Adaptive Pitch Composite Blades For Axial Flow Marine Hydrokinetic Turbines

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.

Marine Composites: Design and Performance presents up-to-date information and recent research findings on the application and use of advanced fibre-reinforced composites in the marine environment. Following the success of their previously published title: Marine Applications of Advanced Fibre-reinforced Composites which was published in 2015; this exemplary new book provides comprehensive information on materials selection, characterization, and performance. There are also dedicated sections on sandwich structures, manufacture, advanced concepts, naval architecture and design considerations, and various applications. The book will be an essential reference resource for designers, materials engineers, manufactures, marine scientists, mechanical engineers, civil engineers, coastal engineers, boat manufacturers, offshore platform and marine renewable design engineers. Presents a unique, high-level reference on composite materials and their application and use in marine structures Provides comprehensive coverage on all aspects of marine composites, including the latest advances in damage modelling and assessment of performance Contains contributions from leading experts in the field, from both industry and academia Covers a broad range of naval, offshore and marine structures

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.

In a trend toward clean energy alternatives, recent years have seen great strides in the marine energy space. Consequently, there is a pressing need for the design, development, and validation of novel energy harvesting technologies such as hydrokinetic devices, which capture kinetic energy from waves, tides, and currents. However, these devices span numerous concepts and designs that often lack solid benchmark research that can be freely referenced throughout their development. This work focuses on the design process of an open-source composite hydrokinetic turbine blade for a three-bladed marine turbine rotor assembly with a diameter of 2.5 m. The proposed blade consists of two structural composite skins that are bonded with an adhesive and filled with a foam core. This study also explores and contrasts the efficiency and resolution of low-fidelity rapid design methodologies and comprehensive high-fidelity approaches in the context of blade design, modeling, and analysis efforts, a key objective in this research. Blade hydrodynamic loads were modeled and applied to finite-element blade models to study deformations and potential failure. Ongoing and upcoming efforts will result in blade manufacture and structural testing at the National Renewable Energy Laboratory. In future work, multiple blades will be deployed at the Living Bridge site at the University of New Hampshire and will be compared to rigid aluminum blades of the same geometry, developed by Sandia National Laboratories. Ultimately, this research will lay foundational groundwork for researchers and manufacturers, establishing a baseline composite blade design that will serve as a benchmark in the development of future hydrokinetic turbine blades.

"Composite materials are gaining interest due to their high strength to weight ratio. This study deals with both experimental and numerical approaches to cover the aspects of the failure of composite materials in hydrokinetic turbine applications. In Part I, the location and magnitude of failure in the horizontal axis water turbine carbon fiber-reinforced polymer (CFRP) composite blades with different laminate stacking sequences were investigated. Two lay-up orientations were adopted for this work ([0°]4 and [0°/90°][sub 2S]). A finite element analysis model was generated to examine the stresses along the blade. Five angles were introduced to study the effect of pitch angle on the CFRP blades. The numerical results showed very good agreement with the experimental results. In Part II, an experimental setup was developed to test the delamination progression in CFRP blades under hydrodynamic loads in a water tunnel. Thermography analysis was employed to scrutinize the propagation of delamination. In addition, a computational fluid dynamics and one-way fluid-structure interaction were developed to predict the stresses along the blade. The unidirectional ([0°]4) blades showed the best performance while the cross-ply blades ([0°/90°][sub 2S]) are prone to delamination. In Part III, the effect of increasing the contact area between the core and facesheet was studied. Two tests (impact and flat-wise tension) were carried out to examine the integrity of the structure. A finite element model was developed to study the damage due to localized load, such as impact load. The results obtained from both the tests (impact and flatwise tension) showed that increasing surface area had improved the structural integrity in regards to damage resistance due to impact, and delamination resistance between the facesheet and the core due to tension"--Abstract, page iv.

Composite tidal turbine blades with bend-twist (BT) coupled layups allow the blade to self-adapt to local site conditions by passively twisting to reduce the angle of incoming flow (feathering). Passive feathering has the potential to reduce the fluid forces on both the blades and support structure, as well as shed power at extreme site conditions. Decreased hydrodynamic thrust and power at extreme conditions means that the turbine support structure, generator, and other components can be sized appropriately for rated conditions, increasing their utilization factor and increasing the device cost effectiveness. This thesis reports the outcomes of research into passively adaptive BT blades. A design tool was developed that couples a finite element model (FEM) and a blade element momentum theory (BEMT) model, to investigate the interactive fluid and structural performance of BT blades. The design tool also incorporated a composite material failure analysis, allowing fast and efficient verification of the structural integrity of different blade designs. Through experimental testing of blades designed using the tool, BT composite blades were shown to have up to 10% lower thrust loads compared to rigid blades, with similar load reductions predicted by the design tool. This proved the concept and demonstrated a design methodology for BT coupling for tidal turbine blades at small-scale. A case study of a full-scale turbine with 4.0 m BT blades with a pre-deformed blade shape (slightly decreased pre-twist distribution along the blade span) was investigated using the design tool. By reducing the pre-twist of the blade by 2.3o at the blade tip, the blade twisted under load to its optimum shape at design conditions, and continued to twist to feather toward extreme flow speeds. These blades were found to have 10% more power capture between the cut-in and design speeds, and a 10% reduction in power and 5% reduction in thrust loads at extreme flow speeds. This makes pre-deformed BT blades a potential solution to structural load reduction, as well as power capture optimization, which would increase the overall cost-effectiveness of the tidal turbine.

A health monitoring approach is investigated for hydrokinetic turbine blade applications. In-service monitoring is critical due to the difficult environment for blade inspection and the cost of inspection downtime. Composite blade designs have advantages that include long life in marine environments and great control over mechanical properties. Experimental strain characteristics are determined for static loads and free-vibration loads. These experiments are designed to simulate the dynamic characteristics of hydrokinetic turbine blades. Carbon/epoxy symmetric composite laminates are manufactured using an autoclave process. Four-layer composite beams, eight-layer composite beams, and two-dimensional eight-layer composite blades are instrumented for strain. Experimental results for strain measurements from electrical resistance gages are validated with theoretical characteristics obtained from in-house finite-element analysis for all sample cases. These preliminary tests on the composite samples show good correlation between experimental and finite-element strain results. A health monitoring system is proposed in which damage to a composite structure, e.g. delamination and fiber breakage, causes changes in the strain signature behavior. The system is based on embedded strain sensors and embedded motes in which strain information is demodulated for wireless transmission. In-service monitoring is critical due to the difficult environment for blade inspection and the cost of inspection downtime. Composite blade designs provide a medium for embedding sensors into the blades for in-situ health monitoring. The major challenge with in-situ health monitoring is transmission of sensor signals from the remote rotating reference frame of the blade to the system monitoring station. In the presented work, a novel system for relaying in-situ blade health measurements in hydrokinetic systems is described and demonstrated. An ultrasonic communication system is used to transmit sensor data underwater from the rotating frame of the blade to a fixed relay station. Data are then broadcast via radio waves to a remote monitoring station. Results indicate that the assembled system can transmit simulated sensor data with an accuracy of ±5% at a maximum sampling rate of 500 samples/sec. A power investigation of the transmitter within the blade shows that continuous max-sampling operation is only possible for short durations (d̃ays), and is limited due to the capacity of the battery power source. However, intermittent sampling, with long periods between samples, allows for the system to last for very long durations (ỹears). Finally, because the data transmission system can operate at a high sampling rate for short durations or at a lower sampling rate/higher duty cycle for long durations, it is well-suited for short-term prototype and environmental testing, as well as long-term commercially-deployed hydrokinetic machines.

Printed collection of 105 full-length, peer-reviewed technical papers.

Aerodynamics of Wind Turbines is the established essential text for the fundamental solutions to efficient wind turbine design. Now in its second edition, it has been entirely updated and substantially extended to reflect advances in technology, research into rotor aerodynamics and the structural response of the wind turbine structure. Topics covered include increasing mass flow through the turbine, performance at low and high wind speeds, assessment of the extreme conditions under which the turbine will perform and the theory for calculating the lifetime of the turbine. The classical Blade Element Momentum method is also covered, as are eigenmodes and the dynamic behaviour of a turbine. The new material includes a description of the effects of the dynamics and how this can be modelled in an ?aeroelastic code?, which is widely used in the design and verification of modern wind turbines. Further, the description of how to calculate the vibration of the whole construction, as well as the time varying loads, has been substantially updated.