• Energy Research

The Oscillating Water Column (OWC) is considered to be one of the most promising Wave Energy Converter (WEC) concepts in terms of practicality, survivability and efficiency. To date, most research has focussed on single–chamber devices, but it is suggested that significant increases in energy extraction can be achieved from dual–chamber devices. This paper investigates, using well–validated 2D and 3D CFD models based on the Reynolds Averaged Navier–Stokes (RANS) equations and the Volume of Fluid (VOF) method, the hydrodynamic performance of various dual–chamber offshore–stationary OWC–WECs and compares the results to single–chamber OWC devices. The effect of chamber lip draught, chamber length in wave propagation direction and the power take–off (PTO) damping on the capture width ratio (power extraction efficiency) of each OWC device was studied over a wide range of wave periods for a constant regular wave height. It was found that all the parameters tested were important for the design of efficient OWC devices, and the dual–chamber device provided superior results to the single–chamber device, especially over the intermediate and long wave periods where the capture width ratio could be improved by a maximum of about 140%; hence extracting significantly more energy. The effectiveness of using the dual–chamber system was more obvious when 3D effects were considered. The findings of this paper contribute towards the design and operation of practical OWC devices for efficiently utilizing ocean waves to produce electricity.

Understanding the hydrodynamic interactions between ocean waves and the oscillating water column (OWC) wave energy converter is crucial for improving the device performance. Most previous relevant studies have focused on testing onshore and offshore OWCs using 2D models and wave flumes. Conversely, this paper provides experimental results for a 3D offshore stationary OWC device subjected to regular waves of different heights and periods under a constant power take–off (PTO) damping simulated by an orifice plate of fixed diameter. In addition, a 3D computational fluid dynamics (CFD) model based on the RANS equations and volume of fluid (VOF) surface capturing scheme was developed and validated against the experimental data. Following the validation stage, an extensive campaign of computational tests was performed to (1) discover the impact of testing such an offshore OWC in a 2D domain or a wave flume on device efficiency and (2) investigate the correlation between the incoming wave height and the OWC front wall draught for a maximum efficiency via testing several front lip draughts for two different rear lip draughts under two wave heights and a constant PTO damping. It is found that the 2D and wave flume modelling of an offshore OWC significantly overestimate the overall power extraction efficiency, especially for wave frequencies higher than the chamber resonant frequency. Furthermore, a front lip submergence equal to the wave amplitude affords maximum efficiency whilst preventing air leakage, hence it is recommended that the front lip draught is minimized.

A comprehensive understanding of the flow within an oscillating water column (OWC) is essential to improving the efficiency of the underwater geometry of this type of wave energy converter. This study aims to investigate the impact of the sidewalls on the flow and the changes in flow across the device. Scale model experiments were performed on a forward facing bent duct OWC to generate two-dimensional (2D) particle image velocimetry (PIV) velocity fields at four longitudinal planes across the width of the device. These fields showed there was substantial variation in the flow at the different planes, with a transfer of flow from the central planes during inflow towards the sidewalls during outflow, in addition to the outer planes spending a greater proportion of time in outflow and vice versa. This identified locations at which there is an even distribution between inflow and outflow. Divergence of the velocity fields was calculated to identify non-2D aspects to the flow revealing a vortex forming on the inner lip of the sidewall demonstrating the devices ability to utilise the volume outside of the extents of the sidewalls to generate power. This study has shown there are significant three-dimensional aspects to the flow within and around the device which must be considered when designing the underwater geometry.

Ocean waves are the most important exciting source acting on marine structures such as ships, offshore platforms, wave energy converters and wavebreakers. In order to efficiently design the aforementioned structures, accurate modelling of these waves is of importance. In this paper a two dimensional Numerical Wave Tank (NWT) has been established based on the Reynoldsaveraged NavierStokes (RANS) equations for viscous, incompressible fluid and Volume of Fluid (VOF) method and a commercial software code ANSYS FLUENT (Release 15.0) has been used to numerically investigate ocean wave generation. Impact of different turbulence models such as standard k-ɛ, realizable k-ɛ, Shear Stress Transport (SST) and Reynolds Stress Models (RSM) on the generated ocean surface waves were investigated. With all uncertainties associated with various numerical setting aspects, experimental wave measurements over a wide range of wave conditions covering intermediate and deep water regimes have been conducted in a physical wave basin to validate the numerical results. The excessive generation of eddy viscosity resulted from using eddy viscosity turbulence models especially at the free surface interface, leads to a significant unphysical damping on the generated waves. Good numerical agreement with both experimental measurements and analytical wave theory was successfully achieved either with the RSM or implementing artificial turbulence damping at the airwater interface with the SST model.

Abstract The principle objective of this paper is to outline the energy transfer processes occurring in a forward-facing bent-duct oscillating water column (OWC). Phase-averaged data obtained from model scale experiments conducted in monochromatic waves was used in conjunction with linear wave theory to investigate the various energy sources, stores and sinks associated with the three-dimensional OWC geometry. The analysis was restricted to energy transfer from the incoming wave and through the device, the intermediate storage mechanisms and losses, and hydraulic work performed on the power-take-off (which was simulated by an orifice plate). Results based on phase-averaged data presented include kinetic and potential energy for both an undisturbed wave and a wave interacting with the OWC geometry and power dissipated by the orifice. Two-dimensional velocity fields experimentally obtained via particle imaging velocimetry (PIV) were used to examine the kinetic energy and vorticity inside and around the device at its centreline. The main conclusion was that damping caused by the orifice (simulated power-take-off) diverts a proportion of the incoming energy around the device during the water inflow part of the cycle.

The hydrodynamic performance of Oscillating Water Column (OWC) wave energy converters depends mainly on the behaviour of the wave–OWC interaction. In this paper, a fully nonlinear 2D RANS–based computational fluid dynamics (CFD) model was used to carry out an energy balance analysis of an onshore OWC. Chamber differential air pressure and free surface elevation from published physical measurements were used to validate the CFD model. Additional validation was carried out via PIV data from available model–scale experiments to validate the CFD model's capability in capturing the flow field and the turbulent kinetic energy. The validated CFD model was then used in an extensive campaign of numerical tests to quantify the relevance of different design parameters such as incoming wave height and turbine pneumatic damping to characterise the hydrodynamic performance and wave energy conversion chain of the OWC. To capture the flow field inside the OWC in good agreement, additional refinement was required at the field of view together with utilizing either SST or RSM turbulence models rather than k-ɛ. It is found that the applied damping has crucial impacts on the energy conversion process. Also, increasing the wave height can lead to a massive drop in the system efficiency. Furthermore, both power take–off (PTO) damping and wave height play an important role in vortex formation around the upper and lower chamber's lips during the in–flow and out–flow stages.

Currently, ocean wave energy technology is in its infancy relative to the mature renewable energy technologies such as wind and solar. Due to its early stage of development, ocean wave energy has high associated Levelised Cost of Electricity (LCOE), a measure of lifetime costs relative to lifetime energy production. Several solutions have been derived in an attempt to reduce this high LCOE, of which breakwater integration of wave energy converters presents a viable option. Current full-scale commercial and demonstration devices indicate that OWC device integrated breakwaters are typically limited to nearshore and onshore operational regions. However, industries such as aquaculture and offshore wind are exploring the viability of placing these structures in deeper waters, where these traditional concepts would not be applicable, providing opportunity for the development of floating offshore multi-purpose structures. This article describes a proof-of-concept for a floating breakwater integrated with Oscillating Water Column (OWC) Wave Energy Converters (WEC). For an integrated device of this type there are multiple key aspects that are inter-related and each must be understood: energy extraction performance, wave attenuation and quantifying platform motions. In order to adequately report on each aspect in a logical manner the study is presented in two parts. This paper covers the energy extraction aspects, while the second part deals with wave attenuation and motion characteristics [1]. The wave energy extraction characteristics of the installed devices are explored across parameters including device configurations, breakwater width, power take-off damping, wave height and motion constraints, all of which was achieved through model scale hydrodynamic experimentation. The major findings indicate that OWC device spacing is a key parameter in the design of multi-device structures, as device-device interaction can have constructive or destructive interferences on the energy extraction. Additionally, the results of the OWC device performance, under the influence of the aforementioned parameters, provides new insights to the development of floating offshore multi-purpose structures and their feasibility.

The performance of wave energy converters using model scale testing has been assessed primarily using regular waves with limited testing in irregular waves. A viable alternative is the use of polychromatic waves, superposition of discrete regular waves with a definite period. Polychromatic waves allow well proven phase-averaging techniques to be applied to a wave which is more random and therefore representing a more realistic sea-state than regular waves. This paper presents results from model test experiments on a generic forward-facing bent-duct oscillating water column in polychromatic waves from a series of experiments using a wave probe array within the device and secondly with the addition of particle image velocimetry. By adapting phase averaging methodology the results showed that more reliable predictions of the device's operation are obtained when testing in polychromatic waves. Results from wave probe array showed that a longitudinal array is required to capture sloshing within the chamber. Velocity fields revealed a reduction in the proportion of kinetic energy within vortices in polychromatic waves compared with regular waves. This study highlights the importance of performing experiments in sea-states that are more realistic than simple regular waves to ensure an accurate representation of the device's performance and operation.

Performance characterization of oscillating water column (OWC) wave energy converters (WEC) is commonly assessed by conducting physical scale model experiments of OWC models fitted with orifice plates to the air chamber to both; simulate the power take off (PTO), and measure the air flow rate. Generally it is assumed that a single calibration factor can be used for bi-directional air flow measurement, however this paper shows the assumption can be in-accurate and that it is necessary to have separate inflow and outflow calibration factors. This paper presents (i) a novel method for in-situ calibration of an orifice and (ii) a simple algorithm to reduce noise during air flow reversal (low air chamber pressure differential). Application of this technique results in more accurate flow rate prediction and consequently, better prediction of the power absorbed by the power take-off for OWCs.