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: :> ! Chapter 3
Basic Concepts of Wind Energy Converters
There are many different ways in which devices to convert the kinetic energy contained
in an air stream into mechanical work can be realised and the most bizarre concepts have
been proposed [1]. Museums and patent offices are filled to the rafters with more or less
promising
inventions of this type. In most cases, however, the practical applicability of
“
these wind power plants” falls far behind the inventors’ expectations.
An attempt to develop an orderly and systematic classification of wind energy converter
types is certainly an interesting task, but it brings little reward as the number of significant
designs is drastically limited by their practical usefulness. When speaking of varying designs one should be aware of the fact that primarily varying designs of the wind energy
converter, the wind rotor, are meant. But the wind rotor is not the only component of a
wind turbine. Other components for the mechanical-electrical energy conversion such as
gearbox, generator, control systems and a variety of auxiliary units and items of equipment
are just as necessary for producing usable electric energy from the wind rotor’s rotational
motion. Many inventors of novel wind rotors, however, do not seem to be aware of this
fact when they are hoping that their invention of a different rotor design will improve
everything.
Wind energy converters can be classified firstly in accordance with their aerodynamic
function and, secondly, according to their constructional design. The rotor’s aerodynamic
function is characterised by the fact of whether the wind energy converter captures its
power exclusively from the aerodynamic drag of the air stream acting on rotor surfaces,
or whether it is able to utilise the aerodynamic
“ lift created by the flow
“ against suitably
shaped surfaces.Accordingly, there are so-called drag-type rotors”
“ and rotors which make
use of the aerodynamic lift”. Occasionally, the“ aerodynamic tip-speed ratio” is used to
characterise wind rotors and one speaks of low-speed and high-speed rotors” in this
case (Chapt. 5.2). These characteristics, however, are of little significance to modern wind
turbines. Apart from the American wind turbine, almost all other wind turbines designs
are of the high-speed type.
Classification according to constructional design aspects is more practicable for obvious reasons and thus more common. The characteristic which most obviously meets
the eye is the position of the axis of rotation of the wind rotor. Thus, it is important to
68
Chapter 3: Basic Concepts of Wind Energy Converters
make a distinction between rotors which have a vertical axis of rotation, and those with a
horizontal axis of rotation.
3.¹
Rotors with a Vertical Axis of Rotation
The oldest design of wind rotors features rotors with a vertical axis of rotation (Fig. 3.1). At
the beginning,
however, vertical-axis rotors could only be built as pure drag-type rotors.
“
The Savonius rotor”, which can be found as ventilator on railroad carriages or delivery
vans, and the cup anemometer used to measure wind velocity are well-known examples
of rotors with a vertical axis of rotation.
It was only recently that engineers succeeded in developing vertical-axis designs which
could also effectively utilise aerodynamic lift. The design proposed in 1925 by the French
engineer Darrieus, in particular,“has been considered as a promising concept for modern
wind turbines (Fig. 3.2). In the Darrieus rotor”, the blades are shaped and“ rotate in the
pattern of a surface line on a geometric solid of revolution, a troposkien (i. e. turning rope”
in Greek), with a vertical axis of rotation. This makes the geometric shape of the rotor blades
complicated and thus difficult to manufacture. As is the case with horizontal-axis rotors,
Darrieus rotors are preferably built with two or three rotor blades.
The specific advantages of vertical axis turbine concepts are that their basically simple
design includes the possibility of housing mechanical and electrical components, gearbox
Figure 3.1. Rotor concepts with a vertical axis of rotation
3.1 Rotors with a Vertical Axis of Rotation
69
Figure 3.2. Darrieus wind turbines of the former American Flowind company (rotor diameter 19 m,
power output 170 kW)
70
Chapter 3: Basic Concepts of Wind Energy Converters
and generator at ground level and that there is no yaw system. This is countered by disadvantages such as its low tip-speed ratio, its inability to self-start and not being able to
control power output or speed by pitching the rotor blades.
A variation of the Darrieus rotor is the so-called H-rotor. Instead of curved rotor blades,
straight blades connected to the rotor shaft by struts are used. Attempts were made particularly in the UK, in the US and in Germany to develop this design to commercial maturity.
Based on plans of the British engineer Musgrove, H-rotors with variable rotor geometry
were also tested in order to permit at least a rough degree of power and speed control [2].
However, to the present day, the production costs of these systems are still so high that
they cannot compete with horizontal-axis rotors (Chapt. 5.7). H-rotors of a particularly
simple structure, with the permanently excited generator integrated directly into the rotor
structure without intermediary gear-box, were developed by a German manufacturer up
Figure 3.3. H-rotor wind turbine (rotor diameter 35 m, 300 kW rated power)
(Heidelberg)
3.2 Horizontal Axis Rotors
71
until the beginning of the nineties [3] (Fig. 3.3) but the development was halted then since
there was no economic success in sight.
Occasionally, the Savonius design is used for small, simple wind rotors, especially for
driving small water pumps. It is not suitable for electricity-generating wind turbines due
to its low tip-speed ratio and its comparatively low power coefficient. With an optimised
aerodynamic design, the Savonius rotor can also make use of aerodynamic lift and its
maximum power coefficient is then of the order of 0.25 (Chapt. 4.2).
Apart from these concepts, a number of proposals for vertical-axis rotors with a great
variety of geometries are known, for example with blades in a V-shaped configuration
and tilted rotor axis. The inventors expect this to be a particularly simple and inexpensive
design but it remains to be seen whether these hopes will be fulfilled. Moreover, rotor
designs such as these inevitably have a much lower power coefficient, which, in turn, has
economic repercussions despite the potentially lower installation costs.
Altogether, it can be said that wind rotors with vertical axes and among these primarily
the Darrieus rotor, might still have a potential for development which has not been exhausted yet. Whether the basic advantages of this design can prevail over its disadvantages
and whether it will become a serious rival to the horizontal-axis rotors cannot be foreseen
for the long-term. In any case, this will still require a relatively long period of development.
3.²
Horizontal Axis Rotors
Wind energy converters which have their“axis of rotation in a horizontal position are realised almost exclusively on the basis of propeller-like” concepts (Fig. 3.4). This design,
which includes European windmills as much as the American wind turbine or modern
wind turbines, is the dominant design principle in wind energy technology today. The
undisputed superiority of this design to date is largely based on the following characteristics:
– In propeller designs, rotor speed and power output can be controlled by pitching the
rotor blades about their longitudinal axis (blade pitch control). Moreover, rotor blade
pitching is the most effective protection against overspeed and extreme wind speeds,
especially in large wind turbines.
– The rotor blade shape can be aerodynamically optimised and it has been proven that
it will achieve its highest efficiency when aerodynamic lift is exploited to a maximum
degree.
– Not least, the technological lead in the development of propeller design is a decisive
factor.
Together, these advantages are the reason why almost all wind turbines for generating
electricity built to date have horizontal-axis rotors.
Fig. 3.5 shows the schematic arrangement of a horizontal-axis wind turbine. The components and their configuration are typical of a large modern wind turbine. Naturally, designs
differing from this standard concept are also possible and constructional simplifications
such as the absence of pitch control can be found, particularly in small wind turbines.
72
Chapter 3: Basic Concepts of Wind Energy Converters
Figure 3.4. Horizontal-axis wind turbine: Bonus/Siemens Wind Power, (rotor diameter 107 m,
rated power 3.6 MW) prototype, 2005
(Siemens)
3.2 Horizontal Axis Rotors
73
Figure 3.5. Components of a
horizontal-axis wind turbine
74
Chapter 3: Basic Concepts of Wind Energy Converters
3.3
Wind Energy Concentrators
Before discussing the technology of the propeller type in more detail, some innovative concepts will be described, as they play a role in this discussion and some are also being tested
“in experimental programs. It is doubtful, however, at least in some cases, whether these
wind power plants” will ever achieve practical significance. The individual assessment of
inventions in the field of wind energy is a thankless task and the author will, therefore,
refrain from doing so in this book.
The basic idea common to all these concepts is to increase the power yield in relation
to the rotor-swept area. Basically, this can be achieved by static, i. e. non-rotating structures
which produce an acceleration in the flow velocity to the rotor or, in some cases, even
generate concentrating vortices (Fig. 3.6). The intention is to achieve a drastic reduction
in
“ rotor size whilst at the same time hoping that the additional construction required for
pre-concentrating” the wind energy will not become too expensive.
Ducted rotor
The simplest method for increasing rotor efficiency is to enclose it in a duct. The duct
prevents narrowing of the flow tube before it reaches the converter, which is unavoidable
with a rotor in a free air stream. The achievable power coefficient exceeds the Betz value
and is about cP = 0.66 [5]. Instead of using a full duct, effects similar to those of a ducted
flow can, to a smaller degree, be achieved with the help of endplates at the blade tips [4].
Turbine with a diffuser duct
“
An obvious idea aimed at capturing more wind” is to mount a funnel in front of the
rotor. However, theoretical and experimental investigations have shown that this does not
achieve an increase in power capture in practice. Apparently, the airflow through the funnel
is determined by the smaller opening, and the funnel additionally produces a circulatory
flow counteracting the wind stream.
It is more effective to place the rotor in a duct in the shape of a reversed funnel, a diffuser.
This results in an additional circulatory flow the speed components of which in the diffuser
have the same direction as the wind stream, thus reinforcing it. The power coefficient of
the rotor rises to values of 2.0 to 2.5 relative to the rotor-swept area [5] but, to obtain fair
results, the power coefficient must now be related to the maximum cross-sectional area of
the diffuser. This reduces the power coefficient to about 0.75, which is still a modest gain
compared to the free-stream rotor.
Vortex tower
An increase in wind concentration can also be achieved by superimposing a stationary
vortex on the wind flow so that the velocity field of the vortex has an extra driving effect
on the rotor. “This effect can be“ produced by various types of concentrator. One idea is
the so-called vortex tower” or tornado tower” [6]. In a tower with shutters arranged on
the cylinder jacket, the wind flows tangentially into the interior of the duct where it forms
3.3 Wind Energy Concentrators
Figure 3.6. Wind rotor concepts combined with static structures for concentrating wind energy
75
76
Chapter 3: Basic Concepts of Wind Energy Converters
a tornado-like air vortex. Due to the low pressure in the vortex centre, air is sucked in
from the bottom of the tower into the duct from the outside, thus driving a turbine with a
diameter which is about a third of the tower diameter. However, this principle has only been
examined in the wind tunnel so far. Applying it to a real, full-sized wind turbine would
probably meet with considerable problems, for example noise emission. The conclusion
drawn by theoretical assessments of this design concept is that the power coefficient related
to the maximum plan-view swept area of the entire structure reaches values of only 0.1 [5].
Vortex concentration with a “delta wing”
Concentrated air vortices occur as so-called boundary vortices in the flow around an aircraft wing. This occurs to a particularly high degree with delta wings with large angles of
attack. Attempts have been made to utilise this effect for wind energy technology. The wind
rotors are mounted on a static structure in the shape of a delta wing so that they work in
the boundary vortices of the delta wing. A reliable theory for this complex case was not
available but theoretical estimates led to the hope that the power yield would increase by
a factor of 10, compared to a rotor with in a conventional free air stream. In the end, the
result of model measurements in the wind tunnel turned out to be so disappointing that
the project was cancelled [7].
Concentrator wind turbine
The Technical University of Berlin
“ has proposed and investigated another variant of wind
concentrator with the name of Berwian” (Fig. 3.7). A fixed stator wheel with a number of
blades generates a strong vortex in the centre of the concentrator. The six- to eight-fold
increase in wind power is utilised by a small wind rotor in the centre of the stator construc-
“Figure 3.7. Concentrator-type wind turbine
Berwian” [8]
3.3 Wind Energy Concentrators
77
tion. Several variants of this design concept have been tested in the wind tunnel and in the
free atmosphere and have confirmed the concentration factors predicted in theory [8]. One
main problem is the strength of the stator in extreme winds. The blades of the stator have
to be movable so that they can be turned out of the wind in order to avoid extreme wind
loads. Structural complexity and the cost of the static structure are, therefore, considerable
also in this case.
Thermal upwind concept
The so-called thermal upwind power plant is based on the idea of generating an air flow,
as occurs naturally due to a rise in temperature, i. e. due to differences in air density. In this
concept, an updraft or upwind is generated in a high tower surrounded by a ground-level
canopy which absorbs solar radiation (Fig. 3.8) and this upwind drives an air turbine.
Strictly speaking, this is not a wind turbine utilising natural wind but rather a solar
power concept utilising solar radiation.
An advantage of this principle is its application
“
in areas otherwise inaccessible to normal” wind power utilisation. An experimental plant
with a projected power output of 100 kW was tested in Spain, funded by the German
Ministry of Research and Technology (Figs. 3.9 and 3.10). The tests and measurements
carried out in 1982 and 1983 yielded a power output of about 50 kW. The promoters pointed
out, however, that this design achieves its maximum efficiency only with considerably larger
dimensions and that, moreover, cost comparisons would have to be made with plants for
the direct utilisation of solar radiation and not with conventional wind turbines [9].
Figure 3.8. Schematic concept of a Thermal upwind power plant
78
Chapter 3: Basic Concepts of Wind Energy Converters
Figure 3.9. Experimental thermal upwind power plant in Manzanares, Spain, 1985. Tower height
200 m, tower diameter 10 m, diameter of collector roof about 250 m
(Schlaich & Partner)
Figure 3.10. Wind turbine inside the tower
(Schlaich & Partner)
3.4 Terms and Expressions
79
3.4
Terms and Expressions
“
Before you argue, clarify your terms” (Confucius, 551 to 479 BC.). It would certainly be
better to follow the advice of this
“ Chinese philosopher than to justify later confusion with
pleasant-sounding phrases like What’s in a name?”. Clear and unambiguous definitions of
terms are the indispensable prerequisite for a systematic modus operandi and wind energy
technology is no exception in this respect.
The designation
of the subject matter of this book will be considered first. The title of
“
this book is Wind Turbines”. Specialist literature offers a wide variety of similar, but not
quite identical terms of which windmill, windwheel, wind generator, wind energy converter,
wind energy plant, wind power plant are the most common ones.
It is obvious that a term like windmill is unsuitable for a machine generating electricity.
The term windmill, by the way, in many cases was not correct even in its time, as windmills
were by no means used exclusively for milling grain. As to the selection of the remaining
terms, this is a matter
of taste. They capture the“ meaning of the object to a varying degree.
“
The decision for wind turbine”, in contrast to wind power plant”, seemed more appropriate to the author. Considering the relatively modest power output compared to conventional
“
power plants, wind power plant seems somewhat pretentious. It should be noted that wind
turbine” is used
“ for the whole system and not for the turbine in its narrower sense which
is called the rotor”.
The main components of a horizontal-axis wind turbine have already been described
(Fig. 3.5). The technical terms used there also
“ require some description and explanation.
“ The actual wind energy converter, the windwheel” in an old windmill,
“ is called the
rotor” in a“ modern wind turbine. Different rotors have differing numbers of rotor blades”.
The term sails”, still frequently used, should be avoided. Rotor blades should definitely
not go sailing off into the sunset!
“
The
“ rotor blades are connected to the rotor shaft by means of a hub”.
“ In wind turbines
with blade pitch control”, the hub contains the blade bearings and the blade pitch mechanism”. Many smaller wind turbines are not fitted with a blade pitch control. The rotor
blades then
“ have a fixed connection to the hub.
The drive train” of the wind turbine converts the rotor’s
“ mechanical rotational motion
into electrical energy. In its narrower sense, the term drive train” is only used for the
mechanical components,
“ excluding the electrical“ system. The rotor hub, with the blade
pitch mechanism, the rotor shaft”, also called low-speed shaft”, the
“ gearbox and the
shaft”,
generator drive shaft, which, in contrast to the rotor shaft is called the high-speed
“
are all part of the drive train. The drive train components are housed
in the nacelle”.
“
“
The nacelle and rotor is turned into the wind
“ direction
“ by the yaw system”
“ or azimuth
drive”. The nacelle is mounted on top of a tower” or mast”. The term mast” is more
suitable for very small wind turbines.
Apart from these terms, a number of other terms and designations are used in the various chapters of this book. These, however, have nothing to do with wind power technology
per se, but have their roots in other fields such as aerodynamics, electrical engineering or
power plant technology. It is recommended to take notice of this nomenclature and not to
change it at will only because one is not familiar with the specific field. The author certainly
80
Chapter 3: Basic Concepts of Wind Energy Converters
has made an effort to do so. In an age where communication between different scientific
disciplines tends to be replaced by dialogue with a computer screen, the remainders of
commonly understood terms and language should be preserved and protected.
References
1. Molly, J. P.: Windenergie, Theorie, Anwendung, Praxis
Karlsruhe: Verlag C. F. Müller, 2nd ed. 1990
2. Mays, I. D.: The Development Programme for the Musgrove Wind Turbine
London: Fourth International Conference on Energy Options 3 – 6 April, 1984
3. Heidelberg, D.; Kroemer, J.: Vertikalachsen-Rotor mit integriertem Magnetgenerator,
BremTec: Windenergie ’90, Bremen 1990
4. De Vries, O.: Fluid Dynamic Aspects of Wind Energy Conversion
AGARD report No 243, 1979
5. Reents, H.: Windkonzentratoren, Munich, Deutscher Physiker-Tag, 1985
6. Yen, J. T.: Tornado Type Wind Energy Systems: Basis Considerations, BHRA Wind Energy Systems, Workshop 1976
7. Greff, E.: Konzentration von Windenergie in Wirbelfeldern und deren Nutzung zur
Erzeugung elektrischer Energie. Statusbericht Windenergie, VDI Verlag, 1980
8. Rechenberg, I.: Entwicklung, Bau und Betrieb einer neuartigen Windkraftanlage
mit Wirbelschrauben-Konzentrator, Projekt „Berwian“, Lübeck: BMFT-Statusreport
Windenergie 1./2. Mar., 1988
9. Schlaich, J.; Simon, M.; Mayr, G.: Baureife Planung und Erstellung einer Demonstrationsanlage eines atmosphären-thermischen Aufwindkraftwerkes im Leistungsbereich
50 – 100 kW, Technical Report phase I, BMFT ET 4249 A, 1980