Development of an icing simulation code for rotating wind turbines

This study aims to develop a three-dimensional icing simulation code named WISE (Wind turbine Icing Simulation code with performance Evaluation) integrated into OpenFOAM®. The freely available source code can contribute to icing simulations that require parallel computations. The rotational motion is explained by a Moving Reference Frame (MRF) in both aerodynamic and droplet fields. The thin water film theory is applied in the thermodynamic module. To verify WISE, ice accretion shapes on NREL Phase VI under rime and glaze icing conditions were considered. The ice accretion shapes obtained by WISE were compared against FENSAP-ICE and another numerical simulation without the MRF method for the droplet field. For the rime condition, the icing limits, maximum thickness, and its location are well predicted by WISE compared with FENSAP-ICE while the simulation without the MRF method overestimates the icing limits and maximum thickness. For the glaze condition, only WISE and FENSAP-ICE results are compared where the icing limits are slightly different. On the suction side, WISE accurately predicts the maximum thickness, ice growth direction, and icing limits. However, the thickness of ice on the pressure side is underestimated. It might be necessary to have a turbulence model that can predict the flow transition

Boundary-layer transition model for icing simulations of rotating wind turbine blades

Icing simulations for wind turbine blades should consider the roughness-induced flow transition. Adding a transport equation for ‘roughness amplification’ to the Langtry-Menter model, the roughness-induced transition can be predicted for rough flat plates. However, this approach exhibits a limitation that it cannot predict the skin friction in the shadow zone of blunt bodies. Such an approach depends on the boundary condition(s) of specific dissipation rate (ω). Typically boundary conditions for turbulent kinetic energy (k) and ω have been investigated for various roughness heights, but have been applied only for fully turbulent conditions. This study introduces an approach to predict the flow transition and the skin friction for a roughened surface, whereby the Langtry-Menter model including roughness amplification is coupled with the k and ω boundary conditions. The proposed method shows good agreement with the experiments for turbulent onset and the distributions of skin friction and heat convection for a roughened flat plate and a circular cylinder. Using the turbulent models under fully turbulent and transitional assumptions, the effects of the flow transition on the ice accretion shape on a rotating wind turbine are compared. The modified turbulent model showed better performance for the icing simulations without any tuning

Stability margin analysis for PI pitch controllers on large wind turbines

This work addresses blade pitch controllers for variable speed wind turbines for the purpose of maintaining power at rated value during above rated wind speeds. The work proposes a collective proportional integral (PI) pitch controller design that accounts for the effects of low frequency aero-elastic modes to enhance the performance of the basic PI controller used in industry. Validation was performed by testing the proposed controller on a non-linear model for the DTU10MW turbine under turbulent wind conditions. Statistical analysis of fatigue loads at the main shaft bearing were further investigated to verify that the proposed controller does not add excessive loading compared with the basic controller

LQG control for hydrodynamic compensation on large floating wind turbines

This work proposes a novel Linear Quadratic Gaussian (LQG)-based blade pitch control method for floating offshore wind turbines, in which a state-space model of the turbine and water hydrodynamics is included in the LQG design. The actuation considered is collective blade pitch control with the objective of generator power stabilization and platform motion reduction. A linear Kalman filter is used to estimate un-measurable states relating to wave excitation and radiation through measurements of generator speed, platform pitch, and wind disturbance. Controller design models were validated with the full order nonlinear model under various testing conditions. The new controller design is tested on a nonlinear high-fidelity simulation model of the 15 Mega-Watt (MW) floating semi-submersible wind turbine. In simulations with realistic stochastic wind and wave disturbances, the new controller achieves 32% lower generator speed Root Mean Square Error (RMSE) and 16% lower platform pitch RMSE compared to a standard LQG controller that does not include hydrodynamic states, for equivalent levels of pitch actuation and with a 2° /sec rate limit on pitch. The inclusion of hydrodynamics in the controller design not only reduced platform pitching fluctuation, but also had a strong effect of hub-height factors such as the generator speed.</p

A robust gain scheduling method for a PI collective pitch controller of multi-MW onshore wind turbines

This work proposes a robust tuning method for full-load pitch control by deriving new design formulae for collective Proportional Integral (PI) pitch controllers. The paper investigates the frequency domain characteristics of a full state linearized turbine model extracted from the aeroelastic simulation tool HawcStab2, and suggests a reduced order model that captures the low-frequency behaviour and may be used to derive PI tuning formulae that account for collective blade flap modes. The proposed controllers are then compared to traditional PI controllers based on a single degree of freedom (1-DOF) model of the drive-train using linearized NREL5MW and DTU10MW models. The 10 MW model is investigated in more detail through non-linear simulations in HAWC2 using wind step and turbulent wind conditions. The proposed design formulae show robust results, giving more consistent gain and phase margins than 1-DOF designs throughout the above rated wind speed region, and may be used to increase controller bandwidth while maintaining acceptable stability margins, achieving 49% and 63% reductions in standard deviation of the output power for the DTU10MW model in turbulent conditions. Statistical analysis for both controllers was also performed to investigate fatigue loading on the main shaft caused by the pitch actuation.</p

Experimental study on ice intensity and type detection for wind turbine blades with multi-channel thermocouple array sensor

A multi-channel thermocouple array (MCTCA) sensor has been fabricated and tested for in-situ monitoring of the temperature variation under icing conditions for wind turbine blades. The obtained temperature data is analysed as a tool to predict the icing intensity (g m−2) and type (rime and glaze) with respect to the temperature gradients. The tests are performed in an environmental chamber with a water spraying system. The temperature of the chamber is set to −15 °C to ensure the sprayed water is supercooled before reaching the apparatus surface. Based on the various tests, the following conclusions can be drawn. Firstly, the MCATA can successfully predict icing events where the maximum temperature rise is monitored as 11.9 °C for 2 mm and 9.8 °C for 3 mm resin thicknesses with the same amount of accumulated ice. Secondly, this study suggests the temperature change per unit mass as an indicator of the ice intensity. Severe icing events can be expected when the indicator converges to zero. Finally, this study found that the surface temperature gradient is changed over time due to the amount of latent heat released where the two different environmental conditions, −15 °C and −5 °C, are considered. It could be used to evaluate the ice types.</p

Development of an anisotropic co-rotational beam model including variable cross-section

The aim of this article is to expand the general approach of the flexible beam model to consider tapered geometry and anisotropic properties by updating the cross-sectional stiffness matrix. The advantage of this approach is that continuously variable cross-section, as well as irregular axes on the cross-sections, are considered simultaneously by the co-rotational method. Anisotropic and isotropic cantilevered beam cases are simulated. A static force or moment is applied as the external load. NREL 5 MW wind turbine blade is analyzed as a practical example. The results are compared against the existing literature and ABAQUS model, and they show excellent agreement

Development of three-dimensional co-rotational beam model for nonlinear dynamic analysis of highly flexible slender composite blades

The main purpose of this work is to introduce a novel method analyzing the dynamic response of a highly flexible slender composite beam having anisotropic property and variable cross-section. A three-dimensional co-rotational model is developed. The beam deformation in dynamic conditions is obtained by applying HHT-α method as dynamic analysis tool. Validation studies with various boundary conditions, multiple external loads and pre-curved structures are performed. Furthermore, dynamic analysis with a large, flexible, and slender composite blade is performed with and without structural couplings. It is shown the developed beam model accurately predicts nonlinear three-dimensional response and anisotropic structural coupling effects

Initial bead growth and distribution under low speed icing condition

One of the critical issues in recent ice prediction studies is the modeling of roughness formation based on physical phenomena. While a number of experimental studies have been conducted to investigate the fundamental physics of roughness formation, initial bead growth is rarely studied despite its significance to determine bead size and distribution. In the present study, an experiment is conducted to provide physical insight and quantitative data of the initial bead growth. To minimize the uncertainty problem that is inherent in the photographic analysis technique, the experiment was conducted at low speed where beads grow at a more macroscopic scale. In addition, bead identification was conducted through an image processing technique to reduce subjective interpretation. In the data analysis process, the ‘characteristic parameter’ concept was adopted to represent the bead growth properties simply. It was verified that the parameter can be extended to initial bead growth studies through the acquired bead data. It was also found that surface coverage and bead distribution, which are the critical indicators of the bead growth process, were well characterized by the parameter. Finally, the correlation between characteristic parameters and icing condition was made, by using a scaling method that can implicitly represent the icing condition variable. In the process, the scaling approach was modified to reflect the surface coverage characteristics of the bead growth, and an improvement of the correlation was achieved. It is expected that the correlation acquired from this study contribute to the modeling of roughness formation, and the methods introduced for data analysis can be applied to subsequent studies of initial bead growth

Development of a novel multi-channel thermocouple array sensor for in-situ monitoring of ice accretion

A test was performed to determine the efficacy of a novel multi-channel thermocouple temperature sensor employing “N+1” array architecture for the in-situ detection of icing in cold climates. T-type thermoelements were used to fabricate a sensor with six independent temperature sensing points, capable of two-dimensional temperature mapping. The sensor was intended to detect the high latent heat of fusion of water (334 J/g) which is released to the environment during ice formation. The sensor was embedded on a plywood board and an aluminium plate, respectively by an epoxy resin. Three different ice accretion cases were considered. Ice accretion for all cases was achieved on the surface of the resin layer. In order to analyse the temperature variation for all three cases, the first 20 s response for each case was averaged between three cases. A temperature increase of (1.0 ± 0.1) °C and (0.9 ± 0.1) °C was detected by the sensors 20 s after the onset of icing, attributed to the latent heat of fusion of water. The results indicate that the sensor design is well-suited to cold temperature applications and that detection of the latent heat of fusion could provide a rapid and robust means of icing detection