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Increasing the efficiency of wind power conversion into electricity poses major challenges to researchers and developers of wind turbines, who are striving for new solutions that can ensure better use of local wind potential in terms of both feasibility and affordability. The paper proposes a novel concept of wind systems with counter-rotating wind rotors that can integrate either conventional or counter-rotating electric generators, by means of the same differential planetary speed increaser, aiming at providing a comparative analysis of the energy performance of counter-rotating wind turbines with counter-rotating vs. conventional electric generators. To this end, a generalized analytical model for angular speeds and torques has been developed, which can be customized for both system configurations. Three numerical simulation scenarios have been contrasted: (a) a scenario with identical wind rotors in both systems, (b) a scenario with the secondary wind rotors being identical in the two applications, but different from the primary rotors, and (c) a scenario with different secondary rotors in the two wind turbines. The results have shown that the wind systems with counter-rotating generator are more efficient and have a higher amplification ratio, compared to systems with conventional generators. In addition, the analyzed wind system with a counter-rotating generator displays better energy performance with low values for output power and ratio of input speeds, whereas the wind turbine with a conventional generator proves to be more efficient in the high-value range of the above-mentioned parameters.

Planetary transmissions used as speed amplifiers in wind/hydro conversion systems typically rely on the conventional concept of mechanism with one input and one output. This concept is found in the most of the high power wind turbines; however, the need to implement in the built environment smaller wind turbines with speed amplifiers led to new turbine concepts such as counter-rotating rotors and classical generator. The paper presents a comparative analysis of two wind turbines with one rotor and respectively two counter-rotating rotors, which contain identical electrical generators and the same type of mechanical amplifier used as a 1 DOF mechanism and differential one respectively. With the assumption of maintaining the same electrical generator running point for both wind turbines, the steady-state behaviour of the considered turbines is identified and a comparative kinematic and static performance analysis is performed, aiming at highlighting the impact on efficiency and on constructive design and development of the two turbines.

A specific topical issue both for small hydropower plants and wind turbines refers to the discrepancy between the relatively low speed of the water turbine/wind rotor and relatively high speed of the electric generator: the turbine/rotor has higher performances at lower speeds, while the generator performances are increasing with the speed. Usually, this problem can be solved connecting a proper speed increaser between the turbine (increaser input) and generator (increaser output). Besides, the gearboxes used in the renewable energy systems (R.E.S.) allow generating the necessary electric energy while functioning at lower values of wind speeds or water flows. The paper presents and analyzes cinematically and dynamically a counter-rotating system that contains two coaxial turbines and a generator, connected through a bevel-planetary gear with two inputs (the two turbines) and one output (the generator). The system works relatively different from other drive trains used in the renewable energy systems, while having a much improved efficiency. Firstly, the kinematical and dynamic parameters of the planetary transmission are established in the paper. Then, a case study of a small hydropower plant equipped with a counter-rotating transmission that multiplies 5 times the input speed is analyzed. The numerical simulation results are analyzed in comparison with the classical solutions and recommendations concerning the use of counter-rotating systems are settled up.

The improvement of wind turbine performance poses a constant challenge to researchers and designers in the field. As a result, the literature presents new concepts of wind turbines (WTs), such as: counter-rotating wind turbines (CRWTs) with two coaxial wind rotors revolving in opposite directions, WTs with higher-efficiency and downsized transmission systems, or WTs with counter-rotating electric generators (CREGs). Currently, there are a few solutions of WTs, both containing counter-rotating components; however, they can only be used in small-scale applications. Aiming to extend the use of WTs with counter-rotating wind rotors (CRWRs) and CREGs to medium- and large-scale applications, this paper introduces and analyzes a higher-performance WT solution, which integrates two counter-rotating wind rotors, a 1 degree of freedom (DOF) planetary speed increaser with four inputs and outputs, and a counter-rotating electric generator. The proposed system yields various technical benefits: it has a compact design, increases the output power (which makes it suitable for medium- and large-scale wind turbines) and allows a more efficient operation of the electric generator. The kinematic and static computing methodology, as well as the analytical models and diagrams developed for various case studies, might prove useful for researchers and designers in the field to establish the most advantageous solution of planetary speed increasers for the CRWTs with CREGs. Moreover, this paper extends the current database of WT speed increasers with an innovative concept of 1 DOF planetary gearbox, which is subject to a patent application.