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 18 th 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 19 th century electrical generators appeared and people had started to turn electrical generators with the help of windmill rotors. The first half of the 20 th 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
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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.
50
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
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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]
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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
56
was transmitted to solid driveshaft through the moment arm of the rigid frame. It