• Energy Research

This paper presents an experimental and numerical hydrodynamic performance assessment of a 1:50 scale model offshore floating–moored Oscillating Water Column (OWC) wave energy converter. The device is a tension–leg structure with four vertical mooring lines. The performance of the OWC device was investigated for several design parameters including regular and irregular wave conditions of different heights and periods, power take–off (PTO) damping and mooring line pre–tension. A 3D Computational Fluid Dynamics (CFD) model using RANS–VOF approach was constructed and validated against experimental results for regular waves showing good agreement. It was found that the hydrodynamic efficiency of the floating–moored OWC device follows the same general trend as a fully–constrained (fixed) model, but the addition of surge motion in the floating device improved the energy production efficiency over a broader bandwidth around the chamber resonance. Increasing the incoming wave height resulted in a higher efficiency for low–frequency waves, but noticeable reductions in the efficiency were observed in the intermediate– and high– frequency zones. The effectiveness of utilizing offshore OWC devices in deep–water was demonstrated by increasing the extracted energy by a maximum of 7.7 times and 5.7 times when regular and irregular wave heights were doubled, respectively. Decreasing the mooring line pre–tension slightly increases the energy extraction efficiency in the intermediate–frequency zone.

Understanding the hydrodynamic performance of offshore Oscillating Water Column (OWC) devices is essential for assisting the development and optimization processes. The chamber underwater geometry is one of the paramount design aspects that strongly affect the wave–OWC interactions. This paper utilizes a well–validated two–dimensional, fully nonlinear Computational Fluid Dynamics (CFD) model to investigate the impact the underwater front and rear lips have on the hydrodynamic performance of an offshore stationary OWC. An extensive campaign of numerical simulations is performed to discover the relevance of the front and rear lip submergence and thickness to OWC performance. The key finding is that the overall hydrodynamic efficiency can be significantly improved over a broad frequency bandwidth by selecting suitable values for both the submergence ratio of asymmetric lips and the lip thickness. The device that is capable of absorbing a large amount of the incoming wave energy provides the maximum power extraction efficiency and the maximum energy losses. The optimal combination achieved a peak efficiency exceeding 0.79, which represents a massive enhancement over more simplistic, but commonly accepted, geometries that returned peak efficiencies of approximately 0.30.

This paper presents the application of phase averaging to experimental data obtained during scale model testing of a forward facing bent duct oscillating water column (OWC). Phase averaging is applied to both wave probe data and a two dimensional velocity field at the centreline plane of the OWC model obtained using PIV. Results are presented for one monochromatic wave condition. The influence of varied wave frequency is briefly discussed.

Despite the predictability and availability at large scale, wave energy conversion (WEC) has still not become a mainstream renewable energy technology. One of the main reasons is the large variations in the extracted power which could lead to instabilities in the power grid. In addition, maintaining the speed of the turbine within optimal range under changing wave conditions is another control challenge, especially in oscillating water column (OWC) type WEC systems. As a solution to the first issue, this paper proposes the direct connection of a battery bank into the dc-link of the back-to-back power converter system, thereby smoothening the power delivered to the grid. For the second issue, model predictive controllers (MPCs) are developed for the rectifier and the inverter of the back-to-back converter system aiming to maintain the turbine speed within its optimum range. In addition, MPC controllers are designed to control the battery current as well, in both charging and discharging conditions. Operations of the proposed battery direct integration scheme and control solutions are verified through computer simulations. Simulation results show that the proposed integrated energy storage and control solutions are capable of delivering smooth power to the grid while maintaining the turbine speed within its optimum range under varying wave conditions.

In this paper, wave forces on a 1:50 model–scale of an offshore stationary Oscillating Water Column (OWC) device are studied in 3D physical and numerical wave tanks. In total 310 physical experiments including repetition were performed in a wave tank for regular waves of different heights and periods, and several turbine–induced pneumatic damping conditions simulated via orifice plates. A Computational Fluid Dynamics (CFD) model based on the RANS equations and the VOF surface capturing scheme was constructed and validated against the tank measurements. The validated CFD model was then utilized to investigate the effects of tank sidewalls on the predicted wave forces. It was found that the horizontal wave force acting on the OWC device was always larger than the vertical force. The total horizontal and vertical forces were maximum and minimum, respectively at the intermediate wavelength. In addition, the pneumatic damping had a significant impact on the measured vertical force, whereas negligible effects were observed on the horizontal force. Increasing the wave height increased nonlinear effects on the forces measured, especially the vertical force. It was concluded that a tank width of less than five times the OWC device breadth will likely provide misleading wave forces due to blockage effects.

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.

The compromise between Wave Energy Converter (WEC) performance, cost and survival is both a delicate and critical one. A successful WEC design must effectively address the exploitable wave energy, but survive the climate extremes. Bombora Wave Power has focussed on designing a WEC that performs well in less extreme nearshore climates and is able to decouple its working surfaces from extreme waves. Numerical modelling of the performance of their submerged, pneumatic, flexible membrane WEC, the mWave, is presented. The mWave power matrix is found to provide good performance over a broad range of wave periods, with a broad peak in performance at wave periods of 9s for the assumed design parameters. This broad peak corresponds favourably to the sea-state probabilities in a typical near-shore shallow water wave climate, yielding a predicted mean annual electrical power production of 240kW in such conditions. Small scale physical modelling of the relationship between the initial level of inflation of the mWave cell membranes and the system’s power capture has confirmed the possibility of an mWave survival strategy that can potentially allow safe, de-rated performance in extreme conditions. Future work is planned to further improve predicted mWave performance by refinement of power take-off damping and to physically validate these performance modelling results at full scale.

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.

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.