Research on an innovative magnetic helix hybrid excitation rotary generator with remarkable power density and efficiency for wave energy conversion

Wave energy converters (WEC) use indirect drive hydraulic or turbine-type power take-off (PTO) mechanisms which consist of many moving parts, creating mechanical complexity and increasing the installation and maintenance costs. Linear generator-based direct drive wave energy converters could be a solution to overcome this problem, but the efficiency of the single conventional linear generator is not high enough, and it cannot work satisfactorily in the low-frequency range.In this paper, a novel dumbbell-shaped flux-switching linear generator has been proposed and studied as a power take-off unit for ocean wave energy conversion. The linear generator has a dumbbell-shaped stator, and the design is ameliorated by placing the permanent magnet rings of longitudinally alternating magnetic pole directions in the slots of its stator outer surface and is separated by thin wall steel ring shoulders. The linear generator also has a dumbbell-shaped translator. The stator is hollow where the translator slides inside axially inducing current in the coil. A long permanent magnet is inserted inside the hollow steel dumbbell core of the translator and a copper coil is wound around the outer surface of the moving translator core. The addition of permanent magnets on the outer slots of the stator is found to increase the output power significantly. To facilitate the investigation, the modified generator design has been compared to the conventional linear permanent magnet generator for their performances using the finite element method, as both machines are different in their structures. The results show that the double dumbbell-shaped flux-switching linear generator gives higher power output, magnetic flux density, branch current, and induced voltage. The double dumbbell-shaped flux-switching linear generator is then placed in a cylindrical buoy and investigated in a wave energy converter in the ocean environment. The hydrodynamic response of the cylindrical buoy has been investigated through ANSYS AQWA. The dynamic differential equations of the wave energy converter have been developed and solved for regular waves using Matlab codes. The peak output voltage, power, and the relative displacement of the linear generator translator with respect to the stator fixed with the buoy have been calculated through the Fourier Transform in the frequency domain using a Matlab code and through numerical integrations in the time domain using a Matlab Simulink code. The results in the time and frequency domains are compared and verified with each other. The relative displacement between the translator and stator buoy has been used as motion input of the ANSYS Maxwell simulation model, and the output voltage results of the ANSYS Maxwell simulation model have been compared and verified with those of the Matlab simulation models. The linear generator design in the wave energy converter under the regular wave excitation is further optimized for the maximum ratio of the peak output power to peak cogging force using the central composite design-based response surface method (RSM). The ANOVA analysis is used to validate the response surface model where its R2 coefficient of 99.93% has indicated an excellent fit. The methods and results differ from those presented in previous studies.

Wave energy is one of the most attractive forms of renewable energy. The reasons include its promising availability, predictability, persistence, and power density. This study focuses on all linear generator designs and technologies which have been used so far in direct‐drive wave energy converters (DD‐WECs). Currently, linear permanent magnet generators (LPMG) have been proposed as the most advantageous generator system developed for DD‐WECs. After a brief description of linear generator based wave energy converters, all proposed state‐of‐the‐art of LPMG topologies available in the literature are discussed and compared in terms of flux path, core type, location of PMs, and etc. In addition, other linear generator technologies such as linear switched reluctance and linear superconducting generators, as an alternative to LPMGs, are reviewed. Finally, based on the surveyed quantitative comparisons performed in previous works, eight major concepts are evaluated in terms of economic and operational aspects.

Over the past two decades, linear generators have been widely used in direct drive wave energy converters (DD-WECs), thus simplifying the system structure and improving conversion efficiency. While these linear generators are desirable for wave energy converter because of their high reliability, they must be very large to generate significant electrical power from such low-speed motion. These linear generators have demerits of large volume, low power density and low conversion efficiency when they are adopted in DD-WEC. Magnetically geared machine and field-modulated permanent magnet linear generator can significantly decreases the volume and cost of the required generator, also the speed of travelling magnetic field is enhanced and high power density capability is ensured by the ‘magnetic gearing effect’. A field-modulated PM transverse flux linear generator (FM-PMTFLG) is proposed and analyzed in this paper. The field modulation mechanism in FM-PMTFLG with tilted translator is introduced in detail. The electromagnetic characteristics of different combinations of stator slots and translator poles and different tilt angle of magnetic tilted bars and nonmagnetic tilted bars are comparatively investigated. The FM-PMTFLG prototype is manufactured to validate the results of the selected design parameters and the relationship between magnetic gearing effect and tilted translator. A fundamental experiment was conducted. A good agreement between 3D-FEA and experiment results proved the validity of the generator design.

During past three decades, marine energy has become an attractive topic in renewable energy. Among various types of marine energy, present study focuses on wave energy because of its advantages such as high availability, high predictability, high stability, and high power density. Nowadays, most wave energy converters (WECs) installed have complex mechanical interfaces in their power-takeoff (PTO) system which results in complicated system structure and more maintenance requirements. These problems are solved in direct-drive wave energy converters (DD-WECs) by using linear generators, which simplify PTO system. The present review discusses different designs and technologies of linear generators employed in DD-WECs. After a brief explanation about WECs based on linear generators, all proposed topologies of linear permanent magnet generators are discussed and compared based on specifications such as flux path, core type, installation and magnetization of permanent magnets, etc. Finally, the best types of linear generators, in term of being economical and operational, are identified.

Direct-drive wave energy converter (DD-WEC) simplifies the structure and improves the efficiency of wave energy conversion. Permanent magnet transverse flux linear generator (PMTFLG) combines the merits of transverse flux linear generator (TFLG) and permanent magnet linear generator (PMLG), and is suitable for low-speed high thrust density applications, like DD-WEC. However, it suffers from a significant demerit which is the high cost due to the rare earth permanent magnets (PMs). A stator-PM spiral translator PMTFLG (ST-PMTFLG) is proposed in this article. Armature winding and PMs are arranged in outer stator and inner stator, respectively while the translator is made of spiral steel blocks and nonmagnetic blocks. Similar to the magnetically geared machine, the proposed topology is suitable to be used in low-speed long-stroke application. Results show that ST-PMTFLG has advantages of low PM consumption and high thrust density. The thrust per PM volume of 12/11 ST-PMTFLG is 4.18 MN/m3.

Research on linear generators for direct-drive wave energy conversion has mostly focused on inner-secondary sole permanent magnet (PM) linear generators, among which vernier hybrid machine and flux-switching generators are considered as the best selection. In this study, an outer-secondary tubular superconducting hybrid excitation linear generator is presented to increase the power density and reduce the voltage regulation. Double-buoys direct-drive WEC is introduced. The operation under hybrid excitation is analyzed. With consideration to cogging force, the linear generator is optimized. The generator performance is evaluated using the finite element method. Based on the on-load performance, a hybrid excitation method is proposed and is compared with only the PM working. According to the working point of the proposed generator, losses of the superconducting hybrid excitation windings are calculated. A PM flux-switching generator is employed to verify the analysis results. The results show that the proposed generator has the advantages of larger air-gap effective area, higher power density, and more steady output power and voltage.

Designing high energy conversion efficient bio-inspired vitamin assisted single-structured based self-powered piezoelectric/wind/acoustic multi-energy harvester with remarkable power density

Recently, linear generators have been implemented in direct drive wave energy conversion (DD-WEC), thus simplifying the structure and improving system efficiency. Since DD-WEC is a typical low-speed application, linear generators with multi-pole design all encounter problems such as low power density and low efficiency. Similar to the magnetically geared machine, field-modulated permanent magnet transverse flux linear generator (FM-PMTFLG) has been a suitable candidate for DD-WEC, due to the high power density capability ensured by the `magnetic gear effect'. However, for solely PM excited generators, there is a tradeoff between high power density and wide speed operation. Hence, field-modulated hybrid-excitation permanent magnet transverse flux linear generator (FM-HE-PMTFLG) is proposed. In order to improve the thrust performance, the partitioned staggered translator (PST) topology is introduced. The simulation results show that PST topology can achieve better thrust performance than partitioned translator (PT) counterpart. Compared with PT topology, PST topology reduces the magnetic flux leakage and has advantages of higher thrust density, less thrust ripple, higher flux linkage and better total harmonic distortion (THD) of back-EMF.

Object and purpose of research. The object of the study is the wave energy converters (WEC) into electric power. The purpose of the study is to draw up a differential equation describing the operation of the WEC, its solution and determination for the design parameters influence of the WEC on the efficiency of energy conversion. Materials and methods. The theoretical methods adopted in ship mechanics and theory are used to study the operation of complex mechanisms and dynamics of marine objects. The data on the characteristics of electric generators are used. Main results. The differential equation describing the operation of the WEC on regular wave is compiled. The equation relates the characteristics of waves to the design parameters of the WEC. Generalized characteristics of electric generators are obtained. Systematic calculations are carried out; they show the influence of the WEC design parameters on the efficiency of wave energy conversion into electric power. Conclusion. The results can be used in design of the WEC. The resulting differential equation makes it possible to investigate the operation of the WEC in various marine conditions and to evaluate the influence of design parameters on the efficiency of energy conversion. The generalized characteristics of electric generators can be used in design of wave power structures and wind power plants.

The traditional wave energy converters (WECs) use hydraulic or turbine-type power take-off (PTO) mechanisms which consist of many moving parts, creating mechanical complexity and increasing the installation and maintenance costs. Linear generator-based direct-drive WECs could be a solution to overcome this problem, but the efficiency of the single conventional linear generator is not high enough, and it cannot work satisfactorily in the low-frequency range. This article reviews the recent research developments of the linear permanent magnet (PM) generator-based WEC to harness maximum energy from ocean waves. It starts with a brief introduction and background of wave energy converters using linear generators. Following this, the working principle of the WECs with linear PM generators is briefly outlined. Subsequently, the analytical model of the linear PM generator-based WEC is studied. After that, the up-to-date developments of the linear PM generator-based PTO systems are studied. Despite some modifications resulting in complexity in the linear PM generator’s structure and a rise in manufacturing costs, the study shows the systems’ efficiencies increased by increasing magnetic flux and reducing cogging force. The key parameters and improvement issues that can increase the performances and efficiencies of the PTO systems are identified to help future researchers for further development. Moreover, the review discusses the numerical and experimental analysis tools, the typical control systems used by the researchers and the challenges of the linear generator-based wave energy conversion system. Finally, conclusions about the significant beneficial characteristics and design choice of the WEC linear generator structure are provided and related to the application conditions.

Linear permanent magnet generators are commonly used as the power take-off system in direct drive wave energy converters. The electrical power generated is characterized by a high variability in amplitude and frequency, so a power electronic interface is needed for grid connection. In addition to its coupling function, the electronic converter can be used as a tool to improve the efficiency of the wave energy conversion. An approach to this goal is to use the generator-side converter to emulate a resistance as seen from the generator. For this optimization method, this paper describes the performance of three different current controllers of the converter from the point of view of the overall conversion efficiency.

Direct drive wave energy converters have been proposed in view of the disadvantage of mechanical complexity and low conversion efficiencies in conventional wave energy converters. By directly coupling a linear generator to a reciprocating wave energy device, it is suggested that direct drive power take-off could be a viable alternative to hydraulic- and pneumatic-based systems. To further realise the benefits of a direct drive system, a control scheme based on reaction force control to maximise energy extraction is presented. It focuses predominantly on the theoretical analysis of the linear generator reaction force. The modelling, simulation and control of direct drive wave energy conversion are systematically investigated by computer-aided analysis via Matlab/Simulink

A new type of permanent magnet-induction magnetic screw (PMI-MS), as a linear rotary actuator (LRA) with high thrust density and high energy conversion efficiency compared with the traditional direct-drive wave energy conversion (DD-WEC), is proposed and applied in wave energy conversion (WEC). The proposed PMI-MS adopts the method of quasi-Halbach magnetization array and hybrid excitation to overcome the shortcomings of the non-adjustable thrust of the permanent magnet magnetic screw (PM-MS) and the low thrust density of the induction magnetic screw (I-MS). At the same time, the special structure of adjacent teeth with different heights on the mover can improve the electromagnetic thrust generated by the electric excitation, thereby further improving the thrust adjustment capability and thrust density of the PMI-MS. Compared with conventional PMI-MS, it has higher thrust density and higher wave energy conversion efficiency compared with the wave energy linear generators. To optimize and design the PMI-MS, analytical methods and global optimization algorithms are adopted. Finally, finite element simulations are used to verify that the proposed PMI-MS has higher thrust density.

The linear permanent-magnet synchronous generator (LPMSG) for direct-drive wave energy conversion (WEC) suffers from many drawbacks that have not yet been overcome, such as a low power density and a bulky system volume. Therefore, a magnetic field-modulated linear permanent-magnet generator (FMLPMG) with a simple structure has been designed and manufactured. First, the operating principle of the FMLPMG is demonstrated by the equivalent magnetic circuit method, which explains the high power density of the FMLPMG. Second, at a constant speed and a sinusoidal speed, the electromagnetic characteristics of the FMLPMG under the no-load and load conditions are analysed by the finite-element method. Finally, a direct-drive WEC test platform is built to simulate the process of wave action on the FMLPMG. No-load and load experiments of the FMLPMG are conducted on the test platform, and the results are compared with those of an LPMSG with the same volume and operating conditions. The results show that the FMLPMG with a high power density converts wave energy effectively and solves the problem of low power density faced by the LPMSG in direct-drive WEC.

Till date, fabrication of piezoelectric nanogenerator (PNG) with highly durable, high power density, and high energy conversion efficiency is of great concern. Here a flexible, sensitive, cost effective hybrid piezoelectric nanogenerator (HPNG) developed by integrating flexible steel woven fabric electrodes into poly(vinylidene fluoride) (PVDF)/aluminum oxides decorated reduced graphene oxide (AlO‐rGO) nanocomposite film is reported where AlO‐rGO acts as nucleating agent for electroactive β‐phase formation. The HPNG exhibits reliable energy harvesting performance with high output, fast charging capability, and high durability compared with previously reported PVDF based PNGs. This HPNG is capable for harvesting energy from a variety and easy accessible biomechanical and mechanical energy sources such as, body movements (e.g., hand folding, jogging, heel pressing, and foot striking, etc.) and machine vibration. The HPNG exhibits high output power density and energy conversion efficiency, facilitating direct light on different color of several commercial light‐emitting diodes instantly and powers up many portable electronic devices like wrist watch, calculator, speaker, and mobile liquid crystal display (LCD) screen through capacitor charging. More importantly, HPNG retains its performance after long compression cycles (≈158 400), demonstrating great promise as a piezoelectric energy harvester toward practical applications in harvesting biomechanical and mechanical energy for self‐powered systems.