Harvested by advanced technical systems honed over decades of research and development, wind energy has become a mainstream energy resource. However, continued innovation is needed to realize the potential of wind to serve the global demand for clean energy. Here, we outline three interdependent, cross-disciplinary grand challenges underpinning this research endeavor. The first is the need for a deeper understanding of the physics of atmospheric flow in the critical zone of plant operation. The second involves science and engineering of the largest dynamic, rotating machines in the world. The third encompasses optimization and control of fleets of wind plants working synergistically within the electricity grid. Addressing these challenges could enable wind power to provide as much as half of our global electricity needs and perhaps beyond.

Wind turbine blades continue to be the target of technological improvements by the use of better designs, materials, manufacturing, analysis and testing. As the size of turbines has grown over the past decade, designers have restrained the associated growth in blade weight to less than would have been possible through simple scaling-up of past approaches. These past improvements are briefly summarized. Manufacturing trends and design drivers are presented, as are the ways these design drivers have changed. Issues related to blade material choices are described, first for the currently dominant glass fibre technology and then for the potential use of carbon fibres. Some possible directions for future blade design options are presented, namely new planforms, aerofoils and aeroelastic tailoring. The significant improvement in sophistication of stress analysis and full-scale blade testing are also discussed.

Wind turbines have historically had reliability issues, which subsequently increase the overall cost of energy. The majority of these issues are caused by faults in the drivetrain, led by the main gearbox. These issues are widespread, existing across all turbine sizes and manufacturers. One means to mitigate the detrimental effect of reliability issues is through condition monitoring. Condition monitoring is a method to assess a system's health; enabling proactive maintenance planning, reducing downtime, reducing operations and maintenance costs and, to some extent, increasing safety. In this report, vibration, acoustic emission (specifically stress wave), electrical signature, oil cleanliness, oil debris, and oil sample analysis condition monitoring techniques were investigated for two identical 750 kW wind turbine gearboxes, in both a dynamometer test cell and field installation. The two test gearboxes are referred to as Gearboxes 1 and 2 in this report. The strengths and weaknesses of the different techniques were assessed. The feasibility of using oil cleanliness monitoring to determine the length of the run-in procedure was investigated on both gearboxes. Both demonstrated that, to make the run-in process sufficient, longer run-in durations at each torque level may be needed as compared to the current standard run-in procedure of prescribed durations at each torque level. Without fully completing the run-in, surface roughness remains excessive leading to increased contact stresses when the gearbox is placed into service and potentially leading to premature failures. Gearbox 1 was installed in a turbine at the Ponnequin Wind Farm and, after 300 hours of operation, it experienced two oil loss events and excessive temperatures that caused damage to some of its internal components. The gearbox was subsequently removed and inspected. Since the damage to the teeth was not severe, gearbox 1 also was installed and retested in the National Wind Technology Center's (NWTC) dynamometer before it was disassembled. Gearbox 2 was tested only in the dynamometer and was undamaged. The results were compared between gearboxes for each monitoring technique and the technique itself was evaluated for its detection capability. Vibration, acoustic emission, and oil debris monitoring all demonstrated the capability of distinguishing between the healthy and damaged gearbox components. It was possible to identify which stage of the gearbox was damaged, but not exactly which component was damaged for some gears and bearings inside the gearbox. Electrical signature analysis did not show any indication of the gear teeth damage, most likely because the damage to the teeth was not severe. To detect dominant failure modes in a gearbox, a combination of vibration or acoustic emission and oil debris monitoring techniques is recommended. Each technique is sensitive to sensor location and even orientation, and maintenance alerts are specific to each component and damaged part. v Table of Contents Acknowledgements .

IAs l3immadThere have tea many efforts aimed at creatiog adaptive blades using material elastic coupling to replace mechanical mechanismn with passive techniques. Several have used the composite lay-up structure to crate a coupling between the twisting of the blade (directly affecting angle of attack) and various other inherent forces. Karaolis'** illustrates how to achieve power control with such a blade on a small system and even provides optimum composite ply structures to provide maximum coupling. Joose and van den axial coupled spar that will rotate a tip mechanism through large enough angles to control power and provide some over-speed protection. Meld and Fe~chtwang~.~ show how small turbines can have improved s@ regulation w i t h twistlaxial coupling. A ammoo feature in these works is to provide relatively large rotations to achieve substantial amounts of power regulation. Most have used stretch twist coupling on variable speed systems to assist in over-speed control or power regulation and rely on large angles of twist to accomplish complete control of high wind loads.Lobie', however, demonstrated that even with relatively smaU twists that enhance regulation, a stall controlled, fixed pitch system could be operated with a larger rotor and the Same maximum power to achieve net eneqy enhancements greater than the gowth in rotor swept a r e a have publicized efforts to develop a special twist/ An excellent report evaluating a great variety of passive methods of achieving power control (Corbet and Mogan, Reference 8) demonstrates how difficult it is to achieve flat power regulation (constant power versus wind speed) m high winds with passive methods alone. The report does not examine aeroelastic tailoring through composite lay-up structure because it is judged too difficult to produce reliably given current hand-lay-up techniques of blade manufacturing. They infer that manuhcturing improvements are needed before aeroelastic tailoring based on material coupling can be fully implemented on utility scale machines.Here we examine more modest twist angles intended to produce load alleviation and perhaps power regulation or enhancement through bendtwist coupling. It seems quite possible to reduce the rotor dyoamic response by enhancing aeroelastic darnping in critical modes of vibration. Egers, et aL9 have shown how a simple control system, in some ways similar to coupling between root bendmg and blade pitch, can reduce low frequency blades loads substantially. However, when modifications that iofluence aeroelastic behavior are introduced into arotor there is always the possibility of also introducing instabilities. Stability is studied here by mapping out regions of potential coupling, creating a finite element formulation for coupled blades, and evaluating an example blade with added twist coupling. The load alleviating capabilities are not evaluated here but will be the topic of follow-on studies.

This paper introduces the development of a new software framework for research, design, and development of wind energy systems which is meant to 1) represent a full wind plant including all physical and nonphysical assets and associated costs up to the point of grid interconnection, 2) allow use of interchangeable models of varying fidelity for different aspects of the system, and 3) support system level multidisciplinary analyses and optimizations. This paper describes the design of the overall software capability and applies it to a global sensitivity analysis of wind turbine and plant performance and cost. The analysis was performed using three different model configurations involving different levels of fidelity, which illustrate how increasing fidelity can preserve important system interactions that build up to overall system performance and cost. Analyses were performed for a reference wind plant based on the National Renewable Energy Laboratory's 5-MW reference turbine at a mid-Atlantic offshore location within the United States. Three software configurations were used: 1) a previously published wind plant cost model using simplified parametric scaling relationships, 2) an integrated set of wind turbine and plant engineering and cost models that use a "bottom-up" approach to determine overall wind plant performance and cost metrics, and 3) the second set of models plus the addition of a plant layout and flow model for calculation of energy production. Global sensitivity analysis was performed on each analysis set with respect to key wind turbine configuration parameters including rotor diameter, rated power, hub height, and maximum tip speed. The analyses show how the latter approaches capture important coupling throughout the wind plant in a way that has not previously been achieved. In addition, while deficiencies even in the newer model set are readily identifiable, the flexibility of the new framework shows how extension and gradual buildup of model fidelity for various parts of the system provide a powerful tool that enables analysis for an ever-expanding set of wind energy research and design problems.