Power Take-off Research Articles

Innovative Design of a Sea Wave Energy Harnesser

The aim of this research is to design and develop a simple, light and cost-effective wave energy conversion device. The mass to power ratio is of importance to achieve high working efficiency. In addition proper control concept is proposed for various sea wave conditions to manage the system optimally. . their operation. This research addresses the above two problems by considering the system mechanical structure and schemes designed to achieve optimal control. Methods employed to achieve maximum power output are inevitably non-causal and require prediction of the shape of the incident waves. The potential wave resource in South Africa is enormous as there are over 2000 km of coastline with an annual wave height average of 2-3 m. One of the most efficient wave energy extractor is the hinged wave energy converter (HIWEC) whereby the wave force is applied to a responsive mechanism able to resist the working force of the waves. The hinged mechanism used in this project is of the power take-off (PTO) type. The PTO captures the wave energy and then between the wave’s time intervals transfers it to be converted and stored. Control of the response of the HIWEC can be achieved through active tuning of the PTO. As wave energy is intermittent, the supplied electrical power will show strong variations unless an energy storage system is available. The energy storage system consists of several series connected gas accumulators, placed in the hydraulic circuit to produce a smoothing effect.

A 3D BEM-Coupled Mode Model for the Performance Analysis of Wave Energy Converter Parks in Nearshore-Coastal Regions

Estimation on the production capacity of wave energy converter arrays (WECs) of the type of simple floaters deployed in nearshore locations highly depends on the evaluation of their performance. The latter also depends on various factors, including the dimensions and inertial characteristics of the devices, their relevant positioning, and the power take-off (PTO) system characteristics. Studying the system operation, based on the prevailing sea conditions in the region considered for deployment, can ensure that such WEC farms are sized and designed in an effective way. Furthermore, the wavelength and propagation direction of incoming wave fields can be significantly impacted by wave-seabed interactions in coastal areas, which can alter the WECs’ response pattern and ultimately the array’s power output. In this work, a 3D BEM hydrodynamic model is proposed aiming to assess the energy-capturing capacity of WEC arrays, accounting for the hydrodynamic interactions between various identical floating devices, as well as the local seabed topography. The model is supplemented by a Coupled Mode System (CMS) to calculate the incident wave field propagating over variable bathymetry, in order to simulate realistic nearshore environments. Finally, a case study is performed for an indicative geographical area, north of the coast of the island of Ikaria, located in the Eastern Aegean Sea region, where the wave potential is high, using long-term data. The latter study highlights the applicability of the proposed method and suggests its usage as a tool to support optimal WEC park design.

Integrated Design and Optimization of a Direct Drive Axial Flux Permanent Magnet Generator for a Tidal Turbine

The C-GEN is a novel topology of direct drive air-core permanent magnet generator being developed at University of Edinburgh [1]. The topology has many benefits such as; absence of cogging torque, reduced mass and ease of manufacturing. A 20 kW prototype test rig and 15kW machine for a wind turbine has been manufactured and tested previously. Initial sizing studies for wind turbines indicate that the C-GEN concept will be up to 50% lighter than conventional iron cored PM direct drive generators [1]. In addition to the applications wind turbines, C-GEN technology can also be implemented for marine energy power take-off systems. To investigate that, a feasibility study is being undertaken in collaboration with two wave and two tidal energy companies. In this paper, design and optimization method of an axial flux permanent magnet generator for a tidal energy converter device has been investigated. An analytical optimization tool is designed that combines electromagnetic, structural and thermal aspects of the machine design. A genetic algorithm optimization method has been utilized based on the operation conditions of generator and pre-defined constraints on dimensions and material limitations. The output of the analytical design tool is compared with the electromagnetic FEA simulations. The results showed that proposed analytical calculation method is consistent with FEA results

Design and Performance Evaluation of an Enclosed Inertial Wave Energy Converter with a Nonlinear Stiffness Mechanism

In order to enhance the power generation efficiency and reliability of wave energy converters (WECs), an enclosed inertial WEC with a magnetic nonlinear stiffness mechanism (nonlinear EIWEC) is proposed in this paper. A mathematical model of the nonlinear EIWEC was established based on the Cummins equation and the equivalent magnetic charge method, and the joint simulations were carried out using MATLAB/Simulink 2020 and AMESim 2020 softwares. The effect of the magnetic nonlinear stiffness mechanism (NSM) on the performance of the EIWEC system was investigated. The results show that the nonlinear negative stiffness property of NSM can significantly improve the motion response and output power of EIWEC under low-frequency waves. Compared to EIWEC without NSM (linear EIWEC), nonlinear EIWEC has a higher generation efficiency and wider frequency bandwidth. Additionally, the effects of linear spring, internal mass body, and hydraulic power take-off (PTO) system parameters on the energy conversion capability of the system were analyzed to provide a reference for the design of nonlinear EIWECs. In general, the proposed nonlinear EIWEC could provide good development potential for the scale utilization of wave energy resources.

Theoretical modeling of a co-located system with a floating wind platform and vertical truncated cylinders array

Combined floating offshore wind turbines (FOWTs), wave energy converters (WECs), and floating solar photovoltaics (FPVs) systems have the potential to provide cost-effective solutions for offshore multi-energy complementation and structure protection. In this study, a theoretical model based on the potential flow theory and eigenfunction matching method is utilized to study wave diffraction and radiation by a co-located system, in which the main components of the wind platform and WECs are made of vertical cylindrical floats. Based on the displacement constraint matrix, coupled equations of motion are developed to calculate the kinematic response of the co-located systems. After running the convergence analysis and model validation, the present model is employed to perform a multiparameter impact analysis. Case studies are presented to clarify the effects of the WEC radius, draft, layout, power take-off (PTO) system, and incident wave heading and frequency on the hydrodynamic coefficient, wave energy capture width, and motion response of the wind platform. Our findings highlight that several factors play a crucial role in the performance of the co-located system, more importantly, that the theoretical model developed in this study is capable of effectively predicting the wave-structure interactions in wave fields, making it applicable to future wave farm projects.

Nonlinear dynamics of a heaving spar-floater system integrated with inerter pendulum vibration absorber power take-off for wave energy conversion

A power take-off based on the inerter pendulum vibration absorber (called IPVA-PTO) is integrated with a spar-floater system to study its hydrodynamic response suppression and wave energy conversion capabilities in regular waves. The hydrodynamics of the spar-floater system is computed using the boundary element method with linear wave theory. With the wave height and wave frequency as the bifurcation parameters, it is found that the system can undergo two bifurcations: period-doubling bifurcation around the first resonance frequency (spar mode) and secondary Hopf bifurcation around the second resonance frequency (floater mode). The period-doubling bifurcation results in an energy transfer between the spar-floater system and the IPVA-PTO for small electrical damping values. As a result, the IPVA-PTO system simultaneously reduces the maximum response amplitude operator (RAO) of the spar and increases the normalized capture width in comparison with the optimal linear benchmark. Experiments performed on a “dry” single-degree-of-freedom system integrated with the IPVA-PTO where the base excitation is substituted for the wave excitation verify the simultaneous performance enhancement due to the period-doubling bifurcation. The system performance beyond the period-doubling bifurcation is also experimentally investigated. On the other hand, as the wave height approaches and passes the secondary Hopf bifurcation, the pendulum responses transition from primary harmonic responses to quasi-periodic responses to rotations. When the rotations occur, the IPVA-PTO system increases the maximum normalized capture width threefold to fivefold compared with the optimal linear benchmark, yet slightly increases the RAO around the second resonance frequency. Nevertheless, the RAO remains smaller than the global maximum RAO of the optimal linear benchmark. Finally, parametric studies are performed to study the effects of parameters on the bifurcations. It is observed that by varying the electrical damping, the wave height required for achieving the period-doubling bifurcation can be changed significantly, which can be exploited to stabilize the spar.

Achieving optimum power extraction of wave energy converters through tunable mechanical components

Wave energy converters (WECs) play a significant role in harnessing the vast potential of marine renewable energy. To enhance the power extraction of WECs, it is necessary but challenging to design the parameters of the WECs for the time-varying wave conditions. This research explores the utilization of tunable mechanical components within both the power take-offs (PTO) and wave capturing structures. We first briefly review the tunable mechanisms and then conduct a theoretical analysis of their applications for maximizing power extraction under regular and irregular wave conditions. The study investigates the impact of PTO stiffness and inertance on power extraction through an analytical approach. The results reveal that both parameters exhibit a monotonic effect within specific frequency ranges, and showed the optimum power extraction can be achieved through the tunable mechanical components. Furthermore, the research highlights the importance of designing the frequency tuning limit for two-body WECs to coincide with the desired wave conditions, as it enables all PTO parameters to achieve an optimal power output. Based on the analysis, the paper gives insightful suggestions on the design of PTO and WEC systems, including mass ratio between the two bodies, mooring stiffness, and other relevant parameters. It is found that the tuning through the mechanical component can be significant for the single-body WEC yet less effective for the two-body WEC especially at irregular wave conditions.

Hydroelastic theory for offshore floating plates of variable flexural rigidity

We present a theoretical model of the hydrodynamic behaviour of a floating flexible plate of variable flexural rigidity connected to the seabed by a spring/damper system. Decomposition of the response modes into rigid and bending elastic components allows us to investigate the hydroelastic behaviour of the plate subject to monochromatic incident free-surface waves of constant amplitude. We show that spatially dependent plate stiffness affects the eigenfrequencies and modal shapes, with direct consequences on plate dynamics and wave power extraction efficiency. We also examine how plate length and Power Take-Off (PTO) distribution affect the response of the system and its consequent absorbed energy. This work highlights the need to improve existing models of flexible floating energy platforms, especially given their importance in the Offshore Renewable Energy (ORE) sector.

Optimal Constrained Control of Arrays of Wave Energy Converters

Wave Energy Converters (WECs) are designed to be deployed in arrays, usually in a limited space, to minimize the cost of installation, mooring, and maintenance. Control methods that attempt to maximize the harvested power often lead to power flow from the WEC to the ocean, at times, to maximize the overall harvested power from the ocean over a longer period. The Power Take-Off (PTO) units that can provide power to the ocean (reactive power) are usually more expensive and complex. In this work, an optimal control formulation is presented using Pontryagin’s minimum principle that aims to maximize the harvested energy subject to constraints on the maximum PTO force and power flow direction. An analytical formulation is presented for the optimal control of an array of WECs, assuming irregular wave input. Three variations of the developed control are tested: a formulation without power constraints, a formulation that only allows for positive power, and finally, a formulation that allows for finite reactive power. The control is compared with optimally tuned damping and bang–bang control.

Dual-function flapping hydrofoil: Energy capture and propulsion in ocean waves

Bionic flapping hydrofoils can be utilized for either energy extraction or propulsion, which has attracted the interest of many researchers. This paper proposes a semi-active flapping hydrofoil device connected to a spring damper power takeoff (PTO) that can simultaneously capture wave energy and propel under ocean wave excitation. The effects of various PTO stiffnesses, PTO damping, and torsional stiffnesses on energy capture and propulsion are investigated using CFD simulation. The performance maps of power generation and propulsion are presented. Compared with the traditional semi-active flapping hydrofoil, the study discovered that the proposed dual-function flapping hydrofoil system could achieve higher overall efficiency. With increasing PTO damping, the overall efficiency initially rises and then falls. After optimizing the PTO parameters, it is possible to increase the overall efficiency by 32 %–86 %. By adjusting the PTO damping, the proportion of energy absorbed for electricity generation and propulsion can be redistributed. Larger torsional stiffness can compensate for the reduction in energy capture and has slight influence on propulsion power. The proposed flapping hydrofoil is expected to be implemented in small wave-powered marine robots, enabling wave propulsion and wave power generation simultaneously.

Control Strategy Development for an Inverter Controlled Wave Energy Plant.

This paper will introduce the Energetech wave energy conversion system and describe its main elements of structure, turbine, generator, instrumentation and control systems. The concept of a parabolic wall which focuses the wave energy onto an oscillating water column (OWC) will be shown. The OWC converts the wave motion to pneumatic energy and then to kinetic energy in air flows suitable for mechanical conversion by the Energetech variable pitch turbine. The paper will show the main points of the station design which is due to have its first pilot plant commissioned by end 2004, 100km south of Sydney, Australia. The paper will describe the development of a wave to wire simulation used to rapidly test various station configurations and control scenarios. The model described will be shown as the result of several years of rigorous study involving scale systems, tank testing and computational fluid dynamics. Finally, the use of an inverter controlled variable speed drive will be discussed as the means of power take-off for the Energetech turbine. The control strategies developed using this system and their application in wave energy will be presented using the results of the wave to wire simulation.

Генератор пар импульсов противоположной полярности с повышенной мощностью с применением сумматора на нерегулярно-включенных полосковых линиях

A pulse pair generator with a subnanosecond front is considered, in which pulses of opposite polarity are created by separate generators, and then supplied to the bridge combiner of the original design (on suspended striplines). For the first time in devices of this type, an output power of 18 watts was achieved. A distinctive feature of this structure is that a half of the power of the generators is dissipated on the ballast resistor (due to the alternating appearance of pulses at the inputs of the combiner). To partially compensate for this effect, a half-standard (25 ohms) input resistance of the adder is selected. In the current circuit of generators it leads to an increase in power takeoff from them. The generators themselves are made on step recovery diodes (SRD) according to a scheme close to the traditional one, but differ by a diode circuit at the output, which eliminates penetration of parasitic lobes of a pulse of opposite polarity to the output and protects the SRD from the action of the pulse of the neighboring generator. The shaper is used in experiments to study nonlinear scattering.

A Review: Indirect Optimal Control of Wave Energy Converters

Wave energy conversion has been the subject of interest in the past several years. While there are several concepts for converting wave power into electric power, the cost of the electric power harvested from ocean waves remains high. One of the main challenges, though it receives less attention, is the control of the wave energy converter (WEC). This paper presents a treatment for the WEC control problem within the context of optimal control theory. The result is systematic development for an explicit expression for a control that maximizes the harvested energy while meeting operational constraints such as the maximum device stroke and the maximum control force. The control presented here can also be adjusted to meet device design constraints such as a limitation on the amount of reactive power available from the power take-off (PTO) unit; this feature enables a control co-design for the PTO unit. Numerical simulations are presented in this paper for demonstration.

Startup optimization of gas foil bearings-rotor system in proton exchange membrane fuel cells

The startup performance is crucial to the high-efficient operation of the sustainable power generation system. Current literature usually focuses on catalysts and startup strategy to improve startup performance of the proton exchange membrane fuel cell, which ignores the gas foil bearings-rotor system that directly affects startup response. Therefore, in this paper, the effects of bump foil structure, coating, and nominal clearance on the startup response of the bearings-rotor system are first investigated experimentally. Multi-objectives optimization mathematical model considering simultaneously lower power consumption, takeoff speed, and vibration is presented based on grey relational analysis. The optimal bearings-rotor system with radial trisection bump foil, coating MoS2, and nominal clearance of 50 μm is identified. Compared to average values, the power consumption of the optimal system is diminished by 27.2% while the takeoff speed and nonlinear vibration are reduced by 16.2% and 47.3%, respectively. These findings can be used to improve energy efficiency and startup performance for fuel cells. The systematic methodology proposed in this study can be extended to other investigations about the startup.

Hydrodynamic responses and layout optimization of wave energy converter arrays consisting of five-degree-of-freedom truncated cylinders in front of a vertical wall

The hydrodynamic responses and layout optimization of a group of cylindrical wave energy conversion devices (WEC) in front of a fully reflecting vertical wall are investigated. Each truncated floating cylinder can oscillate with five degrees of freedom, i.e., surge, sway, heave, roll, and pitch. Based on the linear water wave theory, an analytical solution is developed for the hydrodynamic problem. The results of specific parameter studies suggest that the wall reflection effect significantly improves the energy extraction performance of the WEC array with the appropriate parameter conditions. A multi-level optimization method based on a genetic algorithm is developed. This paper investigates the optimal layout of the six WEC arrays, composed of 2–7 buoys, respectively. Additionally, the impact of other degrees of freedom (DOFs), besides the heave mode, on the hydrodynamic performance of the array is investigated. For β ≤ π/12, there is no need to consider the impact of other DOFs on the energy extraction in heave mode. The dimensionless amplitudes of other DOFs gradually decrease as the equivalent constraint stiffness increases. For k0a > 1.0, the heave amplitude and energy capture performance of the WEC array are significantly smaller. However, the amplitudes of other DOFs still have considerable magnitudes for k0a > 1.0. Therefore, for the sea area with high-frequency incident waves (k0a > 1.0), setting up a power takeoff system on other DOFs of each buoy to extract energy is a feasible solution to improve the performance of the WEC array.

Embedding parametric resonance in a 2:1 wave energy converter to get a broader bandwidth

The effort to increase the converted power is a common challenge to players in the field of wave energy conversion, both academic and industrial. In the case devices are found to be prone to parametric resonance, it typically has a negative impact on power harvesting and may jeopardize the reliability of the device. This paper makes the case that parametric resonance is not a danger that should be avoided, but rather a chance to achieve a broader system response bandwidth and ultimately increase the amount of power available at the power take-off. Since a time-varying wetted surface causes the highly nonlinear phenomenon of parametric resonance, linear models are unable to fully capture this instability. As a result, nonlinear Froude–Krylov forces are herein implemented via a computationally effective method for prismatic floaters that is compatible with both exhaustive simulation methods and real-time computing, as the whole simulations runs up to 50 times faster than real-time. A novel pendulum-based device is intentionally defined to exhibit a 2:1 ratio between heave and pitch natural frequencies, causing parametric instability. Results demonstrate that linear models predict a single zone of meaningful potential power extraction around the pitch natural frequency, as expected; however, by using the designed attitude to develop parametric instability, a second additional region develops near the heave natural period. As a result, the free response bandwidth is in fact increased, making more energy available at the power take-off axis thanks to the nonlinear instability embedded in the wave energy converter.

Enhancing wave energy harvesting: Performance analysis of a dual chamber oscillating water column

This study investigates the hydrodynamic characteristics of a dual chamber oscillating water column (OWC) setup through a series of experiments, aiming to assess its potential for harvesting wave energy. The investigation involves varying frontwall drafts, power take-off (PTO) dampings, and incident wave conditions, while quantitatively evaluating the OWC system's performance using the capture width ratio (CWR) as the key metric. The findings indicate that the dual chamber configuration consistently exhibits enhanced performance for all combinations of orifice ratios, opening heights, and incident wave frequencies studied. As expected, increasing the opening height of the second chamber leads to improved performance of the OWC. Interestingly, the effect of varying applied PTO damping on improvement of CWR values is found to be relatively insensitive. The most significant improvements in CWR are observed for larger incident wave frequency values. For instance, for dimensionless wave frequency (Kh) values of 1.43 and 1.68, the CWR increases from 0.47 and 0.26 to 0.72 and 0.52, respectively, with the orifice ratio combination of τ1, 0.015, and τ2, 0.018. It is worth noting that the single chamber OWC operates most efficiently within the resonant frequency range of 0.94–1.23, while the dual chamber design extends this frequency range, enabling efficient wave energy capture over a wider spectrum of wave conditions. The dual chamber configuration offers considerable advantages for wave energy conversion applications by increasing the effective frequency bandwidth for optimal energy capture. The research findings suggest that this innovative design has the potential to significantly enhance the efficiency and adaptability of wave energy converters, contributing to the advancement of sustainable and renewable energy technologies.

Characteristics of a two-dimensional periodic wave energy converter array

This study concentrates on the hydrodynamics of wave energy converters (WECs) consisting of an array of periodic buoys. The interaction between the bodies and waves was solved by a precise analytical method. The intrinsic relationships between wave propagation, energy capture, and array response are analyzed. Bragg resonance was found not only in scattering waves but also in radiation waves. The radiation wave is affected by different power take-off (PTO) damping, which could lead to incoming waves travelling freely through the array or being prevented through the array. The results show that Bragg resonance reduces the extraction efficiency significantly, and each device is affected to a different degree. In addition, this study reveals that there is collective behavior in the periodic WECs, all bodies in the array oscillate with identical amplitude at the Bragg resonance frequencies. Furthermore, the parametric effects on the hydrodynamic performance are investigated. The result of the study may provide useful guidance for the practical design of the WEC array.

Machine learning-based diagnosis in wave power plants for cost reduction using real measured experimental data: Mutriku Wave Power Plant

In comparison to wind farms, the relative scarcity of actual operational data from wave power plants has contributed to a significant research gap in the areas of wave farm forecasting and cost reduction. In this context, this manuscript presents a new Machine Learning-based Power Take-Off (PTO) diagnosis for wave energy generation farms which has the potential to serve as an extensive reference for other wave energy farms and offer substantial benefits to both investors and policymakers involved in the advancement of the emerging wave technologies. The suggested method has been employed at the Mutriku Wave Power Plant (WWP) to facilitate the implementation of predictive maintenance strategies and reduce the Levelized Cost of Energy (LCoE). Hence, the research study considers two main extraction methods, namely, Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA), used to select the most relevant features for OWC diagnosis. In addition, two classification methods have been considered: Support Vector Machine (SVM) and Multi-Layer Perceptron (MLP) Artificial Neural Network (ANN). The obtained data show that, although both methods allow to achieve an effective performance with an excellent degree of accuracy, the ANN-based method presents better results with 98% accuracy against 81% for the SVM when using PCA extraction method. Then, the developed classification-based OWC diagnosis has been used for the development of a predictive maintenance strategy at the Mutriku WPP, analyzing its impact on the economic indicators. The results indicate that, using the proposed predictive maintenance strategy, the OpEx may be decreased down to 17%, downtime may be decreased down to 55% and plant availability may be better up to 95%, leading to a 5% LCoE reduction.

Development of Dibble Mechanism for Tractor Operated Two Row Semi – Automatic Vegetable Transplanter

Vegetables play a crucial role in daily human nutrition, and India ranks second in vegetable production globally. Despite this, manual transplantation is prevalent in India, characterized by labor-intensive practices. The study aimed to address this by developing a tractor-operated linear dibble mechanism for tomato and chili seedlings, designed with seedling spacings of 45 and 60 cm. The evaluation for optimal operating conditions included varying engine speeds (700, 800, 900, 1000 rpm), gear positions (L1, L2), and power take-off positions (P1, P2). Among these, L1P1 and L1P2, with seedling spacings of 57.57 cm and 45.20 cm respectively, aligned with the designed spacings, while L2P1 was excluded due to excessive dibble spacing. Evaluation concentrated on 800 and 1000 rpm, with 700 rpm causing jerks. The preference for 1000 rpm was driven by higher field capacity, as exceeding 1000 rpm posed difficulties for workers. Selections were made with a focus on achieving desired dibble spacings while taking into account worker comfort and field capacity.