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Within the framework of the fourth phase of the International Energy Agency (IEA) Wind Task 29, a large comparison exercise between measurements and aeroelastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes, ranging in fidelity from blade element momentum (BEM) to computational fluid dynamics (CFDs) and compared to state-of-the-art field measurements from the 2 MW DanAero turbine. More than 15 different variable types ranging from lifting-line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions, a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD-synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and free vortex wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions, which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of the IEA Wind Technology Collaboration Program (TCP) has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.

Within the framework of the fourth phase of International Energy Agency IEA Wind Task 29, a large comparison exercise between measurements and aero-elastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes ranging in fidelity from Blade Element Momentum (BEM) to Computational Fluid Dynamics CFD and compared to state of the art field measurements from the 2MW DanAero turbine. More than 15 different variable types ranging from lifting line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and Free Vortex Wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of IEA Wind has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.

Ground acoustic impedance is important in the prediction of sound propagation. In this paper, a new measurement technique is developed based on the Image Source method. Two set-ups are used. Set-up 1 consists of a speaker at a height of 0.5 m and two microphones at heights of 0.2 m (or 0.3 m) and 0.5 m with a separation distance of 1.75 m between the speaker and microphones. Set-up 2 has a double separation distance and almost double heights of 0.5 m and 1 m. The impedance measurements were performed at grass-ground and soft soil-ground after harvest in two different wind farms in Denmark. The findings are: (1) The Nordtest method works in Set-up 1, but gives inconsistent results in Set-up 2. (2) The new method gives consistent results in both set-ups. (3) The flow resistivity obtained with an impedance model can only be used with that specific impedance model, and it becomes inaccurate when used with other impedance models. (4) The determination procedure of flow resistivity is suggested to work with any impedance model. (5) A relation between the flow resistivity obtained with the one-parameter model of Delany and Bazley and that obtained with the four-parameter model of Attenborough is derived.

Unsuitable wind farm layout design influences wind energy conversion efficiency as well as wind farm noise radiation. In the past, more attentions were paid on high energy efficiency wind farm design, but little progress was achieved on noise constrained wind farm optimization. The present work aims at developing a numerical optimization framework that combines the modelling of wind farm wake and wind farm noise with the annual energy production in an optimization algorithm. Firstly, a wind turbine wake model is implemented in the optimization framework to obtain the detailed flow field of a wind farm. Secondly, the noise radiation around a wind farm is simulated with an advanced method based on solving the parabolic wave equation. Thirdly, the annual energy production of a wind farm is calculated with respect to a prescribed wind speed probability distribution. Finally, wind farm layout optimization is carried out with a cost model built on the levelised cost of energy and an efficient iterative approach. According to the results, the developed optimization framework is capable of seeking a trade-off between maximum energy yield and minimum negative environmental effect.

This paper investigates the characteristics of turbine wake which is the major factor determining the turbulence in wind farm and overall wind farm performance. Using the lattice Boltzmann method (LBM) and the large eddy simulation (LES) approach, dynamic simulations of detailed fluid flow and wake trajectory induced by the MEXICO wind turbine were carried out. Moreover, with the help of adaptive mesh refinement (AMR) method, the lattice units were allowed to be automatically refined to allocate more grid points on the solid walls and in the wake region. The simulation results were firstly compared with the experimental data for validation of the numerical model, as well as for investigation of the impact of sub‐grid scale (SGS) turbulence models. Within the present modeling framework, the characteristics of transient wake propagation, detailed flow field, and rotor‐tower interaction were investigated, and the effect of tip‐speed ratio was also analyzed. The present work indicates that, with a suitable SGS turbulence model, the LBM‐LES method is an effective way for dynamic simulations of wake structures induced by a wind turbine with a rotating rotor.

Most of the k − e closures for modeling stratified surface layer are derived from the classical similarity functions and might fail in complex terrain due to limitations of the classical similarity functions. Despite the classical similarity functions to limit the flux Richardson number R f , we present new similarity functions estimated from field measurement in full range of R f and propose a k − e model using the new similarity functions to improve predictions of stratified surface layer flows in complex terrain. Measurements show that the classical similarity functions are partially valid in complex terrain and the wind shear under strongly stable conditions is constrained at large R f . According to numerical studies in two areas of complex terrain, models using the classical similarity functions and the new similarity functions both present good predictions of convective airflows in complex terrain. The new similarity functions are shown to significantly improve the k − e model in predicting stably stratified airflows in complex terrain by constraining the wind shear at large R f , while the classical similarity functions without limiting the wind shear lead to significantly misestimating the wind speedup factor under stable conditions. Using the proposed model to predict flows in wind farm could benefit wind resource estimation and wind power forecasting.

The tip loss is an important phenomenon in wind turbine aerodynamics and has to be modelled separately in the blade element momentum (BEM) method for predicting the wind turbine aerodynamic performance. Instead of following the conventional trend of thought, the present study for the first time regards the interference factors as a summation of two parts. One is determined by the downwash due to the three-dimensional effect and the other is determined by rotation. Two corresponding factors of FS and FR are proposed to represent the effects of the two parts, respectively. This way of consideration implies a new model for the BEM method, providing appropriate tip loss predictions. The new model is validated in BEM computations of the NREL Phase Ⅵ rotor, the Swedish WG 500 rotor, and the NREL 5 MW reference rotor, with a radius of 2.675 m, 5.029 m and 63 m, respectively. It shows a good accuracy in a wide range of tip speed ratios from 1.58 to 11.3, and presents a good robustness in the cases involving high angles of attack and in the cases involving high axial interference factors. That makes the new model attractive for tip loss correction in BEM.

The power generated by a wind turbine largely depends on the properties of wind resource. In this work, wind characteristics in different regimes occurring throughout the idealized diurnal cycle and its impact on wind turbine's power performance are investigated systematically by means of large-eddy simulation (LES), and blade element momentum method (BEM), respectively. Through a precursor simulation of the atmospheric boundary layer (ABL) over a homogenous surface throughout a day, it is found that the resulting shapes of wind profiles (including wind speed, wind direction and turbulence level) vary significantly at different time periods, induced by distinct stabilities of the atmosphere. The simulated wind field data are then applied to a NREL 5 MW wind turbine for its power evaluation. Due to variabilities in wind shear and turbulence, the equivalent (disk-averaged) wind speed is introduced for power prediction. It is found that the magnitude and fluctuation of turbine's diurnal power are closely related to the atmospheric stability. In general, the average power production is higher under convective conditions during the day than under stable conditions at night, with a difference approaching 24.4%. This indicates that wind energy resource assessment will close to reality and benefit from increased accuracy if atmospheric stability impacts are considered for turbine's power predictions.

A new wind farm during its development phase requires a technical evaluation of its annual power prediction and noise issue in the area in and around the wind farm. In this paper, for the first time, an advanced numerical method for wind farm assessment is developed, which consists of a Reynolds averaged Navier-Stokes - Actuator Disc model for the flow modelling, a semi-engineering model for the noise source modelling, and a parabolic wave equation model for the sound propagation modelling. The developed method can evaluate both annual energy production of wind farm and noise emission at receivers nearby. The wind farm considered in this study is located in a typical mountainous area. The energy production of the wind turbines is simulated and compared with measured data. The flow simulations over the complex terrain are performed using the in-house developed Navier-Stokes solver. A wind farm noise map is created by solving the parabolic wave equation. The obtained flow results are the inputs to the parabolic wave equation solver for sound propagation. The numerical computations are performed on a standard high-performance computer cluster. The developed numerical method provides a reliable assessment method for wind farm about its energy efficiency and noise features.

Promising technology for the conversion of cellulose to aromatics by catalytic fast pyrolysis (CFP) was investigated using five zeolite catalysts, i.e., 5A, SAPO-34, HY, BETA and HZSM-5. The relationship between the porosity and acidity of different zeolites with product selectivity was studied. The results showed that both the acidity and pore size of the zeolite significantly affected the production of aromatics and coke, especially the bio-oil composition. The bio-oils obtained over 5A or SAPO-34 (small pore

In order to effectively improve the quality of bio-oil from catalytic pyrolysis (CFP) of sewage sludge (SS) and to produce aromatic-rich bio-oil, the evolution mechanisms of nitrogen and formation of aromatics during CFP of SS over metal-loaded HZSM-5 (HZ) were investigated in depth. The results show that the presence of metals caused the slight decrease in oil-C (carbon in bio-oil) yield and the increase in gas yield and total carbon yield of aromatics. Ni-loaded HZ has a better performance than Co-loaded one in improving the carbon yield of aromatics, while the maximum yield of 11.2% was obtained over 0.5Ni-HZ. Metal-loaded HZ promoted the formation of monocyclic aromatics rather than polycyclic aromatics and Ni significantly enhanced the selectivity of benzene. With the addition of Ni and Co, more nitrogen was released as NH3 or fixed in solids. NH3 yield increased by raising the metal loading, and the highest NH3 yield of 36.8% was obtained over 3.0Ni-HZ. Ni and Co promoted the cracking reactions during CFP to generate more H radicals, which have advantages to the ring-opening of N-containing heterocycles in SS to produce aromatics and NH3. A possible catalytic mechanism of CFP of SS over metal loaded HZ was proposed in this study.

The ground surface near a wind turbine has a significant influence on the sound propagation from the turbine, and it is therefore important to determine the ground impedance, a quantity that characterizes the ground surface acoustically. Outdoor ground parameters required by a multi-parameter model used to calculate the ground acoustic-impedance are typically unknown, which brings inconvenience for the model use. This paper introduces a technique to determine the parameters of ground acoustic-impedance for use in a multi-parameter impedance model (for example, the Attenborough four-parameter model). The technique consists of three steps: first, the data for sound-pressure level measured at a distance from two different heights are collected, and the sound-pressure-level difference is calculated; second, in line with the experimental data and the sound-pressure-level calculation formula, the MATLAB optimization tool is used to find the optimal values of the parameters used in the impedance model; and finally, when the optimization is finished, the acoustic impedance of the ground is obtained by substituting the optimal values into the impedance model. To check the performance of the calculation, the calculated sound-pressure-level difference is compared to the experimental one. Compared with a traditional method, the technique can significantly reduce the calculation error.

In this paper, the effects of porous trailing edge on airfoil aerodynamic and aeroacoustic characteristics are investigated. In order to reduce the airfoil trailing edge noise and in the mean while to minimize the negative influence on aerodynamic performance, a new filling method is proposed such that porous medium is only used in the suction side half of the trailing edge which is more sensitive to the noise generation. The Large Eddy Simulation (LES) for flow and Ffocws-Williams and Hawkings (FW-H) methods for acoustics are used. At a Reynolds number of 2.63×105 and various angles of attack, the NACA0018 airfoil profile with 20% chords porous trailing edge is studied under two porous configurations, namely a fully-porous and a suction-side-porous trailing edge types. The results show that the flowfield of the porous airfoils are also influenced due to the presence of the porous medium. The noise radiation from the newly proposed porous airfoil achieves a noise reduction of 4.3dB at an angle of attack α=0°, and 4.07dB at an angle of attack α=2°. On the other hand, the loss of lift force is about 0.8%-1.5% in the angle of attack range of 0~10°.

Recently, actuator line model become popular in studying wind-turbine wakes. However, existing models ignore or inaccurately describe nacelle effects, which have been shown to pose significantly impact on wakes. To address the physics underlying here, we develop the actuator line model with large-eddy simulation by introducing a new anisotropic body-force projection model. We validate the new model against a field experiment and the validation indicates that the new anisotropic model can predict the wake more precise than the existing isotropic model. Furthermore, we extend the study to wake characteristics under various yaw conditions. The results show that the thrust component normal to the flow direction creates a skewed wake behind the turbine, which in turn promotes the wake transition from the two-peak profile to the one-peak profile. The wake skew exacerbates the instability of the tip vortex and causes the wake region to narrow. At small yaw angles, the nacelle vortex radially diffuses and blends with the tip vortex in the far wake. At large yaw angles, the nacelle vortex intercepts the tip vortex in the near wake due to the different spatial distribution of thrust. It is concluded that the nacelle significantly affects wind-turbine wakes especially during yaw condition.

In order to develop super-large wind turbines, new concepts, such as downwind load-alignment, are required. Additionally, segmented blade concepts are under investigation. As a simple example, the coned rotor needs be investigated. In this paper, different conning configurations, including special cones with three segments, are simulated and analyzed based on the DTU-10 MW reference rotor. It was found that the different force distributions of upwind and downwind coned configurations agreed well with the distributions of angle of attack, which were affected by the blade tip position and the cone angle. With the upstream coning of the blade tip, the blade sections suffered from stronger axial induction and a lower angle of attack. The downstream coning of the blade tip led to reverse variations. The cone angle determined the velocity and force projecting process from the axial to the normal direction, which also influenced the angle of attack and force, provided that correct inflow velocity decomposition occurred.

H5 was developed to mesoporous system via alkali treatment, and sulfated zirconia (SZr) modified hierarchical HZSM-5 (AT0.2/H5) catalysts (named AT0.2/SZxH5) were successfully prepared by a series of post-treated method followed by Zr(NO3)4·5H2O and H2SO4 as active component. The innovative approach really enhanced acidity of AT0.2/H5 support, which results from the interaction of the sulfate groups attached to zirconia that doped on MFI framework with silanol groups. Adding active component can not only compensate for the loss of acid sites caused by NaOH treatment, but also re-develop more active sites owing to the formation of SZr. Meanwhile, NH3-TPD showed higher strong acid sites and weak acid sites in AT0.2/SZ6:1H5 than other prepared catalysts. The catalytic activity in converting lignite pyrolysis volatiles to light aromatics, including benzene, toluene, ethylbenzene, xylene and naphthalene (BTEXN), correlates well with the enhanced acidity. Compared with H5, the yield of light aromatics increased significantly from 15.2 to 19.3 mg/g over AT0.2/SZ6:1H5 at 600 °C. According to the characterization of N2 adsorption–desorption, SEM, and XRD, both the S species as SO42− and Z species as ZrO2 improved the textural properties of the support itself. Besides, the molar ratio of S/Zr was in direct proportion to the yield of BTEXN in a certain range, and 6:1 exhibited the optimal catalytic performance. The prepared solid acid catalyst provides a promising strategy to directional prepare light aromatics during lignite catalytic pyrolysis.

PyWake isan AEP calculator for wind farms implemented in Python including a collection of wake models.

Increasing maintenance costs will hinder the expansion of the wind power industry in the coming decades. Training personnel, field maintenance, and frequent boat or helicopter visits to wind turbines (WTs) is becoming a large cost. One reason for this cost is the routine turbine inspection repair and other stochastic maintenance necessitated by increasingly unbalanced figure loads and unequal turbine fatigue distribution in large-scale offshore wind farms (OWFs). In order to solve the problems of unbalanced fatigue loads and unequal turbine fatigue distribution, thereby cutting the maintenance cost, this study analyzes the disadvantages of conventional turbine fatigue definitions. We propose an improved fatigue definition that simultaneously considers the mean wind speed, wind wake turbulence, and electric power generation. Further, based on timed automata theory, a power dispatch approach is proposed to balance the fatigue loads on turbines in a wind farm. A control topology is constructed to describe the logical states of the wind farm main controller (WFMC) in an offshore wind farm. With this novel power control approach, the WFMC can re-dispatch the reference power to the wind turbines according to their cumulative fatigue value and the real wind conditions around the individual turbines in every power dispatch time interval. A workflow is also designed for the control approach implementation. Finally, to validate this proposed approach, wind data from the Horns Rev offshore wind farm in Denmark are used for a numerical simulation. All the simulation results with 3D and 2D figures illustrate that this approach is feasible to balance the loads on an offshore wind farm. Some significant implications are that this novel approach can cut the maintenance cost and also prolong the service life of OWFs.

This paper evaluates the influence of atmospheric stability and topography on wind turbine performance and wake properties in complex terrain. To assess atmospheric stability effects on wind turbine performance, an equivalent wind speed calculated with the power output and the manufacture power curve is proposed and calibrated with the mast hub-height wind speed. After estimating the thrust coefficient and turbulence dissipation, this paper examines wind turbine performance curves and wake profiles segregated by atmospheric stability. Results show that the equivalent wind speed at a given mast wind speed can increase by 2% under stable conditions and decrease by 5% under unstable conditions as compared with that under neutral conditions, yielding about 16% reductions of power output and thrust coefficient from stable conditions to unstable conditions. Due to the lower thrust coefficient and the enhanced turbulence, the wind turbine wakes are found to recover faster under unstable conditions than under other stability conditions. Differences in wind turbine performance and asymmetric wake profiles due to topographic effects are also observed. Results suggest that atmospheric stability and topography have significant influences on wind turbine performance and wake properties. Considering effects of atmospheric stability and topography will benefit the wind resource assessment in complex terrain.

The diurnal variation of atmospheric conditions over land has a significant effect on the wind and temperature distributions which greatly influence the generation and propagation of wind turbine aerodynamic sound. In this paper, a fully consistent unsteady approach is used to study wind turbine noise such that large eddy simulation with a rotational actuator disk wind turbine model is used to model the wind and temperature around a mega-watt scale wind turbine over a diurnal cycle, and time dependent flow and temperature fields are used as input to the coupled wind turbine noise generation-propagation model. Computations are carried out for four different 10 min datasets selected at certain periods of a day for a same hub height wind speed. It is observed that the time dependent as well as the time averaged sound pressure levels in near field do not show large variations during the day. However, as we move away from the turbine, the propagation effects take over and downwind of the turbine the night time levels exceed the day time levels (at 3600 m the averaged difference reaches 6.5 dBA).

It is known that wind turbines actually operate in the lower part of the atmospheric boundary layer (ABL), the modelling of this ABL flow is an important precondition for the simulations of wind turbine wakes. So, the capabilities of various inlet boundary conditions with related modelling methodologies in constructing equilibrium ABL are assessed firstly through cases of ABL flows over uniformly rough terrain, to ascertain that there are no substantial changes in the prescribed profiles throughout the whole computational domain. In this process, six popular turbulence inflow profiles, including four uniform and two non-uniform ones, are considered and investigated. Then, sensitivity studies on inflow profiles for predicting wind turbine wake development are carried out. Through comparing with the Sexbierum field experimental data, in terms of wake velocity and turbulence intensity along the cross-wind direction at several downstream positions, this study finds out that the shape and magnitude of wake velocity and wake turbulence profiles are significantly affected by different inflow profiles. Possible reasons for this sensibility are discussed, and accordingly, some suggestions are given to improve the accuracy of wind turbine wake simulation.

The present review paper provides a comprehensive overview of the research activities on wind turbine aeroacoustics at DTU over the last 20 years, as well as it gives the state-of-the-art of noise prediction models for wind turbines under complex inflow conditions. Various noise generation models developed at DTU are described and analyzed, including models based on the acoustic analogy, flow-acoustics splitting techniques, Amiet's model, and various engineering models. Some of the models are coupled to existing aero-elastic software and computational fluid mechanics models developed at DTU, and implemented in the simulation platform WindSTAR (Wind turbine Simulation Tool for AeRodynamic noise). This simulation platform consists of WindSTAR-Gen, dealing with models for generation of noise and design of low-noise wind turbines, and WindSTAR-Pro, which is developed to handle the modeling of long distance acoustic propagation. As specific features of the WindSTAR-Pro package, the rotation of the noise sources is modeled, the propagation simulations combine the so-called Parabolic Equations (PE) propagation model with numerical flow simulations to take into account effects from wind turbine wakes, atmospheric turbulence and wind shear.

To upgrade pyrolysis vapors derived from the catalytic fast pyrolysis (CFP) of Baiyinhua lignite (BYHL) in a drop tube reactor, various Co-loading (1, 3, 6 and 10 wt%) zeolites were prepared via wet impregnation. The effect of Co-loading and pyrolysis temperature ranged from 400 to 700 °C on product yields and tar properties were investigated in this work. The results showed that the product yield and the composition of pyrolysis tar were significantly affected by Co-modified HZSM-5 and parent HZSM-5 compared with no catalyst. Incorporation of metal-Co in HZSM-5 had an additional effect on the performance of the parent HZSM-5. The total content of benzene (B), toluene (T), ethylbenzene (E), xylene (X) and naphthalene (N) significantly increased and it reached 24.2 mg/g at Co-loading of 3 wt% (3Co-H-5) at 600 °C. 3Co-H-5 exhibited also a promising performance in lowering the oxygen content from 49.8% without a catalyst to 14.6% by GC–MS analysis. The deoxygenation mechanisms over metal-Co active centers and acid-catalyzed reactions, such as isomerization and cracking, over acid sites of HZSM-5 were speculated in this work. In addition, the temperature of 600 °C is not only appropriate for fast pyrolysis of lignite to obtain a higher pyrolysis tar, but also for catalytic pyrolysis to obtain BTEXN.

Floating vertical-axis wind turbines (VAWTs) display considerable advantages over horizontal-axis wind turbines (HAWTs) due to their Omni directionality, better structural scalability, and higher system stability. VAWTs are therefore experiencing a regained interest for use in large-scale offshore wind energy generation. However, the aerodynamic performance of lift-type VAWTs is lower than that of HAWTs. To provide more guidance for the performance improvement of VAWTs, this article reviews the existing approaches to aerodynamic performance enhancement from the perspectives of geometric parameters, flow control methods, blade shape modification, power augmentation devices, hybrid systems, and variable pitch control. Additionally, the findings of various investigations on the performance improvement of VAWTs are summarized.

The structural evolution of coal during pyrolysis process has been studied for a long time but remains unclear, mainly due to the extraordinary complexity of coal structure. The present study aimed to further identify the chemical structural evolution of different parts in the semi-chars obtained by low-temperature pyrolysis of a Chinese low-rank coal (Shenfu coal). Semi-chars and their solvent extracted materials (oil, resin, asphaltene and preasphaltene) were analyzed by 13C nuclear magnetic resonance (NMR) mass spectrometry, diffuse reflectance Fourier transform-infrared spectroscopy (DRIFTs) and ultraviolet (UV) fluorescence spectroscopy, separately. The results showed that 23.8% weight loss occurred during the whole pyrolysis process at 435 °C and half of the weight loss was achieved in the initial five minutes. By contrast, amount of the extracts in semi-chars (solvent extracted metaplast materials) decreased more dramatically in the initial stage of the pyrolysis, and 51.8% of the extracts disappeared in the initial one minute. Oil and preasphaltene showed higher thermal sensitivity than asphaltene and resin. Considerable content of metaplast materials with 1–2 rings (especially in oil and/or asphaltene) were produced along with the pyrolysis process. Condensed aromatic structures in semi-chars were relatively stable and mainly decomposed in the initial stage of pyrolysis which could be attributed to the dramatically decomposition of some active aromatic structures with ≥ 4 rings (especially the components in extracts such as preasphaltene). The results provided a new insight into the structural evolution of coal during low-temperature pyrolysis process, which specially considered the detailed information of the solvent extractable metaplast materials in semi-chars.

The unsteady nature of wind turbine noise is a major reason for annoyance. The variation of far-field sound pressure levels is not only caused by the continuous change in wind turbine noise source levels but also by the unsteady flow field and the ground characteristics between the turbine and receiver. To take these phenomena into account, a consistent numerical technique that models the sound propagation from the source to receiver is developed. Large eddy simulation with an actuator line technique is employed for the flow modelling and the corresponding flow fields are used to simulate sound generation and propagation. The local blade relative velocity, angle of attack, and turbulence characteristics are input to the sound generation model. Time-dependent blade locations and the velocity between the noise source and receiver are considered within a quasi-3D propagation model. Long-range noise propagation of a 5 MW wind turbine is investigated. Sound pressure level time series evaluated at the source time are studied for varying wind speeds, surface roughness, and ground impedances within a 2000 m radius from the turbine.

Most studies on offshore wind farm design assume a uniform wind farm, which consists of an identical type of wind turbines. In order to further reduce the cost of energy, we investigate the design of non-uniform offshore wind farms, i.e., wind farms with multiple types of wind turbines and hub-heights. Given a set of different types of wind turbines with a different default hub height for each type, we can specify the design of a wind farm by the types of turbines, number of turbines for each type, and turbine locations. We consider the optimization of such design to minimize the levelized cost of energy, which is calculated using a capital cost model that covers the turbine cost and the balance of plant cost. An empirical wind turbine design cost and scaling model is utilized to model the cost of turbines with different sizes. Constraints on wind farm boundary, wind turbine proximity and total capacity are also included. We solve the problem with a newly developed extended random search algorithm and tested it in a realistic design optimization problem based on the Horns Rev 1 offshore wind farm in Denmark. The optimized non-uniform designs are compared with their uniform counterparts. We find that a non-uniform design can achieve a lower levelized cost of energy than its uniform counterparts, when the capital cost per MW is slightly lower for the smaller size turbine. Comparison with the mixed-discrete particle swarm optimization algorithm is also carried out for a non-uniform wind farm design problem with a fixed number of turbines, which shows the effectiveness and superiority of the proposed algorithm. Finally, the advantages and possible disadvantages of non-uniform design are also identified and discussed.

In this paper, we investigate the sound propagation from a wind turbine considering the effects of wake-induced velocity deficit and turbulence. In order to address this issue, an advanced approach was developed in which both scalar and vector parabolic equations in two dimensions are solved. Flow field input was obtained using the actuator line (AL) technique with Large Eddy Simulation (LES) to model the wind turbine and its wake and from an analytical wake model. The effect of incoming wind speed and atmospheric stability was investigated with the analytical wake input using a single point source. Unsteady acoustic simulations were carried out with the AL/LES input for three cases with different incoming turbulence intensity, and a moving source approach to mimic the rotating turbine blades. The results show a non-negligible effect of the wake on far-field noise prediction. Particularly under stable atmospheric conditions, SPL amplification reaches up to 7.5 dB at the wake centre. Furthermore, it was observed that when the turbulence intensity level of the incoming flow is higher, the SPL difference between the moving and the steady source is lower.

The blade element momentum (BEM) theory is widely used in aerodynamic performance calculations and optimization applications for wind turbines. The fixed point iterative method is the most commonly utilized technique to solve the BEM equations. However, this method sometimes does not converge to the physical solution, especially for the locations near the blade tip and root where the failure rate of the iterative method is high. The stability and accuracy of aerodynamic calculations and optimizations are greatly reduced due to this problem. The intrinsic mechanisms leading to convergence problems are addressed through both theoretical analysis and numerical tests. A term from the BEM equations equals to zero at a critical inflow angle is the source of the convergence problems. When the initial inflow angle is set larger than the critical inflow angle and the relaxation methodology is adopted, the convergence ability of the iterative method will be greatly enhanced. Numerical tests have been performed under different combinations of local tip speed ratio, local solidity, local twist and airfoil aerodynamic data. Results show that the simple iterative methods have a good convergence ability which will improve the aerodynamic or structural design of wind turbines. Copyright © 2017 John Wiley & Sons, Ltd.