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http://www.iaeme.com/IJMET/index.asp 897 [e-mail]
International Journal of Mechanical Engineering and Technology (IJMET)
Volume 9, Issue 9, September 2018, pp. 897–906, Article ID: IJMET_09_09_099
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=9
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
MODELING AND ANALYSIS OF A SINGLE
SHAFT CO-ROTATING DUAL ROTOR SMALL
WIND TURBINE
Ercan Erturk
Bahcesehir University, Mechatronics Engineering Department
Besiktas, Istanbul, Turkey
Orhan Gokcol
Bahcesehir University, Computer Education and Instructional Technologies Department
Besiktas, Istanbul, Turkey
Selim Sivrioglu and Fevzi Cakmak Bolat
Gebze Technical University, Mechanical Engineering Department
Gebze, Kocaeli, Turkey
ABSTRACT
Dual rotor wind turbines have higher efficiency and compared to single rotor wind
turbines they can harvest more energy from the wind. In the present study a single
shaft co-rotating dual rotor wind turbine is modelled with mechanical, aerodynamical
and electrical components. The considered model includes a permanent magnet
synchronous generator (PMSG). In a dual rotor wind turbine, after the front rotor the
wind speed is decreased since the front rotor extracts some portion of energy from the
wind. The wind with reduced speed downstream of the front rotor acts as an inflow for
the rear rotor. The amount of decrease in the wind speed downstream of the front
rotor affects the power output of the dual rotor wind turbine. The mechanical
dynamics, the aerodynamic power and also the electrical power of the single shaft co-
rotating dual rotor wind turbine are simulated using MATLAB/Simulink software.
Key words: Dual rotor wind turbine, co-rotating wind turbine, wind energy,
permanent magnet synchronous generator.
Cite this Article: Ercan Erturk, Orhan Gokcol, Selim Sivrioglu and Fevzi Cakmak
Bolat, Modeling and Analysis of a Single Shaft Co-Rotating Dual Rotor Small Wind
Turbine, International Journal of Mechanical Engineering and Technology 9(9),
2018, pp. 897–906.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=9

Modeling and Analysis of a Single Shaft Co-Rotating Dual Rotor Small Wind Turbine
http://www.iaeme.com/IJMET/index.asp 898 [e-mail]
1. INTRODUCTION
Wind is clean, renewable, inexhaustible free energy source for electric power generation. The
extracted energy from a wind turbine depends on the wind speed, therefore it is economically
more profitable to install the wind turbines at regions where the wind power density is high.
The extracted energy from a wind turbine also depends on the efficiency of the wind turbine.
Today with the use of better design and analysis softwares and also with the use of better
manufacturing techniques, most of the wind turbines have efficiencies more or less close to
each other with very small differences. If we can increase the efficiency of wind turbines
furthermore not only we would be able to extract more energy from the wind at regions with
high power density but also it would be economically feasible to install wind turbines at
regions with less wind power densities.
Wind turbines that have two rotors, one at the front and one at the rear of the wind turbine,
offers high efficiency [1]. A single rotor wind turbine theoretically can extract maximum
59.3% of the energy available in the wind [2] and this limit is known as the Betz limit. Using
multiple actuator-disk theory one can easily show that a dual rotor wind turbine theoretically
can extract maximum 64% of the energy available in the wind [3]. Therefore, the maximum
energy that a dual rotor wind turbine can theoretically extract from the wind is 8% higher than
that of a single rotor wind turbine.
In the literature it is possible to find many numerical and experimental studies that shows
the efficiency of dual rotor wind turbines. Among these studies, Shen et al. [4] have simulated
a Nordtank 500 kW wind turbine using Actuator Line technique implemented EllipSys3D
Navier-Stokes solver. They [4] have considered the Nordtank 500 kW wind turbine both
having a single rotor and also having a counter rotating dual rotor. Using the real wind data
measurements taken on the island of Spragø in Denmark, they have concluded that a dual
rotor wind turbine can produce 43.5% more energy annually compared to a single rotor wind
turbine.
Habash et al. [5] and Mitulet et al. [6] have performed wind tunnel experiments on dual
rotor wind turbines. Their [5,6] results indicate that a dual rotor wind turbine can produce
60% more energy than a single rotor wind turbine.
In their study, Jung et al. [7] have designed, built and tested a 30 kW dual rotor wind
turbine. In this wind turbine, the motions of both rotors are combined with a gear system with
bevel gears and planet gears and transmitted perpendicular to an electric generator. After the
actual field tests, they [7] stated that the power coefficient of this dual rotor wind turbine is
around 0.5, which is well above the power coefficients of single rotor wind turbines.
These different type of studies, numerical [4], experimental [5,6] and field studies [7] all
reported that dual-rotor wind turbines have efficiencies around 45-50% which is much greater
than that of single rotor wind turbines.
In an experimental study using PIV measurements Ozbay et al. [8] have compared the
single rotor wind turbines with dual rotor wind turbines having two rotors rotating in co- and
counter-directions in order to investigate the effect of the direction of the rotation of the two
rotors on the power extracted from the wind. Ozbay et al. [8] state that a co-rotating dual rotor
wind turbine can generate 48% more power compared to a single rotor wind turbine. The
increase in the generated power when two rotors are used is remarkable.
In order to maximize the efficiency of the wind turbines while minimizing the loads, wind
turbines are operated under control. Since in nature the wind has continuously changing
characteristics with continuously varying wind speeds, the aerodynamic torque and/or power
of the wind turbine is controlled by changing the pitch angle of the wind turbine blades.
Control of wind turbine blades also becomes very important at high wind speeds in variable

Ercan Erturk, Orhan Gokcol, Selim Sivrioglu and Fevzi Cakmak Bolat
http://www.iaeme.com/IJMET/index.asp 899 [e-mail]
speed wind turbines [9]. Linear, nonlinear and intelligent control methods for various wind
turbine systems have been studied by many researchers [10,11,12].
In the literature, No et al. [13] and Farahani et al. [14] studied modeling of counter-
rotating dual rotor wind turbines. In the studies of No et al. [13] and Farahani et al. [14] the
counter motions of the wind turbine rotors are combined into a single rotational shaft motion
by set of gears which then rotates the generator shaft as the same with that of Jung et al. [6].
The mechanical design, the physics and also modeling of a single shaft co-rotating dual rotor
wind turbine is completely different than that of dual rotor wind turbine considered in No et
al. [13] and Farahani et al. [14]. To the best of our knowledge, in the literature there is no
study that attempts to model and simulate a single shaft co-rotating dual rotor wind turbine
this constitutes the main motivation of this study. In order to reliably predict and analyze the
performance of dual rotor wind turbine, it is important to have an accurate and reliable model.
In a dual rotor wind turbine, the wind first passes over the front rotor blades and loses
some portion of its energy. Then downstream of the front rotor, the residual wind with lower
energy passes over the rear rotor blades and loses some portion of its energy again. As one
can imagine, since not only the aerodynamics but also the whole system of dual rotor wind
turbines are much more complex compared to single rotor wind turbines. In their analysis No
et al. [13] and Farahani et al. [14] assumed that the flow is uniform downstream of the front
rotor and upstream of the rear rotor, i.e. between the two rotors. Most probably this
assumption introduces some errors in the aerodynamic part of their simulation model.
However, for dual rotor wind turbines since there is no better aerodynamic approximation in
the literature for the flow between the two rotors of the dual rotor wind turbine, in this study,
following No et al. [13] and Farahani et al. [14] we will also assume that the flow approaching
to the rear rotor is uniform.
We note that the rotational speed and the pitching angle of the front rotor blades affect the
speed of the flow downstream of the front rotor. The rotational speed of the front rotor and the
pitching angle of the front rotor blades can change dynamically during operation of the wind
turbine depending on the incoming wind speed and the wind turbine control algorithm.
Therefore, the wind speed approaching the rear rotor varies during operation of dual rotor
wind turbine. In this study, we will examine the variation of the power output of the rear
rotor, i.e. therefore the total power output of the dual rotor wind turbine, as a function of the
flow speed downstream of the front rotor. For this we will assume different deceleration of
the wind after the front rotor and calculate the total power output of the dual rotor wind
turbine and obtain the variation of the total power as a function of deceleration. The power
output of the rear rotor, i.e. therefore the total power output of the dual rotor wind turbine, is
also a function of the rear rotor blade pitch angle. In this study we will also examine the effect
of the rear rotor blade pitch angle on the total power output by using various pitch angles for
the rear rotor blades.
The aim of this study is then to derive a mathematical model for a single shaft co-rotating
dual rotor wind turbine in MATLAB/Simulink environment. Using this mathematical model,
the power output of the wind turbine is investigated for different pitch angles of the rear rotor
blades in a dual rotor wind turbine. Different case studies are realized in order to analyze the
total power output of the dual rotor turbine model for different deceleration values of the wind
downstream of the first rotor.
2. DERIVATION OF MECHANICAL MODEL
The structure of the considered co-rotating dual rotor wind turbine is schematically shown in
Figure 1 as a cross section view. As seen in this figure, the turbine has two co-rotating rotors
connected to the PM rotor with a single shaft. The rotational speed of the rotor is . The