Frederik Zahle

System modeling frameworks for wind turbines and plants are used by research groups and industry to design wind energy systems that take into account key trade-offs across performance, cost, and reliability at both the turbine and plant level. The frameworks are exercised using a variety of multi-disciplinary design, analysis and optimization (MDAO) methods. To improve inter-operability and foster collaboration, this report proposes a classification system for the frameworks along dimensions of model fidelity and scope. The classification system is first motivated with reviews the state-of-the-art in the development of software frameworks for integrated wind turbine and plant simulation. Within each major wind turbine and power plant subsystem, a matrix is developed for the disciplines used and the fidelity levels with which each discipline can be modeled. The existing frameworks are then classified according to the matrix. Next, an ontology is proposed that will allow for standardizing how data is transferred between the most common discipline-fidelity combinations used in the frameworks. A common representation of data creates the ability to 1) share system descriptions and analysis results, supporting more transparent benchmarks and comparison, and 2) integrate models together into workflows within and across organizations for improving the efficiency and performance of wind turbine and power plant design processes. Ultimately, this integration leads to better overall wind energy system designs with high performance and low costs.

The present investigation used numerical simulations to study the vibrations of a wind turbine blade in standstill. Such vibrations are presumed to affect horizontal axis wind turbine designs and can jeopardize the structural integrity of the machine. The applied numerical methods relied on a fluid–structure interaction (FSI) approach, coupling a computational fluid dynamics (CFD) solver with a multibody finite-element structural solver. A 96-m-long wind turbine blade was studied for a large parametric space, accounting for the variation of both pitch and inclination. The inclination was defined as the angle between the freestream velocity and the cross-sectional plane at the root, allowing for the introduction of a flow component in the spanwise direction. The pitch variation corresponded to the rotation of the inflow around the spanwise axis, steering the angles of attack seen by the airfoils. Two regimes of vibrations were characterized, depending on the considered range of the inclination angle. For high inclinations, the pitch angles leading to vibrations clustered around a particular region of the parametric space, and the appearance of large oscillations was accompanied by the synchronization of the loading with the frequency of motion. At low inclination angles, the mechanism triggering vibrations was relatively similar, even if the excitation spectrum was richer, and the critical pitch angles seemed to be more scattered. Regardless of the inflow, the problem was highly three-dimensional, and several complex flow phenomena such as oblique shedding and phase dislocations were identified.

Post-Optimum Sensitivity Analysis (POSA) extends numerical design optimization to provide additional information on how the design and performance would change if various parameters and constraints were varied. POSA is challenging since it typically requires accurate gradients and gradient-based optimization problems that provide Lagrange multipliers. To overcome this problem, this paper introduces a technique to automatically tune gradients with statistical methods and algorithms to calculate the Lagrange multipliers after an optimization. This allows these methods to be applied to problems with noisy gradients or problems solved with gradient-free optimization algorithms. These methods have been applied to swept wind turbine blades. Swept blades can reduce wind turbine loads by twisting out of the wind when the wind speed increases. The methods have shown that introducing design freedom in the sweep, blade root flap-wise bending moments and blade tip deflection has a weaker influence on the design. Instead, blade root torsion moment and material failure become the driving constraints.

We present the preliminary design of a high capacity LowWind turbine rotor with a specific power of 100 W/m ², a rated power of 3.4MW and a 208m diameter rotor. The turbine is designed for optimal system integration and thus with a considerably increased power production at low to medium wind speed and stopped at 13 m/s. The AEP of the turbine, due to the large swept rotor area, is 40-45% higher than the AEP of a conventional on-shore turbine, the IEA Task 37 reference wind turbine with a 130 m diameter rotor and the same rated power of 3.4 MW. Initial aeroelastic simulations show that the loads of the LowWind turbine are generally higher than on the IEA turbine although comparable for some components at steady wind. For Design Load Case 1.2 (DLC1.2) for normal operation in turbulence we see a typical increase of around 20-40% of tower and rotor fatigue loads. However, this moderate increase for such big increase of rotor diameter is only obtained due to the highly flexible light weight rotor designed with a combination of peak shaving and blade bend twist coupling and for a low stop wind speed of 13 m/s.

In this study, simulation results of two different computational fluid dynamics codes, Nalu-Wind and EllipSys3D, are presented for a wind turbine rotor in complex yawed and sheared inflow. The results are compared to measurements from the DanAero experiments, to validate computed pressures and azimuthal trends. Despite different code methodologies and grid setups, the codes agree well in computed pressures and integrated forces along the blade for all blade azimuthal positions, however with some discrepancy in the very yawed case. Additionally, both codes capture well the azimuthal trends and force levels seen in measurements. Investigation into discrepancies shows that expanding grids before the rotor, lead to smearing of the wind profiles, which is likely the main cause of the differences in the results between the codes. Additionally, omission of the ground constraint cause discrepancies in relative velocity seen by the passing blade, due to an over speeding beneath the rotor.

Reference wind turbines (RWTs) that reflect the state-of-the-art of current wind energy technology are necessary in order to properly evaluate innovative methods in wind turbine design and evaluation. The International Energy Agency (IEA) Wind Technology Collaboration Platform (TCP) Task 37 has recently developed a new RWT geared towards offshore floating-foundation applications: the IEA Wind 15 MW. The model has been implemented in two aeroelastic codes, OpenFAST and HAWC2, based on an underlying common ontology. However, these toolchains result in slightly different structural parameters, and the two codes utilise different structural models. Thus, to increase the utility of the model, it is necessary to compare the aeroelastic responses. This paper compares aeroelastic loads calculated using different fidelities of the blade model in OpenFAST (ElastoDyn and BeamDyn) and HAWC2 (prismatic Timoshenko without torsion and Timoshenko with fully populated stiffness matrix), where both codes use the DTU Basic controller and the same turbulence boxes to reduce discrepancies. The aeroelastic responses to steady wind, step wind and turbulent wind (per IEC 61400-1 wind class IB) are considered. The results indicate a generally good agreement between the loads dominated by aerodynamic thrust and force, especially for the no-torsion blade models. Discrepancies were observed in other load channels, partially due to differences in the asymmetric loading of the rotor and partially due to differing closed-loop dynamics, and they will be the subject of future investigations.