Wave Energy Conversion System Research Articles

Assessment of currently available ocean wave energy conversion systems using technology readiness levels

Ocean waves are a huge, untapped source with a higher energy density than solar and wind energy. Over the years, numerous designs for ocean wave energy converter systems (OWECS) have been proposed. This paper provides a comprehensive review and assessment of currently available OWECS using technology readiness levels (TRLs) scale developed by the US Department of Energy. TRLs are used by numerous industries to assess technical maturity and functional readiness of new technologies. The study finds that there are 20 OWECS which are at various stages of technological development. Out of these, 14, five and two OWECS are intended for deployment at near-shore, off-shore and on-shore locations, respectively. The study shows that OWECS technology is diverse and relatively immature compared to solar and wind energy technology. Finally, the study reveals a lack of convergence towards a single OWECS that is capable of energy extraction from on-shore, near-shore and off-shore locations.

Wave Excitation Force Estimator Using Kalman Filtering Approach for Point Absorber Wave Energy Converters Under Different Modeling and Operation Scenarios

In this research, one of the most important research questions that influence the effectiveness of the used control strategies in heaving wave energy converters (WECs) is addressed. A nonlinear extended Kalman filter (EKF) based state estimator to estimate the wave excitation force and heave velocity in real-time is proposed. A holistic nonlinear model involving both the mechanical and electrical sides of the WEC system is used. The proposed estimator was compared with a simpler linear Kalman filter (KF) estimator under varying sea state environment and electric loading conditions. Generally, both estimators produced "statistically" good estimates, however, the EKF estimators outperformed its counterpart in both the estimation accuracy and maintaining low incident energy drop.

Identification of Park Effect Probability in Wave Energy Conversion System using Multi Criteria Decision Making Method (AHP) and Neural Network Model (GMDH Shell)

The present research work demonstrates the trend of Park Effect to the Wave Energy Conversion system or in wave energy converter. The Park Effect occurred due to various reasons in a real field application of Wave Energy Conversion. Park Effect occurred in wind energy as well as wave energy. All possible factors are considered to find out the Park Effect. To analyze the Park Effect probability, Analytical Hierarchy Process (AHP) is used, from the result a model is generated through Neural Network software named GMDH Shell. There are significant uncertainties arising in particular from the lack of field tested result to calculate the Park Effect proximity on the devices. However, applying various hypotheses for design and physical parameters, it was found that the benefits of Park Effect influenced factors are all non-beneficiary to Park Effect trend. After all the calculations it can predict the proximity of the Park Effect in a Wave Energy Conversion system.

Hydraulic Power Take-Off Concepts for Wave Energy Conversion System: A Review

Ocean wave energy is one of the most abundant energy sources in the world. There is a wide variety of wave energy conversion systems that have been designed and developed, resulting from the different ways of ocean wave energy absorption and also depending on the location characteristics. This paper reviews and analyses the concepts of hydraulic power take-off (PTO) system used in various types of wave energy conversion systems so that it can be a useful reference to researchers, engineers and inventors. This paper also reviews the control mechanisms of the hydraulic PTO system in order to optimise the energy harvested from the ocean waves. Finally, the benefits and challenges of the hydraulic PTO system are discussed in this paper.

Sea level variability in the Swedish Exclusive Economic Zone and adjacent seawaters: influence on a point absorbing wave energy converter

Abstract. Low-frequency sea level variability can be a critical factor for several wave energy converter (WEC) systems, for instance, linear systems with a limited stroke length. Consequently, when investigating suitable areas for deployment of those WEC systems, sea level variability should be taken into account. In order to facilitate wave energy developers finding the most suitable areas for wave energy park installations, this paper describes a study that gives them additional information by exploring the annual and monthly variability of the sea level in the Baltic Sea and adjacent seawaters, with a focus on the Swedish Exclusive Economic Zone. Overall, 10 years of reanalysis data from the Copernicus project have been used to conduct this investigation. The results are presented by means of maps showing the maximum range and the standard deviation of the sea level with a horizontal spatial resolution of about 1 km. A case study illustrates how the results can be used by the WEC developers to limit the energy absorption loss of their devices due to sea level variation. Depending on the WEC technology one wants to examine, the results lead to different conclusions. For the Uppsala point absorber L12 and the sea state considered in the case study, the most suitable sites where to deploy WEC parks from a sea level variation viewpoint are found in the Gotland basins and in the Bothnian Sea, where the energy loss due to sea level variations is negligible.

Observer-Based Unknown Input Estimator of Wave Excitation Force for a Wave Energy Converter

Several energy maximization control approaches for point-absorber wave-energy converter (PAWEC) systems require knowledge of the wave excitation force (WEF) that is not measurable during the PAWEC operation. Many WEF estimators have been proposed based on stochastic PAWEC modeling using the Kalman filter (KF), extended KF (EKF), or receding-horizon estimation. Alternatively, a deterministic WEF estimator is proposed here based on the fast unknown input estimation (FUIE) concept. The WEF is estimated as an unknown input obviating the requirement to represent its dynamics. The proposed observer-based unknown input estimator (OBUIE) inherits the capability of estimating fast-changing signals from the FUIE, which is important when considering irregular wave conditions. Unlike preceding methods, the OBUIE is designed based on a PAWEC model, including the nonlinear viscous drag force. It has been shown that the nonlinear viscous drag force is essential for accurate PAWEC model description, within the energy maximization control role. The performance of the proposed estimator is evaluated in terms of PAWEC conversion efficiency in a single degree-of-freedom PAWEC device operating in regular and irregular waves. Simulation results are obtained using MATLAB to evaluate the estimator under different control methods and subject to parametric uncertainty.

Hydrodynamic performance of a floating breakwater as an oscillating-buoy type wave energy converter

Combined floating breakwater and wave energy converter systems have the potential to provide a cost-effective solution to offshore power supply and coastal protection. This will make wave energy economically competitive and commercial-scale wave power operations possible. This paper investigates the hydrodynamic features of wave energy converters that meet the dual objectives of wave energy extraction and attenuation for such a combined system. A two-dimensional numerical model was established using Star-CCM+ commercial software based on viscous Computational Fluid Dynamics theory to investigate the hydrodynamic performance of an oscillating buoy Wave Energy Converter (WEC) type floating breakwater under regular waves. The model proposed in this paper was verified with published experimental results. The hydrodynamics of symmetric and asymmetric floaters were investigated to demonstrate their wave attenuation and energy extraction performance, including square bottomed, triangular bottomed (with and without a baffle plate), and the Berkley Wedge. The asymmetric floaters were found to have higher power conversion efficiency and better wave attenuation performance, especially the Berkeley Wedge bottom device and the triangular-baffle bottom device. The triangular-baffle bottom device with a simpler geometry achieved similar wave attenuation and energy extraction performance characteristics to that of the Berkeley Wedge device. The maximum energy conversion efficiency of the triangular-baffle bottom floater reached up to 93%, an impressive WEC device among many designs for wave energy conversion. There may be a great potential for this newly proposed triangular-baffle bottom WEC type of floater to be an ideal coastal structure for both coastal protection and wave energy extraction.

Editorial

In 2018, the School of Engineering of Universidad de Antioquia (UdeA) celebrated its 75th anniversary. Within this commemoration, the School organized ExpoIngenieria 2018, a broad forum for the interchange of knowledge between Academia, Industry, and the State. Having “Development and sustainability” as a leitmotif, this event included several parallel activities, namely: an international congress with 3307+ attendants, the business and innovation roundtable Tecnnova 2018 with 2100+ business meetings and the fifth Workshop on Engineering Applications of Computer Science with 130+ attendants and 91 contributed papers, among others.Remarkably, the main Engineering fair (ExpoIngenieria 2018) reached a broad public of more than 28000 attendants, ranging from high school students to researchers and industrial practitioners. The Expo was the opportunity to join together more than 42 research groups from academia and industry. The goal of the Expo was to showcase the contributions generated in the Engineering field by the UdeA School of Engineering as well as other partner universities in a broad range of traditional, new and emerging domains such as: Energy and environment; Materials, chemistry and bioengineering; Infrastructure, mobility and logistics; and, Telecommunication, information technologies and computer science.As part of this celebration, we edited this special issue of Redin - Revista Facultad de Ingeniería. The call was open and received seven submissions from academics and practitioners. After a rigorous review process, we proudly present four papers that were accepted and are published in this issue of Redin. These four papers include one in the renewable energy field (Edwin Chica, Fredys Romero Menco, Ainhoa Rubio Celemnte. “Design of a wave energy converter system for the Colombian Pacific Ocean) and three in alternative (natural) materials, its processing and applications (R. Prakash, R. Thenmozhi, Sudharshan N. Raman, C. Subramanian. “Mechanical behavior of polypropylene fibre reinforced concrete containing waste coconut shell as coarse aggregates and fly ash as partial cement replacement; Darío Alfonso Páez Soto, Luis Jorge Herrera Fernández, Oscar Esneider Acosta Agudelo, Marcelo Herrera Martínez. “Design and construction of an acoustic conditioning panel made from typical Colombian fibres”; Estefanía Echeverri Correa, David Orlando Grajales Lopera, Santiago Gutiérrez Restrepo, Claudia Patricia Ossa Orozco. “Effective sericin-fibroin separation from Bombyx mori silkworms fibers and low-cost salt removal from fibroin solution”).

Design of a wave energy converter system for the Colombian Pacific Ocean

This paper details the mechanical design of a wave power generation system comprising a buoy that houses a set of mechanical devices and a linear generator. The system proposed transforms the heaving movement of the buoy in a rotational movement of the arm that houses the linear generator. Firstly, the conceptual design and the operation of the system is exposed. Secondly, the conditions of the waves in the zone of interest are presented; in this case, the device was designed to work in the Colombian Pacific Ocean. Afterwards, the dynamic model of the buoy and the linear generator are deduced starting off the movement laws. To solve the resulting equation, Ansys Aqwa® numerical software is used to know the behavior of the device under the features of the swell conditions. Finally, the simulation results under real conditions are presented and compared to the response under incident regular wave. To determine the amount of electric power generated, a calculus routine in Matlab® is implemented. Throughout the numerical code, the electric power generated is calculated at each time step, and the average electric power is solved subsequently. It was found that under regular and irregular swell conditions the amount of electricity produced was 1.17 and 0.5 kW, respectively. In this regard, the device proposed here is an interesting option for populations inhabiting near coastlines far from existing electricity grids.

On the improved design of the buoy geometry on a two-body wave energy converter model

The motion of some geometries of a buoy in a two-body wave energy converter (WEC) system is studied under regular wave excitations. The buoy is coupled to a heave damping plate where the coupling system models a simplified power takeoff (PTO) system. A frequency-domain analysis is conducted on the two-body model to predict the wave-induced motion behavior of the bodies considering the mutual effects of two bodies and the PTO system. The designed buoys are axisymmetric and have an identical water-plane area and submerged volume. The model was analyzed in identical conditions for different buoys so that the effect of the buoy shape on its dynamic properties and the hydrodynamic coefficients, as well as the absorption efficiency of the two-body WEC model, could be evaluated. The impact of the buoy shape and its hydrodynamic coefficients on the optimal and suboptimal design of the PTO system is studied. The results indicate that a suitable geometry for the buoy can have beneficial effects on the improved efficiency of the two-body WEC model, increasing the relative displacement between the two bodies and, also, allowing an optimal design of the PTO system. Furthermore, the buoy shape can reduce the difference between the damping coefficients of the PTO system and the buoy in order to create the resonance condition in the WEC system.

Experimental Study on the Hydrodynamic Performance of a Heaving Buoy Assembled on a Net Cage Platform

To take advantage of the abundance of both fishery and wave energy resources in offshore sea areas, a novel floating platform with a heaving buoy-based wave energy converter (WEC) assembled with a set of net cages is presented in this work. The floating WEC system provides a power supply, while the net cages are used for aquaculture. It is designed to have an independent and self-operation breeding function. An experimental study is carried out to investigate the hydrodynamic performance of the device in a wave tank considering the factors of net cages, draft depth, and power take-off, and results show that these variables have significant effects on wave energy capture. Incident waves with short periods cause intense interactions that allow the device to undergo large relative motion. The draft depth could be determined according to wave period rather than wave height. This study also explores the response amplitude operator of the device and determines its resonance scope. The experimental results could provide reliable information for future studies on phase differences and the design of two-body WECs.

A study of the effects of increasing the degrees of freedom of a point-absorber wave energy converter on its harvesting performance

Abstract In this paper, the effects of increasing the number of degrees of freedom through adding submerged bodies to a point absorber wave energy converter (WEC) were studied in an attempt to capture more power at a lower resonant frequency while keeping the same total mass and volume of the system, which is similar to those of the piezoelectric vibration energy harvesters. Novel multiple degrees of freedom wave energy harvesters were proposed, starting from a typical two-body point absorber and increasing the number of degrees of freedom through adding submerged bodies from 2 degrees of freedom (DOF) to 5 DOF while keeping the same total mass and volume of the system. Both spherical and cylindrical submerged bodies were studied. The dynamic model of a multiple degree of freedom wave energy converter system was formulated with linearized viscous damping drag force effect. The hydrodynamic properties were simulated using the commercial software ANSYS AQWA. It was found that increasing the number of degrees of freedom through adding spherical submerged bodies increases the average captured power by 26% for the WECs going from 2 DOF to 4 DOF and decreases the resonant frequency by 19% going from 2 DOF to 5 DOF. These results are important for point absorbers, as they translate to capturing power more efficiently at a lower resonant frequency, which is closer to the realistic ocean wave excitation frequencies. As for the wave energy converters with cylindrical submerged bodies, even though the resonant frequency reduces with the increase of the number of degrees of freedom, the power massively decreases as well because of the dominant effect of the viscous damping drag force on the system’s dynamics.

Wave-to-Wire Power Maximization Control for All-Electric Wave Energy Converters with Non-Ideal Power Take-Off

The research presented in this paper investigates novel ways of optimizing all-electric wave energy converters for maximum wave-to-wire efficiency. In addition, a novel velocity-based controller is presented which was designed specifically for wave-to-wire efficiency maximization. In an ideal wave energy converter system, maximum efficiency in power conversion is achieved by maximizing the hydrodynamic efficiency of the floating body. However, in a real system, that involves losses at different stages from wave to grid, and the global wave-to-wire optimum differs from the hydrodynamic one. For that purpose, a full wave-to-wire wave energy converter that uses a direct-drive permanent magnet linear generator was modelled in detail. The modelling aspect included complex hydrodynamic simulations using Edinburgh Wave Systems Simulation Toolbox and the electrical modelling of the generator, controllers, power converters and the power transmission side with grid connection in MATLAB/Simulink. Three reference controllers were developed based on the previous literature: a real damping, a reactive spring damping and a velocity-based controller. All three literature-based controllers were optimized for maximum wave-to-wire efficiency for a specific wave energy resource profile. The results showed the advantage of using reactive power to bring the velocity of the point absorber and the wave excitation force in phase, which was done directly using the velocity-based controller, achieving higher efficiencies. Furthermore, it was demonstrated that maximizing hydrodynamic energy capture may not lead to maximum wave-to-wire efficiency. Finally, the controllers were also tested in random sea states, and their performance was evaluated.

Integrative Sizing/Real-Time Energy Management of a Hybrid Supercapacitor/Undersea Energy Storage System for Grid Integration of Wave Energy Conversion Systems

Grid-connected operation of an offshore renewable energy source (RES), comprising a wave energy converter (WEC), a hybrid supercapacitor (SC)/undersea energy storage system (HESS), and a set of power electronics converters (PECs), is investigated. Since the HESS components feature distinctive power and energy characteristics, the effect of the operating point on their storage capacity must be considered. Thus, an integrative sizing and real-time energy management strategy (EMS) based on reinforcement learning (RL) is proposed. Effectiveness of the sizing approach is examined by comparing the power and energy capacity of the HESS components under the proposed method and when they are sized individually. A similar WEC output power profile is used and it is assumed that the storage components, as hybrid or individually, counteract the power fluctuations. To verify the adaptability of the RL-based EMS (RLEMS), different power profiles are used in the learning and testing phases. Real-time implementation feasibility is validated through real-time simulations. The obtained results confirm that the power and energy capacity of the HESS components are significantly reduced when EMS is considered in the sizing stage. Further, the WEC output power could be regulated by the RLEMS, even in the presence of serious imbalances between harvested and dispatched wave energies.

Acoustic impact of a wave energy converter in Mediterranean shallow waters

In this study, underwater noise from a full-scale wave energy converter system (ISWEC), installed on the coast of Pantelleria Island (central Mediterranean Sea), was characterized. The noise was measured using an autonomous acoustic recorder anchored to the sea bottom 40 m from the ISWEC hull. Acoustic monitoring continued for 15 months, starting 7 months before (PRE), 2 months during (INST) and 6 months after the ISWEC installation (POST). The levels of noise, assessed with power spectrum density and octave and third-octave band sound pressure levels (BSPLs), were higher during the POST period than during the PRE period at lower frequencies up to 4 kHz and increased with wave height. During the ISWEC activation for energy production (POST_ON) in the wave height range 1–2.9 m, the BSPLs increased much more at lower frequencies up to 4 kHz (the median BSPLs at 63 Hz for the PRE, POST, and POST_ON conditions were 73, 106, and 126 dB re 1μPa, respectively). Considering the biophonies that make up the soundscape of the area, we examined the possible masking of fish choruses due to ISWEC noise and highlighted that at a distance of 1000 m, the 800 Hz peak frequency was 10 dB above the ISWEC signal. Within this distance from ISWEC, a possible masking effect is supposed.

Transient Stability Augmentation of a Wave Energy Conversion System Using a Water Cycle Algorithm-Based Multiobjective Optimal Control Strategy

This paper introduces a novel optimum design of the proportional–integral (PI) controller in the power converter circuits using the water cycle algorithm (WCA) to augment the transient stability of a grid-connected wave energy conversion (WEC) system. The proposed system relies on the Archimedes wave swing device, which is coupled with a linear permanent magnet synchronous generator (LPMSG). The WEC system is interfaced with the power grid via a generator-side converter (GSC) and a grid-side inverter (GSI). The GSC is used to control both of the d -axis and q -axis current of the LPMSG to minimize its power losses and extract its maximum real power, respectively. The GSI is implemented to control the terminal voltage at the point of common coupling and the dc-link voltage through a complete vector control scheme. The proposed optimal WCA-based PI control strategy is applied to both converters. The optimization process depends on the simulation-based optimization approach. In the proposed approach, the criterion of integral squared error is chosen as a multiobjective function. To validate the proposed WEC system model, the simulation results are compared with the practical results, and their error reaches less than 1%. The effectiveness of the proposed WCA-based PI control strategy is tested and compared with that obtained using the genetic-algorithm-based PI control scheme under symmetrical and unsymmetrical grid fault conditions taking into account a successful and unsuccessful reclosure of circuit breakers. The validity of the proposed control strategy is extensively checked based on simulation studies in the PSCAD/EMTDC environment.

A Permanent Magnet Linear Generator for the Enhancement of the Reliability of a Wave Energy Conversion System

In this paper, a linear generator for a highly reliable wave energy conversion system is designed and tested. In order to store energy, the system is able to produce hydrogen. The wave energy conversion system consists of an electrical linear generator, a power conversion system, and a sea-water electrolyzer. A small-scale prototype of the system is designed and built. The design is oriented to the enhancement of the system robustness and reliability and a failure mode and effects and criticality analysis are used. In order to guarantee an easy extension of the power capability of the marine plant, a modular architecture of the system is adopted. The design strategy is described. The robustness and reliability of the proposed solution are discussed. Simulation and experimental results on the prototype are shown.

Experimental Assessment of the Performance of CECO Wave Energy Converter in Irregular Waves

The performance assessment of a wave energy converter (WEC) is a key task. Depending on the layout of the WEC system and type of power take-off (PTO) mechanism, the determination of the absorbed power at model scale involves several challenges, particularly when the measurement of PTO forces is not available. In irregular waves, the task is even more difficult due to the random character of forces and motions. Recent studies carried out with kinetic energy harvesters (KEH) have proposed expressions for the estimation of the power based only on the measured motions. Assuming that the WEC behaves as a KEH at model scale, the expressions for power estimation of KEHs have been heuristically adapted to WECs. CECO, a floating-point absorber, has been used as case study. Experimental data from model tests in irregular waves are presented and analyzed. Spectral analyses have been applied to investigate the WEC responses in the frequency domain and to derive expressions to estimate the absorbed power in irregular waves. The experimental transfer functions of the WEC motions demonstrated that the PTO damping is significantly affected by the incident waves. Based on KEH approach's results, absorbed power and PTO damping coefficients have been estimated. A linear numerical potential model to compute transfer functions has been also implemented and calibrated based on the experimental results. The numerical results allowed the estimation of combined viscous and losses effects and showed that although the KEH approach underestimated the absorbed power, qualitatively reproduced the WEC performance in waves.

A Flower Pollination Method Based Global Maximum Power Point Tracking Strategy for Point-Absorbing Type Wave Energy Converters

To maximize the generated output power under random waves, the control strategy of maximum power point tracking (MPPT) is incorporated in point-absorbing type wave energy converter (WEC) system. However, due to the influence of mutative wave conditions, the curve of the maximum average power appears in multiple peaks, adding complexity to the tracking process. This paper proposes a new MPPT control technique for a wave power generation system, by using the flower pollination algorithm (FPA) instead of the conventional hill-climbing method. Compared with the hill-climbing method, this method has advantages of achieving a smaller peak-to-average ratio and capturing the more average power under the same sea state. The verification has been carried out through the simulations and the experimental results on a lab test bench.

Control of Wave Energy Converters Using A Simple Dynamic Model

This paper derives a control law within the context of optimal control theory for a heaving wave energy converter (WEC) and presents its implementation procedure. The proposed control assumes the availability of measurements of pressure distribution on the buoy surface, buoy position, and buoy velocity. This control has two main characteristics. First, this control is derived based on a simple dynamic model. The forces on the WEC are modeled as one total force, and hence there is no need to compute excitation or radiation forces. Second, this control can be applied to both linear and nonlinear WEC systems. The derived control law is optimal, yet its implementation requires estimation of some force derivatives, which render the obtained control force suboptimal. Numerical testing demonstrates in this paper that the proposed simple model control can achieve levels of harvested energy close to the maximum theoretical limit predicted by singular arc control in the case of linear WEC systems.