• Energy Research

SALT - Simplified Aerodynamic Loss Tool A fast BEM-based tool to predict the loss in annual energy production for a wind turbine, due to aerodynamic deteration of different spanwise sections of its blades. It relies on a simplified BEM model to compute the aerodynamic performance of the rotor and perturbs the sectional lift coefficient and lift-to-drag ratio to assess the losses. This calculation tool is initially made to predict the annual aerodynamic energy loss relative to the starting point. The motivation to formulate this model is that wind turbine owners neither have much information about the wind turbine nor information about the real surface conditions of the blades - apart from photos from inspections. Therefore, this tool basically only requires a few parameters: Rated power, rotor radius, air density, Weibull parameters A and k and categories describing the surface conditions of each blade. The remaining parameters required to describe the rotor operation is assumed. The tool is based on the work described in: Christian Bak 2022 J. Phys.: Conf. Ser. 2265 032038 The tool should be able to predict the energy loss within 10% accuracy according to initial investigations, but further validation of the tool will be carried out. Also, the tool is not made to predict precise absolute annual energy production. It is made for relative changes.

Abstract. Offshore wind farms are more commonly installed in wind farm clusters, where wind farm interaction can lead to energy losses; hence, there is a need for numerical models that can properly simulate wind farm interaction. This work proposes a Reynolds-averaged Navier–Stokes (RANS) method to efficiently simulate the effect of neighboring wind farms on wind farm power and annual energy production. First, a novel steady-state atmospheric inflow is proposed and tested for the application of RANS simulations of large wind farms. Second, a RANS-based wind farm parameterization is introduced, the actuator wind farm (AWF) model, which represents the wind farm as a forest canopy and allows to use of coarser grids compared to modeling all wind turbines as actuator disks (ADs). When the horizontal resolution of the RANS-AWF model is increased, the model results approach the results of the RANS-AD model. A double wind farm case is simulated with RANS to show that replacing an upstream wind farm with an AWF model only causes a deviation of less than 1 % in terms of the wind farm power of the downstream wind farm. Most importantly, a reduction in CPU hours of 75.1 % is achieved, provided that the AWF inputs are known, namely, wind farm thrust and power coefficients. The reduction in CPU hours is further reduced when all wind farms are represented by AWF models, namely, 92.3 % and 99.9 % for the double wind farm case and for a wind farm cluster case consisting of three wind farms, respectively. If the wind farm thrust and power coefficient inputs are derived from RANS-AD simulations, then the CPU time reduction is still 82.7 % for the wind farm cluster case. For the double wind farm case, the RANS models predict different wind speed flow fields compared to output from simulations performed with the mesoscale Weather Research and Forecasting model, but the models are in agreement with the inflow wind speed of the downstream wind farm. The RANS-AD-AWF model is also validated with measurements in terms of wind farm wake shape; the model captures the trend of the measurements for a wide range of wind directions, although the measurements indicate more pronounced wind farm wake shapes for certain wind directions.

A row of wind turbine rotors with a mutual spacing of three diameters is simulated using both Reynolds averaged Navier-Stokes (RANS) simulations and a simple inviscid vortex model. The angle between the incoming wind and the line connecting the turbines is varied between 45 and 90 degrees. The simulations show that the power production of the turbines deviate significantly compared with a corresponding isolated turbine even though there is no direct wake-turbine interaction at the considered wind directions. Nevertheless, both models indicate marked alterations in the upstream flow, which directly link to the turbines' power adjustments. Thus, turbines which are placed laterally relative to the prevailing wind (as seen at various test sites) have, at least numerically, a mutual effect on each other. Therefore, they might not necessarily produce the same power as a stand-alone turbine. Copyright © 2016 John Wiley & Sons, Ltd.

AbstractThe force smearing in the actuator line technique ensures its numerical stability, but also breaks its intended similarity to the lifting line by similarly smearing its vorticity in the flow domain. The wake thus induces lower velocities at the blade, linking the blade forces to the force smearing. A recently developed tuning-free, vortex-based correction recovers this missing induction, regaining the lifting-line behaviour of the actuator line. The interplay of this new smearing correction with grid and blade resolution is studied in uniform and turbulent inflow with respect to the blade forces and wake behaviour. With only 10 grid cells along the blade, the thrust is within 2.8% and the power within 5.7% of the high-resolution reference. With 20 grid cells the difference drops to 1.5% and 2.5%, respectively. The influence of the force smearing on the wake velocities dominates over the choice of correction, yet under turbulent inflow the wake characteristics become nearly independent of force smearing 6 rotor radii downstream of the turbine.

AbstractWind farm blockage effects are currently neglected in the prediction of wind farm energy yield, typically leading to an overestimation of the production. This work presents a novel method to assess wind farm production, while accounting for blockage effects. We apply a vortex model, based on a cylindrical wake, to assess induction effects. We present variations of the model to account for finite wake length, finite tip‐speed ratios, and the proximity to the ground. The results are applied to single rotors in aligned and yawed conditions and to different wind farm layouts. We provide far‐field approximations for faster estimates of the velocity field. Further, this article includes a new methodology to couple the induction model to engineering wake models, such as the ones present in the FLOw Redirection and Induction in Steady State (FLORIS). We compare the results to actuator disk simulations for various operating conditions of a single turbine and different wind farm layouts. We found that the mean relative error of the model in the induction zone is typically around 0.2% compared with actuator disk simulations. The computational time of the velocity field using the analytical vortex model is three orders of magnitude less than the one obtained with the actuator disk simulation.

Abstract. The actuator line is a lifting line representation of aerodynamic surfaces in computational fluid dynamics applications, but with non-singular forces which reduces the self-induced velocities at the line. The vortex-based correction by Meyer Forsting et al. (2019) et al. recovers this missing induction and thus the intended lifting line behaviour of the actuator line. However, its computational cost exceeds that of existing tip corrections and quickly grows with blade discretization. Here we present different methods for reducing its computational cost to the level of existing corrections without jeopardising the stability or accuracy of the original method. The cost is reduced by at least 98 % whereas the power is maximally affected by 0.8 % with respect to the original formulation.

AbstractThe induction zone in front of different wind turbine rotors is studied by means of steady‐state Navier‐Stokes simulations combined with an actuator disk approach. It is shown that, for distances beyond 1 rotor radius upstream of the rotors, the induced velocity is self‐similar and independent of the rotor geometry. On the basis of these findings, a simple analytical model of the induction zone of wind turbines is proposed.

AbstractThe loading of a wind turbine decreases towards the blade tip because of the velocities induced by the tip vortex. This tip loss effect has to be taken into account when performing actuator disc simulations, where the single blades of the turbine are not modeled. A widely used method applies a factor on the axial and tangential loading of the turbine. This factor decreases when approaching the blade tip. It has been shown that the factor should be different for the axial and tangential loading of the turbine to model the rotation of the resulting force vector at the airfoil sections caused by the induced velocity. The present article contains the derivation of a simple correction for the tangential load factor that takes this rotation into account. The correction does not need any additional curve fitting but just depends on the local airfoil characteristics and angle of attack. Actuator disc computations with the modified tip loss correction show improved agreement with results from actuator line, free wake lifting line, and blade element momentum simulations.