Wind turbines are complex systems where component‐level changes can have significant system‐level effects. Effective wind turbine optimization generally requires an integrated analysis approach with a large number of design variables. Optimizing across large variable sets is orders of magnitude more efficient with gradient‐based methods as compared with gradient‐free method, particularly when using exact gradients. We have developed a wind turbine analysis set of over 100 components where 90% of the models provide numerically exact gradients through symbolic differentiation, automatic differentiation, and adjoint methods. This framework is applied to a specific design study focused on downwind land‐based wind turbines. Downwind machines are of potential interest for large wind turbines where the blades are often constrained by the stiffness required to prevent a tower strike. The mass of these rotor blades may be reduced by utilizing a downwind configuration where the constraints on tower strike are less restrictive. The large turbines of this study range in power rating from 5–7MW and in diameter from 105m to 175m. The changes in blade mass and power production have important effects on the rest of the system, and thus the nacelle and tower systems are also optimized. For high‐speed wind sites, downwind configurations do not appear advantageous. The decrease in blade mass (10%) is offset by increases in tower mass caused by the bending moment from the rotor‐nacelle‐assembly. For low‐wind speed sites, the decrease in blade mass is more significant (25–30%) and shows potential for modest decreases in overall cost of energy (around 1–2%). Copyright © 2016 John Wiley & Sons, Ltd.

This study investigates the effect of tip-speed constraints on system levelized cost-of-energy (LCOE). The results indicated that a change in maximum tip-speed from 80 to 100 meters per second (m/s) could produce a 32% decrease in gearbox weight (a 33% reduction in cost), which would result in an overall reduction of 1%-9% in system LCOE depending on the design approach. Three 100-m/s design cases were considered: a low tip-speed ratio/high-solidity rotor design, a high tip-speed ratio/low-solidity rotor design, and a flexible blade design in which a high tip-speed ratio was used along with removing the tip-deflection constraint on the rotor design. In all three cases, the significant reduction in gearbox weight caused by the higher tip-speed and lower overall gear ratio was counterbalanced by increased weights for the rotor and/or other drivetrain components and tower. As a result, the increased costs of either the rotor or drivetrain components offset the overall reduction in turbine costs from downsizing the gearbox. Other system costs were not significantly affected, whereas energy production was slightly reduced in the 100-m/s high-solidity case and increased in the low-solidity case. This situation resulted in system cost-of-energy reductions moving from the 80-m/s design to the 100-m/s designs of 1.5% for the high tip-speed ratio and 5.5% for the final flexible case (the latter result is optimistic because the impact of deflection of the flexible blade on power production was not modeled). The low tip-speed ratio case actually resulted in a cost of energy increase of 2.1%. Overall, the results demonstrated that there is a trade-off in system design between the maximum tip-speed and the overall wind plant cost-of-energy but also that there are several design trade-offs and design constraints that can limit the benefits of higher tip-speed designs. Scope Land-based wind project development has historically limited turbine designs to operate at blade-tip-velocities in the range of 75-80-m/s. The constraint arises from blade-tip aero-acoustic noise generation. The turbine system sound power levels are usually dominated by blade-tip noise when appropriate measures have been taken to mitigate sound emissions and audible tones from the tower head machinery within the nacelle and the power electronic converters often located within the tower base. The study looks at five overall turbine configurations: 1. A baseline 5-megawatt (MW) reference turbine Jonkman et al. (Feb 2009) with maximum tip-speed of 80-m/s 2. An optimized version with the same 80-m/s tip-speed design constraint 3. An optimized design at 100-m/s maximum tip-speed with a high-solidity rotor 4. An optimized design at 100-m/s maximum tip-speed with a low-solidity rotor 5. An optimized design at 100-m/s maximum tip-speed with a low-solidity rotor where the tip-deflection con straint has been removed (as a proxy for a machine that would operate downwind). In each of the design cases, a sequential optimization was performed to design the turbine. The ro...

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