Conventional Oscillating Water Column Research Articles

An experimental study of vortex evolution around an offshore stationary dual-chamber oscillating water column converter

The Oscillating Water Column (OWC) is a promising Wave Energy Converter (WEC) due to its practicality and efficiency. Conventional single-chamber OWCs exhibit constraints in extracting energy over diverse wave periods, while dual-chamber OWCs demonstrate enhanced adaptability, leading to improved energy capture rates. This study investigates an offshore stationary dual-chamber OWC with an underlying transverse channel designed to facilitate wave ingress and augment energy extraction. Experiments across various wave periods reveal that dual-chamber OWCs with transverse channels achieve superior wave energy extraction compared to single-chamber counterparts. Time-resolved particle image velocimetry (TR-PIV) is employed for detailed flow field measurement to elucidate the complex hydrodynamic processes. Analysis of the water column oscillation and flow dynamics reveals an intensified interaction between hydrodynamic processes in the dual chambers, especially with elongated transverse channels. The Ω method is used to examine vortex evolution, indicating heightened complexity in vortex formation within dual-chamber OWCs compared to single-chamber OWCs. Notably, large-scale vortices emerging in the forward chamber impede oscillation and diminish energy extraction efficiency. Optimizing the design of transverse channel entries, frontal walls, and internal chamber corners is crucial to mitigate flow separation and vortex generation, thereby enhancing overall energy extraction performance.

Maximizing oscillating water column efficiency: The impact of vertical plate and guide vane

This study is centered on improving the hydrodynamic efficiency of a conventional Oscillating Water Column (OWC) that has a rectangular cross-section. The OWC chamber is modified by introducing a centrally installed vertical plate, dividing it into two sub-chambers, and further incorporating a quarter-circle guide vane beneath the vertical plate. Physical experiments are conducted under three scenarios: the conventional single-chamber OWC, the OWC with double sub-chambers separated by a vertical plate, and the OWC with both a vertical plate and a guide vane. Hydrodynamic efficiency is examined across varying Power Take-Off (PTO) dampings, incident wave frequencies, and wave heights. Results demonstrate substantial efficiency improvements with the vertical plate, particularly at lower dimensionless wave frequencies. The guide vane also boosts these benefits by preventing vortex formation due to flow separation, resulting in a total average efficiency increase of 42 %. Visual observations highlight the transition from sloshing to piston-like motion induced by the vertical plate, contributing to increased kinetic energy and improved efficiency. The study highlights that basic but effective modifications can substantially enhance OWC efficiency, suggesting promising directions for the development of wave energy conversion technology.

Optimal design of an inclined barrier for performance improvement of an oscillating water column device with an artificial neural network model

An oscillating water column (OWC) device with a surface-piercing inclined barrier has been studied in irregular waves. This study is to determine the optimal shape of an inclined barrier utilizing an Artificial Neural Network (ANN) model for the performance enhancement of the conventional OWC system. We employed the dual boundary element method (DBEM) as our numerical approach, which resolves two separate boundary integral equations (singular and hyper-singular) formulated at the coincident source points situated on either side of the degenerate barrier boundary. The design parameters affecting the performance of the OWC system have been optimized to maximize the power extraction using the trained ANN model. The training dataset was prepared using the DBEM, which is then used to train and validate the ANN model. Subsequently, the trained model was employed to make predictions using a broader dataset to identify the optimal combination of design parameters that maximize the conversion efficiency. The findings reveal that the well-trained ANN model can learn complex nonlinear relationships, make good predictions with high accuracy using a larger dataset of input conditions, and offer the optimal values of an inclined barrier appropriate for the wave conditions at the sea installation site.

Proposed improvements in wave energy converter of oscillating water column (WEC-OWC)

The aim of this paper is to show some design improvements on converters oscillating water column (OWC) versus conventional OWC, focusing on a newly patented design, OWC with Differential Pressure Storage Tanks (DPST). In this research work, a short review of the principles of operation and the results obtained for each one in relation to energy efficiency and regularity of electricity delivered to the grid are described. Apart some known innovations published so far in scientific journals, it will be discussed in more detail a new design (protected by patent) that tries to overcome some major performance problems in both conventional and improved OWC proposed up today such as intermittent flow through the turbine and the bi-directional flow. It is concluded that there are feasible to implement innovations that can lead to improvements with respect to OWC currently in operation

On using Helmholtz-type resonance to reduce the size of dual-purpose offshore oscillating water column wave energy converters

Oscillating water column (OWC) devices with Helmholtz resonator features are analyzed with semi-analytical, numerical, and experimental techniques. Conventionally, an OWC is a type of wave energy converter that consists of a duct with one end submerged, where the device's peak frequency is tuned by the length of the submerged duct. This leads to large devices when the desired peak frequency is low. The size of a conventional OWC can be reduced significantly by synthesizing features of Helmholtz resonators into it, such as using a narrow entrance. When integrated with a breakwater, it has been shown that the Helmholtz-type OWC can generate power while protecting the coastline from low-frequency ocean waves, making them dual-purpose. A systematic study of dual-purpose offshore Helmholtz-type OWC is still wanting. This study considers offshore “buoy-like” Helmholtz-type OWCs by a fast and heuristic semi-analytical model, which incorporated viscosity-related damping by empirical terms. The model shows that the Helmholtz-type OWCs have the potential to protect the coastline from low-frequency (long wavelength) waves, by producing a wide wave shadow behind them. The semi-analytical model is validated against the boundary element method (BEM), as well as experiments. The first experiment compares a Helmholtz-type OWC and a conventional OWC of the same size but different peak frequencies; the second compares a larger conventional OWC with a smaller Helmholtz OWC, which is tuned to have the same peak frequency as the former. In both cases, the semi-analytical model and the results from BEM agree well with the experiments.

Design and output power evaluation for a novel hybrid wave-wind energy converter

The performance of oscillating water column (OWC) wave energy converters (WEC) is highly affected by airflow rate. In this paper, a novel system is proposed that increases airflow rate and, therefore, output power through the integration of a mechanical structure known as a windcatcher with a conventional OWC. To investigate the hydrodynamic behaviour of the proposed system, a non-linear two-dimensional computational fluid dynamics (CFD) model is employed, along with the Reynolds Averaged Navier-Stokes (RANS) approach. The results of a comparison of the proposed OWC to a conventional type reveal a significant increase in airflow rate through the turbine blades, realizing an increase in converter output power. Moreover, the results show a power generation consistency in the proposed hybrid system, as the amplitude of the oscillatory part of the turbine airflow rate is diminished. Therefore, the proposed OWC converter not only generates significantly more power than a conventional type, it also has smoother power generation performance.

A novel concept for reducing wave reflection from OWC structures with application of harbor agitation mitigation/coastal protection: Theoretical investigations

Oscillating water column (OWC) device has the potential application in the field of harbor agitation mitigation/coastal protection. In this paper, we present a theoretical study on the hydrodynamic performance of a novel coastal structure, which combines a conventional harbor/coast-based single-chamber OWC device and a surface-piercing perforated barrier. Based on the linear potential flow theory, a semi-analytical model for wave interaction with the novel coastal structure is developed. The influence of the geometrical/physical parameters on hydrodynamic characteristics of the novel coastal structure is revealed. The results show that, by implementing the perforated barrier, a remarkable reduction of wave reflection from the OWC device can be achieved, without the compromise of the hydrodynamic efficiency. The incident wave energy is absorbed by the OWC device in the low frequency region and is dissipated by the perforated wall in the high frequency region. Comparisons of the conventional OWC structure and the proposed structure indicate that the frequency bandwidth for effective wave attenuation is broadened. In addition, the location of the perforated wall is identified as a key parameter influencing the hydrodynamic performance of the novel coastal structure.

Effect of Oscillating Water Column Chamber Inclination on the Performance of a Savonius Rotor

Abstract The power potential in the waves that hit all the coasts worldwide has been estimated to be of the order of 1 TW. Each wave crest transmits 10–50 kW/m of energy, which is 15–20 times higher than wind or solar energies. The availability of wave energy is 90% compared to 30% for wind and solar energies. The oscillating water column (OWC), which is the most investigated wave energy converter consists of a partially submerged hollow structure positioned either vertically or inclined. The bidirectional airflow above the water column drives a turbine. The conventional OWCs experience flow separation at the sharp corners of the chamber. To address this issue, researchers have proposed inclining the chamber at an angle with respect to the incident waves to improve the flow characteristics. In the present work, the effect of OWC inclination on rotor performance is studied using the computational fluid dynamics (CFD) code ansys-cfx. The results highlight that the 55 deg inclined OWC showed improved performance compared to the conventional OWC and modified OWC (optimized in a previous work). The maximum power for the inclined OWC was 13% higher than that for the rotor in the modified OWC and 28% than that in the conventional OWC at mean wave condition. The 55 deg inclined OWC recorded peak rotor power of 23.2 kW with an efficiency of 27.6% at the mean sea state. The peak power and efficiency at maximum sea state were 26.5 kW and 21.5%, respectively.

Non-isentropic study of a closed-circuit oscillating-water-column wave energy converter

The thermodynamics of the air inside a conventional Oscillating Water Column (OWC) is commonly modelled using the isentropic relationship between pressure and density. The innovative Tupperwave device is based on the OWC concept but uses non-return valves and two extra reservoirs to rectify the flow into a smooth unidirectional air flow harnessed by a unidirectional turbine. The air, flowing in closed-circuit, experiences a temperature increase due to viscous losses across the valves and turbine along the repetitive cycles of the device's operation. In order to study this temperature increase which represents a potential issue for the device operation, a non-isentropic wave-to-wire model of the Tupperwave device is developed taking into account the irreversible thermodynamic processes. The model is based on the First Law of Thermodynamics, and accounts for viscous losses at the valves and turbine as well as solar radiation and heat transfer across the device walls and inner free-surface. The results reveal that the temperature increase in the device remains harmless for its operation. The difference between the power performance of the Tupperwave device based on the non-isentropic and isentropic models is found to be relatively small. Its performance are also compared to the corresponding conventional OWC device.

Wave-to-Wire Model Development and Validation for Two OWC Type Wave Energy Converters

The Tupperwave device is a closed-circuit oscillating water column (OWC) wave energy converter that uses non-return valves and two large fixed-volume accumulator chambers to create a smooth unidirectional air flow, harnessed by a unidirectional turbine. In this paper, the relevance of the Tupperwave concept against the conventional OWC concept, that uses a self-rectifying turbine, is investigated. For this purpose, wave-to-wire numerical models of the Tupperwave device and a corresponding conventional OWC device are developed and validated against experimental tests. Both devices have the same floating spar buoy structure and a similar turbine technology. The models include wave-structure hydrodynamic interaction, air turbines and generators, along with their control laws in order to encompass all power conversion stages from wave to electrical power. Hardware-in-the-loop is used to physically emulate the last power conversion stage from mechanic to electrical power and hence validate the control law and the generator numerical model. The dimensioning methodology for turbines and generators for power optimisation is explained. Eventually, the validated wave-to-wire numerical models of the conventional OWC and the Tupperwave device are used to assess and compare the performances of these two OWC type wave energy device concepts in the same wave climate. The benefits of pneumatic power smoothing by the Tupperwave device are discussed and the required efficiency of the non-return valves is investigated.

Modelling Air Compressibility in OWC Devices with Deformable Air Chambers

Air compressibility effects play an important role in large-scale Oscillating Water Column (OWC) wave energy converters. Air compressibility is however not scalable with Froude similarity law. An existing scaling method enables correctly reproducing the air compressibility at the model scale, but its implementation is effortful and becomes cumbersome for floating devices and tests at relatively large scales (1/15th–1/2th). Air compressibility is therefore commonly ignored in model-scale tank testing of conventional OWC devices, which can lead to substantially unrealistic results on the device performance relative to the full-scale device. In the case of the Tupperwave device, which is a closed circuit OWC device, correctly modelling air compressibility during tank testing is however essential because the device relies on air compressibility to work. In this paper, a new method for modelling air compressibility at the model scale is presented. The method uses variable volume chambers, which mimic air compressibility by storing energy under the form of strain energy. This method reduces the difficulties of implementation and enhances the application of the existing method to larger scales. Various applications to this method are identified and described, including the presentation of a novel OWC concept.

Assessment of Primary Energy Conversion of a Closed-Circuit OWC Wave Energy Converter

Tupperwave is a wave energy device based on the Oscillating-Water-Column (OWC) concept. Unlike a conventional OWC, which creates a bidirectional air flow across the self-rectifying turbine, the Tupperwave device uses rectifying valves to create a smooth unidirectional air flow, which is harnessed by a unidirectional turbine. This paper deals with the development and validation of time-domain numerical models from wave to pneumatic power for the Tupperwave device and the conventional OWC device using the same floating spar buoy structure. The numerical models are built using coupled hydrodynamic and thermodynamic equations. The isentropic assumption is used to describe the thermodynamic processes. A tank testing campaign of the two devices at 1/24th scale is described, and the results are used to validate the numerical models. The capacity of the innovative Tupperwave OWC concept to convert wave energy into useful pneumatic energy to the turbine is assessed and compared to the corresponding conventional OWC.

Modelling Approaches of a Closed-Circuit OWC Wave Energy Converter

The Tupperwave device is a wave energy converter based on the Oscillating Water Column (OWC) concept. Unlike conventional OWC devices, which are opened to the atmosphere, the Tupperwave device works in closed-circuit and uses non-return valves and accumulator chambers to create a smooth unidirectional flow across a unidirectional turbine. The EU-funded OceanEraNet project called Tupperwave was undertaken by a consortium of academic and industrial partners, aimed at designing and modelling the Tupperwave device. The device was numerically modelled using two different methods. It was also physically modelled at the laboratory scale. The various modelling methods are discussed and compared. An analysis of the dependence of the device efficiency on the valves and turbine aerodynamic damping is carried out, using both physical and numerical approaches.

Analytical modelling of an U-Oscillating Water Column and performance in random waves

The oscillating water column (OWC) technology is recognized as one of the most effective solutions for exploiting sea wave energy in a scenario including hundreds of promising devices. In this context, the novel U-OWC device has been recently introduced as a further development of the OWC. This device differs from the conventional OWC as it utilizes a small vertical U-duct for connecting the air pocket to the open wave field. The objective of this paper is to propose a consistent representation of the wave field interacting with the U-OWC in the context of a linearized theory of water waves and, then, an analytical description of the U-OWC dynamics. Such a representation overcomes the drawbacks of the analytical models proposed, at the present time, in the open literature. Moreover, it allows including memory effects previously neglected.Next, the performance of the U-OWC is investigated by Monte Carlo simulations. In this regard, several simulations are produced for testing the efficiency of the device in sea waves compatible with power spectral density of wind-generated waves and of swells. The efficiency is evaluated according to a number of parameters, that allow highlighting the remarkable amount of energy absorbed by the system. The influence of the turbine is investigated, as well, by comparing the performance of monoplane (with and without guide vanes), biplane and contra-rotating turbines.

Performance studies on an oscillating water column employing a Savonius rotor

A new oscillating water column (OWC) design is proposed in this study to incorporate a simpler Savonius type turbine. Conventional OWC devices employ a bi-directional turbine such as a Wells or an Impulse turbine to extract energy from the air. The disadvantages of the Wells turbine include its inability to self start and stalling. The Savonius turbine is much cheaper and is an effective option at low Reynolds numbers. In the current rectangular OWC device, unlike the circular OWC, the width of entry of the capture chamber can be increased without being influenced by the diameter at the turbine section. To improve its primary capture efficiency, the front and rear walls of the OWC are inclined to minimize reflection. The Savonius rotor characteristics are studied with respect to the change in frequency of the incoming waves. The rotor rpm is sensitive to wave period and primary conversion efficiency while changes in depth only affect the rotor rpm at lower frequencies. The Savonius rotor shows promising results and can be incorporated into large scale OWC devices to reduce costs of the turbine component of the system.