Tag Archives: Point absorbers

Wave power absorption: Experiments in open sea and simulation


M. Eriksson, R. Waters, O.Svensson, J. Isberg, and M. Leijon- Journal of Applied Physics, 2007

Abstract

A full scale prototype of a wave power plant based on a direct drive linear generator driven by a point absorber has been installed at the west coast of Sweden. In this paper, experimentally collected data of energy absorption for different electrical loads are used to verify a model of the wave power plant including the interactions of wave, buoy, generator, and external load circuit. The wave-buoy interaction is modeled with linear potential wavetheory. The generator is modeled as a nonlinear mechanical damping function that is dependent on piston velocity and electric load. The results show good agreement between experiments and simulations. Potential wavetheory is well suited for the modeling of a point absorber in normal operation and for the design of future converters. Moreover, the simulations are fast, which opens up for simulations of wave farms.

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The Use of Numerical Modeling to Optimize a New Wave Energy Converter Technology


Brandon E. Green and Daniel G. MacDonald – Marine Technology Society Journal, August 2013

Abstract

A numerical model of a new point-absorber wave energy converter (WEC) technology was designed for simulation purposes using Python. The governing equations were defined to take into account the relevant forces on the buoy in an ideal wave environment as well as any opposing forces due to damping, the power take-off (PTO) mechanism, and alternator. These equations of motion were solved using a high-order iterative process to study the linear kinematics of the buoy, the behavior of the PTO, and the associated power output in an ideal ocean wave environment. The model allows for the adjustment of relevant parameters to explore the behavior of the WEC and optimize system efficiency depending on the wave conditions. The numerical model was designed to run single simulations for a specified time interval; however, an optimization routine was implemented to optimize the mechanical parameters that greatly affect power output. The optimization portion of the model was implemented to study the response of the virtual WEC to a variety of input conditions pertaining to the buoy, PTO, and wave dynamics. This paper explains the development of the prototype WEC and the associated numerical model, in addition to evaluating the response of the WEC to a variety of input conditions. The output of the numerical model is discussed for the associated wave field used for simulation purposes. The design and implementation of the numerical model provides insight into changes in design components to maximize system power output and efficiency. The results of the numerical model and examples of data output for specific input conditions are investigated.

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On the Design, Modeling, and Testing of Ocean Wave Energy Converters


Bret Bosma – PhD Dissertation, Oregon State University, July 2013

Abstract

Ocean wave energy converter technology continues to advance and new developers continue to emerge, leading to the need for a general design, modeling, and testing methodology. This work presents a development of the process of taking a wave energy converter from a concept to the prototype stage. A two body heaving point absorber representing a generic popular design was chosen and a general procedure is presented showing the process to model a wave energy converter in the frequency and time domains. A scaled prototype of an autonomous small scale wave energy converter was designed, built, and tested and provided data for model validation. The result is a guide that new developers can adapt to their particular design and wave conditions, which will provide a path toward a cost of energy estimate. This will serve the industry by providing sound methodology to accelerate the continued development of wave energy converters.

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Modeling of a Point Absorber for Energy Conversion in Italian Seas


Silvia Bozzi, Adrià Moreno Miquel, Alessandro Antonini, Giuseppe Passoni, and Renata Archetti – Energies, June 2013

Abstract

In the present paper, we investigate the feasibility of wave electricity production in Italian seas by the deployment of the Seabased wave energy converter (WEC). A numerical model of the coupled buoy-generator system is presented, which simulates the behavior of the wave energy converter under regular waves of different wave heights and periods. The hydrodynamic forces, including excitation force, radiation impedance and hydrostatic force, are calculated by linear potential wave theory, and an analytical model is used for the linear generator. Two buoys of different radii are considered to explore the effect of buoy dimension on energy conversion and device efficiency. The power output is maximized by adding a submerged object to the floating buoy, in order to bring the system into resonance with the typical wave frequencies of the sites. The simulation results show a very good agreement with the published data on the Seabased WEC. The model is used to estimate energy production at eight Italian offshore locations. The results indicate that the degree of utilization of the device is higher than 20% at the two most energetic Italian sites (Alghero and Mazara del Vallo) and that it can be considerably increased if the floating body is connected to a submerged object, thanks to the resonant behavior of the WEC. In this case, the degree of utilization of the device would be higher than 40% at most of the study sites, with the highest value at Mazara del Vallo. The work enlarges the perspective, to be confirmed by experimental tests and more accurate numerical modeling, on clean electric power production from ocean waves in the Italian seas.

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Finite depth effects on the wave energy resource and the energy captured by a point absorber


Adriana Monarcha Fernandes and Nuno Fonseca – Ocean Engineering, July 2013

Abstract

Most of the wave energy converters under development are planned for operation in shallow to intermediate water depths, typically 30 m to 70 m. However, the limited water depth reduces the wave energy resource as compared to offshore deep water. This paper presents an analysis of the water depth effects on the wave energy resource and on the energy absorbed by a floating device. The analysis is based on a procedure to modify the wave spectrum as the water depth reduces, namely the TMA method (Bouws et al., 1985. J. Geophys. Res. 90 (C1), 975–986). The method is based on the hypothesis that the similarity principle for the saturation range of shallow-water spectrum is not restricted to the equilibrium range, but is valid across the entire spectrum. A point absorber-type wave energy converter is modelled and the wave climatology of Figueira da Foz (Portugal) is used as a case study. Significant reduction has been identified for both the wave energy resource and wave energy converted as the water depth decreases. The presented methodology can be used for preliminary cost benefit analysis and decision making regarding the best water depth for installation of specific wave energy converters.

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Optimal Constant DC Link Voltage Operation of a Wave Energy Converter


V. Kurupath, R. Ekströmemail, and M. Leijonemail – Energies, April, 2013

Abstract

This article proposes a simple and reliable damping strategy for wave power farm operation of small-scale point-absorber converters. The strategy is based on passive rectification onto a constant DC-link, making it very suitable for grid integration of the farm. A complete model of the system has been developed in Matlab Simulink, and uses real site data as input. The optimal constant DC-voltage is evaluated as a function of the significant wave height and energy period of the waves. The total energy output of the WEC is derived for one year of experimental site data. The energy output is compared for two cases, one where the optimal DC-voltage is determined and held constant at half-hour basis throughout the year, and one where a selected value of the DC-voltage is kept constant throughout the year regardless of sea state.

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Development of a point absorber wave energy converter: realisation of power take-off, optimisation of geometry and installation techniques


A. Van de Sijpe – PhD Thesis, Ghent University, 2012

Abstract

The development of renewable energy resources is strongly required due to the increasing energy demand, the shrinking reserves of fossil fuels and the effect of greenhouse gas emissions on the change of the wave climate. At Ghent University, study around the extraction of energy from ocean waves is being performed, more specifically with the aid of point absorber wave energy converters (WECs). To deliver a considerable amount of energy output at one location, large numbers of such devices need to be arranged in arrays or farms at sea. Several performed numerical and experimental studies around point absorbers and WEC-arrays are mentioned, indicating the knowledge gap of large scale physical model tests on WEC-farms, which are necessary to study the near- and far-field effects and to verify and improve numerical models. Within the HYDRALAB IV European programme in the frame of the project WEC wakes, large farms of point absorbers will be tested in the Shallow Water Wave Basin of DHI (Denmark). Continue reading

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Optimal active control of a wave energy converter


E. Abraham and E. Kerrigan – 51st Annual Conference on Decision and Control (CDC), December, 2012

Abstract

This paper investigates optimal active control schemes applied to a point absorber wave energy converter within a receding horizon fashion. A variational formulation of the power maximization problem is adapted to solve the optimal control problem. The optimal control method is shown to be of a bang-bang type for a power take-off mechanism that incorporates both linear dampers and active control elements. We also consider a direct transcription of the optimal control problem as a general nonlinear program. A variation of the projected gradient optimization scheme is formulated and shown to be feasible and computationally inexpensive compared to a standard NLP solver. Since the system model is bilinear and the cost function is non-convex quadratic, the resulting optimization problem is not a convex quadratic program. Results will be compared with an optimal command latching method to demonstrate the improvement in absorbed power. Time domain simulations are generated under irregular sea conditions.

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RANS Simulation of the Heave Performance of a Two-Body Floating-Point Absorber Wave Energy System


Y-H. Yu and Y. Li, Computers & Fluids, October, 2012

Abstract

This paper presents our recent numerical study on a point absorber wave energy conversion (WEC) system using a Reynolds-averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD) method. The device we studied is a two-body floating-point absorber (FPA), which operates in heave and generates energy from the relative motion between the two bodies. A series of numerical simulation was performed to analyze the hydrodynamic response and the power absorption performance of the system in regular waves. Overall, it was successful to use the RANS method to model the complex hydrodynamics interaction of the FPA system. We demonstrated the significance of the nonlinear effects, including viscous damping and wave overtopping. The study showed that the nonlinear effects could significantly influence the power output and the motion of the FPA system, particularly in larger waves.

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A class of globally stabilising controllers for the control of wave energy devices for potable water production


A. Lekka, M.C. Turner, and J.V. Ringwood – 2012 IEEE International Conference on Control Applications, October, 2012

Abstract

This paper provides a stability analysis for a system that captures wave energy in order to produce potable water. The system is a Wave Energy Converter (WEC) of the point-absorber type coupled to a hydraulic Power Take-Off (PTO) that converts wave energy into pressure. Previous work has used a partial state-feedback controller with integral action and feed-forward to provide good nominal control behaviour. Although open-loop stability was proven previously, no guarantees of closed-loop stability were given; in this paper we provide such guarantees for a class of controllers, of which the previously proposed controller is a special case.

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