Relative Capture Width Research Articles

Hydrodynamic and energy-harvesting performance of an isolated oscillating water column device: An experimental study

Most previous experimental studies on the hydrodynamic and aerodynamic performances of an oscillating water column (OWC) wave energy converter were conducted in a wave flume. In this study, an experimental study on an isolated OWC plant is carried out in a wave tank. The interactions between the OWC system and incident regular and irregular waves are analyzed by measuring the surrounding wave fields using a wave gauge array. The wave distribution patterns and reflection ratios of the OWC model are presented. In addition, the free-surface elevation, air pressure varying amplitude, turbine torque output, and relative capture width ratios and efficiencies in regular and irregular wave scenarios are measured and analyzed. The operating performances of the OWC model are analyzed and compared to the results derived from the flume tests. An overall performance reduction of over 20% can be observed in the wave tank tests. In addition, the representative results of the overall capture width ratio of the OWC model derived from irregular wave tests are corrected from the scaling effects caused by the similarity contradiction.

Wave energy converter efficiency based on wave transmission and relative capture width performance

A series of quantitative analyses were performed to identify the wave properties and potential, by means of records collected from an offshore buoy in North-West Ireland. Based on the data collected, a series of quantitative analyses was conducted to determine the dominant wind directions and wave properties on an annual basis. In addition, the wave power is computed based on relevant wave heights and periods, and the Pierson-Moskowitz spectral model was used to generate the maximum wave energy spectra for each year. The results show that waves with wave powers of around 100 kW/m were mostly approaching eastward at a rather narrow frequency. In order to compare the relative capture width and the power absorption capacity of three floating structures, the Wave Dragon, Board Net Breakwater, and Cylindrical Floating Breakwater are analyzed. Also outlined is the impact of the transmission coefficient on the effectiveness of wave energy converters (WEC) throughout the energy harvesting process. This was accomplished by fusing information from three distinct field investigations and experimental research with four different wave transmission coefficient models. The results show that as the wave steepness increases, the transmission coefficient decreases and the hydrodynamic performance of wave energy converters increases. Also, it is found that the hydrodynamic efficiency of wave energy converters is higher in summer than in winter, and the Wave Dragon is the most efficient wave energy converter in regard to relative capture width and power absorption.

Experimental investigation on an OWC wave energy converter integrated into a floating offshore wind turbine

A multi-functional floating foundation integrating an Oscillating Water Column (OWC) Wave Energy Converter (WEC) with a Floating Offshore Wind Turbine (FOWT) is a promising design for achieving cost reduction, adaptability and stability improvements. Yet, such a multi-functional integration is still in a demonstration phase and additional investigations are required to reach industrial maturity. This work assesses the hydrodynamic performance of such a design by carrying out a series of carefully instrumented scaled model tests. Two distinct sets of experimental models were designed accordingly, i.e., a pure FOWT without the OWC device, and the FOWT incorporating the OWC device. The multi-functional foundation is either fixed or allowed to move in heave mode along four vertical sliding rails. The introduction of the OWC device enhances the stability of the FOWT in heave, with a maximum motion reduction rate of 54.1%. The chamber air orifice size and the foundation geometries were optimized. The OWC with an air chamber opening ratio of 3.0% produces the maximum relative capture width. A larger chamber draft and breadth lead to a lower chamber resonance frequency. The main objective of this paper is to guide the design and application of the OWC device integrated into a floating offshore wind turbine.

The Prospect of Combining a Point Absorber Wave Energy Converter with a Floating Offshore Wind Turbine

With recent advances in offshore floating wind and wave energy technology, questions have emerged as to whether the two technologies can be combined to reduce their overall levelised cost of energy. In this paper, the potential for combining a floating offshore wind turbine to a point absorbing wave energy converter is investigated. The focus of the investigation is how much power might be produced by a combined floating wind and wave energy converter system, and the resultant changes in motion of the floating wind platform. A model for the combined wave and wind system is developed which uses the standardised NREL OC3 5 MW spar type wind turbine and a cylindrical buoyant actuator (BA), which is attached to the spar via a generic wave power take-off system (modelled as a spring-damper system). Modelling is conducted in the frequency domain and the tests span a wide range of parameters, such as wave conditions, BA sizes, and power take-off coupling arrangements. It is found that the optimal (with respect to power production) BA size is a draft and radius of approximately 14 m. It is found that this BA can theoretically produce power in the range of 0.3 to 0.5 MW for waves with a significant wave height of 2 m, and has the potential to produce power greater or near to 1 MW for waves with a significant wave height of at least 3 m. However, it is also found that, in terms of the relative capture width, significantly smaller BAs are optimal, and that these smaller BA sizes less significantly alter the motion of the floating wind platform.

Nonlinear hydrodynamic modeling of an offshore stationary multi-oscillating water column platform

The hydrodynamics of an offshore stationary platform consisting of four cylindrical Oscillating Water Column (OWC) Wave Energy Converters (WECs) are considered in this paper. Based on the potential flow theory, a second-order Higher-Order Boundary Element Method (HOBEM) model is developed to simulate the wave interaction with the multi-OWC platform in time domain. Laboratory tests of a carefully instrumented scale model are also conducted, and the results from the two methods are compared for the validation. It is found that the linear model overestimates the relative capture width of the OWC device by approximately 10% around the resonant frequency. Subsequently, the hydrodynamic properties of the multi-OWC platform are investigated systematically in terms of the air pressure and free surface oscillations in the chamber, as well as the relative capture width. The isolated cylindrical OWC device is also considered for the comparison. The effects of the incident wave direction, the column and row spacing between the OWC devices are explored. The surface elevation distribution inside and around the OWC device is discussed. The occurrence of the maximum free surface elevation inside the chambers is found to lag behind the crest of the incident wave. The optimum angle of wave incidence in terms of energy capture is β = 0, i.e. multi-OWC platform aligned with the incoming waves. A strategic design of the column and row spacing can lead to a significant improvement for capturing wave energy.

A Control strategy to improve the efficiency of point absorber wave energy converters in complex sea environments

A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function. The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.

Numerical modelling on the performance of a rolling-type wave energy converter with mooring system

This paper presents the performance results of the numerical study on the three-dimensional rolling-type wave energy converter with the mooring system. The novel device combines an established and reliable mooring system, making it suitable for deeper sea utilisation. The shape of the device is based on the renowned Salter’s Duck, which is capable for converting wave energy into mechanical energy through rolling back and forth. The results are obtained using the Panel Element Method package AQWA, based on linear wave theory and potential flow theory. The hydrodynamic behaviour of the device is assessed in the frequency domain, including the additional mass, the radiation damping and the wave exciting force. In the time domain, the coupling of the device motion and the dynamic response of the mooring system is performed, allowing comparisons with experimental work on a 1:16 scale model. The tension of the anchor chain is reported in detail. The relative capture width of the device under different wave conditions is also examined.

The application of modal analysis to the design of multi-mode point absorber wave energy converters

Point absorbers are one of the most common wave energy converters, which are typically designed to extract power primarily from heave motion. Pure heave motion of an axisymmetric wave energy converter results in a relative capture width limited to a third of the maximum possible. Over the past few decades, an increasing amount of attention has been given to the design of point absorbing wave energy converters operating in multiple oscillation modes, in an attempt to more efficiently extract power from waves. However, it is not a trivial task as wave energy converters operating in multiple modes demonstrate complex coupled vibrational characteristics across several degrees of freedom. This paper addresses this challenge using modal analysis, a modern approach developed for determining, improving and optimising dynamic characteristics of complex engineering systems. Case studies are conducted on three multi-mode submerged point absorber designs, each with distinct modal behaviour to show the generality and efficacy of the approach. Results show that in combination with knowledge of wave power absorption in the frequency-domain, modal analysis can be used as an effective analytical tool to evaluate the vibrational characteristics and the power absorption potential of the multi-mode system, as well as to explore the corresponding working principles and the physical limits of the design.

Enhancement of hydrodynamic performance of an Oscillating Water Column with harbour walls

Abstract A comprehensive experimental investigation on the effect of resonant length and the opening angle of harbour walls integrated with an Oscillating Water Column (OWC) under random sea state has been carried out. The random sea state is entitled to be the reference parameter for analysing the performance of the OWC under controlled conditions. The results on the variations of volume flux of water inside the OWC chamber, pneumatic pressure and relative capture width are depicted as a function of relative water depth for different configurations of the harbour walls in a dimensionless form. The presence of the harbour walls has enhanced the performance characteristics of the OWC. Further, the length and inclination of the harbour walls were varied to achieve a relative capture width, RCW (ratio of output power to the input power) of 75% more than that from the OWC without harbour walls. The details of the models, experimental set-up, testing procedure, results and discussion on the aforementioned study are presented.

Performance of an array of oscillating water column devices integrated with an offshore detached breakwater

A systematic experimental investigation on the effect of different spacing between an array of oscillating water column (OWC) devices integrated with offshore detached breakwater (ODBW) in terms of their hydrodynamic performance has been studied. A Froude scale of 1:20 was chosen for integrating five similar devices of OWC to the ODBW (OWCBW) and a series of experiments was carried out in a shallow wave basin. Three different centre to centre spacings, S, i.e., one, two, and three times the width of the OWC model were considered. The hydrodynamic performance was studied in terms of non-dimensional lip wall pressure, wave amplification factor, non-dimensional air pressure, capture width, and relative capture width. The OWCBW system with thrice the width of the model spacing exhibits a better performance. The convergence of water waves in front of the OWC devices due to obstruction from rubble mound breakwaters to progressive waves enhances the concentration of energy in front of the array of OWC devices. It facilitates the system to absorb a larger amount of wave power of about 2.2 times the given input power at natural frequency of the system. This confirms that the array of OWC devices exhibits better performance than in isolation.

Real-time latching control strategies for the solo Duck wave energy converter in irregular waves

As a point absorber, the solo Duck wave energy converter (WEC) shows high power capture efficiency within a narrow bandwidth around the natural period. In this paper, real-time latching control is applied to the solo Duck WEC in irregular waves to improve its performance in sea states away from the natural period. Two predictive latching control strategies, in which one is close-to-optimal and the other is sub-optimal, and one non-predictive strategy are considered. The improvement of the WEC performance due to latching control is studied. Compared to the performance under simple resistive control, the three latching control strategies show almost equivalent control effect, leading to an average increase of the maximum relative capture width by around 70% and an average decrease of the optimal power take-off (PTO) damping coefficient by around 60%. Since the non-predictive strategy requires no prediction of future excitation force and WEC motion, it can be identified as the best choice for the solo Duck WEC under latching control. Although latching control leads to significant increase of fatigue load on the WEC hull like other advanced controls, it does not cause additional fatigue damage to the PTO.

Latching and Declutching Control of the Solo Duck Wave-Energy Converter with Different Load Types

The solo duck wave-energy converter (WEC) captures power in a point absorber manner, hence it exhibits high power-capture efficiency within only a narrow bandwidth. Passive control is characterized by a unidirectional power flow, and thus its engineering implementation can be simplified. In this paper, two typical passive control strategies, latching and declutching control, are applied to the solo duck WEC to improve its power-capture performance at wave periods larger and smaller than the natural period of the WEC, respectively. Special attention is paid to the peak value of instantaneous WEC performance parameters, including the peak motion excursion, the peak power take-off (PTO) moment, and the peak-to-average power ratio, when the captured power is maximized. Performance differences between the linear and coulomb loads are also investigated. Results show that both latching and declutching control can effectively improve captured power, but also incidentally increase the peak motion excursion and peak-to-average power ratio. When under latching and declutching control, the coulomb load leads to the same maximum relative capture width and peak motion excursion as the linear load, but presents smaller peak PTO moment and peak-to-average power ratio than the linear load, hence making the coulomb load the better choice for the solo duck WEC.

Optimizing the Performance of Solo Duck Wave Energy Converter in Tide

The high efficiency performance of the Edinburgh Duck wave energy converter (WEC) in 2D regular wave tests makes it a promising wave energy conversion scheme. A solo Duck WEC will be able to apply the point absorber effect to further enhance its performance. Since released degree of freedom will decrease the efficiency, a Duck WEC with fixed pitching axis will be a better option. However, for fixed supported WECs, tide is a non-ignorable consideration. In this paper, a movable mass method is utilized in the whole tidal range to not only balance the Duck to appropriate beak angles, but also follow the variation of hydrodynamic coefficients to keep cancelling the reactance of the system impedance so that complex conjugate control can be realized to optimize the power capture performance of the Duck WEC in tide. Results show that the beak angle should be adjusted to as large a value as possible so that the response amplitude of the Duck at maximum relative capture width will be reasonable small, and the lowest weight of the movable mass is found when its designed position locates at the center of the Duck profile.

Control of Wave Energy Converters for Maximum Power Absorption with Time Domain Analysis

A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function. The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.

Point Absorber Wave Energy Converters in Regular and Irregular Waves with Time Domain Analysis

A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function.The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.

Time domain prediction of power absorption from ocean waves with latching control

A three-dimensional panel method using Neumann–Kelvin method is presented for the transient wave-body interaction problems in order to absorb maximum power from the sea. The exact initial boundary value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function. The hydrodynamics part of the solution including radiation and diffraction problem is solved as impulsive velocity problem. A discrete control of latching is used to increase the bandwidth of the efficiency of the wave energy converters (WEC). When latching control is applied to WEC in the case of off-resonance condition it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving hemisphere as a point-absorber WEC is used for the numerical prediction of the different parameters. The calculated hydrodynamics coefficients, response amplitude operator, absorbed power, relative capture width of this device compared with analytical and other published results.

Analytical solutions for the flow fields near funnel‐and‐gate reactive barriers with hydraulic losses

Permeable reactive barriers (PRBs) are a passive in situ technology that is based on the interception and physical, chemical, and/or biological remediation of a contaminant plume through installation of reactive material in an aquifer. The present work is an extension and generalization of a previous paper and derives analytical expressions for flow fields toward PRBs in two dimensions on the basis of the conformal mapping approach. Considered is the classic funnel‐and‐gate configuration with perpendicular funnel arms (PFGs) as well as PRBs with velocity equalization walls. While the aquifer is assumed to be homogeneous in a uniform far field, the hydraulic conductivity of the reactive material is allowed to take arbitrary values above or below the aquifer conductivity. At the up‐ and down‐gradient interfaces between reactor and aquifer, highly permeable gravel packs are assumed to establish constant head conditions. The flow fields are analyzed regarding the widths and shapes of respective capture zones as functions of PRB type and dimensions, reactor hydraulic resistance (including flow divergence and longevity), and ambient groundwater flow direction. Expressions for discharge fields are developed as needed for particle‐tracking algorithms. Charts of relative capture width are given to facilitate PRB design and may be included in more comprehensive PRB design/optimization approaches. An efficient approach is presented to estimate reactor conductivity and capture flow from monitoring the hydraulic loss at the reactor. Inherent assumptions and results are validated against numerical flow field solutions and water level field data for the PFG PRB at the Moffett Federal Airfield, California.

Numerical and experimental modelling of a modified version of the Edinburgh Duck wave energy device

This paper presents recent numerical studies conducted at The University of Edinburgh related to a modified version of the Edinburgh duck wave energy converter. Its purpose is seawater desalination rather than electricity production, but under the assumptions taken the results can be applied to both versions. From the design point of view, the key innovation is the change of the shape of the device. Wave energy can be converted into useful work by the same pitching motion by means of a circular cylinder with an offset axis of rotation without the front beak, allowing significant cost reduction. The results to be presented were obtained using the Boundary Element Method (BEM) package WAMIT, based on linear wave theory. The hydrodynamic behaviour of the device is assessed, allowing comparisons with experimental work on a 1:33 scale model. Full-scale figures are also presented. The influences of both the position of the axis of rotation and the submergence ratio are evaluated. A total of ten different configurations, each for two different submergence ratios, are analyzed. Focus is given to the hydrodynamic coefficients, the wave exciting force, the response amplitude operator (body motions), and the relative capture width.