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Prediction horizon requirement  in control and extreme load analyses for survivability: Advancements to improve the performance of wave energy technologies
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering. (Wave Power Group)
2021 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The main objective of wave energy converters (WECs) is to ensure reliable electricity production at a competitive cost. Two challenges to achieving this are ensuring an efficient energy conversion and offshore survivability.        

This thesis work is structured in three different sections: Control and maximum power optimization, forces and dynamics analysis in extreme wave conditions, and statistical modeling of extreme loads in reliability analysis.       

The need for prediction and future knowledge of waves and wave forces is essential due to the non-causality of the optimal velocity relation for wave energy converters. Using generic concepts and modes of motion, the sensitivity of the prediction horizon to various parameters encountered in a real system is elaborated. The results show that through a realistic assumption of the dissipative losses, only a few seconds to about half a wave cycle is sufficient to predict the required future knowledge for the aim of maximizing the power absorption.         

The results of a 1:30 scaled wave tank experiment are used to assess the line force and dynamic behaviour of a WEC during extreme wave events. Within the comparison of different wave type representations, i.e. irregular, regular and focused waves, of the same sea state, the results show that not all the wave types deliver the same maximum line forces. As a strategy of mitigating the line forces during extreme wave events, changing the power take-off (PTO) damping may be employed. With consideration of the whole PTO range, the results indicate an optimum damping value for each sea state in which the smallest maximum line force is obtained. Although wave breaking slamming and end-stop spring compression lead to high peak line forces, it is possible that they level out due to the overtopping effect. Waves with a long wavelength result in large surge motion and consequently higher and more damaging forces.        

On the investigation of reliability assessment of the wave energy converter systems, computing the return period of the extreme forces is crucial. Using force measurement force data gathered at the west coast of Sweden, the extreme forces are statistically modelled with the peak-over-threshold method. Then, the return level of the extreme forces over 20 years for the calm season of the year is computed.

Place, publisher, year, edition, pages
Uppsala: Uppsala University, 2021. , p. 70
Keywords [en]
control, optimal velocity, non-causality, maximum power output, extreme waves, wave tank experiment, end-stop compression, wave breaking slamming, PTO damping, return level, return period, peak-over-threshold
National Category
Energy Systems Marine Engineering Control Engineering Ocean and River Engineering Energy Engineering
Identifiers
URN: urn:nbn:se:uu:diva-457329OAI: oai:DiVA.org:uu-457329DiVA, id: diva2:1606555
Presentation
2021-12-17, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Supervisors
Available from: 2021-11-25 Created: 2021-10-27 Last updated: 2021-11-25Bibliographically approved
List of papers
1. Considerations on prediction horizon and dissipative losses for wave energy converters
Open this publication in new window or tab >>Considerations on prediction horizon and dissipative losses for wave energy converters
2021 (English)In: IET Renewable Power Generation, ISSN 1752-1416, E-ISSN 1752-1424, IET Renewable Power Generation, Vol. 15, no 14, p. 3434-3458Article in journal (Refereed) Published
Abstract [en]

The non-causal optimal control law for wave energy converters leads to a requirement of predicting waves and wave forces over a future horizon.  Using examples of generic body shapes and oscillation modes, we show through computations of the velocity reference trajectory how the length of prediction horizon required to reach the maximum power output depends on the level of dissipative losses in the conversion chain. The sensitivity to noise is discussed, and so is the use of filtering to improve performance when the available prediction horizon is short or predictions are inaccurate. Considerations are also made for amplitude constraints and other effects encountered in a real system.  With realistic assumptions for the level of dissipative losses, results indicate that the prediction horizon needed to approach the maximum achievable power output for real systems ranges from only a few seconds up to about half a wave period, which is shorter than has generally been assumed earlier.

Place, publisher, year, edition, pages
Institution of Engineering and TechnologyInstitution of Engineering and Technology (IET), 2021
Keywords
Wave energy converter, prediction horizon, dissipative losses, optimal velocity, useful power
National Category
Energy Systems Marine Engineering Control Engineering Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-457294 (URN)10.1049/rpg2.12290 (DOI)000703141600001 ()
Funder
Swedish Research Council Formas, 2020-03634Swedish Energy Agency
Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12Bibliographically approved
2. Experimental investigation of a point-absorbing wave energy converter response in different wave types of extreme sea states
Open this publication in new window or tab >>Experimental investigation of a point-absorbing wave energy converter response in different wave types of extreme sea states
(English)In: Article in journal, Editorial material (Other academic) Submitted
National Category
Energy Systems Marine Engineering Ocean and River Engineering Energy Engineering
Identifiers
urn:nbn:se:uu:diva-457326 (URN)
Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2021-10-27
3. Experimental results of force measurements from a scaled point absorbing wave energy converter subjected to extreme waves
Open this publication in new window or tab >>Experimental results of force measurements from a scaled point absorbing wave energy converter subjected to extreme waves
2021 (English)In: Proceedings of the Fourteenth European Wave and Tidal Energy Conference, European Wave and Tidal Energy Conference (EWTEC) , 2021Conference paper, Published paper (Refereed)
Abstract [en]

To achieve a high reliability and durability for wave energy technologies, the effect of extreme wave conditions on the system must be understood. Wave tank experiments are an essential tool to evaluate this, and provide also a foundation for validation of numerical and analytical methods. However, it is not straight-forward how to design such small scale experiments so that they realistically represent wave energy converters in the ocean. In this paper, wave tank experiments of a 1:30 scaled friction damping linear power take-off (PTO) and cylindrical buoy with ellipsoidal bottom are presented. The linear PTO includes a rod that moves vertically against a Teflon block which introduces friction damping. The damping can be adjusted by changing the spring length that provides the compressive force between the Teflon block and the rod. To study extreme forces and snap loads, two load cells measure the line force both directly beneath the buoy, and at the top of the PTO. The motion of the PTO and the buoy are measured with a wire draw line position sensor and Qualysis system, respectively, and a data acquisition system collects and synchronizes the data. The extreme wave conditions used in the experiments are sea states with 50 years return period at the Dowsing site, North Sea. The waves are modelled as regular, irregular and focused waves. Here, the experimental setup and dry testing experiments are presented, and results of the wave tank test experiment for extreme forces are evaluated and further compared with WEC-SIM, to evaluate the agreement of the numerical and experimental model.

Place, publisher, year, edition, pages
European Wave and Tidal Energy Conference (EWTEC), 2021
Series
Proceedings of the European Wave and Tidal Energy Conference, ISSN 2706-6932, E-ISSN 2706-6940
National Category
Marine Engineering Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-457301 (URN)
Conference
Fourteenth European Wave and Tidal Energy Conference (EWTEC), 5-9 September, 2021, Plymouth, UK
Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12
4. Design and evaluation of linear and rotational generator scale models for wave tank testing
Open this publication in new window or tab >>Design and evaluation of linear and rotational generator scale models for wave tank testing
2019 (English)Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
CRC Press, 2019
National Category
Energy Systems Ocean and River Engineering Marine Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-457307 (URN)9780429505324 (ISBN)9781138585355 (ISBN)
Conference
3rd international conference on renewable energies offshore (renew 2018), 8–10 october 2018, Lisbon, Portugal
Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12Bibliographically approved
5. Wave Energy Converter Power Take-Off System Scaling and Physical Modelling
Open this publication in new window or tab >>Wave Energy Converter Power Take-Off System Scaling and Physical Modelling
Show others...
2020 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 8, no 9, article id 632Article in journal (Refereed) Published
Abstract [en]

Absorbing wave power from oceans for producing a usable form of energy represents an attractive challenge, which for the most part concerns the development and integration, in a wave energy device, of a reliable, efficient and cost-effective power take-off mechanism. During the various stages of progress, for assessing a wave energy device, it is convenient to carry out experimental testing that, opportunely, takes into account the realistic behaviour of the power take-off mechanism at a small scale. To successfully replicate and assess the power take-off, good practices need to be implemented aiming to correctly scale and evaluate the power take-off mechanism and its behaviour. The present paper aims to explore and propose solutions that can be applied for reproducing and assessing the power take-off element during experimental studies, namely experimental set-ups enhancements, calibration practices, and error estimation methods. A series of recommendations on how to practically organize and carry out experiments were identified and three case studies are briefly covered. It was found that, despite specific options that can be strictly technology-dependent, various recommendations could be universally applicable.

Place, publisher, year, edition, pages
MDPI, 2020
Keywords
power take-off damping, wave power device, experimental testing, PTO simulator, uncertainty analysis, wave energy testing, experimental set-up, calibration
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-425480 (URN)10.3390/jmse8090632 (DOI)000582055100001 ()
Funder
EU, Horizon 2020, POCI-01-0145-FEDER-016882EU, Horizon 2020, PTDC/MAR-TEC/6984/2014StandUp, 47264-1
Available from: 2020-11-19 Created: 2020-11-19 Last updated: 2024-03-12Bibliographically approved
6. Offshore measurements of hydrodynamic forces on a 1:5 scale buoy
Open this publication in new window or tab >>Offshore measurements of hydrodynamic forces on a 1:5 scale buoy
Show others...
(English)In: Article in journal, Editorial material (Other academic) Submitted
National Category
Energy Systems Marine Engineering Ocean and River Engineering Energy Engineering
Identifiers
urn:nbn:se:uu:diva-457327 (URN)
Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2021-10-27

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