Counter-rotating Wind Turbine Research Articles

Estimation of operational parameters of the counter-rotating wind turbine with artificial neural networks

The article presents the possibility of using artificial neural networks to model the operating parameters of a counter-rotation mini wind turbine. The work is based on data from wind turbine research results conducted in an aerodynamic tunnel in the Institute of Agricultural Engineering at the University of Environmental and Life Sciences in Wrocław. The correlation between the depended variable (generated power P) and independent variables (average velocity of the air stream v, wedge angle of the first rotor blades, wedge angle of the second rotor blades, distance between rotors) was examined. The quality of the network was verified based on the mean-square error. The construction of the turbine allows to steplessly change of the blades wedge angle and the distance between rotors. Hence in the future the constructed network model can be used for programming the controller allowing to optimize the operating parameters of the wind turbine to maximize the generated power.

Measurements of the wake characteristics of co- and counter-rotating twin H-rotor vertical axis wind turbines

Increasing the efficiency of individual vertical-axis wind turbines (VAWTs) has been the subject of much research. However, wind farms that comprise individual VAWTs are not necessarily the most efficient configuration in accordance with VAWTs’ signature wake pattern. This study aims to identify efficient array configurations and thereby increase the packing density and power output by grouping individual VAWTs as a unit. Systematic measurements of wake aerodynamics of co-rotating and counter-rotating (forward- and backward-rotating) twin VAWTs were taken in a wind tunnel. The wake was measured both along the blade mid-span plane up to 10 turbine diameters (10D) and along the vertical plane. The wake of co-rotating twin VAWTs showed great asymmetry, similar to that of a single VAWT. Interestingly, the wake of counter-rotating twin VAWTs demonstrated elegant symmetric patterns. Two pairs of stationary counter-rotating vortices were found to evolve in the wake. The turbulence intensity downstream was observed to experience drastic changes and slow recovery compared with its velocity counterpart. Finally, two types of array units are proposed for applications in wind farms in accordance with the measurements.

Closed dynamical model of a double propeller HAWT

The mathematical model of a counter-rotating horizontal axis wind turbine is constructed. It is supposed that an electrical generator is connected into a local circuit with variable external resistance. External resistance in a local electrical circuit of the generator is a varied parameter of the model. Dynamic equations of the model contain nonlinear functions that describe aerodynamic torque acting on each propeller. This nonlinearity leads to existence of several fixed points of the dynamic system corresponding to operation modes of the HAWT with different characteristics. Existence and stability of steady solutions of the model equations is studied. Parametric analysis of operation modes corresponding to stable fixed points is performed. Domains of attraction of stable steady points are described. Strategies of control via changing of the parameter which is responsible for the external resistance are discussed.

Wind farms with counter-rotating wind turbines

The objective of this study is to assess the effects of using counter-rotating wind turbines on the performance of a wind farm. Large eddy simulations, coupled with the actuator line model, were conducted to investigate flow through a test wind farm with 48 large-scale wind turbines with the same layout as Lillgrund in Sweden. Two counter-rotating cases were tested; first, an alternate-row wind farm in which each turbine has one rotor, rotating either clockwise or counter-clockwise, with alternating rows of clockwise and counter-clockwise turbines throughout the farm; and second, a wind farm with dual-rotor wind turbines in which each turbine has two rotors, with the first rotor rotating counter-clockwise and the second rotor rotating clockwise. It was found that both counter-rotating configurations were more efficient in power generation than the control case in which all turbines have one clockwise rotor; the alternate-row case was found to produce 1.4% more power and the dual-rotor case was found to produce 22.6% more power than the control wind farm. The wakes of the counter-rotating cases, particularly the wind farm with dual-rotor wind turbines, exhibit different characteristics from those in the control case. These differences are discussed through wind speed distribution, thrust coefficient, and power production of wind turbines.

Improving the energy performance of wind turbines implemented in the built environment using counter-rotating planetary transmissions

Most of wind turbine applications for urban areas use electric generators with counter-rotating rotor and stator, able to ensure a better efficiency than the conventional turbines with one wind rotor and generator with fixed stator, and, hence, a higher production of electricity. These types of power systems have two independent wind rotors that require a complex control of the two independent input speeds to obtain the optimal output speed. This paper deals with the use of a 1DOF (Degree Of Freedom) compound planetary transmission with two inputs and two outputs and three sun gears, meant for the implementation in counterrotating wind turbines, which has the properties of summing the input torques and determined transmission of the independent speed. Firstly, the kinematic and static analysis of the proposed planetary transmission, assuming friction of gears, is performed. Afterwards, the mechanism efficiency model is established depending on the ratio of the two input torques. The transmission efficiency is simulated and analysed, with determination and representation of power flows, in the four distinct operating cases according to the k ratio values. The paper results allowed formulating recommendations on the design of these mechanical planetary transmissions used in wind turbines and broadening a database for the conceptual synthesis of wind systems.

Numerical and experimental analysis of a counter-rotating type horizontal-axis tidal turbine

In order to exploit renewable energies form tidal currents, a unique counter-rotating type horizontal-axis tidal turbine was proposed in this paper. Although a counter-rotating type horizontal-axis wind turbine can be taken as a reference in designing the proposed counterrotating tidal turbine, there are some different characteristics such as the effects of the free surface and the occurrence of cavitation. The unique tidal turbine in this paper was preliminarily designed on the basis of the in-house wind turbine, and then CFD analysis and experimental test were carried out to evaluate its performance. For the extreme conditions where severe flow separation and vortex exist, the comparison of the power coefficients generated by the turbine between the CFD predictions and the experimental data shows not so good. However, good agreement has been obtained for the comfortable blade pitch setting angles and over a range of tip speed ratios, which provides an evidence of validation of CFD analysis. Such results give sufficient confidence that the CFD model set up here is suitable for the further works.

Structural and Kinematic Features of a 2 DOF Speed Increaser for Renewable Energy Systems

The counter-rotating wind turbine is a relatively new concept of wind converter that contains two rotors placed on the same side or on both sides of the nacelle with the aim of increasing the wind energy conversion efficiency. As the rotors are rotating at a lower speed than the generator requires, the wind turbine usually contains a speed increaser to harmonize their running regimes. Similar counter-rotating systems are approached for hydro applications but they are still in the research phase.The paper proposes a novel concept of a 2 DOF speed increaser to be used in counter-rotating systems (wind or hydro). The structural and kinematic features of the 1 DOF and 2 DOF transmission running cases are presented in the paper. The differential transmission is further analyzed and the transmission functions and efficiency are established. Finally, conclusions regarding the use of 2 DOF transmissions in the renewable energy systems are formulated.

Performance Evaluation of Co-Axis Counter-Rotation Wind Turbine

The present work was studied the effect of the tip speed ratio to the starting rotation, rev up rotation, power and torque coefficients of a two-shaft co-axis counter-rotating wind turbine (CR-WT). The prototype of CR-WT was tested in the open type wind tunnel with the velocities of 1.5, 2.0, 3.0, 4.0 and 5.0 m/s. CR-WT consisted of 8 blades. The blade is made of wood. It consists of 2 hubs 2 axis and 4 arms. The inner axis is clockwise rotation and outer axis is anticlockwise rotation. The inner and outer are in the identical axis. Both axes are driven the pulley by using the counter rotating gear. The analysis of the experimental results showed that the cut in wind speed of CR-WT was 1.744 m/s and it is increasing when the tangential force ratio increasing. Rev up rotation period depends on wind speed and tangential force ratio. The average of power coefficient of CR-WT was 14.89%.

Wind Tunnel Testing for a New Experimental Model of Counter-rotating Wind Turbine

The paper presents the achievements of INCDIE ICPE-CA on wind installations provided with counter-rotating turbines. There are envisaged both the development of an experimental model and the results following the testing activities, respectively. The experimental model submits a novelty technological character due to the unconventional innovative aspects. It represents a foregoing constructive version, preceding a wind turbine prototype with the rated power of 10-15kW designed for power autonomy assurance within specific applications.The preliminary calculations, the numerical modelling of the wind rotors and the electric generator stand experiments have confirmed the energy conversion capacity for the double effect micro aggregate equal to 1kW for a 10 m/s wind velocity.

Planetary Gear for Counter-Rotating Wind Turbines

A specific problem of the wind turbines refers to the difference between the low rotation speed of the wind turbine rotor and the high rotation speed needed for the electrical generator. Usually, the adaptation between the speed of the turbine rotor and the electrical generator speed is achieved by means of a speed increaser. A recent alternative relates to the use of coaxial counter-rotating wind turbines, which can achieve higher power and improve the conversion efficiency of the wind energy into electrical energy (up to 25%) with a reduced cost of approx. 20-30% compared to similar single rotor turbines. Conceptually, the counter-rotating wind turbine systems can integrate a particular generator wherein the rotor is coupled to a row of blades and the stator with another row of blades, or a commonly generator, coupled to a differential planetary gear, that allows the summation of the blades motions.The paper describes and analyzes kinematic and dynamic aspects of a system consisting of two coaxial counter-rotating turbines and a generator, interconnected by a planetary gear with two inputs (the two turbines) and an output (the generator). The algorithm is based on the property of the differential planetary gear of adding two input motions into one output motion. The kinematic and dynamic parameters of the planetary gear are established in the paper, and a case study is further presented: a small wind turbine equipped with a transmission enabling input speed multiplication.

10kW급 상반전 풍력터빈 로터의 설계와 성능 평가에 관한 연구

The counter-rotating approach on wind turbine has been recently put in interest for its certain advantages in both design and performance. This paper introduces a study on a counter-rotating wind turbine designed and modeled using NREL airfoils S822 and S823. The aims of the study is to evaluate and discover the performance of the counter-rotating system, and compares to that of single rotor turbine of same design using numerical simulation. The results show higher performance of the counter-rotating system compared with single rotor case at TSR 3 to 5 but lower performance at higher TSR. This is due to the interaction between upstream and downstream rotors. Thus, the counter-rotating turbine is more efficient at low rotor rotational speed.

Optimization of a counter-rotating wind turbine using the blade element and momentum theory

A counter-rotating wind turbine has a front rotor and a rear rotor which rotate in opposite directions on the same axis. Compared to a single rotor, the flow field of the counter-rotating wind turbine is complicated due to the interactions between the front rotor and the rear rotor. The wake induced by the front rotor works on the inflow of the rear rotor and is essentially an unsteady flow state. In order to estimate the performance of a counter-rotating wind turbine, it is necessary to consider more variables than the case of a single-rotor wind turbine and to prepare an estimation system capable of performing delicate predictions. For the optimization of a counter-rotating wind turbine, the pitch angles, radius ratios, and rotation speeds of two rotors are chosen as the design values and variations of the power coefficients and thrust coefficients can be observed in this study. The torque balance of the two rotors due to the kinematic coupling of the generator is considered. Modeling by means of the blade element and momentum theory for the optimization of a counter-rotating wind turbine is developed to predict the front rotor flow and the wake flow generated by the front rotor. The wake flow is then applied for the inflow of the rear rotor. A vehicle test is carried out to validate the prediction. The optimized solution is found using a multi-island genetic algorithm.

Efficiency improvement of straight-bladed vertical-axis wind turbines with an upstream deflector

The effect of an upstream flat deflector on the power output of two counter-rotating straight-bladed vertical-axis wind turbines (VAWTs) was investigated experimentally. Although many studies on the deflector effect have been done for a drag-based Savonius VAWT, there is little information on the performance of a lift-based VAWT with a deflector. If a flat vertical plate is normal to the free-stream wind direction, the local wind speed outside the near-wake region of the plate is larger than the free-stream velocity. Thus, when each of the counter-rotating turbines is positioned outside the near-wake region, its power output increases significantly. The power output increase with the deflector installed is dependent on the width and height of the deflector and the turbine position relative to the deflector.

Velocity interference in the rear rotor of a counter-rotating wind turbine

A counter-rotating wind turbine having two rotors rotating in opposite directions on the same axis is proposed to improve the aerodynamic performance of a wind turbine. In order to predict the aerodynamic performance of the counter-rotating wind turbine, the inflow interference in its rear rotor by the wake of the front rotor needs to be considered because the rear rotor operates inside the wake of the front rotor. In the previous research, the rear rotor was assumed to operate inside the fully developed stream tube of the front rotor to define the inflow condition on the rear rotor. In this study, to consider simultaneously the aerodynamic interaction between the two rotors of the counter-rotating wind turbine without any assumption on the inflow velocity of the rear rotor, an aerodynamic analysis of the counter-rotating wind turbine was carried out by using a free-wake vortex lattice method. The power coefficient and the wake geometry of the counter-rotating wind turbine were compared with those of a single rotor wind turbine, and the induction factors on the rear rotor were compared with that from the BEMT, assuming that the rear rotor operated inside the fully developed stream tube of the front rotor.

Effects of design parameters on aerodynamic performance of a counter-rotating wind turbine

This study investigates the effects of design parameters on the aerodynamic performance of a counter-rotating wind turbine. The counter-rotating wind turbine has two rotors rotating in opposite directions on the same axis. It has been proposed on the basis of the theory which states that a configuration of two rotors having the same swept area on the same axis has a higher maximum power coefficient than a conventional configuration of a wind turbine having a single rotor. More design parameters are involved in the description of the counter-rotating wind turbine than of a wind turbine using a single rotor because of the complex phenomenon arising from the aerodynamic interaction between its two rotors, but influences of these parameters is yet to be fully understood. In this study, a modified blade element momentum theory for the counter-rotating wind turbine is developed to investigate the effects of these design parameters such as the combinations of the pitch angles, rotating speeds and rotors’ radii on the aerodynamic performance of the counter-rotating wind turbine.

Analysis of aerodynamic characteristics on a counter-rotating wind turbine

This study aims to analyze aerodynamic characteristics of a counter-rotating wind turbine. For this purpose, three kinds of rotor configurations which are 2-bladed single, 4-bladed single and counter-rotating rotor were compared by using a numerical method. The numerical method used here was based on vortex lattice method and was validated with measurements of the NREL phase-VI rotor. Through numerical calculations of induction factors and power coefficients for each rotor configuration, the aerodynamic feasibility of a counter-rotating wind turbine was considered.

05/01672 Aerodynamic performance prediction of a 30 kW counter-rotating wind turbine system

In this study a single stage planetary multiplying gearbox with helical gears was tested on a back-to-back gearbox test rig with recirculating power under the operating conditions that matched the tangential velocities and operating contact pressures found in a planetary stage of a 2.5 MW wind turbine. The study was done with four different wind turbine gear oils. The gearbox oil sump temperature was set free.The load and no-load losses were measured and compared with the results yielded by an existing numerical model which takes into account the influence of the lubricant formulation in the load losses.The results have indicated that each gear oil formulation promoted different power loss results resulting in distinct stabilized operating temperatures.

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.

후류영향을 고려한 상반회전 풍력발전 시스템의 공력성능 예측에 관한 연구

본 연구에서는 로터의 후류 효과 및 실속 후 특성을 고려하여 30kW급 상반회전 풍차 시스템에 대한 공력성능 해석을 수행하였다. 기본 공력이론은 모멘텀 이론과 2차원 준정상 공기력 이론을 통합한 형태를 사용하였다. 로터의 후류영향을 고려하기 위해 축소형 풍차 블레이드 모델에 대한 풍동시험 결과를 적절히 이용하였으며, 이로부터 보조로터를 지난 후류의 축속도 및 각속도 성분을 결정하였다. 또한, Glauert의 최적 작동판 이론과 Prandtl의 익단손실 효과를 고려하여 30kW급 풍차 시스템에 대한 최적 시위 및 비틀림 분포를 구하였으며, 기존의 단일 로터 시스템과의 공력성능 비교를 통하여 상반회전 풍차 시스템의 효율성 및 우수성을 입증하고자 하였다. In this paper, the aerodynamic performance prediction of a 30kW counter-rotating (C/R) wind turbine system has been made by using the momentum theory as well as the two-dimensional quasi-steady strip theory with special care on the wake and the post-stall effects. In order to take into account the wake effects in the performance analysis, the wind tunnel test data obtained for a scaled blade are used. Both the axial and rotational inductions behind the auxiliary rotors are determined through the wake model. In addition, the optimum chord and twist distributions along the blades are obtained from the Glauert's optimum actuator disk model considering the Prandtl's tip loss effect. The performance results of the counter-rotating wind turbine system are compared with those of the conventional single rotor system and demonstrated the effectiveness of the counter-rotating wind turbine system.

98/00424 Development of multifuel and biomass cook-stoves

This study investigates the effects of design parameters on the aerodynamic performance of a counter-rotating wind turbine. The counter-rotating wind turbine has two rotors rotating in opposite directions on the same axis. It has been proposed on the basis of the theory which states that a configuration of two rotors having the same swept area on the same axis has a higher maximum power coefficient than a conventional configuration of a wind turbine having a single rotor. More design parameters are involved in the description of the counter-rotating wind turbine than of a wind turbine using a single rotor because of the complex phenomenon arising from the aerodynamic interaction between its two rotors, but influences of these parameters is yet to be fully understood. In this study, a modified blade element momentum theory for the counter-rotating wind turbine is developed to investigate the effects of these design parameters such as the combinations of the pitch angles, rotating speeds and rotors’ radii on the aerodynamic performance of the counter-rotating wind turbine.