Wave Energy Conversion System Research Articles

Experimental study of multiple hemisphere wave energy converters arrayed in a water channel resonator

In this study, the performance of multiple hemisphere Wave Energy Converters (WECs) arrayed in a Water Channel Resonator (WCR) has been investigated in irregular waves. First, we determine the characteristics of wave responses from model tests, using 16 wave probes in a WCR. Second, the pitch motion responses of three WECs in a WCR are determined. Experimental results are compared with the numerical solutions using the time-domain SeaFEM and OrcaFlex commercial code and agree well with the numerical results. Both experimental and numerical results indicate that when the incident peak period is close to one of the natural periods in a closed WCR, resonance occurs in the WCR. This resonance induces a large wave response and causes a high pitch motion response of WECs. The present study highlights the advantages of using a WCR for energy amplification and lays the necessary foundation for the commercialization of the present WEC system.

A Novel Rotor Type Wave Energy Converter Design for Maximum Energy Captured in Low Wave Heights

Wave energy converter system (WEC) is generally used by obtaining of electrical energy. Maximise power capture is needed a range of sea-states. Continuity is improved to control of power take off. But wave energy have o lower force and a higher speed in onhore. Therfore, this paper is investigated the novel rotor type wave energy converter performance in this wave suitable conditional. The performance of this system is tested in laboratory condiotions.This aim of design is maximum power absorption from low wave height of waves. Results show that the higher wave height has caused the efficiency to decrease in all periods. The highest efficiency is obtained at a minimum wave height of 2 cm. At this wave height, the efficiency has reached its highest value when the period is 5 s (20.3%). In this case, it makes this prototype suitable for seas with low wave height and high wave period.

Control of Cascaded Multilevel Converter for Wave Energy Applications

This paper proposes a control scheme for a wave energy conversion system based on a linear generator and a cascaded multilevel converter. The mechanical conversion system is composed of a buoy connected directly to a linear generator. The windings of the generator are individually controlled by a cascaded multilevel power converter using independent maximum power point tracking to improve energy harvesting. The output of the cascaded converter is controlled to keep the DC capacitors balanced and generate a multilevel voltage at the output terminals which reduces the losses in the underwater transmission line. Experimental results show the performance of the proposed control scheme maximizing the power generation by imposing a current with the same waveform of the induced voltage and balancing the DC capacitors.

Modeling and Simulation of a Wave Energy Converter: Multibody System Coupled to Fluid-Film Lubrication Model and Thermal Analysis

Sea wave energy is being increasingly regarded as one of the most promising sources of renewable energy. This paper deals with the modeling and simulation of an onshore wave energy converter system designed by UMBRA GROUP SpA. Several topics are addressed. Starting from the multibody modeling strategy, this paper delves more deeply into the mechanical efficiency evaluation of the ball-screw in the elastohydrodynamic lubrication regime, the core of the energy conversion process, as well as the thermal characterization of the power take-off module, based on the lumped-parameter and finite element method models. High values of ball-screw indirect efficiency have been observed, ranging from 73% to 97%; these results appear even more encouraging when compared to the performance of alternative energy-consuming technologies. Thermal analysis, on the other hand, provided a maximum temperature increase of 40 °C, allowing for the aversion of any structural collapse and the realistic identification of the lubrication regime, which turned out to be mostly mixed. Finally, an inverse multibody dynamic analysis is performed, and the most interesting simulation results are collected to prove the effectiveness of the proposed approach.

A Review on Power Electronic Topologies and Control for Wave Energy Converters

Ocean energy systems (OESs) convert the kinetic, potential, and thermal energy from oceans and seas to electricity. These systems are broadly classified into tidal, wave, thermal, and current marine systems. If fully utilized, the OESs can supply the planet with the required electricity demand as they are capable of generating approximately 2 TW of energy. The wave energy converter (WEC) systems capture the kinetic and potential energy in the waves using suitable mechanical energy capturers such as turbines and paddles. The energy density in the ocean waves is in the range of tens of kilowatts per square meter, which makes them a very attractive energy source due to the high predictability and low variability when compared with other renewable sources. Because the final objective of any renewable energy source (RES), including the WECs, is to produce electricity, the energy capturer of the WEC systems is coupled with an electrical generator, which is controlled then by power electronic converters to generate the electrical power and inject the output current into the utility AC grid. The power electronic converters used in other RESs such as photovoltaics and wind systems have been progressing significantly in the last decade, which improved the energy harvesting process, which can benefit the WECs. In this context, this paper reviews the main power converter architectures used in the present WEC systems to aid in the development of these systems and provide a useful background for researchers in this area.

Modeling, Experimental Analysis, and Optimized Control of an Ocean Wave Energy Conversion System in the Yellow Sea near Lianyungang Port

In this paper, an ocean wave energy conversion system (OWECS) is modeled and experimented in the Yellow Sea near Lianyungang port, and an optimized control method based on the sliding mode control is proposed to improve the efficiency of OWECS. Firstly, a motion model of a double-buoy OWECS is presented using a complex representation method, and the analysis results indicate that the efficiency of converting ocean wave energy into the outer buoy’s mechanical power is highest in a suitable ocean wave period. Secondly, a double-buoy OWECS is constructed and experimented in the Yellow Sea near Lianyungang port, which verified the correctness of the above analysis results. Lastly, in order to further improve the efficiency of the double-buoy OWECS, a sliding mode control method based on a linear generator is proposed to realize the phase synchronization between the outer buoy and ocean waves, and the simulation results may be beneficial for the next ocean test of the double-buoy OWECS.

Abrasion testing of fibre rope on impermeable fabrics for wave energy converter systems

The NetBuoy system is being developed as means of providing low cost buoyancy, typically for use in the wave energy converter (WEC) sector where many devices rely on large, heavy and expensive fabricated steel hulls/structures to absorb the incoming wave energy.The NetBuoy system is an elegant concept in that it utilises an air pressurised impermeable buoyant pod to provide structural rigidity, which is restrained by a synthetic fibre rope net. This provides a structure that is cheaper to produce than steel equivalents and has additional benefits in terms of ease of transport to site and maintenance. However, there are a number of considerations relating to the performance of the buoy and the containing net. It is important that the net does not damage the buoy (which might lead to loss of buoyancy) as wave action may cause relative movement between net and buoy. Equally, the net must retain integrity so that the buoy does not come free from its tether (creating a shipping hazard and loss of device).This paper describes a novel test method developed to investigate the performance of a range of impermeable elastomer membranes (buoy materials) when abraded against a polyester fibre rope. It discusses the methodology of the tests and provides results of testing on a number of elastomer membrane materials, considering both wear volumes and measurement of friction between the rope and the membrane material as a means of assessing performance. Of the five materials assessed, a clear ranking of abrasive resistance in the elastomers was found on test with EPDM and Neoprene being clearly better than all other materials. Natural rubber performed well, whilst Hypalon and Nitrile were found to be least resistant to abrasion from the rope. In all tests the rope was largely unaffected by the abrasion.

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.

A Numerical Investigation of the Energy Efficiency Enhancement of Oscillating Water Column Wave Energy Converter Systems

This work focuses on the geometry effects over the performance of oscillating water column (OWC)-type wave energy converter (WEC) systems and searches for the OWC geometries that enhance the energy efficiencies under the same wave conditions. To analyze the hydrodynamic performances of the WEC systems, an in-house smoothed particle hydrodynamics (SPH) code based on weakly compressible fluid approach is utilized. The energy efficiency enhancement studies of the determined OWC device are carried out with a two-step geometry modification procedure. The first step starts with the validation of the free-surface elevation and orbital velocity time histories. Then, a three-by-three simulation matrix that depends on the geometrical design parameters of chamber length and front wall draft is run at three different wave conditions, and the OWC geometry that produces the maximum energy efficiency is determined. In the second step, the corner regions of the obtained optimal geometry are chamfered, and another simulation matrix is tested at the wave condition that yields maximum wave energy. It is observed in this step that the energy efficiency index can still be improved by 4.3% by only chamfering the back face of the OWC chamber. To scrutinize the physical grounds of this increase, the correlation between the time-averaged vorticity and energy efficiency is presented. Finally, the performance of the best configuration is also examined in three different wave periods, where the suggested geometry shows better performance with respect to base geometry results in all wave conditions.

Hydrodynamic performance and energy absorption of multiple spherical absorbers along a straight coast

The development and utilization of wave energy have great potentiality to alleviate the urgent problem of global energy shortage. Spherical bodies can be used as point absorbers to extract wave energy, and much attention has been paid to the performance of spherical absorbers in an open water domain. This study focuses on the hydrodynamic performance and energy absorption of multiple spherical absorbers in front of a straight coast. The coast is assumed to be a fully reflecting vertical wall, and all the absorbers are restricted to only heave motion. An analytical solution based on linear potential flow theory is developed for the problem of wave diffraction and radiation by multiple absorbers. In the solution procedure, the hydrodynamic problem is transformed into an equivalent problem in an open water domain by applying the image principle. The velocity potential of the fluid motion is solved using the method of multipole expansions combined with the shift of local spherical coordinate systems. Then, the wave excitation force, added mass coefficient, radiation damping coefficient, and energy extraction performance of the absorbers are calculated. Case studies are presented to analyze the effects of the coastal reflection and hydrodynamic interaction among absorbers on the energy extraction performance of the wave energy converter (WEC) system. The effects of wave frequency, incident angle, spacing between the absorber and coast, submergence depth, absorber number, and plane layout are also clarified. The results suggest that the energy extraction performance of an isolated absorber is significantly improved when the motions of the waves and absorber are in resonance, and the coastal reflection can enhance the overall energy extraction performance for a WEC system with multiple absorbers. In addition, when the number of absorbers increases, the effects of the coastal reflection and hydrodynamic interaction become more complicated.

Hybrid Data-Model Predictive Control for Enabling Participation of Renewables in Regulating Reserves Service

A long-horizon hybrid data-model predictive direct power control (HD-MPDPC) scheme is proposed. The hybrid control scheme is for grid-side power electronic converters (PEC) in grid-connected renewable energy sources (RES). Computational burden of conventional MPDPC is alleviated by reducing the number of candidate voltage vectors to be examined in the cost function. This is achieved by employing data-driven forecast of RES output power to identify and eliminate the voltage vectors that do not counteract active and reactive power errors appropriately. RES power is forecasted using recurrent neural networks. The method for eliminating voltage vector candidates is inspired from direct power control, where output of active and reactive power hysteresis controllers along with the sector in which grid voltage vector lies are used to determine the switching states of the grid-side PEC. Thanks to reduced computational complexity, the hybrid predictive control scheme dispatches RES power more reliably over long horizons. This in turn enables RES as a regulating reserves service provider in power systems. Hardware-in-the-loop studies of a grid-connected wave energy conversion system verify reduced computational complexity of HD-MPDPC and its effectiveness in controlling RES output power over long horizons.

Experimental Modelling of an Isolated WECfarm Real-Time Controllable Heaving Point Absorber Wave Energy Converter

To offer point absorber wave energy converters (WECs) as a bankable product on the marine renewable energy market, multiple WECs will be installed together in an array configuration. The wave energy community (research and industrial) has identified the urgent need for available realistic and reliable data on WEC array tests in order to perform a better WEC array optimization approach and in order to validate recently developed (non-linear) numerical models. The ‘WECfarm’ project is initiated to cover this scientific gap on necessary experimental data. The ‘WECfarm’ experimental setup consists of an array of five generic heaving point-absorber WECs. The WECs are equipped with a permanent magnet synchronous motor (PMSM), addressing the need for WEC array tests with an accurate and actively controllable power take-off (PTO). The WEC array control and data acquisition are realized with a Speedgoat Performance real-time target machine, offering the possibility to implement advanced WEC array control strategies in the MATLAB-Simulink environment. The presented article describes the experimental setup, the performed tests and the results of the test campaign using a single, isolated ‘WECfarm’ WEC in April 2021 at the wave basin of Aalborg University (AAU), Denmark. A Coulomb and viscous friction model is determined to partly compensate for the drivetrain (motor, gearbox, rack and pinion) friction. A system identification (SID) approach is adopted considering the WEC system to be composed of two single input single output (SISO) models, the radiation and the excitation model. Radiation tests yield the intrinsic impedance. Excitation tests yield the excitation frequency response function. Adopting an impedance matching approach, the control parameters for the resistive and reactive controller are determined from the complex conjugate of the intrinsic impedance. Both controllers are tested for a selection of regular wave conditions. The performed experimental test campaign using an isolated ‘WECfarm’ WEC allows a full evaluation of the WEC design prior to extending the setup to five WECs. Within the ‘WECfarm’ project, an experimental campaign with a five-WEC array in the Coastal and Ocean Basin (COB) in Ostend, Belgium, is under preparation.

Experimental and numerical analysis of a hybrid WEC-breakwater system combining an oscillating water column and an oscillating buoy

Integrating multi-type Wave Energy Converters (WECs) enables wave energy to be extracted from multiple harvesting manners simultaneously, which presents a more competitive energy conversion technology. In this study, an Oscillating Water Column (OWC) and an Oscillating Buoy (OB) are combined to devise a hybrid WEC system that serves as a floating breakwater. The two devices are aligned in line with the incident wave direction, where the OB is deployed upstream from the OWC. A series of physical experiments are conducted to understand the working principle of the hybrid WEC system. After a fully non-linear time-domain model is validated by the measured data, the comparisons of the hybrid system and its respective isolated devicesare emphasised. Then, the hydrodynamic dependence of the hybrid system is comprehensively tested. The results indicated that the hybrid system design provides higher energy conversion as well as better wave attenuation performance compared to the isolated OWC and OB devices. Additionally, the OB outperforms the OWC in terms of wave energy conversion. The OWC-OB gap resonance has a negative impact on wave energy extraction but has a negligible effect on wave attenuation. The OWC with a deeper draft and symmetric wall performs better both in terms of wave energy extraction and wave attenuation in long-period waves. The overall conversion efficiency decreases with the increase of the OB draft, but the transmission coefficient follows an opposite trend. The PTO dampings of the OB and the OWC dominate the overall conversion efficiency and the transmission coefficient, respectively.

Linear Permanent Magnet Vernier Generators for Wave Energy Applications: Analysis, Challenges, and Opportunities

Harvesting energy from waves as a substantial resource of renewable energy has attracted much attention in recent years. Linear permanent magnet vernier generators (LPMVGs) have been widely adopted in wave energy applications to extract clean energy from oceans. Linear PM vernier machines perform based on the magnetic gearing effect, allowing them to offer high power/force density at low speeds. The outstanding feature of providing high power capability makes linear vernier generators more advantageous compared to linear PM synchronous counterparts used in wave energy conversion systems. Nevertheless, they inherently suffer from a poor power factor arising from their considerable leakage flux. Various structures and methods have been introduced to enhance their performance and improve their low power factor. In this work, a comparative study of different structures, distinguishable concepts, and operation principles of linear PM vernier machines is presented. Furthermore, recent advancements and innovative improvements have been investigated. They are categorized and evaluated to provide a comprehensive insight into the exploitation of linear vernier generators in wave energy extracting systems. Finally, some significant structures of linear PM vernier generators are modeled using two-dimensional finite element analysis (2D-FEA) to compare their electromagnetic characteristics and survey their performance.

Numerical and experimental research on a resonance-based wave energy converter

Inefficiency and intermittency caused by high variabilities of wave properties are common problems in existing wave energy technology. Resonant wave energy conversion technology can effectively solve these problems. However, the nonlinearity induced by the wave energy converter’s radiation and operation inhibits the resonance. After weakening the nonlinearity by hydrodynamic optimization and operation manner design, a novel wave energy converter that can realize resonance was proposed. In this paper, it is verified that this wave energy conversion system is close to linear, even in the case of large amplitude oscillation. For the novel wave energy converter, a radiation hydrodynamic parameter testing method is proposed, which provides parameters to accurately control the wave energy conversion system to achieve resonance; a wave force testing method is proposed, which can provide in-situ wave information to control resonance and judge resonance state; the effects of resonance, damping and nonlinearity on wave energy conversion are revealed, which provides a theoretical basis for the design of wave energy converter. Experimental studies showed that the laws of wave energy conversion are basically the same in regular and irregular waves, the methods and laws investigated in the regular waves also apply to irregular waves. And experiments also showed that the novel wave energy conversion system could be controlled to achieve resonance truly, and the novel wave energy converter can achieve resonance and highly efficient wave energy conversion in different wave conditions. The novel wave energy converter can self-perceive the wave changes and actively control its natural period to achieve resonance with incident waves, thus significantly increasing the wave energy conversion efficiency and improving intermittency.

An Improved Hydraulic Power Take-Off Unit Based on Dual Fluid Energy Storage for Reducing the Power Fluctuation Problem in the Wave Energy Conversion System

The power take-off (PTO) stability is one of the most important concerns for wave energy converters (WECs). The PTO unit converts the mechanical energy produced by the wave absorber (WA) unit into useful electrical energy. Due to the drastic input energy variation of real wave motions, the generated electrical power from the PTO unit significantly fluctuates and is potentially harmful to electrical and electronic appliances. This paper proposes an improved hydraulic PTO (HPTO) for the WECs. An improved HPTO unit comprises a dual high-pressure accumulator (HPA) module and fluid energy control (FEC) module, which significantly enhances the generated electrical power from the generator under irregular wave circumstances. A complete model of wave absorber device with conventional and improved HPTO units was built in MATLAB/Simulink using a Simscape fluids toolbox. The parameters of the FEC control strategy were optimized using a genetic algorithm. The improved HPTO unit model was simulated with five irregular wave inputs to evaluate its performance in irregular conditions. The effects of the HPA pressure constraints on the improved HPTO unit performance were also investigated. Overall, the simulation results indicate that the improved HPTO unit was able to generate a stable power up to 87.3% of WECs in an irregular sea state.

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.

Impacts of mechanical energy storage on power generation in wave energy converters for future integration with offshore wind turbine

While offshore wind energy uptake is growing steadily, offshore wave energy remains largely untapped, despite having significant potential globally. The high cost and complexity of wave energy conversion systems are the main challenges to their commercial development globally, while they can be offset if they are integrated into the well-developed offshore structure of wind turbines. Since the source of the wave energy is associated with wind resources, these two ocean renewable resources have strong synergies that are yet to be combined effectively. Therefore, this paper aims to explore the idea of introducing wave energy converters into the offshore wind system as a hybrid system with a specific focus on wave energy converter (WEC) systems from their power characteristic perspective. In this paper, wave power fluctuations characteristics have been analysed and compared with wind power and two mechanical energy storage strategies, added inertia and gas accumulators, are investigated at the power take-off (PTO) system level on a typical point absorbed WEC. A sensitivity study of the impact of various mechanical storage systems on WEC performance was conducted under a set of real wave data. In addition, the significance of mechanical storage systems at the WEC PTO level in a DC-linked hybrid system is discussed along with enabling a better understanding of a future combined system. The results indicate that both mechanical storage options can effectively enhance energy production, reduce the power variations in the WEC system, and lead to the feasibility of integrating wave energy with the well-known offshore wind energy turbine system.

Improvement of Low-Voltage Ride-Through Capability for Wave Energy Conversion System

With the wave energy conversion system (WECS) integrated to grid, the lack of satisfactory ability to ride through transient voltage faults will reduce the stability and reliability of the grid. In this article, in order to enhance the low-voltage ride-through (LVRT) capability of the WECS, the structure and the mathematical model of a magnetic-gear permanent magnet synchronous generator (MG_PMSG) and a float-type wave energy converter are introduced first. The output power characteristic of the WECS is analyzed. Then, a novel coordinative LVRT control scheme based on supercapacitor energy storage system (SC_ESS) is proposed. The control scheme includes four modes, which are determined by the severity of voltage sags and active power deviation of the WECS. Machine-side converter control, grid-side converter, control and SC_ESS control are used coordinately in the four operation modes. The results of simulation and experiment have verified the effectiveness of the proposed control scheme in normal running conditions.