Oscillating Buoy Wave Energy Converter Research Articles

Model Validation and Hydrodynamic Performance of an Oscillating Buoy Wave Energy Converter

Model Validation and Hydrodynamic Performance of an Oscillating Buoy Wave Energy Converter

Technology, Geometry, Performance and Challenges in Wave Energy Converters

Recently, the worldwide energy scarcity raised the demand for integrated renewable energy in the modern grid. Considering the potential and ocean areas, wave energy-based power generation becomes an essential sustainable source for industry and academia. Notably, the wave energy converter (WEC) is the most common solution for extracting power from wave energy and convert into electricity. Wave electricity generation is the main method of using wave energy, and wave energy converters that are the core of wave energy technology have been widely and rapidly evolved. In this paper, the state-of-art WECs are reviewed with the classification based on different ways and different principle of operation of capturing energy of waves, including oscillating water column WEC (OWC-WEC), oscillating buoy WEC (OB-WEC) and overtopping WECs (O-WEC) are systematically reviewed in terms of operation principles, geometric structure and power take-off (PTO) strategies. Additionally, the energy conversion efficiency between different technologies is compared and summarized. The challenges and optimization aspects are delivered.

Energy capture optimization of heave oscillating buoy wave energy converter based on model predictive control

The optimal constant damping control based on the peak frequency of irregular wave is commonly adopted to improve the wave energy capture efficiency for the oscillating buoy wave energy converter (WEC) while the influence of the wave frequency change cannot be considered. In this paper, a real-time PTO force control optimization method is proposed based on model predictive control (MPC). Firstly, the dynamic model of the buoy is built based on the Cummins equation. Secondly, the additional penalty terms which may make the optimization problem convex are introduced into the original maximized energy objective, and their effects on the efficiency are investigated. Then, the deterministic sea wave prediction method is used to predict the future wave force aiming to make up the shortage that the classical MPC cannot compensate the external disturbance. Finally, the energy capture performance of MPC for the oscillating buoy WEC was compared with the resistive control. The results show that the MPC method can improve the energy capture by about 40%–60% under the irregular wave conditions.

Modeling and Simulation of Buoy Heave Response Under Electric Generator Load Using Simulink

According to the different links of energy transmission, the oscillating buoy wave energy converter is divided into mechanical mode and hydraulic mode. For the oscillating buoy wave energy converter with mechanical transmission, it’s requisite to consider the influence of electric generator and different transmission ratio on the heave response of buoy in its design process. Consequently, the Simulink toolbox is used to establish the simulation model of the buoy heave response under the electric generator load, and the heave response of the buoys with different diameters under the electric generator and transmission ratio is analysed. The results show that as the buoy diameter increases linearly, the heave displacement, heave velocity and generating power curve amplitude do not increase linearly which shows that the increment of amplitude becomes smaller. Therefore, buoys with different diameters have different load capacity.

Numerical study on the improvement design of the oscillating buoy wave energy converter

The oscillating buoy wave energy converter (OBWEC) is a popular and widely used wave energy converter (WEC) due to the simplicity of its structure and its strong adaptability. In this paper, the “Haiyuan No. 1″ OBWEC is taken as the research object, in which the buoys adopt heave mode of motion. In order to further enhance the hydrodynamic performance of the buoys, an improved scheme with longitudinal swing motion is proposed through theoretical analysis for the buoy. A 2D numerical wave tank (NWT) model based on the RANS equation is established by using CFD software. The numerical model is applied to compare and analyze the hydrodynamic performance differences of the buoy under two different motion forms: heaving and longitudinal swing, and the consistency of the numerical results is validated by the experimental data under regular waves. Finally, the irregular wave data obtained during the sea trial of the “Haiyuan No. 1″ OBWEC is used to numerically simulate the two buoys. The results show that the buoy with longitudinal swing motion is more effective in improving the hydrodynamic performance compared to the buoy with heave motion, in both regular and irregular waves. The conclusions of this paper can serve as a guide to improve the design of the hydrodynamic performance of the buoy.

Theoretical investigation on an oscillating buoy WEC‐floating breakwater integrated system

Based on the concept of cost sharing, an oscillating buoy wave energy converter- floating breakwater integrated system is proposed, in which the surge or pitch motion of the front-pontoon equipped with Power Take-off (PTO) system is allowed. In this study, the methods of eigenfunction matching and variables separation are applied to establish a theoretical model on hydrodynamic performance of the proposed integrated system. Under the condition of optimal PTO damping, the effects of the front-pontoon width, draft and pontoon spacing on the capture width ratio, reflection coefficient and transmission coefficient are investigated, respectively. It is found that the integrated system with surging front-pontoon performs better on the wave energy conversion than that with pitching one, especially in the high frequency region. The peaks of capture width ratio Cw are associated with the gap resonance and the increase of front-pontoon motion induced by the ‘hydrodynamic constructive effect’.

Numerical Study on the Optimization of Hydrodynamic Performance of Oscillating Buoy Wave Energy Converter

Abstract The oscillating buoy wave energy converter (OBWEC) captures wave energy through the undulating movement of the buoy in the waves. In the process of capturing wave energy, the hydrodynamic performance of the buoy plays an important role. This paper designed the “Haida No. 1” OBWEC, in which the buoy adopts a form of swinging motion. In order to further improve the hydrodynamic performance of the buoy, a 2D numerical wave tank (NWT) model is established using ADINA software based on the working principle of the device. According to the motion equation of the buoy in the waves, the influence of the buoy shape, arm length, tilt angle, buoy draft, buoy width, wave height and Power Take-off (PTO) damping on the hydrodynamic performance of the buoy is studied. Finally, a series of physical experiments are performed on the device in a laboratory pool. The experimental results verify the consistency of the numerical results. The research results indicate that the energy conversion efficiency of the device can be improved by optimizing the hydrodynamic performance of the buoy. However, the absorption efficiency of a single buoy for wave energy is limited, so it is very difficult to achieve full absorption of wave energy.

Modelling of Unidirectional Oscillating Buoy Wave Energy Converter Based on Direct Mechanical Drive System Under Irregular Wave

The development of wave energy converters (WEC) is increasing over time with various designs. The need for a WEC design that emphasizes the level of simplicity, portability, and closeness of its implementation to the nearshore is quite interesting to discuss. One of the WECs that can meet these criteria is oscillating buoy based on a direct mechanical drive system. This research proposes the concept of oscillating buoy WEC based on a direct mechanical drive system, which is designed with a unidirectional gear system to be able to generate energy both in the upward and downward wave phases. The WEC is also making to produce high rotation on the output side from the small motion of the buoy, especially in the heave direction. Predicting the power generated by this concept is very interesting because up to now, there has not been sufficient analytical model to estimate the power generated by oscillating buoys WEC based on a direct mechanical drive system, especially under irregular waves. In this work, an analytical method is conducting to describe the interaction between the model and irregular waves from the JONSWAP model. Model interaction is simulating numerically using the MATLAB program. The buoy translational motion of the model in the heave direction, which is converting to electric power by increasing the rotation of the gearbox and the energy stored in the flywheel, can produce significant output power. In the future, the proposed model can be used to develop more feasible and efficient models of wave energy converters.

Scaling Orchestration in Physical Model Test of Oscillating Buoy Wave Energy Converter

The physical model test interlinks the concept design and sea trial during the commercial utilization of wave energy converter. Aiming at the oscillating buoy wave energy converter, the energy conversion principle is firstly decomposed. Then, the model scale requirement of fluid motion and corresponding hydrodynamic similarity criterion considered in the physical model test are introduced. Finally, the solution of scaling orchestration problem is proposed considering the overall model scale in different energy conversion processes. The hydrodynamic similarity criterion is selected based on the working mechanism of the energy-capturing body only in contact with water, and the model scale requirements during the other stages of energy conversion structure are determined by the main mechanical factors. The first-stage energy conversion of device is recommended to meet the Froude similarity requirements, while the second-stage and third-stage energy conversions only need to meet the power similarity requirements. The power scale ratio in the three energy conversions is recommended to be consistent with the Froude similarity, and there are no requirements of geometric shape of second-stage and third-stage energy conversions to meet the similarity criteria.

Single Mode Simulation Calculation of Oscillating Buoy Wave Energy Converter with A Slider

This paper presents an oscillating slider wave energy device which is based on a seabed anchoring and uses eagle beak as the absorber. The self-compiled program uses the boundary element theory based on the simple Green’s function to solve the wave forces and hydrodynamic parameters. And the equation of motion, the oscillation of the float and the capture width ratio are obtained by the modal method. The influences of the shape of the eagle beak, the angle of the slider and the wave heading on the capture ability of the device are investigated. According to the calculation results and the wave resources in the sea area, the optimal shape of the eagle beak and external damping can be selected to maximize the wave energy capture capability.

MPPT Control of Hydraulic Power Take-Off for Wave Energy Converter on Artificial Breakwater

Wave energy is a renewable energy source that is green, clean and has huge reserves. In order to develop wave energy resources, an oscillating buoy Wave Energy Converter (WEC) device based on the artificial breakwater is presented in this paper. In order to effectively vent the gas in the hydraulic PTO and to improve the active control capability of the PTO system to guarantee the safety performance of the system under high sea conditions, a hydraulic PTO with an active control circuit is designed. Additionally, for the Power Take-Off (PTO) system, there is a optimal damping point under different sea conditions for PTO system, so the PTO can be controlled by the Maximum-Power-Point-Tracking (MPPT) control algorithms to improve the generated power of the system. At present, the MPPT control algorithms for wave energy are mainly used to control the load of generator. However, a fixed-load storage battery is used for the load of the generator in this paper. Additionally, an MPPT control taken at a hydraulic PTO system is executed to improve the power generated by hydraulic PTO under different sea conditions effectively in this paper. The MPPT control based on the hydraulic system is conducted by controlling the displacement of hydraulic motor to achieve the optimal damping point tracking control. The control flow of the MPPT algorithm is provided. The variable step hill-climbing method is used in MPPT control algorithm in which the big step can reduce the time of tracking and the small step can increase the accuracy of MPPT control algorithm. Due to the slow stability of the hydraulic system, a filter method for hydraulic PTO power is used. In addition, the hydraulic PTO system and MPPT control are verified to be feasible with the simulation. Additionally, MPPT control based on hydraulic variable motor is easier to carry out in practical applications than the traditional control of resistance. Finally, the simulation results demonstrate that it is an effective power control strategy for hydraulic PTO system to improve the generated power.

Hydrodynamic performance of a floating breakwater as an oscillating-buoy type wave energy converter

Combined floating breakwater and wave energy converter systems have the potential to provide a cost-effective solution to offshore power supply and coastal protection. This will make wave energy economically competitive and commercial-scale wave power operations possible. This paper investigates the hydrodynamic features of wave energy converters that meet the dual objectives of wave energy extraction and attenuation for such a combined system. A two-dimensional numerical model was established using Star-CCM+ commercial software based on viscous Computational Fluid Dynamics theory to investigate the hydrodynamic performance of an oscillating buoy Wave Energy Converter (WEC) type floating breakwater under regular waves. The model proposed in this paper was verified with published experimental results. The hydrodynamics of symmetric and asymmetric floaters were investigated to demonstrate their wave attenuation and energy extraction performance, including square bottomed, triangular bottomed (with and without a baffle plate), and the Berkley Wedge. The asymmetric floaters were found to have higher power conversion efficiency and better wave attenuation performance, especially the Berkeley Wedge bottom device and the triangular-baffle bottom device. The triangular-baffle bottom device with a simpler geometry achieved similar wave attenuation and energy extraction performance characteristics to that of the Berkeley Wedge device. The maximum energy conversion efficiency of the triangular-baffle bottom floater reached up to 93%, an impressive WEC device among many designs for wave energy conversion. There may be a great potential for this newly proposed triangular-baffle bottom WEC type of floater to be an ideal coastal structure for both coastal protection and wave energy extraction.

Effect of the PTO Damping Force on the 2D Oscillating Buoy Wave Energy Converter Integrated into a Pile-Restrained Floating Breakwater

The aim of the study is to evaluate the parametric effect of a power take-off (PTO) system on an integration system of an oscillating buoy wave energy device with a pile-restrained floating breakwater. To consider the viscous effect, the study is carried out with three conditions using OpenFOAM, i.e., the linear optimal PTO system, the fixed PTO damping coefficient, and the Coulomb PTO system. The results show that the maximum wave energy capture ratio (CWR) of the device can merely reach 0.35 in the present work, when considering the viscous effect. This result is smaller than that obtained from potential flow. In addition, the linear optimal PTO coefficient under viscous flow should be larger than the theoretical optimal coefficient in terms of the potential flow. A comparison of the Coulomb damping model and the linear damping model also shows that the Coulomb damping model has a good performance in terms of both wave energy capture efficiency and coastal protection. Hence, the effect of the PTO system on such an integration system can be an essential factor to achieve an optimal design.

Hydrodynamic performance and power absorption of a multi-freedom buoy wave energy device

Hydrodynamic performance and power absorption of an oscillating buoy wave energy converter (WEC) with 3 degrees of freedom (DOFs) are studied. The DOFs are surge, heave and pitch. Linear potential theory is utilized. Optimal PTO damping coefficient is selected. Optimal PTO power output and capture width ratios are calculated and compared. Influences of the buoy's radius, draft and geometry are investigated. The geometries are cylinder, hemisphere-bottom cylinder, cone-bottom cylinder, linear-chamfered cylinder and circular-chamfered cylinder. Results show that only surge and pitch interact with each other. The interaction is intensified on the resonant frequency of pitch, and is significant with special radius and draft. Power absorption of 3-DOF WEC increases significantly (at least 46.6% here) than unidirectional WEC and the power absorption band is wider. Heave is the main power capture DOF. Pitch gets the biggest power absorption at the resonant wave frequency. Increasing radius benefits the power capture efficiency at low wave frequencies and is harmful at high wave frequencies, except for pitch motion. Increasing draft benefits the power capture efficiency in surge and heave, but is harmful in pitch. For surge, heave and 3-DOF motion, cylinder is the best shape. For pitch, the hemisphere is the best shape.

Hydrodynamic Investigation of an Oscillating Buoy Wave Energy Converter Integrated into a Pile-Restrained Floating Breakwater

An analytical model is developed based on linear potential flow theory and matching eigenfunction expansion technique to investigate the hydrodynamics of a two-dimensional floating structure. This structure is an integration system consisting of a breakwater and an oscillating buoy wave energy converter (WEC). It is constrained to heave motion, and linear power take-off (PTO) damping is used to calculate the absorbed power. The proposed model is verified against the published results. The proposed integrated structure is compared with the fixed structure and free heave-motion structure, respectively. The hydrodynamic properties of the integrated structure with the optimal PTO damping i.e., the transmission coefficient, reflection coefficient, capture width ratio (CWR), and heave response amplitude operator (RAO), are investigated. The effect of the PTO damping on the performance of the integrated system is also evaluated. Results indicate that with the proper adjustment of the PTO damping, the proposed integrated system can produce power efficiently. Meanwhile, the function of coastal protection can be compared with that of the fixed structure.

Analytical Study on an Oscillating Buoy Wave Energy Converter Integrated into a Fixed Box-Type Breakwater

An oscillating buoy wave energy converter (WEC) integrated to an existing box-type breakwater is introduced in this study. The buoy is installed on the existing breakwater and designed to be much smaller than the breakwater in scale, aiming to reduce the construction cost of the WEC. The oscillating buoy works as a heave-type WEC in front of the breakwater towards the incident waves. A power take-off (PTO) system is installed on the topside of the breakwater to harvest the kinetic energy (in heave mode) of the floating buoy. The hydrodynamic performance of this system is studied analytically based on linear potential-flow theory. Effects of the geometrical parameters on the reflection and transmission coefficients and the capture width ratio (CWR) of the system are investigated. Results show that the maximum efficiency of the energy extraction can reach 80% or even higher. Compared with the isolated box-type breakwater, the reflection coefficient can be effectively decreased by using this oscillating buoy WEC, with unchanged transmission coefficient. Thus, the possibility of capturing the wave energy with the oscillating buoy WEC integrated into breakwaters is shown.