Oscillating Water Column Device Research Articles

Experimental Investigation of Wave Pressure on Breakwater-Integrated Oscillating Water Column Devices with a Perforated Wall

This study investigates the wave pressure on an Oscillating Water Column (OWC) device, which features a perforated wall positioned in front of the OWC chamber and is integrated with a caisson breakwater. The perforated wall is designed to mitigate wave impacts on the OWC device under storm wave conditions. To achieve this, a series of laboratory experiments were conducted using a 1:25 scale model, considering both regular and irregular waves. Wave pressure measurements and analyses were performed on various configurations to assess the effects of the perforated wall on the wave pressure exerted against the front wall of the OWC chamber and the caisson breakwater. The results indicate that the perforated wall effectively reduces the amplitude of water column oscillations within the OWC chamber during storm conditions, thereby significantly reducing wave pressure on the OWC chamber wall by approximately 14% and on the breakwater by about 20.3%.

Experimental and numerical investigation of wave loads on land-based multi-chamber OWC converters

Capturing more wave energy in a cost-effective principle has become a challenging task. Multi-chamber oscillating water column (OWC) devices are gradually gaining favour due to their potentially efficient characteristics. However, there is relatively limited research on dynamic analysis which is crucial for the safety and survival of multi-chamber OWC devices. In the study, model experiments of the hydrodynamic pressures for single-, dual- and three-chamber devices were conducted at a wave flume. A two-dimensional fully nonlinear numerical model for the interaction between waves and the OWC devices was established based on the higher-order boundary element method. The numerical results well cross-validated the experimental data. The wave forces and moments exerted on the devices were numerically calculated. The results indicate that the multi-chamber configurations not only generally reduce the wave loads on front and rear walls in high-frequency waves, but also avoid the sloshing motion of the water column inside the chamber, which induces large wave loads on the converter. However, the additional internal walls impose a redoubled burden on the junction of the top ceiling and the rear wall. The OWC converters sustain the horizontal forces much more than vertical forces. The front lip (i.e., the bottom edge of the front wall), the rear wall and the connection between the seabed and the device are relatively dangerous positions for configurations with any number of chambers.

Experimental Study on the Spring-like Effect on the Hydrodynamic Performance of an Oscillating Water Column Wave Energy Converter

The oscillating water column (OWC) wave energy converter has demonstrated significant potential for converting ocean wave energy. The spring-like effect of air compressibility can significantly affect the hydrodynamic behavior of the device, but it has rarely been investigated through experimental studies. In this study, an experimental test on a model-scaled OWC device was carried out in a wave flume using a series of regular and irregular waves. The spring-like effect was taken into account by the combination of the air chamber with an additional air reservoir of appropriate volume, where the total volume was scaled according to the square of the Froude scale. The hydrodynamic performance was compared with the results obtained without considering the spring-like effect. A phase difference between the air pressure and airflow rate was observed when employing the additional air reservoir. The amplitudes of free surface elevation and airflow rate increased, while the air pressure was reduced when the spring-like effect was considered. The results demonstrate that failure to consider the spring-like effect can lead to overestimation of the hydrodynamic efficiencies, and the errors were mainly affected by the incident wave frequency.

Performance of dual‐chamber oscillating water column device under irregular incident waves using Reynolds averaged Navier–Stokes model

AbstractThe purpose of this study is to analyze the hydrodynamic performance and efficiency of a dual‐chamber oscillating water column (OWC) device. For the Joint North Sea Wave Project (JONSWAP) incident waves spectrum, a two dimensional numerical wave tank is employed with nonlinear Reynolds averaged Navier–Stokes equations model along with the standard turbulence model. The free surface elevation is measured using the volume of fluid method. The numerical simulation demonstrates the streamline and velocity vector profiles throughout an entire pressure fluctuation cycle inside the chambers of the given OWC device. Further, investigation is carried out to analyze the impact of pressure drop and air flow rate through the orifice of the dual‐chamber OWC device on the power generation. Moreover, the power spectral density analysis of the free surface elevation is provided to know the variation of the parameters in the frequency domain. These results demonstrate that the effectiveness of the dual‐chamber OWC device is more near the significant wave height m.

Numerical Study of Hydrodynamic Characteristics of a Three-Dimensional Oscillating Water Column Wave-Power Device

The impact of wave-induced forces on the integrity of stationary oscillating water column (OWC) devices is essential for ensuring their structural safety. In our study, we built a three-dimensional numerical model of an OWC device using the computational fluid dynamics (CFDs) software OpenFOAM-v1912. Subsequently, the hydrodynamic performance of the numerical model is comprehensively validated. Finally, the hydrodynamic performance data are analyzed in detail to obtain meaningful conclusions. Results indicate that the horizontal wave force applied to the OWC device is approximately 6.6 to 7.9 times greater than the vertical wave force, whereas the lateral wave force is relatively small. Both the horizontal and vertical wave forces decrease as the relative water depth increases under a constant wave period and height. In addition, the highest dynamic water pressure is observed at the interface between the water surface and device, both within and outside the front wall of the gas chamber. The dynamic water pressure at different locations on the front chamber increases and subsequently decreases as the wave frequency increases.

Three-dimensional numerical simulation of a cylindrical oscillating water column (OWC) device placed in a wave flume

A three-dimensional computational fluid dynamic (CFD) model is developed for simulating a cylindrical Oscillating Water Column (OWC) device for harvesting wave energy. The single-phase CFD model solves the Reynolds-Averaged Navier-Stokes (RANS) equations using the finite element method for simulating the wave motion and uses an aerodynamic model to simulate the flow through the air turbine. The model is implemented to simulate wave energy harvesting of a cylindrical OWC device. Through harmonic decomposition, it is found that the first harmonic wave surface elevation oscillates like a piston in the OWC chamber. However, both the amplitude and phase of the second harmonic wave surface elevation vary significantly along the wave direction in the OWC chamber, demonstrating a sloshing mode of the second harmonic. This study further proved the reduction of the capture width ratio caused by the transverse sloshing mode when the wavelength is the same as the wave flume width. The effect of the width of the wave flume on the peak CWR is found to be negligibly small when the wave flume width is three times the OWC diameter.

Multi-phase SPH-FDM and experimental investigations on the hydrodynamics of an oscillating water column wave energy device

In this paper, the hydrodynamic performance of a bottom-seated type oscillating water column (OWC) device is investigated by numerical and experimental methods. A coupled SPH (Smoothed Particle Hydrodynamics) - FDM (Finite Difference Method) is proposed to simulate the interaction between waves and the OWC device. The current SPH-FDM model improves grid encryption method, and expand the single-phase SPH-FDM to multiphase SPH-FDM. The results from SPH-FDM align closely with those from the published experimental measurements. Following the successful validations, SPH-FDM and experiment are employed for a series of simulations to investigate the performance of the OWC device. Compared to experimental method, SPH-FDM can obtain global flow field information and avoid the influence of free surface identification and capillarity on vortex determination. In addition, SPH-FDM can naturally captures free surfaces, so it supports further studies on the non-uniformity of water surface in the chamber of OWC which cannot be studied by PIV or pneumatic model.

Numerical Study on the Performance of an OWC under Breaking and Non-Breaking Waves

A numerical model for the simulation of the performance of an oscillating water column (OWC) subjected to non-breaking and breaking waves is proposed in this paper. The numerical model consists of a hydrodynamic model specifically designed to simulate breaking waves and a pneumatic model that takes into account the air compressibility. The proposed numerical model was applied to evaluate the potential mean annual energy production from the waves of two coastal sites characterized by different hydrodynamic conditions: a deep-water condition, where the OWC interacts with non-breaking waves, and a shallow-water condition, where the OWC is subjected to breaking waves. The numerical results show that the effects of the air compressibility can be considered negligible only in numerical simulations of the performances of reduced-scale OWC devices, such as those used in laboratory experiments. We demonstrated that in real-scale simulations, the effect of the air compressibility within the OWC chamber significantly reduces its ability to extract energy from waves. The numerical results show that the effect of the air compressibility is even more significant in the case of a real-scale OWC located in the surf zone, where it interacts with breaking waves.

Experimental and numerical analysis of multi-chamber oscillating water column devices

Experimental and numerical analysis of multi-chamber oscillating water column devices

Development of a Predictive Model for Evaluation of the Influence of Various Parameters on the Performance of an Oscillating Water Column Device.

Oscillating Water Column (OWC) systems harness wave energy using a partially submerged chamber with an underwater opening. The Savonius turbine, a vertical-axis wind turbine, is well-suited for this purpose due to its efficiency at low speeds and self-starting capability, making it an ideal power take-off (PTO) mechanism in OWC systems. This study tested an OWC device with a Savonius turbine in an air duct to evaluate its performance under varying flow directions and loads. An innovative aspect was assessing the influence of power augmenters (PAs) positioned upstream and downstream of the turbine. The experimental setup included load cells, Pitot tubes, differential pressure sensors and rotational speed sensors. Data obtained were used to calculate pressure differentials across the turbine and torque. The primary goal of using PA is to increase the CP-λ curve area without modifying the turbine geometry, potentially enabling interventions on existing turbines without rotor dismantling. Additionally, another novelty is the implementation of a regression Machine-Learning algorithm based on decision trees to analyze the influence of various features on predicting pressure differences, thereby broadening the scope for further testing beyond physical experimentation.

Numerical investigation of the hydrodynamic properties of oscillating water column air chambers

Abstract The oceans occupy nearly 71% of the Earth’s surface area. The ocean is rich in energy, and wave energy is one of its important components. Therefore, the study of wave energy generation devices is an important issue. In this paper, we analyze the effect of the size of the air chamber structure on the OWC (oscillating water column) wave energy conversion device under the action of unidirectional regular waves. To analyze the performance of the OWC device under real sea conditions, the wave climate at Sanmenxia is used as the experimental parameter. The performance and characteristic response with the variation of incident wave height, air chamber width and front wall thickness, are analyzed. The results show that the incident wave height, air chamber width, and front wall draft have significant effects on the performance of OWC. With the increase of incident wave height, the wave nonlinear effect increases. As the air chamber width decreases, the horizontal surface inside the air chamber undulates smoothly and the standing wave phenomenon disappears.

Laboratory modelling of nonlinear power take-off damping and its effects on an offshore stationary cylindrical OWC device

The accurate evaluation of the wave energy conversion of oscillating water column (OWC) wave energy converter has consistently plagued scholars. In this paper, an original and well-validated test platform is proposed for exploring the flow characteristic of a nonlinear power take-off (PTO) damping, with which a more accurate and stable measurement of captured energy is obtained in the laboratory modelling. This study utilizes ten orifices to comprehensively investigate the impact of the PTO damping on an offshore stationary cylindrical OWC device. The joint effect of the PTO damping with wave height (Hi) or draft of OWC (Dr) is examined. The damping coefficient Cf = 10350 is found to be the optimal damping coefficient, in terms of both the peak wave energy conversion and the efficient bandwidth. It is revealed that the variations of the PTO damping can regulate the effect of Dr and Hi on wave energy conversion. An appropriate reduction in PTO damping can alleviate the weak energy output caused by the increased Dr and Hi. The effect of increased Hi on wave energy conversion could be judged by the size relation of the maximum and minimum values of internal pressure data.

Study of the Geometry of an Oscillating Water Column Device with Five Chambers Coupled under Regular Waves through the Constructal Design Method

This research conducts a numerical study of a wave energy converter (WEC) device with five coupled hydropneumatic chambers, operating based on the principle of an oscillating water column (OWC). A turbine was not included, only considering the tube without it. The computational domain was defined by a wave channel housing an OWC device subjected to regular incident waves. The central objective was to assess the impact of chamber geometry on maximizing the total hydropneumatic power in energy conversion. The numerical simulations consider the pressure, mass flow rate, and total hydropneumatic power, with the latter being the performance indicator. To determine the geometries to be analyzed, the Constructal Design method was employed in conjunction with the exhaustive search optimization method to maximize the performance indicator. The degrees of freedom defined were the ratios between the height (Hn) and the length (Ln) of the hydropneumatic chambers (Hn/Ln, where n varies from one to five). Based on the results of the mass flow rate and pressure, their influence on power was evaluated. It was observed that the influence of the degrees of freedom on the pressure difference, mass flow rate, and hydrodynamic power was quite similar, displaying an increase for low ratios of Hn/Ln up to a maximum magnitude and followed by a decrease in magnitude. The best performance was achieved for the geometric configuration with Hn/Ln = 0.2613 (Hn = 5.0625 m and Ln = 15.8219 m), representing an improvement of 98.6% compared to the worst case analyzed.

Multiple moonpools within a vessel free to heave and pitch: An experimental study

Moonpools in marine vessels and offshore platforms are vertical openings through the structure used to support underwater operations. Experimental investigations of multiple moonpool resonant responses coupled with vessel heave and pitch motions are presented in this study. For three moonpools at forward, central, and aft locations of a rectangular vessel, the simultaneous location-dependent free surface response amplitudes and oscillation phases are compared for a range of incident wave frequencies in the piston mode range. For the vessel free to heave and pitch, the central moonpool has the highest response amplitude; whereas, a previous study with fixed vessel (Garad et al. 2023) showed that the largest free surface responses are observed in the forward moonpool. Due to coupled hydrodynamic interactions between the moonpool oscillations and the vessel motions, the moonpool resonance frequency is linked with the vessel heave response. Multiple response amplitudes peaks are observed in the forward and aft moonpools due to body motions. The non-dimensional response amplitude increases nonlinearly with decreasing wave steepness close to the resonance frequency. The multiple moonpool oscillations show distinct phase differences from one another and with the outer wave, the individual values depending on the incident wave frequency. Considering the vessel motions, comparison with experiments without moonpools shows that the pitch response is increased due to multiple moonpool oscillations. The hydrodynamic interactions between multiple moonpools and a floating vessel have prospective applications in the case of marine platforms, where the resonant motions can be used for wave energy conversion through oscillating water column devices.

A new numerical modelling framework for fixed oscillating water column wave energy conversion device combining BEM and CFD methods: Validation with experiments

The Oscillating Water Column (OWC) wave energy converter has been shown to have high potential, thus rendering extensive development in recent years. In order to further accelerate its development, highly accurate yet computationally efficient tools are necessary particularly when studying the interaction of multiple OWC devices. This paper proposes a new framework for fixed OWC devices with an orifice, that uses the input from a high fidelity non-linear numerical model to improve the accuracy of a low fidelity linear numerical model keeping computational costs low. This is done by accounting for the non-linearities in the pressure-flow of an orifice in the input to the linear numerical model. Experimental data is used to validate the framework, thus providing an accurate and computationally efficient linear numerical model, that can be used for the preliminary analysis of fixed OWC devices.

Numerical analysis of 3D hydrodynamics and performance of an array of oscillating water column wave energy converters integrated into a vertical breakwater

Performance and hydrodynamics of an array of Oscillating Water Column (OWC) Wave Energy Converter (WEC) integrated into a vertical breakwater is studied. The FLUENT® software, in which the numerical model is based on the Reynolds-Averaged Navier-Stokes equations and the Volume of Fluid method for free surface flow modeling, is used in a 3D numerical wave tank. Three vertical breakwater configurations subject to the action of incident regular waves with periods from 6 to 12 s are studied: normal breakwater, with vertical walls parallel to the direction along the breakwater length; and two novel breakwater geometries, partially and fully convergent breakwaters, whose converging vertical walls are inclined θ in relation to this direction. Different spacing S from 0 to 20 m between the array of OWC devices and two converging wall angles θ, 30 and 45°, are investigated. Firstly, analysis of the influence of S for the normal breakwater shows that the vertical wall concentrates naturally a higher quantity of the incident wave energy inside OWC chamber devices and, consequently, increases their efficiencies. This effect is intensified as the spacing S increases. Secondly, analyses of the partially and fully convergent breakwaters allow concluding that these novel geometries, which direct an amount of incident wave energy into the OWC chamber, increase significantly the efficiency of the array of the OWC devices at the range of the wave periods. The highest performance of OWC device is obtained by the fully convergent breakwater with S = 20 m and θ = 45°, once 10 OWC devices inserted in a breakwater 300 m long have the same efficiency of 20 OWC devices inserted into the normal breakwater.

Wave Energy Conversion through Oscillating Water Columns: A Review

An oscillating water column (OWC) is designed for the extraction and conversion of wave energy into usable electrical power, rather than being a standalone renewable energy source. This review paper presents a comprehensive analysis of the mathematical modeling approaches employed in OWC systems, aiming to provide an in-depth understanding of the underlying principles and challenges associated with this innovative technology. A prominent classification within the realm of wave energy devices comprises OWC systems, which exhibit either fixed or floating configurations. OWC devices constitute a significant proportion of the wave energy converter prototypes currently operational offshore. Within an OWC system, a hollow structure, either permanently fixed or floating, extends below the water’s surface, creating an enclosed chamber where air is captured over the submerged inner free surface. This comprehensive study offers a thorough assessment of OWC technology in conjunction with air turbines. Additionally, the investigation delves into theoretical, computational, and experimental modeling techniques employed for analyzing OWC converters. Moreover, this review scrutinizes theoretical, computational, and experimental modeling methodologies, providing a holistic understanding of OWC converters. Ultimately, this work contributes a thorough assessment of OWC technology’s current state, accentuating its potential for efficient wave energy extraction and suggesting future research avenues.

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.

Pressure Pulsation Analysis of Oscillating Water Column Rotor Eccentricity Based on the Pulsation Tracking Network Method

An oscillating water column (OWC) is typical of axial rotor turbines, which are used to convert ocean wave energy into electrical energy. This device impacts downstream pressure pulsations when its rotor becomes eccentric. This study compared the details of pressure pulsations downstream of eccentric and non-eccentric rotors under three operating conditions: low flow A, high-efficiency flow B, and high flow C. Computational fluid dynamics (CFD) simulations based on the pulsation tracking network (PTN) method were used for the OWC device to compare the experimental results. The results indicate downstream pressure pulsations were mostly dominated by the blade frequency in non-eccentric low-flow cases. In the other eccentric operating conditions, downstream pressure pulsations were mainly dominated by the 2-, 3.6-, 6-, and 7-times rotation frequencies and the 0.5-times blade frequency. The phase change of downstream pressure pulsations in eccentric and non-eccentric conditions is consistent with the flow direction. The phase change is relatively uniform and steady before eccentricity and becomes turbulent after eccentricity, which affects its steadiness. In this study, the OWC device did not significantly change with or without rotor eccentricity at a 1-time blade frequency intensity; however, at a 1-time rotation frequency, the OWC device showed a significant increase in the pressure pulsation amplitude after rotor eccentricity. The study of the dominant frequency, amplitude, and phase of pressure pulsations in OWC devices with eccentric rotors can help prevent excessive pressure pulsations that can lead to incidents.

Investigating Chamber Geometry and Orifice Diameter of Oscillating Water Column Device by Two Phase CFD Modelling

Investigating Chamber Geometry and Orifice Diameter of Oscillating Water Column Device by Two Phase CFD Modelling