Key Engineering Materials Vols. 569-570

Abstract: A multiple generator drivetrain (MGD), where a single large generator in a wind turbine is replaced by multiple generators with the same or different rated powers, is proposed along with an automatic switch as an alternative to an existing MGD configuration. Qualitative and quantitative comparisons of a MGD with a conventional drivetrain are provided to better understand the advantages and disadvantages of having a MGD in wind turbines. New approaches for improving the efficiency and the reliability, expanding the operational range, and reducing the cost of a wind turbine are mentioned. A simple mathematical model for a MGD with electromagnetic clutches is developed, a novel prototype of a MGD is designed and fabricated, and experiments are conducted on the prototype. It is concluded that a multiple-generator drivetrain with generators operating individually or in parallel has a better potential of improving the efficiency and the reliability, expanding the operational range, and reducing the cost of offshore and onshore wind turbines than the existing MGD configuration.

Abstract: This paper will show the first results of a long term monitoring campaign on an offshore wind turbine in the Belgian North Sea. It will focus on the vibration levels and resonant frequencies of the fundamental modes of the support structure. These parameters will be crucial to minimize O&M costs and to extend the lifetime of offshore wind turbine structures. For monopile foundations for example, scouring and reduction in foundation integrity over time are especially problematic because they reduce the fundamental structural resonance of the support structure, aligning that resonance frequency more closely to the lower frequencies. Since both the broadband wave energy and the rotating frequency of the turbine are contained in this low frequency band, the lower natural frequency can create resonant behavior increasing fatigue damage. Continuous monitoring of the effect of scour on the dynamics of the wind turbine will help to optimize the maintenance activities on the scour protection system. To allow a proper continuous monitoring during operation, reliable state-of-the-art operational modal analysis techniques should be used and these are presented in this paper. The methods are also automated, so that no human-interaction is required and the system can track the natural frequencies and damping ratios in a reliable manner.

Abstract: The aim of this paper is to develop an active structural control scheme to control wind turbine nacelle/tower out-of-plane vibration. An active tuned mass damper (ATMD) is designed an placed inside the turbine nacelle. An EulerLagrangian wind turbine model based on energy formulation is developed for this purpose, which considers the structural dynamics of the system and the interaction between in-plane and out-of-plane vibrations. Also, the interaction between the blades and the tower including the ATMD is considered. The wind turbine is subjected to gravity and turbulent aerodynamic loadings. A three-dimensional (3D) model of a wind turbine foundation is designed and analysed in the finite element geotechnical code PLAXIS. The rotation of the foundation is measured and used to calculate a rotational spring constant for use in wind turbine models to describe the soil-structure interaction (SSI) between the wind turbine foundation and the underlying soil medium. Damage is induced in the soil medium by a loss in foundation stiffness. The active control scheme is shown to reduce nacelle/tower vibration when damage occurs.

Abstract: As Development of smallvertical axis wind turbines (VAWT) for urban use is becoming an interestingtopic both within industry and academia. However, there are few new designs ofvertical axis turbines which are customized for building integration. These aregetting importance because they operate at low rotational speed producing veryless noise during operation, although these are less efficient than HorizontalAxis Wind Turbines (HAWT). The efficiency of a VAWT has been significantlyimproved by H-Darrieus VAWT design based on double airfoil technology asdemonstrated by the authors in a previous publication. Further, it is well knowthat the variation of the blade pitch angle during the rotation improves thepower efficiency. A blade pitch variation is implemented by active blade pitchcontrol, which operates as per wind speed and position of the blade withrespect to the rotor. A double multiple stream tube method is used to determinethe performance of the H-Darrieus VAWT. The power coefficient is compared withthat of a fixed pitch and a variable pitch double airfoil blade VAWT. It isdemonstrated that an improvement in power coefficient by 20% is achieved andthe turbine could start at low wind speed

Abstract: The sinking of wind turbines (WTs) with monopile foundations is one of the major issues in the offshore wind industry nowadays. Dynamic wind and wave loads act on the WTs causing vibrations of the structure. However, grouted connections in the monopiled WTs are not designed well enough to transfer bending moments from the wind loading. When the load capacity of the grouted connection is reached, stress cracks appear in the grout causing transition piece to slide down. Direct measuring of the fatigue load, called the tower bending moment, causing fatigue failures and sinking of the WTs is expensive and practically unfeasible. This paper suggests a low-cost, model-based algorithm for indirect measuring of the tower bending moments from the WT dynamic response measurements. The bending moment is estimated recursively using well-known Kalman filter theory. The method is validated using WT simulated data, assuming different measurement noise levels.

Abstract: The paper consists of two parts. First, Electromechanical Impedance (EMI) method is proven to be able to determine some vibrational characteristics of the investigated structure. In order to verify this statement, Scannig Laser Vibrometry (SLV) is used to correlate frequency peaks of velocity (or displacement) operational deflection shapes with corresponding ones obtained by EMI method. Finally, the influence of moisture content in CFRP sample on resonance peaks is investigated using EMI method. Damage detection indicator in this case is based on shifts of resonant peaks.

Abstract: Structural health monitoring systems are required for detecting damage in structures so as to facilitate their timely maintenance and repair and to prevent catastrophic structural failure. To date, a variety of different sensor platforms (e.g., piezoelectric materials, fiber optics, and wireless sensors) have been proposed for SHM. However, they still suffer from high energy demand, large form factors, and durability issues, particularly when applied for monitoring space structures and reusable spacecraft. In a previous study, a bio-inspired and photocurrent-based strain sensor has been developed. This poly(3-hexylthiophene) (P3HT)-based nanocomposite sensor has been shown to generate photocurrent whose magnitude varies in tandem with applied strain. However, the photocurrent generation performance of the sensor is quite low. In addition, the strain sensing mechanism is not fully understood. In this study, the performance of the photoactive thin films were enhanced, and its strain sensing characteristics were analyzed using ultraviolet-visible (UV-Vis) absorption spectroscopy. First, multilayered photoactive and P3HT-based thin films were assembled via spin coating. The photocurrent generation performance of the films was evaluated using two methodologies, namely its photocurrent time history and current-voltage (IV) response. Uniform coating of the photoactive layer and high purity aluminum electrodes were crucial for improving their photocurrent generation. Second, light absorption properties of the P3HT-based photoactive layer were investigated at different strain levels using a UV-Vis spectrophotometer. Light absorption was shown to vary linearly with applied tensile strains.

Abstract: In reported research piezoelectric sensors were used for damage identicitaion purposes. Piezoelectric sensor was used for specimen excitation. Two techniques were investigated. The Electromechanical impedance (EMI) technique and guided wave based technique. The principle of EMI technique is based on measurement and analysis of impedance of piezoelectric transducers bonded on or embedded in investigated structure. It is assumed that structural change should influence the impedance characteristics of the transducers. The guided wave based technique is based on the guided elastic wave propagation phenomena. This type of waves can be used in order to obtain information about structure condition and possibly damaged areas. In reported investigation piezoelectric sensor was used to excite guided waves in chosen structural element. Dispersive nature of guided waves results in changes of velocity with the wave frequency, therefore a narrowband signal was used to minimize the dispersion phenomenon. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided wave. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This non-contact tool allowed to investigate phenomena related to wave propagation. For both techniques numerical signals processing tools were developed. These numerical tools were designed to extract damage relevant features from EMI measurements and guided wave propagation measurements. The damage index (DI) was introduced on the basis of the extracted features.

Abstract: The investigation was focused on Carbon Fibre Reinforced Polymers (CFRP). In the first part of research the aim was the characterization of CFRP surfaces. These surfaces were influenced by release agent, hydraulic fluid, moisture and overheating. In the second part of research adhesive bond quality was investigated. Three different cases of possible weak bonds were focused on. Weak bond caused by release agent contamination, moisture contamination and poor curing of adhesive. The characterization was conducted using laser vibrometry used as NDT tools. An active element in the form of piezoelectric transducer was used to excite the samples made out of CFRP material. Laser vibroemter was used to register the surface response. Combining the piezoelectric excitation with laser sensing a tool was obtained to measure precisely the propagating elastic waves. The excited waves were measured in defined points by the vibrometer obtaining the wavefield. In order to characterize the surface and bonding quality an indicator was proposed based on propagating wave parameters. The guided elastic wave velocity depends material properties (Young modulus, density, Poisson ratio) and thickness of the sample. It was assumed that comparison of the velocities can provide an information about the bond condition. All the investigated scenarios showed deviation from the reference case.

Abstract: In this paper, a method for the localisation and quantification of impacts on orthotropic plates, based on the measurement of the resulting vibrational energy flow, or structural intensity, is presented. The outputs of a compact array of bonded piezo-electric sensors are digitally filtered and combined to yield a local estimate of the instantaneous vibrational energy flow. In the case of transient vibration due to an impact, two or more such estimates can be used to localise the point of impact. The use of energy flow vectors for localisation requires a priori knowledge of the material properties, but may provide more direct access to position information than conventional techniques. Furthermore, the magnitude of the measured energy flow, when combined with knowledge of the impact location, can provide a quantitative measure of impact severity. The approach is based on Kirchhoff orthotropic plate theory, and is thus applicable to low frequencies, and utilises instantaneous estimates of the local wave field to allow the estimation of the various forces, moments and velocities that are required to calculate the vibrational energy flow. The theoretical background and principles of implementation of the approach are outlined, and the effects of sensor array design on systematic errors and sensitivity to measurement noise are discussed. Numerical simulations are used to assess the effectiveness of the technique and to determine its sensitivity to noise and other errors. These simulations suggest that the obtained energy flow estimates can be used for impact localisation.