Study of a point absorber wave energy converter technology: modeling, simulation, control and experimental validation of the system

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Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library

In this paper we employ two implementations of the fictitious domain (FD) method to simulate water-entry and water-exit problems and demonstrate their ability to simulate practical marine engineering problems. In FD methods, the fluid momentum equation is extended within the solid domain using an additional body force that constrains the structure velocity to be that of a rigid body. Using this formulation, a single set of equations is solved over the entire computational domain. The constraint force is calculated in two distinct ways: one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method and another using a fully-Eulerian approach of the Brinkman penalization (BP) method. Both FSI strategies use the same multiphase flow algorithm that solves the discrete incompressible Navier-Stokes system in conservative form. A consistent transport scheme is employed to advect mass and momentum in the domain, which ensures numerical stability of high density ratio multiphase flows involved in practical marine engineering applications. Example cases of a free falling wedge (straight and inclined) and cylinder are simulated, and the numerical results are compared against benchmark cases in literature.Comment: The current paper builds on arXiv:1901.07892 and re-explains some parts of it for the reader's convenienc

An adaptive and energy-maximizing control of wave energy converters using extremum-seeking approach

In this paper, we systematically investigate the feasibility of different extremum-seeking (ES) control schemes to improve the conversion efficiency of wave energy converters (WECs). Continuous-time and model-free ES schemes based on the sliding mode, relay, least-squares gradient, self-driving, and perturbation-based methods are used to improve the mean extracted power of a heaving point absorber subject to regular and irregular waves. This objective is achieved by optimizing the resistive and reactive coefficients of the power take-off (PTO) mechanism using the ES approach. The optimization results are verified against analytical solutions and the extremum of reference-to-output maps. The numerical results demonstrate that except for the self-driving ES algorithm, the other four ES schemes reliably converge for the two-parameter optimization problem, whereas the former is more suitable for optimizing a single-parameter. The results also show that for an irregular sea state, the sliding mode and perturbation-based ES schemes have better convergence to the optimum, in comparison to other ES schemes considered here. The convergence of PTO coefficients towards the performance-optimal values are tested for widely different initial values, in order to avoid bias towards the extremum. We also demonstrate the adaptive capability of ES control by considering a case in which the ES controller adapts to the new extremum automatically amidst changes in the simulated wave conditions

A submerged point absorber wave energy converter for the Mediterranean Sea

Regarding the serious environmental and energy problem existing in the world, renewable technology has taken the global attention. The unexploited huge potential of wave energy challenges the world scientific community, to achieve a sustainable wave energy absorption. Mediterranean is a closed sea with lower energy potential compared to the oceans. However, an efficient WEC that maximizes the power output not with respect to the available power but to its characteristics and cost, could be a feasible solution. A preliminary assessment has been carried out of a submerged point absorber installed in the coast of the Pantelleria island. For the design and selection of the technical characteristics of the WEC a method proposed by Falnes is followed. In order to control and test the performance of the device in different situations, a mathematical model has been constructed in Matlab Simulink based on the Cummins Equation

On-board sea state estimation method validation based on measured floater motion

This paper presents a method to estimate the sea state PSD (Power Spectral Density) of the current wave climate, by using the measured floater motion and the body hydrodynamic response. The knowledge of PSD and the sea state synthetic parameters derivable from the PSD, such as the Significant Wave Height and the Energy Period, is fundamental for the navigation and operation in naval field and also for the control strategies of the Wave Energy Converters (WEC). The ISWEC (Inertial Sea Wave Energy Converter) is used as case study for the validation of the sea state estimation method. ISWEC is a floating device using the inertial effects of a gyroscopic system to convert a floater motion into electric energy. Sea state parameters are used in the control of the device to tune gyroscope speed and the generator torque law to achieve maximum power absorption. The heave measurements are used to estimate the PSD of the incoming wave and it is compared with the wave PSD measured by a wave measurement system. The method is studied and validated for three different sea state cases. At this stage the method presents satisfying results, with an accuracy under the 10% of the estimated parameters. Such accuracy is comparable with the short term (1-3h) wave forecast produced by ECMWF

Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library

In this paper we employ two implementations of the fictitious domain (FD) method to simulate water-entry and water-exit problems and demonstrate their ability to simulate practical marine engineering problems. In FD methods, the fluid momentum equation is extended within the solid domain using an additional body force that constrains the structure velocity to be that of a rigid body. Using this formulation, a single set of equations is solved over the entire computational domain. The constraint force is calculated in two distinct ways: one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method and another using a fully-Eulerian approach of the Brinkman penalization (BP) method. Both FSI strategies use the same multiphase flow algorithm that solves the discrete incompressible Navier-Stokes system in conservative form. A consistent transport scheme is employed to advect mass and momentum in the domain, which ensures numerical stability of high density ratio multiphase flows involved in practical marine engineering applications. Example cases of a free falling wedge (straight and inclined) and cylinder are simulated, and the numerical results are compared against benchmark cases in literature

A passive control strategy applied to the iswec device: numerical modelling and experimental tests

The ISWEC is a floating, slack-moored, wave energy converter, absorbing energy through an electric-mechanical power take-off (PTO), moved by a gyroscopic, activated by the pitching motion of the floater. The system is torque-controlled in order to keep the gyroscope in the desired position range and to maximise productivity. At present, a proportional-derivative (PD) control law regulates the torque on the PTO, comprising a linear stiffness term to recall the gyroscope in the vertical position and a linear damping term to extract power. However, the recall task demands high torques to the PTO, inducing an undesirable flux of reactive power. This paper discusses a technological innovation to address such issues, consisting of an additional eccentric mass to provide the restoring action instead of the stiffness term. The mass and distance from the precession axis should to be optimally designed. Two configurations are considered, one with fixed distance, and one with distance tuneable according to the incoming sea state. Time-domain nonlinear numerical simulations inform the optimization and design of the eccentric mass. Simulations demonstrate the effectiveness of the strategy, inducing a high reduction of the PTO torque levels while maintaining similar power conversion efficiencies. Similar results are obtained via experimental tests performed on a Hardware-in-the-loop (HIL) test bench