Floating Oscillating Water Column Research Articles

Hydrodynamic Investigation and Energy Extraction from a Floating Oscillating Water Column

Hydrodynamic Investigation and Energy Extraction from a Floating Oscillating Water Column

Hydrodynamic performance of a hybrid system of a floating oscillating water column and a breakwater

The floating oscillating water column (FOWC) is an interesting wave energy extraction device attributable to its ability to be used in vast areas of the ocean, especially in offshore regions with large energy reserves. However, high construction costs render the technology less competitive when compared to other energy extraction systems. Adding a breakwater to the FOWC in a hybrid system helps to overcome this problem. Although many recent studies have focused on the hydrodynamic performance of a WEC-breakwater hybrid system, there is a dearth of research about a FOWC-breakwater hybrid system. This study looks into the hydrodynamic performance of a FOWC-breakwater hybrid system. Variables such as the wave period, gap between FOWC and breakwater, and breakwater size were tested to determine the optimal performance. A numerical wave tank with a k-epsilon turbulence model was used to calculate hydrodynamic coefficients such as the reflection, amplification, pressure, and transmission coefficients, energy extraction efficiency, gap wave elevation, and gap velocity ratio. The results demonstrate that the optimal performance for the studied FOWC-breakwater hybrid system was obtained for the highest wave frequency or lowest wave period, from the ranges studied. As for the FOWC-breakwater gap and breakwater size, a narrow gap and a small breakwater size, from the ranges studied, show a more favorable result.

CFD study of a Dual Chamber Floating Oscillating Water Column device

A comprehensive Computational Fluid Dynamics study of a Dual Chamber Floating Oscillating Water Column device fixed in space is presented. Firstly, the numerical wave flume model is calibrated and validated for a simple case of a floating box fixed in space. Spectral components of the vertical wave force on the box are evaluated and the Computational Fluid Dynamics numerical model uncertainty is assessed. Secondly, the numerical wave flume is used to study the hydrodynamics around the Dual Chamber Floating Oscillating Water Column. The wave forces acting on the device when fixed in space are calculated, also free surface elevations and air pressures inside the chambers are evaluated and the efficiency of the device is determined. The present numerical results show that the model can describe properly the two main components of the vertical force of a fixed floating box. Efficiency results of the device show that it is the same whether it is moving or not for the low wave height. For higher wave heights, the efficiency improves when the device is moving due to its resonance mechanisms.

Mooring Analysis of a Floating OWC Wave Energy Converter

This investigation focuses on the modelling of a floating oscillating water column (FOWC) wave energy converter with a numerical code (ANSYS AQWA) based on potential flow theory. Free-floating motions predicted by the numerical model were validated against experimental data extrapolated from a 1:36 scale model device in regular and irregular sea states. Upon validation, an assessment of the device’s motions when dynamically coupled with a four-line catenary mooring arrangement was conducted at different incident wave angles and sea states ranging from operational to survivable conditions, including the simulation of the failure of a single mooring line. The lack of viscosity in the numerical modelling led to overpredicted motions in the vicinity of the resonant frequencies; however, the addition of an external linear damping coefficient was shown to be an acceptable method of mitigating these discrepancies. The incident wave angle was found to have a limited influence on the magnitudes of heave, pitch, and surge motions. Furthermore, the obtained results indicated that the mooring restoring force is controlled by the forward mooring lines under the tested conditions.

Hydrodynamic optimisation of an axisymmetric floating Oscillating Water Column type wave energy converter with an enlarged inner tube

Wave Energy Converters (WECs) that are classified as Floating Oscillating Water Columns (FOWCs) of point absorber type are designed to achieve resonance, at frequencies that are favourable for wave energy extraction. The resonance frequency of a FOWC type WEC is highly dependent on the added mass of its OWC, which often requires a long spar tube to achieve well-performing hydrodynamics. A proposed measure for driving down the cost of the converter is to reduce its length, while finding another way of achieving the desired OWC added mass. Frequency-domain modelling and hydrodynamic optimisation of the Spar-buoy OWC was carried out for an enlarged spar tube segment inside the floater. Results are compared to those of buoys with equal spar and inner tube diameters. Frequency responses of optimised buoy geometries - with and without enlarged inner tubes - explore the differences in their hydrodynamics. Expanding the inner tube diameter of the Spar-buoy OWC may increase energy extraction by up to 6.7% and reduce the buoy mass by up to 11.4%, when operating in a Portuguese offshore location. A sensitivity analysis on mean annual pneumatic power to changes in inner tube diameter and spar tube length was performed.

Efficiency and Survivability of a Floating Oscillating Water Column Wave Energy Converter Moored to the Seabed: An Overview of the EsflOWC MaRINET2 Database

Floating oscillating water column (OWC) type wave energy converters (WECs), compared to fixed OWC WECs that are installed near the coastline, can be more effective as they are subject to offshore waves before the occurrence of wave dissipation at a nearshore location. The performance of floating OWC WECs has been widely studied using both numerical and experimental methods. However, due to the complexity of fluid–structure interaction of floating OWC WECs, most of the available studies focus on 2D problems with WEC models of limited degrees-of-freedom (DOF) of motion, while 3D mooring effects and multiple-DOF OWC WECs have not been extensively investigated yet under 2D and 3D wave conditions. Therefore, in order to gain a deeper insight into these problems, the present study focuses on wave flume experiments to investigate the motion and mooring performance of a scaled floating OWC WEC model under 2D wave conditions. As a preparatory phase for the present MaRINET2 EsflOWC (efficiency and survivability of floating OWC) project completed at the end of 2017, experiments were also carried out in advance in the large wave flume of Ghent University. The following data were obtained during these experimental campaigns: multiple-DOF OWC WEC motions, mooring line tensions, free surface elevations throughout the wave flume, close to and inside the OWC WEC, change in the air pressure inside the OWC WEC chamber and velocity of the airflow through the vent on top of the model. The tested wave conditions mostly include nonlinear intermediate regular waves. The data obtained at the wave flume of Ghent University, together with the data from the EsflOWC tests at the wave flume of LABIMA, University of Florence, provide a database for numerical validation of research on floating OWC WECs and floating OWC WEC farms or arrays used by researchers worldwide.

Experimental Study of a Moored Floating Oscillating Water Column Wave-Energy Converter and of a Moored Cubic Box

This paper describes experimental research on a floating moored Oscillating Water Column (OWC)-type Wave-Energy Converter (WEC) carried out in the wave flume of the Coastal Engineering Research Group of Ghent University. This research has been introduced to cover the existing data scarcity and knowledge gaps regarding response of moored floating OWC WECs. The obtained data will be available in the future for the validation of nonlinear numerical models. The experiment focuses on the assessment of the nonlinear motion and mooring-line response of a 1:25 floating moored OWC WEC model to regular waves. The OWC WEC model motion has 6 degrees of freedom and is limited by a symmetrical 4-point mooring system. The model is composed of a chamber with an orifice on top of it to simulate the power-take-off (PTO) system and the associated damping of the motion of the OWC WEC model. In the first place, the motion response in waves of the moored floating OWC WEC model is investigated and the water surface elevation in the OWC WEC chamber is measured. Secondly, two different mooring-line materials (iron chains and nylon ropes) are tested and the corresponding OWC WEC model motions and mooring-line tensions are measured. The performance of these two materials is similar in small-amplitude waves but different in large wave-amplitude conditions. Thirdly, the influence of different PTO conditions is investigated by varying the diameter of the top orifice of the OWC WEC model. The results show that the PTO damping does not affect the OWC WEC motion but has an impact on the water surface elevation inside the OWC chamber. In addition, an unbalanced mooring configuration is discussed. Finally, the obtained data for a moored cubic model in waves are presented, which is a benchmarking case for future validation purposes.

沖合浮体式波力装置「マイティーホエール」

“Mighty Whale” is a floating Oscillating Water Column (OWC) type wave power device which has a ability absorbing the energy from ocean waves and dissipate the wave height behind the device. So, it can supply the electric energy and can produce the calm sea area for utilizing of coastal zone. Japan Marine Science and Technology Center (JAMSTEC) has put into the project for R&D on Mighty Whale since 1987, and is desighing of the prototype from 1995. So for, we could have a insight on basic behavior, characteristices, safety and economics of the Mighty Whale by theoretical, numerical estimation and evaluation, experimental studies in water tank using the scale models and foundamental discussion on the general design. The construction of the prototype of Mighty Whale will be finished by May 1999, and the open sea test will be carried out by the end of March 2001.This paper describes the project and the design of the prototype of Mighty Whale.