On the Design of an Integrated System for Wave Energy Conversion Purpose with the Reaction Mass on Board

In this paper, we investigate the design of an integrated system consisting of two non-rigidly connected bodies: A floating buoy and an emerged offshore structure. When waves excite the buoy to oscillate, the relative motion between the two bodies are converted to useful energy through a spring damper system, resulting in wave energy being absorbed. The parameter to design includes the mass and underwater shape of the buoy. The spring stiffness of the power take-off (PTO) system is constrained to be non-negative with the concerns of complexity in implementation and system stability. Results suggest that a larger mass of the buoy is advantageous due to smaller optimal spring stiffness and damping coefficient of the PTO system, more absorbed wave power, and less motion amplitude of the two bodies. A favorable underwater shape of the buoy is characterized by large diameter to draft ratio, with the section profile preferring a circle or square rather than an equilateral triangle. Investigations on the designed buoy in irregular waves show that the integrated system presents its peak power absorption within the common range of energy period, and the motion amplitude of the offshore structure is larger than the wave amplitude in a certain range of sea states.

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A Control System For Constrained Wave-Powered Oceanographic Buoys

International Marine Energy Journal ◽

10.36688/imej.2.51-61 ◽

2020 ◽

Vol 2 (1 (Nov)) ◽

pp. 51-61

Author(s):

Shangyan Zou ◽

Ossama Abdelkhalik

Keyword(s):

Relative Motion ◽

Wave Energy ◽

Control Strategy ◽

Real Data ◽

Nonlinear Damping ◽

Irregular Waves ◽

Coastal Observatory ◽

Switching Control Strategy ◽

Displacement Constraints ◽

Two Bodies

Wave energy can be used to power oceanographic buoys. A new switching control strategy is developed in this paper for a two-body heaving wave energy converter that is composed of a floating cylinder and two rigidly connected submerged hemispheres. This control strategy is designed to prevent excessive displacement of the floating buoy that may occur due to the actuator force. This control strategy switches the control between a multi-resonant controller and a nonlinear damping controller, depending on the state of the system, to account for displacement constraints. This control strategy is developed using a one-degree-of-freedom dynamic model for the relative motion of the two bodies. Estimation of the relative motion, needed for feedback control, is carried out using a Kalman filter. Numerical simulations are conducted to select the proper mooring stiffness. The controller is tested with stochastic models of irregular waves in this paper. The performance of the controller with different sea states is discussed. Annual power production using this control strategy is presented based on real data in 2015 published by Martha's Vineyard Coastal Observatory.

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Numerical Study on Hydrodynamic Responses of a Two-Body System in Side-by-Side Operation

Volume 7: Ocean Engineering ◽

10.1115/omae2016-54625 ◽

2016 ◽

Author(s):

Shaowu Ou ◽

Shixiao Fu ◽

Wei Wei ◽

Tao Peng ◽

Xuefeng Wang

Keyword(s):

Numerical Study ◽

Low Frequency ◽

Optimal Operation ◽

Hydrodynamic Interactions ◽

Irregular Waves ◽

Mooring System ◽

Time Varying ◽

Body System ◽

The Time Domain ◽

Two Bodies

Typically, in some side-by-side offshore operations, the speed of vessels is very low or even 0 and the headings are manually maneuvered. In this paper, the hydrodynamic responses of a two-body system in such operations under irregular seas are investigated. The numerical model includes two identical PSVs (Platform Supply Vessel) as well as the fenders and connection lines between them. A horizontal mooring system constraining the low frequency motions is set on one of the ships to simulate maneuver system. Accounting for the hydrodynamic interactions between two bodies, 3D potential theory is applied for the analysis of their hydrodynamic coefficients. With wind and current effects included, these coefficients are further applied in the time domain simulations in irregular waves. The relevant coefficients are estimated by experiential formulas. Time-varying loads on fenders and connection lines are analyzed. Meanwhile, the relative motions as well as the effects of the hydrodynamic interactions between ships are further discussed, and finally an optimal operation scheme in which operation can be safely performed is summarized.

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Coupling Methodology for Studying the Far Field Effects of Wave Energy Converter Arrays over a Varying Bathymetry

Energies ◽

10.3390/en11112899 ◽

2018 ◽

Vol 11 (11) ◽

pp. 2899 ◽

Cited By ~ 13

Author(s):

Gael Verao Fernandez ◽

Philip Balitsky ◽

Vasiliki Stratigaki ◽

Peter Troch

Keyword(s):

Numerical Simulations ◽

Wave Energy ◽

Near Field ◽

Coupled Model ◽

Propagation Model ◽

Irregular Waves ◽

Computational Time ◽

Far Field ◽

Field Effects ◽

Numerical Tool

For renewable wave energy to operate at grid scale, large arrays of Wave Energy Converters (WECs) need to be deployed in the ocean. Due to the hydrodynamic interactions between the individual WECs of an array, the overall power absorption and surrounding wave field will be affected, both close to the WECs (near field effects) and at large distances from their location (far field effects). Therefore, it is essential to model both the near field and far field effects of WEC arrays. It is difficult, however, to model both effects using a single numerical model that offers the desired accuracy at a reasonable computational time. The objective of this paper is to present a generic coupling methodology that will allow to model both effects accurately. The presented coupling methodology is exemplified using the mild slope wave propagation model MILDwave and the Boundary Elements Methods (BEM) solver NEMOH. NEMOH is used to model the near field effects while MILDwave is used to model the WEC array far field effects. The information between the two models is transferred using a one-way coupling. The results of the NEMOH-MILDwave coupled model are compared to the results from using only NEMOH for various test cases in uniform water depth. Additionally, the NEMOH-MILDwave coupled model is validated against available experimental wave data for a 9-WEC array. The coupling methodology proves to be a reliable numerical tool as the results demonstrate a difference between the numerical simulations results smaller than 5% and between the numerical simulations results and the experimental data ranging from 3% to 11%. The simulations are subsequently extended for a varying bathymetry, which will affect the far field effects. As a result, our coupled model proves to be a suitable numerical tool for simulating far field effects of WEC arrays for regular and irregular waves over a varying bathymetry.

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BREAKWATER ARMOR DISPLACEMENT THRESHOLDS: A POSSIBLE CORRELATION WITH CUMULATIVE WAVE ENERGY

Coastal Engineering Proceedings ◽

10.9753/icce.v19.186 ◽

1984 ◽

Vol 1 (19) ◽

pp. 186

Author(s):

Daniel L. Behnke ◽

Frederic Raichlen

Keyword(s):

Wave Energy ◽

Wave Train ◽

Wave Length ◽

Simple Wave ◽

Wave Theory ◽

Irregular Waves ◽

Reconstruction Technique ◽

Regular Wave ◽

Regular Waves ◽

Three Dimensional Model

An extensive program of stability experiments in a highly detailed three-dimensional model has recently been completed to define a reconstruction technique for a damaged breakwater (Lillevang, Raichlen, Cox, and Behnke, 1984). Tests were conducted with both regular waves and irregular waves from various directions incident upon the breakwater. In comparison of the results of the regular wave tests to those of the irregular wave tests, a relation appeared to exist between breakwater damage and the accumulated energy to which the structure had been exposed. The energy delivered per wave is defined, as an approximation, as relating to the product of H2 and L, where H is the significant height of a train of irregular waves and L is the wave length at a selected depth, calculated according to small amplitude wave theory using a wave period corresponding to the peak energy of the spectrum. As applied in regular wave testing, H is the uniform wave height and L is that associated with the period of the simple wave train. The damage in the model due to regular waves and that caused by irregular waves has been related through the use of the cumulative wave energy contained in those waves which have an energy greater than a threshold value for the breakwater.

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OPTIMIZATION OF THE WAVECAT WAVE ENERGY CONVERTER

Coastal Engineering Proceedings ◽

10.9753/icce.v33.structures.5 ◽

2012 ◽

Vol 1 (33) ◽

pp. 5 ◽

Cited By ~ 5

Author(s):

Hernan Fernandez ◽

Gregorio Iglesias ◽

Rodrigo Carballo ◽

Alberte Castro ◽

Marcos Sánchez ◽

...

Keyword(s):

Physical Model ◽

Wave Energy ◽

Model Tests ◽

Irregular Waves ◽

Intermediate Water ◽

Plan View ◽

Santiago De Compostela ◽

Wave Energy Converters ◽

Physical Model Tests ◽

The Difference

The development of efficient, reliable Wave Energy Converters (WECs) is a prerequisite for wave energy to become a commercially viable energy source. Intensive research is currently under way on a number of WECs, among which WaveCat©—a new WEC recently patented by the University of Santiago de Compostela. In this sense, this paper describes the WaveCat concept and its ongoing development and optimization. WaveCat is a floating WEC intended for operation in intermediate water depths (50–100 m). Like a catamaran, it consists of two hulls—from which it derives its name. The difference with a conventional catamaran is that the hulls are not parallel but convergent; they are joined at the stern, forming a wedge in plan view. Physical model tests of a 1:30 model were conducted in a wave tank using both regular and irregular waves. In addition to the waves and overtopping rates, the model displacements were monitored using a non-intrusive system. The results of the physical model tests will be used to validate the 3D numerical model, which in turn will be used to optimize the design of WaveCat for best performance under a given set of wave conditions.

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Numerical and experimental analysis of the power output of a point absorber wave energy converter in irregular waves

Ocean Engineering ◽

10.1016/j.oceaneng.2015.11.011 ◽

2016 ◽

Vol 111 ◽

pp. 483-492 ◽

Cited By ~ 13

Author(s):

M.T. Rahmati ◽

G.A. Aggidis

Keyword(s):

Wave Energy ◽

Power Output ◽

Experimental Analysis ◽

Wave Energy Converter ◽

Irregular Waves ◽

Energy Converter ◽

Point Absorber

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

Volume 10: Ocean Renewable Energy ◽

10.1115/omae2018-77686 ◽

2018 ◽

Cited By ~ 1

Author(s):

Claudio A. Rodríguez ◽

F. Taveira-Pinto ◽

P. Rosa-Santos

Keyword(s):

Wave Energy ◽

Transfer Functions ◽

Nonlinear Effects ◽

Model Tests ◽

Experimental Results ◽

Irregular Waves ◽

Scale Model ◽

Experimental Assessment ◽

Simplified Approach ◽

Wave Basin

A new concept of wave energy device (CECO) has been proposed and developed at the Hydraulics, Water Resources and Environment Division of the Faculty of Engineering of the University of Porto (FEUP). In a first stage, the proof of concept was performed through physical model tests at the wave basin (Rosa-Santos et al., 2015). These experimental results demonstrated the feasibility of the concept to harness wave energy and provided a preliminary assessment of its performance. Later, an extensive experimental campaign was conducted with an enhanced 1:20 scale model of CECO under regular and irregular long and short-crested waves (Marinheiro et al., 2015). An electric PTO system with adjustable damping levels was also installed on CECO as a mechanism of quantification of the WEC power. The results of regular waves tests have been used to validate a numerical model to gain insight into different potential configurations of CECO and its performance (López et al., 2017a,b). This paper presents the results and analyses of the model tests in irregular waves. A simplified approach based on spectral analyses of the WEC motions is presented as a means of experimental assessment of the damping level of the PTO mechanism and its effect on the WEC power absorption. Transfer functions are also computed to identify nonlinear effects associated to higher waves and to characterize the range of periods where wave absorption is maximized. Furthermore, based on the comparison of the present experimental results with those corresponding to a linear numerical potential model, some discussions are addressed regarding viscous and other nonlinear effects on CECO performance.

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Experimental Analysis of a Novel Adaptively Counter-Rotating Wave Energy Converter for Powering Drifters

Journal of Marine Science and Engineering ◽

10.3390/jmse7060171 ◽

2019 ◽

Vol 7 (6) ◽

pp. 171 ◽

Cited By ~ 1

Author(s):

Guoheng Wu ◽

Zhongyue Lu ◽

Zirong Luo ◽

Jianzhong Shang ◽

Chongfei Sun ◽

...

Keyword(s):

Wave Energy ◽

Wave Energy Converter ◽

Irregular Waves ◽

Surface Velocity ◽

Small Scale ◽

Power Constraint ◽

Energy Converter ◽

Wave Tank ◽

Wide Range ◽

Rotating Wave

Nowadays, drifters are used for a wide range of applications for researching and exploring the sea. However, the power constraint makes it difficult for their sampling intervals to be smaller, meaning that drifters cannot transmit more accurate measurement data to satellites. Furthermore, due to the power constraint, a modern Surface Velocity Program (SVP) drifter lives an average of 400 days before ceasing transmission. To overcome the power constraint of SVP drifters, this article proposes an adaptively counter-rotating wave energy converter (ACWEC) to supply power for drifters. The ACWEC has the advantages of convenient modular integration, simple conversion process, and minimal affection by the crucial sea environment. This article details the design concept and working principle, and the interaction between the wave energy converter (WEC) and wave is presented based on plane wave theory. To verify the feasibility of the WEC, the research team carried out a series of experiments in a wave tank with regular and irregular waves. Through experiments, it was found that the power and efficiency of the ACWEC are greatly influenced by parameters such as wave height and wave frequency. The maximum output power of the small scale WEC in a wave tank is 6.36 W, which allows drifters to detect ocean data more frequently and continuously.

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