Oscillating Water Column Device Research Articles

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.

CFD modelling of a small–scale fixed multi–chamber OWC device

Wave Energy Converters (WECs) have excellent potential as a source of renewable energy that is yet to be commercially realised. Recent attention has focused on the installation of Oscillating Water Column (OWC) devices as a part of harbor walls to provide advantages of cost–sharing structures and proximity of power generation facilities to existing infrastructure. In this paper, an incompressible three–dimensional CFD model is constructed to simulate a fixed Multi–Chamber OWC (MC–OWC) device. The CFD model is validated; the simulation results are found to be in good agreement with experimental results obtained from a scale physical model tested in a wave tank. The validated CFD model is then used for a benchmark study of 96 numerical tests. These investigate the effects of the PTO damping caused by the power take–off (PTO) system on device performance. The performance is assessed for a range of regular wave heights and periods. The results demonstrate that a PTO system with an intermediate damping can be used for all chambers in the MC–OWC device for most wave period ranges, except for the long wave periods. These require a higher PTO damping. An increased incident wave height reduces the device capture width ratio, but there is a noticeable improvement for long wave periods.

CFD modeling of a ducted Savonius wind turbine for the evaluation of the blockage effects on rotor performance

The idea of investigating the fluid dynamic behavior of a ducted Savonius wind turbine derives from the research about the improvements of Oscillating Water Column Wave Energy Converters in which the air is forced to flow back and forth inside a duct. The Savonius rotors appeared to be very suitable for this kind of applications for many reasons. Indeed, these rotors are very simple to built, they are unidirectional respect to the incoming flow, they works well at low flow speed and they produce high torque which allow them to start at very low cut in velocities. By means of a CFD 2D numerical model, the present paper demonstrates the great potentialities offered by the use a Savonius wind turbine in Oscillating Water Column devices. The CFD model was developed and validated against experimental data, obtained in the subsonic wind tunnel owned by the University of Catania. The ducted rotor was simulated at different flow and rotational speed, in steady unidirectional condition which is of utmost importance as a first step in the more complex oscillating flow study.

Experimental Study on the Effect of Ocean Wave Characteristics on Air Pressure in Oscillating Water Column Device

Ocean wave energy conversion process in Oscillating Water Column (OWC) device is affected by several parameters. Wave characteristics are an important parameter that determines the level of water fluctuations in the water column. The later parameter is a main factor which influences air pressure in the air column. One of the important wave characteristics is wave steepness (H/L), which is ratio of wave height to wave length. This experimental study analyzed the effect of wave steepness to wave energy. Furthermore, air pressure in the air column of the OWC device is influenced the effects of changes in wave energy converted by OWC device. Testing was performed with several variations of height and wavelength for three different water depths. Wave height change data was recorded with ultrasonic sensor and stored on the CPU via a data logger. The test showed that the deeper the water, the smaller the wavelength and the greater the wave height. In addition, wavelength increase with the increasing of wave period, vice versa wave height decrease with the increasing of wave period. This condition produced different wave steepness so that the wave energy also changes. Data analysis showed the wave steepness of the test results with the biggest pressure change that corresponds to the actual wave steepness generated by the wind at ocean achieved at water depths h = 240 mm with wave height H = 82.8 mm and L = 1250 mm.

Experimental hydrodynamic investigation of a fixed offshore Oscillating Water Column device

Oscillating Water Column (OWC) is one of the pioneer devices in harnessing wave energy; however, it is not fully commercialized perhaps due to the complicated hydrodynamic behavior. Previous studies are significantly devoted to OWC devices located in nearshore and coastal regions where incident wave energy would experience dissipation more than offshore. In this paper, a 1:15 scaled fixed offshore OWC model is tested in a large towing tank of National Iranian Marine Laboratory. Wave spectrum shape effect on the efficiency of the OWC model is addressed. Moreover, the paper investigates the effects of the geometric and hydrodynamic factors on OWC device efficiency and uncovers new points in nonlinear interaction occurring inside the chamber; i.e. sloshing. The results indicate that shape of the spectrum inside the chamber is affected by the type of incident wave spectrum, especially for long waves. Pierson–Moskowitz spectrum leaded to higher efficiency rather than JONSWAP spectrum at longer incident wave periods. According to efficiency analysis, increasing wave height may lead to air leakage from the chamber followed by vortex generation, which is a reason for decreasing the efficiency of the OWC device. Furthermore, no shift in the resonant period of the OWC model, due to wave height increase, was observed at the opening ratios equal or smaller than 1.28%. Spectral analysis of water fluctuation inside the OWC chamber illustrates two modes of sloshing. The first mode can be seen at short period waves while the second mode is visible at long period waves. The sloshing modes approximately vanish by increasing draft value.

Experimental investigation of a land-based dual-chamber OWC wave energy converter

Extensive investigation on wave energy converters (WEC) that convert wave power into electricity is required to improve competitiveness of wave energy, one of the most promising renewable energy resources. Better hydrodynamic performance should enable wave devices to compete favorably with conventional power plants in the near future. In this study the hydrodynamic performance of a land-based dual-chamber Oscillating Water Column (OWC) device is investigated experimentally, with emphasis on its overall performance and respective performance of the two sub-chambers of the system. The effects of the chamber breadth and the barrier wall draft on the hydrodynamic efficiency, the free surface elevation and the air pressure inside the chambers are investigated over a wide range of wave periods with a constant wave amplitude. Numerical simulations using fully nonlinear numerical wave tanks (NWTs) within the framework of potential flow theory are also carried out to cross-check the results, and help to further understand the experimental observations. It is found that both the maximum efficiency and the range of wave frequencies that leads to a higher rate of wave energy absorption, i.e. the effective frequency bandwidth, increase in the dual-chamber OWC system when compared to an equivalent typical single-chamber OWC device. The overall efficiency of the system is found to be insensitive to the variation of the sub-chamber breadth, while the efficiency of the two sub-chamber increases with its breadth when the total chamber breadth is kept the same. Additionally, the hydrodynamic efficiency is found to decrease with increasing barrier wall draft. The rear chamber (i.e., on the landward side) outperforms the front chamber (i.e., on the seaward side) for most of the wave frequencies investigated in this study, especially for the wave frequency that is at or close to the resonance frequency of the system.

Analysis of the Geometric Constraints Employed in Constructal Design for Oscillating Water Column Devices Submitted to the Wave Spectrum through a Numerical Approach

This paper aims a numerical investigation about the fluid dynamic behavior of an oscillating water column (OWC) wave energy converter (WEC) into electrical energy. Constructal design is employed to perform a geometric evaluation of an OWC WEC submitted a Pierson-Moskowitz wave spectrum. The objective function is to maximize the energy conversion. The hydropneumatic chamber volume (VHC) and the total OWC volume (VT) are adopted as geometric constraints. In the first stage, the values are constant during the maximization process. However, in a second stage they are changed according to the constraint variation (CV). One of the goals is to analyze the influence of the choice of this geometric constraints value on the OWC performance in relation to the wave spectrum. For this purpose, are considered three different scenarios: 1) VHyd is equal to the minimum incident wavelength (λmin), that is relative to the maximum frequency of the wave spectrum times the significant wave height (HS); 2) VHyd is equal to the peak incident wavelength (λpeak), that is relative to peak frequency of the wave spectrum times the significant wave height (HS); and 3)VHyd is equal to the maximum incident wavelength (λmax), that is relative to minimum frequency of the wave spectrum times the significant wave height (HS). To do so, constructal design is employed varying the degree of freedom (DOF) H1/L (ratio between the height and length of OWC chamber), while the others DOF’s H2/l (ratio between height and length of chimney) and H3 (lip submergence), are kept fixed. It is employed a Pierson-Moskowitz wave spectrum with significant period (TS) equal to 7.5 s and significant wave height (HS) equal to 1.5 m. For the numerical solution it is used the computational fluid dynamic (CFD) code, based on the finite volume method (FVM). The multiphase volume of fluid (VOF) model is applied to tackle with the water-air interaction. The computational domain is represented by the OWC WEC coupled with the wave tank. The results showed that when CV = 2.25 for λmax and (H1/L)O = 0.2152 the highest average for power was obtained, nearly 18,000 W. While for λmin and (H1/L)O = 0.2193 it was smaller than 1,000 W. Besides, it was obtained a theoretical recommendation about the geometric constraints employed for the constructal design application, aiming the maximization of the OWC energy conversion from the incident wave spectrum.

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.

Motion and performance of BBDB OWC wave energy converters: I, hydrodynamics

The Backward Bent Duct Buoy (BBDB) oscillating water column (OWC) wave energy converter (WEC) has been invented following the so-far most successful OWC navigation buoys in wave energy utilisation, with aims to build large and efficient OWC wave energy converters for massive wave energy production. The BBDB device could use its multiple motion modes to enhance wave energy conversion, however, the mechanism of the motion coupling and their contributions to wave energy conversion have not been well understood in a systematic manner. In particular, the numerical modelling has been very limited in exploring how these motions are coupled and how the wave energy conversion capacity can be improved.As in this part of the research of a systematic study using numerical modelling, focus is on the understanding of the hydrodynamic performance for the BBDB OWC wave energy converter. In the study, the boundary element method based on potential flow theory has been applied to calculate the basic hydrodynamic parameters for the floating BBDB OWC structure and the water body in the water column in the BBDB OWC device. With the calculated hydrodynamic parameters and the decoupled and coupled models for the BBDB OWC dynamics, it is possible to examine these hydrodynamic parameters in details and to understand how they interact each other and how they contribute to the relative internal water surface motion, a most important response in terms of wave energy conversion of the OWC devices. All these will provide a solid base for further studying the power performance of the BBDB devices for converting energy from waves as shown in the second part of the research.

Evaluation of the energy extraction of a small-scale wave energy converter

ABSTRACT This work aims to focus on proposals that could stimulate the development of small scale integrated devices for the global challenge to provide electric energy from renewable alternative resources without major interventions. It presents an evaluation of a small-scale wave energy extraction system that can be installed in marine near shore structures, such as fishing piers. The system is characterized by a small oscillating-water-column (OWC) converter composed by tubes tied to the pillars of the structure. A mathematical model of the OWC device was developed. The model relies on two main components. The first uses linear wave theory to describe the water level variation inside the tube as a result of a wave passing by. The second considers the air flux converted to mechanical torque using Wells turbine equations. The simulations were carried out for different water depths and wave parameters, to evaluate the ratio between the input and output energy throughout the year. For the case study presented in this paper, the performance would be better as long as the device is placed in a position where the waves are less influenced by the bottom friction, but it still has the necessary increment of the wave height.

AN EXPERIMENTAL STUDY OF HYDRODYNAMICS OF OWC DEVICE EMBODYING IN CAISSON BREAKWATER

It has currently become an important issue in searching for clean and sustainable renewable energy. Wave power is one of substantial renewable energies. The oscillating water column (OWC) device is the most extensively studies of wave energy converter and the largest number of prototypes so far deployed into the sea. The breakwater-integrated OWC has been constructed successfully in Sakata harbor in Japan, Mutriku port in Spain, and Civitavecchia harbor in Italy etc. The breakwater-integrated OWC has several advantages, especially much easier maintenance of the wave energy plant (Falcão and Henriques, 2016). However, the front wall of typical OWC device may be received large wave force when the storm waves impacted on it. This study proposed an improvement of a caisson type breakwater-integrated OWC device by installing a perforated front wall to reduce the wave impacting on the structure but also to promote the capture efficiency of the wave energy. This study reports the experimental results of both water and air flow characteristics inside the OWC chamber to demonstrate the hydrodynamic performance of the improved OWC device.

Development of a reduced order wave to wire model of an OWC wave energy converter for control system analysis

Wave energy converters (WECs) face difficulties such as low operating range, low power output and, fluctuating power. A seamless synchronization among WEC components is required to get a better performance. For control system studies, the model should capture all the necessary dynamics involved in each conversion stages, however the interlinked complexity in each subsystem increases the computation time. This article presents a reduced order wave-to-wire (WTW) model of an oscillating water column (OWC) based WEC. The approach involves modeling of hydrodynamic and aerodynamic coupling of the capture chamber, aerodynamic and thermodynamic coupling inside the capture chamber, aerodynamic and rotor dynamic coupling in air turbine; and rotor dynamics and generator dynamics in the turbine generator coupling. The result shows that the model retains its fundamental dynamics and reduces the number of unknowns to describe the state space. The model indicates the correlation of each variable represented in the state space. The model predicted power output for different sea state. It also shows that the accuracy and the efficiency of the model are acceptable for OWC-WEC control system studies. The present model can be used as a time domain tool to design an effective control system for OWC device for different sea states, and the overall device performance can be improved significantly.

Multi-chamber oscillating water column wave energy converters and air turbines: A review

The oscillating water column (OWC) is a more common type of wave energy converter (WEC) that has been the subject of the study and development for several decades. Multi–chamber oscillating water column (MC–OWC) devices or arrays have the advantage of being more efficient in energy extraction compared to a single chamber system, particularly in more chaotic sea states. A variety of single and array OWC devices have been proposed and studied on a small–scale, whereas few large–scale devices have been tested under ocean wave conditions. This paper provides a concise review of the current state of MC–OWC device development in laboratory conditions. The review highlights explicitly the main stages of MC–OWC device development for one ongoing study as an example. This review was based on the available information in the literature from 2003 to 2012, in addition, further work is presented as part of the current study at the University of Technology Sydney. This study is from 2015 to 2018. The discussion shows the challenges that a device needs to overcome to be more competitive with other WECs in the global of wave energy converter area.

Hydrodynamic analysis of an oscillating water column wave energy converter in the stepped bottom condition using CFD

The primary efficiency of the Oscillating Water Column (OWC) Wave Energy Converter (WEC) device in the stepped sea bottom condition is investigated using both experimental and numerical approaches. Wave flume tests were undertaken to investigate the hydrodynamic behaviour of the device in regular waves. A 2D numerical model was developed in the open source computational fluid dynamics (CFD) software package OpenFOAM implementing the fully non-linear Reynolds Averaged Navier-Stokes (RANS) equations to simulate the wave power absorption and wave structure interactions. The numerical results have been validated against the experimental data. The influence of the wave characteristic as well as damping of the power take-off unit on the performance of the device, wave reflection coefficient and the energy dissipation rate is evaluated using the results obtained from the numerical simulations. Furthermore, as was proved in previous studies that the application of the stepped sea bottom condition might increase the efficiency of the OWC devices, the CFD simulation results have been implemented to present and discuss about the power absorption mechanism as well as the flow pattern characteristics in the vicinity area of the step.

Numerical evaluation of the power output of an oscillating water column wave energy converter installed in the southern Brazilian coast

Although studies show high wave energy potential in several coastal regions around the world, its evaluation and detailed characterization are necessary to optimize its harvest. This study aims at numerically evaluating the power output of an Oscillating Water Column (OWC) wave energy converter installed in the southern Brazilian coast. The FLUENT® numerical model, based on the Reynolds Averaged Navier-Stokes (RANS) equations and the Volume of Fluid (VoF) technique, is used. The turbine diameter optimization (TDO) model, which considers the motion of the water surface inside the OWC chamber as a piston movement in response to the hydrodynamic forces from the incoming waves, is also employed. The OWC is equipped with a Wells turbine, with pressure control (relief valve) and regulation of the turbine rotational speed. The turbine diameter is chosen by taking into account the wave characteristics in the region where the device is installed. An OWC device 10 m wide, 10 m long, 16 m high (10 m submerged) with a front wall 2.5 m submerged reaches its optimum performance with an annual-averaged power output of 36.48 kW (efficiency of 54.6%), by using a turbine with 2.25 m diameter.

CFD analysis of the energy conversion process in a fixed oscillating water column (OWC) device with a Wells turbine

Oscillating Water Column (OWC) devices, both the fixed structures and the floating ones, are an important class of Wave Energy Converter (WEC) devices. In this work, we carried out a numerical investigation aiming to give a deep insight into the fluid dynamic interaction between waves and a U-shaped OWC breakwater, focusing on the energy conversion process. The U-OWC breakwater under consideration, represents the full-scale plant installed in the Civitavecchia (near Rome) harbour. The adopted numerical method is based on the solution of the unsteady Reynolds Averaged Navier-Stokes equations (URANS). The water-air interaction is taken into account by means of the Volume Of Fluid (VOF) model. A two-dimensional domain has been adopted to investigate the unsteady flow outside and inside the OWC device. In order to simulate the action of an air turbine of the Wells type, the air chamber has been connected to the atmosphere by means of a porous medium able to reproduce its linear relationship between pressure drop and flow rate of the air turbine. Several simulations have been carried out considering periodic waves of different amplitudes in order to analyze the performance of the plant and, in particular to analyze the resonance with incoming waves, when the U-OWC is expected to absorb more energy. In order to characterize the plant efficiency, we split the energy conversion process into three main steps, 1) the primary conversion from wave energy to hydraulic energy the water discharge flowing inside the U-duct; 2) the secondary conversion from the OWC inlet to the oscillating pneumatic power made available to the turbine and, finally, 3) the turbine mechanical power output. To this purpose, the simulations of three different cases, varying wave period and height, have been carried out to quantify the energy captured by the plant and the fluid dynamic losses both in the water and in the air.

Power performance of BBDB OWC wave energy converters

Abstract This research presents the power performance of the backward-bent duct buoy (BBDB) oscillating water column (OWC) wave energy converters (WECs). To achieve that, following steps are detailed: firstly, power conversion from wave power into pneumatic power by coupling the hydrodynamics and the thermodynamics in the air chamber using a linear air turbine power take-off (PTO), with the calculation of the power response curves for the BBDB OWC device in regular waves; secondly, using the power response curve, a power performance curve is then calculated for irregular waves; thirdly, the power matrices for the device is calculated and the determination of the rated power for the device to meet the target capture factor; and finally, the annual energy production will be assessed as the final indicator for the device's power performance. Using the developed approach, some initial optimisations are made to the original design. It is shown that with some simple optimisations, the BBDB OWC device could increase the annual energy production (AEP) significantly. A simple change in making a uniformed water column for the RM6 device could increase the AEP by about 10%, whilst increasing the horizontal duct length by 10 m could increase the AEP by 58%.

Hydrodynamic performance of single–chamber and dual–chamber offshore–stationary Oscillating Water Column devices using CFD

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.

Performance of an array of oscillating water column devices integrated with an offshore detached breakwater

A systematic experimental investigation on the effect of different spacing between an array of oscillating water column (OWC) devices integrated with offshore detached breakwater (ODBW) in terms of their hydrodynamic performance has been studied. A Froude scale of 1:20 was chosen for integrating five similar devices of OWC to the ODBW (OWCBW) and a series of experiments was carried out in a shallow wave basin. Three different centre to centre spacings, S, i.e., one, two, and three times the width of the OWC model were considered. The hydrodynamic performance was studied in terms of non-dimensional lip wall pressure, wave amplification factor, non-dimensional air pressure, capture width, and relative capture width. The OWCBW system with thrice the width of the model spacing exhibits a better performance. The convergence of water waves in front of the OWC devices due to obstruction from rubble mound breakwaters to progressive waves enhances the concentration of energy in front of the array of OWC devices. It facilitates the system to absorb a larger amount of wave power of about 2.2 times the given input power at natural frequency of the system. This confirms that the array of OWC devices exhibits better performance than in isolation.

Three-dimensional Numerical Simulation of Airflow in Oscillating Water Column Device

Lee, K.H. and Kim, T.G., 2018. Three-dimensional Numerical Simulation of Airflow in Oscillating Water Column Device. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1346–1350. Coconut Creek (Florida), ISSN 0749-0208.An Oscillating Water Column (OWC) is a wave energy converting technology that uses the compressed and decompressed airflow induced by the vertical oscillations of water column inside the chamber as a driving force of an airflow turbine. In this study, three-dimensional numerical simulations of a bottom-fixed OWC are performed to understand the interaction of an OWC device with regular waves. The open-source CFD model REEF3D is used, which is able to consider immiscible two-phases (air and water) using the Reynolds-averaged Navier-Stokes equations. The simulation capability of REEF3D is validated by comparison with experimental data for wave deformations caused by the OWC and air-flow fields and the airflow velocity in the chamber. The numerical results revealed that the CFD numerical tool REEF3D can successfully simulate wave deformations caused by wave-OWC interactions, and directly estimate an air velocity inside the chamber. In addition, it was found that a phase difference occurs between time series of free surface evolution and air velocity evolution.