EOLINK have developed an innovative floating wind turbine in which the single tower is replaced by a set of legs providing a pyramidal architecture. A 1/10 th scale prototype of EOLINK’s 12MW concept has been connected to the grid in April 2018 in France. Firstly, the paper describes the technical specifications of this device. Both the turbine and the floater have been designed using Froude scaling, in order to properly represent the EOLINK full scale 12MW concept. The device has been devised from scratch and deploys a Permanent Magnet Synchronous Generator (PMSG) and an individual electric blade pitch system. The patented mooring system comprises a single point mooring (SPM) system able to withstand very high tide ranges in shallow waters. Regarding monitoring, motions have been recorded using both an Inertial Measurement Unit (IMU) and high precision Global Positioning System (GPS) sensors. Mooring lines tensions have also been monitored. Wind is recorded using both an embedded anemometer on the floating turbine and onshore anemometers installed by IFREMER. This Institute has also measured wave height using a wave recorder. Secondly, experimental results during production and storm events are presented. The encountered environmental conditions highlight the capability of the EOLINK design to withstand harsh wind events, and its ability to produce 12MW using a small sized semi-submersible floater. Then, numerical analysis using FAST and Flexcom is compared with experimental results. Static analysis, decay-tests, Response Amplitude Operators (RAOs) and Power Spectral Densities (PSDs) results are detailed. Power production and the embedded control command capabilities are also presented.

At present, over 1500 offshore wind turbines (OWTs) are operating in the UK with a capacity of 5.4GW. Until now, the research has mainly focused on how to minimise the CAPEX, but Operation and Maintenance (O&M) can represent up to 39% of the lifetime costs of an offshore wind farm, mainly due to the assets’ high cost and the harsh environment in which they operate. Focusing on O&M, the HOME Offshore research project ( www.homeoffshore.org ) aims to derive an advanced interpretation of the fault mechanisms through holistic multiphysics modelling of the wind farm. With the present work, an advanced model of dynamics for a single wind turbine is developed, able to identify the couplings between aero-hydro-servo-elastic (AHSE) dynamics and drive train dynamics. The wind turbine mechanical components, modelled using an AHSE dynamic model, are coupled with a detailed representation of a variable-speed direct-drive 5MW permanent magnet synchronous generator (PMSG) and its fully rated voltage source converters (VSCs). Using the developed model for the wind turbine, several case studies are carried out for above and below rated operating conditions. Firstly, the response time histories of wind turbine degrees of freedom (DOFs) are modelled using a full-order coupled analysis. Subsequently, regression analysis is applied in order to correlate DOFs and generated rotor torque (target degree of freedom for the failure mode in analysis), quantifying the level of inherent coupling effects. Finally, the reduced-order multiphysics models for a single offshore wind turbine are derived based on the strength of the correlation coefficients. The accuracy of the proposed reduced-order models is discussed, comparing it against the full-order coupled model in terms of statistical data and spectrum. In terms of statistical results, all the reduced-order models have a good agreement with the full-order results. In terms of spectrum, all the reduced-order models have a good agreement with the full-order results if the frequencies of interest are below 0.75Hz.

This paper addresses experimental and numerical validation of power output efficiency about an approximate complex-conjugate control with considering the copper loss (ACL) method. A bottom-fixed point absorber type wave energy convertor (WEC) model was used for the experiments carried out at National Maritime Research Institute, Japan (NMRI). In order to model a power take-off (PTO) system constructed by a permanent magnet linear generator (PMLG), a liner shaft motor (LSM) was used for the model test. To investigate characteristics of the ACL method, the resistive load control (RLC) method and approximate complex-conjugate control (ACC) method were also tested by the WEC model. A simulation code based on WEC-Sim (Wave Energy Converter SIMulator) v2.0 written by MATLAB/Simulink, which is developed by collaboration works between the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (Sandia), was used for the validation. The simulated results in regular waves have good agreement with measured ones in terms of the float heave motion, the vertical force and the control input force. Through the experiments and numerical simulations in regular waves, the ACL method has advantages in high power production compared with the RLC and the ACC methods for the WEC model. In addition, the power output characteristics of the ACL method in irregular waves were checked experimentally and numerically.

A recently developed dual (coaxial-)cylinder wave-energy converter (WEC) consists of inner and outer-cylinders, with the outer one sliding over the inner one. An effective design was to tension-tether the inner cylinder (Son and Yeung, OMAE2014-#24582) while the outer cylinder acts as a floater heaving in response to incident waves. Even though the idea was a success, there was significant scientific curiosity in our early stage of the design in the following context: if both cylinders were allowed to heave simultaneously and independently, what would be the implications on the energy-extraction performance and power-take-off constraints? In this paper, we report the detailed analysis conducted at the time of the design. To begin with, the hydrodynamic coefficients, namely, the added mass, radiation-damping, and wave-exciting force for the individually moving cylinders were solved using the method of matched eigenfunction expansions (Chau and Yeung, OMAE2012-#83987). We expanded that capability to allow coupling or interference hydro-dynamic coefficients to be computed in the current work. This coupling is shown to lead to two degrees of freedom of motion, one for each cylinder, with excitation forces on each based on reciprocity (Haskind’s) relations. The resulting relative heave motion between the cylinders is used to drive the permanent magnet linear generator (PMLG) to capture electrical energy. The performance of the WEC, in terms of capture width, is calculated for both regular-wave and irregular-wave conditions and is compared with that for the one degree-of-freedom system, fixed inner cylinder and heaving outer cylinder. The change in WEC performance in response to changing generator damping was found to be very different for the two cases. This behavior leads to very different optimal generator damping values in regular and irregular waves. The advantages and shortcomings of the two systems are compared and explained.

This paper begins with a brief review of the equation of motion for a generic floating body with modification to incorporate the influence of a power-take-off (PTO) unit. Since the damping coefficient is considered the dominant contribution to the PTO reaction force, the optimum non time-varying values are presented for all frequencies, recovering the well-known impedance-matching principle at the resonance condition of the coupled system. The construction of a laboratory-scale permanent magnet linear generator (PMLG), developed at the University of California at Berkeley, is discussed along with the basic electromagnetic equations used to model its performance. Modeling of the PMLG begins with a lumped magnetic circuit analysis, which provides an analytical solution to predict the magnetic flux available for power conversion. The voltage generated across each phase of the stator, induced by the motion of the armature, provides an estimate for the electromagnetic damping as a function of the applied resistive load. The performance of the PMLG and the validation of the proposed analytical model is completed by a set of dry-bench tests. Results from the bench test showed good agreement with the described electromechanical model, thus providing an analytical solution that can assist in further optimization of the PMLG.

This study evaluates two significant design modifications of a dual coaxial-cylinder system as a wave-energy extractor reported in Son and Yeung (2014, OMAE2014-#24582). First, a new and stronger power take-off (PTO) unit for a permanent magnet linear generator (PMLG) was built, along with an appropriate supporting structure, so as to match optimality conditions in terms of impedance matching and mechanical to electrical conversion efficiency. Based on a series of (dry-)bench tests, the properties of the PTO were obtained and the optimal operating conditions were determined. Second, the flat-bottom shape of the outer toroidal floater was modified according to “The Berkeley Wedge design” (Madhi et al, 2014, “The Berkeley Wedge: an asymmetrical energy-capturing floating breakwater of high performance,” Journal of Marine Systems and Ocean Technology, vol. 9(1), pp. 5–16). The new bottom shape led to reduction of the floater damping by almost 70%, which yielded a 3-fold increase in the floater motion response. Experiments in a wave-tank validated the response behavior of the dual-cylinder system with the use of the new PTO. The Berkeley-Wedge shape allowed more than 3 times more energy be extracted compared to the flat-bottom geometry, while the new generator also improved the energy conversion efficiency. As a result, the overall system efficiency of the device was enhanced remarkably five times over that of the previous design.

This paper presents the analysis of a hysteresis interior permanent magnet (IPM) motor drive for electric submersible pumps. A hysteresis IPM motor is a self-starting solid rotor hybrid synchronous motor. Its rotor has a cylindrical ring made of composite materials with high degree of hysteresis energy. The rare earth permanent magnets are buried inside the hysteresis ring. A hysteresis IPM motor can self-start without the need of additional position sensors and complex control techniques. It does not have any slip power losses in the rotor at steady state which results in less heat dissipation and low electrical losses. When used in an electric submersible pump (ESP) for oil production, it has the ability to automatically adapt itself to the changes in well conditions. In this paper, a bond graph model of a hysteresis IPM motor ESP drive is used to predict the effect of pump shaft geometry on transient behaviour of the drive during start-up. Simulation results show that the hysteresis IPM motor drive has high efficiency, and is better able to maintain its speed during changes in load. Due to increased efficiency and simplified controller requirements, the hysteresis IPM motor is proposed as a replacement for the standard induction motor currently used for downhole ESPs. This is expected to improve ESP performance and reliability which are critical requirements for use in harsh offshore environments such as Atlantic Canada.

Experiments were conducted to investigate the performance of two newly designed coaxial cylinders as a wave-energy extractor system in regular waves. The coaxial-cylinder design as a point-absorber consists of a tension-tethered vertical inner cylinder and a heaving outer or toroidal cylinder moving in the vertical direction. The relative heave motion between the two cylinders is used to convert the wave-induced response to electrical energy. The first-order heave response of the outer cylinder is used as the mathematical model in the frequency domain and the predicted results are compared with experimental measurements taken in a wave basin. The analytical solutions for the hydrodynamic added mass, damping, and wave-exciting force of the heaving floater are obtained from Chau and Yeung (2012, OMAE2012-# 83987). Experimentally determined hydrodynamic coefficients from free-decay tests at the resonance frequency are obtained to account for the effects of viscosity. Experimental results first reported include the wave-exciting force on the outer cylinder and its free response induced by the incident waves. The permanent magnet linear generator (PMLG) developed in Tom and Yeung (2012, OMAE2012-# 83736) is next installed as a passive power take-off (PTO) system. The electrical power output from the linear generator is measured with the resistance load as a parameter. The measured performances of the coaxial cylinders with and without the PTO are compared with the theoretical predictions. Excellent agreement is found, confirming the effectiveness of guiding theoretical model and of the engineering design.

This paper begins with a brief review of the time-domain equation of motion for a generic floating body. The equation of motion of the floating body was modified to account for the influence of a power-take-off unit (PTO) to predict the hydrodynamic and electromechanical performance of the coupled system. As the damping coefficient is considered the dominant contribution to the PTO reaction force, the optimum non time-varying damping values were first presented for all frequencies, recovering the well-known impedance-matching principle at the coupled resonance frequency. In an effort to further maximize power absorption in both regular and irregular wave environments, nonlinear model predictive control (NMPC) was applied to the model-scale point absorber developed at UC Berkeley. The proposed NMPC strategy requires a PTO unit that could be turned on and off instantaneously, leading, interestingly to electrical sequences where the generator would be inactive for up to 60% of the wave period. In order to validate the effectiveness of this NMPC strategy, an in-house designed permanent magnet linear generator (PMLG) was chosen as the PTO. The time-varying performance of the PMLG was first characterized by dry-bench tests, using mechanical relays to control the electromagnetic conversion process. Following this, the physical set-up was transferred to the wave tank. The on/off sequencing of the PMLG was tested under regular and irregular wave excitation to validate NMPC simulations using control inputs obtained from running the control algorithm offline. Experimental results indicate that successful implementation was achieved and the absorbed power using NMPC was up to 50% greater than the passive system, which utilized no controller. However, after considering the PMLG mechanical-to-electrical conversion efficiency the useful electrical power output was not consistently maximized. To improve output power, a mathematical relation between the efficiency and damping magnitude of the PMLG was inserted in the system model to maximize the electrical power output through continued use of NMPC. Of significance, results from these latter simulations provided a damping time series that was active over a larger portion of the wave period and required the actuation of the applied electrical load connected to the PMLG, rather than a simple on/off type control.

Renewable energy conversion in offshore environments, such as wave, wind and tidal energy, can potentially give a considerable contribution to the global electric energy demand. These harsh environments require robust generators with minimal need for maintenance at competitive costs. To reduce the generator cost, the electromagnetic design must be done with manufacturing in mind. An optimal design provides high electric efficiency, long device life-time, little need for maintenance and low manufacturing costs. Modern simulation tools can be used to optimize the electromagnetic design of a generator for a specific task and operation mode. Hereby both electromagnetic losses and material stresses can be reduced. Industrial robots might provide new possibilities to automate generator-specific manufacturing tasks. A generator design with a cable wound stator, surface mounted permanent magnets on the translator and direct-drive linear technology is investigated in this article. This concept has a simpler and more robust mechanical design, while both the electromagnetic losses and the need for maintenance are reduced. By reducing the number of generator assembly steps, manufacturing might also be facilitated. Further work is however needed in developing automated assembly methods and comparing them to conventional generator manufacturing.

This paper evaluates two aspects of enhancements made to the UC-Berkeley ocean-wave energy extraction device first presented in [1]. First, the differences in hydrodynamic performance between flat- and hemispherical bottom floaters were investigated theoretically using UC Berkeley 2-D viscous-flow solver: FSRVM [2]. The predicted enhancement was compared with experimental results, demonstrating that an increase in motion of over 50% was realizable. Second, important modifications to the design, fabrication, and material of the rotor and stator of the permanent magnet linear generator (PMLG) were made with the aim of increasing both power output and mechanical-to-electrical conversion efficiency, η el . Increased power extraction and efficiency were achieved, doubling what had been previously reported. The non-linear relationship between the generator damping and the magnet-coil gap width was also investigated to verify that the conditions for optimum power extraction presented in [1] were achievable with the PMLG. Experimental results, obtained from testing the coupled floater and PMLG system in the UC-Berkeley wave tank, revealed that measured capture widths were more than double those from the previous design. These results further confirmed that matching of the generator and floater damping significantly increased the global efficiency of the extraction process.

Marine currents are an offshore source of renewable energy of increasing importance, with the development of technology for electricity generation from tidal currents or low-head river currents advancing at a quick pace. Two of the major components of a marine current power plant are the generator and the turbine. It is not sufficient to design these components separately, but a system approach, where the power plant is seen as one entity, must be taken to achieve best overall efficiency. In the present paper, the performance of three different combinations of direct-driven permanent magnet generator with cross-stream axis marine current turbine is examined numerically under the variation of water flow speed. The design case chosen is that of a shallow river or tidal channel, where the cross-sectional area limits the physical size of the power plant. The units are designed for a power output of 10 kW at a water current velocity of 1 m/s. Turbines for three different rotational speeds are considered, each in combination with a corresponding generator. The three turbine-generator systems are designed according to similar design criteria to allow for comparisons. The turbines are modelled using an in-house code, based on the double multiple streamtube model. Corrections are made due to the finite aspect ratio and tip losses of the blades. Experimental data for the lift and drag coefficients for different Reynolds numbers are used in the model. The generators are modelled using a FEM tool that has been validated with experimental results. The three generators are designed for the same nominal voltage and with a low load angle to allow for overload operation. The overall performance of each of the three systems is studied under varying flow velocity. The main conclusion is that all three machines exhibit essentially the same performance behaviour, which means that the choice of nominal operational speed for a power plant will not be a major design constraint. Turbines with higher rotational speed allow for a more compact generator design within the limits of the design parameters used in this study. However, this also entails certain mechanical constraints on the turbine. Due to the restricted cross-sectional area in the channel, it is clear that at least one of the three systems would have to be placed with the axis of rotation in a horizontal rather than vertical position.

For the past several years an inter-disciplinary research group at Oregon State University (OSU), working in conjunction with Columbia Power Technologies (CPT) has been researching innovative direct-drive wave energy systems. These systems simplify the conversion of wave energy into electricity by eliminating intermediate energy conversion processes. In support of this research OSU and CPT have developed a hybrid numerical/physical modeling approach utilizing a large scale linear test bed (LTB), and a commercial coupled analysis tool. This paper will present an overview of this modeling approach and its application to the design of a 10kW prototype wave energy conversion system that was tested in the open ocean in the fall of 2008. The data gathered during ocean testing was used to calibrate the numerical model of the device and predict the energy capture potential of the system.

An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the ocean floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measure station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the existing wave energy system of the Islandsberg project. Special attention will be given to the power absorption by the generator when it is connected to a non linear load.

Marine currents, e.g. tidal currents, ocean currents and unregulated water courses are characterized by a fairly steady or regular flow and pose a significant potential for electricity generation. An ongoing research project at Uppsala University is looking into an energy conversion system with a vertical axis turbine and a permanent magnet direct drive generator placed directly in the water flow, i.e. without using a dam. This choice of technology is intended to provide a simple and robust system with low maintenance needs and minimal environmental impact. During 2006 an experimental setup has been built. It consists of a frequency converter, a motor and a gearbox to drive the generator, the generator itself, and a resistive load to consume the generated power. The generator is two meters in diameter and is built on an elevated structure over the motor and gearbox. A stainless steel structure supports the cable wound stator. The permanent NdFeB magnets are fastened in milled grooves in the rotor. The experimental setup will be used for verification of a simulation and optimization program.

Rapidly moving storm crossing the shelf from shallow water to deep water can generate tsunami-like waves which can cause local flooding and damage to docks when the waves hit the coast. We report on laboratory experiments to examine the reflection of waves generated by a moving disturbance from the shelf. Experiments are performed in a two-layer fluid consisting of a layer of oil based ferrofluid lying on top of a layer of water with step bottom. The disturbance is generated by a permanent magnet moving above the surface of ferrofluid. Digital images of the flow are analyzed to obtain the evolution of the wave field. The experimental flows demonstrate two distinct regimes, namely subcritical when the speed of the magnet is less than the phase speed of the wave, and supercritical when the speed of the magnet is greater than the phase speed of the wave. In subcritical regime the disturbance is localized and its size is determined by the spatial extent of the forcing. In supercritical regime the waves form two beams extending at “Mach angle” with respect to the direction of motion. Oblique wave incident on the shelf can experience total reflection if the angle between the wave front and the shelf is greater than a critical value.

A novel wave energy converter concept is developed at Uppsala University, Department of Engineering Science. The concept is based on a synchronous permanent magnet linear generator, placed on the seabed. The piston of the generator is directly connected to a surface-floating buoy with a rope. The tension in the rope is maintained with springs that pull the piston downward. The three-phase current induced in the stator coil has a varying amplitude and frequency and a conversion is therefore necessary. Research has been carried out in three main areas: generator design, dynamic behavior and grid connection. The generator is modeled by full physics numeric simulations, based on a 2-dimensional finite element formulation of the time dependent electromagnetic field. A first set up is built to experimentally verify the simulated results. The impact of different parameters are estimated with mathematical models and verified by experiments. This paper includes both simulated and experimental results.