Oscillating Water Column Device Research Articles

Wave power extraction by a dual OWC chambers over an undulated bottom

The impacts of sloping bottom and slanted impenetrable wall on dual oscillating water columns (OWCs) with a slanted front wall are examined. To tackle the physical model, a coupled eigenfunction expansion — boundary element system is employed. The concept is based on employing the anticipated amplification of scattered and reflected wave fields generated by the existence of sloping walls to enhance the absorption of wave power driven by the walls’ wave reflections. The study concludes that 100% efficiency occurs for smaller incident wave angles. It is also observed that compared with short waves, the efficiency of the device is more in the long-wave region. OWCs are observed to be more effective over the undulating bottom and sloping impermeable wall with an increased spacing between the water columns and the second OWC to the impermeable wall. Additionally, the chambers’ width and the sloping bottom angle are vital parameters for higher power extraction from the OWCs. Furthermore, the optimal values of slope bottom and wall inclination may be employed to build and implement an efficient OWC device. Moreover, the proposed model can also be used as a breakwater to protect the coastline.

Performance of Resonant Chambers in Oscillating Water Column Devices

The utilization of marine power potential in Colombia holds a great promise, and the oscillating water column (OWC) is one option that deserves to be explored. The wave energy conversion process using an OWC typically involves two stages: the conversion of wave power to pneumatic power in an air chamber, and the conversion of pneumatic power to electricity using a selfrectifying air turbine coupled to an electric generator. In order to improve the efficiency of the primary stage, this study is aiming at determining the hydrodynamic performance of an OWC air chamber using a numerical model based on the Reynolds Averaged Navier-Stokes equations and the Volume of Fluid approach for free surface simulation. The chamber geometry was based on a U-shaped OWC. The maximum efficiency and the mean velocity of the chamber free surface were found to be 66.8% and 0.17 m/s, respectively. The numerical results for the wave height obtained through simulation were validated by comparing them to analytical expressions reported in the existing literature, and a significant level of agreement was observed. Optimizing the chamber shape parameters for the specific wave characteristics is concluded to be crucial to enhance the operating efficiency of the OWC.

Analysis study of the ratio of wave height to column diameter in OWC wave energy converter

Hydro energy is one of the potentials of renewable Energy in Indonesia. Indonesia has a vast sea, in the amount of 3,544,743.9 km2 making Indonesia a suitable country to utilize Oscillating Water Column (OWC) type power plants. The study was conducted to determine the effect of column diameter on the performance of OWC devices, especially in speed and dynamic pressure. There are 5 variations of the diameter of the column studied, namely 0.5 m, 0.75 m, 1 m, 1.25 m, and 1.5 m. Plus there are also variations in the wave period, namely 3s, 4s, and 5s. The result of this study is the column diameter of 0.75 m has an average speed of 0.389 m/s and a dynamic pressure of 2.676 N/m2, making it the best column diameter ratio of the five column diameter ratio options available.

Role of dual breakwaters and trenches on efficiency of an oscillating water column

In this paper, the effects of double-submerged breakwaters and trenches on the hydrodynamic performance of an oscillating water column (OWC) are investigated. The multi-domain boundary element method is used to tackle the physical problem of wave scattering and radiation from the device. The role of the height of the breakwaters, depth of the trenches, width of the breakwaters and trenches, spacing between the structures, length of the OWC chamber, and other wave and structural parameters is investigated on the efficiency of OWC. The study reveals that there is an oscillating pattern of the efficiency curve in the presence of single or double breakwater/trenches; this pattern is absent when the bottom is flat. Moreover, compared to single or no breakwaters/trenches, the occurrence of full OWC efficiency is higher in the presence of double breakwaters/trenches. Furthermore, the amplitude of the oscillating pattern in the efficiency curve increases with an increase in the height and depth of the breakwaters and trenches, respectively. For some particular wave and structural parameters, zero OWC efficiency occurs nearly k0h=3.4 within 0<k0h<5 (k0 wave number and h water depth). This zero efficiency moves toward small wave numbers as the spacing between OWC and rigid breakwater/trench increases. The radiation conductance of OWC decreases with an increase in the barrier height. The findings outline the structural criteria that can be employed to build and deploy an effective OWC device.

Numerical Investigation into the Performance of an OWC Device under Regular and Irregular Waves

A numerical investigation into the hydrodynamic efficiency of an oscillating water column (OWC) device for the production of energy from sea waves under the conditions of regular and irregular waves is proposed. The numerical simulations were carried out using a two-dimensional version of a recently published three-dimensional free-surface nonhydrostatic numerical model, which is based on a conservative form of the contravariant Navier–Stokes equations written for a moving co-ordinate system. The governing equations are spatially discretized by a finite volume shock-capturing scheme based on high-order wave-targeted essentially nonoscillatory reconstructions and an exact Riemann solver. Time discretization was performed by a predictor-corrector method that took into account the nonhydrostatic pressure component. The proposed numerical model allowed us to highlight the significant differences between the hydrodynamic efficiency obtained under irregular waves and those obtained under regular monochromatic waves and provides more realistic evaluations of the OWC device performances. The results of the above comparison showed a reduction in the hydrodynamic efficiency of the OWC from 0.78 to about 0.54 when passing from regular waves to the corresponding irregular ones. The model was applied to assess the potential energy production obtainable by a set of OWCs at the Cetraro harbor (southern Italy). The numerical results show that, by adopting the optimal dimensions of the OWC, the estimated mean annual energy production obtainable at the Cetraro harbor is equal to 1540.52 MWh, which corresponds to the energy production of about 10 wind turbines with a nominal power of 60 KW.

Assessment of the Hydrodynamic Performance of an Oscillating Water Column Device in Front of a V-Shaped Vertical Wall

Abstract In this work, an oscillating water column (OWC) device is considered placed in front of a V-shaped vertical breakwater. The idea conceived, is based on the amplified wave power absorption due to the wave interactions originating from the presence of the breakwater. A theoretical analysis is presented in the realm of linear potential theory, based on the solution of proper diffraction, and radiation problems in the frequency domain, using the eigenfunction expansion method, the method of images, and the multiple scattering approach. Optimum absorption efficiency is examined taking into consideration the characteristics of the power take off (PTO) system and the air compressibility. Numerical results are presented and discussed in terms of the expected power absorption. The effect of the distance between the OWC and the vertical walls, the breakwater’s forming angle, and the wave heading angle, is examined to demonstrate the enhanced absorption ability of the device. It is concluded that the device’s efficiency is strongly dependent on the position of the OWC in front of the walls, as well as the angle of the wave impact, and should be taken into account when determining the optimum device parameters for maximization of its performance.

Performance of a hybrid wave energy converter device consisting of a piezoelectric plate and oscillating water column device placed over an undulated seabed

The present study investigates the hydrodynamic performance of a hybrid wave energy converter device consisting of a piezoelectric plate and the oscillating water column wave energy converter device placed over an undulated seabed. Primary emphasis is given to analyze the power extraction of the hybrid wave energy converter device, piezoelectric plate, and the OWC devices for various values of incident wave parameters and shape parameters associated with the hybrid wave energy converter device and undulated seabed. Moreover, the time-domain analysis is carried out by considering the Bretschneider spectrum as the incident wave spectrum for irregular incident waves. A multi-parameter optimization based on the Taguchi method is provided. The study reveals that the power extraction by the hybrid wave energy converter device consisting of a piezoelectric plate and the oscillating water column device is significantly higher than the power extraction by the standalone piezoelectric plate and the OWC device for a wider range of frequencies. Further, the present study shows that the power extraction by the hybrid wave energy converter device is higher when the piezoelectric plate is placed in the close proximity of the OWC device. Moreover, the time-domain simulation reveals that the piezoelectric plate is able to capture the incident wave energy for a longer period of time in real sea conditions.

The hydrodynamic performance of a shore-based oscillating water column device under random wave conditions

The testing of scale models is crucial if we are to continue the development of wave energy converters. Investigations have examined the interaction of shore-based Oscillating Water Column (OWC) devices with regular water waves that impact the device perpendicularly in flumes or wave basins. However, these simplistic experiments might misrepresent the effectiveness of an OWC system. This study examines the interaction of irregular, oblique, water waves with a shore-based OWC device in terms of its hydrodynamic performance. In a spectral wave basin, a series of experiments were performed on a scale model of a single chamber of the Mutriku Wave Energy Plant. Wave propagation conditions were examined to see their impact on the hydrodynamic performance of a fixed OWC system. The wave amplification factor, hydrodynamic efficiency, non-dimensional air pressure inside the chamber, and non-dimensional water pressures on the chamber walls were evaluated. The results show that when the free surface is close to the front wall lip, the hydrodynamic efficiency increases, while the effect of significant wave height was seen to be more important in the wave amplification. Finally, it was found that the hydrodynamic efficiency under random waves is lower than that in experiments under regular waves reported previously.

Effects of heave motion of an elastically supported floating oscillating water column device on wave energy harvesting efficiency

The main aim of this paper is to analyze the effects of the heave motion of an elastically supported floating oscillating water column device (OWC) on wave energy harvesting efficiency through two-dimensional computational fluid dynamics simulations. After the numerical model is validated using experimental data, it is used to analyze the effect of the natural frequency ratio on the hydrodynamic efficiency of the OWC. The natural frequency ratio is the ratio of the natural frequency to the wave frequency. The numerical results prove that the natural frequency ratio must be greater than 3 for achieving the best hydrodynamic efficiency. The best hydrodynamic efficiency decreases with the decrease in the natural frequency ratio when the latter is 3. If the natural frequency ratio is smaller than 3, both the amplitudes of wave surface elevation and the displacement of the OWC chamber increase but the harvested energy reduces because the relative motion of the wave surface and the OWC device is small. The energy dissipation due to vortices does not have a strong contribution to the reduction of energy at small natural frequencies.

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.

Numerical investigation of a dual cylindrical OWC hybrid system incorporated into a fixed caisson breakwater

In this paper, a hybrid Oscillating Water Column (OWC)system combining with a heaving floater Wave Energy Converters (WEC) was investigated. The traditional cylindrical-type breakwater consists of dual cylinders with an opening inlet located at the outer wavefront wall, allowing a ring-type wave chamber formed between two cylinders. The oscillating buoy OB is hinged at the front of the OWC device. The WAVE Chamber formed between the two breakwater cylinders is to be used as OWC. Based on the Computational Fluid Dynamics (CFD) software Star CCM+, A three-dimensional numerical wave tank is developed to investigate the hydrodynamic performance of the hybrid system, and the numerical model is validated with published experimental results. The power take-off (PTO) damping performance and the hydrodynamic efficiency affected by water conditions and geometrical dimensions of the device are discussed. Results show that the after combining the floater to the breakwater-type WEC, a great improvement on frequency bandwidth was achieved and the efficiency increase to 83.28% compared to the case of a single device, which was of 57%. Furthermore, the effects of the opening inlet height and the volume ratio of the OWC chamber were studied to optimize the certain geometrical parameters. Lower height ratio of the opening inlet of the OWC chamber should be avoid for larger water depth condition while designing such a hybrid system.

A Numerical Investigation of the Energy Efficiency Enhancement of Oscillating Water Column Wave Energy Converter Systems

This work focuses on the geometry effects over the performance of oscillating water column (OWC)-type wave energy converter (WEC) systems and searches for the OWC geometries that enhance the energy efficiencies under the same wave conditions. To analyze the hydrodynamic performances of the WEC systems, an in-house smoothed particle hydrodynamics (SPH) code based on weakly compressible fluid approach is utilized. The energy efficiency enhancement studies of the determined OWC device are carried out with a two-step geometry modification procedure. The first step starts with the validation of the free-surface elevation and orbital velocity time histories. Then, a three-by-three simulation matrix that depends on the geometrical design parameters of chamber length and front wall draft is run at three different wave conditions, and the OWC geometry that produces the maximum energy efficiency is determined. In the second step, the corner regions of the obtained optimal geometry are chamfered, and another simulation matrix is tested at the wave condition that yields maximum wave energy. It is observed in this step that the energy efficiency index can still be improved by 4.3% by only chamfering the back face of the OWC chamber. To scrutinize the physical grounds of this increase, the correlation between the time-averaged vorticity and energy efficiency is presented. Finally, the performance of the best configuration is also examined in three different wave periods, where the suggested geometry shows better performance with respect to base geometry results in all wave conditions.

Hydrodynamic Performance of a Floating Offshore Oscillating Water Column Wave Energy Converter

A floating oscillating water column (OWC) wave energy converter (WEC) supported by mooring lines can be modelled as an elastically supported OWC. The main objective of this paper is to investigate the effects of the frequency ratio on the performance of floating OWC (oscillating water column) devices that oscillate either vertically or horizontally at two different mass ratios (m = 2 and 3) through two-dimensional computational fluid dynamics simulations. The frequency ratio is the ratio of the natural frequency of the system to the wave frequency. Simulations are conducted for nine frequency ratios in the range between 1 and 10. The hydrodynamic efficiency achieves its maximum at the smallest frequency ratio of 1 if the OWC oscillates horizontally and at the largest frequency ratio of 10 if the OWC oscillates vertically. The frequency ratio affects the hydraulic efficiency of the vertical oscillating OWC significantly stronger than that of the horizontal oscillating OWC, especially when it is small. The air pressure and the volume oscillation in OWC is not affected much by the horizontal motion of the OWC but is significantly affected by the vertical motion, especially at small frequency ratios.

Experimental and numerical analysis of a hybrid WEC-breakwater system combining an oscillating water column and an oscillating buoy

Integrating multi-type Wave Energy Converters (WECs) enables wave energy to be extracted from multiple harvesting manners simultaneously, which presents a more competitive energy conversion technology. In this study, an Oscillating Water Column (OWC) and an Oscillating Buoy (OB) are combined to devise a hybrid WEC system that serves as a floating breakwater. The two devices are aligned in line with the incident wave direction, where the OB is deployed upstream from the OWC. A series of physical experiments are conducted to understand the working principle of the hybrid WEC system. After a fully non-linear time-domain model is validated by the measured data, the comparisons of the hybrid system and its respective isolated devicesare emphasised. Then, the hydrodynamic dependence of the hybrid system is comprehensively tested. The results indicated that the hybrid system design provides higher energy conversion as well as better wave attenuation performance compared to the isolated OWC and OB devices. Additionally, the OB outperforms the OWC in terms of wave energy conversion. The OWC-OB gap resonance has a negative impact on wave energy extraction but has a negligible effect on wave attenuation. The OWC with a deeper draft and symmetric wall performs better both in terms of wave energy extraction and wave attenuation in long-period waves. The overall conversion efficiency decreases with the increase of the OB draft, but the transmission coefficient follows an opposite trend. The PTO dampings of the OB and the OWC dominate the overall conversion efficiency and the transmission coefficient, respectively.

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.

Turbulence model effects on the hydrodynamic response of an oscillating water column (OWC) with use of a computational fluid dynamics model

Oscillating water column (OWC) is considered as one of the promising wave energy converters (WECs) due to its simple structure and working mechanism. Integrating OWCs with other marine structures gives additional feasible applications for WEC devices. This study presents a numerical simulation of a circular bottom-sitting OWC device for wave energy extraction using computational fluid dynamics. The numerical model is based on Reynolds Averaged Navier-Stokes equations with different turbulence models and a volume-of-fluid method to track the free surface. The numerical model and setup are validated against a set of wave-flume experimental results. The numerical simulation provides detailed flow-field information, which allows for an analysis of the spatial non-uniformity inside and outside of the OWC chamber. The results show that the SST k - ω turbulence model is recommended in OWC simulation, of this type, comparing the wave elevation and air pressure to experimental data. Moreover, positive results of the integration of an OWC of this type on a breakwater are presented and discussed.

Sea trial results of the biradial and Wells turbines at Mutriku wave power plant

Open sea experience is essential to develop wave energy technology and reduce costs. One of the goals of the EU H2020 OPERA project was to advance the development of the biradial self-rectifying air turbine from TRL4 to TRL7. For this purpose, a 30kW turbo-generation prototype was fabricated and tested at IST variable flow test rig, the shoreline Mutriku wave power plant and the offshore IDOM MARMOK-A5 spar-buoy device. The paper presents and analyses detailed operational data of the OPERA’s 30kW turbo-generation set at the oscillating water column device of the Mutriku harbour breakwater and unveils operational data of the Mutriku’s Wells turbines collected during the testing period. The sea trials at the Mutriku wave power plant demonstrated that the biradial turbine exhibits a mean time-averaged peak efficiency 37% higher than the Wells turbines in operation at the power plant.

Coupled Dynamic Response of an Offshore Multi-Purpose Floating Structure Suitable for Wind and Wave Energy Exploitation

This paper presents the coupled analysis performed in the frequency and time domain, considering a multi-purpose floating structure suitable for offshore wind and wave energy source exploitation. The floating structure encompasses an array of hydrodynamically interacting Oscillating Water Column (OWC) devices, moored through tensioned tethers as a Tension Leg Platform (TLP), and supports a 10 MW Wind Turbine (WT). The analysis is built to incorporate properly the solutions of the diffraction and the pressure- and motion-dependent radiation problems around the floating structure, the mooring lines characteristics, the OWCs characteristics, and the aerodynamics of the WT by accounting for the aerodynamic modelling of the rotor, the elastic modelling of the turbine components, namely the blades, the drive train, and the tower. Numerical results are obtained through the developed analysis methods, presenting the fundamental hydrodynamic properties of the platform, as well as the ultimate and fatigue loads at several locations on the structure expected to be imposed on it over its lifetime. The effect of the installation sites (i.e., environmental excitation) on the lifetime loads is investigated by comparing the estimated loads at various Mediterranean Sea and North Sea locations. Finally, the OWC effect, due to the air pressure oscillation inside the wave energy converters, is discussed, highlighting its significant influence on the hybrid system’s loading conditions.

Application of integrated wave-to-wire modelling for the preliminary design of oscillating water column systems for installations in moderate wave climates

Oscillating water column (OWC) devices are the technical solution more favourably considered for a significant future exploitation of the energy potential of sea waves. While theoretically quite simple, these systems are indeed rather complex in terms of correct tuning between the caisson and turbine. Recent studies showed that only a holistic approach, also including turbine control, is the way of effectively performing their precise design.To this end, the research presents the application of an analytical wave-to-wire model of an OWC device for the preliminary optimisation of these systems for a small installation in a moderate wave climate in the Mediterranean Sea. More specifically, scatter matrices were determined for imposing the wave states of a selected location positioned in Tuscany (Italy). Regarding previous similar studies, a novel joint optimisation of the caisson and controlled turbine is proposed for the specific site. Either monoplane isolated Wells turbines and axial impulse turbines were analysed. The operating curves, performance parameters, and attended annual energy harvesting were calculated, providing an interesting overview of the functioning of the system. The optimised devices with the Wells and impulse turbomachines convert 36.71 and 42.17 MWh/year operating with a global efficiency of 6.82% and 7.84%, respectively.

Numerical investigation of offshore oscillating water column devices

The main objective of this paper is to analyse the hydrodynamic efficiency of offshore stationary OWC devices with different configurations through two-dimensional computational fluid dynamics simulations. The two-dimensional incompressible Reynolds-Averaged Navier-Stokes equations and the Shear Stress Transport (SST) k-ω turbulence model are solved for the simulation of water moetion under regular waves. The airflow through the turbine is calculated using a compressible aerodynamic model. After the numerical model is validated using the experimental data, it is used to analyse the effect of the submerged draught of rear vertical wall of the OWC on the hydrodynamic efficiency, and the improvement of hydrodynamic efficiency by a horizontal bottom plate attached to the back wall. The numerical results prove that increasing the rear wall draught improves the hydrodynamic efficiency if the front wall draught is 0.25 times water depth. If the draughts of the front and rear walls are the same, increasing them simultaneously causes a reduction in the hydrodynamic efficiency. Attaching a horizontal bottom plate on the deeper rear vertical wall improves efficiency. The best efficiency increases with the increase of the length of the bottom plate until the plate length is twice the chamber length. Further increase in the bottom plate length causes a reduction in the best efficiency.