# A Comparative Performance Analysis of Four Wind Turbines with Counter-Rotating Electric Generators
The performances of existing wind turbines have been studied by many researchers considering various indicators (capacity factor, availability, failure rate, downtime, power coefficient, efficiency, reliability, etc.) and using a wide range of data and methods, such as those systematized by [PERSON] et al. [18], the empirical mode decomposition method [19; 20] or methods such as experiments and the Lattice Boltzmann model combined with Large Eddy Simulation [21] applied to a horizontal axis counter-rotating wind turbine. The performances of dual-rotor (co- and counter-rotating) wind turbines were highlighted also by [PERSON] et al. in [22], ideas that were further developed by [PERSON] et al. [23] and [PERSON] et al. [24]. The performance of a CRWT was discussed by [PERSON] et al. [25] in the case of implementing a counter-rotating generator, while [PERSON] et al. compared rotary systems with two or three wind rotors [26]. [PERSON] et al. analyzed in [27] the stationary operating point of a 2-DOF CRWT with a counter-rotating generator. [PERSON] et al. [28] analyzed the influence of three-stage transmission vibrations on the performance of horizontal axis wind systems operating in low-wind speed areas. [PERSON] et al. studied in a wind tunnel the effect of the speed ratios of two counter-rotating rotors of a vertical axis wind turbine [29]; on the same CRWT type, [PERSON] et al. [30] investigated the aerodynamic characteristics, power and torque coefficients. In [31], [PERSON] et al. highlighted the performance of a CRWT with Darius H-type rotors using three-dimensional CFD models based on the K-omega Shear Stress Transport turbulence model; the same method was applied by [PERSON] et al. [32] for a similar turbine on a floating platform. The performance of a horizontal axis CRWT with identical front and rear rotors was highlighted by [PERSON] et al. in [33], while [PERSON] et al. [34] analyzed the performance of the same wind turbine type by considering the ratio of wind rotor diameters as a dimensionless parameter. The main WT subsystems addressed extensively by researchers are wind rotors, electric generators and speed increasers. [PERSON] et al. studied in [35] the effect of the number of blades on the aerodynamic forces of a vertical axis wind turbine, while the design of the WT blade shapes and their influence on the power coefficient in static and dynamic conditions were addressed in [36]. [PERSON] et al. investigated the performance of a counter-rotating electric generator [37]. Several representative speed reducers for which the power flow was reversed (and, hence, used as a speed increaser) were analyzed by [PERSON] et al. in [38], while [PERSON] et al. [39] proposed and modeled a speed increaser able to be operated with one input and one or two outputs. Other novel solutions of planetary speed increasers have been presented and analyzed in the literature: a 2-DOF hybrid transmission for variable speed wind turbines [12] and a two-input one-output cylindrical planetary gear set with satellites in a series [40]. The dynamic properties of a speed increaser and the steady-state operating point were highlighted by [PERSON] et al. [41] by studying the behavior of a wind turbine with counter-rotating rotors using a wind tunnel and the CFD method. [PERSON] et al. [42] classified the gearbox technologies for horizontal axis wind systems into three categories (planetary gearbox, continuous variable transmission and magnetic gearbox) and studied their performances based on indicators such as the torque output, tracking accuracy and durability. [PERSON] et al. summarized the gear mechanisms used in wind turbines [13]. Recent technologies and development trends of mechanical transmissions were systematized by [PERSON] et al. [43], and [PERSON] et al. analyzed the behavior of the gear teeth using artificial neural networks [44] or the load capacity and its influence on the condition of the teeth [45]. The dynamic behavior of speed increasers was addressed by [PERSON] et al.