Hydrodynamics of Multiple Floating Point-Absorber Wave Energy Systems With Inter-Body and Bottom Slack-Mooring Connections

2009 ◽  
Author(s):  
Pedro C. Vicente ◽  
Anto´nio F. de O. Falca˜o ◽  
Lui´s M. C. Gato ◽  
Paulo A. P. Justino
Keyword(s):  
Numerical Simulations ◽  
Wave Energy ◽  
Vertical Plane ◽  
Energy Resource ◽  
Irregular Waves ◽  
Wave Direction ◽  
Mooring Lines ◽  
Incoming Wave ◽  
Slack Mooring ◽  
Offshore Wave

Point absorbers constitute an important class of offshore wave energy converters. If employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. This kind of mooring arrangement is addressed in the paper in a simplified way by considering an array of two buoys. Two opposed slack-mooring lines connect the floater pair to the bottom, while a third line, from whose mid-point hangs a weight, connects the two buoys pulling them towards each other. The centres of the buoys and the mooring lines are in a vertical plane parallel to the incoming wave direction, so that body and mooring motions are two-dimensional. The whole system — buoys, moorings and power take-off systems (PTOs) — is assumed linear, so that a frequency domain analysis may be employed. In the numerical simulations, two identical hemispherical buoys oscillate in heave and surge, acted upon by the waves, the mooring system and their PTOs. The PTO consists of a linear damper whose force is proportional to the heave velocity. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of the converters, for different mooring and PTO parameters. Comparisons with the simpler cases of one single buoy in the moored and unmoored situations show significant differences.

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

Energies ◽  
2018 ◽  
Vol 11 (11) ◽  
pp. 2899 ◽  
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.

Latching Control of an OWC Spar-Buoy Wave Energy Converter in Regular Waves

2012 ◽  
Author(s):  
J. C. C. Henriques ◽  
A. F. O. Falcão ◽  
R. P. F. Gomes ◽  
L. M. C. Gato
Keyword(s):  
Water Column ◽  
Wave Energy ◽  
Phase Control ◽  
Wave Energy Converter ◽  
Irregular Waves ◽  
Wave Direction ◽  
Regular Waves ◽  
Energy Converter ◽  
Chamber Volume ◽  
The Time Domain

The present paper concerns an OWC spar-buoy, possibly the simplest concept for a floating oscillating-water-column (OWC) wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control by latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular such compressibility may remove the constraint of latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modelling is performed in the time domain for a given device geometry, and includes the numerical optimization of the air turbine rotational speed, chamber volume and latching parameters. Results are obtained for regular waves.

scholarly journals Modeling Link for Ocean Wave Energy of Point Absorbers: A Mathematical Framework

2017 ◽  
Author(s):  
Lorenzo Baños Hernandez
Keyword(s):  
Wave Energy ◽  
Radiation Force ◽  
Ocean Wave ◽  
Irregular Waves ◽  
Mathematical Framework ◽  
Wave Direction ◽  
Regular Waves ◽  
Wave Regime ◽  
Ocean Wave Energy ◽  
Point Absorbers

This compendium presents new mathematical techniques for modeling Point Absorbers. A combined frequency-time domain framework is developed. It is used to simulate the energy generated by the wave farms. With Matlab and Fortran as a base, this leads to obtain physical variables of primary importance, namely position, velocity and power to energy net balance relationships of absorption. Integration of different degrees of freedom with heave as main executable leads in turn to a single buoy motion focus. Acquisition of the needed hydrodynamic coefficients is provided through application of potential field solvers with Boundary Element Methodology background. Initially, this Wave-to-motion model is validated by comparison with previous experimental results for a floating cone cylinder shape (Buldra-FO3). A single, generic, vertical floating cylinder is contemplated then, that responds to the action of the passing regular waves excitation. Later, two equally sized vertical floating cylinders aligned with the incident wave direction are modeled for a variable distance between the bodies. For both unidirectional regular and irregular waves as an input in deep water, we approximate the convolutive radiation force function term through the Prony method. By changing the spatial disposition of the axisymmetric buoys, using for instance triangular or rectangular shaped arrays of three and four bodies respectively, the study delves into motion characteristics for regular waves. The results highlight efficient layouts for maximizing the energy production whilst providing important insights into their performance, revealing displacement amplification- and capture width-ratios, while deriving in possible interpretations of scenarios related to the known park effect. These terms are encompassed by the novelty of a new conceptual Post-Processing methodology in the field, which leads to obtain an optimal distance for the separated bodies with effective energy absorption in a regular wave regime. The main objective is to generate a tendency within the hydrodynamic field of study, which is the Wave to motion perspective. More generally, this computational excursion envisions and depicts potential fields of study, which will surely enhance new connections and link this renewable energy form. Therefore, this research delves first into the historical and technical background on Ocean Wave Energy. Next, it is in the section regarding Materials and Methods, where boundaries and related equations are introduced step by step, together with latter mentioned case scenarios, and their corresponding configuration parameters. A separate section frames then the scope of results, while finally, there is an ensuing discussion and conclusions for evaluation assessment.

scholarly journals The Economic Feasibility of Floating Offshore Wave Energy Farms in the North of Spain

Energies ◽  
2020 ◽  
Vol 13 (4) ◽  
pp. 806 ◽  
Author(s):  
Laura Castro-Santos ◽  
Ana Bento ◽  
Carlos Guedes Soares
Keyword(s):  
Wave Energy ◽  
Net Present Value ◽  
Energy Resource ◽  
Economic Feasibility ◽  
Wave Energy Converter ◽  
Energy Converter ◽  
The North ◽  
Cost Of Energy ◽  
Offshore Wave ◽  
North Of Spain

A technique to analyse the economic viability of offshore farms composed of wave energy converters is proposed. Firstly, the inputs, whose value will be considered afterwards in the economic step, was calculated using geographic information software. Secondly, the energy produced by each wave converter was calculated. Then the economic factors were computed. Finally, the restriction that considers the depth of the region (bathymetry) was put together with the economic outputs, whose value depends on the floating Wave Energy Converter (WEC). The method proposed was applied to the Cantabric and Atlantic coasts in the north of Spain, a region with a good offshore wave energy resource. In addition, three representative WECs were studied: Pelamis, AquaBuoy and Wave Dragon; and five options for electric tariffs were analysed. Results show the Wave Energy Converter that has the best results regarding its LCOE (Levelized Cost of Energy), IRR (Internal Rate of Return) and NPV (Net Present Value), and which area is best for the development of a wave farm.

Physical Modelling of Motions and Forces on a Moored Ship at the Leixões Port

2019 ◽  
Vol 396 ◽  
pp. 60-69
Author(s):  
João Alfredo Santos ◽  
Liliana V. Pinheiro ◽  
Hossam S. Abdelwahab ◽  
Conceição Juana E.M. Fortes ◽  
Francisco G.L. Pedro ◽  
...  
Keyword(s):  
Physical Modelling ◽  
Irregular Waves ◽  
Scale Model ◽  
Wave Direction ◽  
Mooring Lines ◽  
Detailed Model ◽  
Significant Wave ◽  
Measurement Analysis ◽  
Wave Heights ◽  
Spring System

This paper describes the physical model, experimental setup and tests performed at the Portuguese Civil Engineering Laboratory (LNEC), to study the motions and forces of a moored ship at the Leixões port, for different sea states in irregular waves. The tests were carried out at one of the wave tanks of LNEC, where the Leixões port layout was implemented at scale 1:80 with the detailed model similar to the prototype bathymetry and surrounding structures. The moored ship is a 3.43 m long scale model of the well-known “Esso Osaka” tanker and is moored to the pier A of the oil terminal at 0.135 m draft. Several types of measurements were recorded in this study. The free-surface elevation and wave direction were measured with a set of resistive wave gauges. The wave velocities at the entrance of the harbour were measured with an acoustic Doppler velocimeter. Motions of the moored ship were measured with the OptiTrackTM motion capture system whereas forces on fenders and mooring lines were measured with load cells attached to a complex spring system developed at LNEC. Several tests were carried out for a number of incident sea states characterized by a JONSWAP spectrum, with different significant wave heights and peak periods. The measurement, analysis and results obtained for the incident wave conditions characterized by a significant wave height of 6 m and a peak wave period of 14 s are presented and discussed in this paper.

scholarly journals A Design Outline for Floating Point Absorber Wave Energy Converters

2014 ◽  
Vol 6 ◽  
pp. 846097 ◽  
Author(s):  
Mohammed Faizal ◽  
M. Rafiuddin Ahmed ◽  
Young-Ho Lee
Keyword(s):  
Wave Energy ◽  
Power Output ◽  
Energy Resource ◽  
Wave Structure ◽  
Floating Point ◽  
Mooring Lines ◽  
Point Absorber ◽  
Wave Energy Converters ◽  
Incident Waves ◽  
Energy Converters

An overview of the most important development stages of floating point absorber wave energy converters is presented. At a given location, the wave energy resource has to be first assessed for varying seasons. The mechanisms used to convert wave energy to usable energy vary for different wave energy conversion systems. The power output of the generator will have variations due to varying incident waves. The wave structure-interaction leads to modifications in the incident waves; thus, the power output is also affected. The device has to be stable enough to prevent itself from capsizing. The point absorber will give optimum performance when the incident wave frequencies correspond to the natural frequency of the device. The methods for calculating natural frequencies for pitching and heaving systems are presented. Mooring systems maintain the point absorber at the desired location. Various mooring configurations as well as the most commonly used materials for mooring lines are discussed. An overview of scaled modelling is also presented.

An Atlas of the Wave-Energy Resource in Europe

1996 ◽  
Vol 118 (4) ◽  
pp. 307-309 ◽  
Author(s):  
M. T. Pontes ◽  
G. A. Athanassoulis ◽  
S. Barstow ◽  
L. Cavaleri ◽  
B. Holmes ◽  
...  
Keyword(s):  
Wave Energy ◽  
Energy Resource ◽  
Wave Model ◽  
Wave Climate ◽  
Weather Forecasts ◽  
Wave Measurements ◽  
Directional Wave ◽  
Offshore Wave ◽  
Climate Statistics

An atlas of the European offshore wave energy resource, being developed within the scope of a European R&D program, includes the characterization of the offshore resource for the Atlantic and Mediterranean coasts of Europe in addition to providing wave-energy and wave-climate statistics that are of interest to other users of the ocean. The wave data used for compiling the Atlas come from the numerical wind-wave model WAM, implemented in the routine operation of the European Centre for Medium Range Weather Forecasts (ECMWF), in addition to directional wave measurements from the Norwegian offshore waters.

scholarly journals Hydrodynamic Performance of a Pitching Float Wave Energy Converter

Energies ◽  
2020 ◽  
Vol 13 (7) ◽  
pp. 1801
Author(s):  
Yong Ma ◽  
Shan Ai ◽  
Lele Yang ◽  
Aiming Zhang ◽  
Sen Liu ◽  
...  
Keyword(s):  
Energy Conversion ◽  
Frequency Domain ◽  
Wave Energy ◽  
Center Of Mass ◽  
Hydrodynamic Performance ◽  
Irregular Waves ◽  
Wave Energy Conversion ◽  
Wave Direction ◽  
Improved Stability ◽  
The Time Domain

This study analyzes the hydrodynamic performance and application of a pitching float-type wave energy conversion device under complex sea conditions in the South China Sea. Potential flow theory and ANSYS-AQWA software are used to establish a method for analyzing hydrodynamic performance in both time and frequency domains, as well as the various factors that influence hydrodynamic performance. The frequency domain characteristics of the conversion device are explored, as well as the time-domain characteristics when exposed to regular and irregular waves. The results show that the frequency domain of hydrodynamic performance conforms to the requirements of an offshore mobile platform. A mooring point that is closer to the center of mass leads to improved stability of the conversion device. The angle arrangement of the anchor-chain mooring method fully conforms to safety requirements. When the wave direction is 45°, the conversion device is highly stressed and its movement is the most strenuous; however, the device can operate safely and stably under all working conditions. These results provide a significant reference for expanding the wave-energy capture range and the hydrodynamic performance of floating wave-energy conversion devices.

scholarly journals Assessment of Offshore Wave Energy Potential in the Croatian Part of the Adriatic Sea and Comparison with Wind Energy Potential

Energies ◽  
2019 ◽  
Vol 12 (12) ◽  
pp. 2357 ◽  
Author(s):  
Andrea Farkas ◽  
Nastia Degiuli ◽  
Ivana Martić
Keyword(s):  
Renewable Energy ◽  
Wind Energy ◽  
Wave Energy ◽  
Adriatic Sea ◽  
Energy Resource ◽  
Wind Farms ◽  
Offshore Wind ◽  
Energy Potential ◽  
Wind Energy Potential ◽  
Offshore Wave

The European Union is a leading patron for the introduction of renewable energy, having set a target that renewable sources will represent at least 27% of total energy consumption by the year 2030. Presently, the most significant Croatian renewable resource is hydropower, which is presently at its peak and will not develop further because of limited hydro resources. Therefore, the share of electricity generation from onshore wind farms in Croatia during in recent years has grown significantly. However, as the Croatian government has already made most of the concessions for possible locations of wind farms, the aim of the present study is to evaluate a different renewable energy resource, wave energy. An assessment of the offshore wave energy potential in the Croatian part of the Adriatic Sea is performed using data taken from WorldWaves atlas (WWA). WWA is based on satellite measurements, validated against buoy measurements and reanalysed by numerical wave modelling. This assessment was done for seven locations, and mean yearly energy is calculated for two offshore wave energy converters. Capacity factors were calculated for annual as well as for seasonal levels, and it was concluded that the bulk of the energy would be generated in autumn and winter. The most probable extreme significant wave height was determined at the investigated locations as well. Furthermore, the offshore wind energy potential was evaluated and compared to the wave energy potential.

scholarly journals Coupled Mooring Analyses for the WEC-Sim Wave Energy Converter Design Tool

2016 ◽  
Author(s):  
Senu Sirnivas ◽  
Yi-Hsiang Yu ◽  
Matthew Hall ◽  
Bret Bosma
Keyword(s):  
Numerical Method ◽  
Wave Energy ◽  
Coupled Model ◽  
The Body ◽  
Wave Energy Converter ◽  
Irregular Waves ◽  
Lumped Mass ◽  
Energy Converter ◽  
Mooring Lines ◽  
Point Absorber

A wave-energy-converter-specific time-domain modeling method (WEC-Sim) was coupled with a lumped-mass-based mooring model (MoorDyn) to improve its mooring dynamics modeling capability. This paper presents a verification and validation study on the coupled numerical method. First, a coupled model was built to simulate a 1/25 model scale floating power system connected to a traditional three-point catenary mooring with an angle of 120 between the lines. The body response and the tension force on the mooring lines at the fairlead in decay tests and under regular and irregular waves were examined. To validate and verify the coupled numerical method, the simulation results were compared to the measurements from a wave tank test and a commercial code (OrcaFlex). Second, a coupled model was built to simulate a two-body point absorber system with a chain-connected catenary system. The influence of the mooring connection on the point absorber was investigated. Overall, the study showed that the coupling of WEC-Sim and the MoorDyn model works reasonably well for simulating a floating system with practical mooring designs and predicting the corresponding dynamic loads on the mooring lines. Further analyses on improving coupling efficiency and the feasibility of applying the numerical method to simulate WEC systems with more complex mooring configuration are still needed.

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