Spar-buoy Oscillating Water Column Research Articles

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.

Comparative study of offshore spar-buoy oscillating water column dynamic models for captured power estimation

The prompt estimation of power and geometrical aspects enables faster and more accurate financial assessment of wave energy converters to be deployed. This may lead to better commercialisation of wave energy technologies, as they require location-based customisation, unlike the mature wind energy technologies with developed benchmark. The adopted approach provides simple and efficient modelling tool allowing the study of the system from different perspective. The aim of this study is to select the optimum dynamic model to predict the captured power of a spar-buoy Oscillating Water Column (OWC) wave energy converter. Four dynamic models were developed to predict the system dynamics and results were validated experimentally. In-depth investigations on the effect of the mass and damping ratios of the oscillating bodies on the accuracy of the adopted models were performed. Such investigations included the proposed one-way coupling model and three two-degree of freedom models and three reduced-scale models, in addition to analytical and numerical solutions. Pneumatic power was calculated for the reduced-scale model where orifices’ covers simulated the power take-off mechanism damping experimentally. Analysis and comparisons between the adopted models are finally provided.

Opening the air‐chamber of an oscillating water column spar buoy wave energy converter to avoid parametric resonance

The oscillating-water-column (OWC) spar-buoy is a type of wave energy converter that may exhibit undesirable large roll and pitch amplitudes caused by a dynamic instability induced by parametric resonance. The occurrence of this phenomenon not only reduces the power extraction but significantly increases the structural loads on the buoy, the turbine rotor and on the mooring system. The paper compares the parametric resonance behaviour of two configurations of an OWC spar-buoy using experimental data obtained in a wave flume at a scale of 1:100. The configurations investigated were: (1) closed and (2) fully open-air chamber. The experimental tests covered a wide range of regular and irregular waves, as well as in free decay experiments. Results showed that opening the air chamber reduces the coupling between the buoy and the OWC within, thus shifting the damped natural heave frequency of the system in comparison with the closed chamber configuration. This effect changes parametric resonance characteristics of the two configurations due to the coupling between roll/pitch and heave modes. Moreover, for specific wave frequencies, the occurrence of parametric resonance observed when the chamber is closed do not occur while the air chamber is fully open. These results suggest the possibility of controlling a pressure relief valve installed on top of the device to reduce parametric resonance whenever this dynamic instability is detected.

Wave energy converter physical model design and testing: The case of floating oscillating-water-columns

The quest for exploiting the ocean resources and understanding its behaviour has been a challenge with increasing needs for innovation and technology. Model testing is an essential step in offshore renewable energy technology development. It involves challenges that require experience and guidance. Costly mistakes might arise with the subsequent waste of time and resources. This paper presents the model design and testing processes as part of wave energy projects and the results of experimental testing of two types of oscillating-water-column (OWC) wave energy converters (WEC). The model design aims at the creation of a reduced-scale model to simulate the physical phenomena found in full-scale devices. It is a process that requires several skills and an adequate compromise among all variables. This design involves several approaches as different physical phenomena do not follow the same similarity conditions, requiring adjustments in scale, materials, and other relevant properties. Besides, the model testing process comprises the necessary planning and actions to execute the tests and post-processing of data. This process is addressed here through model design and testing of two WECs: the coaxial-duct and the spar-buoy OWCs. The configurations have been designed and studied for large-scale energy production and small-scale power in oceanographic applications. Although the devices are both OWCs, the designs exhibit significant differences. The development process of the models and results are presented for the two OWC devices. Free-decay tests, hydrodynamic performance and mooring tension results are presented and discussed. These may serve as guidelines and numerical modelling validation.

Detecting parametric resonance in a floating oscillating water column device for wave energy conversion: Numerical simulations and validation with physical model tests

The wave energy sector has faced enormous technological improvements over the last five decades, however, due to the complexity of the hydrodynamic processes, current numerical models still have limitations in predicting relevant phenomena. In particular, floating spar-type wave energy converters are prone to large undesirable roll and pitch amplitudes caused by a dynamic instability induced by parametric resonance. Detecting this phenomenon accurately is essential as it impacts drastically on power extraction, structural loads and mooring forces. This paper presents the validation of results from a numerical model, capable of detecting parametric resonance, using experimental data. Experiments were carried out for a scaled model of the Spar-buoy OWC (Oscillating Water Column) device at a large ocean basin. The buoy uses a slack-mooring system attached to the basin floor. The scaled turbine damping effect is simulated by a calibrated orifice plate. Two different buoy draft configurations are considered to analyse the effect of different mass distributions. The numerical model considers the nonlinear Froude-Krylov forces, which allows it to capture complex hydrodynamic phenomena associated with the six-degree-of-freedom motion of the buoy. The mooring system is simulated through a quasi-static inelastic line model. Real fluid effects are accounted for through drag forces based on the Morison’s equation and determined from experimental data. The comparison of results from regular-wave tests shows good agreement, including when parametric resonance is detected. Numerical results show that parametric resonance can produce a negative impact on power extraction efficiency up to 53%.

The Effect of Mooring Line Parameters in Inducing Parametric Resonance on the Spar-Buoy Oscillating Water Column Wave Energy Converter

Although it is widely accepted that accurate modeling of wave energy converters is essential for effective and reliable design, it is often challenging to define an accurate model which is also fast enough to investigate the design space or to perform extensive sensitivity analysis. In fact, the required accuracy is usually brought by the inclusion of nonlinearities, which are often time-consuming to compute. This paper provides a computationally efficient meshless nonlinear Froude–Krylov model, including nonlinear kinematics and an integral formulation of drag forces in six degrees of freedom, which computes almost in real-time. Moreover, a mooring system model with three lines is included, with each line comprising of an anchor, a jumper, and a clump weight. The mathematical model is used to investigate the highly-nonlinear phenomenon of parametric resonance, which has particularly detrimental effects on the energy conversion performance of the spar-buoy oscillating water column (OWC) device. Furthermore, the sensitivity on changes to jumper and clump-weight masses are discussed. It is found that mean drift and peak loads increase with decreasing line pre-tension, eventually leading to a reduction of the operational region. On the other hand, the line pre-tension does not affect power production efficiency, nor is it able to avoid or significantly limit the severity of parametric instability.

Time-domain simulation of a slack-moored floating oscillating water column and validation with physical model tests

The development of devices for extracting wave energy from the ocean is largely supported by numerical models, as they allow the simulation of different configurations without the large costs of tank testing. From the different available options, time-domain models offer a very good combination between accuracy, flexibility and computational time. They allow the incorporation of non-linearities from power take-off systems, mooring lines, sophisticated control techniques and other relevant hydrodynamic effects. In this paper, we present a time-domain model to simulate the dynamics and power performance of a slack-moored Spar-buoy OWC (Oscillating Water Column) wave energy converter. The model considers linear hydrodynamics, mean drift forces, viscous drag effects and air compressibility inside the OWC chamber. The mooring system is simulated using a quasi-static approach. The floating structure is defined as a rigid body with six degrees of freedom, whereas the OWC free surface is assumed flat. The converter motion and power extraction from regular and irregular wave simulations are compared with experimental results from small-scale model tests in a wave channel. Numerical results show good agreement with experimental data except when parametric resonance is observed and near the channel cut-off frequencies.

On the dynamics of an array of spar-buoy oscillating water column devices with inter-body mooring connections

The performance of an array of floating Oscillating Water Column (OWC) devices, known as spar-buoy OWC, is analysed for a configuration with inter-body mooring connections. This configuration has the potential of being a more economically viable solution due to drastic reductions in the amount of mooring cables, when compared to independently moored configurations. Numerical simulations for an array of independently moored devices and for an unmoored array are presented and compared. The frequency-domain model considered throughout this paper uses linear hydrodynamic and hydrostatic forces; real fluid viscous effects are accounted for by using a linear approximation, while the mooring connections are linearised using perturbation theory. For regular waves, by including an inter-body mooring system, the average heave response amplitude of the array’s three buoys decreases by approximately 6.8% at the peak frequency, while the average capture width of the three buoys remains approximately the same. The influence of the wave incidence angle on the array performance is evaluated. A stochastic analysis is conducted to assess the behaviour of the array with mooring connections. A comparative analysis between the performance of an array with inter-body mooring connections and an isolated device suggests a positive park effect for a realistic wave climate.

Test results of a 30 kW self-rectifying biradial air turbine-generator prototype

A 30 kW fixed-guide-vanes biradial turbine-generator set was designed by Instituto Superior Técnico (IST) and manufactured by Kymaner, within the framework of the H2020 European project OPERA, for open sea testing at the Mutriku breakwater oscillating water column (OWC) power plant and at the IDOM MARMOK-A-5 OWC spar-buoy, deployed at BiMEP, Basque Country, Spain. The paper reports the design and construction of the prototype, and its dry testing at IST, in a variable-flow test rig, simulating the oscillating flow induced by irregular waves. Results show that the turbine exhibits a peak efficiency of 70% under steady-state conditions and an average efficiency of 56% for the control laws tested.

Development of a free heaving OWC model with non-linear PTO interaction

Abstract This paper presents the development of a Computational Fluid Dynamics (CFD) model for a free heaving Oscillating Water Column (OWC) spar buoy with non-linear Power Take Off (PTO). Firstly, a freely heaving barge was applied to a 2D Numerical Wave Tank (NWT), used to validate a 1 Degree Of Freedom (DOF) modelling methodology. Multiple sets of regular waves were used to assess the heave response compared to previous experimental and numerical studies. In parallel, the NWT was extended to 3D where analyses of incident waves have been conducted to ensure accurate waves are portrayed. A PTO boundary condition was created to replicate a non-linear impulse turbine, typically simulated by an orifice plate in scaled models. The PTO boundary was compared and validated using experimental data. Finally, a comprehensive system comprising of the 3D NWT, 1DOF set-up and non-linear PTO allowed the development of a heave-only OWC spar buoy model with a non-linear PTO. Experiments completed by UCC MaREI centre in LIR-NOTF ocean wave basin under FP7 MARINET project is detailed and used to validate the comprehensive model. A range of regular waves were applied and responses of heave and chamber pressures were compared to experimental data, which showed excellent correlation.

Wave power extraction of a heaving floating oscillating water column in a wave channel

The performance evaluation of a wave energy converter in wave channel is influenced by the hydrodynamic effects caused by the near presence of the side walls. Since this phenomenon is not observed in the open ocean, it is important to assess the walls influence in the converter dynamics when analysing experimental results. This paper studies the dynamics and power extraction of an axisymmetric floating oscillating water column (OWC) device, the Spar-buoy OWC, using experimental data obtained in a wave channel. A two heaving body model (spar-buoy and OWC) based on linear forces is formulated in the frequency domain. Linear hydrodynamic coefficients are obtained from a boundary integral equation method. The presence of the channel side walls is simulated approximately by a periodic array of devices, and alternatively by two finite-length walls. Linearized drag forces are derived from small-scale model tests. Power extraction results are presented for regular and irregular waves. The numerical simulations show that the wall effect may amplify the power capture up to a maximum of 15% for regular waves and 10% for irregular wave conditions, for a channel-width-to-device-diameter ratio equal to 5.25.

Latching control of a floating oscillating-water-column wave energy converter

The OWC spar-buoy is an axisymmetric floating version of an oscillating-water-column (OWC) based device whose power take-off (PTO) system is an air turbine/generator set. Latching has been regarded as one of the most promising techniques to improve the efficiency of wave energy converters. In the case of the OWC spar-buoy, latching control is performed by opening/closing a high-speed stop valve installed in series with the turbine. The present paper has three main objectives. Firstly, to assess the performance improvements that can be achieved with a latching control strategy within a receding horizon framework. Secondly, to establish the practical requirements of this type of control by evaluating the sensitivity of the turbine power output to several receding horizon time intervals. Finally, to test and validate experimentally the proposed algorithms in a small-scale PTO test rig. All the experimental tests were performed considering irregular wave conditions.

Testing and control of a power take-off system for an oscillating-water-column wave energy converter

The paper concerns the development of the PTO (power take-off) control of an OWC (oscillating-water-column) spar-buoy wave energy converter. The OWC spar-buoy is an axisymmetric device consisting of a submerged vertical tail tube open at both ends, rigidly fixed to a floater that moves essentially in heave. The oscillating motion of the internal free surface relative to the floater-tube set, produced by the incident waves, makes the air flow through a novel self-rectifying air turbine: the biradial turbine. To reduce the losses of the PTO system at partial load, an electrical generator with a rated power twice the maximum expected average power conversion of the buoy was adopted. The control of the turbine-generator set under highly energetic sea-state conditions was experimentally investigated by means of tests performed in a PTO test rig. In the reported tests, the hydrodynamics of the OWC spar-buoy and the aerodynamics of the air turbine were numerically simulated in real-time and coupled with the experimental model of the turbine/electrical generator set in a hardware-in-the-loop configuration. The experimental results allowed the dynamic behaviour of the PTO to be characterized and provided validation of the proposed control algorithms that ensure operation within safe limits.

Dynamic modelling of Spar-Buoy oscillating water column wave energy converter

The main objective of this research is to develop an experimentally validated numerical wave power prediction tool for an offshore Spar-Buoy oscillating water column (OWC) wave energy converter (WEC). The numerical tool should be able to predict the spar heave motion and the water column oscillations inside the structure, in order to estimate the power absorber and evaluate the device performance. A two-degree-of-freedom mechanical oscillation model was adopted. The model considers the power take-off (PTO) damping and the stiffness due to air compressibility inside the pneumatic chamber. Experiments were performed in order to calculate the damping coefficients and to validate the predicted results. PTO mechanism was experimentally modelled by orifice plate located on top of the OWC model. Different orifice areas were used to allow the examination of the PTO damping effect on the hydrodynamic behaviour of the system.

Latching Control of an Oscillating Water Column Spar-Buoy Wave Energy Converter in Regular Waves

The present paper concerns an oscillating water column (OWC) spar-buoy, possibly the simplest concept for a floating OWC wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends and fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control through the latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular, such compressibility may remove the constraint of the latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modeling is performed in the time domain for a given device geometry and includes the numerical optimization of the air turbine rotational speed, chamber volume, and latching parameters. Results are obtained for regular waves.

Dynamics and optimization of the OWC spar buoy wave energy converter

The paper concerns the hydrodynamic analysis and optimization of an OWC spar buoy, possibly the simplest concept for a floating oscillating water column (OWC) wave energy converter. It is an axisymmetric device consisting basically of a submerged vertical tail-tube-fixed to an axisymmetric floater that oscillates essentially in heave. The air flow displaced by the water motion inside the tube drives a self-rectifying air turbine. The possible advantages of using a tube of non-uniform inner cross section are investigated theoretically and numerically, especially as a way of reducing the draught of the device without significantly impairing its power performance. The unsteady water flow in the tube is modelled as one-dimensional. The frequency-dependent hydrodynamic coefficients of the tube-floater pair were computed with a boundary-element code. A linear air turbine is assumed. The hydrodynamics of the wave energy absorption is analysed in the frequency domain, including the effect of air compressibility in the chamber; special attention is devoted to optimization. Numerical results are presented for device's performance in regular and irregular waves, including especially optimization of the tube geometry and of the turbine characteristic. Practical implications of these results are discussed.