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NEW BLADE PROFILE FOR DARRIEUS WIND TURBINES
CAPABLE TO SELF-START
N.C. Batista*, R. Melício*†, J.C.O. Matias*, J.P.S. Catalão*†
* University of Beira Interior, Covilha, Portugal, and Centre for Aerospace Science and Technologies
† Center for Innovation in Electrical and Energy Engineering, IST, Lisbon, Portugal;
email of corresponding author: catalao@ubi.pt
Keywords: blade profile; Darrieus wind turbine; self-start
capabilities; performance; simulation.
Abstract
The wind power generation is experiencing a rapid growth,
achieving the highest number of European installations in
2010 comparing to other renewable sources. The need for a
smarter grid capable of integrating several decentralized
sources of energy and the increasing need for energy in urban
areas, has led to an increase interest in wind turbines for the
urban areas. In these environments, vertical axis wind
turbines (VAWT) have several advantages over horizontal
axis wind turbines (HAWT), namely: their ability to operate
closer to the ground; their insensitivity to yaw wind
directions; the smaller number of components; the operation
at low sound emissions; the ability to generate energy from
wind in skew flow. One problem with the lift-type VAWT
(Darrieus wind turbines) is their natural inability to self-start
at low wind speeds without extra components. Hence, a new
blade profile for Darrieus type VAWT is presented in this
paper, capable to self-start at low wind speeds. A
methodology is developed to compare the new blade profile
with other known airfoils. Finally, conclusions are duly
drawn.
1 Introduction
The wind energy systems have been considered as one of the
most cost effective of all the currently exploited renewable
sources, so the demand and investment in wind energy
systems has increased in the last decade [18].
Several studies have been conducted to model, simulate [14]
and characterize [7] the wind behaviour to stimulate the
acceptance of the wind energy in the market, by offering tools
to help and ease the enterprise I&D.
The investment in wind energy for the 27 EU Member States
is expected to grow in the next 20 years, reaching almost €20
billion in 2030 towards 400 GW of installed capacity
(250 GW onshore and 150 GW offshore), aiming to produce
between 26% and 35% of the electricity needs [6]. This
represents the avoidance of 600 million tonnes of CO2 per
year and a save for Europe of €56 billion a year in avoided
fuel costs and €15 billion a year in avoided CO2 costs.
As the penetration level of wind power increases into the
power systems, the overall performance of the electric grid
will increasingly be affected by the characteristics of wind
turbines. One of the major concerns related to the high
penetration level of the wind turbines is the impact on power
system stability and power quality [15].
The decentralized energy generation is an important solution
in a smarter grid with a growing acceptance for the urban
areas. Also, the increasing need for more environmentally
sustainable housing and the new European norms regulating
this, have contributed for the promotion of wind energy
systems in buildings.
If a network connection is available, the energy can be fed in,
thereby contributing to a reduction in electricity costs. In
order to maximize the security of the energy supply, different
types of wind turbines can be supplemented by a photovoltaic
system or a diesel generator in a quick fashion [16], [12].
In urban areas the wind is very turbulent and unstable with
fast changes in direction and velocity, in these environments
the vertical axis wind turbines (VAWT) have several
advantages over horizontal axis wind turbines (HAWT) [5].
These advantages are: their insensitivity to yaw wind
direction changes (so the turbine does not need the extra
components to turn the rotor against the wind); smaller
number of components (the reduced number of components
lead to a more reliable product and a reduced cost in
production and maintenance); it’s very low sound emissions
(ideal for urban areas); the ability to generate energy from
wind in skewed flows (the skewed flow are very usual in
urban areas specially on the roofs) [16]; a three dimensional
structural design easier to integrate in urban architecture; the
ability to operate closer to the ground level.
The Darrieus type VAWT has a natural inability to self-start,
but several solutions have been presented to overcome this
drawback: use of a guide-vane [19], using a hybrid
configuration of a Savonius VAWT (drag type wind turbine)
and a Darrieus VAWT (lift type wind turbine) [10], use of
mechanical system to optimize the blade pitch [17], use of
blades that change their form during operation [2], or a
specific blade profile capable of offering self-start capabilities
to the wind turbine without extra components [13].
The use of extra components, although it speeds the
development phase, it also increases the complexity of the
wind turbine due to the increase of components, that in turn
decrease final product sustainability and lifetime, and increase
production and maintenance costs.
The development of a blade profile for the VAWT capable to
self-start and with a reasonable performance at high TSR is a
very complex and time consuming task, leading to an increase
of time and cost for the wind turbine development.
The recent developments regarding VAWT, and the
associated technological innovation, motivate the work
carried out in this paper. Hence, this paper is based on a
straight bladed Darrieus VAWT and it has the goal to present
a new blade profile capable to self-start.
By considering the time used to develop a new airfoil for the
VAWT capable to self-start, a methodology for fast analysis
was developed and will be presented in this paper. With this
methodology a substantial reduction of time consumed in the
first phases of new blade development is achieved.
Accordingly, simulation studies are carried out in order to
adequately assess the behaviour of the blade profiles. The aim
is to provide self-start capabilities to the VAWT without the
usage of extra components or external electricity feed.
Although the performance of the profile was developed taking
into account a specific VAWT configuration, leading the
studies path in a certain direction and influencing the final
solution choices, it can be used in other lift type VAWT.
Usually with the VAWT, if a wind turbine needs to be self-
start capable its performance in high TSR is rather poor
(turbines used in low wind speeds sites), while if a wind
turbine needs to have a high performance at high TSR it is
usually not able to self-start (turbines used in high wind speed
sites).
In order to demonstrate the new profile capabilities, its
performance is going to be compared with other profiles
commonly used and known.
This paper is organized as follows. Section II shows the new
airfoil profile design for Darrieus VAWT blade. Section III
presents the methodology used for the first stages of the
development of the airfoil design and self-start capabilities.
Section IV provides the performance of the new blade profile.
Finally, Section V outlines conclusions.
2 The new airfoil profile EN0005
The Darrieus VAWT are divided in two types of turbines: the
curved bladed turbine (egg shaped turbine); and the straight
bladed turbine. Since this is a lift type turbine it can operate at
high TSR, but they usually have an inherent difficulty, which
is the inability to self-start since the blades suffer at the same
time with drag and lift.
These forces (drag and lift) usually balance each other leading
to a lack of starting torque at low wind speeds [4].
The study and development of an airfoil capable to self-start
is a very complex task. The new airfoil presented in this paper
is called EN0005. Before its design was developed, several
other blade solutions with better known profiles were used,
such as, trapping vortex cell systems [11], [20], thick blades
[3], and modified profiles [13]. The need to get a more
suitable blade profile to the VAWT in development and the
need to contribute to the scientific community with another
innovative solution was felt.
The new profile developments started with a base profile that
is continually modified by moving each segment of the
profile surface. For each modification the effects of those
modifications to the wind turbine performance are tested by
applying the methodology that will be explained in the next
section. The new blade profile (EN0005) is shown in Fig. 1.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
−0.2
−0.1
0
0.1
0,2
0.3
x/c
y/c
Figure 1: EN0005 blade profile with divided surface.
The upper surface is a high lift surface with a slight
orientation in the desired movement of the blade. This high
lift surface is essential when the wind turbine is working at
both low and high TSR.
The nose of the blade is in a lower position in relation to the
line chord and it has a tip formation in the front to increase de
wind flow over the body and to reduce the drag forces when
the blade is in the upstream zone.
In the lower surface of the blade profile the first 20% of the
length has high lift properties that are essential when the wind
turbine is working at high TSR. The last 80% of the surface
finishes in a cup form, which is essential to increase the drag
forces of the profile when the wind turbine is stopped and the
blade is in the downstream zone of the rotor.
3 Methodology
To study the self-start capabilities of a VAWT blade profile,
there is the need to create a methodology that would give a
closer relation between the wind forces acting in the blade
and the blade profile itself, and that would be fast in
calculation processing, which will be very useful in the first
steps of the studies when developing different profile designs.
The VAWT in order to self-start relying only on the blades
profile, without the help of extra components and external
energy, must take advantage of the drag forces caused by the
wind on the blades when the turbine is in a stopped position,
without compromising the wind turbine performance at high
TSR. If possible, the lift forces should be used in cooperation
with the drag forces to induce the self-start capability of the
wind turbine, especially when the turbine is stopped and the
wind flow starts to achieve higher velocities.
So, it is essential to study the blade profile behavior in
relation to the wind when the wind turbine is stopped. One
problem arises here, since the blade may be positioned at any
given point around the rotor, thus there is the need to study
the blade profile at any angular position from 0º to 360º.
In this situation the dynamic stall behavior, air flow
separation, and any other aerodynamic disturbances must be
taken in consideration [8], [9].
To study these aerodynamic disturbances, takes a high
computational processing time, which leads to a time
consuming situation not advisable in the first steps of the
development studies. So, the analysis methodology that is
present here to demonstrate the developed blade profile is
only suitable for fast analysis when there is the need to
compare several blade profile solutions to start restricting and
eliminating different designs. It is very important not to forget
the analysis of different aspects of the wind flow disturbances
acting on the wind turbine in a more advanced studies stage.
To study the blade profile modifications and the implications
that those modifications bring to the wind turbine
performance, a close relation between the surface of the blade
and the wind flow must be created. In this methodology the
pressure coefficient pr
C is used, which is a dimensionless
number that describes the relative pressure throughout a flow
field and is intimately correlated to the flow velocity, and can
be calculated at any point of the flow field.
The pr
C is useful to study the forces acting on any given
point on the blade profile surface and its relation with
dimensional numbers is given by [1]:
2
21 ∞
∞
−
=
V
pp
Cpr
ρ
(1)
where the p is the pressure of the point where the pr
C is
being evaluated, ∞
p is the pressure of the undisturbed wind,
ρ
is the fluid density, and ∞
V is the undisturbed wind speed.
Since in the operation of VAWT at low TSR the variations of
pressure and speed have little influence in the fluid density,
the flow can be treated as being incompressible. It is assumed
that, when the 0=
pr
C at one point, the pressure at that point
is the same as the undisturbed wind flow; when 1
=
pr
C, that
point is a stagnation point, meaning that the flow velocity at
that point is null (relevant when optimizing the drag forces);
when 0
<
pr
C in the point of study, the wind is moving at a
higher speed than in the undisturbed wind flow (relevant
when optimizing the lift forces).
In the methodology presented in this paper, first there is the
need to perform a segmentation to the blade profile surface,
as shown in Fig. 1, and then to calculate the pr
C in each
segment. The relation between the blade profile segment and
the pr
C is shown in Fig. 2.
Fig. 2. Pressure coefficient acting on the blade profile
surface.
Fig. 2 shows the points i and 1+i of the segment of length
s
in the blade profile surface and their corresponding
Cartesian coordinates in the
x
and y axis. In the triangle
formed by segment
s
in relation to the
x
and y axis, o
represents the opposite side length and a represents the
adjacent side length. The variables o and a are given by:
ii xxa −
=
+1 (2)
⎩
⎨
⎧
+
+
−=
−=
surfacelower
surfaceupper
1
1
ii
ii
yyo
yyo (3)
When o is positive it means that the surface segment is
oriented in the direction to the wind turbine rotation, while
when o is negative the segment is oriented in the opposite
direction.
The blade profile segment length
s
is given by:
2
2oas += (4)
The segment angle
β
in relation to the blade chord line (the
x
axis) is given by:
(
)
aoarctan
=
β
(5)
By having the pr
C exerted in each segment of the blade,
there is the need to determine the pr
C contribution to the
tangential force pr
T and the pr
C contribution to the normal
force pr
N, which are shown in Fig. 3. As shown in Fig. 3,
the angle
ϕ
is the pr
C angle in relation to the blade chord
line, given by:
β
ϕ
−
−
=
º90º108 (6)
Fig. 3. Pressure coefficient, tangential and normal forces
acting on the blade profile segment.
The pr
C contribution to the tangential force pr
T and to the
normal force pr
N can be expressed as in (7) and (8). These
contributions must be multiplied by the blade profile segment
length and are given by:
()
()
⎪
⎩
⎪
⎨
⎧
<−=
≥=
0cos
0cos
owhensCT
owhensCT
prpr
prpr
ϕ
ϕ
(7)
()
()
⎪
⎩
⎪
⎨
⎧
−=
=
surfacelowersCN
surfaceuppersCN
prpr
prpr
ϕ
ϕ
sin
sin (8)
The equations (7) and (8) show the relation between the pr
C,
pr
T, pr
N, the angle
ϕ
and the segment length
s
.
4 Performance of the new airfoil
In order to assess the performance of the new airfoil EN0005,
a comparison to other better known and studied airfoils is
going to be presented. The airfoils chosen in this paper for the
comparison are the NACA0018 and the NACA4418 and they
are presented in Fig. 4, along with the EN0005 airfoil.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
−0.2
−0.1
0
0.1
0,2
0.3
x/c
y/c
↓ EN0005
↑ NACA0018
↓ NACA4418
Fig. 4. Airfoil profiles for EN0005, NACA0018 and
NACA4418.
The NACA0018 is more commonly used in Darrieus type
VAWT for its proved high performance at high TSR in high
wind speeds. The NACA4418 is a cambered version of the
NACA0018, with a maximum camber of 4% of the chord
located 40% from the leading edge, and was chosen as a
comparison to NACA0018 performance, since the cambered
airfoils are more likely to present better self-start capabilities.
To study the self-start capabilities of the airfoils, the
methodology presented in the previous section is used. By
applying the equations (2) and (3) to the given
x
and y
coordinates, the opposite side o and the adjacent side a are
obtained. By applying equation (4), the length of the airfoil
surface exposed to the wind forces is obtained. With the
equations (5) and (6), the pr
C angle in relation to the blade
chord line
ϕ
is obtained. With the data calculated previously
applying the equations (7) and (8) it is possible to determine
the pr
C contribution to the tangential force pr
T and the pr
C
contribution to the normal force pr
N.
Figs. 5 and 6 present the comparison of the pr
C contribution
to the normal and tangential forces, respectively.
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
−4
−3
−2
−1
0
1
2
3
4
5
Angle
Cpr contribution to Normal Force
EN0005 NACA0018 NACA4418
Fig. 5. pr
N at any blade azimuth angle.
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360
−8
−7
−6
−5
−4
−3
−2
−1
0
1
Angle
Cpr contribution to Tangential Force
EN0005 NACA0018 NACA4418
Fig. 6. pr
T at any blade azimuth angle.
The airfoil EN0005 has a higher pr
C exerted in the blade
profile surface contributing to the tangential force, but it
presents a higher contribution to the axial force, which must
be taken in consideration when designing the wind turbine
arms structure. In Fig. 5 the profile NACA0018 show a
symmetrical axial forced exerted in the blades, that can be
compared to the NACA4418 forces, although in this last
airfoil the forces are slightly higher to the outside of the wind
turbine but lower to the inside of the rotor. In Fig. 6 the best
profile is the EN0005 that shows an airfoil with the highest
contribution to the tangential force pr
T.
When analyzing the self-start of a wind turbine when it is still
in a stopped position, there is the need to increase the drag
exerted on the blades when they are positioned in the
downstream zone of the rotor. The drag contribution of the
surface segments of the blade profile that is suffering drag
forces, to the tangential force, is shown in Fig. 7. The profile
EN0005 has the higher drag contributing to the forward wind
turbine rotation movement, offering the best performance of
all the airfoils.
80 100 120 140 160 180 200 220 240 260 280
−0.25
−0.2
−0.15
−0.1
−0.05
0
Angle
Drag contribution to T
pr
EN0005 NACA0018 NACA4418
Fig. 7. Drag contribution to pr
T at rotors downstream zone.
5 Conclusions
This paper focus on the study and development of new blade
profiles for Darrieus type VAWT capable to self-start without
the use of extra components or external energy input. A new
blade profile design, EN0005, has been presented. This blade
design gives the wind turbine the ability to self-start, showing
an excellent performance at low wind velocities and low TSR,
and showing also a good performance at high wind velocities
and high TSR. Hence, this paper offers a significant new
blade design solution for the Darrieus type VAWT.
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