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Sea Level Compensation System for Wave Energy Converters
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The wave energy converter developed at Uppsala University consists of a linear generator at the seabed driven by the motion of a buoy on the water surface. The energy absorbed by the generator is negatively affected by variations of the mean sea level caused by tides, changes in barometric pressure, strong winds, and storm surges.

The work presented in this doctoral thesis aims to investigate the losses in energy absorption for the present generation wave energy converter due to the effect of sea level variations, mainly caused by tides. This goal is achieved through the modeling of the interaction between the waves and the point absorber. An estimation of the economic cost that these losses imply is also made. Moreover, solutions on how to reduce the negative effect of sea level variations are discussed. To this end, two compensation systems which adjust the length of the connection line between the floater and the generator are designed, and the first prototype is built and tested near the Lysekil research site.

The theoretical study assesses the energy loss at about 400 coastal points all over the world and for one generator design. The results highlight critical locations where the need for a compensation system appears compelling. The same hydro-mechanic model is applied to a specific site, the Wave Hub on the west coast of Cornwall, United Kingdom, where the energy loss is calculated to be about 53 %. The experimental work led to the construction of a buoy equipped with a screw jack together with its control, measurement and communication systems. The prototype, suitable for sea level variations of small range, is tested and its performance evaluated. A second prototype, suitable for high range variations, is also designed and is currently under construction.

One main conclusion is that including the compensation systems in the design of the wave energy converter will increase the competitiveness of the technology from an economic point of view by decreasing its cost per kWh. The need for a cost-effective wave energy converter with increased survivability emphasizes the importance of the presented research and its future development.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 71 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1386
Keyword [en]
Ocean energy, Tides, Linear generator, Point absorber, Offshore experiment, Hydro-mechanic modeling, Power absorption, Control system, Communication system, Measurement system, Lysekil research site, Wave Hub.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-295603ISBN: 978-91-554-9613-5OAI: oai:DiVA.org:uu-295603DiVA: diva2:941507
Public defence
2016-09-09, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-08-19 Created: 2016-06-08 Last updated: 2016-08-25
List of papers
1. Influence of Sea State and Tidal Height on Wave Power Absorption
Open this publication in new window or tab >>Influence of Sea State and Tidal Height on Wave Power Absorption
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(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-295597 (URN)
Available from: 2016-06-08 Created: 2016-06-08 Last updated: 2016-06-08
2. Impact of Tidal Level Variations on the Wave Energy Absorption at Wave Hub
Open this publication in new window or tab >>Impact of Tidal Level Variations on the Wave Energy Absorption at Wave Hub
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-295599 (URN)
Available from: 2016-06-08 Created: 2016-06-08 Last updated: 2016-08-25Bibliographically approved
3. Tidal effect compensation system for point absorbing wave energy converters
Open this publication in new window or tab >>Tidal effect compensation system for point absorbing wave energy converters
2013 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 51, 247-254 p.Article in journal (Refereed) Published
Abstract [en]

Recent studies show that there is a correlation between water level and energy absorption values for the studied wave energy converters: the absorption decreases when the water levels deviate from average. The situation appears during tides when the water level changes significantly. The main objective of the paper is to present a first attempt to increase the energy absorption during tides by designing and realizing a small-scale model of a point absorber equipped with a device that is able to adjust the length of the rope connected to the generator. The adjustment is achieved by a screw that moves upwards in the presence of low tides and downwards in the presence of high tides. Numerical results as well as experimental tests suggest that the solution adopted to minimize the tidal effect on the power generation shows potential for further development.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-194181 (URN)10.1016/j.renene.2012.09.043 (DOI)000312922000032 ()
Available from: 2013-02-13 Created: 2013-02-11 Last updated: 2016-08-25
4. Algorithm for the Calculation of the Translator Position in Permanent Magnet Linear Generators
Open this publication in new window or tab >>Algorithm for the Calculation of the Translator Position in Permanent Magnet Linear Generators
2014 (English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012, E-ISSN 1941-7012, Vol. 6, no 6, 063102- p.Article in journal (Refereed) Published
Abstract [en]

A permanent magnet linear generator for direct drive wave energy converters is a suitable power take-off system for ocean wave energy extraction, especially when coupled with a point absorbing buoy via a connection line. The performance of the linear generator is affected by the excursion of the translator along the stator. The optimal stroke is achieved when the midpoint of the oscillations coincides with the center of the stator. However, sea level changes due to, e.g., tides will shift these oscillations. This paper proposes a model able to detect the position of the translator from the generator output voltage. The algorithm will be integrated in the control system of a mechanical device that adjusts the length of the connection line in order to center the average position of the translator with the center of the stator. Thereby, the output power from the wave energy converter increases, and the mechanical stresses on the hull of the generator decrease. The results obtained by the model show good agreement with the experimental results from two linear generators, L2 and L3, deployed in the Lysekil wave energy research site, Sweden. The theoretical results differ from the experimental results by −4 mm for L2 and 21 mm for L3 with a standard deviation of 27 mm and 31 mm, respectively.

Keyword
ocean waves, permanent magnet generators, stators, wave power generation, linear machines
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-237446 (URN)10.1063/1.4900553 (DOI)000347152500003 ()
Available from: 2014-12-02 Created: 2014-12-02 Last updated: 2016-06-08
5. Wireless System for Tidal Effect Compensation in the Lysekil Research Site
Open this publication in new window or tab >>Wireless System for Tidal Effect Compensation in the Lysekil Research Site
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2012 (English)In: Proceedings of the ASME 31st International Conference on Ocean, Offshore and Arctic Engineering, vol. 7, 2012, 293-298 p.Conference paper (Refereed)
Abstract [en]

This paper describes, firstly, the rope adjustment device for wave energy converters (WECs) to minimize the tidal effect on the electricity production and, secondly, a wireless communication network between point absorbing WECs in the Lysekil Research Site and a computer station at the Department of Engineering Sciences at Uppsala University. The device is driven by a motor that activates when the main water level deviates from the average. The adjustment is achieved through a screw that moves upwards during low tides and downwards during high tides. For the purpose of testing the device in the research site, a wireless connection between the buoy in the sea and a computer on land will be designed. A sensor located close to the research site monitors the sea water level and, every time a significant variation is registered, it sends wirelessly a signal to the data logger that controls the power to the motor The position of the screw is observed by a second sensor and the measurements are retrieved back to Uppsala via GSM connection. The full scale device is tested in the lab and it is demonstrated to work properly, requiring less than 750 W to lift and lower different loads. Moreover, the wireless communication network is designed and once it will be built, it will allow to recall and store data, send information from one node of the system to another, monitor the proper functioning of the device and modify the control as desired.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-190207 (URN)10.1115/OMAE2012-83361 (DOI)000324507000036 ()978-0-7918-4494-6 (ISBN)
Conference
31st International Conference on Ocean, Offshore and Arctic Engineering, July 1-6, 2012 Rio de Janeiro, Brazil
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2016-08-25Bibliographically approved
6. Control System for Compensator of Mean Sea Level Variations at the Lysekil Research Site
Open this publication in new window or tab >>Control System for Compensator of Mean Sea Level Variations at the Lysekil Research Site
2014 (English)Conference paper (Other academic)
Place, publisher, year, edition, pages
Japan, Tokyo: , 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-237524 (URN)
Conference
2nd Asian Wave and Tidal Energy Conference
Available from: 2014-12-03 Created: 2014-12-03 Last updated: 2016-06-10
7. Nearshore Tests of the Tidal Compensation System for Point-Absorbing Wave Energy Converters
Open this publication in new window or tab >>Nearshore Tests of the Tidal Compensation System for Point-Absorbing Wave Energy Converters
2015 (English)In: Energies, ISSN 1996-1073, Vol. 8, no 4, 3272-3291 p.Article in journal (Refereed) Published
Abstract [en]

The power production of the linear generator wave energy converter developed at Uppsala University is affected by variations of mean sea level. The reason is that these variations change the distance between the point absorber located on the surface and the linear generator located on the seabed. This shifts the average position of the translator with respect to the center of the stator, thereby reducing the generator output power. A device mounted on the point absorber that compensates for tides of small range by regulating the length of the connection line between the buoy at the surface and the linear generator has been constructed and tested. This paper describes the electro-mechanical, measurement, communication and control systems installed on the buoy and shows the results obtained before its connection to the generator. The adjustment of the line was achieved through a linear actuator, which shortens the line during low tides and vice versa. The motor that drives the mechanical device was activated remotely via SMS. The measurement system that was mounted on the buoy consisted of current and voltage sensors, accelerometers, strain gauges and inductive and laser sensors. The data collected were transferred via Internet to a Dropbox server. As described within the paper, after the calibration of the sensors, the buoy was assembled and tested in the waters of Lysekil harbor, a few kilometers from the Uppsala University research site. Moreover, the performance of the sensors, the motion of the mechanical device, the power consumption, the current control strategy and the communication system are discussed.

National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-255300 (URN)10.3390/en8043272 (DOI)000353963400045 ()
Available from: 2015-06-18 Created: 2015-06-15 Last updated: 2016-08-25Bibliographically approved
8. Tidal Effect Compensation System Design for High Range Sea Level Variations
Open this publication in new window or tab >>Tidal Effect Compensation System Design for High Range Sea Level Variations
2015 (English)Conference paper, Poster (Refereed)
Abstract [en]

The working principle of the wave energy converter (WEC) from Uppsala University is a heaving point absorber with directly driven linear generator placed on the seabed. The heave motion of the buoy is transmitted to the generator via a steel cable. When tides occur, the sea level changes, and thus making the WEC works below optimal condition. This system is designed so that the WEC is able to work at sea level variation up to 8 meters. A compensation system is designed to continuously make the WEC work in its optimal condition even at different sea levels. We present a mechanical system and its control algorithm that monitor and control the length of the connecting line. The connecting line is consist of a steel wire and a steel chain connected together. The mechanical part of the system is the winch that retracts or releases the steel chain that connects the translator and the buoy at the water surface. The rotation of the winch is controlled by a motor with the help of microcontrollers and several sensors for accuracy and feedback. The result from simulation showed that the system works fine. The approach of compensating the wire length connecting the buoy and the translator allow more flexibility to WEC to work in the area with high sea level variation.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-286598 (URN)
Conference
Proceedings of the 11th European Wave and Tidal Energy Conference 6-11th Sept 2015, Nantes, France
Available from: 2016-04-21 Created: 2016-04-21 Last updated: 2016-08-25

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