# A Comparative Performance Analysis of Counter-Rotating Dual-Rotor Wind Turbines with Speed-Adding Increasers

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

- the development of innovative concepts of wind turbines (with horizontal axis and counter-rotating rotors [1,2,3,4], or with multiple and smaller rotors in a spatial arrangement [5]). An example of comprehensive, but not exhaustive overview of research achievements in counter-rotating wind turbine systems development, characterization and use can be found in [4];
- the effect of the number of blades of counter-rotating wind turbines on the system performance [8];

## 2. Problem Formulation

- The same set of rotors R1 and R2 is used in both wind systems and the same set of mechanical characteristics is considered in modeling these systems, implicitly. Conventionally, R1 is the primary wind rotor and R2—the secondary rotor. The mechanical characteristics of wind turbines can be considered as linear functions with constant coefficients in operation at a constant wind speed. However, the values of these coefficients depend on both the value of wind speed and the characteristics of wind rotor, e.g., the pitch angle of blades.
- In both cases, the speed increaser SI has the same structure and the same values for internal kinematic ratio i
_{0}and internal efficiency η_{0}[49]. The mechanical transmission is a differential mechanism (2-DOF) with two inputs that are connected to wind rotors R1 and R2, and one or two outputs, through which the mechanical power is transmitted either to rotor GR of the conventional generator or to rotor GR and stator GS of the counter-rotating generator. - The counter-rotating electric generator is obtained from the conventional one, by setting the stator GS to rotate in opposite direction to the rotor GR; the two generators have the same mechanical characteristics with respect to the relative speed of rotor GR and stator GS, i.e., ω
_{G}= ω_{GR}− ω_{GS}. For the sake of simplicity, the case of direct current (DC) generators is further considered, which are characterized by linear mechanical characteristics with constant coefficients, is further considered. - The ratio of angular speeds of wind rotors is denoted by k
_{ω}= −ω_{R}_{2}/ω_{R}_{1}> 0. The ratio k_{ω}can be adjusted during wind turbine operation by changing the pitch angle of the two rotors R1 and R2, which also changes their mechanical characteristics.

_{H}and ω

_{4}), as sun gear 1, ring gear 4 and carrier H are mobile bodies.

_{R}

_{1}and ω

_{R}

_{2}in opposite directions—a property ensured by setting opposite inclinations of the two rotors’ blades. The two outputs of the speed increaser, by 1 and H, are also counter-rotating due to the kinematic property given by the planetary transmission structure, as highlighted in the next section. Being a differential mechanism, the speed increaser sums up the two input angular speeds ω

_{R}

_{1}and ω

_{R}

_{2}in an output angular speed ω

_{GR}. Angular speed ω

_{GS}of stator GS is identical to the input speed of primary rotor R1 (Figure 2a), as a result of the direct connections GS-H-R1.

_{ω}= −ω

_{R}

_{2}/ω

_{R}

_{1}on the behavior and performance of the two wind systems. Notations in Figure 3 are: T

_{x}and ω

_{x}for torque and angular speed of kinematic body x; i

_{0}and η

_{0}for internal kinematic ratio and internal efficiency of the speed increaser—all of which representing the intrinsic parameters of the planetary transmission [33]. In particular, carrier H is connected to primary rotor R1 by HR shaft and to the generator stator GS by HS shaft (see Figure 3a).

## 3. Analytic Modeling of Wind Systems

#### 3.1. Kinematic Modeling

_{H}and ω

_{4}). For this purpose, the speed increaser is characterized by a function of transmitting angular speeds, while each connection (represented by a double line in Figure 3) between the components of the wind system is described through a distinct kinematic equation. Thus, the following system of equations can be written [33]:

_{0}can be calculated from the particular case when H is assumed to be fix, given by relation (2):

_{xy}—angular speed of body x relative to body y, z

_{j}—the number of teeth of gear j; i

_{0}is defined as the kinematic ratio of the fixed axes mechanism that is associated to the planetary gear train, being obtained by reversing the motion relative to the carrier H [49].

_{ω}:

_{G}, can be obtained:

#### 3.2. Torque Correlation and Efficiency Modeling

- two torque transmitting functions that are obtained from the static modeling of the differential planetary speed increaser; the number of functions is equal to the mechanism degree of freedom (M = 2). Additionally, the condition of static equilibrium of the planetary transmission can be added as dependent equation for verification purposes;
- a static equilibrium equation for each of the other five components of the wind system, which are obtained after breaking the connections;
- the static equilibrium equation of the electric generator, described from the condition that the torque values at rotor GR and stator GS are equal and in opposite direction.

_{g}is the efficiency of a component gear pair; in this case, the three cylindrical fixed-axis gear pairs (1‒2, 2‒3, and 3‒4) are considered to have identical efficiency values.

_{GS}+ T

_{HS}= 0 in system (7) has to be replaced by T

_{HS}= 0, as the connection of stator GS to wind rotor R1 is broken and then fixed to the frame (ω

_{GS}= 0).

#### 3.3. Steady-State Operational Point

- the mechanical characteristics of the two wind rotors R1 and R2, described as linear functions with constant coefficients under stationary conditions (constant wind speed, same values of pitch angles):

- the mechanical characteristic of DC generator, represented by a linear function with constant coefficients, describing generator’s torque T
_{G}in relation to angular speed ω_{G}. By convention, the torque of the generator is T_{G}= T_{GR}:

_{F}= ω

_{G}and the torque T

_{F}= T

_{G}. Thus, for the two analyzed wind systems, the following expressions of parameters ω

_{F}, T

_{F}and power P

_{F}are obtained:

- for the wind system with counter-rotating generator, Figure 2a:

- for the wind system with conventional generator, Figure 2b:

#### 3.4. Input Angular Speeds Ratio k_{ω}

_{ω}depends on the mechanical characteristics of the motor and effector sub-systems and on the intrinsic parameters of the speed increaser (i.e., i

_{0}and η

_{0}); in practical applications, ratio k

_{ω}can be adjusted to a given value by modifying appropriately at least one mechanical characteristic through different approaches, e.g., by changing the pitch angle of the blades, or controlling the electric generator.

_{ω}for the two wind systems considered in this comparative analysis can be obtained by processing Equations (1), (3), (4), (6), (7), (12) and (13). Thus, in the case of a system with counter-rotating generator (Figure 2a), it can be concluded from Equation (7) that:

_{ω}:

_{ω}expression is the result of solving the system of Equation (20):

_{ω}on the behavior of the two wind turbines is herein considered by adjusting only the mechanical characteristic of the secondary wind rotor R2 and keeping unchanged the mechanical characteristics of wind rotor R1 and the electric generator. Under these assumptions, coefficients a

_{R}

_{2}and b

_{R}

_{2}become variables which are dependent on both ratio k

_{ω}and the constant coefficients of the mechanical characteristic of primary rotor R1.

_{R}

_{2}= −T

_{R}

_{1}from Equation (16):

_{R}

_{1}is the independent parameter; this equality is mathematically satisfied only if the following two conditions are met simultaneously:

_{R}

_{1}:

_{R}

_{2}and b

_{R}

_{2}are determined:

_{HS}= 0) and fixing the stator SG to the frame (i.e., ω

_{SG}= 0). The analysis of the relations from Table 1 can highlight the following properties of wind systems of 2 in-2 out type compared to 2 in-1 out type systems:

- -
- achieve a kinematic amplification ratio, in absolute value, higher than 1 (as both the ratios i
_{0}and k_{ω}have positive values); - -
- have a higher efficiency, which does not depend on the kinematic configuration of the speed increaser and which is equal to the internal efficiency η
_{0}; - -
- ensure the operation with higher angular speeds ω
_{G}and powers P_{G}of the electric generator, for the same power of the primary wind rotor P_{R}_{1}= T_{R}_{1}ω_{R}_{1}; - -
- the wind rotor R2 operates at lower torques and powers.

## 4. Numerical Simulations and Discussions

_{0}and η

_{0}), is performed under equivalence conditions, by considering the following three numerical simulation scenarios:

_{ω}.

_{0}= 10 and${\mathsf{\eta}}_{0}={\mathsf{\eta}}_{g}^{3}={0.95}^{3}=0.857$.

#### 4.1. Scenario A

- -
- wind rotors R1 and R2 are identical, since they have the same mechanical characteristics: T
_{R}_{1}= −a_{R}_{1}ω_{R}_{1}+ b_{R}_{1}= −18.763 ω_{R}_{1}− 204.81 and T_{R}_{2}= −a_{R}_{2}ω_{R}_{2}+ b_{R}_{2}= −18.763 ω_{R}_{2}+ 204.81; - -
- electric generators have the same mechanical characteristic −T
_{G}= −a_{G}ω_{R}_{1}− b_{G}= −;0.4ω_{G}− 35.

#### 4.2. Scenario B

- -
- the same wind rotor R1, with the mechanical characteristic T
_{R}_{1}= −a_{R}_{1}ω_{R}_{1}+ b_{R}_{1}= −18.763 ω_{R}_{1}− 204.81 - -
- the same wind rotor R2, with the mechanical characteristic T
_{R}_{2}= −a_{R}_{2}ω_{R}_{2}+ b_{R}_{2}= −18.763 ω_{R}_{2}+ 204.81; established from the condition that the two wind turbines generate equal power; - -
- the same electric generator, with the mechanical characteristic T
_{G}= −a_{G}ω_{R}_{1}− b_{G}= −0.4ω_{G}− 35.

_{R2}) is a challenging task for designers, which can lead to changing the option for one or the other type of wind turbine. Parameters marked by asterisk (*) in the following diagrams, Figure 5, refer to the wind turbine with a conventional generator.

_{R}

_{2}< 13.78, and generates power values significantly close to those of the turbine with a counter-rotating generator (PG curve), for values of coefficient a

_{R}

_{2}> 13.78. The efficiency of the conventional turbine decreases with the increase of values for a

_{R}

_{2}(see Figure 5b). In all instances, the wind turbine with a counter-rotating generator is stabilized at higher values of ratio k

_{ω}, i.e., a greater difference between input speeds, but with less significant differences for a

_{R}

_{2}> 13.78 (see Figure 5c).

_{R}

_{1}= a

_{R}

_{2}.

#### 4.3. Scenario C

_{ω}for both wind turbines. Under the conditions of maintaining the same primary wind rotor R1 and the same electric generator as in Scenarios A and B, this goal can only be achieved if the secondary wind rotors R2 are different, i.e., a

_{R}

_{2}≠ a*

_{R}

_{2}and b

_{R}

_{2}≠ b*

_{R}

_{2}. As a case study, the target value of ratio k

_{ω}= 2.5 is further considered and, therefore, the coefficients of the mechanical characteristics of rotors R2 can be calculated, by means of Equations (24) and (27). The values of the steady-state operational points for the two wind turbines are displayed in Table 4 below. As in Scenario B, the parameters marked by * refer to the wind system with a conventional generator.

_{ω}—described by Equations (10) and (11) and illustrated in Figure 6a—highlights the fact that efficiency of a 2 in-2 out wind system is constant and always higher than that achieved by a 2 in-1 out system, whose values tend to infinity toward this constant value. Thus, the efficiency of the two wind systems can have close values only for very high values of k

_{ω}, which is not justified in the operation of a wind system with counter-rotating rotors.

_{ω}and output power, implicitly (Figure 6b). Instead, the wind turbine with a conventional generator proves to be more efficient for higher values of ratio k

_{ω}and output power.

## 5. Conclusions

- The differential wind system with a counter-rotating electric generator, which is characterized by in-parallel transmission of power at both input and output, always ensures better efficiency of mechanical power transmission from inputs to outputs, compared to the wind system with a conventional generator. In the case of counter-rotating outputs, the efficiency of the wind system is equal to the internal efficiency η
_{0}of the planetary transmission; instead, the efficiency of the wind turbine with a conventional generator is significantly influenced by system parameters (i.e., ratio k_{ω}, coefficients of mechanical characteristics, kinematic ratio i_{0}), the value η_{0}being the upper limit of efficiency variation for this type of wind turbine. - The energy response of the two types of wind system depends significantly on the characteristics of the selected wind rotors and electric generator. Thus, the advantage of better mechanical efficiency of wind turbines with a counter-rotating generator is accompanied by higher energy performance only in certain system configurations—generally, in the range of lower power. Considering the higher complexity of counter-rotating electric generators, due to their mobile stator, the designers’ decision to choose a type of counter-rotating wind turbine seems to be a compromise between technical, energy and economic performance.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

SI | Speed increaser | DOF | Degree of freedom |

R1 | Primary wind rotor | M | Mechanism mobility |

R2 | Secondary wind rotor | L | Total number of inputs and outputs |

P | Power | G | Electric generator |

ω | Angular speed | GR | Electric generator rotor |

T | Torque | GS | Electric generator stator |

k_{ω} | Ratio of the input angular speeds | i | Kinematic ratio |

z | Number of gear teeth | i_{0} | Internal kinematic ratio |

H | Satellite carrier | i_{a} | Amplification kinematic ratio |

F | Operational point | η | Efficiency of the speed increaser |

a | Angular speed coefficient | η_{0} | Internal efficiency |

b | Torque coefficient | η_{g} | Efficiency of a gear pair |

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**Figure 1.**Block diagrams of the differential counter-rotating wind systems that are considered in the comparative analysis: (

**a**) with counter-rotating generator; (

**b**) with conventional generator.

**Figure 2.**Structural and block diagrams for wind systems with two counter-rotating inputs, a differential planetary speed increaser and: (

**a**) counter-rotating generator; (

**b**) conventional generator.

**Figure 3.**Block diagram of the differential wind system with two inputs and: (

**a**) two outputs; (

**b**) one output.

**Figure 4.**Algorithm for analytical modeling of the generalized 2-DOF dual-rotor counter-rotating wind systems.

**Figure 5.**Variation with respect to a

_{R}

_{2}coefficient of: (

**a**) output power, (

**b**) efficiency and (

**c**) ratio k

_{ω}, in both wind systems.

**Figure 6.**Variation with respect to ratio k

_{ω}of: (

**a**) efficiency and (

**b**) output power, in both wind systems.

**Figure 7.**Power flow in the Scenarios A (continuous red line), B (yellow dashed line) and C (continuous green line) through: (

**a**) 2 in-2 out wind system; (

**b**) 2 in-1 out wind system.

Parameter | Symbol | Wind System 2 in-2 out, Figure 2a | Wind System 2 in-1 out, Figure 2b |
---|---|---|---|

Amplification kinematic ratio | ${i}_{a}=\frac{{\omega}_{G}}{{\omega}_{R1}}$ | $-{i}_{0}\left(1+{k}_{\omega}\right)$ | $1-{i}_{0}\left(1+{k}_{\omega}\right)$ |

Efficiency | $\mathsf{\eta}$ | ${\mathsf{\eta}}_{0}$ | ${\mathsf{\eta}}_{0}^{}\frac{1-{i}_{0}\left(1+{k}_{\omega}\right)}{{\mathsf{\eta}}_{0}-{i}_{0}\left(1+{k}_{\omega}\right)}\text{}$ |

Angular speed of the generator | ${\omega}_{G}={\omega}_{GR}-{\omega}_{GS}$ | $-{\omega}_{R1}{i}_{0}\left(1+{k}_{\omega}\right)$ | ${\omega}_{R1}\left[1-{i}_{0}\left(1+{k}_{\omega}\right)\right]$ |

Generator torque | ${T}_{G}={T}_{GR}$ | $-{T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{\mathsf{\eta}}_{0}-{i}_{0}}$ | $-{T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{\mathsf{\eta}}_{0}-{i}_{0}}$ |

Generator power | ${P}_{G}={T}_{G}\text{}{\omega}_{G}$ | ${T}_{R1}{\omega}_{R1}{\mathsf{\eta}}_{0}\frac{{i}_{0}\left(1+{k}_{\omega}\right)}{{\mathsf{\eta}}_{0}-{i}_{0}}$ | $-{T}_{R1}{\omega}_{R1}{\mathsf{\eta}}_{0}\frac{1-{i}_{0}\left(1+{k}_{\omega}\right)}{{\mathsf{\eta}}_{0}-{i}_{0}}$ |

Angular speed of the generator rotor | ${\omega}_{GR}$ | ${\omega}_{R1}\left[1-{i}_{0}\left(1+{k}_{\omega}\right)\right]$ | ${\omega}_{R1}\left[1-{i}_{0}\left(1+{k}_{\omega}\right)\right]$ |

Generator rotor torque | ${T}_{GR}$ | ${T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ | ${T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ |

Generator rotor power | ${P}_{GR}$ | ${T}_{R1}{\omega}_{R1}\frac{{\mathsf{\eta}}_{0}\left[1-{i}_{0}\left({k}_{\omega}+1\right)\right]}{{i}_{0}}$ | ${T}_{R1}{\omega}_{R1}\frac{{\mathsf{\eta}}_{0}\left[1-{i}_{0}\left({k}_{\omega}+1\right)\right]}{{i}_{0}}$ |

Angular speed of the generator stator | ${\omega}_{GS}$ | ${\omega}_{R1}$ | 0 |

Generator stator torque | ${T}_{GS}$ | $-{T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ | $-{T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ |

Generator stator power | ${P}_{GS}$ | $-{T}_{R1}{\omega}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ | 0 |

Angular speed of the carrier H | ${\omega}_{H}$ | ${\omega}_{R1}$ | ${\omega}_{R1}$ |

H carrier torque | ${T}_{H}$ | ${T}_{R1}$ | ${T}_{R1}$ |

H carrier power | ${P}_{H}$ | ${T}_{R1}{\omega}_{R1}$ | ${T}_{R1}{\omega}_{R1}$ |

Angular speed of the shaft HR | ${\omega}_{HR}$ | ${\omega}_{R1}$ | ${\omega}_{R1}$ |

HR shaft torque | ${T}_{HR}$ | ${T}_{R1}\frac{{i}_{0}-{\mathsf{\eta}}_{0}}{{i}_{0}}$ | ${T}_{R1}$ |

HR shaft power | ${P}_{HR}$ | ${T}_{R1}{\omega}_{R1}\frac{{i}_{0}-{\mathsf{\eta}}_{0}}{{i}_{0}}$ | ${T}_{R1}{\omega}_{R1}$ |

Angular speed of shaft HS | ${\omega}_{HS}$ | ${\omega}_{R1}$ | 0 |

HS shaft torque | ${T}_{HS}$ | ${T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ | ${T}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ |

HS shaft power | ${P}_{HS}$ | ${T}_{R1}{\omega}_{R1}\frac{{\mathsf{\eta}}_{0}}{{i}_{0}}$ | 0 |

Angular speed of gear 4 | ${\omega}_{4}$ | $-{k}_{\omega}{\omega}_{R1}$ | $-{k}_{\omega}{\omega}_{R1}$ |

Torque on gear 4 | ${T}_{4}$ | $-{T}_{R1}$ | $-{T}_{R1}\frac{{i}_{0}}{{i}_{0}-{\mathsf{\eta}}_{0}}$ |

Power of gear 4 | ${P}_{4}$ | ${k}_{\omega}{T}_{R1}{\omega}_{R1}$ | ${\omega}_{R1}{T}_{R1}\frac{{k}_{\omega}{i}_{0}}{{i}_{0}-{\mathsf{\eta}}_{0}}$ |

Parameter | Wind System 2 in-2 out, Figure 2a | Wind System 2 in-1 out, Figure 2b |
---|---|---|

${\omega}_{R1}$[s^{−1}] | −5.470 | −5.989 |

${T}_{R1}$[kNm] | −102.176 | −92.438 |

${P}_{R1}$[kW] | 558.905 | 553.613 |

${\omega}_{R2}$[s^{−1}] | 5.470 | 5.527 |

${T}_{R2}$[kNm] | 102.176 | 101.106 |

${P}_{R2}$[kW] | 558.905 | 558.818 |

${\omega}_{G}$[s^{−1}] | 109.401 | 109.171 |

${T}_{G}$[kNm] | −8.760 | −8.669 |

${P}_{G}$[kW] | −958.382 | −946.366 |

$\mathsf{\eta}$ | 0.8573 | 0.8507 |

${k}_{\omega}$ | 1.000 | 0.923 |

${i}_{a}={\omega}_{G}/{\omega}_{R1}$ | −20.000 | −17.209 |

_{R}

_{1}= a

_{R}

_{2}= 18.763 [kNms]; b

_{R}

_{1}= b

_{R}

_{2}= −204.81 [kNm]; a

_{G}= 0.4 [kNms]; b

_{G}= 35 [kNm]; i

_{0}= 10; η

_{0}= 0.857.

Parameter | Wind System 2 in-2 out, Figure 2a | Wind System 2 in-1 out, Figure 2b |
---|---|---|

${\omega}_{R1}$[s^{−1}] | −4.754 | −5.283 |

${T}_{R1}$[kNm] | −115.604 | −105.693 |

${P}_{R1}$[kW] | 549.622 | 558.332 |

${\omega}_{R2}$[s^{−1}] | 6.474 | 6.474 |

${T}_{R2}$[kNm] | 115.604 | 115.604 |

${P}_{R2}$[kW] | 748.372 | 748.372 |

${\omega}_{G}$[s^{−1}] | 112.279 | 112.279 |

${T}_{G}$[kNm] | −9.912 | −9.912 |

${P}_{G}$[kW] | −1112.868 | −1112.868 |

$\mathsf{\eta}$ | 0.8574 | 0.8516 |

${k}_{\omega}$ | 1.362 | 1.225 |

${i}_{a}={\omega}_{G}/{\omega}_{R1}$ | −23.616 | −21.255 |

_{R}

_{1}= 18.763 [kNms]; ]; b

_{R}

_{1}−204.81 [kNm]; a

_{R}

_{2}= 13.780 [kNms]; b

_{R}

_{2}= 204.81 [kNm]; a

_{G}= 0.4 [kNms]; b

_{G}= 35 [kNm]; i

_{0}= 10; η

_{0}= 0.857.

Parameter | Wind System 2 in-2 out, Figure 2a | Wind System 2 in-1 out, Figure 2b |
---|---|---|

${\omega}_{R1}$[s^{−1}] | −3.367 | −3.529 |

${T}_{R1}$[kNm] | −141.630 | −138.592 |

${P}_{R1}$[kW] | 476.908 | 489.117 |

${\omega}_{R2}$[s^{−1}] | 8.418 | 8.823 |

${T}_{R2}$[kNm] | 141.630 | 151.589 |

${P}_{R2}$[kW] | 1192.301 | 1337.455 |

${\omega}_{G}$[s^{−1}] | 117.857 | 119.992 |

${T}_{G}$[kNm] | −12.143 | −12.997 |

${P}_{G}$[kW] | −1431.138 | −1559.516 |

$\mathsf{\eta}$ | 0.8574 | 0.8538 |

${i}_{a}={\omega}_{G}/{\omega}_{R1}$ | −35.000 | −34.000 |

_{R}

_{1}= 18.763 [kNms]; ]; b

_{R}

_{1}−204.81 [kNm]; a

_{R}

_{2}= 7.505 [kNms]; b

_{R}

_{2}= 204.81 [kNm]; k

_{ω}= 2.5; a*

_{R}

_{2}= 8.209 [kNms]; b*

_{R}

_{2}= 224.016 [kNm]; a

_{G}= 0.4 [kNms]; b

_{G}- 35 [kNm]; i

_{0}= 10; η

_{0}= 0.857.

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## Share and Cite

**MDPI and ACS Style**

Saulescu, R.; Neagoe, M.; Jaliu, C.; Munteanu, O.
A Comparative Performance Analysis of Counter-Rotating Dual-Rotor Wind Turbines with Speed-Adding Increasers. *Energies* **2021**, *14*, 2594.
https://doi.org/10.3390/en14092594

**AMA Style**

Saulescu R, Neagoe M, Jaliu C, Munteanu O.
A Comparative Performance Analysis of Counter-Rotating Dual-Rotor Wind Turbines with Speed-Adding Increasers. *Energies*. 2021; 14(9):2594.
https://doi.org/10.3390/en14092594

**Chicago/Turabian Style**

Saulescu, Radu, Mircea Neagoe, Codruta Jaliu, and Olimpiu Munteanu.
2021. "A Comparative Performance Analysis of Counter-Rotating Dual-Rotor Wind Turbines with Speed-Adding Increasers" *Energies* 14, no. 9: 2594.
https://doi.org/10.3390/en14092594