Fixed-pitch Blades Research Articles

Power and thrust control by passive pitch for tidal turbines

Tidal turbines operate in unsteady and non-uniform flows and thus experience large load fluctuations. In this study, we explore the effects of changes in freestream velocity and yaw misalignments on the performance of a turbine equipped with passively pitching blades. We compare the experimental results from a 1.2-m-diameter turbine in a recirculating open channel facility and numerical results based on blade-element-momentum theory. The thrust remains approximately constant with increasing free stream speed, meaning that the structural load on the turbine is minimised. While not held constant, the power increases at less than half of the rate of a turbine with fixed pitch blades. This finding shows that power can be controlled effectively by changing the tip speed ratio, allowing, for example, the power to be capped at a rated value by marginally increasing the angular velocity. Furthermore, the power and thrust are found to be negligibly sensitive to yawed incoming flow compared to a fixed-pitch turbine. Overall, these results demonstrate that passive pitch is a viable alternative to active pitch, and pave the way for further development of this technology.

Numerical study on wake recovery of vertical-axis wind turbines through fixed blade pitch offsets: H vs X rotors

This study assesses the wake recovery mechanism between an H-type Darrieus and an X-type vertical-axis wind turbine, named H-Rotor and X-Rotor respectively for different blade pitch offset configurations. The analysis is conducted in OpenFOAM using the actuator line method to model the turbines to present qualitative (velocity and vorticity contours) and quantitative (available power) studies for three different fixed blade pitch offsets. The results demonstrate that the H-Rotor recovers the wake much faster than the X-Rotor at positive pitch offsets. Overall, applying fixed pitch offsets to VAWT blades helps recover the wake faster than having no blade pitch offset.

Aerodynamic model comparison for an X-shaped vertical-axis wind turbine

Abstract. This article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free vortex wake model (from CACTUS), a free vortex wake model with turbulent vorticity (from QBlade), a blade-resolved unsteady Reynolds-averaged Navier–Stokes (URANS) model, and a lattice Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other, and the differences when compared against the URANS and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor, and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.

Understanding the fixed pitch RPM-controlled rotor modeling for the conceptual design of UAM vehicles

AbstractMost of the introduced eVTOL models in the UAM market utilize electrically driven rotors with fixed pitch blades, controlled by their rotational speed. This design approach brings new features and challenges that need to be considered during the conceptual design stage, taking new aspects into account, among them the rotor dynamic response. This paper presents an initial parameterization of the rpm-controlled rotors along with the electric motor, based on the conducted literature research. Several isolated rotor variants are modeled and analyzed in terms of performance, and the dynamic response of the rotor rotational speed. The relations between aerodynamics and inertia are discussed, and their effects on the rotor dynamic response are elaborated on.

Development of a Hardware-in-the-Loop Platform for the Validation of a Small-Scale Wind System Control Strategy

The use of renewable energies contributes to the goal of mitigating climate change by 2030. One of the fastest-growing renewable energy sources in recent years is wind power. Large wind generation systems have drawbacks that can be minimized using small wind systems and DC microgrids (DC-µGs). A wind system requires a control system to function correctly in different regions of its operating range. However, real-time analysis of a physical wind system may not be feasible. An alternative to counteract this disadvantage is using real-time hardware in the loop (HIL) simulation. This article describes the implementation of an HIL platform in an NI myRIO 1900 to evaluate the performance of control algorithms in a small wind system (SWS) that serves as a distributed generator for a DC-µG. In the case of an SWS, its implementation implies nonlinear behaviors and, therefore, nonlinear equations, and this paper shows a way to do it by distributing the computational work, using a high-level description language, and achieving good accuracy and latency with a student-oriented development kit. The platform reproduces, with an integration time of 10 µs, the response of the SWS composed of a 3.5 kW turbine with a fixed blade pitch angle and no gear transmission, a permanent magnet synchronous generator (PMSG), and a three-phase full-bridge AC/DC electronic power converter. The platform accuracy was validated by comparing its results against a software simulation. The compared variables were the PMSG currents in dq directions, the turbine’s angular speed, and the DC bus’s voltage. These comparisons showed mean absolute errors of 0.04 A, 1.9 A, 0.7 rad/s, and 9.5 V, respectively. The platform proved useful for validating the control algorithm, exhibiting the expected results in comparison with a lab-scale prototype using the same well-known control strategy. Using a well-known control strategy provides a solid reference to validate the platform.

Development of a passive blade-pitch mechanism to reduce the loads on a tidal turbine in high-flow conditions

Maintenance of tidal turbines is expensive, and relies upon suitable weather conditions and vessel availability, which can lead to costly delays. Turbine reliability can be improved by eliminating complexity in the design. The use of fixed-pitch blades, for example, can greatly reduce maintenance costs by avoiding complex active blade-pitch mechanisms which are the main source of failure in wind turbines.
 Pitching rotor blades to feather in high-flow conditions is, however, the most effective means of reducing loads on the rotor. Turbines with fixed-pitch blades therefore necessarily have smaller rotor diameters compared to those with active-pitch systems, in order to keep the thrust force, flap-wise bending moment, and power output within allowable limits. In low-flow conditions, power output is proportional to the square of the rotor diameter, so turbines with small diameter rotors capture less energy over their lifetime.
 This study seeks to develop a novel passive-pitch mechanism for tidal turbine blades. This mechanism must be reliable, and should act to reduce the loads on the rotor in high-flow conditions so that a larger diameter rotor can be installed, increasing power capture.
 An initial design for a passive-pitch mechanism, which is actuated by the hydrodynamic forces developed by the rotor such that the blades pitch-to-feather in high-flow conditions, has been developed. The design is compatible with proven passive reversible rotor blade technology, eliminating the requirement for a maintenance sensitive yaw mechanism.
 Since initial concept design, work has been undertaken to build a tool, based on NREL’s OpenFAST blade element momentum code, which models the forces acting on the rotor blades, coupled with the mechanical response of the passive blade-pitch mechanism. This tool has been used to predict the performance of the turbine for a range of operating conditions, allowing the influence of parameters such as blade geometry, rotor diameter and passive-pitch response to be analysed in terms of rotor loading and turbine performance.
 Initial analysis suggests that installing blades with a passive-pitch mechanism could reduce the loads on the rotor in high-flow conditions down to just 25% of the loads that would act on an equivalent rotor with fixed-pitch blades. This would allow larger diameter rotors to be installed which could improve annual energy yield by over 40% at a typical site.
 Despite these significant potential performance improvements, the challenges of developing a passive blade-pitch mechanism for tidal turbines are not well understood, as tidal developers have all so far implemented fixed or active pitch mechanisms. These challenges will be addressed throughout the remainder of this study, which will involve detailed design followed by physical tank testing.

Control and dynamic analysis of a 10 MW floating wind turbine on a TetraSpar multi-body platform

The role of wind turbine controllers in stabilizing floating offshore wind turbines (FOWTs) is critical in the above-rated operational regions. In this paper, three types of control methods are tested on a 10 MW FOWT mounted on a TetraSpar multi-body platform to numerically investigate the rotor and platform dynamics. A validated aero-hydro-servo-elastic coupling tool based on FAST and AQWA is used to analyse the integrated floating wind turbine model. The results show that the baseline controller designed for the onshore application yields the highest blade pitch and platform pitch fluctuations primarily due to the negative damping effect. Conversely, the detuned controller obtained by derating the control gains reduces these fluctuations and sufficiently mitigates the damping problem. It is also observed that the instabilities caused by the higher frequency baseline controller and the resulting structural penalties are reduced when the detuned controller is employed. On the other hand, the open-loop type fixed blade pitch control method resulted in the highest rotor speed and surge translation. Further analysis found that the tension responses on some of the tendons are dominated by the coupled pitch mode when implementing the detuned and the fixed blade pitch control options.

Integrated Design and Experimental Validation of a Fixed-Pitch Rotor for Wind Tunnel Testing

In this paper we report about the design and validation of a 1.2 m wind turbine rotor with fixed blade pitch. The wind turbine is a scaled version of the DTU 10 MW. Integrated design of dimensional scaling laws, blade aerodynamics, and turbine control is carried out to reproduce blade loading and interaction with atmospheric boundary layer of the reference turbine, despite challenges posed by the great reduction in chord-based Reynolds number. The rotor is verified with numerical simulations in OpenFAST and wind tunnel testing. The servo-aerodynamic design approach proposed in this article is shown to be successful for small-scale wind turbine models for use in experiments about wakes and floating wind.

An Experimental Study of the Aeroacoustic Properties of a Propeller in Energy Harvesting Configuration

The aim of the present manuscript is to investigate the noise footprint of an isolated propeller in different flight configurations for the propulsion of a hybrid-electric aircraft. Experimental tests were performed at the Low-Turbulence Tunnel located at Delft University of Technology with a powered propeller model and flush-mounted microphones in the tunnel floor. The propeller was investigated at different advance ratios in order to study the noise impact in propulsive and energy harvesting configurations. For brevity, this work only reports the results at the conditions of maximum efficiency in both propulsive and energy harvesting regimes, for a fixed blade pitch setting. Comparing these two configurations, a frequency-domain analysis reveals a significant modification in the nature of the noise source. In the propulsive configuration, most of the energy is related to the tonal noise component, as expected for an isolated propeller; however, in energy harvesting configuration, the broadband noise component increases significantly compared to the propulsive mode. A more detailed analysis requires separation of the two noise components and, for this purpose, an innovative decomposition strategy based on proper orthogonal decomposition (POD) has been defined. This novel technique shows promising results; both in the time and in the Fourier domains the two reconstructed components perfectly describe the original signal and no phase delays or other mathematical artifices are introduced. In this sense, it can represent a very powerful tool to identify noise sources and, at the same time, to define a proper control strategy aimed at noise mitigation.

Comparative Study of 3-Bladed and Sсissors Tail Rotors Aerodynamics in Axial Flow

The paper is focused on the numerical study of the aerodynamic characteristics of 3-bladed and scissors tail rotors used on Mi-8/17 series helicopters. The hovering and axial flow modes, including the Vortex Ring State (VRS) area are considered. The free wake model developed by authors is used in the research. The study has been performed for two approaches: with a fixed blade pitch angle and with a fixed time-average thrust value. The dependences of the rotor thrust and torque coefficients vs. axial flow velocity and vs. number of revolutions of a rotor have been obtained. The vortex wake shapes and flow images via streamlines have been analyzed. The comparative analysis of the results has been performed. Scissors rotor demonstrates better aerodynamic characteristics in comparison with 3-bladed rotor. Among them: increased hovering efficiency, lower level of thrust and torque pulsations in the VRS modes, smaller power consumption while constant thrust. The authors came to the conclusion about the greater efficiency of the aerodynamic design of scissors tail rotor compared to 3-bladed rotor both in hover and in the VRS modes area.

Numerical and Experimental Investigation into the Aerodynamic Benefits of Rotorcraft Formation Flight

The use of formation flight to achieve aerodynamic benefit applied to rotorcraft has, unlike its fixed-wing counterpart, received little attention in the literature. This document presents a proof-of-concept of rotorcraft formation flight from two independent investigations: a numerical study of a fully articulated helicopter influenced by an upstream helicopter wake and a wind-tunnel experiment featuring two small-scale helicopter models with fixed-pitch blades. Both cases feature a representation of two helicopters in a diagonal, staggered formation aligned on the advancing side of the main rotor, but do not simulate directly comparable flight conditions. The vertical and lateral alignment of the two helicopters is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yield an estimated maximum total power reduction for the secondary aircraft of approximately 24%, while the numerical models yield reductions between 20% and 34% dependent on flight velocity. Both experiments predict a higher potential for aerodynamic benefit than generally observed for fixed-wing formations, which is attributed to the asymmetric velocity profile induced by the wake of the upstream rotor. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas, rather than tip-to-tip alignment, as a result of the circular rotor disk area. Experimental data show an optimal vertical alignment of the secondary rotorcraft below the primary, due to the self-induced vertical displacement of the rotor wake, which is absent from the numerical results due to the application of a flat wake assumption. The results show a promising potential for rotorcraft formation flight, though due to the limited nature of the models used, conclusions cannot be generalized. The potential aerodynamic benefit indicated by the present study invites further research in the field of rotorcraft formation flight.

Dynamic responses of a 10 MW semi-submersible wind turbine at an intermediate water depth: A comprehensive numerical and experimental comparison

A series of experiments and numerical simulations were conducted to investigate the dynamic responses of a 10 MW floating wind turbine concept named SPIC, at an intermediate water depth of approximately 60 m. A fully coupled numerical model was established using the aero-hydro-servo-elastic-moor method. The experimental and numerical dynamic responses under operational conditions, including irregular waves, steady and turbulent winds, and combined wind–wave conditions, generally agree well. The mean values are well predicted, and the powers at motion resonant frequencies, wave frequency and tower vibration frequency are captured. Discrepancies between the numerical and experimental results are further addressed by in-depth numerical analyses. Under turbulent winds, the discrepancies of dynamic responses at 1P and 2P frequencies are induced mainly by individual blade installation error, while the discrepancies at 3P components are caused mainly by the nonuniformity of the wind field. In addition, in the turbulent wind case with variable blade pitch angles and rotor speed control system, the mean tower-base bending moment is smaller, and the low-frequency responses are larger than those in the case with fixed blade pitch angles and a constant rotor speed. The reveal of uncertainties is helpful for further guiding the model test of floating wind turbines.

Performance Analysis and Optimization of a Vertical-Axis Wind Turbine with a High Tip-Speed Ratio

In this work, the aerodynamic performance and optimization of a vertical-axis wind turbine with a high tip-speed ratio are theoretically studied on the basis of the two-dimensional airfoil theory. By dividing the rotating plane of the airfoil into the upwind and downwind areas, the relationship among the angle of attack, azimuth, pitch angle, and tip-speed ratio is derived using the quasi-steady aerodynamic model, and aerodynamic loads on the airfoil are then obtained. By applying the polynomial approximation to functions of lift and drag coefficients with the angle of attack for symmetric and asymmetric airfoils, respectively, explicit expressions of aerodynamic loads as functions of the angle of attack are obtained. The performance of a fixed-pitch blade is studied by employing a NACA0012 model, and influences of the tip speed ratio, pitch angle, chord length, rotor radius, incoming wind speed and rotational speed on the performance of the blade are discussed. Furthermore, the optimization problem based on the dynamic-pitch method is investigated by considering the maximum value problem of the instantaneous torque as a function of the pitch angle. Dynamic-pitch laws for symmetric and asymmetric airfoils are derived.

A Ffowcs Williams-Hawkings numerical aeroacoustic study of varied and fixed-pitch blades of an H-Rotor vertical axis wind turbine

The effects of sound pressure level at two observation positions of a fixed and varied blade pitch angle at Low-Mach unsteady incompressible Reynolds-Average Navier-Stokes flow approach, on an H-rotor Vertical Axis Wind Turbine. The objective of the study is to compare the noise dissipation and output power/energy of the airfoil blades design of the vertical axis wind turbine in residential zones. The Ffowcs Williams-Hawkings (FHWH) techniques were applied to validate the output noise and vortex shedding of the different angles of attacks (AoA). The study postulated that the time history of the calculated sound pressure level at two observers positions: the aeroacoustic, blade vortex interaction noise, flow separations, dynamic stall experience from varied angled of attacks are found to produces less noise and vortex shedding compared to the fixed angle of attack.

An investigation on the aerodynamic performance of a co-axial contra-rotating vertical-axis wind turbine

In this study, a conceptual co-axial contra-rotating vertical axis wind turbine (CR-VAWT) is conceived by splitting the single conventional vertical axis wind turbine (C-VAWT) blade length into two equal lengths forming two rotors separated by a small distance about the same axis of rotation. The available literature lacks much systematic investigation to find the best possible numerical setting and to extract an in-depth understanding of dominant factors limiting the aerodynamic performance of the CR-VAWT using CFD, constitutes the primary focus for the present study. Results from the CFD were compared with the analytical model formulated using FAST along with literature results. The power output of the CR-VAWT agrees well in the low wind speed and variation increases gradually as the wind speed increases compared to C-VAWT, however, the thrust is in good agreement at all wind speeds. The wake velocity directly behind the C-VAWT and CR-VAWT follows the general trend but differences were noticed to a distance accordingly, with the wind speed. The power performance reduces gradually with increasing speed for a fixed blade pitch angle. The present adopted concept of CR-VAWT are found to have negligible aerodynamic torque and side-side force action at the tower base.

Design of GEMSTAR tidal current fixed pitch rotor controlled through a Permanent Magnet Generator (PMG) de‐fluxing technique

This work reports a study on a tidal current system named GEMSTAR, composed of two hydrokinetic turbines supported by a floating submerged structure. A flexible mooring cable links the floating support to the seabed allowing the free rotation of the system and its alignment to changing current direction. The work is mainly focused on the design of the turbine blade with two objectives: keeping a constant power output above the rated power condition and limiting the thrust increase between rated and maximum operating speed. A fixed pitch blade was considered, in order to reduce the costs associated to a complex pitch control mechanism. An analytical procedure for blade design is proposed and validated by comparison to other numerical methods. Above the rated current speed, the power is held constant while the rotational speed of the turbine and of the generator increases, assuming an over-speeding control strategy. The adopted design configuration relies on the possibility of the generator to operate in de-fluxing working conditions. As reported in the last part of this study, a numerical model of the generator has also been developed and applied to a test case in order to analyse the electric behaviour of the system.

Effect of blade attachments on the performance of an asymmetric blade H-Darrieus turbine at low wind speed

ABSTRACT Wind energy is a significant source of renewable energy and vertical axis wind turbine (VAWT) is one of the advanced wind energy harvesters. Maximum extraction of energy from wind harvesters is a major goal. Researchers are faced with the challenge of enhancing the performance of VAWTs in low wind speed conditions. To this end, researchers have contributed substantially by doing research on the blade design parameters thereby generating innovative and improved VAWT designs. However, there is hardly any work on the performance augmentation of H-Darrieus VAWT with blade attachments on asymmetric blades, which is the main objective of this work. Further, this work investigates the combined effect of turbulence intensity and flow dissimilarity on turbine performance for low wind speed condition. In this paper, blade attachment of length 20 cm (equal to 1/2.5 times blade span) is attached to asymmetric NACA 63–415 blade turbine having fixed blade pitch angle (+5°). NACA 63–415 blade profile is one of the prominent asymmetric series of NACA blades; hence it is considered in this work. The performances of seven different configurations of the turbine with blade attachments have been investigated experimentally at 5.0 m/s and 6.0 m/s using wind tunnel testing. The performance of the VAWT design is systematically improved by changing the number and position of these attachments on the turbine blades. The result shows that both side attachments on turbine blade on its top position have maximum power coefficient (Cp) of 0.13 at tip speed ratio (TSR) 1.8 at wind speed 6.0 m/s among all the turbine configurations tested. There is an improvement of power coefficient by 4.34% when attachments are placed at top of the turbine blade compared to turbine blade without attachment at wind speed 6.0 m/s. Further, maximum power coefficient is increased by 3.28% when turbine is operated from 5.0 m/s to 6.0 m/s for both side attachments at top of the turbine blade. Moreover, due to the combined effect of turbulence intensity and flow dissimilarity, the power coefficient of the turbine configuration having both side attachments has also improved a little. In this case, an improvement of 0.6% in power coefficient of turbine having both side attachments on top of the blade is achieved. As the turbine is a prototype having smaller size hence the effect of turbulence intensity and flow dissimilarity is less, in case of actual turbine of bigger size the effect of turbulence intensity and flow dissimilarity will be more and hence the percentage enhancement of power coefficient will also be more. Hence, it can be concluded that a turbine blade with higher degree of thickness (by using blade attachments) at top portion will increase the performance of an asymmetric blade H-Darrieus VAWT.

Performance Evaluation of a 1 kW Variable Pitch-Straight Blade Vertical Axis Wind Turbine

Abstract This research work represents a study of the design, analysis, and experimental study of a 1 kW variable pitch-straight blade vertical axis wind turbine (VAWT) using natural fiber reinforced composite. Wind turbine which is an emerging technology is of great interest for researchers nowadays. The VAWT was chosen for this study because of its numerous advantages over horizontal axis wind turbine (HAWT). A new concept of variable pitch was implemented by the introduction of a pitching mechanism associated with the turbine blades which helps the blade to maximize the generation of torque and power. For this purpose, the straight blade H-rotor design was chosen. The analytical calculations were performed for variable pitch and the fixed pitch blade followed by the computer aided design modeling of the rotor exhibiting the variable pitching mechanism. Computational fluid dynamics (CFD) analysis of the blade at the azimuth position of 0–360 deg was performed and the CFD results were imported into static structural module of ansys for the finite element analysis of the blade. The blade was 3D-printed at a reduced scale and tested in a wind tunnel for aerodynamic properties including lift, drag, and aerodynamic forces. A comparison was done between the analytical, software, and experimental values. Furthermore, basalt fiber which is a natural fiber was used as the material for the turbine blade and analysis was performed to obtain high strength to weight ratio of the composite material. The structure was analyzed under the damage tolerance study to determine for how long the structure can bear damage. The experimental results showed a good agreement with the analytical and numerical values. The introduction of the variable pitching mechanism resulted in an increase in the cumulative torque as compared with the fixed pitching mechanism which in turn enhanced the resulting power.

Lifetime Analysis of IGBT Power Modules in Passively Cooled Tidal Turbine Converters

Thermal cycling is one of the major reasons for failure in power electronic converters. For submerged tidal turbine converters investigating this failure mode is critical in improving the reliability, and minimizing the cost of energy from tidal turbines. This paper considers a submerged tidal turbine converter which is passively cooled by seawater, and where the turbine has fixed-pitch blades. In this respect, this study is different from similar studies on wind turbine converters, which are mostly cooled by active methods, and where turbines are mostly pitch controlled. The main goal is to quantify the impact of surface waves and turbulence in tidal stream velocity on the lifetime of the converter IGBT (insulated gate bipolar transistor) modules. The lifetime model of the IGBT modules is based on the accumulation of fatigue due to thermal cycling. Results indicate that turbulence and surface waves can have a significant impact on the lifetime of the IGBT modules. Furthermore, to accelerate the speed of the lifetime calculation, this paper uses a modified approach by dividing the thermal models into low and high frequency models. The final calculated lifetime values suggest that relying on passive cooling could be adequate for the tidal converters as far as thermal cycling is concerned.

Blade Tip-Vortices of a Four-Bladed Rotor with Axial Inflow

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)'s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Göttingen. Algorithms deriving the vortex position, swirl velocity, circulation, and core radius were implemented. Hover-like conditions with a fixed blade pitch were analyzed giving further physical insights of the static vortex system. These results are used to understand the vortex development for the unsteady pitching conditions, which can be described as a superpositioning of static vortex states. The use of a zonal detached-eddy simulations approach improved physical modeling of the vortex development by resolving finer scales than URANS. Trimmed cases agree well with differences less than 0.5% in the circulation and swirl velocity.