Design, Development, and Field Testing of a Scalable Nearshore Wave Energy Converter Optimized for Sri Lanka’s Shallow Sea Conditions

High efficiency and wideband wave energy capture of a bistable wave energy converter with a displacement amplifier

Wave energy converters (WEC) have been researched extensively to harvest large amounts of untapped ocean renewable energy. Floating-type point absorbers are a type of WEC that converts the ocean wave energy to electrical energy, by utilizing the relative motion of multi-body system mainly consisting of a floating section and a reaction section. The design of floating-type point absorbers is not straightforward and needs special attention to understand the dynamics of the multi-body system, and power take-off (PTO) mechanism placed in the reaction section. The purpose of this study is to develop a numerical simulation tool which can help understand various dynamics of a floating-type WEC and aid in the practical design of PTO systems. The PTO mechanism is represented using a spring-damper system and theoretical modelling of mean absorbed power is presented. The numerical simulation is developed in Orcaflex and is carried out on a demonstration model to show the usefulness of the developed tool. The heave displacement shows the effect of damping (PTO) on the motion response of the floating-type WEC. It is found that the maximization of absorbed power is possible using the developed simulation tool for a specific sea state. Furthermore, the results show the potential use of the developed tool for other floating-type WEC concepts.

This paper presents a practical design of a wave energy converter (WEC). The WEC is based on an innovative mechanical device power take-off (PTO) which can convert wave energy into electric energy by using a floating buoy. By using cable and pulley mechanism, the WEC can work in high efficiency by capturing wave energy in both surge and heave mode and converting the bidirectional motion of ocean waves into one-way rotation of an electric generator. First, the configuration is presented to describe working principle of the system. Then, the hydrodynamic forces are analyzed in the time domain. Moreover, the coupled mechanical and hydrodynamic model are simulated in Matlab/Simulink to investigate the performance of the proposed design. Finally, simulation results indicate that the proposed WEC can absorb wave energy in high efficiency and can be applied in the realistic condition.

As global energy demands and climate concerns continue to grow, the need for renewable energy is becoming increasingly clear and wave energy conversion (WEC) systems are receiving growing interest. Over the years, many WEC systems have emerged but most of these systems are designed to extract energy from a single direction of motion. In reality, there are six degrees of freedom that a conversion device can potentially harness and a device that can operate on multiple axes, should be able to more effectively and consistently produce power. However, the design and control of the power take off (PTO) system for a multi-axis device is challenging due to the system complexity and nonlinearity. WEC systems often utilize optimal control techniques for PTO operation and leverage a prediction of the upcoming wave force to ensure power optimization. Prior work has clearly demonstrated that high power production can be achieved when an exact system model is used and the upcoming wave conditions are known, but uncertainty in the underlying model or the wave prediction can degrade performance. PTO control on a multi-axis WEC must leverage predictions of forces in multiple directions and if model predictive strategies are used, must leverage a simplified model of the WEC dynamics to be able to optimize in real time. The uncertainty in these predictions and the model could severely degrade the WEC’s power output. This work examines the control of a multi-axis WEC system, TALOS, and leverages machine learning to predict wave forces over the upcoming time horizon. TALOS is a point-absorber type WEC with multi-axis PTO system. The PTO uses a heavy ball that is attached to the hull with springs and hydraulic cylinders. When the hull is pushed by the external waves, the relative motion between the ball and hull moves the hydraulic cylinders causing them to pump a fluid through a circuit, thereby driving a hydraulic motor to produce electricity. This design has shown promising results in energy output but is more challenging to control since the PTO can move over six degrees of freedom. This paper seeks to quantify wave prediction uncertainty and its seasonal variation and to examine the impact of the uncertainty of the prediction on a model predictive controller’s ability to optimize the power output of TALOS.

Environmental-Sensing and adaptive optimization of wave energy converter based on deep reinforcement learning and computational fluid dynamics

Integrating floating bridges with wave energy converters can provide a viable option for developing economical wave energy resources, and solving problems of the high cost of wave energy devices and the unstable motion of floating bridges. This paper proposes a novel wave energy floating bridge with an integrated wave energy conversion module (WECM), and its energy capture and hydrodynamic stability problems are investigated. A nonlinear stiffness power take off (PTO) mechanism is used. A hybrid dynamics model for the system is established and the accuracy of numerical model is verified. The energy capture and motion stability of the linear PTO and nonlinear PTO wave energy floating bridge under regular and irregular wave conditions are compared. Results show that WECM can enhance the floating bridge with wave power generation capability and better motion stability in waves. The linear PTO mechanism can reduce the motion response of the floating bridge by choosing the appropriate PTO damping value under regular waves. The nonlinear PTO mechanism increases the double-peak width to broaden the energy capture frequency band. The nonlinear PTO is more suitable for low-frequency wave sea conditions, while the linear PTO is more suitable for high-frequency wave sea conditions. This article can provide favorable reference data for floating bridge design and practical application in the future.

The proper design of wave energy converters (WECs) is crucial for ensuring robustness in harsh wave climates without incurring the additional expense of unnecessary overdesign. The power take-off (PTO) mechanism, serving as a vital link between the moving body and the electric generator, is a key component in the design load analysis of WECs. However, the setting of PTO system parameters significantly impacts the dynamic behavior of the entire WEC system, leading to alterations in estimated loads. This work is dedicated to studying the influence of PTO control strategies on the identification of extreme loads of a heaving point absorber WEC. A nonlinear time-domain model is established to estimate the dynamic responses and loads of the WEC. Both PTO loads and end-stop loads under extreme conditions are examined, considering the wave climate of a realistic sea site. The results suggest that the PTO setting strategies significantly impact the extreme load exerted on both the PTO system and the end-stop system. Varying the PTO damping within a certain range could lead to a difference of 57% and 63% in short-term extreme loads for the PTO system and the end-stop system, respectively. Furthermore, the impacts of the PTO control strategy appear to be specific to each WEC component. The PTO parameters selected for reducing the extreme PTO loads might increase the extreme end-stop loads. A holistic examination is therefore recommended for estimating the extreme loads of WECs.

The increasing demand for renewable energy, driven by the urgent need to mitigate climate change and achieve net-zero emissions, has highlighted ocean energy as one of the most sustainable and promising resources. Wave energy converters (WECs) are pivotal technologies for harnessing marine wave power, providing substantial environmental benefits by reducing dependence on environmentally harmful, non-renewable energy sources such as fossil fuels. Efficient wave energy utilization holds significant potential to contribute to a cleaner, more sustainable energy future. To minimize ecological disruptions while preserving marine biodiversity, various innovative WEC concepts have been proposed. Among them, flexible wave energy converters (FlexWECs) attract great attention due to their lightweight deformable structures and reduced construction costs, significantly resulting in a lower environmental impact than traditional rigid-body WECs. Furthermore, the flexible design of FlexWECs enables adaptation to diverse environmental scenarios, enhancing energy extraction efficiency.In contrast to rigid-body WECs, FlexWECs are characterized by their rubber-like, deformable structures, allowing broadband power absorption and simpler WEC designs. Recent trends in device development have focused on FlexWECs, where primary energy-absorbing components, power take-off (PTO) systems, and other subcomponents are constructed flexibly. Over 20 FlexWEC devices have been developed to date, most of which are still in the concept or laboratory test stages, indicating substantial potential for further development and research. Among these, deformable airbag-based converters and flexible membrane systems stand out for their adaptability and energy absorption capability. Leveraging flexible materials that can deform in response to varying wave conditions, these devices are capable of effectively capturing wave energy across a broad range of frequencies.As a key contributor to wave energy research and the development of FlexWECs, the University of Plymouth has made significant strides in promoting more adaptable, resilient, and environmentally sustainable wave energy solutions. This study focuses on a deformable airbag-based FlexWEC, engineered to optimize wave energy capture by adjusting its form in response to ocean conditions. Using high-fidelity computational fluid dynamics (CFD) simulations, the research explores the airbag’s dynamic behaviour under wave interaction, primarily analyzing multi-physics fluid-structure interactions (FSI) within a multiphase setting. The study examines essential relationships between structural deformation, hydrodynamic response, and energy capture efficiency, aiming to illuminate the underlying interaction mechanisms between wave energy devices and waves. Building on these insights, this research provides valuable perspective on the development of novel FlexWECs that harness renewable marine energy while minimizing environmental impact, achieving a balance between sustainable ocean resource utilization and the preservation of marine ecosystems. Keywords: Renewable marine energy; Environmental benefits; Preservation of marine ecosystems; Flexible wave energy converters; Fluid-structure interactions

This paper presents a hydrodynamic investigation carried out on the “Wave Attenuator” device, which is a new type of floating breakwater anchored with piles and equipped with a linear Power Take Off (PTO) mechanism, which is typical for wave energy converters. The device is tested in the wave flume, under regular waves, in slightly non-linear conditions. The PTO mechanism, that restrains one of the two degrees of freedom, is simulated through an actuator and a programmable logic controller with preassigned strategy. The paper presents the system identification procedure followed in the laboratory, supported by a numerical investigation essential to set up a credible control strategy aiming at maximizing the wave energy harvesting. The maximum power conversion efficiency under the optimal PTO control strategy is found: it is of order 50–70% when the incident wave frequency is lower than the resonance one, and only of order 20% for higher frequencies. This type of experimental investigation is essential to evaluate the actual efficiency limitations imposed by device geometry.

The Power Take-Off (PTO) system is the key component of a Wave Energy Converter (WEC) that distinguishes it from a simple floating body because the uptake of the energy by the PTO system modifies the wave field surrounding the WEC. Consequently, the choice of a proper PTO model of a WEC is a key factor in the accuracy of a numerical model that serves to validate the economic impact of a wave energy project. Simultaneously, the given numerical model needs to simulate many WEC units operating in close proximity in a WEC farm, as such conglomerations are seen by the wave energy industry as the path to economic viability. A balance must therefore be struck between an accurate PTO model and the numerical cost of running it for various WEC farm configurations to test the viability of any given WEC farm project. Because hydrodynamic interaction between the WECs in a farm modifies the incoming wave field, both the power output of a WEC farm and the surface elevations in the ‘near field’ area will be affected. For certain types of WECs, namely heaving cylindrical WECs, the PTO system strongly modifies the motion of the WECs. Consequently, the choice of a PTO system affects both the power production and the surface elevations in the ‘near field’ of a WEC farm. In this paper, we investigate the effect of a PTO system for a small wave farm that we term ‘WEC array’ of 5 WECs of two types: a heaving cylindrical WEC and an Oscillating Surge Wave Energy Converter (OSWEC). These WECs are positioned in a staggered array configuration designed to extract the maximum power from the incident waves. The PTO system is modelled in WEC-Sim, a purpose-built WEC dynamics simulator. The PTO system is coupled to the open-source wave structure interaction solver NEMOH to calculate the average wave field η in the ‘near-field’. Using a WEC-specific novel PTO system model, the effect of a hydraulic PTO system on the WEC array power production and the near-field is compared to that of a linear PTO system. Results are given for a series of regular wave conditions for a single WEC and subsequently extended to a 5-WEC array. We demonstrate the quantitative and qualitative differences in the power and the ‘near-field’ effects between a 5-heaving cylindrical WEC array and a 5-OSWEC array. Furthermore, we show that modeling a hydraulic PTO system as a linear PTO system in the case of a heaving cylindrical WEC leads to considerable inaccuracies in the calculation of average absorbed power, but not in the near-field surface elevations. Yet, in the case of an OSWEC, a hydraulic PTO system cannot be reduced to a linear PTO coefficient without introducing substantial inaccuracies into both the array power output and the near-field effects. We discuss the implications of our results compared to previous research on WEC arrays which used simplified linear coefficients as a proxy for PTO systems.

Wave energy converters (WEC) use indirect drive hydraulic or turbine-type power take-off (PTO) mechanisms which consist of many moving parts, creating mechanical complexity and increasing the installation and maintenance costs. Linear generator-based direct drive wave energy converters could be a solution to overcome this problem, but the efficiency of the single conventional linear generator is not high enough, and it cannot work satisfactorily in the low-frequency range.In this paper, a novel dumbbell-shaped flux-switching linear generator has been proposed and studied as a power take-off unit for ocean wave energy conversion. The linear generator has a dumbbell-shaped stator, and the design is ameliorated by placing the permanent magnet rings of longitudinally alternating magnetic pole directions in the slots of its stator outer surface and is separated by thin wall steel ring shoulders. The linear generator also has a dumbbell-shaped translator. The stator is hollow where the translator slides inside axially inducing current in the coil. A long permanent magnet is inserted inside the hollow steel dumbbell core of the translator and a copper coil is wound around the outer surface of the moving translator core. The addition of permanent magnets on the outer slots of the stator is found to increase the output power significantly. To facilitate the investigation, the modified generator design has been compared to the conventional linear permanent magnet generator for their performances using the finite element method, as both machines are different in their structures. The results show that the double dumbbell-shaped flux-switching linear generator gives higher power output, magnetic flux density, branch current, and induced voltage. The double dumbbell-shaped flux-switching linear generator is then placed in a cylindrical buoy and investigated in a wave energy converter in the ocean environment. The hydrodynamic response of the cylindrical buoy has been investigated through ANSYS AQWA. The dynamic differential equations of the wave energy converter have been developed and solved for regular waves using Matlab codes. The peak output voltage, power, and the relative displacement of the linear generator translator with respect to the stator fixed with the buoy have been calculated through the Fourier Transform in the frequency domain using a Matlab code and through numerical integrations in the time domain using a Matlab Simulink code. The results in the time and frequency domains are compared and verified with each other. The relative displacement between the translator and stator buoy has been used as motion input of the ANSYS Maxwell simulation model, and the output voltage results of the ANSYS Maxwell simulation model have been compared and verified with those of the Matlab simulation models. The linear generator design in the wave energy converter under the regular wave excitation is further optimized for the maximum ratio of the peak output power to peak cogging force using the central composite design-based response surface method (RSM). The ANOVA analysis is used to validate the response surface model where its R2 coefficient of 99.93% has indicated an excellent fit. The methods and results differ from those presented in previous studies.

Power performance and dynamic response of the WLC wave energy converter based on hydroelastic analysis

In response to the need for efficient, small-scale power sources for applications such as ocean observation and navigation, this paper presents the design, modeling, fabrication, testing, and analysis of a compact point-absorber wave energy converter (PAWEC) equipped with a mechanical direct-drive power takeoff (PTO) mechanism. The motivation is to address the mismatch between the natural frequencies of conventional PAWECs and dominant ocean wave frequencies, which limits energy capture. The primary objective is to enhance the efficiency of small-scale wave energy converters (WEC) without increasing the buoy size. To achieve this, we introduce a novel design element: an added mass plate (AMP) attached to the buoy. The AMP is devised to increase the WEC added mass and natural period, thereby aligning its natural frequency with dominant ocean wave frequencies. In our case study of a scaled model (1:2.2), the AMP effectively doubled the added mass of the WEC and increased its natural period by 32%. The WEC incorporates a rack and pinion mechanical motion rectifier-type PTO to convert the heave oscillations of the buoy into unidirectional rotation. The scaled model was tested in a wave basin facility with regular waves at zero angle of incidence. The WEC with AMP achieved a maximum root mean square power of 9.34 W, a nearly 30% increase compared to the conventional configuration without AMP, which produced 7.12 W under similar wave conditions. Numerical analysis using the boundary element method in the frequency domain for regular waves confirmed these findings. Finally, it has been derived that the proposed WEC, equipped with an AMP, offers enhanced efficiency in longer wave periods without the need for a larger buoy, establishing its viability as a power source for navigational buoys. This paper also offers a comprehensive guide to experimental techniques for characterizing a PAWEC in a laboratory setting, contributing valuable insights into the wave energy community.

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

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.