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Modelling and phase controlof wave-energy converters
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply.

Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings.

This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used.

The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments.

A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum.

Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller.

An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

Place, publisher, year, edition, pages
NTNU, 2010.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2010:23
Identifiers
URN: urn:nbn:no:ntnu:diva-11952ISBN: 82-471-2003-3 (printed ver.)ISBN: 82-471-2004-0 (electronic ver.)OAI: oai:DiVA.org:ntnu-11952DiVA: diva2:403616
Available from: 2011-03-14 Created: 2011-02-03 Last updated: 2011-03-15Bibliographically approved
List of papers
1. Modelling and simulation of sea wavepower conversion systems
Open this publication in new window or tab >>Modelling and simulation of sea wavepower conversion systems
2007 (English)In: Proceedings of the 7th European Wave and Tidal Energy Conference,, 2007Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-11953 (URN)
Conference
7th European Wave and Tidal Energy Conference - EWTEC
Available from: 2011-02-03 Created: 2011-02-03 Last updated: 2011-03-14Bibliographically approved
2. Dynamics of A Force-Compensated Two-Body Wave Energy Converter in Heave with Hydraulic Power Take-Off Subject To Phase Control
Open this publication in new window or tab >>Dynamics of A Force-Compensated Two-Body Wave Energy Converter in Heave with Hydraulic Power Take-Off Subject To Phase Control
2007 (English)In: Proceedings of the 7th European Wave and Tidal Energy Conference, 2007Conference paper (Other academic)
Identifiers
urn:nbn:no:ntnu:diva-4108 (URN)
Conference
7th European Wave and Tidal Energy Conference
Available from: 2008-10-14 Created: 2008-10-14 Last updated: 2011-03-14Bibliographically approved
3. Experimental and numerical investigation of non-predictive phase-control strategies for a point-absorbing wave energy converter
Open this publication in new window or tab >>Experimental and numerical investigation of non-predictive phase-control strategies for a point-absorbing wave energy converter
Show others...
2009 (English)In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 36, no 5, 386-402 p.Article in journal (Refereed) Published
Abstract [en]

Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system's dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices. In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement. A mathematical model of the system was established and applied in numerical simulation. In the case the OWC's diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results. (C) 2009 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
Ocean waves, Wave energy, Latching control, Point-absorber, Threshold unlatching, Oscillating water column
Identifiers
urn:nbn:no:ntnu:diva-11955 (URN)10.1016/j.oceaneng.2009.01.015 (DOI)000265813900009 ()
Note
The article is reprinted with kind permission from Elsevier, sciencedirect.comAvailable from: 2011-02-03 Created: 2011-02-03 Last updated: 2011-03-14Bibliographically approved
4. Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter
Open this publication in new window or tab >>Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter
2011 (English)In: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme, ISSN 0892-7219, E-ISSN 1528-896X, Vol. 133, no 1, 011401- p.Article in journal (Refereed) Published
Abstract [en]

The question of optimal operation of wave-energy converters has been a key issue since modern research on the topic emerged in the early 1970s, and criteria for maximum wave-energy absorption soon emerged from frequency domain analysis. However, constraints on motions and forces give the need for time-domain modeling, where numerical optimization must be used to exploit the full absorption potential of an installed converter. A heaving, semisubmerged sphere is used to study optimal constrained motion of wave-energy converters. Based on a linear model of the wave-body interactions, a procedure for the optimization of the machinery force is developed and demonstrated. Moreover, a model-predictive controller is defined and tested for irregular sea. It repeatedly solves the optimization problem online in order to compute the optimal constrained machinery force on a receding horizon. The wave excitation force is predicted by use of an augmented Kalman filter based on a damped harmonic oscillator model of the wave process. It is shown how constraints influence the optimal motion of the heaving wave-energy converter, and also how close it is possible to approach previously published theoretical upper bounds. The model-predictive controller is found to perform close to optimum in irregular waves, depending on the quality of the wave force predictions. An absorbed power equal to or larger than 90% of the ideal constrained optimum is achieved for a chosen range of realistic sea states. Under certain circumstances, the optimal wave-energy absorption may be better in irregular waves than for a corresponding regular wave having the same energy period and wave-power level. An argument is presented to explain this observation. [DOI: 10.1115/1.4001431]

Keyword
reactive control, amplitude constraints, wave prediction, model-predictive control
Identifiers
urn:nbn:no:ntnu:diva-11954 (URN)10.1115/1.4001431 (DOI)000284112600007 ()
Available from: 2011-02-03 Created: 2011-02-03 Last updated: 2011-03-14Bibliographically approved
5. A comparison of selected strategies for adaptive control of wave energy converters.
Open this publication in new window or tab >>A comparison of selected strategies for adaptive control of wave energy converters.
2011 (English)In: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme, ISSN 0892-7219, E-ISSN 1528-896XArticle in journal (Other academic) Accepted
Place, publisher, year, edition, pages
ASME, 2011
Identifiers
urn:nbn:no:ntnu:diva-12049 (URN)
Available from: 2011-02-17 Created: 2011-02-17 Last updated: 2011-03-14Bibliographically approved

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