IRONLESS PERMANENT MAGNET GENERATORS FOR DIRECT-DRIVEN OFFSHORE WIND TURBINES

Since the beginning of this century, the offshore wind power industry has witnessed fast development, as the result of the increasing awareness of climate change and the need for diversifying power supply. Offshore has vast area available with high wind speed, which is an ideal place for large-scale wind power exploitation. However, these advantages come with technological challenges. One of the key challenges is to develop high-power cost-effective wind turbines with high reliability. Previous publications have demonstrated the potential of using ironless permanent magnet generator (iPMG) to reduce the total weight and cost of a wind turbine. Unfortunately, before the start of the research reported in this thesis, there was no systematic investigation of the iPMG concepts. As shown in this thesis, if the traditional direct-driven high-power iron-cored PMG is used for offshore wind turbines, instead of iPMG concept, a significant amount of the total weight goes to the supporting structures (inactive parts). The supporting structures maintain the machine reliability by counter-reacting to various forces, such as the machine gravity, electromagnetic torque, rotor-to-stator force (for iron-cored machine), rotor-torotor force (if there are multiple rotors), and force caused by faults. The “secret” of iPMG roots at the negligible normal stress between its rotor and stator (active parts). By using non-magnetic material in the stator, the rotor will not exert attracting force on stator. Having low rotor-to-stator force means lower requirement to the supporting structures, i.e., the supporting structures can be lightweight and low-cost. Due to the low air gap field and thus low tangential stress, iPMG has low torque production. Therefore, iPMG is normally designed with large diameter to produce higher torque with longer force arm. Classical machine design theory has concluded that the machine torque is roughly proportional to its volume. A large-diameter iPMG will therefore have short axial length, which means small aspect ratio (the machine axial length divided by its outer diameter). This ring-shape feature requires 3D approach to analyse the magnetic and thermal fields. Traditionally, it is a common practice to do machine preliminary design and optimization with analytical methods. The 2D or even 3D finite element method (FEM) is considered to be accurate but time-consuming, though the performances of computing codes and hardware are being steadily improved. Multi-core computers and distributed-memory clusters are normally available to most machine designers. Some academic users even have the access to super computers. The objective of this research is to investigate the iPMG technologies with the help of advanced modelling approaches. The main results reported in this thesis are • An overview of the ironless permanent magnet machines (iPMM) (Chapter 1). • A comprehensive overview of the generator technologies for operational offshore wind turbines, the comparison of the efficiencies for different drive trains, and a review of the new generator concepts (Chapter 2). • A method for evaluating the goodness of any generator design and predicting the generator total weight, and a design and optimization strategy for investigating the weight variation of iron-cored high-power PMGs (Chapter 3). • A design and optimization strategy for investigating the performances of various single-stage and multi-stage iPMGs (Chapter 4 and 5). • A comparison of 10-MW iron-cored PMG and iPMG (Chapter 4). • A 3D multiphysics design approach where all the calculations are done with open source codes (Chapter 6). • An investigation of the machine efficiency improvement with the instantaneous ABC theory borrowed from power system theory (Chapter 7). It is also the aim to explore the frontier of machine modelling techniques. In this thesis, different calculation codes are used, including both open source codes and commercial codes. The machine modelling covers 1D, 2D, and 3D approaches. The computing resources used range from ordinary PCs, multiple workstations, a computing server, and a super computer (tried). To assist this research and other computation-demanding projects, a scientific computing lab was built with the financial support from the Department of Electric Power Engineering. The modelling approach was validated with an existing 23-kW axial-flux iPMG provided by SmartMotor AS. The main conclusions are: Geared drive trains with induction generators are the dominant solutions in offshore wind farms. Direct-driven iron-cored PMGs are heavy and expensive, and most of the weight and cost go to their inactive parts, which makes this solution not weight-/cost-effective. iPMMs are normally used in low-power (several kW) high-speed (above 1000 rpm) applications. Using iPMM technology in high-power (e.g. 10 MW) low-speed (e.g. 12 rpm) offshore wind turbines can significantly reduce the generator cost and weight. It is preferred to build a machine at single stage rather than multiple stages in terms of torque density and efficiency. For a single-stage iPMG solution, two-rotor conventional-array PMGs have the optimal performances among all the investigated topologies, and the performance difference between radial flux and axial flux PMGs gets reduced when their outer diameters are greater than 25 m. Multi-stage solution may outperform the single-stage solution, if the machine outer diameter is constrained and the design has multiple objectives. At 10 MW level, a 12-rpm single-stage iPMG with a diameter of 20 m is lighter and less expensive than a 12-rpm iron-cored PMG with a diameter of 9.9 m. Even though the state of the art commercial finite-element-analysis and genetic-algorithm codes are used, and the specially developed design and optimization strategy can reduce the total calculation time, it takes a week to ten days to solve an optimization with five free variables. A 3D multiphysics design and optimization strategy can take into account the effects due to small aspect ratio of iPMGs, and executing such a strategy with open source codes on a super computer can reduce the computational cost, but it is still a challenge to parallelize the calculation and optimization. An active shunt filter controlled with the instantaneous ABC theory can reduce the generator loss, but the system efficiency is not significantly improved. Nonetheless, it is still attractive to use the diode rectifier in a converter as a trade-off of cost, reliability, and power yield

Mechanical Parameter Inversion in Sandstone Diversion Tunnel and Stability Analysis during Operation Period

A large number of experimental studies show that the mechanical parameters of deep buried surrounding rock show significant attenuation characteristics with the increase of strain from the rheological acceleration stage to the attenuation stage. However, the existing numerical models all take mechanical parameters as constants when describing the rheological behavior of surrounding rocks, which can only be applied to the stability analysis of the shallowly buried tunnel. Therefore, this work proceeding from the actual project, improved the sandstone rheological constitutive model and optimized the algorithm of parameter inversion, and put forward a long-term stability analysis model that can accurately reflect the rheological characteristics of surrounding rocks under the complex geological condition including high stress induced by great depth and high seepage pressure. In the process, a three-dimensional nonlinear rheological damage model was established based on Burgers rheological model by introducing damage factors into the derivation of the sandstone rheological constitutive model to accurately describe the rheological behaviors of the deep buried tunnel. And BP (Back Propagation) neural network optimized by the multi-descendant genetic algorithm is used to invert the mechanical parameters in the model, which improves the efficiency and precision of parameter inversion. Finally, the rheological equation was written by using parametric programming language and incorporated into the general finite element software ANSYS to simulate the rheological behavior of the tunnel rock mass at runtime. The results of the model analysis are in good agreement with the monitoring data in the later stage. The research results can provide a reference for the stability analysis of similar projects

New Insights on Relieving Task-Recency Bias for Online Class Incremental Learning

To imitate the ability of keeping learning of human, continual learning which can learn from a never-ending data stream has attracted more interests recently. In all settings, the online class incremental learning (CIL), where incoming samples from data stream can be used only once, is more challenging and can be encountered more frequently in real world. Actually, the CIL faces a stability-plasticity dilemma, where the stability means the ability to preserve old knowledge while the plasticity denotes the ability to incorporate new knowledge. Although replay-based methods have shown exceptional promise, most of them concentrate on the strategy for updating and retrieving memory to keep stability at the expense of plasticity. To strike a preferable trade-off between stability and plasticity, we propose a Adaptive Focus Shifting algorithm (AFS), which dynamically adjusts focus to ambiguous samples and non-target logits in model learning. Through a deep analysis of the task-recency bias caused by class imbalance, we propose a revised focal loss to mainly keep stability. By utilizing a new weight function, the revised focal loss can pay more attention to current ambiguous samples, which can provide more information of the classification boundary. To promote plasticity, we introduce a virtual knowledge distillation. By designing a virtual teacher, it assigns more attention to non-target classes, which can surmount overconfidence and encourage model to focus on inter-class information. Extensive experiments on three popular datasets for CIL have shown the effectiveness of AFS. The code will be available at \url{https://github.com/czjghost/AFS}.Comment: 12 pages,15 figure

Design optimization of ironless multi-stage axial-flux permanent magnet generators for offshore wind turbines

Direct-driven ironless-stator machines have been reported to have low requirements on the strength of the supporting structures. This feature is attractive for offshore wind turbines, where lightweight generators are preferred. However, to produce sufficient torque, ironless generators are normally designed with large diameters, which can be a challenge to the machine’s structural reliability. The ironless multi-stage axial-flux permanent magnet generator (MS-AFPMG) has the advantages of ironless machines but a relatively small diameter. The objective of this article is to present the design optimization and performance investigation of the ironless MS-AFPMG. An existing design strategy, which employs two- and three-dimensional static finite element analyses and genetic algorithm for machine optimization, is improved with the aim of reducing the calculation load and calculation time. This improved design strategy is used to investigate the optimal ironless MS-AFPMG. Some intrinsic features of this kind of machine are revealed

The effect of ions doping on the rheological properties of ferrite ferrofluids

A series of ferrite nanoparticles were synthesized via ion doping and then were coated by surfactant and dispersed in perfluorinated polyether oil (PFPE-oil), and the various ferrite ferrofluids were obtained. The scanning electron microscope was used to characterize the morphology of particles and the dispersed state of ferrofluid, energy-dispersive spectroscopy was used to study the chemical composition of particles, fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis were used to study the coated effect of PFPE-acids on particles, vibrating sample magnetometer was used to research the magnetization curves of ferrite particles, and the rheological property of the ferrite ferrofluids was studied by a rheometer. The results show that Zn2+, Mn2+/Zn2+, and Dy3+ ions were doped in the ferrite nanoparticles with a size less than 50 nm. The four kinds of ferrite nanoparticles have the characteristics of super-paramagnetic materials, and the M-T curves decrease with increasing temperature, while their decline rates are notably different. The ferrite particles are coated with PFPE acids chemically, and the ferrofluids have well dispersion stability. The rheological properties of the ferrite ferrofluids change with the variation of ion doping, magnetic field strength, temperature, etc. The magnetism and viscosity of ferrite ferrofluids are regularly affected by ion doping, and the results will have a great significance on basic research and related applications

Fenofibrate suppresses corneal neovascularization by regulating lipid metabolism through PPARα signaling pathway

Purpose: The purpose of this study was to explore the potential underlying mechanism of anti-vascular effects of peroxisome proliferator–activated receptor α (PPARα) agonist fenofibrate against corneal neovascularization (CNV) through the changes of lipid metabolism during CNV.Methods: A suture-induced CNV model was established and the clinical indications were evaluated from day 1 to day 7. Treatments of vehicle and fenofibrate were performed for 5 days after suture and the CNV areas were compared among the groups. The eyeballs were collected for histological analysis, malondialdehyde (MDA) measurement, terminal deoxynucleotidyl transferase 2′-deoxyuridine 5′-triphosphate nick end labeling (TUNEL) staining, western blot, quantitative real-time PCR (qRT-PCR) assays and immunohistochemical (IHC) staining to elucidate pathological changes and the underlying mechanism.Results: Lipi-Green staining and MDA measurement showed that lipid deposition and peroxidation were increased in the CNV cornea while the expression of long-chain acyl-coenzyme A synthetase 1 (ACSL1), carnitine palmitoyltransterase 1A(CPT1A) and medium-chain acyl-coenzyme A dehydrogenase (ACADM), which are key enzymes of fatty acid β-oxidation (FAO) and targeted genes of peroxisome proliferator-activated receptor alpha (PPARα) pathway, were decreased in CNV cornea. Fenofibrate suppressed lipid accumulation and peroxidation damage in the CNV cornea. Fenofibrate upregulated the expression levels of PPARα, ACSL1, CPT1A, and ACADM compared with vehicle group. IHC staining indicated that fenofibrate also decreased the expression of VEGFa, VEGFc, TNFα, IL1β and CD68.Conclusion: Disorder of lipid metabolism may be involved in the formation of suture-induced CNV and fenofibrate played anti-neovascularization and anti-inflammatory roles on cornea by regulating the key enzymes of lipid metabolism and ameliorating lipid peroxidation damage of cornea through PPARα signaling pathway

Effect of post-peak cyclic load on mechanics and seepage characteristics of sandstone under different confining pressures

The mining disturbance causes the rock mass in a certain range of the coal seam floor to be in a post-peak state. Due to the excavation of adjacent roadways and the mining of coal seams, the post-peak rock mass undergoes a cyclic loading-unloading process. In order to explore the influence of post-peak cyclic loading on rock mass structure under different in-situ stress environments and clarify the gestation process of disasters such as water inrush from coal seam floor, post-peak cyclic loading tests of sandstone under 5,10,15,20,25 MPa confining pressure were carried out based on Rock Top multi-field coupling tester. The results show that: ① Before the post-peak cyclic loading, the unit permeability of rock shows a rapid decline-tend to be stable-sudden rise-sudden decline-tend to be stable. In the post-peak cycle stage, the unit permeability of rock is almost inverted with the axial load. ② Rock elastic modulus, crack closure stress, crack initiation stress, damage stress, peak stress and residual stress are positively correlated with confining pressure, while Poisson 's ratio increases first and then decreases with the increase of confining pressure. ③ Under the confining pressure of 5 and 10 MPa, the intermittent failure of rock occurs, and the brittle failure characteristics are weakened. Under the confining pressure of 15, 20 and 25 MPa, the brittle failure characteristics of rock are obvious, and the failure characteristics of rock are determined by the properties of rock itself. ④ The post-peak axial load mainly promotes the increase of rock permeability, but the promotion effect is weaker than that of confining pressure on rock permeability, and confining pressure is the dominant factor affecting the post-peak permeability change of rock. ⑤ Under different confining pressures, a through-shear crack occurs in all rocks, but with the increase of confining pressure, the degree of rock failure gradually weakens and the failure mode tends to be simple

Molecularly imprinted polymer based on MWCNTs-QDs as fluorescent biomimetic sensor for specific recognition of target protein

A novel molecularly imprinted optosensing material based on multi-walled carbon nanotube-quantum dots (MWCNT-QDs) has been designed and synthesized for its high selectivity, sensitivity and specificity in the recognition of a target protein bovine serum albumin (BSA). Molecularly imprinted polymer coated MWCNT-QDs using BSA as the template (BMIP-coated MWCNT-QDs) exhibits a fast mass-transfer speed with a response time of 25 min. It is found that the BSA as a target protein can significantly quench the luminescence of BMIP-coated MWCNT-QDs in a concentration-dependent manner that is best described by a Stem-Volmer equation. The K-SV for BSA is much higher than bovine hemoglobin and lysozyme, implying a highly selective recognition of the BMIP-coated MWCNT-QDs to BSA. Under optimal conditions, the relative fluorescence intensity of BMIP-coated MWCNT-QDs decreases linearly with the increasing target protein BSA in the concentration range of 5.0 x 10(-7)-35.0 x 10(-7) M with a detection limit of 80 nM

IRONLESS PERMANENT MAGNET GENERATORS FOR DIRECT-DRIVEN OFFSHORE WIND TURBINES

Since the beginning of this century, the offshore wind power industry has witnessed fast development, as the result of the increasing awareness of climate change and the need for diversifying power supply. Offshore has vast area available with high wind speed, which is an ideal place for large-scale wind power exploitation. However, these advantages come with technological challenges. One of the key challenges is to develop high-power cost-effective wind turbines with high reliability. Previous publications have demonstrated the potential of using ironless permanent magnet generator (iPMG) to reduce the total weight and cost of a wind turbine. Unfortunately, before the start of the research reported in this thesis, there was no systematic investigation of the iPMG concepts. As shown in this thesis, if the traditional direct-driven high-power iron-cored PMG is used for offshore wind turbines, instead of iPMG concept, a significant amount of the total weight goes to the supporting structures (inactive parts). The supporting structures maintain the machine reliability by counter-reacting to various forces, such as the machine gravity, electromagnetic torque, rotor-to-stator force (for iron-cored machine), rotor-torotor force (if there are multiple rotors), and force caused by faults. The “secret” of iPMG roots at the negligible normal stress between its rotor and stator (active parts). By using non-magnetic material in the stator, the rotor will not exert attracting force on stator. Having low rotor-to-stator force means lower requirement to the supporting structures, i.e., the supporting structures can be lightweight and low-cost. Due to the low air gap field and thus low tangential stress, iPMG has low torque production. Therefore, iPMG is normally designed with large diameter to produce higher torque with longer force arm. Classical machine design theory has concluded that the machine torque is roughly proportional to its volume. A large-diameter iPMG will therefore have short axial length, which means small aspect ratio (the machine axial length divided by its outer diameter). This ring-shape feature requires 3D approach to analyse the magnetic and thermal fields. Traditionally, it is a common practice to do machine preliminary design and optimization with analytical methods. The 2D or even 3D finite element method (FEM) is considered to be accurate but time-consuming, though the performances of computing codes and hardware are being steadily improved. Multi-core computers and distributed-memory clusters are normally available to most machine designers. Some academic users even have the access to super computers. The objective of this research is to investigate the iPMG technologies with the help of advanced modelling approaches. The main results reported in this thesis are • An overview of the ironless permanent magnet machines (iPMM) (Chapter 1). • A comprehensive overview of the generator technologies for operational offshore wind turbines, the comparison of the efficiencies for different drive trains, and a review of the new generator concepts (Chapter 2). • A method for evaluating the goodness of any generator design and predicting the generator total weight, and a design and optimization strategy for investigating the weight variation of iron-cored high-power PMGs (Chapter 3). • A design and optimization strategy for investigating the performances of various single-stage and multi-stage iPMGs (Chapter 4 and 5). • A comparison of 10-MW iron-cored PMG and iPMG (Chapter 4). • A 3D multiphysics design approach where all the calculations are done with open source codes (Chapter 6). • An investigation of the machine efficiency improvement with the instantaneous ABC theory borrowed from power system theory (Chapter 7). It is also the aim to explore the frontier of machine modelling techniques. In this thesis, different calculation codes are used, including both open source codes and commercial codes. The machine modelling covers 1D, 2D, and 3D approaches. The computing resources used range from ordinary PCs, multiple workstations, a computing server, and a super computer (tried). To assist this research and other computation-demanding projects, a scientific computing lab was built with the financial support from the Department of Electric Power Engineering. The modelling approach was validated with an existing 23-kW axial-flux iPMG provided by SmartMotor AS. The main conclusions are: Geared drive trains with induction generators are the dominant solutions in offshore wind farms. Direct-driven iron-cored PMGs are heavy and expensive, and most of the weight and cost go to their inactive parts, which makes this solution not weight-/cost-effective. iPMMs are normally used in low-power (several kW) high-speed (above 1000 rpm) applications. Using iPMM technology in high-power (e.g. 10 MW) low-speed (e.g. 12 rpm) offshore wind turbines can significantly reduce the generator cost and weight. It is preferred to build a machine at single stage rather than multiple stages in terms of torque density and efficiency. For a single-stage iPMG solution, two-rotor conventional-array PMGs have the optimal performances among all the investigated topologies, and the performance difference between radial flux and axial flux PMGs gets reduced when their outer diameters are greater than 25 m. Multi-stage solution may outperform the single-stage solution, if the machine outer diameter is constrained and the design has multiple objectives. At 10 MW level, a 12-rpm single-stage iPMG with a diameter of 20 m is lighter and less expensive than a 12-rpm iron-cored PMG with a diameter of 9.9 m. Even though the state of the art commercial finite-element-analysis and genetic-algorithm codes are used, and the specially developed design and optimization strategy can reduce the total calculation time, it takes a week to ten days to solve an optimization with five free variables. A 3D multiphysics design and optimization strategy can take into account the effects due to small aspect ratio of iPMGs, and executing such a strategy with open source codes on a super computer can reduce the computational cost, but it is still a challenge to parallelize the calculation and optimization. An active shunt filter controlled with the instantaneous ABC theory can reduce the generator loss, but the system efficiency is not significantly improved. Nonetheless, it is still attractive to use the diode rectifier in a converter as a trade-off of cost, reliability, and power yield