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New blade profile for Darrieus wind turbines capable to self-start


Abstract and Figures

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.
Content may be subject to copyright.
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:
Keywords: blade profile; Darrieus wind turbine; self-start
capabilities; performance; simulation.
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
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
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
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]:
where the p is the pressure of the point where the pr
C is
being evaluated,
p is the pressure of the undisturbed wind,
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=
C at one point, the pressure at that point
is the same as the undisturbed wind flow; when 1
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
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
Fig. 2 shows the points i and 1+i of the segment of length
in the blade profile surface and their corresponding
Cartesian coordinates in the
and y axis. In the triangle
formed by segment
in relation to the
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)
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
The blade profile segment length
is given by:
2oas += (4)
The segment angle
in relation to the blade chord line (the
axis) is given by:
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
is the pr
C angle in relation to the blade chord
line, given by:
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:
sin (8)
The equations (7) and (8) show the relation between the pr
T, pr
N, the angle
and the segment length
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
Fig. 4. Airfoil profiles for EN0005, NACA0018 and
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
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
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
contribution to the normal force pr
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
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
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
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
Drag contribution to T
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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... That is to say aerofoils with these properties were predicted to increase the ability of VAWTs to self-start while not causing a drop in performance. Efforts to develop blade profiles that improve the ability of VAWTs to self-start while still giving reasonable performance at high TSRs were continued in [33]. ...
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A lift-driven vertical axis wind turbine (VAWT) generates peak power when it is rotating at high tip-speed ratios (TSR), at which time the blades encounter angles of attack (AOA) over a small range from zero to 30 degrees. However, its ability to self-start is dependent upon its performance at low TSRs, at which time the blades encounter a range of AOAs from zero to 180 degrees. A novel vented aerofoil is presented with the intention of improving the performance of a lift-driven VAWT at low TSRs without hampering the performance of the wind turbine at high TSRs. Computational fluid dynamics (CFD) simulation is used to predict the aerodynamic characteristics of a new vented aerofoil based on the well documented NACA0012 profile. Simulations are performed using the SST turbulence model. The results obtained show a reduction in the coefficient of tangential force (the force that generates torque on the wind turbine) at low AOAs (less than 90 degrees) of no more than 30%, while at high AOAs (more than 90 degrees) an improvement in the tangential force of over 100% is observed. Using a simple momentum based performance prediction model, these results suggest that this would lead to an increase in torque generation by a theoretical three-bladed VAWT of up to 20% at low TSRs and a minor reduction in coefficient of performance of up to 9% at TSR of 2 and closer to 1% at higher TSRs.
... It should also be noted that the primary challenge with VAWTs is complicated structural dynamics and a larger amount of blade and tower mass for the same swept area. These deficiencies have led to considerable research aimed at developing new designs, materials, and techniques to increase the scalability, durability, and efficiency of VAWTs [4][5][6][7]. ...
This paper presents a comprehensive study of the dynamic behavior of small vertical axis wind turbines (VAWTs) based on local fabricated Savonius VAWTs, which is suitable for countries that have moderate wind speed. The merits of this design are cleanliness, silent, start-up under low wind speed, independent wind directions, adaptability and ease of manufacturing. Also, this paper presents an experimental validation study for the optimized Savonius VAWT. Four verification test configurations of the optimized VAWT composite blades are designed, simulated and fabricated of Glass – Polyester with different stacking sequence layout for each. Modified mechanical parameters are introduced to improve the scalability, reliability, and accuracy of the developed models. Based on wind energy conversion system basics, aerodynamic characteristics (tip speed ratio (λ) and coefficient of power (Cp)), dynamic characteristics (natural frequencies and mode shapes) of Savonius-rotor models are presented and simulated within SOLIDWORKS Simulation 2020 software. The dynamic characteristics such as frequency, mode shape and damping factor are extensively investigated using Fast Fourier Transformer (FFT) analyzer. The results show that the role of composite material blades in improving the dynamic performance of a wind turbine is significant.
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Wind Energy, Convergent omnidirectional nozzle guide, Vertical Axis Wind Turbine, Energy Efficiency Wind energy, considered a stable alternative, can be implemented in cities by means of vertical axis wind turbines, which have better performance against turbulent flow compared to horizontal axis turbines. However, this type of turbine has not evolved technologically significantly in the last few centuries, being the horizontal axis turbines more studied and developed, due to the theoretical better efficiency of these turbines, which creates room for improvement. Therefore, vertical axis wind turbine will be studied and the performance of some enhancements will be analyzed aiming a more efficient harvesting of wind energy. In this regard, a flow augmentation system is proposed to be integrated with the wind turbine. In addition, the Lenz 2, S815 and JShaped airfoil shapes will be analyzed by Computational Fluid Dynamics – CFD technique on the ANSYS software for comparison of static torque generated by the wind turbine against wind flow for different angular positions of the turbine. Analyzing the gains obtained with the integration of the flow augmentation system proposed, achieving, this way, results regarding to cut in speed and overall efficiency of the shapes. Results show that the use of the convergent omnidirectional nozzle guide increased the overall static torque of all turbines, which would decrease the cut in speed, as well as its increased effectiveness on drag driven airfoils.
To alleviate the energy crisis, tidal energy extraction has become a hot topic in recent years. As a commonly utilized energy converter, a primary concern of vertical axis tidal turbines (VATTs) is the incapability to self-start. Previous studies have found that the starting performance has a great correlation with the number of blades. However, the detailed effects of blade number on starting characteristic are not fully understood. Therefore, it is necessary to further assess the influences of the number and azimuthal angle of blades on starting characteristics. In this study, we investigate the effects of the blade number on start-up performance of VATTs by means of the commercial Computational Fluid Dynamic (CFD) software CFX. It can be seen that the increase of the blade number results in a more homogeneous flow field, and the varying angles can lead to a high average torque. Besides, the region of 100 •-120 • is proved that all turbines with three different blade numbers can achieve a better starting performance.
The effects of airfoil shapes on the power coefficient and the torque coefficient have been studied for an H-type Darrius vertical axis wind turbine (VAWT). Different types of airfoils were analyzed, and eight of them were selected and divided into two groups. The first group includes the airfoils with camber, including S815, NACA9418, and NACA9415, while the second group including S1048, NACA0018, and NACA0015 have symmetric geometries. The focus of the current study is on two-blade VAWTs because they have higher power coefficient than three or four blades VAWTs. The two-blade VAWTs with selected airfoils were simulated with Computational Fluid Dynamic (CFD) method, and k-\(\upomega \) SST was used as a turbulence model and then grid independency was checked. The numerical investigation indicates that the cambered airfoils produce a higher static torque coefficient than symmetric ones, up to 79.8%, and are qualified for self-starting purposes. In addition, the symmetric airfoils produce higher power coefficient than cambered ones, up to 68.7%, and are qualified for power extraction purposes.
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Application of wind turbines on roofs of higher buildings is a subject of increasing interest. However the wind conditions at the roof are complex and suitable wind turbines for this application are not yet developed. This paper addresses both issues: the wind conditions on the roof and the behavior of a roof-located wind turbine with respect to optimized energy yield. Vertical Axis Wind Turbines (VAWTs) are to be preferred for operation in a complex wind environment as is found on top of a roof. Since the wind vector at a roof is not horizontal, wind turbines on a roof operate in skewed flow. Thus the behavior of an H-Darrieus (VAWT) is studied in skewed flow condition. Measurements showed that the H-Darrieus produces an increased power output in skewed flow. The measurements are compared with a model based on Blade Element Momentum theory that also shows this increased power output. This in contradiction to a HAWT in skewed flow which suffers from a power decrease. The paper thus concludes that due to this property an H-Darrieus is preferred above the HAWT for operation on a flat roof of higher buildings.
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In this paper we present a dynamical systems model and control algorithms for a small, vertical axis wind turbine (VAWT). The wind turbine is designed for the domestic market, including regions without very favorable wind conditions. Good performance at low wind speeds is an important requirement for developing an economically viable, suburban VAWT. The performance of a VAWT can be greatly enhanced by incorporating estimation and control capabilities. Individual blade pitch and camber controls are considered in our VAWT design. Pitch control is achieved by rotating each individual blade about its vertical axis, while camber control is realized using a trailing edge flap on each blade. Using camber and pitch controls help in creating a greater force differential across the turbine than using pitch control alone. In this paper we present a simple strategy for implementing pitch control and demonstrate the resulting efficiency improvement through a simulation.
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We present a Large-Eddy simulation of a vortex cell with circular shaped. The results show that the flow field can be sub divided into four important zones, the shear layer above the cavity, the stagnation zone, the vortex core in the cavity and the boundary layer along the wall of the cavity. It is shown that the vortex core consits of solid body rotation without much turbulence activity. The vortex is mainly driven by high energy packets that are driven into the cavity from the stagnation point region and by entrainment of fluid from the cavity into the shear layer. The physics in the boundary layer along the cavity's wall seems to be far from that of a canonical boundary layer which might be a crucial point for modelling this flow. Keywords—Turbulent flow, Large eddy simulations, boundary layer and cavity flow, vortex cell flow.
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An analytical-numerical study is presented for an innovative lift vertical axis turbine whose blades are designed with vortex trapping cavities that act as passive flow control devices. The unsteady flow field past one-bladed and two-bladed turbines is described by a combined analytical and numerical method based on conformal mapping and on a blob vortex method.
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The aerodynamic behavior of a vertical axis wind turbine (VAWT) is analyzed by means of 2D particle image velocimetry (PIV), focusing on the development of dynamic stall at different tip speed ratios. The VAWT has an unsteady aerodynamic behavior due to the variation with the azimuth angle θ of the blade’s sections’ angle of attack, perceived velocity and Reynolds number. The phenomenon of dynamic stall is then an inherent effect of the operation of a VAWT at low tip speed ratios, impacting both loads and power. The present work is driven by the need to understand this phenomenon, by visualizing and quantifying it, and to create a database for model validation. The experimental method uses PIV to visualize the development of the flow over the suction side of the airfoil for two different reference Reynolds numbers and three tip speed ratios in the operational regime of a small urban wind turbine. The field-of-view of the experiment covers the entire rotation of the blade and almost the entire rotor area. The analysis describes the evolution of the flow around the airfoil and in the rotor area, with special focus on the leading edge separation vortex and trailing edge shed vorticity development. The method also allows the quantification of the flow, both the velocity field and the vorticity/circulation (only the results of the vorticity/circulation distribution are presented), in terms of the phase locked average and the random component.
The implementation of wind energy conversion systems in the built environment has renewed the interest and the research on Vertical Axis Wind Turbines (VAWTs). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack and perceived velocity with azimuth angle. The phenomenon of dynamic stall is then an intrinsic effect of the operation at low tip speed ratios, impacting both loads and power. The complexity of the problem and the need for new design approaches for VAWTs for the built environment have driven the authors to focus this research on the CFD modeling of VAWTs on: Comparing the results between commonly used turbulence models: Unsteady Reynolds Averaged Navier-Stokes – URANS (Spalart-Allmaras and k-ϵ) and large eddy models (Large Eddy Simulation and Detached Eddy Simulation). Verifying the sensitivity of the model to its grid refinement (space and time). Evaluating the suitability of using Particle Image Velocimetry (PIV) experimental data for model validation. The current work investigates the impact of accurately modeling the separated shed wake resulting from dynamic stall, and the importance of validation of the flow field rather than validation with only load data. The structure and magnitude of the wake are validated with PIV results, and it demonstrated that the accuracy of the different models in simulating a correct wake structure has a large impact in loads.
The objective of this study is to show the effect of guide vane geometry on the performance. In order to overcome the disadvantages of vertical axis wind turbine, a straight-bladed vertical axis wind turbine (S-VAWT) with a directed guide vane row has been proposed and tested by the authors. According to previous studies, it was clarified that the performance of the turbine can be improved by means of the directed guide vane row. However, the guide vane geometry of S-VAWT has not been optimized so far. In order to clarify the effect of guide vane geometry, the effects of setting angle and gap between rotor blade and guide vane on power coefficient and starting characteristic were investigated in the experiments. The experimental study of the proposed wind turbine was carried out by a wind tunnel. The wind tunnel with a diameter of 1.8m is open jet type. The wind velocity is 8 m/s in the experiments. The rotor has three straight blades with a profile of NACA0018 and a chord length of 100 mm, a diameter of 0.6 m and a blade height of 0.7 m. The guide vane row consists of 3 arc plates.
Conference Paper
Wind power is the fastest growing renewable energy technology and is becoming a significant component of the energy mix. The secure and reliable operation of the power system implies the need for scheduling in advance the energy sources that will produce, so that the power system is balanced. Therefore, the use and importance of the wind power is strictly dependent on the ability to predict the wind in advance. In this paper, ARMA models are used to forecast the wind speed in terms of a medium-term prediction. Furthermore, an investigation on the benefits of pre-filtering the wind speed time series using wavelets is carried out. Some simulations are done with the twofold purpose of evaluating the performance of ARMA models as compared with reference models and investigating whether the wavelet pre-filtering technique leads to an improvement of the forecast results.
Since ancient past humans have attempted to harness the wind energy through diversified means and vertical axis wind turbines (VAWTs) were one of the major equipment to achieve that. In this modern time, there is resurgence of interests regarding VAWTs as numerous universities and research institutions have carried out extensive research activities and developed numerous designs based on several aerodynamic computational models. These models are crucial for deducing optimum design parameters and also for predicting the performance before fabricating the VAWT. In this review, the authors have attempted to compile the main aerodynamic models that have been used for performance prediction and design of straight-bladed Darrieus-type VAWT. It has been found out that at present the most widely used models are the double-multiple streamtube model, Vortex model and the Cascade model. Each of these three models has its strengths and weaknesses which are discussed in this paper.