Rotor Power Research Articles

Effect of surge motion on rotor aerodynamics and wake characteristics of a floating horizontal-axis wind turbine

Aerodynamics of a floating horizontal-axis wind turbine (FHAWT) is subject to its platform’s six degrees of freedom (DOFs) motion, especially the surge motion, and may become much more complicated than that of a fixed-base wind turbine. This paper aims at investigating the aerodynamics of the FHAWT’s rotor and the characteristics of its wake under surge motion. To explore this, a CFD method with the improved delayed detached eddy simulation (IDDES) is applied to a 1:50 model FHAWT. First, it is demonstrated that the rotor’s aerodynamics including the thrust, torque and rotor power may be greatly affected by surge motion even though it is small. During surge motion, the light dynamic stall and rotor-wake interaction phenomena are observed. In addition, the near and far wake may be obviously influenced by surge motion. More importantly, the wake-recovery process under surge motion is slower than the one without surge motion. In general, the surge motion impacts the aerodynamics of the FHAWTs’ rotor and the characteristics of its wake greatly, therefore should receive due attention in the design procedure and the arrangement of wind farms.

NUMERICAL INVESTIGATION OF THE BLADE PROFILE EFFECT ON THE AERODYNAMIC PERFORMANCE OF A VERTICAL-AXIS WIND TURBINE DARRIEUS H-ROTOR

A thorough understanding of the various parameters that affect the vertical axis wind turbine performance and lifespan will make design this model of turbine not difficult task. This article presents a study of the blade profile effect on the aerodynamic performance of a vertical axis wind turbineDarrieus H-rotor. A complete series of simulations based on complete unstable URANS calculations are performed for a three-blade Darrieus wind turbine. Fluent’s software is used for the numerical solution. The aerofoils of blade chosen for testing were NACA 0012, 0015, 0018, 0021 and -60 pitch angle is taken as the reference case for comparison. In order to quantify the influence of the profile of the blade on the performance, the characteristics of the flow field around the rotor of the different configurations are studied. For different values of the tip speed ratio, dynamic quantities, such as torque and rotor power, are presented and analyzed. Also in this investigation of the flow flied, two parameters are carefully investigated: mesh resolution and time step size. In the analysis, it appears that these parameters affect result accuracy. Finally, the numerical result shows that the variation of blade profile directly affected directly the power production.

Computational Fluid Dynamics Analysis of Floating Offshore Wind Turbines in Severe Pitching Conditions

Abstract The unsteady aerodynamics of floating wind turbines is more complex than that of fixed-bottom turbines, and the uncertainty of low-fidelity predictions is higher for floating turbines. Navier–Stokes computational fluid dynamics (CFD) can improve the understanding of rotor and wake aerodynamics of floating turbines, and help improving lower-fidelity models. Here, the flow field of the NREL 5 MW rotor with fixed tower, and subjected to prescribed harmonic pitching past the tower base are investigated using blade-resolved CFD compressible flow COSA simulations and incompressible flow FLUENT simulations. CFD results are also compared to predictions of the FAST wind turbine code, which uses blade element momentum theory (BEMT). The selected rotor pitching parameters correspond to an extreme regime unlikely to occur without faults of the turbine safety system, and thus relevant to extreme aerodynamic load analysis. The rotor power and loads in fixed-tower mode predicted by both CFD codes and BEMT are in very good agreement. For the floating turbine, all predicted periodic profiles of rotor power and thrust are qualitatively similar, but the power peaks of both CFD predictions are significantly higher than those of BEMT. Moreover, cross-comparisons of the COSA and FLUENT predictions of blade static pressure also highlight significant compressible flow effects on rotor power and loads. The CFD analyses of the downstream rotor flow also reveal wake features unique to pitching turbines, primarily the space- and time-dependence of the wake generation strength, highlighted by intermittency of the tip vortex shedding.

Design of High-Speed Permanent Magnet Motor Considering Rotor Radial Force and Motor Losses

Different from the design of conventional permanent magnet (PM) motors, high-speed motors are primarily limited by rotor unbalanced radial forces, rotor power losses, and rotor mechanical strength. This paper aimed to propose a suitable PM motor with consideration of these design issues. First, the rotor radial force is minimized based on the selection of stator tooth numbers and windings. By designing a stator with even slots, the rotor radial force can be canceled, leading to better rotor strength at high speed. Second, rotor power losses proportional to rotor frequency are increased as motor speed increases. A two-dimensional sensitivity analysis is used to improve these losses. In addition, the rotor sleeve loss can be minimized to less than 8.3% of the total losses using slotless windings. Third, the trapezoidal drive can cause more than a 33% magnet loss due to additional armature flux harmonics. This drive reflected loss is also mitigated with slotless windings. In this paper, six PM motors with different tooth numbers, stator cores, and winding layouts are compared. All the design methods are verified based on nonlinear finite element analysis (FEA).

Determination of Rotor Power Consumption in a Thin Film Scraped Surface Heat Exchanger

Scraped surface heat exchangers (SSHE) are prevalent in the food industry to heat/cool viscous fluids and to provide enhanced mixing. The present study was undertaken to determine the power consumption for different blade assembly configurations for the first stage and second stage SSHE. Milk was concentrated from 15% to 26% total solids using first stage SSHE and 32 to 52% total solids using second stage SSHE. Effect of number of blades (2,4,6) weight of blades (1.6, 1.75, 2.0 kg) and (1.75, 2.0 kg, 2.25 kg ) and rotor speed (150, 175, 200 rpm) and (125,150, 175 rpm) was investigated on power consumption (W) using Response Surface Methodology for the first stage and second stage SSHE. The results show that power consumption increased linearly with increase in number of blades as well as rotor speed. Power consumption varied from 420 to 734 W for the first stage SSHE and it varied from 448 to 755 W for the second stage SSHE for all weight of the blades.

Helicopter Shipboard Landing Simulation Including Wind, Deck Motion and Dynamic Ground Effect

A first principles physics-based simulation framework that accounts for wind-over-deck (WOD) and ground effects during approach and landing of a helicopter on a ship deck is developed. The WOD velocities are obtained from unsteady detached-eddy simulation of the flow over a simple frigate shape version 2 ship. The HeliUM2 flight dynamics code was modified to include WOD, static and dynamic ground effects, and landing gear models. A dynamic ground effect model that combines a finite-state wake model with a ground source distribution was also incorporated. Simulations are carried out to determine the combined effect of WOD and deck motion. Representative ship motion data are extracted from the Systematic Characterization of the Naval Environment database. A simple linear-quadratic-regulator-based flight control system is used to follow a prescribed trajectory during approach and landing. High-frequency oscillations are induced by the WOD. The entire helicopter has to be modeled to capture the complete WOD effect. In-ground-effect simulations show up to 10% reduction in rotor power consumption as well as good agreement with experimental data for a stationary deck. Deck heaving motion produces an additional change in rotor power consumption relative to the static ground effect, highlighting the importance of modeling the dynamic ground effect. The controller maintains the desired approach trajectory; and successful landing is demonstrated on level, inclined, and moving decks in the combined presence of WOD and ground effects.

Effect of active camber morphing on rotor performance and control loads

A computational investigation of a helicopter main rotor with an active-camber morphing mechanism was conducted to identify the capability to simultaneously save rotor power and reduce pitch-link loads using as low as possible camber deflection magnitudes. Comprehensive rotor aeromechanics analysis with elastic blade modeling and a free vortex wake for the aerodynamics model was used to ensure computational efficiency. The investigation was based on a full-scale Bo 105 helicopter main rotor in level flight condition at μ=0.3 and CT/σ=0.089. In addition to a variation of the radial position and length of the active-camber section, the capabilities and system behavior were investigated in an extensive parametric study using practical actuation inputs. The same computational framework was used to obtain optimal control inputs that led to best performance in terms of power savings using two-per-rev (2P) individual blade control (IBC) via pitch-link inputs and 2P active-twist control. The relative potential of the three active mechanisms and the aerodynamic phenomenology was compared. All of the investigated active-rotor mechanisms contributed to rotor power savings via a more uniform distribution of thrust over the rotor disk. Both IBC and active twist yielded maximum performance improvement of about 1.8% over the baseline in terms of power reduction. With active camber, simultaneous rotor power reduction of 3.8% and a peak-to-peak pitch-link load reduction of 20% were obtained using a nonharmonic camber deflection deployment schedule with a half-peak-to-peak magnitude of 2∘. The global maximum of power savings observed with active camber was 4.4%, which resulted in an increase of peak-to-peak pitch-link loads by 35% and a required half-peak-to-peak camber deflection magnitude of 3.6∘.

On the enhancement of Savonius Bach-type rotor performance by studying the optimum stator configuration

The purpose of the present study is to enhance significantly the Savonius Bach-type rotor performance. This was achieved by adding a new stator around it. The stator is composed of three parts: shield, obstacle, and guide plate. A comprehensive parametric study was performed in which eight geometrical parameters for the stator were varied. One factor at a time method was utilized to find the stator optimum shape that achieves the maximum rotor power coefficient value. A 2D numerical model was employed using ANSYS Fluent 18.1. The dominant parameters that affect the aerodynamic performance of the turbine were demonstrated. According to the examined conditions, the results revealed that the maximum power coefficient for the rotor with optimum stator configuration achieved a value of 6 times greater than that of the bare rotor. It was concluded from this parametric study that the main effective element in the turbine stator is the stator obstacle. This study may be an important issue for further enhancement of the Savonius turbine's performance.

Cross-comparative analysis of loads and power of pitching floating offshore wind turbine rotors using frequency-domain Navier-Stokes CFD and blade element momentum theory

Reliable predictions of the aero-and hydrodynamic loads acting on floating offshore wind turbines are paramount for assessing fatigue life, designing load and power control systems, and ensuring the overall system stability at all operating conditions. However, significant uncertainty affecting both predictions still exists. This study presents a cross-comparative analysis of the predictions of the aerodynamic loads and power of floating wind turbine rotors using a validated frequency-domain Navier-Stokes Computational Fluid Dynamics solver, and a state-of-the-art Blade Element Momentum theory code. The considered test case is the National Renewable Energy Laboratory 5 MW turbine, assumed to be mounted on a semi-submersible platform. The rotor load and power response at different pitching regimes is assessed and compared using both the high-and low-fidelity methods. The overall qualitative agreement of the two prediction sets is found to be excellent in all cases. At a quantitative level, the high-and low-fidelity predictions of both the mean rotor thrust and the blade out-of-plane bending moments differ by about 1 percent, whereas those of the mean rotor power differ by about 6 percent. Part of these differences at high pitching amplitude appear to depend on differences in dynamic stall predictions of the approaches.

Benchmarking different fidelities in wind turbine aerodynamics under yaw

This paper analyses the aerodynamics of wind turbines under yaw with different modelling fidelities (BEM, BEM with skewed wake model, UVLM and LES-AL). First of all, models are compared in a zero-yaw case to demonstrate their accuracy in prediction of out-of-plane loads and the discrepancy of UVLM in the in-plane loads due to the lack of viscous drag. Secondly, the yaw aerodynamics are described through the advancing/retreating and skewed wake effects, which are appropriately captured by UVLM and LES-AL and lead to an incorrect prediction of the location of maximum and minimum loading along a revolution by BEM. Further, when a skew-wake model is included in BEM, it predicts the correct locations but exhibits overly large loading variations. These predictions are consistent for all yaw angles studied (γ = 10° − 30°). All solvers predict similar decrease of root-bending moments, rotor power and thrust coefficients up to a yaw angle of 10°. However, at larger yaw angles, BEM overpredicts this decrease of coefficients with the yaw angle due to the unsuccessful performance of yaw corrections as opposed to UVLM that inherently accounts for three-dimensional effects. This study demonstrates the need to use computational models that can account for three-dimensional effects in the computation of aerodynamic loads for yaw angles above 10°.

CFD Simulation of Two Tandem Floating Offshore Wind Turbines in Surge Motion

High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulation is employed to investigate the variations in the power performance of two tandem in-line floating offshore horizontal axis wind turbines for the scenario in which the upstream rotor is oscillating in surge motion and the downstream rotor is positioned in a distance of 3D (D: turbine diameter) and is stationary. The rotors are the NREL-5MW reference turbine. The platform surge period and wave amplitude are 9 s and 1.02 m, respectively. The results are presented for 100 full surge periods. It is found that the surge motion of the upstream rotor results in: (i) sinusoidal fluctuations in the power and thrust coefficients (CP and C T) of the upstream rotor with a standard deviation (std) of 9.7% and 5.5%, respectively; (ii) such fluctuations in CP and CT are less regular with a std of 4.2% and 2.8% for the downstream rotor, respectively. A low-frequency oscillating mode with a period nearly 10 times the surge period is also observed for the downstream rotor. The mean Cp and Ct of the downstream rotor are 28.9% and 38.5% of the upstream one.

The 3D effects of a vertical-axis wind turbine: rotor and wake induction

This paper deals with the 3D effects of a vertical-axis wind turbine caused by the tip vortices. In this study, the VAWT rotor is simplified by the infinitely bladed actuator cylinder concept. The loads are prescribed to be uniform and normal to the surface and are distributed between the upwind and downwind half. Depending on the load configuration, the tip vortices are shed at different locations. This causes the wake induction field and the induction at the rotor to be significantly different. The 3D effects cause a power loss that may go up to 15% depending on the load configuration and aspect ratio. Starting from an aspect ratio of 5, the rotor induction and power is approaching 2D results.

Validation of FAST.Farm Against Full-Scale Turbine SCADA Data for a Small Wind Farm

FAST.Farm is a new midfidelity engineering tool developed by the National Renewable Energy Laboratory targeted at accurately and efficiently predicting wind turbine power production and structural loading in wind farm settings, including wake interactions between turbines. FAST.Farm is based on some of the principles of the Dynamic Wake Meandering model—including passive tracer modeling of wake meandering—but addresses many of the limitations of previous Dynamic Wake Meandering (DWM) implementations. Previous FAST.Farm verification studies show the similarities and differences between FAST.Farm and large-eddy simulations for rigid and flexible turbines. In this validation study, FAST.Farm turbine responses are compared to multiturbine measurements from a subset of a full-scale wind farm. FAST.Farm predictions of turbine generator power, rotor speed, and blade pitch for five-turbine simulations are compared to supervisory control and data acquisition results. Results reveal that FAST.Farm generator power mean and standard deviation results reasonably match measured data for upstream and downstream turbines, as well as the mean rotor speed and blade pitch above rated wind speeds. However, FAST.Farm generally underpredicts the mean rotor speed and overpredicts the mean blade pitch below rated operation. These errors are likely related to inaccuracies in the generic controller simulated. Despite controller differences, FAST.Farm predicts the same overall relative rotor power trends for all waked turbines at all wind speeds.

System Stability Analysis in Two Area System Integration with Three Renewable Energy Resources

Stability is the most important feature required in the modern electrical system. In recent years, grid stability problems have been detected due to the rapid growth of electrical and electronic loads. To study the system performance under the effect renewable energy based generating units the kundur’s two area system has been taken as test system. The direct integration of these resources were studied for various instability issues like rotor angle stability, power stability at the generating points of machines and distortion level in the voltage and current waveforms of the grid system. The work has proposed a universal dynamic system optimizing control for system stability enhancement in all the aspects. The MATLAB/SIMULINK environment being the platform for the system designing and implementation, The effects on the two area four machines system has been studied by integrating wind energy system without dynamics optimization control in area 1and then systems with both solar and wind with the dynamic optimization controller at area 2 was developed. The study is further carried to integration of a fuel cell system in area 1 as well. The rotor angle stability and power stability at the point of generation bus was also stabilized by the proposed control in the power system.

A low-power prototype of contactless field power controlled BLAC and BLDC motors

AbstractIn this paper, a new design configuration has been proposed in which a prototype of resonant inductive power transfer-based contactless power transfer to wound rotor has been developed which provides field power to brushless alternating current (BLAC) or brushless direct current (BLDC) motors without the use of permanent magnets in the rotor. Further, wound field in the rotor of DC motor can be powered without carbon brushes. The proposed design facilitates motor performance improvement by adding an extra dimension of field flux control, while the armature circuit is conventionally fed from position detection and commutation schemes. It contains a primary multilayer concentrated coil fed with high-frequency resonating AC supply or switched mode supply. A single layer helical secondary coil coaxially fixed on the shaft receives high frequency wireless AC power transmitted from primary coil. Fast rectifier inside the hollow shaft and DC filter provides the transferred DC power to field terminals in the rotor. It has been verified that rotor power can be varied linearly with linear variation in input DC power with the highest efficiency at the resonant frequency. Available power to the rotor remains invariable with rotational speed and angle, which is a necessary requirement for rotor field. DC voltage on the rotor terminals can be effectively controlled during standstill as well as during rotation at any speed.

Indirect Vector-Controlled Brushless Doubly-Fed Twin-Stator Induction Generator for Wind Energy Conversion Application

Wind energy conversion systems (WECSs) seem certain to play a major part in the world’s energy future due to their known high power capacity. The maximum power tracking is unavoidable due to the wind velocity variation and the non-linear relation with the turbine mechanical power. Commercial wind turbines are commonly coupled to either doubly-fed induction generators (DFIGs), wound rotor synchronous generators (WRSG) or permanent magnet synchronous generators (PMSGs). The DFIG-based WECS has several advantages over others. One of which is the power converter in such systems only deals with rotor power, hence the converter rating can run at reduced power rating. However, DFIG has the famous disadvantage of the presence of slip rings which leads to increased maintenance costs and outage times. Hence, brushless doublyfed induction machines (BDFIMs) can be considered as a viable alternative at the penalty of complicated controller requirement and limited decoupling control capability due to the machine’s non-linearity. In this paper, an enhanced performance indirect vector controller is proposed for WECS based on brushless doubly-fed twin-stator induction generator (BDFTSIG). The presented controller offers (i) simplified implementation, (ii) decoupled active-reactive power control, and (iii) a wide range of operation. The proposed controller performance is investigated under various loading conditions showing enhanced transient and minimal steady-state oscillations in addition to complete active/reactive power decoupling. The rigorous simulation and experimental results verify the claimed controller effectiveness under all possible operating conditions for sub- and super-synchronous speed range.

Performance and Airloads Analyses for a Rigid Coaxial Rotor of High-Speed Compound Unmanned Rotorcrafts

This study investigates the performance and blade airloads for a rigid coaxial rotor of high-speed compound unmanned rotorcrafts. The present compound unmanned rotorcraft uses not only a rigid coaxial rotor, but also wings and propellers for high-speed flights. For the rigid coaxial rotor in this work, CAMRAD II, a rotorcraft comprehensive analysis code, is used to study the performance at a flight speed of up to 250 knots and blade section lift forces at 230 knots. As the flight speed increases, the rotor power decreases; however, the power of propellers increases to overcome the drag force of a rotorcraft in high-speed flight. The effective lift-to-drag ratio of a rotor has the maximum value of about 11.6 which is much higher than the value of the conventional helicopter. The blade section lift forces of the upper and lower rotors at 230 knots show the similar variation trends for one rotor revolution, and the impulses because of the aerodynamic interaction between both rotors are observed. Keywords: 강체 ë™ì¶•ë°˜ì „ 로터, 복합형 무인 íšŒì „ìµê¸°, 성능, ë¸”ë ˆì´ë“œ ê³µë ¥ 하중 Keywords: Rigid Coaxial Rotor, Lift-Offset, Compound Unmanned Rotorcraft, Performance, Blade Airloads

A Highly Effective Fault-Ride-Through Strategy for a Wind Energy Conversion System With a Doubly Fed Induction Generator

This article proposes an improved fault-ride-through (FRT) system for a wind turbine with a doubly fed induction generator (DFIG) that is based on the proper stator voltage control to address symmetrical as well as unsymmetrical and unbalanced grid voltage sags. This is accomplished by adopting a properly modified topology of the conventional wind energy conversion system (WECS) with DFIG that provides the ability to regulate the stator voltage through the system of the rotor power converters. Therefore, significant improvement of the FRT capability is attained, since any oscillations of both the stator and rotor currents that may be caused by the voltage dip can be considerably reduced and they can remain within predefined safety limits. The implementation of the new topology as well as the corresponding control system is cost effective, since no additional hardware is required, and it is accomplished by the reconfiguration of the conventional topology. Selective simulation and experimental results obtained by a high- and low-scale WECS with DFIG, respectively, are presented to validate the effectiveness of the proposed FRT control method and demonstrate the operational improvements.

The energy yield potential of a large tidal stream turbine array in the Alderney Race

This research provides an updated energy yield assessment for a large tidal stream turbine array in the Alderney Race. The original array energy yield estimate was presented in 2004. Enhancements to this original work are made through the use of a validated two-dimensional hydrodynamic model, enabling the resolution of flow modelling to be improved and the impacts of array blockage to be quantified. Results show that a range of turbine designs (i.e. rotor diameter and power capacity) are needed for large-scale development, given the spatial variation in bathymetry and flow across the Alderney Race. Array blockage causes a reduction in flow speeds in the array of up to 2.5 m s-1, increased flow speeds around the array of up to 1 m s-1 and a reduction in the mean volume flux through the Alderney Race of 8%. The annual energy yield estimate of the array is 3.18 TWh, equivalent to the electricity demand of around 1 million homes. The capacity factor of the array is 18%, implying sub-optimal array design. This result demonstrates the need for turbine rated speed to be selected based on the altered flow regime, not the ambient flow. Further enhancement to array performance is explored through increases to rotor diameter and changes to device micro-siting, demonstrating the significant potential for array performance improvement. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Aeromechanics Analyses of a Modern Lift-Offset Coaxial Rotor in High-Speed Forward Flight

This study conducts an aeromechanics analysis of a modern lift-offset coaxial rotor in high-speed flight. A lift-offset coaxial rotor of the Sikorsky X2 technology demonstrator (X2TD) is considered for the present study. For the analyses of rotor performance, blade airloads, and hub vibratory loads, a rotorcraft comprehensive analysis code, CAMRAD II, is used. For the rotor performance analysis at a flight speed of up to 250 knots, which is the maximum level flight speed of X2TD in flight test, the present prediction of rotor power is compared well with the flight test data. For the blade section airload analysis at 250 knots, the impulses of the section lift forces are observed because of the aerodynamic interactions between the upper and lower rotors. The 4/rev hub vibratory loads are predicted, and its variation trend is moderately compared with the previous analysis results. Furthermore, the rotor vibration index increases significantly in high-speed flight with an increase in the flight speed, and this is well correlated with the previous rotorcraft comprehensive analysis result. This study shows that the present modeling and analysis techniques are appropriate to analyze the aeromechanics, including the performance, blade airloads, and hub vibratory loads of a modern lift-offset rotor in high-speed flight.