Backward Bent Duct Buoy Research Articles

Improving wave energy conversion performance of a floating BBDB-OWC system by using dual chambers and a novel enhancement plate

In this study, a novel floating dual-chamber backward bent duct buoy oscillating water column (BBDB-OWC) wave energy converter (WEC) is introduced, featuring a horizontal plate at the bottom of the front chamber to act as an enhancement plate. A three-dimensional computational fluid dynamics (CFD) model is developed and validated by comparing its results with existing experimental measurements. The validated model is employed to investigate the hydrodynamic performance and power generation characteristics of the dual-chamber BBDB-OWC WEC under various conditions, including variations in the length of the horizontal plate (lp/lf) and different regular wave conditions. Key performance metrics, including peak to average ratio of power (PTARP), wave energy capture width ratio (ξtotal), and its wave period respond bandwidth (indicated by Pξtotal > 0.5 and Pξtotal > 0.7), are analyzed and compared with those of a traditional single-chamber BBDB-OWC WEC. The results reveal that, compared to the single-chamber WEC, the dual-chamber WEC with a specific horizontal plate length reduces the average PTARP from 2.88 to a minimum value of 1.82 for lp/lf = 0.5, improves the average ξtotal from 0.55 to a maximum value of 0.64 for lp/lf = 2.5, and increases Pξtotal > 0.5 and Pξtotal > 0.7 from 71 % and 14 % to maximum values of 86 % and 43 % for lp/lf = 2.5, respectively. An explanation for these observations is also provided in the context of structure motion and flow fields.

Fully coupled simulation of dynamic characteristics of a backward bent duct buoy oscillating water column wave energy converter

In this paper, a fully coupled model for a backward bent duct buoy oscillating water column (BBDB-OWC) wave energy converter (WEC) is established, with the consideration of strong transient nonlinear interactions among ocean waves, BBDB hull motion, mooring system, internal water column oscillation, aerodynamics, turbine rotation and the damping effect of the generator. The model is validated by comparison with experimental results, and then used to investigate the transient characteristics and periodic average performance of the WEC under regular wave conditions. The differences in transient hydrodynamic characteristics and air pressure-flow rate relationship between the fully coupled model and the orifice model are compared. The transient characteristics of an impulse turbine under reciprocating and transient airflows are studied. And the effect of wave period, wave height and load torque coefficient on the average performance of the WEC is investigated.

A Statistical Analysis for Optimisation of a Hybrid BBDB-PA in Mantanani Island, Sabah

The hybrid form of wave energy converter (WEC) is a recent advancement in research concerning harvesting energy from the ocean. This study investigates the effect of size and position of the point absorber integrated with a backward bent duct buoy. The aim of this optimisation is to maximise the WEC-absorbed power and heave response amplitude operators (RAO) at a specific sea site. The optimisation process was applied based on the data collected over a one-year period about sea characteristics for a nearshore region of the Mantanani Island. We present a methodology for optimising the Hybrid BBDB-PA based on a statistical analysis and the hydrodynamics of the system in the frequency and time domain. We used the ANSYS/AQWA software for the hydrodynamic diffraction analysis, and the design of experiments method was applied through the statistical software to determine the optimised parameters. We found that the diameter and gap length between PA and BBDB were found to significantly influence two characteristics, namely, heave RAO and maximum power absorption of PA. This observation shows that the PA size was directly proportional to the performance because a higher diameter has more contact with the ocean’s wet surface area with the ocean and absorbed higher wave energy. Moreover, the gap length between the PA and BBDB was directly correlated with a wavelength, which followed the theoretical value for peak-to-trough length, where the maximum wave height occurs. Despite the condition parameter, we discovered that the WEC position and arrangement were responsible for the highest value of the power, regardless of the PA position used in the experiment. The results of this research provide recommendations for optimising the ocean energy harvesting in order to fully utilise ocean space for energy.

Research on Laboratory Test Method of Wave Energy Converter Wave-Wire Conversion Ratio in Irregular Waves

The laboratory test of the wave energy converter model is an important means to evaluate the performance of the device. At present, there are few performance tests for complete specifications under the irregular wave. Referring to the test methods and standards at home and abroad, combined with the actual test work experience in the laboratory, using the irregular wave power calculation formula with the effective wave height and the spectral peak period as parameters, then the wave-wire conversion ratio test method of the wave energy converter physical model under irregular waves in the laboratory is proposed. The method is applied to the basin test experiment of the physical model of the horn-shaped backward bent duct buoy (BBDB) wave energy converter. The research results show that the established test method and process of wave-wire conversion performance have achieved good application results in the irregular waves laboratory test, and can better reflect the device operating characteristics in real sea conditions. The test results provide data support for the model design of the wave energy converter in the next test stage, the demonstration test of the prototype, and the prediction of power generation in real sea conditions.

Numerical study on a hybrid WEC of the Backward Bent Duct Buoy and Point Absorber

This study aims to investigate a wave energy converter that utilizes heave motion as the primary driver for energy harvesting. The main objective is to examine the effect of applying a new form of hybrid Backward Bent Duct Buoy (BBDB) with Point Absorber (PA) at various wave conditions. The effects of single BBDB, PA, and a new hybrid form of BBDB-PA on heave Response Amplitude Operator (RAO) and power absorption were examined within various ranges of wave periods under regular wave conditions using ANSYS-AQWA. The results reveal that the characteristics of excitation force, radiation damping coefficient, and added mass of the BBDB and PA are significantly influenced by the new hybrid form. This observation is due to the advantage of the hybrid form that would have more contact with the wet surface area. Moreover, the hybrid BBDB-PA yields higher heave RAO and power absorption compared to the non-hybrid counterpart. The results offer recommendations for optimizing the design of hybrid WECs and imply the possibility of synergy between BBDB and PA to fully utilize ocean space for energy.

Experimental research on three parallel backward bent duct buoys

Backward bent duct buoy (BBDB) is a high-efficiency wave energy converter. It can be paralleled to form a wave energy farm. In this paper, a wave energy converter (WEC) array composed of three parallel BBDBs was tested. The array's performance and properties were reported. And, an effective method to improve the arrays' energy-capturing ability by setting the spacing above zero was proposed and verified. This method is helpful for the construction of BBDB wave energy farm.

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.

Backward bent-duct buoy or frontward bent-duct buoy? Review, assessment and optimisation

This work focuses on the investigation of bent-duct buoy oscillating-water-column (OWC) wave energy converters (WEC), which have been generally studied in their backward configuration (BBDB). This work studies both backward and frontward configurations through systematic phases starting from a reference model of a BBDB OWC WEC. Firstly, an appropriate numerical model is defined to assess the performance of BBDBs equipped with linear turbines. Secondly, having variations from the reference model, a parametric sensitivity analysis is carried out to study the influence of the main geometric variables, when designing a bent-duct buoy in both backward and frontward configurations. Results show a better performance of frontward configurations in most cases. Afterwards, an optimisation study is performed for a specific site. Results confirmed that the frontward configuration for the optimised geometry outperformed the backward configuration, and the selected geometry also performs better than the original reference at the site selected. Finally, a techno-economic assessment is carried out for the reference BBDB and for the optimised bent-duct buoy. Despite the substantial increase in annual mean power output, results confirm that the wave commercial projects are not economically feasible under current assumptions. This requires more efforts from developers and from regulators/policy makers to develop mechanisms to promote wave energy projects.

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

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

Power performance of BBDB OWC wave energy converters

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

Electrical cable utilization for wave energy converters

This paper investigates the suitability of sizing the electrical export cable based on the rating of the contributing WECs within a farm. These investigations have produced a new methodology to evaluate the probabilities associated with peak power values on an annual basis. It has been shown that the peaks in pneumatic power production will follow an exponential probability function for a linear model. A methodology to combine all the individual probability functions into an annual view has been demonstrated on pneumatic power production by a Backward Bent Duct Buoy (BBDB). These investigations have also resulted in a highly simplified and perfunctory model of installed cable cost as a function of voltage and conductor cross-section. This work solidifies the need to determine electrical export cable rating based on expected energy delivery as opposed to device rating as small decreases in energy delivery can result in cost savings.

Flume experiment study on capture width ratio of a new backward bent duct buoy with a pentagon buoyancy cabin

In this paper, the flume tests on a new backward bent duct buoy (BBDB) with a pentagon buoyancy cabin and single-point mooring are presented. Influences of the incident wave height, incident wave period, orifice area, mooring chain length, and mooring point position on its capture width ratio (η) are also investigated. In experiments it was discovered that: η is high in a wide wave period range; η reaches maximum 130.78% when BBDB is moored at the intersection of its front and the symmetric line of the bottom; and when other factors are set near their optimal values, the mooring point position has a weak influence on η, that is, this BBDB may has a low mooring point position requirement.

Experimental study on primary efficiency of a new pentagonal backward bent duct buoy and assessment of prototypes

To improve the wave energy conversion efficiency of backward bent duct buoy (BBDB), a pentagonal BBDB model was designed and fabricated for experiments in a wave flume (2D) as well as in a wave basin (3D). Incident wave height, wave period, water elevation in the air chamber and pressure difference between indoor and outdoor of the air chamber were measured. The test results show that the maximum value of capture width ratio (CWR) of the model in the wave flume is about 119.1%, which is much better than the highest value 79.1% obtained under the same experimental conditions in historical documents. Under regular wave, the maximum value of CWR tested in the wave basin is about 146.8%, which is equivalent to the highest value in historical documents and has relatively wide bandwidth. Under irregular waves, the maximum average value of CWR tested in the wave basin is about 87.2%, which is superior to the highest value 52% in historical documents. The combination of experimental results of the model and the latest research achievements of air turbine can help to break the thoughts that wave energy utilization technology can't balance low cost and high efficient conversion.

Levelized cost of energy for a Backward Bent Duct Buoy

The Reference Model Project, supported by the U.S. Department of Energy, was developed to provide publically available technical and economic benchmarks for a variety of marine energy converters. The methodology to achieve these benchmarks is to develop public domain designs that incorporate power performance estimates, structural models, anchor and mooring designs, power conversion chain designs, and estimates of the operations and maintenance, installation, and environmental permitting required. The reference model designs are intended to be conservative, robust, and experimentally verified. The Backward Bent Duct Buoy (BBDB) presented in this paper is one of three wave energy conversion devices studied within the Reference Model Project. Comprehensive modeling of the BBDB in a Northern California climate has enabled a full levelized cost of energy (LCOE) analysis to be completed on this device.

Nonlinear time-domain NWT simulations for two types of a backward bent duct buoy (BBDB) compared with 2D wave-tank experiments

A fully nonlinear time-domain NWT-simulation tool has been developed with nonlinear viscous-damping and pneumatic pressure terms. The tool was applied to the performance evaluation of fixed/floating sharp-corner and round-corner BBDB WECs. Shape-induced and motion-induced viscous damping for the BBDB system was considered in the potential-flow-based simulation with quadratic pneumatic pressure and fully nonlinear free-surface conditions. Systematic experiments were also conducted in a two-dimensional wave tank to verify the NWT-simulation results. The numerical results were further verified by checking the total power conservation of the entire system. The nonlinear simulations were also compared with linear ones. Since the round-corner BBDB has less energy loss caused by body shape and motion, the available power of the round-corner BBDB is generally bigger than that of the sharp-corner BBDB. The phenomenon of negative drift motion for only the sharp-corner BBDB for a certain wave-length range was also reproduced by the fully nonlinear BBDB simulations. This numerical simulation tool can be used for the reliable design of prototype systems for a given site and environmental conditions.

Effect of duct extension on pneumatic efficiency of Backward Bent Duct Buoy (BBDB)

The Backward Bent Duct Buoy (BBDB) is a wave energy converter which works on the principle of the Oscillating Water Column theory. The air displaced due to the rising and falling water surface in the oscillating water column is used to drive an electric machine to generate power. A physical model of the prototype BBDB with a geometric scale of 1:8 made of acrylic glass of size 412 × 280 × 255 mm was tested in the wave flume at the Centre for Water Resources laboratory, Anna University to determine the pneumatic efficiency of BBDB. The pneumatic efficiency, which is the ratio of output power of the displaced air to the input power due to waves, is primarily dependent on the amplitude of oscillation of the water column in the vertical air chamber. The effect of increasing the length of the horizontal duct on pneumatic efficiency has been studied. It was found that the duct length affects the primary conversion efficiency considerably and longer ducts may not necessarily improve the efficiency. It was observed that the BBDB model with the duct length of 412 mm shows better primary conversion efficiency of 5.3% and the area of the orifice opening also has an effect on the pneumatic efficiency.

規則波中における後ろ曲げダクトブイ型波力発電装置の一次変換性能に関する実験的研究

A primary conversion efficiency of a floating OWC-type wave energy converter Backward Bent Duct Buoy (BBDB) in regular waves is investigated in 2D and 3D wave tank tests. Firstly, the amplitude and phase difference between the internal wave height and motions are investigated. In a 3D tank test, surge, heave and pitch become in-phase at efficiency become maximum. The phase of internal wave height is close to motion. Secondary, the efficiency of duct-extended buoy is investigated in 2D test. Efficiency is reduced because the pitch becomes small by duct extension. Thirdly, the effect of a moment of inertia is investigated. In case of total mass and draft is constant, frequency band of high-efficiency increases as the moment of inertia become large. Fourthly, the effect of the shape of the air chamber is also investigated.

Fully nonlinear time-domain simulation of a backward bent duct buoy floating wave energy converter using an acceleration potential method

A floating Oscillating Water Column (OWC) wave energy converter, a Backward Bent Duct Buoy (BBDB), was simulated using a state-of-the-art, two-dimensional, fully-nonlinear Numerical Wave Tank (NWT) technique. The hydrodynamic performance of the floating OWC device was evaluated in the time domain. The acceleration potential method, with a full-updated kernel matrix calculation associated with a mode decomposition scheme, was implemented to obtain accurate estimates of the hydrodynamic force and displacement of a freely floating BBDB. The developed NWT was based on the potential theory and the boundary element method with constant panels on the boundaries. The mixed Eulerian-Lagrangian (MEL) approach was employed to capture the nonlinear free surfaces inside the chamber that interacted with a pneumatic pressure, induced by the time-varying airflow velocity at the air duct. A special viscous damping was applied to the chamber free surface to represent the viscous energy loss due to the BBDB's shape and motions. The viscous damping coefficient was properly selected using a comparison of the experimental data. The calculated surface elevation, inside and outside the chamber, with a tuned viscous damping correlated reasonably well with the experimental data for various incident wave conditions. The conservation of the total wave energy in the computational domain was confirmed over the entire range of wave frequencies.

부유식 진동수주형 파력발전기(BBDB)의 유체 동역학적 성능 실험 연구

An experimental study on the hydrodynamic performance of a backward bent duct buoy (BBDB) was performed in a 2D wave tank. The BBDB is one of the promising oscillating water column (OWC) types of floating wave energy converters. Two different corner-shaped BBDBs (sharp-corner and round-corner) were used to measure the maximum chamber surface elevations and body motions for various incident wave conditions, and their hydrodynamic characteristics were compared. In order to investigate the effect of the pneumatic pressure inside the chamber, the heave and pitch angle interacted with elevations were compared for both open chamber and partially open chamber BBDBs. From the comparison study, the deviation in the chamber surface elevations between the two shapes of BBDBs was found to be significant near the resonance period, which may be explained by viscous energy loss. It was also found that the pneumatic pressure noticeably affected the chamber surface elevation and body motions.

Numerical Investigation of 2D Optimal Profile of Backward-Bent Duct Type Wave Energy Converter

A floating type backward-bent duct buoy (BBDB) is a wave energy conversion device with an oscillating water column (OWC) at the front side. The device captures the wave energy using the heaving, the pitching, the surging motion of BBDB, and the heaving motion of OWC. Investigations are carried out to find more reasonable devices than the traditional OWC type floating device. An eigenfunction expansion method is introduced for analyzing the BBDB with OWC. It is confirmed that these solutions give good agreement with several experimental results in this paper. It is shown in a design method how to make BBDB match the turbine characteristics. This feature is being able to select the optimum profile of the turbine and the BBDB individually from each characteristic before comprehensive evaluation of the BBDB and the turbine in the design. After grasping the element characteristics, which are appropriate for the wave energy conversion system, the synthetic design method is built. The BBDB size and the turbine diameter are determined by considering the cost corresponding to the smallest size under the same output. In this way, we can obtain the optimal profiles considering the construction cost including the turbines.