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.

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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.

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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.

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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].

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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>