Heaving Buoy Wave Energy Converter Research Articles

Hydrodynamic response analysis of a hybrid TLP and heaving-buoy wave energy converter with PTO damping

In the present study, the numerical investigation is performed to analyse the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving point absorber wave energy converter (WEC) around a Frustum Tension-Leg Platform (FTLP) based on potential flow theory. The responses of the single FTLP and the FTLP-WEC hybrid system are analysed for the rated wind speed of a 5 MW wind turbine to observe the influence of the WECs on wind power absorption of wind turbines supported on FTLP. The presence of the FTLP floating wind turbine platform and other WECs affects the hydrodynamic coefficients of the WEC. The influence of the hybrid system on the hydrodynamic coefficients is analysed on determining the ratio of the hydrodynamic coefficients for a single WEC system to those for a hybrid system. Further, the study analyses the instantaneous wave power absorption for the WECs arranged around the FTLP in a circular and concentric pattern. The hydraulic power take-off for the hybrid system with two different control strategies is then discussed to improve the wave power absorption of the WECs. The study observed higher wave power absorption of the WECs with the influence of the PTO system. The mean interaction factor and the capture width ratio of the hybrid system are further studied to understand the influence of array arrangement for the WECs. The hybrid system is noted to have favourable dynamic responses for different environmental factors and contributes positively in increasing power output.

Performance investigation of a wave-driven compressed air energy storage system

The intermittent nature of waves causes a mismatch between the energy supply and demand. Hence an energy storage system is essential in the utilization of wave energy. This paper proposes a novel wave-driven compressed air energy storage (W-CAES) system that combines a heaving buoy wave energy converter with compressed air energy storage. Wave drives the heaving buoy to convert the wave energy to mechanical work that pumps water into a water-air compression chamber to form a liquid piston compressor. Air is compressed in the chamber and the wave energy is stored in the air. A numerical model was first developed in ANSYS-AQWA and validated using experimental data. Then this model was applied to evaluate the hydrodynamic characteristics, power generation capacity and thermodynamic behaviour of the W-CAES system under different wave conditions. Furthermore, the impacts of geometric parameters of the wave energy converter and compression chamber on the system performance were investigated. Results indicated that energy storage power was improved as the hydraulic cylinder area and storage pressure increased. The energy storage efficiency and round-trip efficiency could reach 60.5 % and 47.1 %, respectively under the isothermal compression process. By comparing the proposed W-CAES system with the conventional wave energy converter prototype under the same conditions, it showed that the proposed system yielded an increase in power output by up to 30 % under Tasmania wave conditions under an isothermal compression. The overall system wave capture factor is more sensitive to wave height than wave frequency. The capture factor can be as high as 20 % under Tasmania wave conditions. This study provides a new method for efficient wave energy conversion and storage.

Numerical simulation on the energy capture spectrum of heaving buoy wave energy converter

Based on the Wave Energy Capture Spectrum (ECS) theory, the characteristics of the ECS of a cylindrical heaving buoy WEC with consideration of linear PTO damping are studied in this paper. A 3D numerical wave tank (NWT) based on the RANS equations and the two-phase VOF model was established using the CFD software Flow-3D and validated by the corresponding experimental data. The ECSs of the heaving buoy WEC with different settings were calculated by the energy capture fluctuation signal, which was based on the incident wave elevation and the power output of the heaving buoy. The procedures of ECS solution using numerical method were proposed, and the effects of irregular wave, linear PTO damping, and relative draught on the ECS of the heaving buoy were studied, respectively. The results showed that the average period of incident wave is the key control factor of the peak frequency of the ECS. Compared to linear PTO damping, the relatively draught has little influence on the peak frequency of ECS. With the evaluation of the ECS method, the cylindrical heaving buoy WEC shows good adaptability to variable wave conditions and could be further improved by adjusting its ECS.

Experimental and numerical investigation on an optimization method of heaving buoy wave energy converter arrays based on a given target wave spectrum

In this paper, an active optimization method of a wave energy converter (WEC) array that focuses on the characteristics of the random sea condition is proposed. Physical model tests were conducted in both regular and irregular wave conditions. Three representative regular wave cases were determined from the target wave spectrum using the method. A rigid cylindrical heaving buoy model with different constant power take-off (PTO) damping forces and drafts was applied to find the optimal isolated WEC parameter under each regular wave condition. Three equal buoys with respective optimized parameters were arranged rectilinearly into a WEC array for irregular wave tests. The power output of arrays with different settings are compared, indicating the advantages of the different optimized array and the significance of unit order. To amplify the optimization effect, a supplementary numerical investigation of a 5-body WEC array was carried out using the boundary element method (BEM). The results demonstrate the method and show the influence of the number of different units.

Numerical modelling of wave run-up heights and loads on heaving buoy wave energy converter under the influence of regular waves

The effect of non-linear phenomenon such as wave run-up and wave impact on the heaving buoy wave energy converter may lead to its efficiency decrease and overtopping. In this study, a hydrodynamic simulation of wave run-up heights and maximum wave loads on a cylindrical heaving buoy is constructed and preliminarily analysed using the Reynolds-averaged Navier–Stokes equations. The numerical results show that kinetic energy is mainly concentrated in the wave-ward and leeward sides of the buoy. Further, the maximum pressure at the bottom of the buoy occurs on the leeward side when the wave hits the buoy, increasing by 47% over the initial pressure and 32% over the maximum pressure at the centre of the buoy. For a heaving buoy with the power take-off mechanism (PTO), the damping coefficient is the main parameter that affects the wave run-up and wave loads. By contrast, for a heaving buoy without PTO, the wave height and the radius of the buoy are the main parameter. The accuracy of the numerical results is verified through model tests of the motion response of the buoy and the wave surface characteristics around it. The computational method can guide future buoy designs in terms of safety and reliability.

System integration and coupled effects of an OWT/WEC device

This paper uses a fully nonlinear model to comprehensively explore the coupled effects caused by the system integration between an offshore wind turbine and a heaving buoy wave energy converter. Parameters of the study include the pitch motion of wind turbine, the tension on the mooring line, the contact pressure between these two devices, and the energy absorbed by power take-off (PTO) system. Results demonstrated that the buoy stabilises/reduces the pitch motion of wind turbine when its metacentric height is positive. A buoy with a negative metacentric height will increase the pitch amplitude of wind turbine and the tension of the mooring line, which is undesirable. The relative vertical motion between heaving buoy and spar could buffer their maximum contact pressure. The Coulomb PTO could offer a higher peak output power of WEC than the linear PTO. The relationship between contact pressure and WEC peak power is quantified to inform the PTO design. The wind/wave device is evaluated at a representative site with suitable wind and wave conditions off the US West Coast. The WEC does significantly reduce the wind turbine pitch motion by at least 60%/50% for the modelled average/max wave conditions with increased power production (14%/80%).

Optimisation of heaving buoy wave energy converter using a combined numerical model

This optimisation study focused on the mass, dimension, hydrodynamic response, and power take-off (PTO) damping for heaving buoy wave energy converters (WEC). A numerical model, consisting of a potential flow (PF) model and a computational fluid dynamics (CFD) model, is designed and applied to address numerical errors caused by liquid viscosity after physical validations. Comparing results from the PF and CFD models, it is evident that the liquid viscosity will produce additional radiation damping for the oscillator near its resonance frequency, leading the decrease of the device's energy absorption. Optimisations based on the combined model were conducted, including non-PTO and PTO cases with both regular and irregular incident waves. The results of non-PTO cases indicate that a buoy with a relatively larger mass is more sensitive to the liquid viscosity, but it still can obtain a better optimal hydrodynamic performance with a higher wavelength-to-diameter ratio. The PTO cases compare the energy absorption caused by the linear PTO and the Coulomb PTO. The comparison results demonstrate that the Coulomb PTO can reduce the effects of viscosity and absorb more energy under identical wave conditions. This paper presents the methods and considerations for working towards the overall optimisations of the heaving buoy WEC. The work will be useful for practitioners and researcher working on wave energy utilization.

Experimental investigation on hybrid mooring systems for wave energy converters

Abstract This study presents a comprehensive experimental assessment of three hybrid mooring systems for a heaving-buoy wave energy converter. Free decay model tests are conducted to study the natural periods in heave, pitch and surge modes of the buoy when it is moored with different mooring configurations. A series of regular and irregular wave model tests are carried out to investigate the hydrodynamic responses of the point absorber as well as mooring dynamics. The effects of wave period, incident wave height and mooring configuration are evaluated. Besides, the influence of mooring configuration on buoy motion is discussed. To study the short term extreme dynamic tension, the traditional Weibull distribution, Weibull distribution based on tail data and peaks-over-threshold method are applied. The results showed that the Weibull distribution failed to present accurate extreme dynamic tension prediction when the snap events occur frequently, while the other two methods showed good performance despite of number of snap events. Based on the results of energy production performance and extreme dynamic tension, a novel mooring line design is suggested, which helps to reduce mooring dynamic tension.

Simulation of the Power Take-off System for a Heaving Buoy Wave Energy Converter

The heaving buoy wave energy device is popular for wave conditions with small wave heights and short periods. This paper presents the design of a wave energy converter composed of a gyro buoy, a hydraulic power take-off, and an electricity generation system. The energy accumulators are used to improve the output power with different control strategies. The quantity relations between each component are established using theoretical methods. The simulation model describes the energy conversion and calculates the operation time, energy capacity, and output power of the device under different initial pressures, release pressures, and electric resistances. As a result, the capacity is determined by the electronic load, and the maximum output power can be obtained when the accumulators are turned on and off at the same time. This study investigates the factors influencing the electric energy production and can be used to guide the optimization of this type of device.

Development of a Three-Dimensional Fully Nonlinear Potential Numerical Wave Tank for a Heaving Buoy Wave Energy Converter

The hydrodynamic performance of a vertical cylindrical heaving buoy-type floating wave energy converter under large-amplitude wave conditions was calculated. For this study, a three-dimensional fully nonlinear potential-flow numerical wave tank (3D-FN-PNWT) was developed. The 3D-FN-PNWT was based on the boundary element method with Rankine panels. Using the mixed Eulerian–Lagrangian (MEL) method for water particle movement, nonlinear waves were produced in the PNWT. The PNWT can calculate the wave forces acting on the buoy accurately using an acceleration potential approach. The constant panels and least-square gradient reconstruction method were applied to regridding of computational boundaries. An artificial damping zone was employed to satisfy the open-sea conditions at the end free surface boundaries. The diffraction and radiation problems were solved, and their solutions were confirmed by a comparison with previous studies. The interaction of the incident wave, floating body, and power take-off (PTO) behavior was examined in the time domain using the developed 3D-FN-PNWT. From comparison, the difference between the conventional linear analysis and the nonlinear analysis in large-amplitude waves was examined.

Numerical calculation and experiment of a heaving-buoy wave energy converter with a latching control

Latching control was applied to a Wave Energy Converter (WEC) buoy with direct linear electric Power Take-Off (PTO) systems oscillating in heave direction in waves. The equation of the motion of the WEC buoy in the time-domain is characterized by the wave exciting, hydrostatic, radiation forces and by several damping forces (PTO, brake, and viscous). By applying numerical schemes, such as the semi-analytical and Newmark B methods, the time series of the heave motion and velocity, and the corresponding extracted power may be obtained. The numerical prediction with the latching control is in accordance with the experimental results from the systematic 1:10-model test in a wave tank at Seoul National University. It was found that the extraction of wave energy may be improved by applying latching control to the WEC, which particularly affects waves longer than the resonant period.

Hydrodynamic responses and efficiency analyses of a heaving-buoy wave energy converter with PTO damping in regular and irregular waves

Abstract Experimental investigation on the power performance of a heaving-buoy wave energy converter (WEC) with power take-off (PTO) damping was conducted under regular and irregular waves. The effects of the main influential parameters, including the incident wave height, wave frequency and PTO damping, on the maximum heave displacement, phase difference between the buoy velocity and wave elevation, and capture width ratio were quantitatively studied. For regular waves, with decreasing incident wave height or increasing PTO damping, the nonlinearity between the heave motion and surrounding wave elevation became pronounced and three modes of the buoy, i.e., linear motion, non-linear motion and non-motion, can be found. Based on analyses of the capture width ratio in both regular and irregular waves, the present WEC can obtain an optimal power efficiency at frequency ratio of ω / ω n ≈ 0.8 and PTO damping ratio of ζ p ≈ 0.5. It has been examined that H 1/10 can generally provide better approximation of the incident wave energy than H 1/3 and H AVG for irregular waves based on the linear wave theory. The statistical power performance of the WEC in irregular waves generally has the same trend as that in regular waves. The capture width ratio in irregular waves is found to be (approximately 5–40%) higher than that in regular waves for the same wave parameters, though the absolute incident and absorbed wave power in irregular waves are only half of those in regular waves. Finally, the flow structures around the heaving buoy are analyzed. The formation of vortices around the bottom corner provides flow interpretation on the viscous loss of wave energy for a heaving-buoy WEC with a flat bottom.

Theoretical study on the power take-off estimation of heaving buoy wave energy converter

Heaving buoy wave energy converter is applicable to small wave height and short wave period, which are the main characteristics of the China Sea. The device captures the wave power by buoy's heaving motion, transfers it by hydraulic system, and converts it into the electric power by generator. Power Take-Off (PTO) of the device is related to the resistance of the hydraulic system. However, effects of the hydraulic damping on the motion of buoy are often neglected and the study on the optimal hydraulic system is rare. In this paper, a new theoretical analysis has been developed, which is more applicable to the buoy considering hydraulic system. The paper establishes the governing equation considering buoyancy, wave dynamic force and hydraulic damping. The displacement and the velocity of the buoy are both obtained. The time-averaged power captured is calculated by integrating the square-velocity expression derived, and the existence of the maximum PTO is proved. Theoretical result is compared with the experimental result. The paper demonstrates the displacements and average output power of the buoy with different values of damping and inertia coefficients. The results of the study could be a guidance for the PTO design of the heaving buoy sets.

Effects of Damping Plate and Taut Line System on Mooring Stability of Small Wave Energy Converter

Ocean wave energy can be used for electricity supply to ocean data acquisition buoys. A heaving buoy wave energy converter is designed and the damping plate and taut line system are used to provide the mooring stability for better operating conditions. The potential flow assumption is employed for wave generation and fluid structure interactions, which are processed by the commercial software AQWA. Effects of damping plate diameter and taut line linking style with clump and seabed weights on reduction of displacements in 6 degrees of freedom are numerically studied under different operating wave conditions. Tensile forces on taut lines of optimized mooring system are tested to satisfy the national code for wire rope utilization.

Prediction of long-term fatigue damage of a hydraulic cylinder of a wave energy converter subjected to internal fluid pressure induced by wave loads

This paper deals with fatigue analysis of the high-pressure hydraulic power take-off (PTO) machinery in a heaving-buoy wave energy converter (WEC) exposed to irregular incident waves, with a focus on the hydraulic cylinder. To estimate the fatigue damage of the relevant critical joints of the cylinder, time-domain simulations are required to obtain the pressure loads inside the hydraulic pump. In this paper, the relevant dynamic responses are calculated by using the previously established nonlinear WEC model. Long-term pressure loads are obtained by combining the short-term time-simulation results and the occurrence probability of each sea state, and two models, Weibull and generalized Gamma distributions, are applied to fit the long-term rainflow-counted pressure cycles. Based on the pressure cycle distribution, the long-term fatigue damage of the cylinder with two different kinds of material, carbon, low alloy steel and nickel–chromium–molybdenum–iron alloy, is obtained by using the SN-Miner Palmgren approach. Two main parameters, cylinder diameter and thickness-diameter ratio, are investigated. Moreover, the parameter combination with which the cylinder has an expected fatigue life and possesses minimum material volume is supplied. Finally, the relative contribution of fatigue damage from different sea states is obtained and the most important wave conditions are identified.

Dynamic analysis of wave energy converter by incorporating the effect of hydraulic transmission lines

A heaving-buoy wave energy converter equipped with hydraulic power take-off is studied in this paper. This wave energy converter system is divided into five subsystems: a heaving buoy, hydraulic pump, pipelines, non-return check valves and a hydraulic motor combined with an electric generator. A dynamic model was developed by considering the interactions between the subsystems in a state space form. The transient pressures caused by starting/stopping the buoy or closing/opening the check valves were predicted numerically using the established model. The simulation results show that transmission line dynamics play a dominant role in the studied wave energy converter system. The length of the pipeline will not only affect the amplitude of the transient pressures but also affect the converted power. The variation of the time-averaged converted electric power with the pipeline length is estimated using the simulation method for the buoy exposed to one irregular sea state. Finally, it is suggested how reduced power efficiency due to the pipelines may be ameliorated.

A Novel Heaving Buoy Wave Energy Converter

This paper brings forward a novel heaving buoy wave energy converter, which can be installed in underwater robots to supply control and circuit energy consumption during their navigation and executing other tasks. This converter can absorb wave energy on the ocean surface and transform it into electric energy for a DC generator is driven by wave energy. Then the structure and basic principle of wave energy transform methodology is presented. Finally, theoretical model of the converter’s kinetic and dynamic principle is deduced and an factual example is calculated.

Estimation of Stress in the Inner Framework Structure of a Single Heaving Buoy Wave Energy Converter

This paper details a method for strain measurements in the inner framework structure of the single heaving buoy wave energy converter (WEC). This type of the WEC consists of a linear direct drive generator placed on the seafloor and connected to the floating buoy through the connection line. The study focuses on estimation of stress in the inner framework structure of the WEC using strain measurements in the material. Stress in ocean structures is as important as maximum stress. The offshore experiment took place at the Lysekil research site a few kilometers from the Swedish west coast. Stresses had the maximum value when the translator hit an end stop. Interesting observations were made: compressive stress occurred in the framework crossbar at a sea state of 1.32-m waves, but both compressive and tensile stresses occurred at a sea state of 3.2-m waves. A computational procedure employs good results. Error estimation is calculated and represented. This work is a step that may influence future design of wave energy devices in terms of the material aspect, survivability in a hard wave climate, and cost-effective renewable energies.

Numerical Modeling of Wear Damage in Seals of a Wave Energy Converter with Hydraulic Power Take-Off under Random Loads

Understanding and managing the safety and reliability issues of ocean energy converters is a key factor in the development of viable technologies. The present article deals with wear in the high-pressure hydraulic power take-off machinery in a heaving-buoy wave energy converter. In this study, an abrasive wear model for such machinery is developed for the steady-state operation by considering the effects of sealed pressure, relative velocity, material properties, time-varying environmental conditions, and lubrication, with a focus on the piston ring. In particular, the dependence of the wear damage on the sea state condition is investigated. The dynamic parameter named instantaneous wear work rate (Load × Velocity) is numerically calculated in the time domain. An estimate of the average wear work rate in each sea state is computed on the basis of a wave scatter table (significant wave heights versus peak period). Long-term wear damage can then be accounted for by summing up the short-term wear damage multiplied with the occurrence probability of each sea state. Finally, the relative contribution of wear damage from different sea states is obtained.

Optimal Damping Profile for a Heaving Buoy Wave Energy Converter

This paper explores optimal damping profiles for a heaving buoy wave energy converter (WEC). The approach is mathematical and the model of Eidsmoen (1995) is used as a basis. In order to permit analytical development, the model is initially simplified and an optimal damping profile is determined using numerical optimization. Having found the optimal damping profile, a semi-analytical solution methodology is developed to determine the optimal damping parameters. Finally, the procedure is validated on the original model and some aspects related to the control problem are addressed.