Water Column Type Wave Energy Research Articles

Effects of air chamber and curtain wall geometries on energy conversion performance in a fixed oscillating water column-type wave energy converter

Abstract An oscillating water column-type wave energy extractor is composed of an air chamber, an air turbine and a generator. The energy conversion process is compounded of the primary conversion in an air chamber and the secondary conversion of an air turbine. The oscillating water column-type wave energy extractor is considered to be safe even under dirty weather conditions. For societal implementation, there is a need to make a design method which can consider the water surface motion, the pressure fluctuation in an air chamber, the rotations of an air turbine and a generator. At here, the authors carried out the 2-dimensional wave tank tests using the model oscillating water column equipped with the impulse turbine and a generator to obtain the experimental data needed to make this design method. In the experiments, the effects of the air chamber and curtain wall geometries on energy conversion performance in regular wave conditions were investigated. As the results, it is noticed that the geometry of the curtain wall and the vortex shedding from the corners of curtain wall affect the primary conversion efficiency.

Effect of the installation position on the primary conversion performance of an oscillating water column wave energy converter attached on double slit caisson break water

A ouble-slit breakwater has two water columns, one at the sea side and the other at the land side. an oscillating water column type wave energy converter has been proposed to utilize these chambers. In this presentation, the performance of a wave generator installed in both or one of the two chambers was evaluated in a water tank experiment. In the experiment, a 1/25 scale model of the actual system was used to evaluate the performance in regular waves. The regular waves ranged from 0.6 to 2.4 seconds with a wave height of 5 cm. Air chambers were installed on both the sea and land sides, and it was found that the water level amplitude of the seaward air chamber was larger at the short periods of 0.6 and 0.8 seconds, while the amplitude of the land side air chamber increased as the period increased. In other words, more energy is absorbed in the seaward air chamber in waves with short periods, and more energy is absorbed by the land-side air chamber in waves with long periods. The phase difference between the two air chambers was found to decrease as the period becomes longer. It was also confirmed that when air chambers were installed only on the seaward side or only on the landward side, the air chambers exhibited characteristics similar to a dynamic damper.

Hydrodynamic Investigation of a Novel Concept of Oscillating Water Column Type Wave Energy Converter Device

AbstractThe hydrodynamic performance of a novel and efficient concept of a floating oscillating water column device has been investigated. The new concept consists of two chambers. These chambers are positioned on the upstream (fore chamber) and on the downstream (rear chamber) of the incident wave direction. The rear chamber acts mainly similar to a backward bent duct buoy system, while the design of the fore chamber follows conventional types of oscillating water column systems with the harbor plates (bottom plates and side plates) elongated outside of the fore chamber. The primary efficiency of the devised concept has been investigated in the frequency domain. In this context, to solve the corresponding diffraction and radiation problems due to the influence of the air pressure inside the chambers as well as motions of the body, an in-house code has been developed in two-dimensional using the boundary element method based on the linear wave theory. The obtained numerical results show that the introduced concept has advanced hydrodynamic efficiency in a broad range of waves.

Power Performance Analysis According to the Configuration and Load Control Algorithm of Power Take-Off System for Oscillating Water Column Type Wave Energy Converters

A power take-off (PTO) system for an oscillating water column (OWC) wave energy converter comprises a turbine-generator-power converter. In this study, only the topologies of the power converter that affect the load control algorithm are compared. A power converter for renewable energy is composed of a diode-dc/dc converter and a pulse-width modulation (PWM) converter operating at small and large capacities, respectively. However, selecting a power converter according to the capacity based on the characteristics of the wave energy converter, in which the input energy is highly fluctuating, can significantly reduce the power performance. Thus, to verify load control characteristics according to the topology of the power converter, the turbine-generator-power converter was incorporated in the modeling, and the power performance based on the power converter topology under various wave conditions was analyzed. Further, torque control to obtain the maximum power among load control algorithms was applied under irregular wave conditions, and the power performance and PTO system characteristics according to the torque coefficient were analyzed. The results of this study suggested an increase in the torque coefficient of the maximum power control for the operational stability of the OWC-WEC, and it was confirmed that the RPM characteristics of the PTO system were reduced.

Hydrodynamic optimisation of an axisymmetric floating Oscillating Water Column type wave energy converter with an enlarged inner tube

Wave Energy Converters (WECs) that are classified as Floating Oscillating Water Columns (FOWCs) of point absorber type are designed to achieve resonance, at frequencies that are favourable for wave energy extraction. The resonance frequency of a FOWC type WEC is highly dependent on the added mass of its OWC, which often requires a long spar tube to achieve well-performing hydrodynamics. A proposed measure for driving down the cost of the converter is to reduce its length, while finding another way of achieving the desired OWC added mass. Frequency-domain modelling and hydrodynamic optimisation of the Spar-buoy OWC was carried out for an enlarged spar tube segment inside the floater. Results are compared to those of buoys with equal spar and inner tube diameters. Frequency responses of optimised buoy geometries - with and without enlarged inner tubes - explore the differences in their hydrodynamics. Expanding the inner tube diameter of the Spar-buoy OWC may increase energy extraction by up to 6.7% and reduce the buoy mass by up to 11.4%, when operating in a Portuguese offshore location. A sensitivity analysis on mean annual pneumatic power to changes in inner tube diameter and spar tube length was performed.

Experimental Analysis of a Biplane Wells Turbine under Different Load Conditions

The oscillating water column type wave energy converters equipped with Wells turbine are one of the popular wave energy conversion devices. In most of the numerical and experimental studies, the Wells turbine characteristics are examined in no-load condition or with a fixed loading to achieve a fixed rotational speed. In the present work, a biplane Wells turbine is designed and tested in an experimental test facility. The test facility consists of a piston-chamber assembly that can generate sinusoidal airflow inside a duct. The turbine is placed inside the duct and tested for different stroke lengths and time periods of the piston, which produces a sinusoidal inlet airflow of different amplitude and time period. The turbine characteristics are studied at the no-load condition and for different values of resistive loading connected with the generator. The hysteresis behavior of the turbine is studied for two different flow coefficients based on experimentally observed and numerically calculated volume flow rates. Based on the experimental results, a detailed analysis of the turbine performance is presented for different operating conditions.

Experimental investigations on the performance of a fixed-oscillating water column type wave energy converter

Oscillating water column (OWC) type wave energy converters are the most promising candidates to harvest untapped ocean wave energy. OWC front wall opening and power take-off (PTO) damping optimization are very significant for feasible energy extraction. A comprehensive experimental investigation was performed for a bottom-fixed oscillating water column to determine the influence of the underwater opening height of the chamber, power take-off damping and wave steepness on the energy converter efficiency. A broad range of opening heights and power take-off dampings was utilized in physical experiments under various wave steepness values. Water column displacements, velocities and motion behaviors were also examined. Optimal orifice ratios were determined to obtain maximum efficiency under different wave steepness values. Based on the results, the key finding of this study is, for a certain range of wave steepness, optimal damping that should be applied on the system does not only depend on the wave characteristics, but also opening height of the chamber. The motion of water column surface behavior also affects the performance of the converter considerably.

Use of the Mps method to estimate the energy conversion efficiency of the owc-wec (first report)

In recent years, ocean renewable energy has been increasingly in the spotlight due to global warming and energy issues etc. Research on ocean renewable energy devices is being carried out by many researchers, companies and countries. Additionally, many sea tests are being carried out by countries with real sea test sites. Ocean renewable energy devices still need to research about a power generation method, an energy conversion efficiency and costs etc. In this paper, Authors focus on the oscillating water column type wave energy converter (called below OWC-WEC). It is considered that the OWC-WEC is useful because a movable part which becomes a cause of trouble is few and can also be used as a breakwater device. In this study, the MPS (Moving Particle Semi-implicit) method is applied to the performance analysis of the OWC-WEC. It is because the performance analysis by various parameters and the reduction of an experimental cost is needed. In this paper, these following items are shown; 1) the development of analysis program code which can simulate the multiphase flow by three phases of gas, liquid and solid, 2) the development of air duct model, 3) verification of the usefulness of the improved MPS program code and 4) verification of the applicability to the performance analysis of the OWC-WEC. This program code was applied to the simulation of the fixed type OWC-WEC. Following this, the reproducibility of the primary conversion efficiency of the fixed type OWC-WEC by this program code was verified and discussed. Finally, the usefulness and applicability of this program code were discussed and recommendations for future work were made.

Basic Characteristics of the Primary Conversion of an Oscillating Water Column Type Wave Energy Converter Installed on a Wave-Dissipating Double Caisson

Several types of oscillating water column (OWC) type wave energy converters (WECs) are researched and developed in the world. They are floating types and fixed types. In case of a fixed type, wave-dissipating caissons could be replaced by WECs of an OWC type. In OWC types, installation of the projecting walls (PWs) is useful in order to improve power take-off (PTO) performance. In this study, a double-dissipating caisson was used as an OWC type WEC with PWs. A front caisson of the double caisson seems to be the area surrounded by PWs and a back caisson can be seen as an OWC. The paper studied the basic property of the primary conversion from wave power to pneumatic power from model tests in a wave tank. It was found that the wave height strongly affects the behaviors of OWC motion and air pressure. Finally, the primary conversion was affected by wave height. Besides, the concept of use of a double caisson was useful for the primary conversion over 80% evaluated using test data.

Experimental and analytical investigation on chamber water surface fluctuations and motion behaviours of water column type wave energy converter

One of the most promising wave energy converter type is Oscillating Water Column (OWC) system. Fluctuation amounts and the motion behavior of the water column inside the chamber are very important parameters effecting the energy extraction. Therefore, predicting these parameters with respect to varying wave characteristics and geometric design parameters is of great importance. In this study, physical experiments are conducted for a bottom-fixed OWC system with seven different sizes of opening heights under various regular wave series. Average fluctuations inside the chamber are measured. It is found that there is a critical relative opening height ratio (α) that makes the fluctuations maximum regardless of wave parameters used. Exponential and linear relationships are found between average fluctuations and dimensionless parameters ‘dimensionless wave frequency’ and ‘captured wavelength’ defined, respectively. Mathematical models are developed to predict water column fluctuations under varying relative opening heights and wave parameters. The results of mathematical models indicated good aggrement with experimental data. Also chamber water surface profiles are observed and related to defined dimensionless wave parameters. Another factor (named as excessive harmful energy) is determined which also induces sloshing motion inside the chamber after the critical ratio value α is exceeded. It is found that under all wave series, the highest relative average column water surface fluctuations occur at relative opening height α is equal to 0.67 which is a unique value. It can be concluded that mathematical models can be used to estimate water column fluctuations from relative opening height and wave parameter data in the chamber.

Pressures and Forces on an Oscillating Water Column–Type Wave Energy Caisson Breakwater

Pressures and Forces on an Oscillating Water Column–Type Wave Energy Caisson Breakwater

Experimental Research on Primary and Secondary Conversion Efficiencies in an Oscillating Water Column-Type Wave Energy Converter

For the practical application of a fixed oscillating water column (OWC)-type wave energy converter, it is necessary to develop a design method which can consider the characteristics of incident wave motion, the motion of the internal free surface affected in the structure such as a partly submerged wall, the fluctuation of air pressure in an air chamber, and the rotation of the air turbine. On the other hand, the impulse turbine as the secondary conversion device in the OWC unit is expected to achieve high efficiency. In this paper, firstly, the steady air flow tests for a single-impulse turbine were conducted to grasp the characteristics of the turbine without the effect of water waves. Secondly, the 2-dimensional wave tank tests in regular waves for the performance evaluation of the fixed OWC-type wave energy converter with the same impulse turbine were conducted to obtain the data needed to make this design method. As the results, the effects of the air chamber length and the guide vane’s setting angle on the primary and secondary conversion efficiencies are clarified experimentally.

Improvement of Performance of Wave Power Conversion Due to the Projecting Walls for Oscillating Water Column Type Wave Energy Converter

This paper describes a method to improve the performance of primary conversion of wave power takeoff. The wave energy converter (WEC) used here was of oscillating water column (OWC) type. This method for improvement has been already proposed in past research and its usefulness has been confirmed. It involves projecting walls (PWs) being attached to the front of the inlet–outlet of the OWC. The prediction method of hydrodynamic behaviors for the OWC type WEC with PWs installed is explained in this paper. The boundary element method with the Green's function is applied, and influence of air pressure and free surface within every air-chamber was directly taken into consideration in the prediction method based on linear potential theory. Validity of the prediction method was proved by comparing the results with the results of model experiments. Series calculations are performed with the prediction method. Behaviors of air pressure, water elevation, and the efficiency of primary conversion of wave power were investigated. From the calculations, length of the PWs was shown to affect the efficiency of primary conversion. It was possible to equip the PWs so as to enable improvements in oblique waves to beam sea conditions as well as in the head sea conditions. This paper examined not only the PWs but also the application and effects of the end walls (EWs) with the slit. The EWs were very useful to improve the efficiency.

Experimental Study on Primary and Secondary Conversion Efficiencies of a Fixed Oscillating Water Column Type Wave Energy Converter

Experimental Study on Primary and Secondary Conversion Efficiencies of a Fixed Oscillating Water Column Type Wave Energy Converter

Wave energy extraction by an oscillating water column with a gap on the fully submerged front wall

In this paper, the theoretical analysis of an alternative oscillating water column type wave energy converter is presented. The problem geometry differs from earlier two dimensional studies by a modification on the front wall where the surface piercing barrier is replaced with a fully submerged barrier with a gap. Linear Wave Theory is used to govern the fluid motion. Matched eigenfunction expansions and a Galerkin approximation are utilized and results are presented comparatively. Two optimization ideas for power conversion are discussed. First, optimization for all frequencies is considered with the usual assumption of linear, Wells-type air turbine characteristics. Next, results following the practical optimization idea of Lovas et al. [16] are obtained where it is found that only one value for the turbine parameter can be sufficient to obtain efficiency very close to theoretical maximum for any wave frequency. It is also shown that by choosing geometrical parameters of the current problem appropriately, it is possible to obtain improvements on the efficiency curve of the OWC with a surface piercing barrier type front wall where both problems have the same resonant frequency.

1402 振動水柱型波浪エネルギー変換装置負荷における非線形性の影響と等価線形化(OS14-1 再生可能流体エネルギー,オーガナイズドセッション)

The air turbine installed in the oscillating water column type wave energy converter is analyzed linearizing in order to usually work as nonlinear load. On the other hand, it is possible solving directly nonlinear characteristic in more detailed analysis, and the nonlinear time series analysis using the impulse response function is performed. The advantage of the formation of equivalent linearization is treatment by the frequency domain, and is that statistical processing is possible, there are few amounts of calculation, and a solution is uniquely determined to an irregular wave. In this paper the error treated by the formation of equivalent linearization is validated by comparison with the nonlinear time series analysis for the orifice which is of nonlinear load damping.