Oscillating Water Column Wave Energy Research Articles

A novel geometry of an onshore Oscillating Water Column wave energy converter

This study aims proposing a novel geometry of the onshore Oscillating Water Column (OWC) device to increase its efficiency. A comparison between standard OWC's and the new proposal ones, in which the exterior front wall is inclined in opposite direction from that of the incident wave, is carried out. Simulations of 2D flows, with incompressible water and compressible air, are performed by means of the FLUENT® software, which is based on Reynolds-Averaged Navier-Stokes equations. The k-ϵ turbulence model and the Volume of Fluid method are employed. Analyses of the hydrodynamic and aerodynamic behavior, run-up/down on the exterior front wall, sloshing inside the chamber and the energy balance of OWC devices equipped with the Wells turbine and with different wall slopes (40° − 90° from horizontal) are carried out for incident regular waves with periods from 6 to 12 s and height of 1.5 m. The new proposal OWC's provide higher extracted energy than standard ones. The best performances are reached by wall slope of external front wall of 40° and internal wall slopes of 40° and 52°, although the more efficient solution depends on the sea state characteristics of the region where the OWC will be installed.

Numerical study on free-spinning performance of oscillating water column Wells turbine in reciprocating airflows

The axial-flow Wells turbine is one of the most widely used air turbines in oscillating water column wave energy converters. By Wells turbine, we mean a reaction air turbine developed by A. A. Wells of Queen's University Belfast in the late 1970s. A comprehensive understanding of its free-spinning performance is crucial for determining control strategies for output power enhancement in practical engineering applications. In the present study, a three-dimensional (3D) transient model was established on an ANSYS-Fluent® platform to simulate the time-varying flow field and motion state of the rotor during the free-spinning process. After the model was validated with our experimental data, it was used to investigate the operation patterns in airflows with various profiles. The magnitude and phase features of the pressure difference and turbine torque were examined to identify the mechanism for overcoming the gradually ascending stage and maintaining dynamic equilibrium in the stable state. Additionally, the 3D flow-field details for several instants were demonstrated, including the severe vortex from the suction side in the post-stall region, strong tip leakage vortex downstream of the rotor, downstream helical strip vortex, and cyclic-asymmetric surface pressure distributions over the turbine. Furthermore, the effects of the cyclic volume flux on the free-spinning performance were investigated.

Hydrodynamics of an U-shaped OWC device in a two-layer fluid system

In this article, the effectiveness of an U-shaped “oscillating water column wave energy converter device (OWC-WEC)” is investigated in a two-layer fluid system. To solve the BVP, the “dual boundary element method” (DBEM) is employed. Further, the response of shape parameters on the effectiveness of OWC-WEC is studied in “surface mode” (SM) and “internal mode” (IM), respectively. It is seen that in the surface mode, the variation in the efficiency curve as a consequence of the change in chamber’s length and the lip wall’s draft adopt a similar resonating pattern as in single layer fluid. Furthermore, negligible change is appeared in the efficiency curve due to the change in density ratios in SM mode. Moreover, for smaller and moderate wavenumbers, the variation in the efficiency curve due to the various chamber length, draft of the lip wall and density ratios in internal mode shows highly oscillatory behavior. In addition, this oscillatory behavior vanishes in the short-wave zone.

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.

Model tests on a floating coaxial-duct OWC wave energy converter with focus on the spring-like air compressibility effect

The spring-like air compressibility effect in oscillating-water-column (OWC) wave energy converters may significantly affect the device’s power performance. Yet, this effect has rarely been accounted for in physical model testing, especially in floating OWCs. Quantifying this effect in the development and analysis of OWCs is crucial for i) making better estimates of their performances and decreasing uncertainties, ii) improving design and testing processes, iii) reducing development costs, and iv) accelerating decision-making on wave energy projects. This paper presents experimental results of a floating coaxial-duct OWC at 1:40-scale. The experimental campaign comprised tests in which the Froude scale was applied to the whole converter structure and, for comparison, tests where the air chamber volume was expanded to comply with the aero-thermodynamic similitude. Tests included regular and irregular waves and various damping conditions. Experimental results show that compressibility effects can increase, decrease, or have neutral contributions to the mean absorbed power, depending on wave frequency. Another effect is the presence of reactive power. The capture width ratio is higher for the compressible experimental configuration in several cases. A new compressibility factor is proposed to aid in designing experimental and numerical campaigns.

Theoretically based correction to model test results of OWC wave energy converters to account for air compressibility effect

The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive development. The spring-like effect of air compressibility in the chamber is related to the density–pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is rarely accounted for in model testing. A method is presented to correct results from physical model testing for effects of air compressibility. It combines linear theory in the frequency domain and hydrodynamic coefficients, together with air thermodynamics. A new concept of linear turbine simulator aerodynamically equivalent to the orifice simulator is introduced in the theory. Corrections for non-linear real-fluid effects may be accounted for from comparisons with data from wave tank testing. The method was validated in a case study involving published data from OWC model testing with regular waves of different periods and amplitudes in a wave flume. Theoretically based corrections were found to well predict the air compressibility effects upon power performance. Air compressibility may negatively or positively affect power conversion depending on whether the wave period is larger or smaller than a critical value accurately predicted by the theory.

A Study on the Viscous Damping Effect According to the Shape of the Inclined OWC Chamber Skirt

In this study, numerical analysis and experiments were performed to analyze the viscous damping effect according to the shape of the chamber skirt of the breakwater-linked inclined oscillating water column wave energy converter. Experiments were conducted using a two-dimensional mini wave tank and verified by comparing the results of a computational fluid dynamics numerical analysis. Pointed and rounded skirts were modeled to compare the effect of viscous damping when incident waves enter the chamber, and the difference in the displacement of the water surface in the chamber was compared according to the wave period for the two skirt shapes. The wave elevation in the chamber in the rounded-skirt condition was larger than the pointed-skirt condition in all wave periods, which was approximately 47% greater at 0.9 s of the incident wave period. Therefore, extracting the maximum energy through the optimal orifice is possible while minimizing the energy attenuation in the rounded-skirt condition.

Experimental measurements of wave-induced scour around a scaled gravity-based Oscillating Water Column Wave Energy Converter

Experimental measurements of wave scour are reported for a 1:20 scaled model of an operating 200 kW Oscillating Water Column (OWC) Wave Energy Converter (WEC) in King Island, Tasmania, Australia. Test cases include different wave conditions (Keulegan-Carpenter (KC) number range of 0.45-1.02) and multiple geometric changes to the device, which are compared to scour measurements taken in the field around the King Island OWC WEC device. The measurements therefore provide the first ever comparison of field data with a scaled laboratory model of scour around an OWC WEC.Results highlight that scour forms at the back corners of the device before undermining the structure with depths up to 64 mm (scour depth to structure width ratio, S/B, of 0.094), although the magnitude of this scour is shown to be reduced by 23% by curving the back corners. Scour at the front of the device is highly sensitive to the configuration of the front chamber door, with very different scour scenarios depending on whether the chamber door is open, closed, or removed entirely. Comparison with field data shows that undermining scour at the back of the device is reproduced in both settings, and this leads to settlement of the structure in both the laboratory and field. This suggests that the physical modelling forms an accurate representation of a real-world site, and therefore demonstrates scour processes with applicability to OWC WECs and other gravity-based bluff body structures such as caissons or gravity-based foundations.

A coupled numerical framework for hybrid floating offshore wind turbine and oscillating water column wave energy converters

Integrating floating offshore wind turbines with oscillating-water-column wave energy converters has been seen as a promising solution for hybrid offshore renewable energy production, as the cost-effective wave energy devices could possibly help increase the overall power absorption, reduce platform dynamic responses, and mitigate loads for critical wind turbine structures etc. As most existing research works on dynamic analysis of these hybrid concepts are based on frequency-domain simulations or scale model experiments, this work focuses on establishing an aero-hydro-elastic-servo-mooring coupled numerical framework for integrated time-domain dynamic analysis. In particular, the water column dynamics are characterised based on an equivalent virtual oscillating body approach so that the time-domain analysis capability for oscillating-water-columns with power take-off control is enabled. For validation, a novel combined concept is designed, and its time-domain numerical results under various environmental conditions have been compared against the 1:50 scale model wave basin test data. Good agreement has been observed between the numerical and experimental results, demonstrating the feasibility of the proposed numerical framework. Furthermore, different power take-off control strategies for the oscillating-water-column wave energy converters have been proposed, and it is found that the designed gain-scheduling control schemes are more beneficial for mitigating the platform motion responses and wind turbine structural loads compared with traditional linear damping control, resulting in 15% platform pitch motion mitigation and 6% tower base fatigue load reduction. Further studies on multi-objective optimal power take-off control design regarding both load reduction and power maximisation could be conducted for hybrid energy platforms based on the established numerical framework.

Experimental investigation of a novel OWC wave energy converter

Wave energy, one of the safest and currently growing forms of renewable energy, requires the development of the appropriate wave energy converters (WECs). This study presents a recently patented WEC; and proves the feasibility of this concept using a series of experiments. To achieve this, two rectangular chambers of an oscillating water column (OWC) system were arranged in series, and the wave with a propagation direction that forms a relatively small angle of 30° with the front walls was referred to as obliquely incident wave. The effects of the wave amplitude, front wall draughts, and opening ratios were investigated in relation to the overall hydrodynamic performance and the respective performances of the two sub-chambers. It was found that the overall efficiency of the system was greatly reduced as the front wall draught in the high-frequency zone increased. Furthermore, the opening ratios of the individual chambers significantly affected the overall hydrodynamic performance for all frequency bandwidths under obliquely incident waves, and the optimal efficiency achieved at the opening ratio of ε = 1%. The rear chamber is inferior to the front chamber for most of the wave frequencies.

Fluid dynamics inside a U-shaped oscillating water column (OWC): 1D vs. 2D CFD model

Several mathematical models have been proposed in the last decades, focusing on peculiar aspects of dynamic behaviors of OWCs Wave Energy Converters (WEC). Computational fluid dynamics (CFD) studies have been widely diffused in the recent past, thanks to the strong increasing of computing power. Numerical wave flumes, both 2D or 3D, are become refined tools of investigations. However, there are tasks performed effectively and conveniently through 1D mathematical models, which require much less time of setup and running, in respect to CFD. Tuning the eigenperiod of a plant with most energetic waves is the most important of these tasks. It requires several trials, changing the size of some geometric parts of the device. In this work, we improved a conventional 1D mathematical model, by removing the assumption of polytropic transformation of the air mass inside the plenum, which is physically not consistent, and considering the actual asymmetry of thermodynamics during inhalation and exhalation phases. The results were validated through a numerical experiment carried out in a 2D numerical wave flume, considering a U-OWC at a full scale, achieving an excellent agreement of pressure and temperature variations in the plenum chamber.

Hydrodynamic performance and optimization of a pneumatic type spar buoy wave energy converter

In order to optimize the geometry of the spar buoy wave energy converter and improve its capture performance, numerical calculation and experimental research were carried out. The spar buoy oscillating water column wave energy technology can be regard as one of the pneumatic types oscillating single floating body. According to the study of the hydrodynamic characteristic and capture performance of the floater and the water column in the central pipe, the spar buoy mainly utilizes its heave motion to absorb wave energy. By comparing the numerical calculation results of the hydrodynamic performance and Capture Width Ratio (CWR) of five spar buoy models with different floater shapes, it is found that the flat-bottomed spar buoy has the best capture performance. The influence of the model's total mass on its capture performance was studied. The results show that an appropriate increase in the total mass increases the optimal response period and helps to improve the capture performance. The maximum CWR can reach 0.984. The research results show that the tail tube length has a minimal effect on the wave energy conversion characteristic, but the water column in the tube is greatly affected by its length.

WCSPH simulation of the forced response of an attenuator oscillating water column wave energy converter

In this study, a weakly compressible smoothed particle hydrodynamics (WCSPH) model was used to simulate the forced piston motion of water inside a fixed oscillating water column (OWC) wave energy converter. The considered device was a chamber with a complex entrance geometry attached to the rear wall of the wave flume. The effect of an impulse turbine on the chamber was modeled by an equivalent orifice damping force applied on a thin plate. This way, a balance is established between numerical accuracy and efficiency. The simulations were performed for two different wave steepness values of 0.025 and 0.04. To validate the model results, the experimental work performed in the Technical University of Denmark was used. Fourier Transform analysis was performed on the SPH model results and experimental data to obtain the first-harmonic amplitude of the desired parameters. In general, a good correspondence was found between the SPH calculations and the experimental results for the piston motion of the water inside the chamber. However, the SPH results were slightly larger, especially around the resonance period. A possible reason is the placing of the plate above the free surface which might reduce the small sloshing motions of water inside the chamber. Finally, from the heaving velocity of the plate, the flux and pressure drop through the orifice were evaluated. It is shown that all three parameters reach their maximum around the resonance period of the chamber.

Design Optimization of a Cross-Flow Air Turbine for an Oscillating Water Column Wave Energy Converter

A cross-flow air turbine, which is a self-rectifying, air-driven turbine, was designed and proposed for the power take-off (PTO) system of an oscillating water column (OWC) wave energy converter (WEC). To predict the complicated non-linear behavior of the air turbine in the OWC, numerical and experimental investigations were accomplished. The geometries of the nozzle and the rotor of the turbine were optimized under steady-flow conditions, and the performance analysis of the model in bi-directional flows was conducted by commercial computational fluid dynamics (CFD) code ANSYS CFX. Experimentation on the full-scale turbine was then undertaken in a cylindrical-type wave simulator that generated reciprocating air flows, to validate the numerical model. The optimized model had a peak cycle-averaged efficiency of 0.611, which is 1.7% larger than that of the reference model, and a significantly improved band width with an increase in flow coefficients. Under reciprocating-flow conditions, the optimized model had a more improved operating range with high efficiency compared to the performance derived from the steady-flow analysis, but the peak cycle-averaged efficiency was decreased by 4.3%. The numerical model was well matched to the experimental results with an averaged difference of 3.5%. The proposed optimal design having structural simplicity with high performance can be a good option to efficiently generate electricity.

Hydrodynamic Investigation on a Land-Fixed OWC Wave Energy Device under Irregular Waves

The hydrodynamic response of a land-based oscillating water column (OWC) wave energy converter under various irregular wave conditions is investigated numerically. Based on the potential flow theory, a two-dimensional fully nonlinear numerical wave flume (NWF) is developed to model the hydrodynamic characteristics using the time-domain higher-order boundary element method (HOBEM). The inner-domain sources method and JONSWAP energy spectrum is used to generate the irregular incident waves, and a linear pneumatic model is used to determine the pneumatic pressure inside the chamber. The free surface elevations at the chamber centre and the oscillatory pneumatic pressures induced by the vertical motion of the water column are recorded. The influence of irregular waves on the hydrodynamic characteristics of the OWC device is carried out by comparison with the regular waves, and a number of significant wave heights and peak wave periods are considered. The hydrodynamic efficiency of the OWC device in irregular wave conditions is observed to be lower than that in regular waves for most wave frequencies, especially near the resonant frequency.

Field Observations of Scour Behavior around an Oscillating Water Column Wave Energy Converter

This study provides the first ever published measurements of scour and morphological change around an Oscillating Water Column (OWC) Wave Energy Converter (WEC) device at a real-world site, with the intention of informing future designs to reduce costs of the technology. A 200-kW prototype OWC WEC was deployed at King Island, Tasmania, Australia in January 2021, providing a unique opportunity to monitor the device using a combination of dive footage, multi-beam surveys and bedrock surveys. Settlement of the device was observed and monitored before ceasing once the foundation made contact with the underlying bedrock at the site. It is hypothesized that the settlement is caused by scour undermining the gravity structure’s foundations. The processes causing this scour are explored and possible future design modifications are suggested to reduce the risk of scour and settlement.

A CFD-based wave-to-wire model for the oscillating water column wave energy Convertor

The construction of the prototype OWC plant proposed development requirements of the wave-to-wire model for the overall performance predictions, which should balance the computational cost and accuracy. In this study, a CFD-based wave-to-wire model was developed to couple the air chamber and air turbine. The steady numerical model was employed to derive the relationship between the air pressure drop and the incident airflow velocity, which was induced to the air chamber model in the numerical wave tank using the porous zone module. The numerical predictions on the free surface elevation and air pressure variation in the chamber, the torque output of the turbine in regular and irregular wave scenarios, and the efficiencies were validated by the experimental data. By considering the air compressibility, the wave-to-wire model was employed to predict the performance of the 500 kW Yongsoo prototype plant. The interaction mechanism among energy converting processes and the overall performance of the plant under the real-sea condition were carefully analyzed and reported.

Performance analysis of a coast – OWC wave energy converter integrated system

In this paper, a cylindrical dual-chamber oscillating water column (OWC) wave energy converter semi-embedded in a coastline or breakwater is proposed. A theoretical model based on linear potential flow theory is developed to evaluate the power absorption performance of the integrated system in the finite water depth. According to the matched eigenfunction expansions, the fluid domain is divided into five sub-domains and unknown coefficients in the diffraction and radiation problems are separately solved by matching the velocity and pressure between adjacent sub-domains. A linear power take-off system considering air compressibility is adopted to connect air volume flux with the chamber pressure. The reliability of this model is assessed by general identities (i.e., far-field relations). Then the model is performed to investigate the effects of wave conditions, draft, breadth and opening angle of the chambers on the wave power extraction. It is shown that the proposed integrated system can improve the total hydrodynamic efficiency in both regular and irregular waves.

Experimental investigation into laboratory effects of an OWC wave energy converter

Reproducibility advances science and de-risks engineering. In ocean wave energy, developing wave energy converters (WECs) requires model test experiments, but there is limited knowledge on the consistency of results if an experiment is reproduced in multiple wave basin laboratories. To better understand reproducibility in WEC experiments, in particular laboratory effects, we reproduced a 1:30 scale model experiment of a case study WEC in two laboratories. This paper compares results between laboratories and evaluates whether, or the degree to which, each experimental parameter contributed to laboratory effects. Performance assessment tests were conducted in intermediate-shallow water regular waves of a bottom-fixed oscillating-water-column (OWC) WEC with a nonlinear, unidirectional flow power take-off (PTO). Despite conducting the experiments according to international guidelines, we found significant differences (15–30%) between laboratories in incident waves and capture width ratio, due to parameters associated with the test environment (wave generation and calibration, and ambient conditions of the air) and the model (deployment position, and PTO modelling). Wave-WEC nonlinear interactions likely amplified the interlaboratory differences. In conclusion, laboratory effects can be significant and, therefore, should be accounted for in WEC model test experiments, especially when there are nonlinearities. We close with recommendations to address laboratory effects relevant for WECs.

Analysis of a Two-Degree-of-Freedom Vibration System Model of an Oscillating water column wave energy converter installed on a double-slit breakwater

Breakwaters protect beaches by absorbing wave energy. Double-slit breakwaters can be used as air chambers for oscillating water column wave energy converters. In this study, the oscillating water column-type wave power generator using the double-slit breakwater was treated as a two-degree-of-freedom oscillating system model. The differential equations of motion were derived from Newton's second law, and the frequency response characteristics of the vibration of the oscillating water column- type wave energy converter using the double-slit breakwater were investigated using Simulink in MATLAB.