Wave Energy Conversion Devices Research Articles

Mini-DAQ: A lightweight, low-cost, high resolution, data acquisition system for wave energy converter testing

As part of the development process, scaled testing of wave energy converter devices are necessary to prove a concept, study hydrodynamics, and validate control system approaches. Creating a low-cost, small, lightweight data acquisition system suitable for scaled testing is often a barrier for wave energy converter developers’ ability to test such devices. This paper outlines an open-source solution to these issues, which can be customized based on specific needs. This will help developers with limited resources along a path toward commercialization.

Data-driven control of a Pendulum Wave Energy Converter: A Gaussian Process Regression approach

The energy coming from the motion of the waves of seas and oceans could be an important component in the solution of the energy problem related to the pursuit of alternatives to fossil fuels. However, wave energy is still technologically immature and it has not reached the economic feasibility required for economy of scale. One of the major technological challenges for the achievement of this goal is the development of control strategies capable of maximizing the extracted energy, adapting to the conditions of the seas and oceans that surround the Wave Energy Converter (WEC) devices. To perform this task, control systems often adopt explicitly control-oriented models, that are by nature affected by uncertainties. On the contrary, to address the problem a data-driven solution is proposed here. The presented strategy applies an optimization approach based on a Gaussian Process Regression (GPR) metamodel to learn the control strategy to be applied. In order to accelerate the learning process, we present a novel method that exploits in the initial phase a previous knowledge given by simulations with the system model and based on the co-kriging concept. To test this approach the Pendulum Wave Energy Converter has been adopted as a case study. To differentiate the previous knowledge and the real system behaviour, a simplified linear model is used to obtain the prior knowledge, while a complex nonlinear one acts as the environment in which simulate the behaviour of the real system. A month-long simulation is used to validate the effectiveness of the proposed strategy, showing the ability of adapting to a real system different from the simplified model on the basis only of data, and overcoming the model-based strategy in terms of performance.

Experimental study on heave performance of a new wave energy power generation device based on regular waves

As an important form of ocean energy, wave energy is gradually being developed and utilized by coastal countries around the world. Researchers have developed various forms of wave energy conversion devices. In this paper, a new type of horizontal rotor wave energy power generation device is designed and manufactured. The device is in a moored floating state when working at sea. The heave belongs to the vertical movement up and down, which directly affects the stability of the device. Based on the engineering prototype making model and conducting regular wave experiments, four test conditions with the same period and different wave heights were designed to verify the influence of wave height changes on heave plates of different shapes and different installation distances on the stability of the device. Determining the shape of the heave plate and the optimal installation distance under the test conditions provides a theoretical basis for the manufacture of engineering prototypes and sea trials.

The Shape Optimization and Experimental Research of Heave Plate Applied to the New Wave Energy Converter

The development and utilization of wave energy is inseparable from the wave energy converter, and its stability is an important condition for operation. Heave is the biggest factor affecting the stable power generation of wave energy converters. The key method to solve this problem is to install a suitable heave plate. Therefore, the design of the heave plate is particularly important. Based on a new type of horizontal rotor wave energy converter, this paper proposes three different shapes of heave plate design schemes and completes the calculation and modeling of the engineering prototype. First, the three types of heave plate devices were numerically simulated using hydrodynamic calculation software to compare their stable performances and verify the feasibility of the scheme. Subsequently, an experimental model was made according to the parameters of the engineering prototype, and a tank experiment was carried out under the same working conditions to further study the influence of the heave plate installation distance on the stability of the wave energy generator. The results showed that when the distance was between 10 mm and 20 mm, the average amplitude change was large, and when the distance was between 20 mm and 30 mm, the average amplitude change was small. Therefore, the installation distance should be between 20 mm and 30 mm. In the case of the same heave plate area and installation distance, the average amplitude of the chamfered heave plate device was smaller than the other two types, indicating that its stability was better. The optimization of the shape and installation distance of the heave plate proposed in this study has obvious effects on improving the stability of the device and provides a reference for the design of the wave energy converter device.

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.

Performance enhancement for a dual-chamber OWC conceived from side wall effects in narrow flumes

Performance enhancement is always the focus for the exploration in oscillating water column (OWC) wave energy converter devices. In the present study, the hydrodynamic characteristics of a dual-chamber OWC system composed of two 3D circular sub-units is examined under the framework of OpenFOAM. The model-scale simulations are firstly carried out in a wave flume with different widths under the excitation of regular waves. Then, conceived from the side wall effects in narrow flumes, a pair of harbor walls parallel to the wave propagation direction is introduced; by tuning its deployed location and geometric dimension, the enhancement of wave power extraction can be expected. The results indicate that significant estimation errors in the capture width ratio may occur when the incident wave length is nearly equal to the flume width or the resonant frequency of water column is close to the wave frequency in a narrow flume. In addition, the integrated system consisting of OWC and harbor walls shows that the distance between above two structures can be tuned to a threshold value to maximize the capture width ratio by strengthening the fluids and structures interaction. The medium submerged depth of harbor walls may be an alternative when the trade-off between performance enhancement and economic costs has to be considered during practical construction. Moreover, a well agreement between the model-scale and prototype-scale results proves that the performance enhancement can be scalable to large scales by integrating OWC device with harbor walls.

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.

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.

Power Constrained Optimal Control of Wave Energy Converters

The general goal of control methods developed for Wave Energy Converter (WEC) technologies is to improve the amount of energy captured by the WEC from the wave. However, most WEC control methods require some power to be supplied from the grid (reactive power) to drive the floater towards resonance at intervals to improve the device's overall performance. Such controls’ reactive power and motion requirements sometimes become large and unrealistic. Additionally, a power take-off (PTO) unit capable of fulfilling a sizeable reactive power requirement will be expensive and complex, if not impossible. In this work, an optimal control formulation that aims to maximize the harvested energy while constraining the reactive power not to exceed a realistic threshold is derived using Pontryagin Minimum Principle. The optimal control formulation is derived for a single WEC device with irregular excitation. Low fidelity numerical simulations are presented comparing the proposed power-constrained Bang Singular Bang (PCBSB) control to an Optimal Resistive Loading (ORL) control.

Inclusion of a pitching mid-wall for a dual-chamber oscillating water column wave energy converter device

The concept of a dual-chamber oscillating water column (OWC) device consisting of a pitching mid-wall (restrained by an angle spring stiffness) is proposed and the corresponding theoretical model is established under the framework of potential flow theory. By employing the matched eigenfunction method along the common interfaces in terms of the velocity and pressure continuity, the present model can be numerically solved. A numerical strategy of successive approximation is utilized to determine the optimal turbine parameters for the optimal energy extraction. Some potential influential factors, including the linear density, installed location and draft of the mid-wall, chamber breadth and angle spring stiffness, are explored to analyze their impacts on the power extraction performance. It is found that the mid-wall in the dual-chamber structure with a relatively larger linear density and smaller draft is more beneficial for energy extraction. Under the condition that the dual-chamber structure is designed with a relatively wider size, an asymmetry configuration with a narrower front but a wider rear chamber is more recommended. In addition, a relatively smaller angle spring stiffness is advisable due to the occurrence of more satisfactory peak efficiency.

Experimental Validation of a Gyroscope Wave Energy Converter for Autonomous Underwater Vehicles

Power technology has long been the main problem that has plagued the realization of ocean exploration by autonomous underwater vehicles (AUVs). This paper introduces a new wave energy conversion device for AUV, which is sealed inside a closed floating body to avoid interaction with the marine environment. The system uses the gyroscopic effect to continuously convert the pitching motion of waves into electrical energy through flywheel rotation, and thus theoretically extend the endurance time of AUVs. In this paper, a mathematical model of the power generation device is established, and the effects caused by different parameters on the system behavior and energy output are analyzed. In order to reduce the cost of experiments, the energy conversion device is installed on an experimental platform that can simulate wave motion to observe its energy generation performance. The experimental results show that the established mathematical model can accurately reflect the real behavior of the power generation device on the platform under different wave conditions, and the energy output error is only 9.91%.

Numerical simulation of fluid structure interaction in free-surface flows: the WEC case

In this work we present a numerical framework to carry-out numerical simulations of fluid-structure interaction phenomena in free-surface flows. The framework employs a single-phase method to solve momentum equations and interface advection without solving the gas phase, an immersed boundary method (IBM) to represent the moving solid within the fluid matrix and a fluid structure interaction (FSI) algorithm to couple liquid and solid phases. The method is employed to study the case of a single point wave energy converter (WEC) device, studying its free decay and its response to progressive linear waves.

Air pressure forecasting for the Mutriku oscillating‐water‐column wave power plant: Review and case study

The high variability and unpredictability of renewable energy resources require optimization of the energy extraction, by operating at the best efficiency point, which can be achieved through optimal control strategies. In particular, wave forecasting models can be valuable for control strategies in wave energy converter devices. This work intends to exploit the short-term wave forecasting potential on an oscillating water column equipped with the innovative biradial turbine. A Least Squares Support Vector Machine (LS-SVM) algorithm was developed to predict the air chamber pressure and compare it to the real signal. Regressive linear algorithms were executed for reference. The experimental data was obtained at the Mutriku wave power plant in the Basque Country, Spain. Results have shown LS-SVM prediction errors varying from 9% to 25%, for horizons ranging from 1 to 3 s in the future. There is no need for extensive training data sets for which computational effort is higher. However, best results were obtained for models with a relatively small number of LS-SVM features. Regressive models have shown slightly better performance (8–22%) at a significantly lower computational cost. Ultimately, these research findings may play an essential role in model predictive control strategies for the wave power plant.

HYDRODYNAMICS OF A DUAL-CHAMBER OWC WAVE ENERGY CONVERTER IN HEAVING MOTION

The oscillating water column (OWC) wave energy conversion device have been recognized as the most promising wave energy conversion technology due to the advantages of its simple structure, convenient assembly and easy maintenance. A heave-only dual-chamber OWC device was numerically investigated by a well-developed open source software OpenFOAM coupled with a wave generation and absorption toolbox Waves2Foam. The volume of fluid (VOF) method tracking the water-air interface and the six-degree-freedom (6DOF) Dynamic Mesh solver duplicating the heave motion of the OWC device were employed to examine the influences of the relative width of the front and rear chambers and the spring elastic coefficient on the energy capture width ratio and hydrodynamic characteristics of the device under the actions of different incident regular waves. Through comparing the present results with the existing ones of a fixed dual-chamber OWC wave energy converter, and examining the free-decay motion of a cylinder, the rationality and effectiveness of the present numerical model has been revealed. The results show that the wider rear chamber can make for the extraction of wave energy of the dual-chamber OWC in heave motion. The heave-only dual-chamber OWC device can improve the device performance as the relative width ratio of the front and rear chambers is 1/2, and the rear chamber has larger capture width ratio in the middle and high frequency wave bands, compared with that of the fixed one. Multiple-peak values of the relative water surface elevation and the relative pressure occur in the whole test wave frequency bands due to the phase gap between the dual-chamber OWC device and the water columns in the front and rear chambers. In addition, it is found that by adjusting the spring stiffness coefficient, the wave-frequency bandwidth of high-efficiency can be significantly broadened and larger energy capture width ratio can be achieved.

Geometric Analysis through the Constructal Design of a Sea Wave Energy Converter with Several Coupled Hydropneumatic Chambers Considering the Oscillating Water Column Operating Principle

The work presents a numerical study of a wave energy converter (WEC) device based on the oscillating water column (OWC) operating principle with a variation of one to five coupled chambers. The main objective is to evaluate the influence of the geometry and the number of coupled chambers to maximize the available hydropneumatic power converted in the energy extraction process. The results were analyzed using the data obtained for hydropneumatic power, pressure, mass flow rate, and the calculated performance indicator’s hydropneumatic power. The Constructal Design method associated with the Exhaustive Search optimization method was used to maximize the performance indicator and determine the optimized geometric configurations. The degrees of freedom analyzed were the ratios between the height and length of the hydropneumatic chambers. A wave tank represents the computational domain. The OWC device is positioned inside it, subject to the regular incident waves. Conservation equations of mass and momentum and one equation for the transport of the water volume fraction are solved with the finite volume method (FVM). The multiphase model volume of fluid (VOF) is used to tackle the water–air mixture. The analysis of the results took place by evaluating the performance indicator in each chamber separately and determining the accumulated power, which represents the sum of all the powers calculated in all chambers. The turbine was ignored, i.e., only the duct without it was analyzed. It was found that, among the cases examined, the device with five coupled chambers converts more energy than others and that there is an inflection point in the performance indicator, hydropneumatic power, as the value of the degree of freedom increases, characterizing a decrease in the value of the performance indicator. With the results of the hydropneumatic power, pressure, and mass flow rate, it was possible to determine a range of geometry values that maximizes the energy conversion, taking into account the cases of one to five coupled chambers and the individual influence of each one.

Energy‐maximising moment‐based constrained optimal control of ocean wave energy farms

Successful commercialisation of wave energy technology inherently incorporates the concept of an array of wave energy converters (WECs). These devices, which constantly interact via hydrodynamic effects, require optimised control that can guarantee maximum energy extraction from incoming ocean waves while ensuring, at the same time, that any physical limitations associated with device and actuator systems are being consistently respected. This paper presents a moment-based energy-maximising optimal control framework for WECs arrays subject to state and input constraints. The authors develop a framework under which the objective function (and system variables) can be mapped to a finite-dimensional tractable quadratic program (QP), which can be efficiently solved using state-of-the-art solvers. Moreover, the authors show that this QP is always concave, i.e. existence and uniqueness of a globally optimal solution is guaranteed under this moment-based framework. The performance of the proposed strategy is demonstrated through a case study, where (state and input constrained) energy-maximisation for a WEC farm composed of CorPower-like WEC devices is considered.

Real-Time Nonlinear Model Predictive Controller for Multiple Degrees of Freedom Wave Energy Converters with Non-Ideal Power Take-Off

An increase in wave energy converter (WEC) efficiency requires not only consideration of the nonlinear effects in the WEC dynamics and the power take-off (PTO) mechanisms, but also more integrated treatment of the whole system, i.e., the buoy dynamics, the PTO system, and the control strategy. It results in an optimization formulation that has a nonquadratic and nonstandard cost functional. This article presents the application of real-time nonlinear model predictive controller (NMPC) to two degrees of freedom point absorber type WEC with highly nonlinear PTO characteristics. The nonlinear effects, such as the fluid viscous drag, are also included in the plant dynamics. The controller is implemented on a real-time target machine, and the WEC device is emulated in real-time using the WECSIM toolbox. The results for the successful performance of the design are presented for irregular waves under linear and nonlinear hydrodynamic conditions.

A Comparative Study of Model Predictive Control and Optimal Causal Control for Heaving Point Absorbers

Efforts by various researchers in recent years to design simple causal control laws that can be applied to WEC devices suggest that these controllers can yield similar levels of energy output as those of more complex non-causal controllers. However, most studies were established without adequately considering device and power conversion system constraints which are relevant design drivers from a cost and economic point of view. It is therefore imperative to understand the benefits of MPC compared to causal control from a performance and constraint handling perspective. In this paper, we compare linear MPC to a casual controller that incorporates constraint handling to benchmark its performance on a one DoF heaving point absorber in a range of wave conditions. Our analysis demonstrates that MPC provides significant performance advantages compared to an optimized causal controller, particularly if significant constraints on device motion and/or forces are imposed. We further demonstrate that distinct control performance regions can be established that correlate well with classical point absorber and volumetric limits of the wave energy conversion device.

In-Ocean Validation of a Deterministic Sea Wave Prediction (DSWP) System leveraging X-Band Radar to Enable Optimal Control in Wave Energy Conversion Systems

Advanced controls optimization offers a promising pathway to improve performance, reduce structural loads and thereby reduce the Levelized Cost of Electricity (LCOE) for wave energy conversion (WEC) systems. Model predictive control (MPC), which remains the preferred algorithm framework for WEC devices requires a wave excitation force forecast on the order of 30 seconds into the future to be effective. To establish the feasibility of forecasting the wave resource within this time-horizon, we benchmarked a wave prediction system that leverages an X-band ship navigation radar using a set of field campaigns carried out offshore Santa Cruz in California. For quantitative assessment of the prediction accuracy, we evaluated error metrics based on correlation and normalized mean absolute error between measurement and prediction. The relationship between prediction accuracy and key environmental factors prevalent during data collection (wave height and wind-speed) was analyzed through a set of sensitivity studies. Overall, the results show that wave prediction accuracy is good enough to enable MPC based control, which establishes a credible technology pathway to utilize these control techniques on commercial devices at sea.

Mathematical modeling of breakwater-integrated oscillating water column wave energy converter devices under irregular incident waves

In the present study, the performance of breakwater-integrated OWC (oscillating water column) wave energy converter devices is analyzed under the action of unidirectional regular and irregular incident waves. Two different types of OWC devices: (i) LIMPET device (sloping-face OWC device) and (ii) quarter-circle-shaped front wall OWC device are considered for the present study. Detailed derivations of the parameters associated with the performance of the OWC devices under the action of irregular waves are provided. To analyze the efficiency of the OWC devices in real sea conditions, the Bretschneider Spectrum is taken as the incident wave spectrum along with nine sea states represent the local wave climate at the OWC plant site Pico, Portugal. The annual-averaged plant efficiencies of the two aforementioned OWC devices are analyzed as a function of chamber length, submergence depth, turbine rotor diameter, and rotational speed of the Wells turbine. It is observed that the annual-averaged plant efficiency can be enhanced significantly with appropriate combinations of chamber length, submergence depth, and turbine characteristics.