Direct Drive Wave Energy Converter Research Articles

Simple and Low-Model-Dependent Strategy for the Economic and Safe Control of Direct-Drive Wave Energy Converters

For engineering application and commercialization, a wave energy converter (WEC) should harness as much wave energy as possible to improve its economy and maintain each component works within their physical constraints for safe operation. Model predictive control (MPC) can maximize wave energy extraction under constraints for the direct-drive WEC. MPC requires accurate modelling parameters, wave prediction and complex computation, while these can hardly be guaranteed in real application. Besides, researchers generally take position and force constraints into consideration, while power constraint is neglected. In this work, we analyze the power extraction characteristics under constraints and propose a strategy to realize economic and safe control of WECs by decoupling it into different control modes of a permanent magnet synchronous generator (PMSG). In this way, the control is simple and low-model-dependent and power constraint can be also maintained. The strategy is put through comprehensive simulation tests with MPC as the benchmark, and it is proven to be effective under various wave conditions and constraint settings. Furthermore, experimental tests are carried out for a prototype device in a wave tank laboratory and these tests validate the applicability, effectiveness, and robustness of our strategy.

Resonance control to eliminate reactive power of an improved dual-port direct-drive wave energy converter

The single-port direct-drive wave energy converter (SPDDWEC) is widely utilized for harvesting ocean power among the existing wave energy converters (WECs) designs. Due to its working principle of generating reactive power inevitably and intermittent nature of the waves, the SPDDWEC is unable to provide continuous and stabilized energy to the grid. This paper proposed a dual-port resonance (DPR) control, which can achieve an improved dual-port direct-drive WEC (DPDDWEC) resonance by adjusting the dual electromagnetic force and electromagnetic spring force. The proposed DPR control can effectively avoid reactive power flow of the DPDDWEC, which can generate more energy than the SPDDWEC. Based on the established DPDDWEC kinematic state space model, the proposed DPR control is simulated and verified. The study results confirm the effectiveness of the proposed control method and correctness of theoretical analysis.

A Robust Faster Joint Control of a Direct-Drive Wave Energy Converter Combined With Supercapacitor and Battery Energy Storage

A Robust Faster Joint Control of a Direct-Drive Wave Energy Converter Combined With Supercapacitor and Battery Energy Storage

Biofilm prevention in the generator of a direct drive wave energy converter

Power take off in a wave energy converter has a number of unique requirements. It must convert low speed oscillating motion into electricity in a reliable, low maintenance manner. A direct drive system, where the electrical machine is optimised to operate at low speed, has the potential to offer a mechanically robust and simple solution. Similar to a hydraulic power take off, the only regular maintenance would be to inspect and replace the seal between moving parts. One strategy for removing regular maintenance is to have an unsealed system, i.e. one where sea water is allowed throughout the electrical machine. A fully flooded electrical machine has benefits in terms of cooling, but poses challenges relating to reliability, corrosion, biofouling and lubrication.
 Biofilm refers to a thin layer of fouling organisms which can interfere with the operation of components. Recent work has found that submerged surfaces can be kept free of biofouling using projected ultraviolet (UV) light from LEDs. This paper discusses the testing procedure and impact of the UVC irradiation on biofilm prevention within the active part of an electric generator in a systematic manner, with a view to accelerate its translation to full-scale applications.
 A prototype generator is being developed which will be installed in the North Sea, consisting of a submerged linear tubular electrical machine. A magnetic tubular translator will oscillate within a cylinder that houses stator coils. Lubrication will be by way of solid polymer bearings. In order that the active part of the electrical machine can oscillate smoothly, it is imperative that biofilm is prevented from colonising on the bearing surface, which also makes up the magnetic gap of the electrical machine.
 The system will have a slow reciprocating oscillation, with a peak speed of perhaps 2m/s. For most wave energy converters there will be brief static periods twice in every wave, and in calm seas these could be prolonged to several hours or even days. In low energy sea states oscillation amplitude could be less than the fully rated amplitude, meaning different parts of the bearing surface could be exposed for different amounts of time.
 Early-stage work is underway to investigate the use of UV irradiation in the active part of the electrical machine and bearing surface as biofilm prevention. Flat panels (600mm x 220mm) are used to simulate the original surfaces between moving parts. To achieve biofilm growth, an artificial slime farm was deployed which allows test panels to be subjected to a continuous dynamic flow. The light source of UV irradiation was provided by Light Emitting Diodes (LEDs) with 278nm wavelength. The effectiveness of the biofilm prevention by UVC were evaluated by Image Analysis
 The results indicate that UVC can significantly control biofilm presence on the panels. It also has demonstrated that intermittent UV can achieve successful biofilm prevention on submerged surfaces. However, observations indicate the actual UVC light intensity may perform below the manufacturer’s specifications, and this could lead to a detrimental effect on its biofilm control performance.

Design and Analysis of a Field-Modulated PM Transverse Flux Linear Generator With Tilted Translator

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.

Maximum power point tracking control based on inertia force for underwater direct-drive wave energy converter

Underwater direct-drive wave energy converter (UDDWEC) using linear-rotating axial flux permanent magnet generator (LR-AFPMG) can change form of energy conversion and improve conversion efficiency. This paper proposed a maximum power point tracking (MPPT) control based on inertia force in electromagnetic force for UDDWEC which can control the form and magnitude of electromagnetic force including damping force, spring force and inertia force. By adding inertia force in electromagnetic force, the pole location and dynamic response performance can be changed and adjusted. This paper studied the influence of inertial force in electromagnetic force and system response in detail. The research also involved the influence of buoys with different shapes on wave energy capture, the expression of q-axial reference current when inertia force is considered, the necessity of additional damping in the system and power capture width in different sea conditions. According to the hydrodynamic parameters and wave characteristics, the proposed MPPT control was verified through simulation in regular wave and JONSWAP spectrum and the results showed that MPPT control based on inertia force can capture the maximum wave energy and achieve maximum power capture width. Above on this, this control strategy can be a decent candidate for UDDWEC. Finally, a series of flume experiments were carried out to verify the accuracy of hydrodynamic parameters of underwater buoy.

Quantitive Harmonic Analysis and Force Ripple Suppression of a Parallel Complementary Modular Linear Reluctance Machine

Considering the fluctuating price of permanent magnet, magnetless machine is a new trend in development. Variable reluctance linear machines (VRLM) with simple structure, good robustness, and high dynamic performance, is one of the potential candidates for linear direct-drive wave energy conversion (WEC) application replacing PM-excited linear machine. To ameliorate the fault-tolerance performance of VRLM, the researchers have come up with modular or segmented design, denoted as modular variable reluctance linear machines (MVRLM) However, suffering from large thrust ripple, the design of MVRLM still needs to be improved. To relieve the force ripple of MVRLM, a novel H-shaped modular variable-flux linear reluctance machine (HMVF-LRM) is proposed in this paper. The principle of force ripple suppression by using parallel-complementary structure is analyzed deeply. In this paper, the analytical calculation of induced voltage is developed quantitively with equivalent magnetic circuit method (EMC) and harmonics analysis. Then the design mechanism of HMVF-LRM is illustrated. Further, the performance and fault-tolerant capability of the proposed machine are simulated and evaluated by finite element analysis (FEA), and the prototype is tested to verify the effectiveness of the machine design.

Design and Optimization of Linear Permanent Magnet Vernier Generator for Direct Drive Wave Energy Converter

A novel linear permanent magnet vernier generator (LPMVG) for small-power off-grid wave power generation systems is proposed in this paper. Firstly, in order to reduce the cogging force and the inherent edge effect of the linear generator, a staggered tooth modular structure is proposed. Secondly, in order to improve the output power and efficiency of the LPMVG and reduce the fluctuation coefficient of electromagnetic force, the relationship between the parameters of the generator is studied, and a method combining multi-objective optimization and single parameter scanning based on the response surface model and particle swarm optimization algorithm is proposed to obtain the optimal structural parameters of the generator. Thirdly, the output power and efficiency of the optimized generator are calculated and analyzed based on the two-dimensional finite element method, and the effectiveness of the multi-objective optimization design method based on the response surface model and particle swarm optimization algorithm is verified. Finally, a prototype is developed, and the calculated results and the measured results are shown to be in good agreement.

A study of a modified design of dumbbell-shaped flux switching tubular linear generator for regular 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.

Research on an improved model predictive current control for direct‐drive wave energy converters with linear generators

AbstractThis paper deals with the current control strategy of a permanent magnet linear generator (PMLG) for a direct‐drive wave energy converter (DDWEC). As the mover is in a linear reciprocating motion, the reference of the controller of the generator is dynamic fluctuations with the mover movement under maximum power point tracking (MPPT) conditions. Therefore, the dynamic requirements of the controller become higher. To increase dynamic control performance, an improved model predictive current control (IMPCC) strategy is proposed here. Through a three‐step cascaded optimization algorithm, the target voltage vector is optimized from the three dimensions of the sector of the voltage vector, the angle of the voltage vector and the length of the voltage vector. The target voltage vector is output to the three‐phase rectifier by using SVPWM to complete the entire control process. Simulation and experimental results show that the proposed control strategy effectively reduces the current ripple and the vibration of PMLG and improves the stability of DDWEC.

A Novel Multiport Hybrid Wave Energy System for Grid-Connected and Off-Grid Applications

Direct drive wave energy converters (DDWECs) have gradually become the mainstream of wave energy converters (WECs). In order to make better use of wave energy, energy storage devices and other renewable energy sources are often used to suppress power fluctuation in DDWECs. However, the addition of other energy sources will increase the complexity of the converter system and the number of power switches. Considering the flexibility of nine-switch converters (NSCs), this paper proposes a novel nine-switch grid-connected/off-grid multiport hybrid wave energy system (HWES). First, the system structure and modulation principle are described. Then, a model for a generator, a grid and energy storage are built, including a control strategy of each part. Finally, a simulation for the grid-connected/off-grid application and an experiment on the off-grid HWES are carried out. The results show that the multiport wave energy system can achieve the objective of stable and reliable power transmission by reducing power devices.

Detent Force Reduction in Linear Interior Permanent Magnet Generator for Direct-Drive Wave Power Conversion

The permanent magnet linear generator is widely applied in the direct-drive wave energy converter (DD-WEC) because of its high power density. In this paper, a novel tubular permanent magnet linear generator, which consists of multilayer and interior permanent magnets (MI-TLPMGs), is presented for DD-WEC, which improves the output power and back electromotive force (back EMF) through the flux concentrating effect. However, MI-TLPMGs with multilayer embedded permanent magnets have severe problems regarding force ripples and detent force, which affect the DD-WEC’s dynamics. Therefore, a DD-WEC system with MI-TLPMGs is proposed, and the effect of the detent force on the dynamic performance of the DD-WEC is analyzed theoretically. Then, the L-type auxiliary teeth and magnetic barriers, which are optimized by the Taguchi method, are introduced to minimize the detent force of the MI-TLPMGs. After optimization using the Taguchi method, the amplitude of the detent force is reduced from the initial 21.7 N to 5.2 N, which means it has weakened by nearly 76.1%. Finally, a prototype has been manufactured and measured in the wave tank to verify the optimization results.

Direct‐Drive wave energy conversion with linear generator: A review of research status and challenges

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.

Design and experiment of a direct-drive wave energy converter using a variable stiffness mechanism

This paper proposes a new method to achieve the resonant behavior of a point absorber floating buoy type of Wave Energy Converter using direct drive power take-off system. The system has four main components: a frame; a buoy with a guiding shaft; a supplementary mass coupled with a linear generator and a variable stiffness structure. Firstly, the motion in the vertical direction of the buoy in the sea is converted to an oscillating motion of the supplementary mass. Next, the mass is coupled with a copper coil. The coil motion inside a magnetic wall induces an electrical current in the coil as a linear generator. Besides, a pair of pre-tension springs creates a negative stiffness phenomenon that acts on the motion of the mass and increases the performance of the system. Finally, a variable stiffness structure is installed to control the trajectory of the mass, it can increase the velocity of the mass motion thus the performance of the system is improved. Configuration and working principles of WEC were introduced, studied and analyzed. The mathematical model was presented to explain system behavior. The numerical simulation had been carried out to evaluate the operation of the system. The experimental test rig had been set up and implemented to verify the effectiveness of the proposed structure.

Finite-time cascade-like tracking control of direct-drive wave energy converters

In this paper, sufficiently addressing power take-off dynamics, high-accuracy tracking problem of the direct-drive wave energy converter (DWEC) which is inevitably disturbed by complex unknowns is solved by creating a finite-time cascade-like tracking control (FCTC) scheme. More specifically, a cascade structure of the DWEC system is initially built by finely taking d-axis current dynamics into account such that accurate tracking of wave energy can be hierarchically accommodated by independently designing cascade-like control laws for d- and q-voltages. Then, to exactly compensate complex unknowns including unmodeled dynamics and disturbances, a finite-time observer is devised within the foregoing cascade structure, and facilitates the cascade-like controller synthesis whereby the backbone can be rationally decoupled. Within the entire FCTC scheme, d-axis current tracking dynamics can be sufficiently addressed so as to significantly enhance tracking accuracy of wave energy. Eventually, rigorous analysis ensures that both wave displacement and velocity tracking errors are globally exponentially stable. Simulation results and comparisons with typical methods demonstrate remarkable performance and superiority of the proposed FCTC scheme.

Design optimization of a novel linear transverse flux switching permanent magnet generator for direct drive wave energy conversion

Linear transverse flux permanent magnet machines (LTFPMs) are categorized as high force density topologies, which are suitable candidates for direct drive and low-speed applications. This paper introduces a novel LTFPM topology, which also proposes advantages of the flux-switching machines. This proposed linear transverse flux switching PM machine (LTFSPM) has two long passive translators located at the armature's top and bottom sides. The topology offers a high value of force density, and also, it is a cost-effective topology due to its double-sided passive translator. Furthermore, the PMs are fully utilized in the magnetic circuit, the end winding length is negligible, and the iron cores can be laminated easily. In this paper, the operation principle of the topology is explained and its magnetic circuit is discussed. To reduce the computational time of optimization procedure, a double-layer method is adopted, which provides high robustness, low computational time, and high precision. Results of the finite element method indicate that the proposed machine can be considered a potential candidate for linear motion applications especially direct drive wave energy conversion systems, which require high force density values.

A Review of the Linear Generator Type of Wave Energy Converters’ Power Take-Off Systems

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.

A Novel Back-Stepping Sliding Mode Control Strategy of Direct-Drive Wave Energy Converters

In this paper, a maximum energy extraction and tracking strategy for direct-drive wave energy converters (DDWECs) based on back-stepping and sliding mode control strategies are developed. It is well known that the existence of the chattering phenomenon degrades the control performance of conventional sliding mode control (SMC). To mitigate the effects of flutter and external disturbances, a back-stepping sliding model control (BSMC) scheme is proposed. The main features of the proposed methodology are as follows: (1) By using the proposed BSMC, the maximum wave energy of DDWEC can be captured. Moreover, the speed tracking of the permanent magnet linear generator (PMLG), which is a subsystem of DDWEC, tracked in real-time. (2) By virtue of the proposed BSMC, the closed-loop control system is asymptotically stable in finite time. (3) With the superior controllability of the BSMC, it can handle disturbances that the SMC cannot handle. Comprehensive and comparative studies are proved to be superior to the most advanced method

Comparative Analysis of Tubular Permanent Magnet Linear Generator With Equidirectional Toroidal Windings and Conventional Toroidal Windings

Tubular permanent magnet linear generators (TPMLGs) are widely used in direct drive wave energy conversion system due to their simple structure, friendly maintenance, and high efficiency. Unfortunately, the power density of this type of generator is relatively small due to their low operation speed. To improve the power density, a TPMLG with equidirectional toroidal windings (ED-TWs; ED-TPMLG) is proposed in this article. First, the structure of the ED-TPMLG is introduced, and its windings configuration and windings coefficient are analyzed and compared with those of TPMLG with conventional toroidal windings (C-TPMLG). Furthermore, to investigate the performances and the unique advantages of the ED-TPMLG, comparative analysis of the ED-TPMLG and C-TPMLG is conducted based on the similar dimensions. The results indicate that the proposed ED-TPMLG has superiority in detent force and power density than the C-TPMLG. Then, the unstable ac electricity generated by the generators is rectified, which can be transformed to the stable dc electricity for supplying the equipment. Finally, a planar linear generator prototype with the same ED-TWs is manufactured to validate the reasonable of the proposed winding configuration (ED-TWs) and the credible of the above finite element analysis method.

Assessment of Electrical Power Generation of Wave Energy Converters With Wave-to-Wire Modeling

Direct-drive wave energy converter (WEC) and buoy control algorithms have shown great potential for renewable wave energy extraction in ideal conditions. However the actual power take-off (PTO) impacts are barely considered in the WEC design. This paper highlights the demands of designing the WEC wave-to-wire control from a global point of view by studying the actual PTO impacts. A permanent magnet linear electrical machine (LEM) PTO unit is simulated and controlled to fulfill the WEC buoy control requirements. Several state of the art control algorithms, which include singular-arc (SA) control, shape-based (SB) control, model predictive control (MPC), and proportional-derivative (PD) control, are applied to maximize the wave energy production (mechanical energy). Multiple types of electrical PTOs, including ideal PTO, unlimited PTO and limited PTO, are all implemented to evaluate WEC wave-to-wire performances. Further, the PTO copper loss model and the PTO actual efficiency maps are introduced and studied to improve the electrical PTO operation efficiency. To further assess the control schemes in various wave conditions, one-year PacWave ground-truth data is applied as well. Numerical simulations are conducted using MATLAB/Simulink and the Simscape toolbox. The electrical PTO unit is composed of a LEM, an ideal inverter, and an ideal energy storage system. The results show that the actual PTO will impact the constrained controls (MPC and SB) less comapring to unconstrained controls (SA and PD). Although SB can produce the maximum energy with the limited PTOs, it is not robust for all wave conditions. At the end of the paper, the possible solutions for improving the WEC wave-to-wire performances are also provided.