Aarhus University – Engineering College of Aarhus

Vertical Wind Turbine Creative Offshore Challenge 2008

Tommy Bødker Jørgensen and Helle Ørum Nielsen

19-12-2008

Vertical Wind Turbine 2

Preface This is a summary of the scheme design for the Master of Science thesis in Architectural Engineering,

Aarhus University. As well as a part of the participation in the Creative Offshore Challenge 2008.

In this report the study done regarding the Vertical Wind Turbine design development is explained, and the

idea for the further development of the turbine. The project is still in the idea phase.

The work period for this scheme design is approximately 7 weeks, beginning October 27th and to be handed

in December 17th.

Summery and abstract 3

Summery and abstract The focus on environmentally friendly energy (green energy) has during the last decade increased a lot. The

wind energy is already the mainly supplier of green energy, and the 3-bladed Horizontal Axis Wind Turbines

(HAWT) is already producing a lot of green energy.

The HAWT have been growing in size, during many years, and have reach a limit, where it seems that the

strength of the material are the limiting factor, and in some way also the possibility to transport the

components from the factory to the construction site.

A short description of the idea behind the Vertical Wind Turbine (VWT) is described here.

The VWT is working on the same principles as the Darrieus rotor (See known Technology). The use of lift

forces from aerodynamic profiles will give the structure the rotation.

Imagine the cut-out of the Darrieus rotor in Figure 1, as seen in Figure 2, with a supporting circle in the top

and the bottom. The structure with a top- and bottom-ring is rotating, and the structure is supported at the

bottom ring. Between the two rings there can be more than just four blades.

To be able to understand the behaviour of a VWT, the most well known technologies are found and

described. The aerodynamic influence from the profiles in windward side on the leeward side profiles are

discussed as well as the preconditions for the aerodynamic in this project.

There are many parameters to be adjusted and discussed. The discussion of the complex nature of the

structure is a subject that can tell where the next step can be taken on the way to make a design and a

model of the structure.

Figure 1 - Darrieus Wind Turbine - with 4 blades

Figure 2 - Cut out of the Darrieus rotor

4 Vertical Wind Turbine

Content Preface ................................................................................................................................................................ 2

Summery and abstract ....................................................................................................................................... 3

Table of figures ................................................................................................................................................... 5

1 Wind Energy – Existing technologies.......................................................................................................... 6

1.1 3 bladed Horizontal Axis Wind Turbine ............................................................................................. 6

1.2 Darrieus rotor .................................................................................................................................... 6

1.3 Giromill or H-rotor ............................................................................................................................. 7

1.4 Savonius ............................................................................................................................................. 7

1.5 Kite ..................................................................................................................................................... 7

2 Idea and design .......................................................................................................................................... 9

2.1 Choice of design ................................................................................................................................ 9

3 Brainstorm ................................................................................................................................................ 12

3.1 Reference model ............................................................................................................................. 13

4 Parameter discussion ............................................................................................................................... 14

4.1 Wind ................................................................................................................................................ 14

4.2 Loads ................................................................................................................................................ 15

4.3 Supports ........................................................................................................................................... 16

4.4 Profiles and wind profiles ................................................................................................................ 16

4.5 Other variables ................................................................................................................................ 17

4.6 Materials .......................................................................................................................................... 17

5 Power calculation ..................................................................................................................................... 18

5.1 Graphs ............................................................................................................................................. 18

6 Finite Element Method............................................................................................................................. 20

7 Control parameters .................................................................................................................................. 22

7.1 Flywheel ........................................................................................................................................... 22

7.2 Pitch ................................................................................................................................................. 22

7.3 Stall .................................................................................................................................................. 22

8 Conclusions ............................................................................................................................................... 23

9 Full report ................................................................................................................................................. 25

10 Bibliography .......................................................................................................................................... 26

5 Table of figures

Table of figures Figure 1 - Darrieus Wind Turbine - with 4 blades .............................................................................................. 3

Figure 2 - Cut out of the Darrieus rotor ............................................................................................................. 3

Figure 3 - Design proposal ................................................................................................................................. 5

Figure 4 - Darrieus Wind Turbine ...................................................................................................................... 6

Figure 5 - Giromill or H-rotor ............................................................................................................................. 7

Figure 6 - Savonius rotor ................................................................................................................................... 7

Figure 7 - Kite Gen power plant ......................................................................................................................... 8

Figure 8 - Laddermill .......................................................................................................................................... 8

Figure 9 - Simple design ..................................................................................................................................... 9

Figure 10 - Wire support between rings .......................................................................................................... 10

Figure 11 - Model of a cross section with two rings at the bottom ................................................................ 10

Figure 12 - Vertical Wind Turbine design proposal ......................................................................................... 11

Figure 13 - Mind map for Vertical Wind Turbine ............................................................................................ 12

Figure 14 - Wind profile ................................................................................................................................... 14

Figure 15 - Reduced wind ................................................................................................................................ 15

Figure 16 - Lift and drag force ......................................................................................................................... 15

Figure 17 - Wind profile rebuilding process .................................................................................................... 17

Figure 18 - Graph ............................................................................................................................................. 18

Figure 19 - Tangential force - 9.7 m/s ............................................................................................................. 19

Figure 20 - Tangential force - 25 m/s .............................................................................................................. 19

Figure 21 - Sketch of the profiles used in FEM model ..................................................................................... 20

Figure 22 - Stress, 4 constraints, 25 m/s ......................................................................................................... 21

Figure 23 - Stress, 8 constraints, 25 m/s ......................................................................................................... 21

Figure 3 - Design proposal

6 Vertical Wind Turbine

1 Wind Energy – Existing technologies In the start of the project a search of known technologies was done, to find out which other technologies is

using the wind to produce power, and to find out if there is other type of turbines like our idea.

1.1 3 bladed Horizontal Axis Wind Turbine

The most popular turbine at the time is the 3 bladed Horizontal Axis Wind Turbine (HAWT). They are

designed in many sizes; the largest one producing in December 2006 is an offshore HAWT, testing in the

North Sea 15 miles off the East coast of Scotland. The turbine is erected in a water depth of 44 metres. The

German RePower turbine has a power output of 5 megawatts with a rotor diameter of 126 meter.

(www.reuk.co.uk, 2008). Another large one is the Enercon E-126 which should be able to produce 6 MW.

(www.thewindpower.net, 2007)

The HAWT have to be yawed into the wind, so the wind direction is perpendicular to the rotor plane. When

the turbine is yawed into the wind, the lift force from the aerodynamic profiles will make the rotor turn.

1.2 Darrieus rotor

The French inventor Georges Jean Marie Darrieus files the first patent for a modern type of a Vertical Axis

Wind Turbine (VAWT) in 1925 in France (Darrieus, 1927), then in 1931 in the United States (Darrieus, 1931).

At that time the invention did not get so much attention.

The VAWT is independent on the wind direction, and do therefore not need to be yawed into the wind.

Figure 4 - Darrieus Wind Turbine

The common Darrieus rotor has two or three blades, there are shaped like an egg-beater. The Darrieus

Wind Turbine is also seen with four blades.

7 Wind Energy – Existing technologies

In the past the Darrieus rotor has been tested in different dimensions. The driving force in the Darrieus

rotor is the use of the lift force on the aerodynamic profiles.

Another difference is that the generator can be placed on the ground, and does not need a tall tower. A

disadvantage is that to maintain the components, the rotor has to be taken of the Darrieus wind turbine, to

get access.

1.3 Giromill or H-rotor

Another version of the Darrieus Wind Turbines is the Giromill, also called H-rotor. The main difference from

the egg-beater shape is the straight aerodynamic profiles.

Figure 5 - Giromill or H-rotor

1.4 Savonius

An invention, that was patented in 1930 by Sigurd J. Savonius from Finland (Savonius, 1930). The

aerodynamic drive from this turbine is only using the drag force from the wind. The speed of a Savonius

rotor can never be higher than the wind speed.

Figure 6 - Savonius rotor

1.5 Kite

An innovating idea is the use of kites to produce power. There are at least two projects working on

developing a wind power plant using kites to catch the power in the wind.

One project is Kite Gen. Kites are through a set of wires connected to a big structure turning around in a

circle (like the wind turbine this report is looking at). (Kite Gen , 2008)

8 Vertical Wind Turbine

Figure 7 - Kite Gen power plant

Another project which also is using kites is the Laddermil. In this all kites are connected to the same two

wires. (Ingeniøren A/S, 2008), (www.ockels.nl, 2008), (www.guardian.co.uk, 2008)

Figure 8 - Laddermill

The kites can catch the wind there is high up in the air, and far away from the roughness of the ground.

The power will be made from generators driven by the two wires the kites are connected to. The kites in

the right side of Figure 8 are having an angle of attack to the wind that gives the kite an upward going

direction. In the top of the circle, in an altitude of about 800 meters, the kite is changing the angle of attack

so it is gliding down again, with help from the wires.

9 Idea and design

2 Idea and design The idea for this vertical wind turbine has started many years ago. But one evening after dinner in the

spring 2007, Erik Ørum took an old piece of paper from his office, and showed it to his daughter and asked

if such a structure could be build. – They talked about how it could be build and designed, how it would

work, advantages and disadvantages etc.

Erik Ørum is Bachelor of Science in mechanical engineering (1977), and his daughter, Helle was at that time

working on her final project for Bachelor of Science in civil and construction.

With more than 20 years of experience in working with power plants, some of the demands of the Danish

power grid have been taken into account when this idea was developed.

One of the demands is to look at some kind of production flexibility. The power suppliers need to be able to

put extra power on the grid when needed, so the same frequency is kept.

Another demand is to be able to offer a different solution for producing electricity of wind energy than the

3 bladed HAWT.

2.1 Choice of design

With the experiences from the Darrieus wind turbines and some key issues in mind, such as to use the lift

forces from aerodynamic profiles and to consider the stability of the structure, the design of a bigger and

hopefully better turbine can begin.

A weak point at the Darrieus turbine is the axis. If the dimension of the turbine and rotor is increasing, then

the loads on the blades and the shaft are also increasing. If the axis can be a weak point, why not try to

make a turbine without the axis in the centre?

Figure 9 - Simple design

The first and most simple design was one ring in top and bottom – with aerodynamic profiles in between.

The stability in this structure is to use the two rings to support the profiles and just use simple wires to

move the forces between the two rings, see Figure 10. By replacing some of the vertical profiles with a

diagonal profile, the stability could be solved without using the wires.

10 Vertical Wind Turbine

Figure 10 - Wire support between rings

If the structure have a huge diameter, the cross section of the structure could be difficult to make stable.

The cross section would almost look like a wall, and then the height of the profiles will be limited.

A solution could be to make two rings in the bottom.

Figure 11 - Model of a cross section with two rings at the bottom

In the model in Figure 11 the bottom ring is supposed to be two rings.

The top of the blades might still need a top ring. Some issues about having two rings in the bottom are the

torsion. The torsion in the two rings is difficult to handle, and the structure need double support. When the

cross section is like a triangle the support gets horizontal load as well. To be able to take the horizontal load

at the rings, a connection between the two rings might be needed, and that connection will probably give a

lot of drag forces and no lift forces to help the rotation.

There are many things to consider with the two rings in the bottom. To get a simple model the design with

one ring in top and bottom with no wires was decided.

The support can be a bridge between the supports or just the support so the rotating structure only will be

supported on the top of the columns. The supports are not a subject to be discussed into details in this

project.

11 Idea and design

The simple design used for the calculations in this report will have the focus on is drawn in Figure 12. The

rotating structure is the blades, the top- and bottom-ring.

Figure 12 - Vertical Wind Turbine design proposal

Because of the complex nature of the optimization process, there have only been looked at one design.

12 Vertical Wind Turbine

3 Brainstorm To have an idea of which subjects that could fit in a final project, we used a mind map (Figure 13) to see

how we could make delimitation for this project so it could be possible to keep an overview instead of just

go out of one track.

It was quick clear for us that this mind map could continue and continue. Some of the topics we would like

to look at are the power and the stability. The design is a topic we have to consider, but that is not the main

topic.

Figure 13 - Mind map for Vertical Wind Turbine

Later in the process other mind map trees have been made. They are used as a tool to create an overview

during the working period. We used them as well to find the topics to discus later on, and to go further into

details with those we decided to do so with. The mind maps is placed in the full report.

There are many questions from the mind map we have had to assume, that there is a solution. For

example, how do we transfer the energy from the rotating structure to the grid? – With a background as

civil and structural engineers, we decided early in the process that this is not our topic, but whether it could

be done with magnetic track or a generator or…? That it is possible.

The assembly of the structure and the transportation of the structures is also topics there are not

considered in this project.

13 Brainstorm

Our search for literature started with known technology, both about vertical axis wind turbines and the

aerodynamics for them. The search was also among patents.

3.1 Reference model

The model is designed as simple as possible so it is easier to get some results. If there is a simple reference

model, it can be shown how much one change can do to this design.

The support is chosen to be free standing supports, so there is no connection, like a bridge, between the

columns.

All profiles are vertical. The profile is a symmetric NACA 0018 with a chord length of 3 m. Number of

profiles is 10. Diameter of the structure is 200 m. The only connection between the two rings is the profiles,

so no wires. See Figure 12.

Issues about the control system will not be discussed in details.

14 Vertical Wind Turbine

4 Parameter discussion In the power calculations there are 7 parameters that can vary. The 7 parameters are the wind speed, tip

speed, numbers of profiles, radius of the wind turbine, the height of the wind turbine, chord length of the

profiles and the temperature.

Parameters from the structure itself are the number of support, type of support (bridge vs. column), the

width of the profile is used for stability and material.

4.1 Wind

The wind data is based on the information found about Horns Rev. To find some design criterion for the

turbine, the mean wind for Horns Rev is used (9.7 m/s). At 13 m/s the 2MW HAWT produces the 2 MW.

And the turbines have a cut out at 25 m/s.

To have an idea of a possible cut out for the VWT the cut out criterion is chosen to 25 m/s. This cut out

criterion can later on be changed due to more work and studies with the VWT. If the speed is controllable

and the loads do not exceed the limit, then it might not be necessary to stop the turbine completely.

4.1.1 Profile of the Wind

The wind used in the calculation is assumed to be equal distributed in height. Figure 14 shows how a real

profile and the assumed profile of the wind looks like. The reason to use the equal distribution is because

the turbine is supposed to be located at sea. Here is the terrain roughness not as large like the roughness

terrain on land. Therefore will the gradient of the wind profile at sea level be much larger at sea than on

land. This means that the wind faster will achieve its full speed in a lower height.

Figure 14 - Wind profile

4.1.2 Wake effect and reduced wind

Wake effect has not been taken into account in the power calculation. But there will be some turbulence in

the wind because the previous profile interferes with the wind.

In the calculation it is assumed that all profiles are receiving the same amount of wind and from the same

direction. This assumption is not correct, but was made to simplify the calculation. In reality the wind speed

15 Parameter discussion

will be reduced when it passes the first half of the turbine. Besides the reduced speed the wind will also be

deflected a little bit, as shown at Figure 15.

Figure 15 - Reduced wind

4.2 Loads

The loads from the wind are calculated from the relative wind speed and an angle of attack. The lift force is

perpendicular to the relative wind and the drag force is parallel.

Figure 16 - Lift and drag force

The lift and drag force is calculated with this equation:

� � � · ½ · � · �� · · � �

C is lift-coefficient or the drag-coefficient. The coefficient is a function of the angle of attack and the

Reynolds number. The profile used is NACA 0018, and the coefficients are from (Sheldahl & Klimas, 1981)

The centrifugal force is depended on the rotational speed, so that force can be much bigger than the force

from the wind. The radius of the structure also has an effect on the centrifugal force. The rotational speed

has a maximum speed. So how does the radius affect on the centrifugal force?

16 Vertical Wind Turbine

Tip speed � � ������ · �

Centrifugal force ����������� � �� · !��� · ������ ��

��"��#· !���

If the radius is increased, the centrifugal force is decreased. But to make a bigger structure, the mass would

probably not be the same. This is yet another parameter to say something about the factors to be included

in an optimization process, which will not be included in this project.

The dynamic load from the rotation and the unsteady wind is a really important factor, due to deformation

and fatigue problems. To get to know how this structure behaves, those dynamic loads are not considered

in this project.

4.3 Supports

If the tolerances in the support are a sensitive point, the structure could be stable with only three supports.

Due to the dead load from the structure, and the span between three supports, that would demand a really

strong structure. To decrease the deformation, more supports can be added. So can a whole bridge as the

support. If the support is made with a bridge, the dimension of the rotating structure can be reduced, and

the deformation can be seen like when a train is crossing a bridge. The reason for the reduction of the

dimension in the rotating structure is because of the more uniformly placed support, compared with the

columns.

4.4 Profiles and wind profiles

The number of profiles does affect both the power production and the structure stability. How close is it

possible to place one profile next to another without making too much turbulence for the next profile? This

is an area of the literature which we have had problems finding information about for a vertical profile.

The possibility for the wind profile to be rebuilt during the distance between the profiles in the windward

side and the profiles in the leeward side exist. It is sketched in Figure 17.

17 Parameter discussion

Figure 17 - Wind profile rebuilding process

If the wind profile can be rebuild on that distance, how big should the diameter of the structure be? If the

wind profile is rebuild, is it then right to calculate the swept area as the frontal area? Or can it be changed,

because the wind meets the profile twice?

4.5 Other variables

The height of the profiles does also affect both the power production and the structure stability. The higher

profile the more power is produced, but the higher profile is also causing higher dead load of the structure.

The variables due to temperature differences are not included. That means that we have fixed the

temperature at 15 degree Celsius. In the power calculation it is possible to change from minus 50 to plus 50

degree Celsius. The expansion from temperature differences is therefore also not included.

Because of the complexity of the optimization process of the structure some parameters are chosen to be a

starting point. The number of profiles, the number of supports and the diameter are chosen. The height

was chosen after few FEM calculations.

4.6 Materials

The material used in the FEM calculation is steel for the whole structure. The goal for the FEM calculation

was to get to know how the structure behaves. Other materials are more suitable for the structure than

steel.

The dead load of the structure is also a great part of the loads on the structure, so yet another reason to

consider other materials.

18 Vertical Wind Turbine

5 Power calculation

5.1 Graphs

On the basis of the chosen design, there have been created some graphs, which shows how much power

the turbine is producing as a function of the wind speed and the power coefficient. The graphs have been

created with an interval from 1 m/s to 25 m/s. The tip speed ratio is fixed at the ratio 4, until the tip speed

reaches a limit at 50 m/s, hereafter becomes the tip speed ratio variable.

Figure 18 - Graph

The green graph shows the power that the turbine is producing. The graph shows the turbine stalls when

the wind speed reaches a wind speed at around 15.5 m/s.

The blue graph shows the power coefficient. The power coefficient is far from keeping within Betz limit (red

graph) at 59.3%. The graph shows that the power coefficient is exceeding the limit in almost every wind

speed. This is a problem, because it is not possible to pull that much energy out of the wind. And thereby

does the graph tell that the chosen design is not okay. A solution for the problem could be to reduce the

numbers of blades or blade area, or to reduce the angular speed. Another solution could be to enlarge the

diameter of the turbine to extend the swept area and thereby the energy of the wind. A mutual downside

with these solutions is that the distance between the profiles will be larger, and so will the dimension of the

structure.

The problem that the turbine stalls with a wind speed at 15.5 m/s can be solved by adding a pitch system to

the turbine. With a pitch system, it is possible to turn the profile into a position where a better angle of

attack is achieved.

19 Power calculation

When the turbine begins to stall, it means that the angle of attack for some of the profiles has become so

large that there is established turbulence around the profile. Because of the turbulence, the lift force

become small and the whole turbine will lose some of its power. Figure 19 and Figure 20 shows an example

of how the two situations are looking like. The two figures show how the tangential force around the

turbine is distributed. Figure 19 shows the situation where the wind is 9.7 m/s. In this situation the force is

smooth and contiguous around the circle, this indicate that there are no profiles that stalls.

Figure 19 - Tangential force - 9.7 m/s

Figure 20 shows the situation where the wind is 25 m/s. In this situation, the force is starting by following

the same pattern like Figure 19. Until the profiles is beginning to stall. It clearly is shown that the profiles

from 55 to 155 degree and again from 200 to 300 degree are stalling, because the profiles in these interval

is losing a lot of power compared to if it had followed the same pattern like Figure 19.

Figure 20 - Tangential force - 25 m/s

20 Vertical Wind Turbine

6 Finite Element Method For the displacement that is found for the simple model with SHS 800x40 and double HE500M profiles was

too big (see sketch of the model) The displacement found from the structure with 4 supports gave a

displacement of 3 m. With 8 supports it almost made it half as big, it gave a displacement of 1.7 m. The free

span is half the size it was before, so it makes sense that it gives such a big change.

Figure 21 - Sketch of the profiles used in FEM model

The displacements found in the calculations are not acceptable. To add more support might help, but the

next problem is to make a construction there respect the constraints in the model.

In Figure 22 and Figure 23 the stress distribution for the beam model is shown. The structure is deformed in

the figures.

21 Finite Element Method

Figure 22 - Stress, 4 constraints, 25 m/s

When adding more supports, the weak points are in the joints between the profiles and the bottom ring.

The Profiles also need to be stronger, or there is a need for a design change.

Figure 23 - Stress, 8 constraints, 25 m/s

The deformation of the structure is depended on the stiffness of the connections between the rings and

profiles. In the model these connections are fixed which affect also the deformations.

The top ring is deformed from the moment added from the profiles, and the top ring is also working like a

string around the profiles.

22 Vertical Wind Turbine

The deformation in the top ring can be compared with the tip of a HAWT tip of a blade. They deflect a lot

when operating. The bottom ring has less tolerance, and need a taller and stronger cross section.

7 Control parameters

7.1 Flywheel

If the structure has a heavy mass, and the structure is rotating, it takes some times for it to stop again. This

flywheel concept is useable in the situations where there for example is a need for more power distributed

to the electricity network.

And when it is not needed, the speed of the rotation can be increased, to later on, when there is need for

power regulations. The turbine is therefore flexible and not depended on the wind power as strongly as a

normal horizontal axis wind turbine.

This flywheel concept has not been further examined due to the many variables in the design process.

7.2 Pitch

If the profiles are able to be pitched, it can have an effect on the loads as well as on the power output. How

the profiles can be pitched is not further discussed, but it can also have an effect on the stability of the

structure. The opportunity to control the turbine with the pitching of the profiles, and to be able to move

the stall limit, that can make the turbine produce more, when the wind increase.

7.3 Stall

The fact that the turbine stalls at a certain wind speed combined with a certain rotation velocity can give a

design feature that have to be examined.

The influence of change in the rotation velocity and the wind speed can give a view over the possibility for

control of the turbine without having a pitch control system. Can it be controlled as wanted, and what is

the affect?

An optimal control feature for the rotational velocity would be, to be able to keep the turbine balancing

between the maximum utilisation of the structure, generator etc. (included the Betz limit) and the stall

characteristics, which function as a security brake.

23 Conclusions

8 Conclusions During the process with developing a design and to try to find out how this structure is behaving with the

loads from the wind, some answers where found as well as more new questions appeared.

Known technology

During the search for known technology, there was a lot to find about the 3-bladed HAWT. The fact that

this turbine design is so well documented makes it hard to convince people about this idea.

The thought of the design as a possible competitor to the HAWT can sometimes be hard to see. The VAWT

found in the literature has had a lot of disadvantages, such as the access for maintenance of big

components could only be done if the rotor was detached (Darrieus).

The clever thing in the Darrieus turbine is that there is only tension in the blades when operating. The

tension is then changed to compression in the axis, which therefore is a weak point, as well as the

connections to the thin profiles (due to fatigue).

One way is to be near ground with easy maintenance, another is to get up high in the air, far away from the

roughness of the surface. The two examples with kites do that.

Aerodynamics in theory

For the aerodynamics in the turbine, the literature about a vertical wind turbine have not been easy to find,

and as mentioned, some of the preconditions are chosen, as for example the wind to have the same

direction in windward and leeward side of the turbine. If a more realistic model was found and used, it can

help to make the precision of the turbines power output.

Supervision from an aerodynamic teacher would be helpful, or with representatives from the industry with

experiences in aerodynamic. To find out more exactly what is happening with more or less profiles next to

each other.

To find out how the wind profile is behind the turbine, some experiments could be performed in a wind

tunnel. A CFD model could as well help to find out how the flow is behind the turbine. To be able to make a

CFD model it takes a lot of time and knowhow to build a 3D model to use in a CFD program, and the fact

that the turbine is time dependent as well, does not make the simulation easier.

Pitch control

The control feature of the turbine is not a subject in this report. From the power curve the turbine stall at

about 15 m/s (see Figure 18). The preconditions for this situation are that the rotation velocity does not

exceed 50 m/s, and before that speed limit, the speed varies with a tip speed ratio at 4. This demands some

control to keep these limits. The possibility for finding a better control area is open, but the need for

control is still there.

A possible control system is the pitch control. In that way the turbine can spin with the profiles having an

optimum angle of attack. (See opportunities instead of eliminations…)

24 Vertical Wind Turbine

Stall control

The need for a control for the turbine exists. To examine how the rotational velocity can have an influence

on the limit for stall, and therefore also for the loads and the production is a subject to be examined further

in the future. The power curve in Figure 18 show where the stall limit is in this situation, but the rotational

velocity is fixed in these calculations, they can be changeable as well.

Design change

With the information gathered about the structure at this moment, the structure needs to be stronger. A

possible change of design should be considered, or at least other cross sections for the bottom ring. The

deformation of the bottom ring does affect the supports, and when the dynamic loads are taken into

account, the displacement should be looked at carefully.

The simple change is to find a better cross section for the two rings, for example a taller cross section in the

bottom, and maybe straight beams between the profiles in the top. Another change could be adding a

bridge as the support.

Structure

In this project the focus was to find out how the structure works. The optimization of the structure has not

really started. The whole optimization can take long time and include different designs as well.

To find the loads to use in a FEM model as the one used in this project, can be done much more precise

because of the unknown dynamic loads, such as turbulence from the other profiles in the turbine.

Power output

The power calculation is based on the mentioned preconditions. A weakness of this calculation is that it is

the theoretical production, or almost. Betz’s limit is not included in the calculations, so the control for this

limit has to be done afterwards.

The optimization for the power output can still be done with the spreadsheet, but the connection between

the calculations and some tests would be helpful.

The optimization for the whole structure has to be seen together. Both an optimization of the structure for

stability and deformation, as well as for how much the design change affects the power output.

Economic

The economics in this project is as well a big topic. It all depends on cost of energy in the end, whether this

turbine can manage somehow to be a competitor to the 3 bladed HAWT. At this stage there is not a final

design, and therefore no economics involved.

Let us first see whether this structure can be build and function properly with controls and security as a

limit.

25 Full project

Offshore

To place the VWT offshore make sense because of the steep gradient of the wind profile and because the

turbine is expected to have big dimensions, and therefore is also the transportation of big elements easier

at sea than at land.

If the diameter of the turbine becomes large, the angular speed becomes smaller. The wanted speed at the

profiles is not allowed to exceed the limit of the sound, which are about 70 m/s. If this speed is kept at a

certain level, the angular speed must be smaller. And so does the centrifugal force as well.

Horns Rev Offshore wind farm consist of 80 HAWT placed on an area of 20 km2. When there is a design

ready of this turbine, the comparison between Horns Rev fitted with HAWT and fitted with VWT can be

done. From this comparison, the differences is cost can be seen, advantages and disadvantages as well.

9 Full project The 3rd semester project at Master of Science in Architectural Engineering is a scheme design for the master

thesis. The full report and other material can be fund at a temporary homepage.

http://userportal.iha.dk/~20074071/

If there are any questions, please write an email:

Helle Ørum Nielsen 20070595@iha.dk

Tommy Bødker Jørgensen 20074071@iha.dk

26 Vertical Wind Turbine

10 Bibliography

Darrieus, G. J. (1927). Patent No. GB259558. France.

Darrieus, G. J. (1931). Patent No. US1835018. United States of America.

Ingeniøren A/S. (2008, August 4). Mediehuset Ingeniøren. (K. Krøyer, Producer) Retrieved August 2008,

from Ingeniøren: http://ing.dk/artikel/89907-drager-i-800-meters-hoejde-skal-hoeste-billig-

vindenergi?highlight=tr%E6dem%F8lle

Kite Gen . (2008). Retrieved from http://www.kitegen.com/index_en.html

Savonius, S. J. (1930). Patent No. US1766765. Finland.

Sheldahl, R. E., & Klimas, C. P. (1981). Aerodynamic Characteristics of Seven Airfoil Sections through 180

Degrees Angle of Attack for Use in Aerodynamic Analysis of Vertical Axis Wind Turbines. Albuquerque:

Sandia National Laboratories.

www.guardian.co.uk. (2008, August 3). Retrieved from

http://www.guardian.co.uk/environment/2008/aug/03/renewableenergy.energy

www.ockels.nl. (2008). LadderMill. Retrieved from http://www.ockels.nl/

www.reuk.co.uk. (2008, December 15). The Renewable Energy Website. Retrieved October 23, 2008, from

http://www.reuk.co.uk/print.php?article=Worlds-Largest-Wind-Turbine-Generator.htm

www.thewindpower.net. (2007, October 19). The wind power. Retrieved October 23, 2008, from

http://www.thewindpower.net/wind-turbine-datasheet-223-enercon-e126.php