Influence of Generator Damping on Peak Power and Variance of Power for a Direct Drive Wave Energy Converter

The first offshore prototype of a wave energy converter system has been launched off the Swedish west coast. The concept is based on a point absorber directly coupled to a linear generator located on the ocean floor. The wave energy converter is part of a research project that will study the electric system of ten units forming a small farm of wave power plants as they are linked and connected to an electric grid. A full scale farm will consist of a large number of interconnected units. The chosen direct drive system reduces the mechanical complexity of the converter but has repercussions on the electric system. The output from the generator will vary with the speed of the point absorber, leading to large fluctuations of power on the second scale. This has implications on both the individual generator and on the system as a whole. The hydrodynamic behavior of the point absorber depends, to a large extent, on the damping of the generator. The damping, in turn, can be remotely controlled by changing the load resistance. It has previously been shown that this has a large influence on the power absorbed by the wave energy converter. This paper investigates the peak power, the translator speed, and the variance of the power at different sea states and for different levels of damping. The peak power has an impact on the design of the generator and the required ability, for a single unit, to handle electric overloads. The momentum of the translator is directly proportional to its speed. The speed is thus important for the design of the end stop. The variance of the power of one unit will have an impact on the farm system behavior. The study is based on two and a half months of experimental measurements on the prototype wave energy converter and a wave measurement buoy. The aim is to analyze whether load control strategies may influence the dimensioning criteria for the electric system and the generator. The results are compared to previously investigated relationships between the absorbed mean power and the load resistance as a function of sea state. In the study, it was found that the maximum power is approximately proportional to the average power, while the maximum translator speed and standard deviation decrease as the damping factor is increased.

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Influence of Generator Damping on Peak Power and Variance of Power for a Direct Drive Wave Energy Converter

Volume 5: Ocean Space Utilization; Polar and Arctic Sciences and Technology; The Robert Dean Symposium on Coastal and Ocean Engineering; Special Symposium on Offshore Renewable Energy ◽

10.1115/omae2007-29291 ◽

2007 ◽

Author(s):

Magnus Sta˚lberg ◽

Rafael Waters ◽

Oskar Danielsson ◽

Mats Leijon

Keyword(s):

Wave Energy ◽

Peak Power ◽

Load Resistance ◽

Full Scale ◽

Wave Energy Converter ◽

Direct Drive ◽

Electrical System ◽

Energy Converter ◽

Swedish West Coast ◽

Point Absorber

The first full-scale offshore prototype of a novel wave energy converter system has been launched off the Swedish west coast. The concept is based on a point absorber directly coupled to a linear generator located on the ocean floor. The wave energy converter is part of a research project that will study the electrical system of 10 units forming a small farm of wave power plants as they are linked and connected to an electric grid. A full scale farm will consist of a large number of interconnected units. The chosen direct drive system reduces the mechanical complexity of the converter but has repercussions on the electrical system. The output from the generator will vary with the speed of the point absorber, leading to large fluctuations of power on the second scale. This has implications on both the individual generator and on the system as a whole. The hydrodynamic behavior of the point absorber depends, to a large extent, on the damping of the generator. The damping, in turn, can be controlled remotely by changing the load resistance. It has previously been shown that this has a large influence on the power absorbed by the wave energy converter. This paper investigates the peak power, the translator speed and the variance of the power at different sea states and for different levels of damping. The peak power has an impact on the design of the generator and the required ability, for a single unit, to handle electrical overloads. The speed of the translator is directly proportional to its momentum. It is thus important for the design of the end stop. The variance of the power of one unit will have an impact on the farm system behavior. The study is based on two and a half months of experimental measurements on the prototype wave energy converter and a wave measurement buoy. The aim is to analyze whether load control strategies may influence the dimensioning criteria for the electrical system and the generator. The results are compared with previously investigated relationships between absorbed mean power and load resistance as a function of sea state. In the study it was found that the maximum power is approximately proportional to the average power while maximum translator speed and standard deviation decrease as the damping factor is increased.

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Performance analysis of a point-absorber wave energy converter with a single buoy composed of three rigidly coupled structures

International Marine Energy Journal ◽

10.36688/imej.4.37-45 ◽

2021 ◽

Vol 4 (2) ◽

pp. 37-45

Author(s):

Aldo Ruezga ◽

José M. Cañedo C. ◽

Manuel G. Verduzco-Zapata ◽

Francisco J. Ocampo-Torres

Keyword(s):

Wave Energy ◽

Wave Energy Converter ◽

Floating Body ◽

Analysis Tool ◽

Direct Drive ◽

Energy Converter ◽

Drive Power ◽

Point Absorber ◽

Single Body ◽

Coupled Structures

A single-body point absorber system is analysed to improve its power absorption at a finite water depth. The proposed wave energy converter consists of a single floating body coupled to a direct-drive power take-off system placed on the seabed. The structure of a cylindrical buoy with large draft is changed by a single body composed of three structures rigidly coupled, reducing its volume and improving its frequency-dependent hydrostatic parameters that are obtained through a numerical analysis tool called NEMOH. The undamped natural frequency of the oscillating system is tuned to a specified wave period and the performance of the WEC system is obtained assuming a linear Power Take-Off system. In time domain, the performance of the WEC device is carried-out under a regular (sinusoidal) and irregular incident wave profile. Comparing the performance of the WEC system using the cylindrical and the proposed buoy outcomes that the system with the proposed buoy is able to absorb more energy from incident waves with a wider frequency range, whereas the oscillating system is kept as simple as possible.

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Frequency/time domain modeling of a direct drive point absorber wave energy converter

Science China Physics Mechanics and Astronomy ◽

10.1007/s11433-013-5200-8 ◽

2013 ◽

Vol 57 (2) ◽

pp. 311-320 ◽

Cited By ~ 8

Author(s):

ZhengShun Cheng ◽

JianMin Yang ◽

ZhiQiang Hu ◽

LongFei Xiao

Keyword(s):

Wave Energy ◽

Time Domain ◽

Wave Energy Converter ◽

Domain Modeling ◽

Direct Drive ◽

Energy Converter ◽

Point Absorber ◽

Time Domain Modeling

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Experimental Results From an Offshore Wave Energy Converter

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4001443 ◽

2010 ◽

Vol 132 (4) ◽

Cited By ~ 14

Author(s):

Cecilia Boström ◽

Erik Lejerskog ◽

Simon Tyrberg ◽

Olle Svensson ◽

Rafael Waters ◽

...

Keyword(s):

Wave Energy ◽

Wave Energy Converter ◽

Energy Converter ◽

Nonlinear Load ◽

Swedish West Coast ◽

Point Absorber ◽

Measuring Station ◽

Grid Connection ◽

Offshore Wave ◽

Voltage Conversion

An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the sea floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measuring station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the Lysekil research site. Special attention will be given to the power absorption by the generator when it is connected to a nonlinear load.

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Passive Model Predictive Control on a Two-Body Self-Referenced Point Absorber Wave Energy Converter

Energies ◽

10.3390/en14061731 ◽

2021 ◽

Vol 14 (6) ◽

pp. 1731

Author(s):

Dan Montoya ◽

Elisabetta Tedeschi ◽

Luca Castellini ◽

Tiago Martins

Keyword(s):

Model Predictive Control ◽

Predictive Control ◽

Wave Energy ◽

Power Flow ◽

Linear Constraints ◽

Wave Energy Converter ◽

Control Technique ◽

Computational Time ◽

Energy Converter ◽

Point Absorber

Wave energy is nowadays one of the most promising renewable energy sources; however, wave energy technology has not reached the fully-commercial stage, yet. One key aspect to achieve this goal is to identify an effective control strategy for each selected Wave Energy Converter (WEC), in order to extract the maximum energy from the waves, while respecting the physical constraints of the device. Model Predictive Control (MPC) can inherently satisfy these requirements. Generally, MPC is formulated as a quadratic programming problem with linear constraints (e.g., on position, speed and Power Take-Off (PTO) force). Since, in the most general case, this control technique requires bidirectional power flow between the PTO system and the grid, it has similar characteristics as reactive control. This means that, under some operating conditions, the energy losses may be equivalent, or even larger, than the energy yielded. As many WECs are designed to only allow unidirectional power flow, it is necessary to set nonlinear constraints. This makes the optimization problem significantly more expensive in terms of computational time. This work proposes two MPC control strategies applied to a two-body point absorber that address this issue from two different perspectives: (a) adapting the MPC formulation to passive loading strategy; and (b) adapting linear constraints in the MPC in order to only allow an unidirectional power flow. The results show that the two alternative proposals have similar performance in terms of computational time compared to the regular MPC and obtain considerably more power than the linear passive control, thus proving to be a good option for unidirectional PTO systems.

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