Tag Archives: Control

Model predictive control of sea wave energy converters – Part II: The case of an array of devices


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Guang Li and Mike R. Belmont, Renewable Energy – August 2014

Abstract

This paper addresses model predictive control (MPC) of highly-coupled clusters of sea wave energy converters (WECs). Since each WEC is not only a wave absorber but also a wave generator, the motion of each WEC can be affected by the waves generated by its adjacent WECs when they are close to each other. A distributed MPC strategy is developed to maximize the energy output of the whole array and guarantee the safe operation of all the WECs with a reasonable computational load. The system for an array is partitioned into subsystems and each subsystem is controlled by a local MPC controller. The local MPC controllers run cooperatively by transmitting information to each other. Within one sampling period, each MPC controller performs optimizations iteratively so that a global optimization for the whole array can be approximated. The computational burden for the whole array is also distributed to the local controllers. A numerical simulation demonstrates the efficacy of the proposed control strategy. For the WECs operating under constraints explored, it is found that the optimized power output is an increasing function of degree of WEC–WEC coupling. Increases in power of up to 20% were achieved using realistic ranges of parameters with respect to the uncoupled case.

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Model predictive control of sea wave energy converters – Part I: A convex approach for the case of a single device


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Guang Li and Michael R Belmont, Renewable Energy – September 2014

Abstract

This paper investigates model predictive control (MPC) of a single sea wave energy converter (WEC). By using control schemes which constrain certain quantities, such as the maximum size of the feedback force, the energy storage for actuators and relative heave motion, it is possible for control to not only improve performance but to directly impact strongly on design and cost. Motivated by this fact, a novel objective function is adopted in the MPC design, which brings obvious benefits: First, the quadratic program (QP) derived from this objective function can be easily convexified, which facilitates the employment of existing efficient optimization algorithms. Second, this novel design can trade off the energy extraction, the energy consumed by the actuator and safe operation. Moreover, an alternative QP is also formulated with the input slew rate as optimization variable, so that the slew rate limit of an actuator can be explicitly incorporated into optimization. All these benefits promote the real-time application of MPC on a WEC and reduced cost of hardware.

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An Intelligent Fuzzy Logic Controller for Maximum Power Capture of Point Absorbers


Mohammed Jama, Addy Wahyudie, Ali Assi, and Hassan Noura – Energies, June 2014

Abstract

This article presents an intelligent fuzzy logic controller (FLC) for controlling single-body heaving wave energy converter (WEC) or what is widely known as “Point Absorber”. The controller aims at maximizing the energy captured from the sea waves. The power take-off (PTO) limitations are addressed implicitly in the fuzzy inference system (FIS) framework. Continue reading

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Grid Connected Three-Level Converters


Remya Krishna – Uppsala University, Doctoral Dissertation, March 2014

Abstract

This thesis presents an electrical system analysis of a wave energy converter (WEC) for the objective of grid connection. To transfer the enormous amount of power from waves to the load centers, efficient power electronic systems are essential. This thesis includes the modeling of a buoy–translator dynamics and the modeling of a linear permanent magnet generator along with simulation and experimental validation. Continue reading

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An Adaptive Nonlinear MPPT Controller for Stand Alone Marine Current Energy Conversion Systems


Khan, N.; Rabbi, S.F. ; Hinchey, M.J. ; Rahman, M.A. – 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013, November 2013

Abstract

This paper presents an online estimation based adaptive nonlinear maximum power point tracking (MPPT) controller for a stand-alone permanent magnet synchronous generator (PMSG) based marine current energy conversion system. The proposed control strategy requires no flow sensor and also does not need the parameters of a PMSG. A nonlinear control algorithm for the load side converter to extract maximum power has been proposed to adequately deal with the inherent nonlinearities in the energy conversion system. A Lyapunov based online estimation approach is used to continually estimate the time varying input voltage and the output load resistance of the converter. Detailed simulation results of the proposed nonlinear controller namely adaptive backstepping are presented and fully analyzed. Simulation results demonstrate that the proposed nonlinear controller can incessantly extract maximum power from the ocean current at various flow speeds.

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Adaptive damping power take-off control for a three-body wave energy converter


Zhe Zhang, Brekken, T., Rhinefrank, K., Schacher, A., Prudell, J., Hammagren, E., and Lenee-Bluhm, P. – 2013 IEEE Energy Conversion Conference and Exposition (ECCE), September 2013

Abstract

The performance of the power take-off (PTO) system for a wave energy converter (WEC) depends largely on its control algorithm. This paper presents an adaptive damping control algorithm which improves power capture across a range of sea states. The comparison between this control algorithm and other active control approaches such as linear damping is presented. Short term wave elevation forecasting methods and wave period determination methods are also discussed as pre-requirements for this method. This research is conducted for a novel WEC, developed by Columbia Power Technologies. All hydrodynamic models are validated with their 1:7 and 1:33 scale tests.

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Wave prediction and its implementation on control systems of Wave Energy Converters


Peter Kracht, Boris Fischer, Sebastian Perez-Becker, and Jean-Baptiste Richard – MARINET Technical Report, 2013

Abstract

Many advanced control schemes have been proposed for wave energy converters (WEC), which offer the chance of significantly increasing the energy yield. Assessing the available literature so far most of these control schemes have only been investigated by simulations. These simulations are often based on assumptions such as that linear wave theory is applicable, that perfect models for the WEC/PTOs etc. are available or that an ideal prediction of the wave excitation force some distance in the future is available. Continue reading

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On wave energy focusing and conversion in open water


Umesh A. Korde and R. Cengiz Ertekin – Renewable Energy, February 2014

Abstract

This paper investigates wave energy conversion in open water where the goal is to utilize the wave-field focusing effect of a stationary disc submerged a short depth beneath the water surface. Dynamic interaction of the disc with additional coupled, submerged inertias is used to minimize its oscillation. The method used to enable and extend this favorable dynamic coupling is discussed here. An oscillating water column in a submerged duct attached under a small circular opening in the disc and driven by the wave-field over the disc is used for wave energy conversion. Non-real-time reactive control of the water column response to enhance energy absorption is examined. Added mass, radiation damping, and exciting force values for the submerged disc are computed, and the focusing effect of a submerged stationary disc is confirmed with numerical calculations of surface elevation over the disc. Calculations of hydrodynamic performance suggest that energy absorption from the oscillating water column is significantly greater under control holding the disc stationary, and can be improved further by applying reactive loads tuned to the optimal susceptance and conductance associated with the oscillating water column. Although the control forces involved in holding the disc stationary may be large at lower wave numbers, the maximum deflection amplitudes of the compensation system are found to be within reasonable limits.

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Modeling, Simulation, and Analysis of Two Hydraulic Power Take-off Systems for Wave Energy Conversion


S. Casey – Masters Thesis, Oregon State University, June 2013

Abstract

Hydraulic power take-off (PTO) systems have been implemented in several wave energy converter (WEC) designs in recent years. Two popular hydraulic PTO configurations coupled to a point absorber hydrodynamic model are simulated in waves representative of an energetic sea state likely to be found in deep waters off the coast of Oregon. The first hydraulic PTO configuration is a passive sys­tem tuned to the dominant forcing period of the sea state. The second system is an actively controlled hydraulic PTO topology tracking an optimal power absorption trajectory. A linear quadratic tracking controller is developed to follow the reference trajectory and performs well despite the nonlinear elements of the system dynamics. Simulation results of the system dynamics are presented for both models. A loading analysis is conducted and loading distributions of the two systems in irregular waves are compared. The distributions show the active system to have a larger variability in loading, while the distributions of the passive sys­tem indicate more frequent mean loading components. Parameters of the passive system components are varied in order to understand the effect on power output and loading.

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Influence of control strategy on the global efficiency of a direct wave energy converter with electric power take-off


Thibaut Kovaltchouk, Bernard Multon, Hamid Ben Ahmed, François Rongère, Judicaël Aubry, and Alain Glumineau – 2013 8th International Conference and Exhibition on Ecological Vehicles and Renewable Energies (EVER), June 2013

Abstract

The choice of control strategy for Direct Wave Energy Converters (DWEC) is often discussed without taking into account the limitations of electric Power Take-Off (PTO): limits of torque or force and power, as well as losses in the electric chain. These assumptions leads to large and expensive electric systems, that prevent leading to a global minimization of the per-kWh cost. We propose herein a simple loss model in order to design a better control strategy.

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