Design optimization of a magnetically-geared tidal turbine generator 

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Tidal stream turbines drivetrain availability

Tidal stream turbines and wind turbines systems are analogous, which allows to assess their reliability and compares qualitatively between the different existing drivetrainconfigurations. Although the tidal stream turbine failures data are not available, last years some failures have been reported. The first turbine failure concerns the 1 MW OpenHydro direct-drive tidal stream turbine in Canada (Bay of Fundy) in 2009 (fig. 2.1). The problem have been reported after three weeks of the deployment because of blades failure due to fatigue issues. Another blade fault have been reported at the Atlantis Resources AK1000 tidal stream turbin in 2010 due to manufacturing fault [2]. Therefore, to predict such failures and avoid long downtimes a condition monitoring scheme is needed [2, 28, 29]. However, condition monitoring for tidal stream turbine should be specific to consider the marine current load variability and amplitude [4,30]. Indeed, tidal stream turbines input torque is 50% much greater than wind turbines one for the same rated power due to the high water density compared to air [31]. Moreover, it has been shown that the shaft speed variations are greater for tidal stream turbines than for wind turbines even though wind current fluctuations are greater [31]. Accordingly, the high loading torque fluctuations highly affects the reliability of tidal stream turbine mechanical subsystems especially the gearbox. In the study [2], tidal turbine components criticality are evaluated according to their Risk PriorityNumber (RPN) (see equation (2.1)), which is the multiplication of occurrence (Occ), severity (Sv), and probability of failure detection (Pf ) [32].

Gearbox and geared turbines

Last decades, gearbox failures affected the wind turbine industry through downtime and cost of repair (fig. 2.4). However, geared systems components, especially the generator, are standardized, available in the market, and cheaper [35]. Moreover, geared wind turbines are widely deployed during last decades which offers a large theoretical and practical knowledge. Such knowledge presents a great advantage to accelerate geared tidal turbines development [36]. Gearboxes, as a mature technology, are widely used in transport, energy, and process industries. They are vital The availability of wind turbine gearboxes, based on the National Renewable Energy Laboratory reliability database, has been improved from 2013 [3]. Such improvements can be qualitatively considered although the logistic delay time and MTTR of offshore tidal stream turbines are higher than onshore wind turbines, mainly due to the difficulty of on-site access. Concerning the gearbox failures rate, it is less than 15%, of which 76% are due to bearing faults, however electrical sub-assemblies faults are more than 25% [37, 38].

Gearbox issues mitigation

Typical operations and maintenance actions, such as the fine filtration of lubricating oil and the use of remote condition monitoring, are required to detect incipient faults and ensure the good functioning of gearboxes particularly in offshore applications [39]. Besides that, manufacturers try to prolong the service life of wind turbines by developing more robust gearbox systems that withstand the varying load torque. In this context, the Pure Torque concept is proposed by GE/Alstom [8]. In fact, in addition to the rotational forces, wind turbine rotor transmits side forces to the main shaft and gearbox due to the high load input. In the Pure Torque design, a cast iron frame supports the rotor shaft as an extension of the tower structure (fig. 2.8). Hence, the deflective loads are diverted to the front frame not to the drivetrain. Moreover, even under extreme load cases, gearbox misalignment and displacements stay steady with low amplitudes. On the other hand, according to an Alstom study covering 930 wind turbines with rated power from 1.67 MW to 3.0 MW and during 5.5 years, gearbox failure rate causing replacements was lower one order of magnitude than in other drivetrain configurations. This concept is chosen to design the offshore wind turbine GE/Alstom 6MW Haliade 150 (fig. 2.9) [8, 9, 40, 41].

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Integrated drivetrain options : Multibrid

The Multibrid concept is an integrated drivetrain option which is the intermediate between the conventional geared concept and the direct-drive one avoiding their extreme characteristics. In this hybrid solution, the single-stage planetary gearbox (sometimes two-stages), the medium speed permanent magnet generator, the main shaft, and the shaft bearing are all integrated in the same housing. In this case both the generator and the gearbox have approximately the same size which leads to more balanced drivetrain arrangement. This technology, also known as semi direct-drive, was first introduced by AREVA Wind (figs. 2.14 and 2.15) [39, 55, 56].

Table of contents :

Abstract
Acknowledgements
1 Introduction 
1.1 Overview
1.2 Issues and challenges
1.2.1 Cost-effectiveness
1.2.2 Biofouling
1.2.3 Environmental concerns
1.3 Thesis outline
2 Tidal Stream Turbine Drivetrain Configurations 
2.1 Background
2.2 Tidal stream turbines drivetrain availability
2.3 Geared vs. gearless tidal turbine : progress on both sides
2.3.1 Gearbox and geared turbines
2.3.2 Gearbox issues mitigation
2.3.3 Direct-drive turbines
2.3.4 Integrated drivetrain options : Multibrid
2.3.5 Hydraulic transmission
2.4 Geared vs. gearless tidal turbine: performances
2.5 Magnetically-geared tidal stream turbines
2.5.1 Magnetic gears
2.5.2 Magnetically-geared tidal stream turbine
Pseudo direct-drive generator
Magnetically-geared inner stator permanent magnet generator
Axial flux magnetically-geared generator
2.5.3 Challenges
2.6 Comparison summary
2.7 Conclusions
3 Grid-connected tidal stream turbine design 
3.1 Introduction
3.2 Renewable resource and turbine modeling
3.2.1 Turbine modeling
3.2.2 Annual energy production
3.2.3 bi-directional fixed axis direction tidal turbine
3.2.4 Yaw drive-based tidal turbine
3.3 Gearbox modeling
3.3.1 Parallel shaft gearbox
3.3.2 Planetary gearbox
3.3.3 Two-stage gearbox design
3.3.4 Gearbox cost estimation
3.3.5 Gearbox losses
3.4 Permanent magnet generator design
3.4.1 Electromagnetic torque
3.4.2 Air-gap
3.4.3 Magnet height
3.4.4 Slot height
3.4.5 Stator and rotor yoke height
3.4.6 Teeth pitch ratio
3.4.7 Maximum magnetic field
Iron and copper losses
Electromotive force
Synchronous inductance
Equivalent per-phase circuit
3.4.8 Power electronic converter design
3.5 Conclusions
4 Optimal design of a tidal stream turbine 
4.1 Introduction
4.2 Design optimization method
4.2.1 Generator cost
Generator model inversion
4.2.2 Optimization constraints
4.2.3 Pole pair number
4.2.4 Slot depth to slot width ratio
4.2.5 Mechanical air-gap
4.2.6 Maximum magnetic field
4.2.7 Current density and loading current
Generator efficiency Phase voltage
4.3 Design results and discussion
4.3.1 Two-stage gearbox driven generator
4.3.2 Single-stage gearbox driven generator (Multibrid)
4.3.3 Comparison: direct-drive, Multibrid, tow-stage gear drive .
4.3.4 Power rating variation: Multbird Vs. Direct-drive
4.3.5 Optimal drivetrain configuration
4.4 Conclusions
5 Design optimization of a magnetically-geared tidal turbine generator 
5.1 Introduction
5.2 Overview on the pseudo-direct drive generator
5.2.1 Pseudo-direct drive components
Flux-modulated magnetic gear operating principles .
5.2.2 Gear ratio
5.2.3 Adaptation between the generator and the gearbox
5.3 Design optimization methodology
5.3.1 Fixed design parameters
Power rating
Pole pairs and stator tooth number
Operating electrical frequency
Mechanical air-gap
5.3.2 Initial sizing
Stator sizing
Rotors sizing
5.3.3 Finite element modeling
5.3.4 Constraints
5.4 Design results
5.5 Conclusions
Conclusion and Perspectives

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