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45
Chapter 3
LITERATURE REVIEW
It is important to discuss history of wind turbine. The discussion of wind energy history
expresses the issues related to wind energy systems still face today. It also presents
insight into how the present status of wind turbines is reached.
As per the record, first windmills were built by the Persians in around 900AD. These
windmills had vertical axes and were drag driven systems.
The horizontal axis windmills were appeared in Europe in middle Ages. All they were
basically used to perform mechanical tasks such as grain grinding, water pumping wood
sawing and powering tools. Early wind mills normally had four blades. The number of
blades and size of rotor were presumable based on ease of construction and empirical
relations [MMR 2002].
In 18th century the important development took place and scientific testing and evaluation
method was introduced by the John Smeaton. He discovered three basic rules which are
still acceptable,
1. The speed of the blade tips is ideally proportional to the wind speed
2. The maximum torque is proportional to the square of rotor diameter
3. The maximum power is proportional to the third power of wind sped
At the end of 19th century electrical generators appeared and people had started to turn
electrical generators with the help of windmill rotors. The first half of the 20th century a
lot of developments took place and world saw the construction of the large wind turbines
which considerably influenced the progress of today’s wind turbine technology. Denmark
was the most pioneer in the dev
Poul La Cour built more than 100 wind turbine to generate electricity in the range of 20
35 kW. After the World War II, Johannes Jull built a 200 kW three bladed wind turbine
in Denmark. Ulrich Hutter, o
aerodynamics principles to the wind turbine design. Many of his concepts are still in use
in some form.
After the energy crises of mid
sources. Soon they realize that wind energy may be one of the promising alternative
source of energy and development of wind energy started rapidly.
After this the size of the largest commercial wind turbines has increased from 50 kW to
7,500 kW. Fig. 3.1 shows incr
development many design standards and certification procedures have been set. This
increases reliability and performance of large wind turbines and they have achieved a
respectable position in renewable energ
Fig. 2.1:
It is important to note that some researchers have worked on different options instead of
increasing rotor diameter.
mills and multi rotor wind
and researchers.
46
was the most pioneer in the development of wind turbines. Between the 1891 and 1918
Poul La Cour built more than 100 wind turbine to generate electricity in the range of 20
35 kW. After the World War II, Johannes Jull built a 200 kW three bladed wind turbine
in Denmark. Ulrich Hutter, one of the pioneers in Germany, tried to apply modern
aerodynamics principles to the wind turbine design. Many of his concepts are still in use
After the energy crises of mid-1970s many people have stated to find alternative energy
on they realize that wind energy may be one of the promising alternative
source of energy and development of wind energy started rapidly.
After this the size of the largest commercial wind turbines has increased from 50 kW to
7,500 kW. Fig. 3.1 shows increasing size of modern wind turbine. During this
development many design standards and certification procedures have been set. This
increases reliability and performance of large wind turbines and they have achieved a
respectable position in renewable energy market.
Fig. 2.1: Increasing size of Modern Wind Turbines
It is important to note that some researchers have worked on different options instead of
increasing rotor diameter. This chapter presents the detailed review of multi rotor wind
and multi rotor wind turbine systems proposed and developed by different inventors
elopment of wind turbines. Between the 1891 and 1918
Poul La Cour built more than 100 wind turbine to generate electricity in the range of 20-
35 kW. After the World War II, Johannes Jull built a 200 kW three bladed wind turbine
ne of the pioneers in Germany, tried to apply modern
aerodynamics principles to the wind turbine design. Many of his concepts are still in use
1970s many people have stated to find alternative energy
on they realize that wind energy may be one of the promising alternative
After this the size of the largest commercial wind turbines has increased from 50 kW to
easing size of modern wind turbine. During this
development many design standards and certification procedures have been set. This
increases reliability and performance of large wind turbines and they have achieved a
Increasing size of Modern Wind Turbines
It is important to note that some researchers have worked on different options instead of
This chapter presents the detailed review of multi rotor wind
turbine systems proposed and developed by different inventors
47
1. Literature Review
The literature review is carried out in the perspectives to understand development in
multi rotor wind turbine technology. The Multi Rotor Wind Turbine (MRWT) consists of
two or more rotors placed on a single tower. The concept of MRWT has been raised in
the 18th century in Denmark. Few models are built, tested and used based on this concept.
Based on the literature available, first multi rotor wind mill were installed in Denmark in
1873as shown in Fig. 3.2. A wind turbine consists of two rotors and each rotor in this
wind mill consists of six blades. These rotors are mounted on two separate shafts.
Fig. 2.2: A multi rotor wind mill installed in Denmark in 1873
In the 18th century, Wallace Amos [Wal 1890] has proposed the concept of numbers of
blades rotating in different planes, mounted on a horizontal driveshaft in his patent,
Windmill as shown in Fig. 3.3. He used many curved blades similar in shape in his wind-
wheel (rotor). A tail vane was attached in the downstream region. The purpose of the
windmill was water pumping. As per the theory of wake rotation developed in the 19th
century, the angular velocity of stream in the opposite direction after passing the first
blade resists rotation of next blades. This wake rotation was not considered by Wallace.
Carlson John [Car 1911] has proposed a windmill that consists of two rotors on a
common driveshaft, with the objective of the actuating pumping rod as shown in Fig. 3.4.
He used one upwind and one downwind rotor. The horizontal driveshaft was extended on
both sides of the tower. These rotors consist of a spiral shaped blade connected to the
shaft by means of spokes. The tail vane was placed above rotors.
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Fig. 2.3: A wind mill proposed by Wallace with plurality of curved blades rotating in
different planes [Wal 1890]
Fig. 2.4: A dual rotor wind mill proposed by Carlson [Car 1911]
Miller [Mill 1924] has proposed a wind power generator to convert wind energy into
electricity as shown in Fig. 3.5. Similar to Carlson, he used two rotors of conical shapes
on opposite sides of the tower. He proposed the gradually increasing rotor diameter from
one end of the shaft to the opposite end. The rotors were mounted on a horizontal shaft.
49
Power was transmitted to another horizontal shaft to drive a generator through a vertical
shaft. He claimed shaft not aiming parallel to the wind direction but inclined to the wind
direction. The structural stability was also anticipated by inventor in this patent.
Fig. 2.5: Conical shaped dual rotor wind turbine proposed by Miller [Mill 1924]
Hermann Honnef in 1931 [Hon 1934] [KS 13c] tried to build a multi-rotor wind turbine
system of a large-capacity as shown in Fig. 3.6. His most famous design comprises of a
250 m high lattice type tower fitted with three double-rotors arranged with a 120 m
diameter front rotor and two 160 m diameter rotor at the rear. A striking design feature
was that the complete rotor assembly could pivot into a safe horizontal position during
stormy weather. Also unique at the time was the application of ring generators. Honnef
never succeeded in building a complete multi-rotor turbine due to the outbreak of the
Second World War, but he manufactured parts of the 150 m high tower.
Roberts and Edmonds [RE 1939] have proposed a dual rotor (propeller) wind turbine to
generate electricity as shown in Fig. 3.7. They used two upwind rotors rotating in the
opposite direction. One rotor was used to drive the armature of generator and the second
rotor was used to rotate the field of generator in the opposite direction. They used rotors
of different sizes. The first small rotor was used to drive the generator housing in a
clockwise direction, while the second big rotor was used to drive the armature in a
counterclockwise direction. The second rotor was made slightly longer with intention to
avoid shielding effect and turbulence caused because of first rotor. Also, it was
anticipated to access unimpeded air to the second rotor because of longer blades.
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Fig. 2.6: A multi rotor wind turbine partly built by Hermann Honnef [Hon 1934]
Fig. 2.7: Contra rotating wind turbine proposed by Roberts and Edmonds [RE 1939]
Rotors Tail vane
51
Hachmann H. [Hac 1962] has put the concept of a wind power plant containing a
plurality of rotors (wind-wheels) mounted on a main drive shaft as shown in Fig. 3.8.
These rotors are mounted and spaced apart on the main drive shaft. He proposed each
rotor consisting of a series of elongated rectangular blades extending outward from the
main drive shaft. The inventor claimed this invention as a solution to the large scale wind
turbines available during that period. Instead of using conventional blades, he used three
sets of rectangular blades in a rotor and each set comprised three blades. In this way each
rotor contained nine blades.
Fig. 2.8: A multi rotor wind turbine proposed by Hachmann H. [Hac 1962]
Allison William [All 1977] has claimed a multi-vane windmill having many pairs of
diametrically opposite vanes mounted on horizontal axis shafts on both, upwind and
downwind sides as shown in Fig. 3.9. It was proposed with two rotors i.e. primary and
secondary of the same diameter. In a rotor, seven pairs of vanes are uniformly spaced
apart axially, relatively close to prior pair and successively indexed circumferentially in
the direction of rotation relative to the preceding pair. A proposed rotor consists of a
number of pairs rotating in different planes. The primary and secondary rotors were
placed on the two separate coaxial shafts. These shafts were connected to both the ends of
52
the generator through gearboxes in order to multiply the rotational speed. The inventor
was anticipating the maximum utility of available wind. The inventor suggested the
length of the vane as six times the vane width and the axial distance between the vanes as
at least one half of vane width. Also, the inventor claimed self yawing of wind turbine
because of secondary rotor assembly.
Fig. 2.9: Contra rotating wind turbine proposed by Allison William [All 1977]
Fry and Hise [FH 1978] have invented the coaxial wind turbine with many wind rotors
mounted on a flexible shaft. The lowermost end of the shaft was connected to the
electrical generator supported at the ground, whereas the second end of the driveshaft was
supported at a great height by means of a swivel. Because of long unguided flexible shaft
relatively low power is transmitted to the generator.
Are Endel [Are 1980] has proposed a wind energy conversion device with two rotors
rotating in opposite direction, mounted on two different coaxial shafts. The rotors
comprised a number of blades and rotating in the opposite directions as shown in Fig.
3.10. The inventor claimed good stability of this wind energy conversion device. Also the
faster response is claimed with respect to changes in wind direction, when compared to
the single rotor wind turbine with tail vane.
vanes vanes
generator
53
Fig. 2.10: Contra rotating wind turbine proposed by Are Endel [Are 1980]
Three-Rotor Wind Turbine by Lagerwey, a Netherlands based company [KS 13c].
During the mid-1980s Multiwind and the former Lagerwey erected the 300 kW Quadro,
comprising four 75 kW two-blade Lagerwey 15/75 turbines as shown in Fig. 3.11. After
some teething problems the installation performed well for about 15 years at the
Maasvlakte industrial area near the port of Rotterdam. In the same decade a wind turbine
erected with three rotors along with individual generator together on a tower as shown in
Fig. 3.12 and Fig 3.13.
Fig. 2.11: Four rotor array wind turbine [KS 2013c]
54
Fig. 2.12: Three rotor array wind turbine [KS 2013c]
Fig. 2.13: Three rotor array wind turbine [KS 2013c]
The Windship systems were developed by William Heronemus, at the University of
Massachusetts at Amherst as shown in Fig. 3.14. Combined with onboard hydrogen
production through electrolysis, it was planned that one million of the Windships could
completely power and fuel the U.S. Heronemus is a main originator of the Multi-Rotor
Turbine concept, the Offshore Turbine concept, and the Floating Offshore Turbine
concept.
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Fig. 2.14: Windship multi-rotor wind turbine
Krolick et al. [KSL 1987] has invented a collapsible structure comprising a non-rigid
helicoidal wind turbine as shown in Fig. 3.15. It also suffered from relatively low power
transmission.
Fig. 2.15: Helicoidal wind turbine proposed by Krolick et al. [KSL 1987]
Harburg Rudy [Har 1991] has invented coaxial multi-turbine generator to convert wind
energy into electricity as shown in Fig. 3.16. In this system light weight rotors were used
instead of conventional rotors to convert wind energy into rotational mechanical energy.
These coaxial rotors were arranged in a series by lines attached to their extremities. A
central line was used at centers of each rotor to assist alignment and suspension. Power
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was transmitted to solid driveshaft through the moment arm of the rigid frame. It
employed a durable suspension support system.
Fig. 2.16: Coaxial multi-turbine generator proposed by Harburg Rudy [Har 1991]
McCauley Richard [MaC 1994] has invented wind line power system as shown in Fig.
3.17. The invention claimed a series of rigid rods along with a rotor, connected end-to-
end to allow relative transmission of axial torque from one rod to the next and finally to
the driveshaft of electrical generator. Thus it formed a unidirectional coaxial multi rotor
wind turbine to drive an electric generator. It included two main spaced supports above
the ground. One support was stationary and the other movable one to face the change in
wind direction. Also, these rods were intermediately supported from the ground with the
help of guy lines. It also claimed flexibilities such as folding, easy assembly and
disassembly, easy mobility.
Fig. 2.17: A wind line power system proposed by McCauley Richard [MaC 1994]
light weight rotors
57
Shin Chan [Shi 2002] has proposed a contra-rotating wind turbine system comprised
combined bevel-planet gear assembly to combine the power of two shafts rotating in
opposite directions as shown in Fig. 3.18. This combined power was used to drive an
electrical generator in order to produce electricity. In addition to this he proposed a multi
rotor wind turbine system comprised of a number of small rotors mounted on a tower.
Fig. 2.18: Contra rotating and multi rotor wind turbine system [Shi 2002]
Kari [Kari 2003] and Kanemoto & Galal [KG 2006]developed twin rotor in series turbine
for use with synchronous generators. The power developed by the upstream rotor would
drive the internal armature while the downstream rotor would provide power for the
external armature. It produces more power than single rotor wind turbine.
Kari (2003) developed counter rotating wind turbine having two separate generators
associated with two rotors.
In 2004 Jung S N, et. al. [JNR 2005] have developed a 30kW counter-rotating wind
turbine system as shown in Fig. 3.19. They found that a turbine with two rotors produced
higher power than a single rotor turbine, depending on the distance between the rotors. It
was found that there is a 21% increase in power coefficient (up to 0.50) when the
distance between the rotors is half of the main rotor.
Fig. 2.19:
Selsam Douglas [Sel 2006
consists of a horizontal drive shaft
at spaced intervals as shown in Fig. 3.
Rotors are mounted on both, upwind and downwind sides.
directly aimed into the wind, but at a slightly offset angle, allowing
fresh wind, considerably undisturbed
said invention is mentioned by passive yawing arrangement in the form of tail vane. In
high wind turbine is allowed to turn across the wind or furl
over speed.
Fig. 2.20: A side
58
Fig. 2.19: A 30 kW counter rotating wind turbines [JNR 2005]
Selsam Douglas [Sel 2006a] has proposed a side-furling co-axial multi
consists of a horizontal drive shaft with plurality conventional rotors
as shown in Fig. 3.20. The generator is mounted near
Rotors are mounted on both, upwind and downwind sides. This wind turbine is not
directly aimed into the wind, but at a slightly offset angle, allowing
fresh wind, considerably undisturbed by the wake of upwind rotors.
said invention is mentioned by passive yawing arrangement in the form of tail vane. In
high wind turbine is allowed to turn across the wind or furl sideways to protect it from
A side-furling co-axial multi-rotor wind turbine
[JNR 2005]
axial multi-rotor wind turbine
with plurality conventional rotors coaxially mounted
The generator is mounted near to the tower.
This wind turbine is not
directly aimed into the wind, but at a slightly offset angle, allowing each rotor to access
by the wake of upwind rotors. The offset angle in
said invention is mentioned by passive yawing arrangement in the form of tail vane. In
sideways to protect it from
rotor wind turbine [Sel 2006a]
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Selsam Douglas [Sel 2006b] has proposed a multi-rotor wind turbine with generator as
counterweight as shown in Fig. 3.21. In this invention he claimed many of conventional
rotors mounted on along a long driveshaft at spaced interval. This driveshaft makes
inclination with horizontal axis in the vertical plane. This shaft inclination and space
between rotors help to get fresh wind flow for all rotors, without affecting from the wake
of previous rotors. To protect the wind turbine from over speed, the shaft is allowed to
make zero inclination or is parallel to the wind. It also comprised of rotors on both
upwind and downwind sides.
Fig. 2.21: A multi-rotor wind turbine with generator as counterweight proposed by
Selsam Douglas [Sel 2006b]
In the year 2010 a multi-rotor wind turbine consists of seven rotors as shown in Fig. 3.22,
is tested in NASA laboratory, delivered encouraging results for future work [JB 2012],
[KS 13c].
Fig. 2.22: Seven rotor array wind turbine [JB 2012]
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2. Summary of literature review
The above study shows that inventors and researchers have tried to find alternative
solution to conventional Danish wind turbines by using number of rotors on a tower with
following two objectives,
1. To replace a big rotor with number of small rotors
2. To use the energy available in the wake of first rotor in order to enhance practical
coefficient of performance
Two different models are proposed and developed with first objective are,
1. Co-planer Multi Rotor Wind Turbine and,
2. Unidirectional Co-axial Series rotor wind turbine
The contra rotating wind turbine is developed to meet the second objective.
Some arrangement proposed from the end of 18th century to first few decades of 19th
century are practicable because of complicated construction. Also they have not
considered aerodynamics of wind turbine and wake rotation. After the development of
aerodynamic theory some modifications are proposed in multi rotor wind turbine systems
after the mid of 19th century.
This section discusses the various aspects of Co-planer Multi Rotor Wind Turbine,
Unidirectional Co-axial Series rotor wind turbine and contra rotating wind turbine. The
multi rotor wind turbines are presented with the help of simplified figures.
2.3.1 Co-planer multi rotor wind turbine
Co-planer multi rotor wind turbine consists of number of rotors mounted on tower as
shown in Fig. 3.22. It consists of a number of small rotors of equivalent area to replace a
big rotor. Each rotor is supported by a separate support and transmits power to a separate
generator to produce electricity. The generated electricity is used for further purpose.
This forms a cluster of conventional small wind turbines mounted on a structure. In the
last quarter of 19th century some wind turbines of this type were built. But they have not
attracted the market because of the problems associated with the yawing of the system.
Now a days, for large power output systems electronic yaw control can be used, but for
small capacity it is not economical.
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(a) Three rotors array wind turbine (b) Four rotors array wind turbine
(c) Three rotor array wind turbine (d) Seven rotor array wind turbine
Fig. 2.23: Co-planer multi rotor wind turbines [KS 2013b]
Jamieson [Jam 2011] has explained that, when a system of n rotors is compared with a
single large rotor of equivalent capacity, the ratio of total mass and cost of rotors and
drive trains of multi rotor system to that of single rotor system is given by1/ n . Wind
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turbine size is growing day by day and manufacturers are always involved in
development of larger size blades along with associated parts. The product life cycle is
very short. So standardization concept is not adopted at desired level. In case of multi-
rotor wind turbines the great advantage of preferred size is possible and further work
could be carried for reliability related aspects. For installing large size rotors cranes
working at more height are required. For multi-rotor wind turbines comparatively small
height cranes can be used. Transportation of small rotors and associated parts is simple
compare to large rotors.
Working principle of multi-rotor wind turbine systems is similar to conventional
horizontal axis wind turbine. In case of co-planer multi-rotor wind turbine system small
rotors can be placed appropriately to avoid the effect of wake among themselves after
CFD or wind tunnel analysis. CFD investigations carried out in NASA have shown
positive results for multi-rotor array wind turbine.
The scaling relationship between conventional single rotor and multi rotor wind turbine
shows that economical advantage increases with rotor numbers. The large modern wind
turbines depend on strength of composite material for their rotor systems, which also
increases cost. The cost of additional support structure of multi-rotor wind turbines can
be well compensated by cost reduction of blades, drive-trains. Total cost of small
generator used in co-planer system is quite same that of used in large turbine. Further by
taking advantage of standardization cost of the preferred size parts could be reduced.
The synchronization and conditioning of electrical power produced by different generator
will be important in case of co-planer multi rotor wind turbine.
After considering various aspects of co-planer multi rotor wind turbine it can be
concluded that this system can be effectively used to replace mid to high range small
wind turbine and large wind turbine with small rotors with electronic yaw control,
conditioning of power output from different generators. Transportation of small rotors
and associated parts is simple compare to large rotors. But it does not seems effective to
replace micro or mini wind turbines because of cost of electronics involved in yawing
arrangement and power conditioning.
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2.3.2 Counter rotating horizontal axis wind turbine
Counter rotating horizontal axis wind turbine are developed to use the wake energy of
first rotor by placing the second rotor behind it in order to enhance the practical
coefficient of performance as shown in Fig. 3.24. The power of rotors rotating in opposite
direction is used by three different methods as follows,
1. The power developed by the upstream rotor is used to drive the internal armature
while the downstream rotor is used to drive the external armature. Hence, increase
the relative speed between armatures more power is obtained than single rotor
wind turbine.
2. The mechanical power output of both rotors is combined by gears and used to
drive a generator.
3. Using two separate generators associated with two rotors.
Fig. 2.24: Counter rotating wind turbines [KS 2013b]
Though counter weight of the rotors at both sides of the tower improves the stability of
the wind turbine system, the counter rotating wind turbine may not feasible at MW scale
because of overhang shaft in order to maintain appropriate distance between rotors.
Scaling economy is not too much useful for counter rotating wind turbine because it uses
only two rotors comparing to other two multi rotor wind turbines.
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Counter rotating wind turbines as per first method needs complex gearing arrangement
and subsequent synchronization. Twin rotor generator has complex construction.
2.3.3 Unidirectional co-axial series rotor wind turbine
The unidirectional co-axial series rotor wind turbine consists of a number of upwind and
downwind rotors mounted in different planes on a common driveshaft to drive a
generator as shown in Fig. 3.25. They offers following advantages compare to single
rotor wind turbine, co-planer multi rotor wind turbine and Counter rotating horizontal
axis wind turbine ,
a. For unidirectional co-axial series rotor wind turbine needs small size direct drive
generator because of higher rpm.
b. The cost of high rpm generator is less compare to low rpm generator.
c. It has good structural stability as it contains both, upwind and downwind rotors. It
also improves yawing performance.
d. Compare to single rotor wind turbine low bending and tensile stresses are
induced.
The unidirectional co-axial series rotor wind turbine may not feasible at MW scale
because of overhang shaft in order to maintain appropriate distance between rotors.
Fig. 2.25: Unidirectional co-axial series rotor wind turbine [KS 2013b]
65
In the first decade of 21st century, Selsam Douglas [Sel 2006a] has proposed a side-
furling co-axial multi-rotor wind turbine consists of a horizontal drive shaft with plurality
conventional Danish rotors coaxially mounted at spaced intervals at both, upwind and
downwind sides and aiming slightly offset in order to get fresh air for each rotor, without
affecting from the wake of previous rotor. The offset angle in said invention is mentioned
by passive yawing arrangement in the form of tail vane. This angle plays important role
for better system performance. In high wind turbine is allowed to turn across the wind or
furl sideways to protect it from over speed.
In addition to above, Selsam Douglas [Sel 2006b] has also proposed and built a multi-
rotor wind turbine with plurality of conventional Danish rotors mounted along a long
inclined driveshaft at spaced interval. This shaft inclination and space between rotors
help to get fresh wind flow for all rotors, without affecting from the wake of previous
rotors. To protect the wind turbine from over speed, the shaft is allowed to make zero
inclination or parallel to the wind. It also comprised rotors on both upwind and
downwind sides.
3. Problem definition
Following are the major limitations of the system proposed by Selsam Douglas,
1. The rotor spacing and shaft inclination play important role in the performance of
unidirectional co-axial series rotor wind turbine. The shaft inclination and spacing
between two successive rotors should be arranged to avoid the wake effect of
previous rotor in order to ensure fresh air access to each rotor. Increase in the
rotor spacing increases the cantilever portion. The distance between the rotors can
be reduced by increasing the rotor inclination. The performance of rotor reduces
with increasing shaft inclination with wind direction. He has not used any
computational technique or experimentation to determine the shaft inclination and
rotor spacing. Hence, it is required to determine the rotor spacing and inclination
in order to get better performance.
2. When wind speed increases, its force pushes the shaft in downward direction and
brought it in to horizontal position. He claimed that system stops its working in
66
this position because of opposite directional wake effect. In actual practice the
system rotates at considerably high speed and burnt out the generator.
3. The complete tower top system is again pivoted on the yawing mechanism
reduces rigidity and stability of the system. This also increases moving parts in
the system. He used springs and shock absorber in this pivoting mechanism. The
wind force applied on spring and shock absorber causes pivoting action of system.
This increases a possibility of instability of the system. Also, it is difficult to
design spring and shock absorber for all operating conditions. Hence, there is a
need of simple and stable system to hold number of rotors easily and steadily.
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