Wave Energy Extraction Research Articles

Effects of power take-off parameters and harvester shape on wave energy extraction and output of a hydraulic conversion system

Considering nonlinear hydrodynamic forces of a harvester in irregular waves and a nonlinear hydraulic power take-off system, a time-domain nonlinear motion model and nonlinear hydraulic power take-off models are established to investigate the harvester motion response, the hydraulic system dynamic performance, the power extraction, the motor output power and efficiency. Matlab/Simulink is used to establish and simulate the system models. The convolution identification of nonlinear radiation force is based on a state-space model. The parameters of the state-space model and the infinity frequency added mass are fitted based on the least square method. The nonlinear hydrostatic storing force is considered for cone and hemisphere. The Pierson–Moskowitz 2-parameter spectrum is adopted for the irregular waves. The Coulomb friction force and viscous friction force based on the pressure difference and hydraulic cylinder parameters are modeled in the power take-off force. Under a given sea state, for cone, cylinder and hemisphere, the optimal hydraulic power take-off parameters are predicted based on a genetic algorithm for the maximum motor output power. The effects of the hydraulic cylinder piston working area, the motor displacement, the high pressure accumulator pre-charge pressure and initial volume, the significant wave height and energy period, the harvester diameter, draft and shape on the extracted power, the extraction efficiency, the motor output power and the hydraulic conversion efficiency are investigated. The extraction ability of the cone harvester is the highest among three shapes under the same diameter and draft. The conversion efficiency increases as the motor displacement decreases.

Hydrodynamic Performance of an Asymmetry OWC Device Mounted on a Box-Type Breakwater

To share the construction and maintenance cost, an asymmetric oscillating water column (OWC) device integrated with a pile-fixed box-typed offshore breakwater is considered experimentally and numerically. A fully nonlinear numerical wave tank is established and validated with the open source solver OpenFOAM. The effects of the width and draft of rear box, and the incident wave height on the wave energy conversion efficiency, reflection and transmission coefficients, and energy dissipation coefficient are examined. In addition, the superiority of the present coupling system, compared to the traditional box-type breakwater, is discussed. With well comparisons, the results show that the existence of the rear breakwater is beneficial for the formation of partial standing waves and further wave energy conversion. In the range of wave heights tested, the higher the incident wave height, the larger the energy absorption efficiency except for the short-wave regimes. Moreover, the OWC-breakwater coupling system can obtain a similar wave blocking ability to the traditional one, and simultaneously extract wave energy and decrease wave reflection.

Experimental investigation of a triple pontoon wave energy converter and breakwater hybrid system

In this paper, a novel hybrid wave energy converter (WEC)-floating breakwater system consisting of three floating pontoons with power take-off (PTO) modules is developed to extract wave energy from heave motion and attenuate waves to protect the coast. Unlike previous studies of this subject, the present three pontoons are in close proximity so that they may share the same foundation to reduce the cost further and both pontoons with step change drafts and same drafts are investigated. A systematic experimental study was carried out in a wave flume to examine the hydrodynamic characteristics of the hybrid system under regular waves. The experimental results are used to assess the influences of the WEC-breakwater integration and the configuration of pontoons on the hydrodynamic performance of the hybrid system as a WEC and a breakwater. It was found that the integration of multiple energy-extracting pontoons is robust in smoothing out the power output fluctuations, and the floating pontoons with same drafts gave arise to higher energy conversion efficiency while the floating pontoons with step change drafts are more effective in attenuating wave energy. A parametric analysis is able to identify the balanced performance of the system as a WEC and a breakwater.

Modelling and analysis of inertia self‐tuning phase control strategy for a floating multi‐body wave energy converter

Wave energy converters (WECs) exhibit higher energy capture capacity when the wave frequency is close to the nature frequency of the device. A possible way to enhance energy capture efficiency is to modify the nature frequency of a WEC to match the incoming wave frequency. This paper presents an inertia self-tuning phase control strategy which can optimize the wave energy capture efficiency of a floating multi-body (FMB) WEC by regulating automatically the total mass of the capture energy part according to the incoming wave conditions. A variable-frequency inertia self-tuning control system is designed to conduct the proposed control strategy. The mathematical model of rotational motion of the FMB WEC is built and the dynamic performance of the control system is analysed in detail. Simulation results illustrate the proposed control strategy can achieve a phase optimization and hence significantly improve wave energy extraction of the FMB WEC in both regular and irregular waves. For regular waves, the maximum capture width ratio)CWR( under the control strategy is 7.3 times higher than that no control strategy is used. For irregular waves, the maximum CWR under the control strategy is 4.4 times that without the control strategy at the same wave frequency.

Study on the capture spectrum of wave energy conversion

As various WECs are invented and tested, establishing wave energy farm of multiple devices is becoming a possible way to realize large-scale development of wave energy resources. Previous studies into array effects have focused on a single type of device. However, such an approach doesn't consider the potential advantages of using different device types that operate at different efficiencies for different parts of a wave spectrum within a single array to maximize the extraction of wave energy. To realize this approach, a reliable method of quantifying the energy capture performance of any WEC in real sea state is necessary. To define the energy-capture unit in an array, a new concept, named Energy Capture Spectrum (ECS) is presented. It can evaluate the energy capture performance of any WECs in real sea state by converting the power signal of the WEC into an equivalent fluctuation signal related to the incident waves at the location of the WEC and finally generates the Energy Capture Spectrum (ECS) of the WEC to compare with the incident wave spectrum. The definition of the ECS and theoretical derivation is also presented in this paper. A specific test method with no restrictions on WEC types is developed. To validate the correctness of the ECS and to study the influence on the ECS by the PTO and draught, a physical model test based on heaving buoy is introduced and analyzed. The conclusion of this paper figures out the characteristics of the ECS and optimization method of the heaving buoy device accordingly.

Wave energy extraction for an array of dual-oscillating wave surge converter with different layouts

Array configuration of oscillating wave surge converter (OWSC) devices in nearshore is a preferable option for realizing a cost-balance of extracting wave energy and reducing installation expense due to closer installed place to coastline. The goal of the present work is to assess energy extraction of an array of dual-OWSC system with different layout schemes in comparison with an isolated OWSC, which can be regarded as a guideline for multi-array configuration in realistic wave farm. The coupled three-dimensional (3-D) hydrodynamic model is established based on the potential flow theory with fully nonlinear boundary condition in time domain. A non-dimensional approach is conducted to focusing on the accurate effects of multi-body interaction, wave nonlinearity, wave resonance, mechanical damping, layout scheme and oblique incidence as optimization design. For a front-back array system, wave resonance in dual-module gap enhances significantly the energy extraction of the front OWSC but does not contribute much to that of the back OWSC. Furthermore, wave resonance in wide gap has a positive effect on the capture efficiency in large wave periods. An in-line array system has a beneficial performance in small wave periods, while a staggered array system realizes more energy extraction in resonance region. A strong wave disturbance between flap sides of an in-line and staggered system, leads to the increase of energy extraction for the back OWSC with imposing small incident wave heading. Therefore, the combination of multi-triple-array OWSC with different gap distances will provide a desirable configuration which is independent on oblique wave conditions.

Cost-Benefit Analysis of a Hybrid Biophysical Approach to Wave Energy Extraction: Bio-Oscillator

A cost-benefit analysis (CBA) is described for a novel wave energy converters (WEC) design based on a marine hybrid bio-structure—a combination of macroalgae, shellfish or other species on a built frame. The Bio-Oscillator design utilises a hard “skeleton” (e.g., carbon fibre, wood) on which biological organisms (e.g., shellfish, large macroalgae) are grown. As waves pass by, the load generated by the oscillating drag and inertia is transferred through mooring lines to power takeoff technology. This novel approach essentially reverses the typical marine engineering view that “bio-fouling is bad” and instead leverages off the added-drag of biological growth on structures. The approach results in a structure that is largely biodegradable, naturally self-replicating and synergistic with the background environment, self-de-risking in terms of failure impact and can leverage off its own form to enhance energy capture beyond a conventional design. This reduces impact while connecting with conventional marine industries such as aquaculture. A CBA examines the economic pros and cons of this approach, focusing on installation and material costs, along with benefits from synergistic production. The analysis suggests that in addition to typical wave energy obstacles (e.g., cable length, capture width, and power take off) the benefits (biodegradability, harvestability, and carbon reduction) of replacing much of the mass of the structure with living biological material can be included.

Performance characteristics and parametric analysis of a novel multi-purpose platform combining a moonpool-type floating breakwater and an array of wave energy converters

Integration of Wave Energy Converters (WECs) with floating breakwater system provides a multi-function solution to wave energy extraction and offshore infrastructural protection. The contribution of this work is to guide the optimal size and configuration of a multi-purpose platform including a moonpool-type floating breakwater and an array of heaving oscillating-buoy (OB) WECs. The investigation is performed using a developed time-domain numerical wave tank (NWT) based on the three-dimensional (3D) potential flow theory with fully nonlinear boundary conditions on transient wetted body surfaces and free surfaces. The comparison of the hydrodynamic performance among the multi-purpose platform, the isolated array WECs, and the isolated floating breakwater are examined. The internal fluid motion in the moonpools has a positive effect on the wave energy absorption of WECs, which in turn enhances the wave attenuation capacity of the floating breakwater. WECs with larger diameter have a larger water-plane area, which leads to more extracted wave energy. The wave nonlinearity reduces the optimal PTO damping value and has an adverse effect on the wave energy extraction. However, when wave nonlinearity becomes prominent, the wave attenuation capacity is improved with increasing PTO damping. For an unequal layout of moonpools, the thinner moonpools are the major contributor to wave energy extraction, especially in the short wave region. As a result of mass exchange of fluid from the moonpool to the outer domain, the multi-purpose platform indicates favorable performance of wave energy absorption. The novel floating system makes the utilization of wave energy over a wider frequency range.

Theoretical analysis on hydrodynamic performance for a dual-chamber oscillating water column device with a pitching front lip-wall

For the sake of extracting wave energy over a broader range of wave frequencies, dual-chamber OWC devices are more beneficial than the single-chamber one. In this paper, the concept of a dual-chamber OWC device with a pitching front lip-wall is proposed and by utilizing the matched eigenfunction method along the adjacent interfaces, present theoretical model developed under the framework of potential flow theory can be numerically solved. To determine the optimal turbine parameters combination of front and rear chamber for the optimal wave power extraction efficiency, a numerical method of successive approximation is employed. Parameters, including lip-wall drafts, installed location of mid lip-wall and angle spring stiffness are changed to explore effects on the hydrodynamic performance. It is found that when the device is designed with an asymmetry structure configuration, i.e., narrow front but wide rear, is more desirable and the optimal breadth ratio is 3:7. There exists a medium lip-wall draft condition which can satisfy stable operation and efficient performance simultaneously by including the pitching motion of front lip-wall. In addition, a relatively smaller spring stiffness (within the range of 105) is more recommended as the pitching motion of lip-wall can be adequately utilized to obtain a more satisfactory energy efficiency curve.

Effects of narrow gap wave resonance on a dual-floater WEC-breakwater hybrid system

The effects of gap wave resonance on the performance of a dual-floater hybrid system consisting of an oscillating-buoy type wave energy converter (WEC) and a floating breakwater are important for the design of such a hybrid system. This paper investigates the gap wave resonance by employing a two-dimensional numerical wave flume developed using the Star-CCM + software. The maximum wave elevation in the WEC-breakwater gap and the effects of the gap wave resonance on the performance of the dual-floater hybrid system were studied. The influence of the WEC motion and the geometrical parameters of the hybrid system on the maximum wave elevation were analyzed. The maximum gap wave elevation is essentially controlled by the vertical velocity of the free surface in the WEC-breakwater gap. The gap wave resonance was found to significantly improve the wave energy extraction performance of the hybrid system. This allowed the maximum conversion efficiency to exceed the well-known limit of 0.50 for a symmetric body in single degree-of-freedom motion. The wave resonance frequencies in the WEC-breakwater gap decreased with the increase of the gap width and the WEC draft. Due to the energy extraction of the WEC, the horizontal and vertical forces on the breakwater were reduced by up to 0.79 and 0.59, respectively.

Feasibility Study for the Extraction of Wave Energy along the Coast of Ensenada, Baja California, Mexico

Mexico is one of the countries with the highest emissions of greenhouse gases. In order to reduce the emission of contaminants due to fossil fuels, the state of Baja California has recently launched several research projects for the optimization of facilities for the exploitation of renewable sources, and in particular wave energy. In this work a first-level feasibility study of energy extraction from wave motion is presented for the Ensenada coast, along a complex distance of more than 200 km. The methodology proposed provides good spatial and temporal resolution for wave heights and periods calculation and consequently for the wave power. The methodology is based on the application of the coupled Simulated Waves Nearshore and Advanced Circulation (SWAN + ADCIRC) model for generation, propagation and dissipation of waves. To take into account the meteorological variability within a 21-year dataset, the Typical Meteorological Year method was applied. Results show that overall, the most persistent energy potential during the year is >2 kW/m, with peaks of 5 and 10 kW/m during few months. Given the theoretical energy potential calculated, the Ensenada coast could produce hundreds of GWh per year. The proposed methodology can be applied for the exploration of other coasts with energy potential.

Optimal Sliding Mode Control of Permanent Magnet Direct Drive Linear Generator for Grid-Connected Wave Energy Conversion

the key goal of this article is on the design and optimum sliding mode control for Grid-Connected direct drive extraction method of ocean wave energy by Multi-Objective Particle Swarm Optimization (MOPSO). A Linear Permanent Magnet Generator simulates the ocean wave energy extraction system, driven by an Archimedes Wave Swing. Uncontrolled three-phase rectifiers, a three-level buck-boost converter and 3 level neutral point clamped inverter are planned grid integration of Wave Energy Conversion device. The technique monitors the three-level buck-boost converter service cycle linked to the PMLG output terminals and decides the optimum switching sequence of 3 level neutral point clamped inverter to enable the grid relation. Simulations using Matlab/Simulink were carried out to test working of the wave energy converter after the suggested optimal control method was applied under various operating settings. Various simulation test results indicate that the proposed optimum control system is tested in both normal and irregular ocean waves. And it has been shown that the control method of the MOPSO sliding mode is ideal for maximizing energy transfer efficiency. Better voltage management at the DC-link and for achieving greater controllability spectrum was accomplished by the proposed Duty-ratio optimal control system.

Optimal Sliding Mode Control of Permanent Magnet Direct Drive Linear Generator for Grid-Connected Wave Energy Conversion

the key goal of this article is on the design and optimum sliding mode control for Grid-Connected direct drive extraction method of ocean wave energy by Multi-Objective Particle Swarm Optimization (MOPSO). A Linear Permanent Magnet Generator simulates the ocean wave energy extraction system, driven by an Archimedes Wave Swing. Uncontrolled three-phase rectifiers, a three-level buck-boost converter and 3 level neutral point clamped inverter are planned grid integration of Wave Energy Conversion device. The technique monitors the three-level buck-boost converter service cycle linked to the PMLG output terminals and decides the optimum switching sequence of 3 level neutral point clamped inverter to enable the grid relation. Simulations using Matlab/Simulink were carried out to test working of the wave energy converter after the suggested optimal control method was applied under various operating settings. Various simulation test results indicate that the proposed optimum control system is tested in both normal and irregular ocean waves. And it has been shown that the control method of the MOPSO sliding mode is ideal for maximizing energy transfer efficiency. Better voltage management at the DC-link and for achieving greater controllability spectrum was accomplished by the proposed Duty-ratio optimal control system.

Inter- and intra-annual wave energy resource assessment in the south-western Black Sea coast

This study focuses on the inter- and intra-annual variation in theoretical wave power along the south-western coast of the Black Sea. Long-term (1979–2009) hindcast developed using the calibrated three-layer nested SWAN model was used for a detailed assessment of wave energy resource. Annual change, monthly and seasonal wave power variability indices were evaluated, and the Optimal Hotspot Index (OHI) was calculated for 62 sites along the coast to rate the hotspot locations. The assessment shows the highest wave energy resources in western Karaburun with higher variability. However, the western Sinop sites have lower wave energy resources compared to western Karaburun sites, in spite of considering the lower variability, these sites are more appropriate for wave energy extraction. The location with the highest potential (KA5 100 m depth) has an average of 8.4 kW/m and a maximum of 1015 kW/m wave power capacity. The east sites of Sinop and Filyos are comparatively less dynamic. Moreover, it was found that the average peak values of wave heights are in February, and the lowest sea state was in June. 2003 was the most, and 1989 was the least energetic years in the 31-year period.

Analytical investigation on the hydrodynamic performance of a multi-pontoon breakwater-WEC system

Based on linear potential flow theory and the matching eigenfunction expansion method, an analytical model is developed to investigate the hydrodynamics of two-dimensional multi-pontoon floating breakwaters that also work as oscillating buoy wave energy converters. The pontoons are constrained to independent heave motion and the linear power take-off damping is used to calculate the absorbed power. The proposed model is verified using the principle of energy conservation. Performance of the system with different numbers of pontoons is studied, and the results show that the wave attenuation performanceper, the energy capture performance, and the effective frequency bandwidth are superior in the multi-pontoon system as compared to the single-pontoon systems with the same volume. Bragg resonance dominates the wave energy extraction performance at certain frequencies, which reduces the hydrodynamic efficiency. Conversely, in the frequency region away from Bragg resonance, the hydrodynamic efficiency is enhanced due to the constructive hydrodynamic interactions of the multi-body system.

Oscillating Water Column Wave Energy Converter for Low Wave Height Conditions

For continuous and uninterrupted extraction of energy from marine wave motion, majority of wave energy extraction systems employ bi-directional turbines. The aerodynamic loss due to symmetrical placement of guide vanes causes a major loss in power output of the system. To optimize this problem, researchers have developed and tested a variety of oscillating water column air turbines. Most commonly used are the axial turbines because of their simplicity and operational ease. The present work focuses upon design of an oscillating water column energy extraction system based on uni-directional axial impulse turbine with a novel rectifying system arrangement which primarily focuses to minimize the losses due to downstream guide vanes and in turn shoot up the power output for low wave height conditions. A numerical model was developed and investigated using RANS equations and k-omega SST turbulence model. The results obtained portrays that the proposed flap based bi-directional impulse system is operationally viable and produces substantially greater power output in comparison to conventional bi-directional and uni-directional turbine systems under low wave height conditions. A hybrid choking scheme was followed which resulted in additional power output. Results show that full choking of the idle turbine is ensured during the inhalation cycle, which was not possible in existing models. During the exhalation cycle, additional power output was also obtained from the secondary turbine i.e., front end turbine during exhalation cycle. The proposed system generates maximum total power output at around flow coefficient of 1.

Wave Energy Extraction by Flexible Floaters

We present a novel mathematical model to investigate the extraction of wave power by flexible floaters. The model is based on the method of dry modes, coupled with a matched eigenfunction expansion. Our model results compare satisfactorily with preliminary data obtained from a demonstrator device, developed at the University of Groningen. We show that the role of elasticity is to increase the number of resonant frequencies with respect to a rigid body, which has a positive effect on wave power output. The mathematical model is then extended to irregular incident waves, described by a JONSWAP spectrum. Our results show that the peak capture factors decrease in irregular waves, as compared to the monochromatic case. However, the system becomes more efficient at non-resonant frequencies. This work highlights the need to scale-up experimental investigations on flexible wave energy converters, which are still a small minority, compared to those on rigid converters.

Hydroelastic Response of a Flexible Submerged Porous Plate for Wave Energy Absorption

The application of flexible horizontal porous structure has a significant impact on the design of breakwaters and wave energy absorption devices for coastal protection and wave energy extraction, respectively. This type of structure is more economical compared to a rigid type structure. Therefore, the hydroelastic response of the flexible porous structure can be investigated to widen the influence of structural deformations in design parameters. This paper presents a generalized expansion formula for the said problem based on Green’s function in the water of finite depth (FD) and infinite depth (ID). The series form of the velocity potentials for the wave-maker problem is also derived using Green’s second identity. The derived expansion formula is applied to a real physical problem and the analytical solution is obtained utilizing a matched eigenfunction expansion method under velocity potential decompositions. The convergence study of the series solution is checked. The present results and the published experimental datasets, as well as analysis, are compared. The effect of design parameters on the hydroelastic response of the submerged flexible porous plate is analyzed. It is observed that the analysis of the results will be useful for gaining insight into how to design a wave energy absorption device.

Three-dimensional hydrodynamic analysis and efficiency optimization of submerged multi-cylindrical oscillating wave surge converter

The intense surge motion, the low installation costs and the less environmental threat, promote the development of the nearshore wave energy technologies which are often classified as Oscillating wave surge converters (OWSCs). This paper investigates the hydrodynamic performance of a multi-cylindrical fully submerged OWSC by developing a 3-D numerical model based on fully nonlinear potential flow theory and time-domain higher-order element method (HOBEM). A local change in the geometric configuration is verified as an effective approach to enhance wave energy extraction in long waves. Then, it is found that a larger incident wave height not only reduces the peak value of the capture efficiency, but it also leads to a change in the real resonant frequency. The geometrical parameters have a significant influence on the energy extraction, with the thicker, wider and shallower device demonstrating both higher average generated power and capture efficiency in low-frequency region. Meanwhile, the maximum efficiency is mainly related to the optimal PTO parameters. A two-value optimization strategy on PTO damping is applied to industriously maximize the absorbed energy over whole wave frequency region, and is easier to be implemented in practical application compared with ideal optimal scheme at each frequency component of incident wave spectrum.

Sustainability of wave energy resources in the South China Sea based on five decades of changing climate

In this study, 55 years of simulated waves using SWAN were utilized to investigate the long-term change of the resources in the South China Sea as a new measure for locating suitable areas for wave energy extraction. The results showed that there are mainly three regions with higher potential in their nearshore areas: Luzon Strait, southern coasts of China, and southeast of Vietnam. However, the intra-annual fluctuation of the wave power is considerably high in southeast Vietnam ranging from 2 to 40 kW/m, and the long-term change of wave energy resources during the 55 years is high in Luzon Strait (around −0.2 kW/m per year). Moreover, the decadal variability has a significant increase in the ‘70s and then, a slight decrease during the next decades in wave power potential in southeast Vietnam, while the southern coasts of China include the most stable wave resources in both short and long-term evaluations. The investigation in the selected points indicated that the rate of change of wave power is higher in areas with higher wave energy. The results of this study highlight the importance of including the long-term change in detecting the appropriate areas for wave energy exploitation in line with sustainable development.