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.

48

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

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.

55

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

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]

59

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]

60

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.

61

(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

62

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.

63

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.

64

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.