Direct-drive Linear Generator Research Articles

Novel proposal of PTO direct-drive linear generator, an Azimuthal Multi-translator Switched Reluctance Machine (AMSRM): mechanical, characterization and performance tests.

Research and development on power take-off (PTO) in wave energy has been gaining relevance in recent years, appearing as a priority in various forums (JCR reports [1], SRIA in wave energy development [2], specific research funding program in WES, etc.).
 The new proposal of a linear electric generator to act as a PTO in wave energy extraction applications has been developed in the European project SEATITAN [3].
 The linear generator is based on a switched reluctance machine type topology in which the magnetic flux is azimuthal. This allows eliminating the yoke, reducing the amount of magnetized material needed and increasing the force density, besides having a cylindrical shape that allows adapting perfectly to the spar of point absorber type WECs.
 The Azimuthal Multi-translator Switched Reluctance Machine (AMSRM) has been developed, designed and a prototype has been manufactured in the project. The prototype is composed of two modules of 35 kN of force and 3 m/s of peak velocity.
 The complete manuscript will detail the testing facility, as well as the tests performed for the static characterization of the force, electrical losses, and air gap achieved during manufacturing, as well as the dynamic characterization of frictional losses and a preliminary setting of force control parameters.
 The dry test facility (figure 1) consists of the necessary electronic converters to conveniently operate the generator and impose a reference force controlling current per generator coil. In addition, an end-stopper is provided to ensure the safety of the tests (this equipment is essential for dynamic tests). For static tests, a load cell for force measurement as well as precision electrical power meters are used.
 
 A test protocol has been defined for each test, as well as an analysis procedure to obtain the required results: for example, for the evaluation of friction, dynamic tests based on a back-and-forth motion with drifting sections are proposed to evaluate the braking force with the generator off at various speeds.
 The description of the test protocol, the raw data obtained, the data analysis carried out, and de results obtained will be presented and detailed in the manuscript, as well as the description of the test-based parameterization of a simplified AMSRM model to be used in the dynamic modeling of WECs. The simplified model has been previously developed and presented in EWTEC 2021.
 [1] D. Magagna, R. Monfardini, and A. Uihlein, “JRC Ocean Energy Status Report 2016 Edition,” 2016
 [2] J. L. Villate et al., “Strategic Research and Innovation Agenda for Ocean Energy,” Etipocean, no. 826033, p. 64, 2020.
 [3] WEDGE GLOBAL, “SeaTitan,” 2018. [Online]. Available: https://seatitan.eu/. [Accessed: 29-Nov-2021].
 [4] Marcos Blanco, Miguel Santos‐Herran, Gustavo Navarro, Jorge Jesus Torres, Jorge Najera, and Luis García-Tabarés, “Simplified model of a novel direct-drive PTO based on an azimuthal linear switched reluctance generator,” 2021.

Investigation of the dynamics of a multibody wave energy converter excited by regular and irregular waves

The dynamics and the performance of a novel multibody wave energy converter is investigated, which is based on inclined single modules connected to a frame. The frame floats on the sea surface and the modules each move translationally along inclined guidance rods. Direct-drive linear generators or rotation based generators convert the relative translational motion between the frame and the modules into electrical power. This paper studies the conditions which influence the performance of the converters in regular and irregular waves. Different design layouts are investigated numerically, whereby the wave excitation is modeled by a random non-white Gaussian stochastic process.

The Novel Low Reluctance Superconducting Permanent Magnet Linear Generator for Oceanic Wave Energy Extraction

In this paper, the low reluctance superconducting direct drive linear generator (DDLG) for harvesting oceanic wave is presented. At first, a permanent magnet DDLG is designed in the ANSYS/Maxwell environment. The DDLG with both the copper and superconducting winding are characterized with finite element analysis. The DDLG is analyzed with the conventional and recently developed M5 Carlite magnetic cores. High graded neodymium iron boron permanent magnet, N46SH is selected for the DDLG to create strong magnetic field. It is analyzed that the proposed superconducting DDLG with M5 Carlite magnetic core and N46SH permanent magnet produces 294 kW more electrical power compared to that of the DDLG with copper conductor. It is also found that the stator size of the proposed DDLG is much smaller than the conventional one for designing the winding with superconducting tape.

Coupled Modeling and Advanced Control for Smooth Operation of a Grid-Connected Linear Electric Generator Based Wave-to-Wire System

The perpetual oscillations of ocean waves produce potential energy, which can be converted to electrical energy with the help of direct-drive linear generators. The fluctuating generated power poses a major challenge when it is supplied to the power grid. In this article, a supercapacitor provides the short-term energy storage to buffer and smooth out the power fluctuations. A new coupled model of a wave energy converter and a linear generator is proposed for its response characterization under varying system conditions. The developed model and an advanced control strategy are used to exhibit a smooth and stable operation of the wave-to-wire system. The generator-side converter is controlled to extract the maximum power from the waves and to minimize the generator losses by controlling its d -axis and q -axis currents. The grid-side converter is controlled to keep the dc-link voltage constant and to generate the required voltage waveforms at the point of common coupling. The performance of the proposed control strategy for the wave-to-wire system is investigated under different applied diffraction forces. The simulation results show that with the use of the proposed control scheme and the supercapacitor, the wave-to-wire system can operate in a smooth and stable operation under normal and fault conditions.

Performance estimation of resonance-enhanced dual-buoy wave energy converter using coupled time-domain simulation

This paper presents the modeling methodology and performance evaluation of the resonance-enhanced dual-buoy WEC (Wave Energy Converter) by HEM (hydrodynamic & electro-magnetic) fully-coupled-dynamics time-domain-simulation program. The numerical results are systematically compared with the authors’ 1/6-scale experiment. With a direct-drive linear generator, the WEC consists of dual floating cylinders and a moon-pool between the cylinders, which can utilize three resonance phenomena from moon-pool dynamics as well as heave motions of inner and outer buoys. The contact and friction between the two buoys observed in the experiment are also properly modeled in the time-domain simulation by the Coulomb-friction model. Moon-pool resonance peaks significantly exaggerated in linear potential theory are empirically adjusted through comparisons with measured values. A systematic comparative study between the simulations and experiments with and without PTO (power-take-off) is conducted, and the relative heave displacements/velocities and power outputs are well matched. Then, parametric studies are carried out with the simulation program to determine optimum generator parameters. The performance with various wave conditions is also assessed.

Performance of a Direct-Driven Wave Energy Point Absorber with High Inertia Rotatory Power Take-off

An alternating rotatory generator using an eddy current break is developed as a physical scale model of a direct-driven floating point absorber power take-off (PTO) for wave tank tests. It is shown that this design is a simple and cost-effective way to get an accurate linear damping PTO. The device shows some beneficial characteristics, making it an interesting option for full scale devices: For similar weights the inertia can be significantly higher than for linear generators, allowing it to operate with natural frequencies close to typical wave frequencies. The influence of the higher inertia on the power absorption is tested using both a numerical simulation and physical wave tank tests. With the increased inertia the PTO is able to absorb more than double the energy of a comparable direct-driven linear generator in some sea states. Moreover, the alternating rotatory generator allows the absorption characteristic to be tuned by changing the inertia and the generator damping.

Experimental investigation and performance analysis of a direct-driven linear generator

Electricity generation of renewable sources has got great development to reduce the usage of fossil fuels. Linear generator is a newly-developing orientation to utilize the power with reciprocating movement. In the paper, the structure and principle of the direct-driven linear generator (DLG) are introduced. The induction electromotive force is given in detailed based field & circuit method. Meanwhile, the paper introduces the design, fabrication and testing of a DLG prototype, theoretical model of the free-piston engine is presented, and the DLG operation driven by Simulated Power Source (SPS) is discussed. Furthermore, the performance of the system is investigated in detail under different conditions. The results show that, there is an optimal capacitance to get the highest coefficient, which is 25μF here. The frequency has obvious effect on the output at lower load resistance, but it has little effect on the efficiency, the efficiency depends more on the ratio of the load resistance and generator internal resistance. Lastly, according to the analysis results with the phasor algorithm analysis, the proposed model and analytical method are correct and effective. The DLG has a good application prospect in the energy conversion field.

A fully floating system for a wave energy converter with direct-driven linear generator

Wave energy is one of the most promising renewable energy for power generation. This research develops a novel power take-off methodology to surmount the problems associated with mooring, seawater corrosion and access for maintenance in conventional WEC (wave energy converters) with direct-driven linear generators. Its prototype consists of two bodies, the floating body acting as a buoy to extract the wave energy, while the inner body undergoes a forced oscillation, whose relative motion generates the electronic power. Its feasibility is investigated theoretically by coupling the dynamics of the wave, the floating and the inner bodies and the electromagnetic characteristics of the linear generator. As a result, the generator can induce a highly sinusoidal voltage. Furthermore, the performance of the system is investigated in detail under different conditions. The results show that, a resonance has been achieved in the case with the spring constant of 12,633 N/m, with a maximum power capture ratio of 57%. The performance of the system is shown to be sensitive to the load resistance, the wave height, and the spring constant.

Multivariable direct-drive linear generators for wave energy

This study presents a multivariable linear generator which can convert wave energy directly into electrical energy. Based on the varying induction segment length over time, this study established an operational simulation of the multivariable linear generator while simultaneously constructing a multivariable generator device that considers the relationship between the number of vertical movements, the change in induction coils, the magnetic flux density, and other coefficients. Using finite element analysis, this study then analyzed the magnetic flux distribution for a single-stem linear generator and optimized the wiring. This method can be used to determine the wiring direction and coil thickness. The attraction between the magnet and the iron core is used to overcome the alternating magnetic fields, and linear motion with less stress is used to generate power. The generator developed in this study can generate 21.21W power in 0.36 square meters area. We built a low cost, simple structure and multi-purpose human kinetic energy linear generator first for demonstration.

A resonant two body system for a point absorbing wave energy converter with direct-driven linear generator

Based on an earlier conceptual model of a two body system point absorbing wave energy converter tuned to resonance in Swedish west coast sea states, an extended coupled hydrodynamic, mechanic, and electromagnetic model has been developed. The hydrodynamic characteristics of the two body system are studied in the frequency and time domain, while its response to real Swedish west coast sea states are studied in the time domain, by using a wave energy converter model with two independently moving bodies connected to a direct driven linear generator with non-linear damping. The two body system wave energy converter gives nearly 80% power capture ratio in irregular waves. The resonant behaviour is shown to be sensitive to the shape of the spectrum, and the distance between the two bodies is shown to have a large effect on the power absorption.

Operation analysis of a wave energy converter under different load conditions

This study analyses the electrical behaviour of a direct-driven linear generator under different load conditions. The studied generator is used in a wave energy converter (WEC) that converts the energy in ocean waves into electric energy. To enable a grid connection of a WEC, the voltage must be converted, and thereby, the generator will be subjected to a non-linear damping. Depending on how the conversion system is designed, the damping will be different. In the case studied, the voltage is first rectified, and on the dc-side of the rectifier the voltage is kept constant by controlling the power through a converter. In order to study the electrical behaviour of the generator in this operation mode, a simulation model was made in MATLAB Simulink. The model of the generator was verified with experimental data from an offshore operating WEC. The result of the study shows that the model of the generator agrees with the real generator and can be used for analysing the electrical behaviour of the WEC. Moreover, the results show that the operation with a non-linear load will be different compared to a linear load case.