# A Comparative Performance Analysis of Four Wind Turbines with Counter-Rotating Electric Generators
## 5 Conclusions  
This paper presented a comparative analysis of the performances of four different types of wind turbines with a counter-rotating electric generator. To this end, a differential planetary transmission with a variable structure, derived from an innovative solution proposed by the authors, was firstly proposed. By appropriate combinations of engaging/disengaging of the clutches, the transmission can operate in various structures, of which four cases were selected for analysis: a system with three rotors and a 2-DOF speed increase (Case A), a 1-DOF system with two counter-rotating wind rotors (Case B), a 2-DOF system with two counter-rotating wind rotors (Case C) and 1-DOF system with a single wind rotor. Archetype models for speeds, torques, powers and the efficiency of the planetary transmission with a variable structure used as speed increase in the general case of differential transmission with three inputs (Case A) were developed. The analytical models of the other three cases (B, C and D) were the result of customizing the archetype models based on the correlations specific to each case according to the engaging/disengaging of the component clutches. The analysis of the numerical results obtained by the simulations of the analytical models of the four _WSCGs_, considering a set of representative values for the simulation ratios \(k_{t}\) and \(k_{\omega}\), allowed drawing the following conclusions:  
* the wind turbine with three wind rotors (Case A) allows the increase of the output powers (towards the electric generator) and of the input one compared to those with two counter-rotating wind rotors (Cases B and C); in turn, the systems in Cases B and C are shown in the same figure. * the wind turbine with three wind rotors (Case B) allows the increase of the output powers (towards the electric generator) and of the input one compared to those with two counter-rotating wind rotors (Cases B and C); in turn, the systems in Cases B and C are shown in the same figure. C can ensure a better use of the wind potential compared to traditional single-rotor wind turbines (Case D);  
- the reduced input powers (corresponding to secondary wind rotors R2 and R3) and the reduced output power, as well as the configuration of the power flows, depend, to a large extent, on the values of ratios \(k_{t}\) and \(k_{\omega}\);  
- the transmission efficiency is constant in Cases C and D, because it does not depend on the operating speed nor on the transmitted power; instead, the efficiency in Cases A and B changes with ratio \(k_{t}\);  
- thanks to the property of \"summing up\" the speeds, the 2-DOF systems (Cases A and C) can offer higher amplification ratios (\(i_{a}\)) than the 1-DOF ones and can implicitly ensure a higher power supply by the stator GS;  
- the turbines with two wind rotors (Cases B and C) can have comparable power performances, the 1-DOF system (Case B) being advantaged with superior powers and efficiency in the vicinity of the value \(k_{t}=1\). The differential system (Case C) achieves higher amplification ratios, accompanied by relatively high powers, as the ratio \(k_{\omega}\) increases;  
- the maximum input power supply being brought about by rotor R3; the most interesting energy aspects are found in Case A in the vicinity of ratio value \(k_{t}=1\) for the situation \(k_{\omega}=0\). Practically, this means a 2-DOF system with two rotors, R1 and R3, were obtained from the case with three wind rotors by removing rotor R2 (i.e., Case B); it is also interesting that, in this system (Figure 1a without rotor R2), planetary gear set I idles (it does not participate in the transmission of the torque and of the power implicitly). The authors intend to develop this topic more in the future by analyzing the CRWT behavior in dynamic conditions, the transient effects of changing the wind speed, by considering wind turbines and electric generators with known functional characteristics. The experimental validation of these theoretical results is also a future purpose of the authors.