design
Designing a wind energy conversion system is a complex process.
• The environmental conditions to which the turbine is exposed can be severe and
unpredictable.
• Structural dynamics
• Material science and economics are to be applied to develop machines which are
reliable.
• Efficient and cost effective.
In this section, a simple procedure for an approximate design of a wind rotor is discussed, based on the
fundamental aerodynamic theories. Following input parameters are to be identified for such a design.
1. Radius of the rotor (R)
2. Number of blades (B)
3. Tip speed ratio of the rotor at the design point (λD)
4. Design lift coefficient of the airfoil (CLD)
5. Angle of attack of the airfoil lift (α)

1-Radius of the rotor
The radius of the rotor primarily depends on the power expected from the turbine and the
strength of the wind regime in which it operates. Various losses involved in the energy
conversion process are also to be considered. If the power expected from the turbine (PD) at its
design point is known, with the relationship
where CPD is the design power coefficient of the rotor, is the drive train efficiency, is the
generator efficiency and VD is the design wind velocity. For a well designed system, the design
power coefficient (CPD) may be in the range of 0.4 to 0.45 and the combined efficiency of the
drive train and generator may be taken as 0.9.

If the design is to be based on the energy required for a specific application
(EA), the rotor radius can be calculated by
where, is the overall system efficiency, VM is the mean wind velocity over a period and T is
the number of hours in that period.
2-Tip speed ratio
Design tip speed ratio depends on the application for which the turbine is being developed. For
example, when we design the rotor for a wind pump which require high starting torque, low tip
speed ratio is chosen. On the other hand, if our intention is to generate electricity, we require a
fast running rotor and hence high tip speed ratio.
3-Number of blades
Number of blades in a rotor is directly related to the design tip speed ratio. The higher the tip
speed ratio, the lower would be the number of blades. Fig. 2.18 gives a guideline for choosing
the number of blades based on the design tip speed ratio.

4-Angle of attack and corresponding lift coefficient

Thank you for your kind attention

Aerodynamic,rotor design and rotor performance of horizontal axis wind turbine(HAWT)

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Aerodynamic,rotor design and rotor performance of horizontal axis wind turbine(HAWT)