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Master Program in Wind Power Project Management
Wind Turbine Concepts and Applications
Most suitable wind turbine design concept for your wind project
Jacopo Antonelli
Advisor: Bahri Uzunoglu
Summary
Most suitable wind turbine design concept
Jacopo Antonelli
This report presents a comparative analysis between three different design concepts
of wind turbines: Horizontal Axis Wind Turbines, Vertical Axis Wind Turbines, and Counter
Rotating wind turbines.
The analysis is conducted describing the design characteristics of the different
turbines, focusing on the differences concerning the generator and gearbox, the yaw system,
and the blades, which are some of the main components of a wind turbine, pointing out
qualitatively the possible advantages deriving from the choice of one solution instead of the
others.
A more technical comparison is carried out looking at the main parameters that
identify the aerodynamic performances of a wind turbine, the power coefficient and the tip
speed ratio; furthermore is compared the estimated annual energy production for two
turbines, taken as example the wind statistics of the island of Sprogø, in Denmark.
The report is aimed as a preliminary tool for people interested in the development of
wind power projects, in order to gain a wider knowledge of the technologies available in this
field, and to be able to identify, at least from a qualitative point of view, the most effective
wind turbine concept for a specified application.
Keywords: HAWT; VAWT; H-rotor; counter rotating; dual rotor; wind turbine design; tip
speed ratio; power coefficient.
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Contents
Introduction .......................................................................................................................................... 4
Description ........................................................................................................................................... 5
Design characteristics ........................................................................................................................... 7
Generator .......................................................................................................................... 7
Yawing ............................................................................................................................. 9
Blades design and structural loads ................................................................................. 10
Aerodynamic characteristics .............................................................................................................. 11
Applications ....................................................................................................................................... 14
Wind farms ..................................................................................................................... 14
Residential areas ............................................................................................................. 15
Conclusion .......................................................................................................................................... 16
References .......................................................................................................................................... 17
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Introduction
In this report is carried out a comparison between the possible uses of three kind of
wind turbines in wind farms: HAWTs, VAWTs, and Counter Rotating wind turbines.
This analysis is important since the increasing development of wind power all over the
world has led to the study of new design concepts in order to achieve the best performances at
the lowest costs.
In most, if not all, wind power projects, HAWTs are the preferred solution, because of
the strong development and reliability of this technology. Nevertheless VAWTs and CR have
some technical characteristics that might indicate them as a future development in wind
power technology.
A complete and accurate analysis should also take into account, in addition to the
aerodynamic characteristics, some objective economic parameters, such for instance the Price
of Energy, defined as
considered over a significant lifetime period, usually 20 years.
Even if the data about installation and maintenance costs for HAWTs are easy to find,
such comparative economic analysis is anyway hard to carry on, because of the very poor
experience around VAWTs and CR: the former is an old unsuccessful concept which is
experiencing some new interest during the last years, thanks to new improvements in the
technology used, and all the economic estimations available are 20 years or more old; the
latest on the contrary is a very new concept without any large scale application for a reliable
cost analysis.
Anyway, a rough idea of the possible advantages of the proposed alternative
technologies can be given with the analysis of the two most important aerodynamic
parameters, the power coefficient Cp and the tip speed ratio λ, and the relative curves.
5
Description
HAWTs are the most known and used turbines in wind power projects. The main rotor
shaft and the generator are placed at the top of the tower, horizontal to the ground. The plane
of rotation of the blades is perpendicular to the ground, and so the axis of rotation is parallel
to the ground. This type of wind turbines has been used since the first windmills were built in
the Middle Ages, and then the knowledge around this technology is quite strong, leading them
to dominate the wind power market. Today this turbines have reached the size of 6 MW [1].
VAWTs are so named because the axis of rotation of the blades and the drive shafts of
the generator and power train are vertical with respect to the ground. This concept was
invented in the early „20s by the Finnish engineer S.J. Savonius, who designed the concept of
a two-curved-bladed wind turbine. Later on this concept of vertical axis was drawn on by
Darrieus, that realized a vertical turbine with straight bent blades; this idea was afterwards
developed leading to the H-rotor or Giromill wind turbine discussed on this paper, which so
far is the most diffused VAWT.
It consists of two or more vertical blades connected through metallic arms to the tower
where the generator is located. This technology found its golden period during the 1970s and
1980s, when were built several prototypes up to 500 kW in UK and Germany [2]. Due to the
higher experience gained with HAWTs, this concept so far is limited to some few low power
applications.
The Counter Rotating wind turbine, instead, is a quite new design concept. It is a
horizontal axis wind turbine with two rotors separated by an appropriate distance. One of the
rotors is rotating in counterclockwise direction and the other in clockwise direction on the
same axis. The rotors can be both upwind, or on the two sides of the tower. Also the blade
length can be different for the two rotors. Since it is an extremely young technology, most of
them are prototypes of small size (within 30 kW) under study [3], even though the Korean
manufacturer Kowintec is launching on the market models of counter rotating wind turbines
of 100 kW and 1 MW [4].
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Figure 1: HAWT 2 MW Vestas V80
Figure 2: VAWT 200 kW Vertical Wind
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Figure 3: Counter Rotating wind turbine a) 30 kW prototype; b) 1MW commercial type
Design characteristics
In this section the attention is focused on the most important aspects that distinguish
the different wind turbine concepts analyzed, evaluating the main design characteristics of a
wind turbine, and pointing out the main differences and advantages of each design concept
compared to the others.
Generator
The conversion of the rotational kinetic energy in electrical energy is made by the
generator, that is connected to the rotating shaft through the gearbox, that scales its speed to
match the frequency of the generator. In HAWTs (and then also in C.R. wind turbines) the
generator is therefore placed close to the rotor inside the nacelle, at the same height of the
rotor. The generator is one of the heaviest and biggest components of a wind turbine, and thus
its elevated position is a major issue for maintenance and installation: the large dimensions of
the nacelle and the heavy structure of the tower, that must be able to support such big masses,
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affect sensibly the operation costs of the turbine, since it is necessary the need of installing
efficient low mass generators [2].
Thanks to the vertical orientation of the axis of VAWTs, the generator can be easily
placed at the bottom of the tower, on ground level, as well as the control system for
maintenance, reducing the cost of operations; with this solution the attention on the choice of
the generator can be focused on efficiency and costs, without taking into consideration the
mass of it. Furthermore, the tower is not subject to high structural loads, since the nacelle is
much less heavy, making the construction of the turbine easier.
The possibility to place the generator at ground level allows to use direct drive
systems, in which the generator is directly connected to the shaft, without any gearbox, that is
one of the main breakdown causes. This type of generator has higher mass and dimensions,
since the lack of the gearbox necessitates the generator to have a high number of electric poles
to reach the grid frequency with the low rotational speed of the shaft. Its use in high power
HAWTs is not much widespread, while it is a solution often adopted for VAWTs.
Recently, in some Counter Rotating wind turbines, has found an application the Bevel-
Planetary gear system, a type of gear that allows to transfer the rotation of the two horizontal
shafts of the rotors to the generator placed vertically inside the tower [3]. This is a solution
widely used in the C.R. turbine systems, and allows to combine some advantages of HAWTs
and VAWTs. The complexity of the gearbox in a CRWT is generally reduced, since it
requires a smaller gear ratio than other gearboxes to step up the speed of the shaft to match
the speed of the generator, because of higher tip speeds achieved by smaller blade length in
comparison with the conventional system for a fixed power output.
However, it is necessary to point out that the design of the generator for a CR wind
turbine might have some additional issues, related to the necessity to connect one rotor to the
generator stator, and the other one to the generator rotor. So far there are no studies regarding
the complexity of such a design, and so it is not possible to evaluate the impact on costs.
9
Figure 4: Schematic of a bevel-planetary gear system
Yawing
The yaw mechanism is used in upwind HAWTs (the most common type) so that the
rotor plane is always perpendicular to the wind, and the relative wind speed has the optimal
direction for which the blades are designed. Through gearboxes and servo-hydraulic motors,
when the wind changes direction, the entire nacelle is rotated to face the wind.
On the contrary, the other two typologies of wind turbine don‟t need any yaw
mechanism. In the CRWT the nacelle is free to rotate around the tower axis, and as the wind
changes direction, it rotates the turbine, but since the turbine consists of two rotors on both
side of the tower, the auxiliary rotor is always facing the wind, maintaining the optimal
direction for the relative wind speed. The main rotor, on the contrary, is always downwind.
Since the H-rotor has a horizontal plane of rotation, instead, it can accept wind from
any direction, without the necessity to orient the rotor to capture the best wind.
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For both turbines, the absence of the yaw mechanism means lower costs of
construction and maintenance, and lower complexity that lead to a higher efficiency, since
there are less failure sensitive parts, and isn‟t registered any power loss to run the drive
system. VAWTs, furthermore, are completely indifferent to gusts or turbulence, since they are
omni-directional, and then could find application in areas with very unstable winds, where the
other turbines might experience mechanical problems due to the high frequency of change in
orientation.
Blades design and structural loads
One of the main design differences between HAWTs and VAWTs lies in the profile of
the blades. In order to maintain the same optimal angle of attack all over the length, and
therefore maximize the lift, the blades of an HAWT need to be twisted. They are also tapered
from the root to the tip, to improve torque and reduce wind resistance at the tip of the blade.
The root of the blade generally has a cylindrical profile, and has high mass and
thickness, because it suffers of the highest bending loads and fatigue stress due to gravity
during the rotation, and then is the most subject to fatigue failures. The more the blade length
is, the stronger the root has to be. Furthermore, the inner portion of the blade, since it doesn‟t
have an optimal aerodynamic profile, plays less significant role in generating torque due to its
low sweeping speed [3].
In CRWT this power loss is partially avoided thanks to the auxiliary rotor, of usually
one-half of the main rotor diameter, that provides to compensate for the so called “dead zone”
of the main rotor, and to generate additional torque. In this type of rotors, the “dead zone”
(about the inner 30% of main rotor diameter) of the blade is sometimes replaced with an
extension bar that connects the blade to the hub [3].
The blades of the H-rotor are, on the contrary, generally straight, with the same profile
all over the length, and then are much more easier to manufacture and with a lower cost. The
gravity loads on this kind of blades are better distributed, since they are connected to the
tower through support arms; anyway, this kind of turbines needs a larger blade area than
HAWTs of the same rated power in order to capture the wind at the best, increasing the mass
of the structure.
The blades of a VAWT cannot avoid, however, the sensible torque ripple caused by
the continuous change of the angle of attack during the rotation, thus reducing the fatigue life
of the turbine; this effect can be anyway reduced by increasing the number of blades.
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Aerodynamic characteristics
The power that a wind turbine can extract from the wind can be expressed by the
formula:
where Cp is the power coefficient. This parameter indicates the aerodynamic efficiency of a
wind turbine, and is a function of the tip speed ratio λ, defined as
and indicates the ratio between the rotational speed of the rotor and the wind speed.
The maximum theoretical limit for the power that a single turbine can extract from the
wind, i.e. the power coefficient, is referred to as the Betz limit, approximately 0.59. The real
value of the power coefficient, however, can be much lower, due to the losses in the energy
conversion.
Typical values of Cp for a HAWT is usually between 0.40 and 0.50. In the figure
below is plotted a typical power curve for a HAWT, where is shown the dependency of the Cp
from the tip speed ratio [5].
Figure 5: Cp-λ curve for a HAWT
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It is more difficult to evaluate the Cp for a Vertical Axis Wind Turbine, since there is
only a small amount of operating rotors of this type. The data available come mainly from
theoretical study or experimental results of small scale applications, and give values of Cp
around 0.40, even if some of them are quite old, because of the decreased research on this
technology in recent times. The picture shows the power coefficient for the British VAWT
260, a 100 kW two bladed H-rotor. In the same graph are also plotted the curves for a HAWT
and a Darrieus rotor [2].
Figure 6: Cp-λ curve for the H-rotor British VAWT 260
For the counter rotating wind turbine system, is even harder to evaluate the power
coefficient, since this technology is extremely new, and most of the data available refers to
numerical analysis or small prototypes.
According to the momentum theory, the maximum efficiency in extracting power from
the wind by two rotors of the same area rotating around the same axis increases from 59% to
64% [6]. Thus, placing a second rotor after the main one allow to convert an higher amount of
the wind energy content. As the wake behind the first rotor is rotating in the opposite
direction to the rotational direction of the rotor, the second rotor is advisable to rotate in the
same direction as the wake, i.e. counter rotate in respect to the first rotor, in order to extract
more efficiently the available energy in the wake.
Anyway, the laboratory studies conducted have obtained a maximum Cp value of
about 0.50; the lower value than the theoretical maximum is due to the complexity of
optimizing such a system, because of the strong interactions and influences of the two rotors
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on each other: the flow behind the upwind rotor is turbulent, and furthermore, in case of the
second rotor downwind, the flow is also disturbed by the presence of the tower.
In figure 7 is plotted the numerically predicted Cp- λ curve for a C.R. wind turbine,
with two 2-bladed rotors; in the same graph is also possible to evaluate the differences with
the curves of a 4-bladed single rotor and a 2-bladed single rotor [7].
Figure 7: Cp- λ curve for a counter rotating wind turbine
The power curves for the three different turbines analyzed show that they operate at
different optimum tip speed ratios. The best results are obtained with the HAWT, due to the
large experience gained in optimizing this design concept.
Looking closer at the curves, both the H-rotor and the Counter-rotating turbines
exhibit competitive results at low tip speed ratios: the C.R. rotor reaches the maximum Cp for
values of λ around 5 and the H-rotor has the Cp of 0.40 at a tip speed ratio of about 4, against
the optimum tip speed ratio of HAWTs of 7. This means that the C.R. and the H-rotor have
lower rotational speed at the optimum, with therefore a lower noise level and lower stresses.
In particular, comparing only the HAWT and the C.R. wind turbine, it can be seen that
the latest shows almost the same optimum power coefficient as the HAWT but at a tip speed
ratio of 5: for the same wind speed, the power output of the two turbines is almost the same,
but the counter rotating rotor has a rotational speed about 30% lower.
Furthermore, since the power extracted is proportional to the rotor area, in order to
increase the output of a HAWT is necessary to increase the size and the blade length,
increasing as a result the mechanical loads on the structure. Thanks to the peculiarity of the
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dual rotor, for a fixed power output, the C.R. turbine has a smaller size, and therefore lower
stresses.
A deeper look at the predicted Annual Energy Production for a Counter Rotating wind
turbine of commercial size of 1 MW, allows to get a clear idea of their efficiency. A
numerical simulation conducted with the wind data of the island of Sprogø in Denmark for a
C.R. wind turbine consisting of two 3-bladed 500 kW Nordtank rotors, shows an estimated
AEP of 2965.9 MWh, against the AEP for a single rotor of the same type of 2066 MWh, with
an increase in the energy produced of about 43% [8]. The results are shown in the picture
below.
Figure 8: AEP for a C.R. wind turbine of 1 MW and for a single rotor
Applications
Previously in this paper have been highlighted the main differences in design and
aerodynamics of the three wind turbine concepts investigated. Anyway, the analysis cannot be
completed without comparing their possible usage in different applications, in order to
identify the most suitable design concept for a wind power project.
Wind farms
As previously stated, so far almost every wind farm is equipped with HAWTs, due to
the strong knowledge gained during the years around this technology. With this type of
turbines has been possible to build wind farms of very high capacity, and of considerable size.
15
VAWTs or C.R. wind turbines need to be investigated and developed much more to reach
such results.
However, the more the rated power of a turbine raises, the more the size of it
increases. This lead to some issues with the size of a wind farm: due to the strongly disturbed
flow created behind a wind turbine by its rotor, that would affect the efficiency of the other
turbines of the wind park, the distance between each turbine must be of several rotor
diameters. Therefore, there is a limit caused by the usable area to the maximum capacity of a
wind farm.
C.R. wind turbines, on the contrary, for the same rated power have smaller dimensions
than a traditional HAWT, and consequently the distance between turbines can be smaller
without affecting sensibly the total power output. That means that in theory for a fixed land
area it could be possible to realize C.R. wind turbines farms of higher rated power than
HAWTs wind farms [8].
Residential areas
One of the main limitations to the strong development of wind farms in some sites
with good energy potential is the proximity to urban areas, that lead to visual and noise issues.
One of the sources of noise is of aerodynamic nature, caused by the rotation of the blades.
The level of noise is proportional to the rotational speed.
As highlighted previously, both VAWTs and C.R. wind turbines work with lower
values of λ than HAWTs, and therefore have lower rotational speeds at the optimum.
Anyway, the optimal power coefficients for both VAWT and C.R. wind turbine are not
drastically lower than the one for HAWTs: for the same power output, VAWT and C.R. wind
turbine have 30% - 40% lower rotational speed, consequently generating less noise.
VAWT can also rely on less mechanical moving parts, thanks to the absence of a yaw
system or a gearbox, sources of mechanical noise. Also the possibility to place the generator
at ground level can hinder the propagation of noise.
This characteristics allow C.R. wind turbines and VAWTs to find application on a
smaller scale and where environmental issues have created problems to the development of
wind farms.
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Conclusion
In this work three different design concepts of wind turbine have been analyzed,
describing their characteristics and focusing on the design and aerodynamic differences, in
order to identify the most suitable wind turbine for a project.
The discussion has pointed out that even if HAWT is the most common wind turbine
concept in most of applications, the C.R. wind turbines and the VAWTs show some
interesting characteristics that could suggest to investigate further in these technologies.
More specifically, VAWTs have proven to have a much more simple structure,
making their manufacture cheaper than a typical HAWT, and they lack of many of the
sensitive elements that lead HAWTs to failures, like yaw mechanism or gearbox, thus keeping
their maintenance easier and at lower costs.
Anyway, even if it is a very old turbine concept, the development of this technology is
still far away from reaching the efficiency of HAWTs, and making them competitive on the
wind power market in the short term, even if during the last years some energy producers
showed interest in realizing some wind farms equipped with H-rotors, mainly in Sweden [9].
Counter Rotating wind turbines, on the other hand, are a very new technology that
showed some promising results on the small scale laboratory testing. On one side, they can
count on all the strong knowledge gained with decades of experience with HAWTs, and have
basically many of the advantages of this turbine concept, but on the other side the peculiarity
of the double rotor poses some more issues on the aerodynamic behavior, that needs to be
investigated deeper.
So far this new concept has mainly found application in many low capacity
applications, such as home energy generation, but the positive results showed by the
prototypes running increased the interest in this technology for their possible use in high
power applications. As stated previously, this new concept of wind turbine allows to
theoretically realize high capacity wind farms in limited areas, with lower visual impact and
less noise issues.
Furthermore, some studies conducted in California have identified some old wind
farms in which the HAWTs could be converted into C.R. wind turbines, thus increasing the
overall capacity containing the land usage and the costs [10].
17
References
[1] Earnest Joshua, Wizelius Tore, Wind Power Plants and Project development, 2011
[2] Sandra Eriksson, Hans Bernhoff, Mats Leijon,
Evaluation of different turbine concepts for wind power. Renewable and Sustainable
Energy Reviews, Volume 12, Issue 5, June 2008, Pages 1419-1434
[3] Sung Nam Jung, Tae-Soo No, Ki-Wahn Ryu, Aerodynamic performance prediction of
a 30 kW counter-rotating wind turbine system. Renewable Energy, 2005, 30, (5), 631–
644
[4] <http://www.kowintec.com/english/products/main.htm>
[5] P.W. Carlin, A.S. Laxson, E.B. Muljadi, The History and State of the Art of Variable-
Speed Wind Turbine Technology. February 2001, NREL/TP-500-28607
[6] Newman BG. Actuator-disc theory for vertical-axis wind turbines. Journal of Wind
Engineering and Industrial Aerodynamics 1983; 15: pp. 347-355
[7] Seungmin Lee, Hogeon Kim, Soogab Lee, Analysis of aerodynamic characteristics on
a counter-rotating wind turbine. Current Applied Physics 10 (2010) S339–S342
[8] Kari Appa, Counter Rotating Wind Turbine System. Eisg Final Report, 2002
[9] W.Z.Shen, V.A.K Zakkam, J.N.Sørensen, K.Appa, Analysis of Counter-Rotating Wind
Turbines. Journal of Physics: Conference Series 75 (2007) 012003
[10] <http://www.verticalwind.se>