Estimating the potential yield of small building‐mounted wind turbines

The wind profile in the urban boundary layer is described as following a logarithmic curve above the mean building height and an exponential curve below it. By considering the urban landscape to be an array of cubes, a method is described for calculating the surface roughness length and displacement height of this profile. Firstly, a computational fluid dynamics (CFD) model employing a k‐? turbulence model is used to simulate the flow around a cube. The results of this simulation are compared with wind tunnel measurements in order to validate the code. Then, the CFD model is used to simulate the wind flow around a simple pitched‐roof building, using a semi‐logarithmic inflow profile. An array of similar pitched‐roof houses is modelled using CFD to determine the flow characteristics within an urban area. Mean wind speeds at potential turbine mounting points are studied, and optimum mounting points are identified for different prevailing wind directions. A methodology is proposed for estimating the energy yield of a building‐mounted turbine from simple information such as wind atlas wind speed and building density. The energy yield of a small turbine on a hypothetical house in west London is estimated. The energy yield is shown to be very low, particularly if the turbine is mounted below rooftop height. It should be stressed that the complexity of modelling such urban environments using such a computational model has limitations and results can only be considered approximate, but nonetheless, gives an indication of expected yields within the built environment. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigations of a building‐integrated ducted wind turbine module

So far, wind energy has not played a major role in the group of technologies for embedded generation in the built environment. However, the wind flow around conventional tall buildings generates differential pressures, which may cause an enhanced mass flow through a building‐integrated turbine. As a first step, a prototype of a small‐scale ducted wind turbine has been developed and tested, which seems to be feasible for integration into the leading roof edge of such a building. Here an experimental and numerical investigation of the flow through building‐integrated ducting is presented. Pressure and wind speed measurements have been carried out on a wind tunnel model at different angles of incident wind, and different duct configurations have been tested. It was confirmed that wind speeds up to 30% higher than in the approaching freestream may be induced in the duct, and good performance was obtained for angles of incident wind up to ±60°. The experimental work proceeded in parallel with computational fluid dynamics (CFD) modelling. The geometry of the system was difficult to represent to the required level of accuracy, and modelling was restricted to a few simple cases, for which the flow field in the building‐integrated duct was compared with experimental results. Generally good agreement was obtained, indicating that CFD techniques could play a major role in the design process. Predicted power of the proposed device suggests that it will compare favourably with conventional small wind turbines and photovoltaics in an urban environment. Copyright © 2002 John Wiley & Sons, Ltd.     

Aerofoil optimization for vertical‐axis wind turbines

An expression for the aerodynamic optimization of aerofoils for 2D lift driven vertical‐axis wind turbines is derived as a function of lift slope and drag. As lift slope is proportional to aerofoil thickness, the aerodynamic optimum is found in thick aerofoils, which are also structurally advantageous. Using a genetic optimization algorithm, the objective function is used to generate aerofoils whose performance in a vertical‐axis wind turbine is calculated using a potential flow solution of the induction field and 2D polars calculated with XFOIL. The results demonstrate power and structural gains. This approach can lead to reductions in rotor mass due to the thicker and thus stiffer aerofoils, without compromising aerodynamic performance. Copyright © 2014 John Wiley & Sons, Ltd.     

An enhanced analytical model for airfoil‐based shrouded wind turbines

This paper provides a simple yet accurate lower order model for predicting the performance of shrouded wind turbines for a range of shroud airfoil shapes varying in length, camber, and thickness. The model employs classic thin airfoil aerodynamic principals and is an enhancement of a previous related model developed for shrouded propellers. The new method's accuracy is assessed using inviscid and viscous computational studies employing an actuator‐disc representation for the turbine rotor. Detailed comparisons are made for three shrouds over a full range of rotor loadings. For the range of configurations assessed, it was found that the algebraic solutions from the new method provided very good engineering approximations at all operating conditions considered, thereby enabling rapid preliminary design of shrouded power systems. Additionally, the new model is used to establish a Betz‐like power performance limit for airfoil‐based shrouded turbines.     

Implementation of a non‐linear foundation model for soil‐structure interaction analysis of offshore wind turbines in FAST

Bottom‐fixed offshore wind turbines (OWTs) involve a wide range of engineering fields. Of these, modelling of foundation flexibility has been given little priority. This paper investigates the modelling of bottom‐fixed OWTs in the non‐linear aero‐hydro‐servo‐elastic simulation tool FAST v7. The OWTs considered is supported on a monopile. The objective of this paper was to implement a non‐linear foundation model in this software. The National Renewable Energy Laboratory's idealized 5MW reference turbine was used as a base for the analyses. Default modelling of foundation in FAST v7 is by means of a rigid foundation. This implies that soil stiffness and damping is disregarded. Damping may lead to lower design loads. A softer foundation, on the other hand, will increase the natural periods of the system, shifting them closer to the frequencies of the environmental loads. This may in turn lead to amplified moments at the mudline. Therefore, it is important to include soil stiffness and damping in analyses. In this paper, a non‐linear foundation model is introduced in FAST v7 by means of uncoupled parallel springs. To verify that the implementation is successful, non‐linear load‐displacement curves of the foundation spring are presented. These show the typical hysteresis loops of an inelastic material, which confirms the implementation. Copyright © 2016 John Wiley & Sons, Ltd.     

Modelling and control of synchronous generators for wide‐range variable‐speed wind turbines

The modelling and control of a wide‐range variable speed wind turbine based on a synchronous generator are presented. Two different methods to control the operation of the synchronous generator are investigated, i.e. load angle control and instantaneous vector control. The dynamic performance characteristics of these control strategies are evaluated and compared using three model representations of the generator: a non‐reduced order model including both stator and rotor transients, a reduced order model with stator transients neglected, and a steady‐state model that neglects generator electrical dynamics. Assessment on the performance of grid‐side controller is shown during network fault and frequency variation. A simplified wind turbine model representation is also developed and proposed for large‐scale power system studies. Simulation results in Matlab/Simulink are presented and discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

SSCI mitigation of grid‐connected DFIG wind turbines with fractional‐order sliding mode controller

To mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)‐based wind farm, this paper proposes a robust controller for rotor‐side converter (RSC) using fractional‐order sliding mode controller (FOSMC). The proposed FOSMC can improve robustness and convergence properties of the controlled system, thus achieving SSCI damping under various operating conditions. Impedance‐based analysis and time‐domain simulation are performed to check the capability of the designed FOSMC as compared with conventional sliding mode control (SMC) and subsynchronous damping control (SSDC). Simulation results demonstrate that FOSMC can mitigate SSCI within shorter time and effectively reduce the fluctuation range of system transient responses under various operating conditions of wind speeds and compensation levels. Moreover, FOSMC also improves system robustness against parameter uncertainties and external disturbances, which is important for safe operation of realistic wind farms.     

Modelling the aerodynamics of vertical‐axis wind turbines in unsteady wind conditions

Most numerical and experimental studies of the performance of vertical‐axis wind turbines have been conducted with the rotors in steady, and thus somewhat artificial, wind conditions—with the result that turbine aerodynamics, under varying wind conditions, are still poorly understood. The vorticity transport model has been used to investigate the aerodynamic performance and wake dynamics, both in steady and unsteady wind conditions, of three different vertical‐axis wind turbines: one with a straight‐bladed configuration, another with a curved‐bladed configuration and another with a helically twisted configuration. The turbines with non‐twisted blades are shown to be somewhat less efficient than the turbine with helically twisted blades when the rotors are operated at constant rotational speed in unsteady wind conditions. In steady wind conditions, the power coefficients that are produced by both the straight‐bladed and curved‐bladed turbines vary considerably within one rotor revolution because of the continuously varying angle of attack on the blades and, thus, the inherent unsteadiness in the blade aerodynamic loading. These variations are much larger, and thus far more significant, than those that are induced by the unsteadiness in the wind conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Harmonic balance Navier‐Stokes aerodynamic analysis of horizontal axis wind turbines in yawed wind

Multimegawatt horizontal axis wind turbines often operate in yawed wind transients, in which the resulting periodic loads acting on blades, drive‐train, tower, and foundation adversely impact on fatigue life. Accurately predicting yawed wind turbine aerodynamics and resulting structural loads can be challenging and would require the use of computationally expensive high‐fidelity unsteady Navier‐Stokes computational fluid dynamics. The high computational cost of this approach can be significantly reduced by using a frequency‐domain framework. The paper summarizes the main features of the COSA harmonic balance Navier‐Stokes solver for the analysis of open rotor periodic flows, presents initial validation results on the basis of the analysis of the NREL Phase VI experiment, and it also provides a sample application to the analysis of a multimegawatt turbine in yawed wind. The reported analyses indicate that the harmonic balance solver determines the considered periodic flows from 30 to 50 times faster than the conventional time‐domain approach with negligible accuracy penalty to the latter.     

Aero‐structural dynamics of a flexible hub connection for load reduction on two‐bladed wind turbines

In this study, an innovative concept for load reduction on the two‐bladed Skywind 3.4 MW prototype is presented. The load reduction system consists of a flexible coupling between the hub mount, carrying the drive train components including the hub assembly, and a nacelle carrier supported by the yaw bearing. This paper intends to assess the impact of introducing a flexible hub connection on the system dynamics and the aero‐elastic response to aerodynamic load imbalances. In order to limit the rotational joint motion, a cardanic spring‐damper element is introduced between the hub mount and the nacelle carrier flange, which affects the system response and the loads. A parameter variation of the stiffness and damping of the connecting spring‐damper element has been performed in the multi‐body simulation solver Simpack. A deterministic, vertically sheared wind field is applied to induce a periodic aerodynamic imbalance on the rotor. The aero‐structural load reduction mechanisms of the coupled system are thereby identified. It is shown that the fatigue loads on the blades and the turbine support structure are reduced significantly. For a very low structural coupling, however, the corresponding rotational deflections of the hub mount exceed the design limit of operation. The analysis of the interaction between the hub mount motion and the blade aerodynamics in a transient inflow environment indicates a reduction of the angle of attack amplitudes and the corresponding fluctuations of the blade loading. Hence, it can be concluded that load reduction is achieved by a combination of reduced structural coupling and a mitigation of aerodynamic load imbalances. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison and testing of power reserve control strategies for grid‐connected wind turbines

The stability of the electrical grid depends on enough generators being able to provide appropriate responses to sudden losses in generation capacity, increases in power demand or similar events. Within the United States, wind turbines largely do not provide such generation support, which has been acceptable because the penetration of wind energy into the grid has been relatively low. However, frequency support capabilities may need to be built into future generations of wind turbines to enable high penetration levels over approximately 20%. In this paper, we describe control strategies that can enable power reserve by leaving some wind energy uncaptured. Our focus is on the control strategies used by an operating turbine, where the turbine is asked to track a power reference signal supplied by the wind farm operator. We compare the strategies in terms of their control performance as well as their effects on the turbine itself, such as the possibility for increased loads on turbine components. It is assumed that the wind farm operator has access to the necessary grid information to generate the power reference provided to the turbine, and we do not simulate the electrical interaction between the turbine and the utility grid. Copyright © 2013 John Wiley & Sons, Ltd.     

Ultimate load design of jacket‐type offshore wind turbines under tropical cyclones

Tropical cyclones are a high risk to offshore wind turbine (OWT) support structures, so the design conditions, including this risk, are necessary for tropical cyclone frequent occurrence zones. This study developed a computer program to carry out a critical ultimate load analysis and determine the optimum design for a Jacket‐type OWT support structure. The total weight of the OWT support structure after the optimal steel design with the yaw operative condition is always considerably smaller than that without for the steel design results under the loads of the GL Tropical Cyclone Technical Note (GL TCTN). This paper studies OWTs under the tropical cyclone classes 1 to 3 and the terrain categories A, to C, where the 1‐minute wind speed at 10‐m height is gradually increased from classes 1 to 3, and the surface roughness decreases from A to C. When the yaw can operate, the total steel weight consumption due to the tropical cyclone 1C, 1B, and 2C loads is lower than that for the IEC 61400‐3 loads. In the case of 1A, the overall steel consumption is only slightly higher than that of the IEC 61400‐3. However, for other conditions, the design should include the GL TCTN loads. For the yaw inoperative condition, the GL TCTN results are always largely dominant in the steel design, so the use of only the IEC 61400‐3 condition will result in extremely high risk to OWT support structures.     

A finite element model for pre‐stressed or post‐tensioned concrete wind turbine towers

Wind turbine support towers have been traditionally formed of structural steel tubular sections, being fabricated in large sections under factory conditions before being transported to site for erection. Given the trend towards developing turbines with hub heights in excess of 90 m, it is now necessary to develop towers of concrete and other materials that can provide improved dynamic properties and ease transportation difficulties over the structural steel solutions. Concrete towers of this height require pre‐stressing to overcome high vertical stresses induced in bending, which would otherwise lead to cracking in the concrete, with a resulting reduction in the tower's natural frequency. In order to properly understand the behaviour of concrete towers, it is necessary to take account of both material and geometric non‐linearity. Material non‐linearity of concrete is well understood, and geometric non‐linearity arises because of the imposition of an initial stress into the concrete by way of the application of pre‐stress. In this paper, a finite element model is proposed, which will describe the concrete as a continuum of four‐noded, two‐dimensional Reisser–Mindlin shell elements. The pre‐stressing tendons are to be represented by one‐dimensional bar elements, with an imposed pre‐stress. For the numerical examples considered in the paper, tendons are modelled to be post‐tensioned and debonded. The effect of varying the design parameter of magnitude of pre‐stress and the time dependence of pre‐stress force has been investigated using the model described. The impact that concrete compressive strength had on overall tower stiffness was also investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

Integrated design of downwind land‐based wind turbines using analytic gradients

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.     

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

This paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley & Sons, Ltd.     

Performance modelling for wind turbines operating in harsh conditions

This paper presents a new method of online wind turbine performance modelling (recursive parameter estimation) that addresses the nonlinearity associated with wind turbine performance characteristics. A sliding linearization algorithm is implemented to track changes in the turbine operating environment. A multivariate polynomial approximation of the turbine power coefficient is developed to produce a linear process model approximating the operating wind turbine. The estimated model parameters are recursively calculated to compensate for changes in both the turbine operating environment and the condition of the wind turbine. The algorithm models both the steady‐state and dynamic wind turbine performance throughout the entire operating range, producing a continuously valid turbine linearization with applications in gain scheduling and turbine performance optimization. Copyright © 2016 John Wiley & Sons, Ltd.     

Downwind pre‐aligned rotors for extreme‐scale wind turbines

Downwind force angles are small for current turbines systems (1–5 MW) such that they may be readily accommodated by conventional upwind configurations. However, analysis indicates that extreme‐scale systems (10–20 MW) will have larger angles that may benefit from downwind‐aligned configurations. To examine potential rotor mass reduction, the pre‐alignment concept was investigated a two‐bladed configuration by keeping the structural and aerodynamic characteristics of each blade fixed (to avoids a complete blade re‐design). Simulations for a 13.2 MW rated rotor at steady‐state conditions show that this concept‐level two‐bladed design may yield 25% rotor mass savings while also reducing average blade stress over all wind speeds. These results employed a pre‐alignment on the basis of a wind speed of 1.25 times the rated wind speed. The downwind pre‐aligned concept may also reduce damage equivalent loads on the blades by 60% for steady rated wind conditions. Even higher mass and damage equivalent load savings (relative to conventional upwind designs) may be possible for larger systems (15–20 MW) for which load‐alignment angles become even larger. However, much more work is needed to determine whether this concept can be translated into a practical design that must meet a wide myriad of other criteria. Copyright © 2017 John Wiley & Sons, Ltd.     

Airfoil optimisation for vertical‐axis wind turbines with variable pitch

To advance the design of a multimegawatt vertical‐axis wind turbine (VAWT), application‐specific airfoils need to be developed. In this research, airfoils are tailored for a VAWT with variable pitch. A genetic algorithm is used to optimise the airfoil shape considering a balance between the aerodynamic and structural performance of airfoils. At rotor scale, the aerodynamic objective aims to create the required optimal loading while minimising losses. The structural objective focusses on maximising the bending stiffness. Three airfoils from the Pareto front are selected and analysed using the actuator cylinder model and a prescribed‐wake vortex code. The optimal pitch schedule is determined, and the loadings and power performance are studied for different tip‐speed ratios and solidities. The comparison of the optimised airfoils with similar airfoils from the first generation shows a significant improvement in performance, and this proves the necessity to properly select the airfoil shape.     

Response load extrapolation for wind turbines during operation based on average conditional exceedance rates

The paper explores a recently developed method for statistical response load (load effect) extrapolation for application to extreme response of wind turbines during operation. The extrapolation method is based on average conditional exceedance rates and is in the present implementation restricted to cases where the Gumbel distribution is the appropriate asymptotic extreme value distribution. However, two extra parameters are introduced by which a more general and flexible class of extreme value distributions is obtained with the Gumbel distribution as a subclass. The general method is implemented within a hierarchical model where the variables that influence the loading are divided into ergodic variables and time‐invariant non‐ergodic variables. The presented method for statistical response load extrapolation was compared with the existing methods based on peak extrapolation for the blade out‐of‐plane bending moment and the tower mudline bending moment of a pitch‐controlled wind turbine. In general, the results show that the method based on average conditional exceedance rates predicts the extrapolated characteristic response loads at the individual mean wind speeds well and results in more consistent estimates than the methods based on peak extrapolation. Copyright © 2011 John Wiley & Sons, Ltd.