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.     

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.     

Large-scale experiments on the behaviour of a generalised Oscillating Water Column under random waves

This work investigates wave reflection and loading on a generalised Oscillating Water Column (OWC) wave energy converter by means of large scale (approximately 1:5–1:9) experiments in the Grosse Wellenkanal (GWK), in which variation of both still water depth and orifice (PTO) dimension are investigated under random waves. The model set-up, calibration methodology, reflection analyses and loadings acting on the OWC are reported. On the basis of wave reflection analysis, the optimum orifice is defined as that restriction which causes the smallest reflection coefficient and thus the greatest wave energy extraction. Pressures on the front wall, rear wall and chamber ceiling are measured. Maximum pressures on the vertical walls, and resulting integrated forces, are compared with available formulations for impulsive loading prediction, which showed significant underestimation for heaviest loading conditions.The present study demonstrates that a OWC structure can serve as a wave absorber for reducing wave reflection. Thus it can be integrated in vertical wall breakwaters, in place of other perforated low reflection alternatives. The possibility to convert air kinetic into electric energy, by means of a turbine, may give an additional benefit. Thus the installation of such kind of energy converters becomes interesting also in low energy seas.     

A twin unidirectional impulse turbine topology for OWC based wave energy plants

Experimental results from near shore bottom standing OWC based wave energy plants in Japan and India have now been available for about a decade. Historically the weakest link in the conversion efficiency of OWC based wave energy plants built so far has been the bidirectional turbine. This is possibly because a single turbine has been required to deliver power when the plant is exposed to random incident wave excitation varying by a factor of 10. A new topology that uses twin unidirectional turbines (which features a high efficiency spanning a broad range) is proposed. Using the Indian Wave Energy plant as a case study, it is shown that the power output from such a module considerably exceeds existing optimal configurations including those based on a fixed guide vane impulse turbine, linked guide vane impulse turbine or a Well's turbine. A wave to wire efficiency of the order of 50% over the incident range is shown to be feasible in a credible manner by showing the output at all stages of the conversion process. A frequency domain technique is used to compute the OWC efficiency and a time domain approach used for the power module with the turbine pressure being the pivotal variable.     

Performance estimation of bi-directional turbines in wave energy plants

Oscillating water column(OWC)based wave energy plants have been designed with several types of bidirec-tional turbines for converting pneumatic power to shaft power.Impulse turbines with linked guide vanes andfixed guide vanes have been tested at the Indian Wave Energy plant.This was after initial experimentation withWell's turbines.In contrast to the Well's turbine which has a linear damping characteristic,impulse turbines havenon-linear damping.This has an important effect in the overall energy conversion from wave to wire.Optimizingthe wave energy plant requires a turbine with linear damping and good efficiency over a broad range of flow co-efficient.This work describes how such a design can be made using fixed guide vane impulse turbines.The In-dian Wave Energy plant is used as a case study.     

Design and performance analysis of impulse turbine for a wave energy power plant

Wave energy is the most abundant source of renewable energy in the World. For the last two decades, engineers have been investigating and defining different methods for power extraction from wave motion. Two different turbines, namely Wells turbine and impulse turbine with guide vanes, are most commonly used around the world for wave energy power generation. The ultimate goal is to optimize the performance of the turbine under actual sea conditions. The total research effort has several strands; there is the manufacture and experimental testing of new turbines using the Wave Energy Research Team's (WERT) 0.6 m turbine test rig, the theoretical and computational analysis of the present impulse turbine using a commercial software package and finally the prediction of the performance of the turbine in a representative wave power device under real sea conditions using numerical simulation. Also, the WERT 0.6 m turbine test rig was upgraded with a data acquisition and control system to test the turbine in the laboratory under real sea conditions using the computer control system. As a result, it is proven experimentally and numerically that the turbine efficiency has been raised by 7% by reducing the hub‐to‐tip ratio from 0.7 to 0.6. Effect of tip clearance on performance of the turbine has been studied numerically and designed tip clearance ratio of 1% has been validated. From the numerical simulation studies, it is computed that the mean conversion efficiency is reduced around 5% and 4.58% due to compressible flow and damping effects inside OWC device. Copyright © 2005 John Wiley & Sons, Ltd.     

A modified Wells turbine for wave energy conversion

The method of wave energy conversion utilises an oscillating water column (OWC). The OWC converts wave energy into low-pressure pneumatic energy in the form of bi-directional airflow. Wells turbine with its zero blade pitch setting has been used to convert this pneumatic power into uni-directional mechanical shaft power. Measurements in OWC based wave energy plants in India and Japan show that the airflow velocity is not equal in both directions. The velocity is more when the airflows out to the atmosphere (exhalation) than in the reverse direction. It may be advantageous to set the rotor blade pitch asymmetrically at a positive pitch so as to achieve a higher mean efficiency in a wave cycle. Towards this objective, performance characteristics of a turbine with different blade setting angles in steady flow were found by experimentation. Quasi-steady analysis was then used to predict the mean efficiency for a certain variation of air velocity with time. This variation with time was taken as pseudo-sinusoidal wherein the positive part of the cycle was taken as a half sine-wave whose amplitude is greater than that of the negative half sine-wave. Such a variation is representative of what happens in reality. For exhalation velocity amplitude to inhalation velocity ratios 0.8 and 0.6, a rotor blade setting angle of 2° was found to be optimum.     

Aerodynamic conversion of ocean power from wave to wire

The action of the ocean’s waves can be converted into a vertical motion by submerging an opening in the base of a chamber into the sea. The entrained water forces an oscillating airflow through the top of the chamber which vents to atmosphere via an air turbine. An electric generator completes the conversion chain. A design methodology that focuses on the aerodynamic stage which couples the oscillating water column (OWC) and the Wells air turbine will be presented. The turbine’s most influential design variables will be considered within the context of maximising the plant’s annual performance within the wave regime.     

Effects of turbine damping on performance of an impulse turbine for wave energy conversion under different sea conditions using numerical simulation techniques

This paper presents the work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes with 0.6 hub to tip (H/T) ratio under real sea conditions.This enhances the earlier work done by authors on the subject by including the effects of damping applied by the turbine. Real wave data for different wave sites were used as the input data. A typical oscillating water column (OWC) geometry has been used for this simulation. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. Considering the quasi-steady assumption, uni-directional steady flow experimental data were used to simulate the turbine characteristics under irregular unsteady flow conditions. The test rotor used for this simulation consisted of 30 blades with elliptical profile with a set of symmetric, fixed guide vanes on both up-stream and down-stream sections of the rotor, with 26 vanes each. The results show that the performance of this type of turbine depends on the level of damping applied by the turbine and the prevailing wave site conditions. The objective of this paper is to predict the effects of applied damping on the behavior of impulse turbine under irregular, unsteady conditions for wave power conversion using numerical simulation.     

波能利用型圆筒透空堤水动力特性实验研究

将波能装置与防波堤等海洋结构物相结合,将有助于提升其经济性,促进其应用。以一定间距平行排布多个圆筒振荡水柱装置（OWC）形成波能利用型圆筒透空堤,并基于二维波浪水槽物理模型实验对其水动力特性展开研究,重点关注筒间距、OWC吃水、波高对于波浪防护和波能转换的影响规律。结果表明：圆筒较为紧密排布时,高效波能转换的波频范围显著拓宽;较浅OWC吃水在获得近似波浪防护效果的同时波能转换性能更佳;波浪防护效果及波能转换性能受波高影响较小。波能利用型圆筒透空堤在实际应用时,应采用较小的筒间距和OWC吃水,以同时兼顾较好的波浪防护效果和波能转换性能。     

Application of Numerical Simulation Method to Predict the Performance of Wave Energy Device with Impulse Turbine

This paper presents the work carried out to predict the behavior of a 0.6 m Impulse turbine with fixed guide vanes with 0.6 hub-to-tip (H/T) ratio under real sea conditions. In order to predict the true performance of the actual Oscillating Water Column (OWC), the numerical technique has been fine tuned by incorporating the compressibility effect. Water surface elevation verses time history based on Pierson Moskowitz Spectra was used as the input data. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. The effect due to compressibility inside the air chamber and turbine performance under unsteady and irregular flow condition has been analyzed numerically. Considering the quasi-steady assumptions, unidirectional steady flow experimental data was used to simulate the turbine characteristics under irregular unsteady flow conditions. The results show that the performance of this type of turbine is quite stable and efficiency of air chamber and the mean conversion     

Performance of OWC wave energy converters: influence of turbine damping and tidal variability

The performance of oscillating water column (OWC) systems depends on a number of factors in a complex manner. The objective of this work is to analyse the influence of the wave conditions, the damping caused by the turbine and the tidal level on the efficiency of the conversion from wave to pneumatic energy that occurs in the OWC chamber. To achieve this, a comprehensive experimental campaign is carried out, involving in total 387 tests of a model OWC under varying wave conditions (both with regular and irregular waves), damping coefficients and tidal levels. It is found that the damping exerted by the turbine is the factor that most affects the chamber efficiency—even more than the wave conditions. It follows that a proper selection of the turbine is crucial not only to the performance of the turbine itself but also to that of the chamber, which reflects the importance of the turbine–chamber coupling in OWC systems. The next factor in order of importance is the wave period. Finally, we find that the influence of the tidal level, which is examined in this work for the first time, is significant under certain conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling and optimization of the chamber of OWC system

Due to their non-polluting nature and environment friendliness, Renewable Energies have gained great deal of attention and deserve a substantial body of both theoretical and empirical research. Amongst other factors, the low operational cost and simple maintenance procedures attributed the Oscillating Water Column (OWC) are perhaps the main reasons why this system is the most used concept for the ocean wave energy capture.In this paper, through extensive experimental research various geometrical designs of an OWC system is investigated and the optimized set up for the maximum energy harness is obtained.The initial chamber dimensions were 10 × 50 × 53 cm with the chamber being placed in an open channel with wave-simulating equipment with dimensions of 16 × 0.7 × .05 m. For various chamber geometries, with the aid of a air rotameter and a Pitot tube equipped with a digital manometer, the outlet air flow and velocity from the chamber was measured and registered.The measurements were then interpreted to provide design data for the optimal geometry of the chamber that may yield the maximum conversion of wave energy to useful energy.     

A twin unidirectional impulse turbine topology for OWC based wave energy plants – Experimental validation and scaling

The twin unidirectional turbine topology was recently proposed with the promise of very significant improvements in the energy capture in Oscillating Water Column (OWC) based wave energy plants. Here, we present the initial results of the experimental validation of the twin unidirectional impulse turbine topology. A scale model of the concept was built and tested using simulated bidirectional flow. The model consists of two 165 mm impulse turbines each individually coupled to 375 W grid connected induction machines. An oscillatory flow test rig was used to simulate bidirectional flow to test the model. The results of the experiments validate the concept of the twin turbine configuration. The proposed topology utilizes no moving parts and achieves more than 50% efficiency over a broad range of flow coefficients. A comparison with other competing turbines (viz, a twin Wells’ turbine, a linked guide vane impulse turbine and a fixed guide vane impulse turbine) is done, based on actual measurements in the Indian wave energy plant. The results from the experiments are scaled to evaluate the design features of a 50 GWh wave energy plant.     

A novel radial self-rectifying air turbine for use in wave energy converters

The flow through the air turbine of an oscillating water column (OWC) wave energy converter is reciprocating and is random and highly variable. It is not surprising that the time-averaged efficiency of the air turbine is substantially lower than that of a conventional turbine working in nearly steady conditions. A new type of radial-flow self-rectifying turbine (named here biradial turbine) is described in the paper. The two inlet/outlet openings of the rotor are axially offset from each other and face radially the surrounding space. The turbine is symmetrical with respect to a plane perpendicular to its axis of rotation. The rotor blades are surrounded by a pair of radial-flow guide-vane rows. Each guide vane row is connected to the rotor by an axisymmetric duct whose walls are flat discs. A two-dimensional flow method is used first as a preliminary design tool for the turbine geometry. More detailed numerical results are then obtained with the aid of a commercial three-dimensional real-fluid CFD code, which allows a more refined geometry optimization to be carried out, and yields results for flow details through the turbine and for the turbine overall performance under several operating conditions.     

Wells turbine blade profile optimization for better wave energy capture

Despite the fact that wave energy is available at no cost, it is always desired to harvest the maximum possible amount of this energy. The axial flow air turbines are commonly used with oscillating water column devices as a power take‐off system. The present work introduces a blade profile optimization technique that improves the air turbine performance while considering the complex 3D flow phenomena. This technique produces non‐standard blade profiles from the coordinates of the standard ones. It implements a multi‐objective optimization algorithm in order to define the optimum blade profile. The proposed optimization technique was successfully applied to a biplane Wells turbine in the present work. It produced an optimum blade profile that improves the turbine torque by up to 9.3%, reduces the turbine damping coefficient by 10%, and increases the turbine operating range by 5%. The optimized profile increases the annual average turbine power by up to 3.6% under typical sea conditions. Moreover, new blade profiles were produced from the wind turbine airfoil data and investigated for use with the biplane Wells turbine. The present work showed that two of these profiles could be used with low wave energy seas. Copyright © 2017 John Wiley & Sons, Ltd.     

Nonlinear 2D analysis of the efficiency of fixed Oscillating Water Column wave energy converters

This paper reports on the development of a two-dimensional, fully nonlinear Computational Fluid Dynamics (CFD) model to analyse the efficiency of fixed Oscillating Water Column (OWC) Wave Energy Conversion (WEC) devices with linear power take off systems. The model was validated against previous experimental, analytical and numerical results of others. In particular, the simulation results show excellent agreement with the analytical results obtained by Sarmento and Falcão [1] for linear waves in a 2D channel and with previous experiments by others on the interaction between nonlinear waves and a fixed barge. Results are presented for linear waves on the influence of the seaward wall draft and thickness of the OWC device on the resonant frequency and the capture efficiency of the OWC. The key outcome of the present work is that for fully nonlinear waves a substantial decrease in the hydrodynamic capture efficiency of the OWC device was observed with increasing wave height, which represents a significant departure from the linear wave case. The optimal pneumatic damping coefficient for the OWC was also found to be dependent on the wave height. By analysing the magnitude of the first and higher order components of the incident nonlinear waves and the response of the OWC it was found that the first order capture efficiency decreases with increasing wave height, which in turn implies that the OWC hydrodynamic system is fully nonlinear and that the behaviour of an OWC in a nonlinear wave train cannot be accurately represented by the superposition of the linear response to a number of component linear waves. These results have significant implications for the design and operation of practical OWC systems.     

Floating type ocean wave power station equipped with hydroelectric unit

The authors have invented the unique ocean wave power station,which is composed of the floating type platform with a pair of the floats lining up at the interval of one wave pitch and the counter-rotating type wave power unit,its runners are submerged in the seawater at the middle position of the platform.Such profiles make the flow velocity at the runner is twice faster than that of the traditional fixed/caisson type OWC,on the ideal flow conditions.Besides,the runners counter-rotate the inner and the outer armatures of the peculiar generator,respectively,and the relative rotational speed is also twice faster than the speed of the single runner/armature.Such characteristics make the runner diameter large,namely the output higher,as requested,because the torque of the power unit never act on the floating type platform.At the preliminary reseach,this paper verifies to get the power using a Wells type single runner installed in the model station.The runner takes the output which is affected by the oscillating amplitude of the platform,the rotational speed and the inertia force of the runner,etc.     

A linearized model for estimating the performance of submerged resonant wave energy converters

A realistic performance analysis of oscillating water column wave energy converters (WECs) addresses to a set of non-linear differential equations that need to be integrated in time, by using a stochastic approach, under the hypothesis of random wind-generated sea waves, for all the sea states which characterize the location of the system. Non-linearities of the differential equations have several origins:

• minor and major losses of the unsteady flow of water and air;

• compressibility of air and heat exchange with the walls of the air chamber;

• non-linear characteristics of the turbine.

Under the hypothesis of random sea waves with Gaussian distribution, the authors propose an original methodology for linearizing the differential equations that describe the flow motion inside a wholly submerged WEC. Under such hypothesis, the linearized model can be used for predicting the power output by means of the calculations in the frequency domain and for control design. The developed methodology has been applied to the estimation of the performance of the new “resonant sea wave energy converters”, called REWEC, patented by Boccotti in 1998, and consisting of several caissons, characterized by a structure similar to the caissons of the traditional breakwaters and placed on the seabed, close one to each other, to form a submerged breakwater. Each caisson is connected to a vertical duct wholly beneath the sea level, where a hydraulic Wells turbine is placed.The matching between turbine and resonance characteristic of the system is carefully analysed in order to maximize the energy conversion efficiency.Some results, given for a small installation in the Mediterranean sea, confirm that the REWEC system is able to absorb a large share of the incident wave energy due to a very simple regulation system which permits the tuning on sea states with different significant heights.     

Numerical modelling in wave energy conversion systems

This paper deals with a numerical modelling devoted to predict the flow characteristics in the components of an oscillating water column (OWC) system used for the wave energy capture. In the present paper, the flow behaviour is modelled by using the FLUENT code. Two numerical flow models have been elaborated and tested independently in the geometries of an air chamber and a turbine, which is chosen of a radial impulse type. The flow is assumed to be three-dimensional (3D), viscous, turbulent and unsteady. The FLUENT code is used with a solver of the coupled conservation equations of mass, momentum and energy, with an implicit time scheme and with the adoption of the dynamic mesh and the sliding mesh techniques in areas of moving surfaces. Turbulence is modelled with the k–ε model. The obtained results indicate that the developed models are well suitable to analyse the air flows both in the air chamber and in the turbine. The performances associated with the energy transfer processes have been well predicted. For the turbine, the numerical results of pressure and torque were compared to the experimental ones. Good agreements between these results have been observed.