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Ocean wave energy is sustainable and renewable energy, wave energy conversion (WEC) effectively solves the disadvantage of traditional power system with fossil energy. WEC system is the power generation process that converts wave kinetic energy of undulation and swing into electric energy, and direct‐drive WEC system (D‐DWEC) converts wave energy into electric power directly by linear generator to improve the efficiency. At present, the research of WEC system has the characteristic of interdisciplinary, multi‐system integration, engineering application, and commercial operation. This paper summarizes the types, principle, and classification of WEC system, mainly focusing on the aspects of the kinds and characteristics of linear generators used in D‐DWEC system, power post‐processing, and power control methods. At the end of the paper, giving the development trend and challenges of D‐DWEC technology, the research of ocean complementary energy generation platform with offshore wind, solar and ocean wave energy is analysed. It shows that the platform has advantages owing to stable power output, efficient utilization, ocean saving‐space, and power generation cost reduction. [ABSTRACT FROM AUTHOR]

It is commonly acknowledged that a direct‐drive linear wave energy converter (DDLWEC) will achieve maximum power extraction in the resonance state, which can be realised through impedance matching under the assumption that the wave is regular. While the real ocean waves are hardly regular. Considering swell, a relatively regular type of real ocean wave, by approximating it with a sum of multiple frequency components, the DDLWEC can be forced resonance to the swell through velocity control, the optimal velocity is derived based on the dynamical model of the DDLWEC through frequency domain analysis. The stability of velocity control is proven and the float equilibrium position deviation due to the velocity control is analysed and countered through compensation control. In real‐time control, the excitation force is approximated by the sum of four sinusoids, simulation and experiment results demonstrate that the proposed method is valid, stable, capable of achieving a high‐power extraction efficiency, and superior to some existing methods. [ABSTRACT FROM AUTHOR]

Snake‐like wave energy conversion (WEC) is a new type of WEC that combines raft WEC and magnetoelectric transducer. This type of WEC uses the magnetoelectric transducer, which is designed based on the permanent magnet linear generator and Halbach permanent magnet array, as its power take‐off unit. The purpose of this study is to design a matching magnetoelectric transducer for WEC in common wave conditions and test its performance. The hydrodynamic model of the snake‐like WEC is established to study the motion of WEC in regular waves. The equivalent magnetic circuit model of the magnetoelectric transducer is set up to optimise its ability to harvest wave energy in the ocean. Taking the maximum damping coefficient (cm·max) as the optimisation goal of the magnetoelectric transducer, the induced voltage and output power of the magnetoelectric transducer in relatively uniform and sinusoidal motion can be calculated by simulation software. Finally, a prototype of the magnetoelectric transducer was made and its performance was tested. The experiment shows that the magnetoelectric transducer has a good ability to harvest wave energy, and snake‐like WEC has wide applicability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ocean waves are an abundant source of energy. This energy of the ocean can be converted into useful electrical energy using electrical generators. Linear generators have received tremendous attention in energy harvesting technology due to its unique ability to convert the energy without any intermediate converter. The principal objective of this study is to present the various types of linear electrical generators which have been investigated so far for direct drive ocean wave energy conversion and describe the working principle of each type with the difference. In this study, after a brief description of the basic electrical generator, various types of generators, including the linear and flat generators available in the literature are reviewed and discussed based on the design configuration of different types of magnet arrangements. Linear generators have been compared in terms of core type, flux path, the location of PMs, etc. The research gaps have been identified and future research directions have been suggested. [ABSTRACT FROM AUTHOR]

Wave Energy Converters (WECs) generally use a Mechanical Power-Take-Off (MPTO) involving hydraulics or gearing to optimise the extraction of energy from the incoming waves before converting it into electrical energy via a high speed rotating generator. This simplifies the design of the Electrical Power Conversion System (EPCS). Further, it facilitates the use of mechanical energy storage such as hydraulic accumulators to reduce the peaky nature of the power flow and allows the WEC resonant frequency to be tuned for maximum energy capture. This work compares two power electronic converter topologies for an Electrical PTO (EPTO). This EPTO is intended to replace the aforementioned MPTO and generator with a Permanent Magnet Linear Machine (PMLM) directly coupled to the WEC. The compared topologies comprise either a Current Source Converter (CSC) or a Voltage Source Converter (VSC) as the generator interface working in combination with a DC-DC converter and an Energy Storage System (ESS). The principle differences between the two topologies are explored and losses are evaluated in a modular EPCS working with a WEC. Wide-bandgap power electronic switches are assumed for both topologies over a range of switching frequencies. The evaluation concludes that the CSC topology is advantageous at higher switching frequencies.

This work presents a new wave energy converter power take off (PTO) system that combines two recent developments in drive train technology, the consequent pole linear vernier hybrid permanent magnet machine and the linear magnetic gear. The combined system is fully realised in three-dimensional finite element method and compared to a similar direct drive machine. While the resulting system is lighter, reducing the active material mass by 30%, and more efficient, the system requires larger amounts of high cost permanent magnet material and increased PTO complexity. It is summarised that while such a system could be recommended in certain conditions, it is not viable for general application.

Free-piston engines have the potential to offer a compact machine capable of delivering electrical power. In this work, the design and modelling of a linear tubular generator are presented and used to investigate the effect of the electrical machine design on engine performance. A cogging-force-reduction technique is presented and used as a case study to demonstrate the importance of the integrated design.

This study proposes a linear magnetic gear (LMG). Unlike conventional LMGs, the proposed one purposely uses high‐temperature superconducting (HTS) bulks replace permanent magnets (PMs). The high‐speed mover of the magnetic gear is connected to the secondary of the linear permanent magnet generator (LPMG), which constitutes a direct‐drive wave power system that converts wave energy into electrical energy. First, the LMG structure is presented according to the principle of magnetic field modulation, and the authors briefly analyzed the number of pole pairs of inner, outer, and ferromagnetic modulators. Second, they also established a finite element analysis model to analyze the performance of magnetic gear, including magnetic field distribution, radial air gap magnetic flux density, and force characteristic. An LMG with HTS bulks not only increased the radial air gap magnetic flux density but also improved the speed of LPMG. In conclusion, this experiment verifies the efficacy of direct‐drive wave power take‐off system and effectively converts wave energy into electrical energy. [ABSTRACT FROM AUTHOR]

Free-piston engines have the potential to offer a compact machine capable of delivering electrical power. In this work, the design and modelling of a linear tubular generator are presented and used to investigate the effect of the electrical machine design on engine performance. A cogging-force-reduction technique is presented and used as a case study to demonstrate the importance of the integrated design. [ABSTRACT FROM AUTHOR]

The development and reasonable utilisation of wave energy have great realistic significance for solving environment pollution and energy crisis. A direct‐drive system with no middle conversion devices such as gear and turbine, which has a simple structure and good stability, is widely applied to wave power systems. First, the study analysed the structure of a tubular permanent magnet linear generator. The structure of short primary and long secondary is chosen for wave power systems because of the convenience of installation and higher power density. Then, based on finite‐element analysis, the structure of the generator is optimised by parametric analysis and genetic algorithm. A prototype is manufactured according to the optimised size, and the electromagnetic parameters are measured to verify the accuracy of simulation. Finally, a double floating direct‐drive wave power system is installed in the port of Lian Yungang, China, and the system is tested by still and dynamic water; the results show that the system can effectively convert wave energy into electricity. [ABSTRACT FROM AUTHOR]

This study considers the design and testing of a laboratory scale test rig for wave energy converters (WECs). The main element of the test rig is a double‐sided permanent magnet linear generator (PMLG). In this study, the authors describe the detailed design of the PMLG. The objective of the design is to find the detailed parameters of the PMLG to meet the targeted electromotive force (EMF) voltage with respect to the designed physical constraints. The design procedure is easy to follow and emphasises the practical aspects to construct the PMLG. A spreadsheet table was generated using the procedure to find the design parameters. Therefore, a designer can easily modify the parameters based on the physical constraints and the targeted EMF voltage. In addition to that, the authors explain the procedure to find the rating temperature for the generator. Finally, this PMLG is integrated with other components to form the test rig. The experiment is conducted to show how close the performance of the constructed PMLG is, in term of its EMF voltage and rated thermal, to the designed values. Additional tests were also conducted to test the performance of the test rig using various scenarios similar to ocean wave profiles. [ABSTRACT FROM AUTHOR]

This study considers maximising conversion between the mechanical and electrical powers for heaving point absorbers (HPAs). The objective is implemented by generating the buoy's velocity reference by designing the intrinsic resistance. The authors designed the intrinsic resistance using a rapid procedure involving the mechanical and electrical models of HPAs. The electrical power conversion can be improved by tuning a weighting constant with the constraints on the maximum value of the control force and the power take‐off utilisation index. The value of the intrinsic resistance is varied based on irregular sea states, which are characterised by their significant heights and peak angular frequencies. A simple robust proportional‐integral‐derivative (PID) controller is utilised in the servo feedback control system to follow the velocity reference. The PID controller is designed using the complex polynomial stabilisation to convert the robust performance into a set of linear programming problems. A set of admissible PID controller is obtained to satisfy the robust performance specifications. The authors tested the proposed method in various nominal and perturbation scenarios, and its performance was compared with existing reference and non‐reference based HPA control strategies. [ABSTRACT FROM AUTHOR]

In this paper, a new linear primary permanent magnet vernier (LPPMV) machine is developed for direct‐drive wave energy conversion. Owing to the magnetic gearing effect, the LPPMV machine possesses the advantages of high thrust force and low speed motion. After describing the machine structure, the operation principle of the machine is analysed and compared using two methods, namely, the magnetic gearing principle and the straightforward magnetic path. Then the characteristics are analysed by using the finite element method. Considering the unbalance of the no‐load flux linkage and the inductance among the three phases, the mathematical model of the proposed machine is established, which is significantly different to that of the traditional permanent magnet machine. Finally, the machine is prototyped and tested so as to experimentally verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper discusses about the voltage output ripple reduction and error minimization for the direct-drive, linear switched reluctance generator (LSRG)-based wave power generation system. First, the concept of the LSRG-based wave power generation system is studied. According to the characteristics of the LSRG, the suitable drive circuit dedicated to proper current excitation and generation is established. Second, the reasons that cause voltage output ripples are investigated. To reduce the remarkable ripples from phase current commutations, a current distribution function (CDF) is proposed based on the minimized copper loss principle. Third, the dual-loop control strategy with current and voltage as the inner and outer loop is constructed, implemented with the proposed CDF. Theoretical bases of the control strategy are derived. The simulation results prove that the proposed control algorithm is capable of voltage ripple suppression and error reduction within the range of ±0.5 V over the entire operation speed for wave energy extraction, validated by experimental verification. [ABSTRACT FROM AUTHOR]

An external tubular linear permanent magnet generator (ETLPMG) is proposed for direct drive wave energy conversion. ETLPMG with quasi‐Halbach array is larger in the air gap magnetic flux density than the internal and radial magnetisation one. An assistant tooth and fractional slot are implemented to decrease the detent force. The power density of ETLPMG is about 7–8 times for that of the internal one and the efficiency reached 90.03% at 18 Ω condition. The characteristics of the ETLPMG are analysed with a finite element analysis. Finally, a prototype is manufactured to verify the simulation results by some experiment tests. [ABSTRACT FROM AUTHOR]

This study presents the dynamic modelling of a linear permanent magnet generator for extracting energy from ocean waves. Translator position, calculated from measured generator voltage, is used as input for the simulation model. Instantaneous power from the simulation model has been compared with the measurements from the Lysekil research site. The power output from the model considering the air gap flux variation is precisely matching with the measured values before core saturation. The generator dynamic model is modified by including the saturation effect. Although a simple mathematical expression is considered for representing the saturation, the model gives accurate power spectrum close to the experimental results. The presented model is a first step towards the system model that can simulate the entire electric system including electric grid. As such, the generator model can be used for further analysis of the wave energy conversion system. [ABSTRACT FROM AUTHOR]