Experimental Study Of Active And Passive Blade Pitch Control Strategies For Axial Flow Marine Current Turbines

Cost and reliability remain among the main barriers limiting widespread adoption ofriverine, estuarine, or ocean current turbine power generation. In particular, structural loads are significantly greater than for wind turbines with equivalent power output, which contributes to higher costs. Compounded with uncertainties about hydrodynamic loads, this can contribute to structural failure or excessive and expensive safety factors. Consequently, control strategies to mitigate structural loads and reduce cost are of considerable importance. Load reduction is of particular interest when currents exceed a certain threshold (i.e., theturbine-specific “rated speed”), and a control strategy is implemented to maintain a constant power output. Most fixed-pitch turbines will use a speed control strategy, increasing or decreasing the rotation rate to achieve the efficiency required for power regulation. However, these “overspeed” and “underspeed” control strategies correspond to large increases in thrust or torque, respectively, that require overdesigning the turbine blades or generator. Blade pitch control circumvents this trade-off, as decreased angles of attack simultaneously reduce thrust and torque. This does, however, require actuators to change blade pitch. While active pitch control is the conventional standard for wind turbines in these above-rated conditions, similar variable blade pitch mechanisms have not yet been uniformly adopted by marine current technology developers due to the higher cost of inspection, maintenance, and repairs relative to wind turbines. For this reason, passive adaptive blade pitch control, in which blades are designed to elastically deform under load without an actuator, sensor, or control logic, is conceptually attractive. Improved understanding of the loading associated with both speed and pitch control strategies is critical to optimizing a design for minimal cost and maximal reliability. Therefore, the overarching goal of this work is to experimentally investigate active and passive pitch control methods, characterize their potential for load reduction, and establish appropriate scaling relations for passive adaptive blades. The three underlying objectives supporting this goal are outlined below. The first objective is to demonstrate active blade pitch control in above-rated flow conditionsand compare the measured turbine loads to those observed with overspeed and underspeed control in order to develop our understanding of the trade-offs associated with each. To this end, we experimentally characterized power performance and turbine loading over a range of blade pitch settings and tip-speed ratios for a three-bladed axial-flow turbine. We then implemented a control strategy to maintain power output in time-varying currents using blade pitch control and compared the turbine performance under this control strategy to overspeed and underspeed control strategies for a fixed pitch turbine. The experiments were conducted with a laboratory-scale 0.45-m diameter turbine in an open channel flume with a 35% blockage ratio. During pitch characterization experiments, inflow velocity was maintained at 0.8 m/s with 4% turbulence intensity. During time-varying inflow experiments, currents varied from 0.7-0.8 m/s over a 20-minute period, while a proportional controller regulated either blade pitch or rotor speed, and we recorded turbine power output and turbine loads. In this velocity range, where turbine performance is independent of Reynolds number, we demonstrate that pitch control substantially reduces torque requirements relative to underspeed control and streamwise turbine loads relative to overspeed control. Additional tests were conducted for underspeed control and pitch control in a Reynolds-dependent regime with time-varying inflow between 0.4-0.5 m/s and 0.5-0.6 m/s. These cases suggest that blade pitch control could provide even greater benefits relative to speed control in small-scale applications. The second objective is to develop our understanding of passive adaptive blade fabricationand the effect of fiber orientation to inform a passive pitch control design. By tailoring the ply angle in a unidirectional carbon fiber blade, a desired twist can be induced in response to bending of the blade under load. In developing this form of passive adaptive control, a fundamental question is how to non-dimensionalize the fluid-structure interaction to make laboratory-scale experiments relevant to full-scale applications. To address these questions, we first conducted an experimental investigation into the effect of fiber angle on blade performance and blade deformation during turbine operation. The composite blades were fabricated with 0°, 2.5°, 5°, and 10° fiber orientations, where a positive fiber orientation results in a reduced angle of attack as load increases (i.e., a “pitch-to-feather” control strategy). Blades were tested in a recirculating flume at 0.7 m/s (Rec = 5.3 · 104 − 2.0 · 105) while measuring force and torque on the rotor. Simultaneously, a high-speed camera observed in-situ deflection and twist at the blade tip. Results show a greater reduction in CP and CT for blades with larger fiber orientations relative to the neutral blade set, while even small fiber orientations were observed to limit thrust at high tip-speed ratios. To explore the correct non-dimensional scaling for this physical process, we performed a set of Cauchyscaled experiments using blades with identical bend-twist couplings but different bending stiffness. These results demonstrate that the Cauchy number is a meaningful parameter for scaling passive adaptive current turbine blades and to model steady-state hydrodynamic and hydroelastic behavior. The third and final objective is to implement passive pitch control to develop our understandingof the trade-offs between speed, active pitch, and passive pitch control methods. Two passive blade pitch control strategies for the same lab-scale turbine were developed and tested experimentally in a recirculating flume. The goal of the control is to regulate mechanical power, while minimizing rotor loads, when flow conditions exceed the rated condition. Both strategies used the 5° fiber blade set from the aforementioned study. One control strategy combined passive adaptive blades with overspeed control (actuating rotational speed above the tip-speed ratio corresponding to peak efficiency) while the other combined passive adaptive blades with active pitch control (actuating blade pitch using motors at the blade root). Both strategies were implemented in linearly increasing inflow from 0.7 m/s to 0.8 m/s and compared to control strategies using rigid, aluminum blades under the same flow conditions. The passive adaptive blades combined with active pitch control show no improvement in steady-state load reductions relative to rigid blades used with active pitch control. However, the passive adaptive blades combined with overspeed control show reduced torque and only a 12% increase in thrust relative to the rated flow condition. This indicates that passive adaptive blades combined with overspeed control can be an effective strategy in currents above the rated flow speed, removing the need for an active pitch mechanism in some applications. In addition to measuring turbine loads, deflection and twist of the passive adaptive blades during experimental testing were observed using a high-speed camera to support our understanding of the bend-twist behavior during turbine operation over a range of flow speeds, rotation rates, and preset pitch angles. Overall, active and passive pitch control strategies for Region III are shown to offer significantload reductions in thrust and torque relative to rigid blade speed control strategies. While controller selection is discussed primarily relative to their associated loads, we discuss additional considerations including blade design, channel blockage, range and frequency of flow variation, and Reynolds-number. These discussions underline the value of future investigations into active and passive pitch control for smoothing high-frequency loads and scaling between lab- and full-scale passive adaptive rotors, among other work.

Marine hydrokinetic (MHK) turbines are quickly becoming a viable and valuable method of generating renewable energy from ocean, tidal, and river currents. Modern MHK turbine blades are typically constructed from fiber reinforced polymer (FRP) composites, which provide superior strength- and stiffness-to-weight ratios and improved fatigue and corrosion resistance compared to traditional metallic alloys. Furthermore, it is possible to hydroelastically tailor the design of an FRP composite blade by manipulating the anisotropic nature of the material, creating a load-dependent adaptive pitch mechanism. With this strategy, the blade geometry is able to passively adjust to the instantaneous inflow, and system performance can be modified over the expected range of operating conditions. Adaptive blade designs have demonstrated the potential to increase performance, reduce hydrodynamic instabilities, and improve structural integrity in aerospace and other marine applications; however, previous research specific to adaptive MHK turbine blades has been preliminary. Further work is needed to better understand and model the behavior of these systems. To that end, the research presented here combines numerical and experimental modeling to develop greater insight into the potential benefits to be gained by the use of adaptive pitch MHK turbine blades. In this work, a well-validated boundary element method-finite element method solver is used to develop a numerical strategy for predicting the performance and structural response of adaptive turbine blades under a wide range of site-specific operating conditions. The behavior of adaptive MHK turbine blades under normal as well as cavitating conditions is analyzed; results suggest numerous advantages possible with the use of adaptive pitch blades. Following the numerical study, an experimental program is outlined in which a flume-scale turbine system is tested under steady and fluctuating inflow conditions. Loading and performance trends found in the experimental study agree well with numerical predictions. Finally, numerical and experimental results are synthesized into a complete analysis of the potential benefits to be gained with the use of adaptive blades in MHK turbine systems. Future research directions are identified with the goal of further evolving adaptive blade technology.

This thesis investigates the use of blade-pitch control and real-time wind measurements to reduce the structural loads on the rotors and blades of wind turbines. The first part of the thesis studies the main similarities between the various classes of current blade-pitch control strategies, which have to date remained overlooked by mainstream literature. It also investigates the feasibility of an estimator design that extracts the turbine tower motion signal from the blade load measurements. In turn, the second part of the thesis proposes a novel model predictive control layer in the control architecture that enables an existing controller to incorporate the upcoming wind information and constraint-handling features. This thesis provides essential clarifications of and systematic design guidelines for these topics, which can benefit the design of wind turbines and, it is hoped, inspire the development of more innovative mechanical load-reduction solutions in the field of wind energy.

"The purpose of this study was to determine by analysis and full-scale experiments, whether a two-bladed wind turbine rotor with passive cyclic pitch variation, automatically yawed for rotor torque and speed control, is potentially cost-effective for wind energy conversion." cf. Document Control Sheet.

Wind turbines will play a crucial role in the global energy transition from a fossil fuel-based world to a renewable-based one. Horizontal-axis wind turbines (HAWTs) currently dominate the commercial sector, although a recent resurgence in interest and research has shown exciting opportunities for the future of vertical-axis wind turbines (VAWTs). Unique cyclical fluid physics results in additional complexity to VAWT blade aerodynamics. Unlike their horizontal-axis counterparts, the rotating blades of a VAWT produce varying torque depending on their location in the circular cycle. The aerodynamic relationship between the incoming wind flow and blade motion is such that the peak power is extracted when the blade and incoming wind are nearly perpendicular to each other during the windward side of the rotor. Recognizing this, the recently conceived dual-vertical-axis wind turbine (D-VAWT) extends a typical VAWT's windward region by having the blades rotate about two vertical axes. Introduced is a new path of purely rectilinear motion connecting the two axes, wherein the blade is designed to achieve optimal aerodynamic efficiency. Initial investigations into the D-VAWT's operation shows promising potential, with power coefficient values in the range of the most efficient VAWTs and even HAWTs. The current study seeks to further improve the performance of the D-VAWT through the incorporation of active blade pitch control throughout the blade path's distinct rotation and rectilinear regions. Computational fluid dynamics (CFD) is used to model a single-blade D-VAWT, and a user-defined method is devised to implement the blade pitch actuation as a function of cycle time location and blade centroid position. Numerical results reveal that strategic blade pitching can indeed increase performance in a specific region of the D-VAWT cycle, however the improvement can create undesired impacts on the flow field in other regions of the rotor. Emphasis is focused on the upstream and downstream flow interaction during active-pitch operation, offering important physical insight to the D-VAWT and VAWTs of similar geometric sizing.

Green and renewable energy technologies are becoming more and more necessary as demand for energy grows exponentially around the world. Recently, there has been increased interest in using marine hydrokinetic turbines to generate energy from ocean currents and tidal flows. The blades of these turbines are slender and are subjected to large, dynamic fluid forces; for that reason they are typically constructed from fiber-reinforced composites. The bend-twist deformation coupling behavior of these materials can be hydroelastically tailored such that the pitch angle of the blades will passively change to adapt to the surrounding flow, creating an instantaneous reaction that can improve system performance over the expected life of the turbine. Potential benefits of this passive control mechanism include increased lifetime power generation, reduced hydrodynamic instabilities, and improved load shedding and structural performance. There are practical concerns, however, that increase the complexity of the design of these bend-twist coupled blades. Large inflow variations in viable locations for turbine implementation combined with system component limitations such as restrictions on the generator and maximum rotational speed require consideration of practical and site-specific constraints. Using a previously validated boundary element method-finite element method solver, this work presents a numerical investigation into the capabilities of passive pitch adaptation under both instantaneous and long-term variable amplitude loading to better describe potential benefits while considering practical design and operational restrictions.

Wind turbines are designed to operate optimally at a certain set of ambient conditions, termed the design point. An unavoidable consequence of wind turbine design is that away from this point, efficiency can drop drastically which inhibits energy conversion, especially in highly varying winds. Numerous strategies have been employed, for example blade pitch and torque control systems, which mitigate these losses, but such systems are economically costly and are usually only present in larger scale applications. For small Horizontal-Axis Wind Turbines (HAWTs) as well as most Vertical Axis Wind Turbines (VAWTs), pitch control strategies in particular are either too complex or simply not economically feasible. Recent research into flexible bladed wind turbine rotor design has shown that continuous, passive blade morphing can increase aerodynamic lift, reduce drag, and even delay stall for two-dimensional airfoil sections. These studies, however, do not take into account centrifugal or gravitational loadings which can change the morphing direction of the flexible material. In this thesis, a fully three-dimensional Fluid-Structure Interaction (FSI) solver is developed and used to simulate a flexible HAWT rotor, with comparisons to experimental results, also conducted herein. The experimental findings, which compare geometrically identical rigid and flexible rotors, show a marked increase in average torque and operational envelope over a wide range of flow conditions. Through the analysis of the associated FSI simulations, these performance increases are attributed to small changes in local attack angles, acting to mitigate losses associated with flow separation. After validation with experimental data, the FSI solver is then utilized to investigate the effects of flexible blade design on a VAWT, the first analysis of its kind. Results indicate that efficiency increases are realized not by improving the maximum rotor torque, which varies along VAWT rotation, but by increasing the torque minima through passive deflection of the blades. This morphing action augments the local attack angle to decrease the magnitude of low-pressure regions created due to blade stall, acting to improve average rotor torque and increase energy capture of the flexible VAWT design when compared to a geometrically identical rigid one.

The aim of this dissertation was to develop passive techniques for vibration control and structural health monitoring applications in structures operating in conditions of significant ambient noise. These goals were accomplished through the use of an experimental wind turbine blade that represents the materials and design of a full scale wind turbine with the inclusion of devices simulating the presence of known manufacturing defects. The vibration control aim was accomplished using a shunted array of piezoelectric elements tuned to the vibration properties of the blade skin. The macro-fiber composite (MFC) composite piezoelectric transducer was bonded to the wind turbine blade skin and connected to the tuned shunt circuit. The damage detection techniques were developed using both an aluminum plate and the experimental wind turbine blade. Two approaches using a reconstructed impulse response function are presented. The first examines the break in reciprocity of impulse response functions. A single similarity damage index was proposed and then extended into a multi-feature analysis. The second method applied linear and nonlinear beamforming techniques using an experimentally generated replica field and cross-spectral signal processing techniques. The passive reconstruction of the impulse response function between two sensors is an important topic in NDE and SHM. Previously studied methods using active pitch-catch approaches between any two sensors is well suited for structures such as wind turbine blades that experience significant amounts of noise during operation. The study of these approaches advances the understanding of passive damage detection using reconstructed impulse response functions.

This handbook provides both a comprehensive overview and deep insights on the state-of-the-art methods used in wind turbine aerodynamics, as well as their advantages and limits. The focus of this work is specifically on wind turbines, where the aerodynamics are different from that of other fields due to the turbulent wind fields they face and the resultant differences in structural requirements. It gives a complete picture of research in the field, taking into account the different approaches which are applied. This book would be useful to professionals, academics, researchers and students working in the field.

Wind energy is gaining critical ground in the area of renewable energy, with wind energy being predicted to provide up to 8% of the world’s consumption of electricity by 2021. Advances in wind turbine blade design and materials reviews the design and functionality of wind turbine rotor blades as well as the requirements and challenges for composite materials used in both current and future designs of wind turbine blades. Part one outlines the challenges and developments in wind turbine blade design, including aerodynamic and aeroelastic design features, fatigue loads on wind turbine blades, and characteristics of wind turbine blade airfoils. Part two discusses the fatigue behavior of composite wind turbine blades, including the micromechanical modelling and fatigue life prediction of wind turbine blade composite materials, and the effects of resin and reinforcement variations on the fatigue resistance of wind turbine blades. The final part of the book describes advances in wind turbine blade materials, development and testing, including biobased composites, surface protection and coatings, structural performance testing and the design, manufacture and testing of small wind turbine blades. Advances in wind turbine blade design and materials offers a comprehensive review of the recent advances and challenges encountered in wind turbine blade materials and design, and will provide an invaluable reference for researchers and innovators in the field of wind energy production, including materials scientists and engineers, wind turbine blade manufacturers and maintenance technicians, scientists, researchers and academics. Reviews the design and functionality of wind turbine rotor blades Examines the requirements and challenges for composite materials used in both current and future designs of wind turbine blades Provides an invaluable reference for researchers and innovators in the field of wind energy production