Wave Energy Conversion System Research Articles

Nonlinear Modeling and Real-Time Simulation of a Grid-Connected AWS Wave Energy Conversion System

In this paper, we use a nonlinear model of a grid-connected Archimedes wave swing (AWS) to explore the actual generation potential of this wave energy conversion system. Our model has never been used before for a grid-connected AWS. The control system employs six proportional-integral (PI) controllers to maximize the energy harvest from waves, to minimize the generator power losses, and to maintain both the grid and DC link voltages at their reference values. The PI controller parameters were selected using a hybrid augmented grey wolf optimizer and cuckoo search (AGWO-CS) algorithm. The results obtained from MATLAB Simulink for different types of grid fault, with successful and unsuccessful reclosing of the circuit breakers, were compared with results from the particle swarm optimization (PSO) and COOT algorithms. To verify the accuracy of our control system, a grid-connected AWS was tested experimentally using a real-time RT-LAB simulator combined with OP4510. The results validated the efficiency and superiority of the control system using AGWO-CS and the results from simulation and experiment were similar.

Cogging Force Optimization of Double-Sided Tubular Linear Machine With Tooth-Cutting

The double-sided tubular linear machine (DSTLM) has a more compact structure than general single-sided motors, resulting in a higher power density, typically at the cost of a higher cogging force. To reduce this aspect and make it more suitable for applications in wave energy conversion systems or elevator driver systems, a new tooth-cutting topology is proposed in this article. The Schwarz–Christoffel mapping method is used to model and analyze the magnetic field and cogging force of the DSTLM. Based on the analytical model and the relationships between the cutting dimensions and the force performances, optimization is carried out by using a differential evolution algorithm. The optimization and the experiment results show that the proposed tooth-cutting topology can effectively reduce the cogging force at the small cost of weakening average thrust.

Facilitating optimal operations of wave energy converter using a preeminent mooring line: an entropy weight-VIKOR method

This study employed viable methods for the selection of a preeminent mooring line amongst other alternatives for the mooring of a floating wave energy converter (WEC) in shallow water. Conventional mooring lines for mooring WEC are identified for optimal selection exercise. A combination of Entropy Weight and Visekriterijumska Optimizacijia I Kompromisno Resenje (VIKOR) methods is utilized in the aforementioned exercise. The two methods are effectively used in assessment of the attributes and performance of the various mooring lines in practical application. The result obtained, demonstrated that steel wire rope is the best mooring line suitable for a WEC system operations. It is good references to the marine and offshore engineering industries in decision making for optimal mooring line suitable for the mooring of a WEC system in shallow waters.

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.

Mechanical Reliability Analysis of Flexible Power Cables for Marine Energy

Marine power cables connected to moving devices at sea may experience millions of load cycles per year, and thus they need to be flexible due to the movements of the cable and designed for mechanical loads. In this study, the focus is on the mechanical life of flexible low- and medium voltage power cables connecting devices to hubs. The reliability design method Variational Mode and Effect Analysis (VMEA) is applied, based on identifying and quantifying different types of uncertainty sources, including scatter, model and statistical uncertainties. It implements a load–strength approach that combines numerical simulations to assess the loads on the cable and experimental tests to assess the strength of the cable. The VMEA method is demonstrated for an evaluation of bending fatigue, and is found to be a useful tool to evaluate uncertainties in fatigue life for WEC (Wave Energy Converter) system cables during the design phase. The results give a firm foundation for the evaluation of safety against fatigue and are also helpful for identifying weak spots in the reliability assessment, thereby motivating actions in the improvement process. Uncertainties in terms of scatter, statistical uncertainty and model uncertainty are evaluated with respect to the WaveEL 3.0, a WEC designed by the company Waves4Power, and deployed in Runde, Norway. A major contribution to the overall uncertainty is found to originate from the fatigue life model, both in terms of scatter and model uncertainty.

Nonlinear Model Reduction by Moment-Matching for a Point Absorber Wave Energy Conversion System

This paper presents a data-driven model reduction by moment-matching approach to construct control-oriented models for a point absorber device. The methodology chosen and developed generates models which are input-to-state linear, with any nonlinear behaviour confined to the output map. Such a map is the result of a data-driven approximation procedure, where the so-called moment of the point absorber system is estimated via a least-squares procedure. The resulting control-oriented model can inherently preserve steady-state properties of the target WEC system for a user-defined class of input signals of interest, with the computation only dependent upon a suitably defined set of input-output data.

Structural design of multi-body heave wave energy conversion system and analysis of energy efficiency of floating body on water surface

Abstract. Due to the defects of the internal structure and energy supply carrier, conventional deep sea unoccupied marine equipment cannot meet the requirements of low power consumption. In this paper, the whole structure of a multi-body heave wave energy conversion system was designed to capture and convert wave energy. The conversion system consists of a floating body, an underwater absorber and a power takeoff system (PTO). The dynamic model of the energy conversion system and the mathematical model of energy efficiency evaluation were established according to the dynamic analysis. Based on the real service environment in the South China Sea, the energy efficiency characteristics of floating bodies with different shapes were simulated, and the amplitude response operator (RAO), radiation damping, added mass and Froude–Krylov force of floating bodies with different shapes were compared. Then, the optimal energy efficiency parameters of surface floating body were explored. Finally, the correctness of the conclusion was verified by the energy efficiency test. The results show that, under the limitation of low power consumption and space scale, the energy conversion system of an axisymmetric rotary body with the same sea conditions, same material and the largest scale can significantly improve the conversion efficiency, and the spherical rotary body performs the best, which makes the unoccupied marine equipment have a broad prospect for development.

Recent advances in wave energy conversion systems: From wave theory to devices and control strategies

Harnessing energy from ocean waves, although not a new concept, is beginning to gain traction in the renewable energy research community. This largely untapped energy resource has considerable potential; however, researchers are still seeking to understand how to make it economically viable. This paper presents an overview of wave energy conversion as follows. It identifies various advantages of wave energy conversion as well as challenges that researchers and industry developers must overcome before large-scale installations can be fully realized. This paper also reviews the devices that have been designed to achieve efficient energy conversion. Multiple studies concerning wave energy converters placed in an array are reviewed and discussed, focusing specifically on consistent trends concerning array performance. The paper also reviews recent control methods for wave energy conversion.

Numerical investigation on flow collector with guide vane for tidal energy conversion

Abstract In ocean-based renewable energies, such as wave motion, ocean current, tidal current and ocean thermal energy, many studies have been conducted on the wave energy conversion system and they have been tested in many countries. In this study, a tidal current energy is focused. Collecting a tidal flow is very important because it is generally known that a power available from a stream of water is proportional to the cube of the free stream velocity of the current. In this research, a bi-directional turbine system is investigated for a tidal flow that changes the direction periodically. Numerical investigations are conducted on a bi-directional flow collector with guide vane to collect the flow, in which the effects of vane skew angle and blade number are investigated. Calculated results showed that the collector with eight guide vanes was the highest angular moment at turbine inlet. The axial velocity at turbine inlet was larger for smaller value of the vane skew angle, whereas the tangential velocity at the turbine inlet was larger for larger value of the vane skew angle. As the results of both the profiles of axial and tangential velocity, the angular moment took the local maximum value at around the angle of 180 deg. in this study.

Optimization of Power Cable Layout Scheme for Point Absorption Wave Energy Converters

This paper studies the power cable of the point absorption wave energy converters (WECs). As bearing the cyclic loads induced by WECs motion, waves and ocean currents, cable is prone to damage and failure. Therefore, it is necessary to optimize the design of the power cable to improve its performances and ensure the service safety of the WECs system. The idea of setting buoyancy modules on different positions of the power cable is proposed in this paper. The length and weight of the power cable and the length and position of buoyancy modules are considered to optimize the design of the power cable in the numerical simulation. The optimal design is evaluated based on the maximum effective tension and curvature of the power cable and the captured power of the WEC. The schemes of the cable with buoyancy modules and without buoyancy modules are compared based on the maximum effective tension and curvature of the power cable and the captured power of the WEC. The results show that the cable performance is significantly improved after setting the buoyancy modules, but the improvement for the captured power of the WEC is little.

An Experimental Comparison between an Ironless and a Traditional Permanent Magnet Linear Generator for Wave Energy Conversion

Permanent Magnet Linear Generators (PMLGs) are currently being studied for sea wave energy harvesting. Typically, a PMLG consists of an iron-made armature and a moving translator. The permanent magnets adoption produces parasitic effects, such as cogging force, and the machine weight increment. A solution could be the adoption of an ironless configuration, accepting a power density reduction. This paper investigates the use of ironless PMLGs in sea wave energy conversion systems by an experimental comparative analysis between an iron PMLG prototype and an ironless PMLG prototype, which share the same geometry. The main electrical and mechanical parameters (resistance, mass, and magnetic fields) were preliminarily measured. Subsequently, open-circuit and load tests were carried out to compare the induced voltages, the energy transferred to a resistive load, efficiency and the load average power. The reported comparison shows that iron PMLG performances are significantly superior to the ironless ones during the open-circuit tests, as expected. However, the analysis carried out through the load tests shows that the cogging force significantly limits the energy production, obtaining similar values in both machines. Therefore, the experimental tests justify the use of ironless machines in sea wave energy harvesting, where the maximization of energy production is a relevant target.

An effective solution to boost generation from waves: Benefits of a hybrid energy storage system integration to wave energy converter in grid-connected systems

Background: Wave energy represents one of the most promising renewable energies due to its great theoretical potential. Nevertheless, the electrical compliance of grid-connected systems is a great issue nowadays, due to the highly stochastic nature of wave energy. Methods: In this paper, a Hybrid Energy Storage System (HESS) consisting of a Li-ion battery and a flywheel is coupled to a Wave Energy Converter (WEC) that operates in grid connected mode. The study is performed using real yearly wave power profiles relating to three different sites located along the European coasts. The Simultaneous Perturbation Stochastic Approximation (SPSA) principle is implemented as real-time power management strategy for HESS in wave energy conversion systems. Results: Obtained results demonstrate how the proposed HESS and the implementation of the SPSA power management coupled to a WEC allow a reduction of more than 80% of power oscillations at the Point of Common Coupling (PCC), while proving the robustness of the developed management strategy over the investigated sites. Moreover, the average energy penalty due to the HESS integration results slightly higher than 5% and battery solicitation is reduced by more than 64% with respect to the flywheel solicitation, contributing to extend its lifetime. Conclusions: HESS integration in renewable generation systems maximizes the WEC production while smoothing the power at the PCC. Specifically, flywheel-battery HESS together with the implemented power management strategy could provide a great flexibility in the view of increasing power production from waves, strongly mitigating the variability of this source while enhancing grid safety and stability.

Experimental analysis of an enhanced design of float with inverted cup for wave energy conversion

One of the clean energy available annually is wave energy which has massive energy stored but is not used widely due to locations, depths, near-shore and offshore. There are many different wave energy conversion systems patented worldwide, such as the point absorber, which is used in this work. An innovative design for the float has been developed with an inverted cup. This system aims to generate power effectively from irregular and regular waves. One of the features of this system is a mechanical unidirectional motion under the effect of the reciprocating wave motion. This work includes an experimental assessment of the efficiency of this system by using an innovative float design. The outcomes indicate the high compatibility between the mechanical system and the moving floats. Modifications are executed for the float by inserting a baffle around it with 1.6 times the float diameter. Experimental results indicated that the captured efficiency for the float 30 cm with a baffle is 19% instead of 6% for the float 50 cm in diameter without a baffle. The efficiency is increased 3 times more than conventional design as well as superior performance under the effect of regular wave patterns was obtained.

Numerical Analysis of Velocity Magnitude on Wave Energy Converter System in Perforated Breakwater

Waves are an alternative energy source that can be used for electricity generation. Wave Energy Converter (WEC) system in perforated breakwater is potentially applicable WEC system for coastal area. The magnitude of wave energy generated is determined by the volume of sea water inside the perforated breakwater. This volumetric flow rate is calculated using the flow velocity at perforated holes on the structure slope. Therefore, this research aims to study the velocity magnitude by analyzing the interrelation among wave steepness, wave run-up and relative velocity. The method used consists of applying numeric 3D flow model in the perforated structure of the breakwater with the variation of wave height, wave period and structure slope. The result shows that, the steeper the structure, the bigger is the relative run up (Ru/H). The higher the relative run up, the higher are the relative run-up velocities (V/Vru). As the velocity increase, the volumetric flow rate inside perforated breakwater will be higher, which leads to higher wave energy. Hence, it can be concluded that the higher the velocities (V/Vru), the higher is the wave energy generated.

Hydrodynamic Performance of A Porous-Type Land-Fixed Oscillating Water Column Wave Energy Converter

A hybrid, porous breakwater—Oscillating Water Column (OWC) Wave Energy Converter (WEC) system is put forward and its hydrodynamic performance is investigated using the fully nonlinear, open-source computational fluid dynamics (CFD) model, OpenFOAM. The permeable structure is positioned at the weather side of the OWC device and adjoined to its front wall. A numerical modelling approach is employed in which the interstices within the porous structure are explicitly defined. This permits the flow field development within the porous structure and at the OWC front wall to be observed. The WEC device is defined as a land-fixed, semi-submerged OWC chamber. A range of regular incident waves are generated at the inlet within the numerical tank. The OWC efficiency and the forces on the structure are examined. Results are compared for the simulation cases in which the porous component is present or absent in front of the OWC chamber. It is found that the incorporation of the porous component has minimal effect on the hydrodynamic efficiency of the OWC, reducing the efficiency by less than 5%. Nevertheless, the forces on the front wall of the OWC can be reduced by up to 20% at the higher wave steepness investigated, through inclusion of the porous structure at the OWC front wall. These findings have considerable implications for the design of hybrid OWC—breakwater systems, most importantly in terms of enhancing the durability and survivability of OWC WECs without significant loss of operational efficiency.

Transient stability improvement of wave energy conversion systems connected to power grid using anti-windup-coot optimization strategy

This paper introduces an enhancement to the transient stability of a wave energy conversion system (WECS) by using the coot optimization algorithm (COA) combined with an anti-windup method. This combination helps in elimination of the windup issue in the integral term of the proportional-integral (PI) controller during power system faults leading to a significant enhancement for the transient stability of the WECS connected to the grid. The COA is utilized to select the PI controller parameters and the anti-windup method back-calculation coefficients. The WECS converts the linear vertical motion into electrical energy by using an Archimedes wave swing device connected to a linear synchronous generator. Minimization of the generator's stator losses and maximization of the generator's real power is accomplished by the utilization of a generator-side converter (GSC). Also, both the point of common coupling voltage and the capacitor link voltage are set at their reference values by utilizing a grid-side inverter (GSI). An optimal design for the PI controllers in both converters is achieved by direct application of the COA to the MATLAB/Simulink model. A comparison is made among the results obtained by the COA and those obtained by other recent optimization algorithms under different grid fault conditions.

An Adaptive Load Frequency Control for Power Systems with Renewable Energy Sources

The frequency of power systems is very sensitive to load variations. Additionally, with the increased penetration of renewable energy sources in electrical grids, stabilizing the system frequency becomes more challenging. Therefore, Load Frequency Control (LFC) is used to keep the frequency within its acceptable limits. In this paper, an adaptive controller is proposed to enhance the system performance under load variations. Moreover, the proposed controller overcomes the disturbances resulting from the natural operation of the renewable energy sources such as Wave Energy Conversion System (WECS) and Photovoltaic (PV) system. The superiority of the proposed controller compared to the classical LFC schemes is that it has auto tuned parameters. The validation of the proposed controller is carried out through four case studies. The first case study is dedicated to a two-area LFC system under load variations. The WECS is considered as a disturbance for the second case study. Moreover, to demonstrate the superiority of the proposed controller, the dynamic performance is compared with previous work based on an optimized controller in the third case study. Finally in the fourth case study, a sensitivity analysis is carried out through parameters variations in the nonlinear PV-thermal hybrid system. The novel application of the adaptive controller into the LFC leads to enhance the system performance under disturbance of different sources of renewable energy. Moreover, a robustness test is presented to validate the reliability of the proposed controller.

On Real-Time Hybrid Testing of Ocean Wave Energy Conversion Systems: An Experimental Study

The growing wave energy sector requires an efficient and flexible testing process for the development phase of wave energy systems. Real-time hybrid testing is a promising technique for the accelerated testing of wave energy conversion systems. This article presents an experimental study on developing a hybrid testing platform for wave energy systems at the Wallace Energy System and Renewables Facility (WESRF) at Oregon State University. The wave energy conversion system is broken down into numeric (i.e., virtual) and physical (i.e., hardware) components. The numeric component involves software components such as the control algorithm for Wave Energy Converter (WEC) and controller for the power electronic converters and numerical models for the WEC device hydrodynamics. The hardware involves an ocean wave emulator testbed, Power Take-Off (PTO) mechanism, power electronics, and instrumentation. The numeric components are implemented in a real-time target machine and are interfaced with the experimental system. A case study implementation of Nonlinear Model Predictive Control (NMPC) is presented for a single degree of freedom heaving nonlinear WEC model with a Permanent Magnet Synchronous Generator (PMSG) as a PTO system. A Field-Oriented Control (FOC) algorithm controls the PMSG-PTO generation using a three-phase Integrated Intelligent Power (IIP) module converter. A demonstration of the proposed hybrid testing setup is provided.

Energy-maximising experimental control synthesis via impedance-matching for a multi degree-of-freedom wave energy converter

We present, in this paper, an experimental framework for design and synthesis of impedance-matching-based (IM) controllers capable of maximising energy extraction in inherently multi degree-of-freedom wave energy converter (WEC) systems, and its subsequent application to the Intertial Sea Wave Energy Converter (ISWEC) device, by incorporating recent advances in IM-based theory. In particular, we consider a 1/20th scale prototype of the ISWEC system, tested as part of a larger experimental campaign conducted within the tank facilities available at Università degli Studi di Napoli Federico II, subject to a variety of wave conditions. We adopt two different control structures to realise an approximation of the IM principle, fully tuned based upon interpolation of a particular (experimentally obtained) non-parametric empirical transfer function estimate, which defines the optimal frequency-domain input-output response for energy-maximising behaviour. Furthermore, a performance comparison between controller tuning based upon traditional linear boundary element method models, and the presented experimental approach, is also offered, showing that the latter can consistently outperform the former in realistic scenarios, for the set of analysed sea-states.

Dynamic analysis and performance assessment of the Inertial Sea Wave Energy Converter (ISWEC) device via harmonic balance

Given the particular energy-maximising performance objective, wave energy converter (WEC) systems are prone to exhibit highly nonlinear behaviour. We present, in this paper, a detailed dynamic analysis and control synthesis for the Inertial Sea Wave Energy Converter (ISWEC) system, deriving and considering a comprehensive associated nonlinear model. In particular, we adopt a harmonic balance (HB) method to achieve this objective, producing the so-called amplitude-frequency curves (AFC) for the corresponding ISWEC nonlinear model, derived via a Lagrangian approach. We demonstrate that the system can present a variety of different behaviours which are completely neglected by its linear model counterpart. Leveraging both the efficiency, and convenient representation of the HB method, we synthesise so-called ‘passive’ (i.e. proportional) energy-maximising controllers using a variety of input conditions. We provide a comparison of the obtained control parameters with those arising from standard linear modelling, showing a consistent improvement in performance by effectively considering the relevant nonlinear ISWEC dynamics.