Auxiliary Rotor Research Articles

Experimental study of the effect of the duct on dual co-axial horizontal axis wind turbines and the effect of rotors diameter ratio and distance on increasing power coefficient

Due to the recent growing interest in wind energy in urban industry, the present research investigates the increase in power of a micro-wind system using a duct and an auxiliary rotor. The system has the capacity to be employed separately to generate electricity or to be used as part of other systems such as INVELOXes. A duct increases the air flow rate through the turbine, while an auxiliary rotor allows the extraction of the wake kinetic energy. For this purpose, the effect of the duct on increasing wind speed is first studied, and then increasing wind turbine power is investigated. The built duct increases the wind speed up to 42% (from 5 m/s to 7.1 m/s); consequently, the wind turbine power increases over two times. Next, in order to achieve more power, a rotor is placed inside a duct within the front rotor wake. The effect of rotor diameters and the rotors' distance have been investigated by evaluating different configurations to obtain the highest system power. The results show that in the case of adding an auxiliary rotor with the same diameter in a duct, the power coefficient of the rotary part increases to 16%. Moreover, if the two rotors have different diameters (the front rotor has a smaller diameter), the power coefficient will increase up to 13% compared to two rotors with the same diameter. Importantly, as the system is designed to test rotors with independent rotation speed, using the same TSR for both rotors increases the power coefficient of the rotary part by up to 23% compared to the case where two rotors operate at the same rotational speed. Considering the frontal area of the system instead of the rotor area, the power coefficient of the system for ducted wind turbines can also be computed. Accordingly, it is found that its value will be significantly reduced.

Effect of the yaw angle on the aerodynamics of two tandem wind turbines by considering a dual-rotor wind turbine in front

The innovative dual-rotor counter-rotating wind turbine (CRWT) shows great importance in the development of offshore wind farms. In this paper, the performance and wake characteristics of the independent CRWT and two tandem wind turbines equipped with a CRWT at different yaw angles were numerically studied using the open-source software SOWFA (Simulator fOr Wind Farm Applications). The actuator line model (ALM) and large-eddy simulation (LES) were used to model wind turbines and turbulent flows. The third generation of vortex identification criterion was used to analyze wake vortex characteristics. The results indicated the auxiliary rotor had a negligible effect on the main rotor in terms of the dominant frequency and fluctuations. In comparison with the rotational blade frequency (BF), it was found that one BF and double BF were equivalent to the dominant frequency of the SRWT and CRWT, respectively. The CRWT suffers a greater velocity deficit and has a faster velocity recovery. In the case of tandem wind turbines, the upstream wind turbine equipped with a CRWT performs minimal influence on the optimal yaw angle of 30°. In addition, the effects of the yaw angle on wake center deflection, wake width, and performance fluctuation for the downstream wind turbine were investigated.

Numerical Investigation on Aerodynamic Characteristics of Dual-Rotor Wind Turbines

Improving power output and reducing costs are crucial to the sustainable development of offshore wind power. In the present study, a dual-rotor wind turbine (DRWT) is proposed to improve wind energy capture efficiency by adding an auxiliary rotor behind the main rotor. The two rotors can be the same size or different sizes. This will result in different aerodynamic characteristics for DRWTs. In this paper, the NREL Offshore Baseline-5 MW and the NREL 750 kW single-rotor wind turbines (SRWTs) are used to configure three different types of DRWTs. The power output and wake characteristics of three different DRWTs with co-rotating (CO-DRWT) and counter-rotating (CR-DRWT) configurations on an actual scale are compared. The Reynolds-averaged Navier–Stokes (RANS) model with k-ω SST (shear stress transport model) is used to simulate the unsteady flow generated by the DRWT’s rotation. The present numerical results show that the power coefficient of the 5 MW-5 MW CO-DRWT can reach 1.22 times that of the 5 MW SRWT. Moreover, a faster wake velocity deficit recovery is found in the 5 MW-5 MW DRWTs because the high-velocity flow caused by the merging and mixing of the trailing vortices of the 5 MW-5 MW DRWTs brings an energy supplement to the wake velocity deficit.

A particle image velocimetry study of dual-rotor counter-rotating wind turbine near wake

This experimental work studied the flow characteristics in the near wake region behind dual-rotor wind turbines using two-dimensional particle image velocimetry. Two auxiliary rotors of 50% and 80% scale of the main rotor were installed upwind and operated in counter-rotating condition, which are compared to the conventional single-rotor turbine. In all the three configurations, a constant Reynolds number 9.5times 10^4 was applied, and all the rotors operated at a fixed tip speed ratio of 3.46. The mean and phase-averaged velocity fields were investigated together with the turbulence kinetic energy. It was found that the two auxiliary rotors do not result in a significantly different wake flow property. The configuration implementing the 50% auxiliary rotor sees a slightly better wake characteristics, in terms of weaker main rotor tip vortices and a counter-rotating swirling shear region at the mid-span behind the main rotor. The decay rates of the peak vorticity of the main rotor tip vortices and their circulation are found to follow an exponential manner.Graphic abstract

Small wind turbine augmentation: Experimental investigations of shrouded- and twin-rotor wind turbine systems

An increase in the efficiency of Small Wind Turbines (SWTs) by aerodynamic optimisation of the blade geometry is limited (low Reynolds number influence). Solutions such as the Diffuser-Augmented Wind Turbine (DAWT) and the twin-rotor systems are of increasing interest. A diffuser promotes an increase in the wind mass flow rate through the turbine, whereas an auxiliary rotor enables extraction of the wind kinetic energy in the wake.The paper summarizes the measurements of wind turbine systems performance conducted at the Institute of Turbomachinery, Lodz University of Technology (IMP TUL). The research incorporated a spectrum of wind turbine configurations for open and shrouded, single- and twin-rotor systems. The objective was to compare the performance of the same rotor in different configurations. The influence of a low Reynolds number flow on the rotor performance is also discussed and quantified.The study shows that, while augmenting the wind turbine performance (as much as twofold increase), shrouding rises significantly the rotor loading. A remedy for that may be an application of the second rotor. Although it provides a rather modest efficiency increase (11–13% for the unshrouded-, 4–5% for shrouded turbine), it allows loads to be distributed more evenly on turbines.

An experimental study on the aeromechanics and wake characteristics of a novel twin-rotor wind turbine in a turbulent boundary layer flow

The aeromechanic performance and wake characteristics of a novel twin-rotor wind turbine (TRWT) design, which has an extra set of smaller, auxiliary rotor blades appended in front of the main rotor, was evaluated experimentally, in comparison with those of a conventional single-rotor wind turbine (SRWT) design. The comparative study was performed in a large-scale wind tunnel with scaled TRWT and SRWT models mounted in the same incoming turbulent boundary layer flow. In addition to quantifying power outputs and the dynamic wind loadings acting on the model turbines, the wake characteristics behind the model turbines were also measured by using a particle image velocimetry system and a Cobra anemometry probe. The measurement results reveal that, while the TRWT design is capable of harnessing more wind energy from the same incoming airflow by reducing the roots losses incurred in the region near the roots of the main rotor blades, it also cause much greater dynamic wind loadings acting on the TRWT model and higher velocity deficits in the near wake behind the TRWT model, in comparison with those of the SRWT case. Due to the existence of the auxiliary rotor, more complex vortex structures were found to be generated in the wake behind the TRWT model, which greatly enhanced the turbulent mixing in the turbine wake, and caused a much faster recovery of the velocity deficits in the turbine far wake. As a result, the TRWT design was also found to enable the same downstream turbine to generate more power when sited in the wake behind the TRWT model than that in the SRWT wake, i.e., by mitigating wake losses in typical wind farm settings.

Improved Transverse Flux Type Permanent Magnet Reluctance Generator With Auxiliary Rotor Pole Inserted Permanent Magnet

This paper proposes an improved transverse flux type permanent magnet reluctance generator (TFPMRG) for the purpose of reducing the leakage flux and improving the back EMF. This improved TFPMRG has auxiliary rotor poles with permanent magnet (PM) added between the main rotor poles. The auxiliary rotor pole is effective to obtain high-back EMF voltage source under no-load condition and to increase the amplitude and region of the generated current under load condition. To obtain these desired performances, PMs are inserted into both ends of auxiliary rotor pole. The auxiliary rotor and main rotor poles works at the same instance under load condition. Characteristics of the improved TFPMG are analyzed using the finite element analysis and it is verified that the improved TFPMRG has an excellent performance.

Aerodynamic performance prediction of a 30 kW counter-rotating wind turbine system

In the present work, the aerodynamic performance prediction of a unique 30 kW counter-rotating (C/R) wind turbine system, which consists of the main rotor and the auxiliary rotor, has been investigated by using the quasi-steady strip theory. The near wake behavior of the auxiliary rotor that is located upwind of the main rotor is taken into consideration in the performance analysis of the turbine system by using the wind tunnel test data obtained for scaled model rotors. The relative size and the optimum placement of the two rotors are investigated through use of the momentum theory combined with the experimental wake model. In addition, the performance prediction results along with the full-scale field test data obtained for C/R wind turbine system are compared with those of the conventional single rotor system and demonstrated the effectiveness of the current C/R turbine system.

이중 로터 풍력발전 시스템 모델링 및 시뮬레이션에 관한 연구

본 논문에서는 로터 블레이드, 고/저속 회전축, 발전기, 기어 시스템 등 다수의 몸체가 서로 상대적인 운동을 하며 연결되어 있는 이중 로터 수평축 풍력발전 시스템을 다몸체 시스템으로 간주하고, 다몸체 역학을 이용한 풍력발전 시스템 모델링 기업을 제안하였다. 이를 기반으로 풍력발전 시스템의 성능 해석을 위한 시뮬레이션 소프트웨어 WINSIM을 개발하였고, 다양한 시뮬레이션을 통해 제안된 풍력발전 시스템의 과도 및 정상 상태 특성의 연구에 적용할 수 있음을 예시하였다. In this paper, an efficient method for modeling a dual-rotor type wind turbine generator system and simulation results are presented. The wind turbine is treated as a collection of several rigid bodies, each of which represents, respectively, main and auxiliary rotor blades, high/low speed shafts, generator, and gear system. Simulation software WINSIM is developed to implement the proposed modeling method and is used to investigate the transient and steady-state performance of the wind turbine system.

Brushless and self-excited 3-phase synchronous machine

The paper describes a brushless and self-excited synchronous machine, which uses current compounding in auxiliary stator and rotor windings to produce an excitation current which increases as the load current is increased. With the machine used as an isolated alternator, the effect is sensitive to the power factor of the load, and the increase in excitation current is greater as the load becomes more inductive. The constancy with which the terminal voltage of an experimental machine is controlled as the load varies is comparable with that obtained with typical static current-compounded excitation schemes, and a feature of the new scheme is the very rapid recovery of the terminal voltage following a sudden increase in load. As with static schemes, the new system is readily adapted to control either the power factor or the reactive power of the input to the machine, operated as a synchronous motor on an infinite busbar. Results obtained from the experimental machine again indicate an accurate control, from no load to well beyond full load.