ELEKTRICITETSLÄRA

Wave data – Islandsberg – What is the current wave climate?

The wave measurement buoy that was installed during the spring of 2004 continually measures the waves that enter the research site at Islandsberg. From the gathered information on wave height and length, one can calculate something that may be thought of as a mean wave height, through a so called spectral analysis. This is called the significant wave height. The significant wave height is not only important for the knowledge of how large the waves are that fall in towards the coast, but it is also needed for the calculations of the amount of power flux that enters the research area, and for calculations on the amount energy that is carried by the waves.

Previous Milestones:

On Thursday 25 June, 2009 the substation is launched after being repaired and the three generators are connected to the substation. In the evening, for the first time, the voltage, which is rectified in the substation, and the power from the three linear generators are transmitted to measurement station simultaneously. During May 2009 new buoys are attached to L2 and L3 and the donut-shaped buoy back to L1. During Mars the substation is deployed in the sea. The power cable is lifted from the seabed cut in two halves and attached to the substation. A communication cable is deployed in the direction towards the observation tower for later installation if needed. The substation During late autumn/winter 2008 the construction of the two linear generators L2 and L3 are completed and shipped down to Lysekil. In February 2009 they are deployed at the project site. The linear generator L2 and L3   In May 2008 it is discovered that most of the equipment on the observation tower has been destroyed during storms the previous autumn and winter. The equipment is dismantled and repaired and reinstalled at a greater height in the tower, together with new batteries, camera, charger, router and a modem. The camera system was run for the first time on the 4th July, enabling observation of the work at the buoy field from Uppsala. During spring 2008 new DC-loads are installed in the measurement station to lower the resistive value (and through that enhance the damping). In July 1 kV loads are installed to be used when the three generators have been connected to the substation. The substation is tested for the first time in June in the laboratory in Uppsala and in July the transformer and rectifier are tested together in the substation. During the spring of 2008 the construction of four new buoys is finished, one buoy is shaped like a donut and the rest has cylindrical shape. The donut-shaped buoy is attached to the linear generator in May. The four different buoys An observation tower is erected during the summer 2007 on Klammerskäret and equipped with wind turbine and solar panels. The purpose of this equipment is to power a network camera. Pictures showing the tower, solar panels and wind turbine Inspections of the biology buoys are made during July and August 2007. 2007, the weather continued to be harsh the first months but since early March the generator is operating again. New measuring techniques enables us to compare actual energy affecting the buoy with what the generator produces monumentally during different wave regimes. The measuring station on Gullholmen has also been modified and complemented with new measuring techniques. In April the WEC deliver power to DC-load through a rectifier for the first time. The construction of two new generators starts in Uppsala. From late March to May a large number of new environmental buoys are launched in the project area. New biology buoys are launched   2006, data collection goes on continuously. During the autumn the generator is out of operation due to corrections and exchange of some equipment. Also, the buoys included in the environmental studies are removed and reinforced. The exceptionally rough weather during the late autumn and early winter makes quick replacement impossible. The generator is loaded on the barge.   The generator is shipped off to Lysekil. Late winter/early spring 2006, the sea cable is successfully laid out from Gullholmen on to the project site. During two days of work the measuring station is connected to the project site via the sea cable. The end of the cable is marked with a buoy. Three weeks later, 13th of Mars, the generator is mounted on the concrete foundation, loaded on to a barge, and thereafter towed away to the project site. The slow lowering of the generator to the seabed takes approximately 4 hours. In the evening after 9 pm, power is measured on land at the station on Gullholmen. Sea cable is laid out from Gullholmen to the project site Summer/autumn 2005, ecological tests and research continues, focusing on the issue of fouling by mussels, barnacles and algae. The inventorying of the seabed fauna continues. During summer, an informative poster is made and put up in Grundsund and on Gullholmen. There is also an information meeting with the permanent and summer residents on Gullholmen. The four buoys are finally deployed after some delays due to holiday periods and windy weather. In the end of December an intense period of assemble the first linear generator together begins. The assembling of the first linear generator starts in December 2005 Spring 2005, the building of 4 buoys for environmental studies starts: 1 full-scale buoy and 3 smaller ones (about 2 m in diameter) that have a special design for the study of fouling, among others. The construction of four biology buoys March 2005, the first full-scale buoy (diameter 3 m) is deployed with a 40-ton foundation. The foundation and the buoy were loaded in Lysekil harbor and towed to the site. The foundation is lowered slowly to the seabed. Divers check the site after one week and finds that the foundation is lying steadily on the sandy seabed without getting buried deeper. The force measurement starts.

The force-measuring buoy is deployed

2004, the final site is determined and the National Maritime Administration (Sjöfartsverket) lays out the marker buoys. The wave-measuring buoy is deployed after receiving permit from the County Administrative Board. Later, the first buoys arrive at the Uppsala Laboratory. During November the biological site investigation is started and during December the project has consultations with the County Administrive Board about the drawing of a sea cable from the project site to Gullholmen.

The first buoys arrive to Uppsala in October

The wave-measuring buoy is deployed in April

2003 During the summer; site and seabed exploration are carried out. Project consultation documentation and choice of the site is sent in to the County Administrative Board (Länsstyrelsen) in Västra Götaland. In December the experimental generator is finished in the Uppsala laboratory.

2002 Start of the project: background, goals and planning

Project History and Purpose

The planning of the wave power project Islandsberg started early spring 2002 by researchers and PhD students at the Department for Engineering Science, Division for Electricity at Uppsala University .

The project has two goals. One is to verify that the basic technology for a new wave power concept is successful. The concept is based on a linear generator standing on a foundation on the seabed, and that will be tested under realistic, natural conditions. Another aim is to evaluate alternative solutions. This means, e.g. testing several buoys varying in material, size and design. The project will also develop generators when knowledge and experience grows. The connected generators will be studied as a unified system for electricity production.

The project has permission to use a maximum of 10 generators, which will be deployed successively from 2005 to 2010. Every generator will have an installed capacity of 10 kW. The complete installation of 10 units (100 kW) will, once it is fully operational, produce about 300,000 kWh per year. This amount of electricity is the equivalent of the yearly consumption of about 20 households.

Another, and equally important goal of the study is to gain knowledge of the effects of this new type of wave power plant (smaller and larger ones) on the local environment. This implies e.g. commercial and leisure fishing, effects on birds and other marine species and effects on other marine biological systems.

To study possible nature conservation and other environmental effects, the project is expanded with up to 30 dummy buoys. Consequently, questions about single units and those about the area effects of wave power plants can be investigated. As a result, the knowledge gained from these studies can be used to make environmentally friendly adjustments at an early stage. The generator technology and the simulation models on which the construction is based, allow for various construction alternatives. Environmental and nature conservation can be taken into account without having a negative effect on the technical and economical performance. Even the matter of acceptance, i.e. the view of the general public on wave power will be monitored (see more below)

Choice of location and coordinates

The search for a suitable site for the research installation started late 2002. The choice fell on an area in the sea off Islandsberg in the municipality of Lysekil .

This site provides an acknowledged good wave climate, access to harbours, other modes of transportation and other necessary facilities. It is close to Uppsala University 's Biological Station "Klubban", as well as to Kristineberg Marine Research Station, both of which are co-operators in the project. Furthermore, the closeness to and possibility for connection to the main grid was a decisive factor in the choice of the location.

The project area is relatively close to land, as this simplifies access and reduces costs. Even the average depth (25m) was a factor in the choice of the location, as well as the actual bottom substrate, which is a flat sandy seabed. The depth makes diving relatively easy and diving will be required on a regular basis.

Smaller areas close to the project location that are of a similar nature will be used as control areas, which is important for the biological studies.

A full-scale commercial installation, however, would probably be located further offshore and in deeper, completely open water away from islands and skerries. Such a location was not considered necessary though to achieve the project's scientific objectives.

The research installation is expected to be completed 2009-2010. The project will be concluded in 2013-2014, after which all the equipment will be removed. The purpose and advantage of a slow deployment is that knowledge can be gained and lessons learnt and that adjustments can be made gradually.

Coordinates: (updated during the project's development)
Marking buoy north: Lat N 58º 11'850, Long O 11º 22'460
Marking buoy south: Lat N 58º 11'630, Long O 11º 22'460
Measuring buoy: Lat N 58° 11'740, Long O 11° 22'340

Nautical chart of the project site, which lies in the municipality of Lysekil and directly west of Islandsberg (2 km). The dashed area in the northwest indicates the flat sandy seabed area that was considered suitable. The dots give a preliminary plan of how the buoys will be deployed. The yellow dot furthest west is the measuring buoy, (shown as *). and the marks to the north and south are the marker buoys. The dashed and dotted line to Gullholmen/Härmanö indicates the planned course of the sea cable that will connect the wave power plant to the main grid on land.

Why Wave Power?

The obvious advantage of all forms of renewable energy (with the exception of bio-energy) is that no fuel is required, which eliminates the emission of carbon dioxide. However, it is not always mentioned that even renewable energy sources have a "cost" factor. Sometimes they require large areas and people living in the vicinity are affected in various ways. Renewable resources may also have an impact on their natural environment and on the organisms living there. The scale of these "costs" depends on the choice of the site and the type of energy production (amongst others). The potential of "green" energy sources varies as well, not in the least because of differences in energy content/energy density and in the amount of hours the energy source (sun, wind, waves, etc.) can be used during the year. When these and other physical/technical, economical and environmental factors are weighed up, wave energy stands out as being very competitive.

As the above figure shows, the energy content per unit increases in intensity from solar power, to wind power, to water. The reason for this is that nature concentrates solar energy in wind and then further amasses this energy in sea waves.

Direct use of solar power is limited because the degree of utilization is only 10-12 percent. Moreover, in Sweden the sun only shines 1000 of a year's 8760 hours. The rest of the time it is night, dawn, dusk or overcast.

Wind power plants are dependent on wind. The wind speed needs to be at least 13-15 m/s to fully utilize a wind power plant to its rated effect. For smaller types 10 m/s might suffice. Because it is not always windy, the degree of utilization is 25% on average (on land in Sweden ), going up to 30% if the plants are located at sea.

Waves continue to roll, even after the wind stops blowing, which leads to a higher degree of utilization than for wind power. The circumstances in Sweden , with moderate wave conditions, indicate a degree of utilization of 35% to 50% (depending on differences between the Baltic Sea and the west coast of Sweden ). However, in bigger seas and large oceans, this can go up to 70%. Furthermore, the energy density is a lot higher than for wind or solar power. The physical conditions for wave power are therefore very good and the relatively high degree of utilisation makes waves a predictable source.

Energy generation from wave power should thus have a considerable potential to contribute to our electrical energy production. This is especially the case along the coastlines of the big oceans, but is an option even for Swedish waters, provided that suitable technologies can be developed. About 70 percent of the earth's surface is covered by water. Various estimations show that the world's potential for wave energy is 10,000-15,000 TWh per year. That is about the same as the economic potential of hydropower in the world. In the Baltic Sea alone, the potential is calculated to be 24 TWh, which is much more than the planned Swedish development of renewable energy in the next 5-10 years.

Hydropower, which is based on stored, potential energy, is of course one of the best sources of renewable energy, since hydropower plants can be utilised close to 100 percent of time (provided it rains enough). This high degree of utilisation is not achieved that often, because hydropower is used to regulate the power production (i.e. adjust the production to the consumption which varies during the day). Fossil fuels, nuclear power and bio fuel are used in the same manner. Their energy generation is usually only interrupted because of maintenance or low energy consumption (e.g. during summer)

Technical Background

Research into wave power and wave power systems has been performed for decades. There have been several proposals on how to convert the energy in the ocean's waves into electricity and several research installations have been built all over the world. So far, none have been commercially viable. The main reason has been that all experiments used complicated mechanics and standard generators that are optimised for speeds up to 100 times higher than those generated by ocean waves. A wave rolls 10-15 times per minute. However, a standard generator typically spins 1500 rotations per minute (rpm).

The result has been bulky and expensive installations; often placed in or close to the water surface and including gearboxes as well as other complex and vulnerable sub-systems. Because of their size and emplacement, they cannot cope with the harsh north-Atlantic wave climate they are intended for.

No serious endeavours have been made on wave power plants for the considerably milder, but steadier wave climate in the Baltic Sea . Since the 1980ies, the wave energy of these waters has been considered to be too small for the types of systems tested so far.

However, through a thorough analysis of the physical prerequisites and through applying material science and other relevant advances in the construction, a new system concept for wave power has been developed. The new concept is environmentally friendly and commercially interesting in a manner that is very different from five or ten years ago.

The wave power concept being tested in the project differs in many ways from earlier attempts. Instead of adjusting rotating standard generators to rolling waves, a totally new type of generator has been developed at Uppsala University , which is specially designed for "standard" waves.

The concept is based on a generator situated on the seabed. The other components are a rope that couples the generator to a buoy on the surface. The rotor exists of a piston that moves up and down in the stator. Therefore, the rotor does not spin, but is driven directly via the rope by the buoy motion on the surface. This results in a host of system advantages. A generator placed on the seabed is protected from harsh weather. Should a buoy break adrift, it should float to land. This, together with the rope, would only constitute a minor cost.

This new direct-driven linear generator with a uniquely low pole width means that electrical power can be generated even with the low velocity of ocean waves. It also entails that the power supply can be connected to the main grid on land by standard transmission. Moreover, this type of generator allows for very simple device mechanics (buoy and rope). These can also cope with high loads in a cost effective way.

Computer simulations indicate that this solution can compete commercially with established methods of producing electrical energy, without long-term subsidies. This is an important prerequisite, because renewable energy should be able to contribute to the energy supply without negative socio-economic consequences.

What is more, the technology is expected to have little or limited effect on the environment. Wave power does not produce any emissions, will not be visible from land and may protect the marine environment. The solution contains only well known materials. It is not dependent on shallow banks and will contribute to fulfilling at least 5 of the 15 environmental goals that the Swedish Parliament approved:

• Limited environmental effects (emission of greenhouse gases)
• Fresh air
• Only natural acidification
• Protecting ozone layer
• Safe radiation environment

Besides, wave power plants may protect and even improve the marine environment where they are located, as well as limit over-fishing. In this way, they fulfil 2 more environmental goals:

• Ocean in balance
• Living coast and archipelago

The new technology is expected to have a substantial commercial potential and in case of industrialisation, to create about 3000 jobs. For that reason, cost analyses will be made in parallel with the other project studies. The various alternatives will be assessed against each other and costs and maintenance needs can be weighed against e.g. environmental costs.

© Oskar Danielsson, Division for Electricity . Uppsala University

Above is a schematic sketch of a linear generator for the extraction of energy from wave power. A buoy follows the waves' motions up and down. About 20 percent of the incoming energy can be absorbed and turned into electrical power. The buoy's motions are transferred via a rope or cable to the generator's moveable part, which in this case consists of a piston. The piston is equipped with very strong neodymium-iron-boron (Nd-Fe-B) magnets and induces currents in the stator's windings. In addition, the piston is connected to a spring system, which gives the generator additional power also when the buoy is mowing down a wave.

Calculations show that for a unit, like the one above, to give 10 kW power, one needs a linear generator with a stroke length of about 2m. The unit's total weight will be about 20 ton, the bulk of that being the foundation. For Swedish waters, which have relatively small waves, powers of 10 kW per unit are suitable. For bigger waves, like the ones off the coast of Norway or Scotland , bigger units with powers of 100 kW or higher are more advantageous.

Another advantage of the technology is that it is modular. Wave power plants can consist of a suitable amount of units, which share the systems of transfer of energy to land. More units can be added afterwards when demand grows. In the same way, one or more units can be disconnected and lifted out without the whole installation being forced out of service. Future offshore installations are expected to consist of some hundred units up to several thousand, depending on the electricity need and location.

Graphical illustration of a wave power plant
© Karl Åstrand and Division for Electricity, Uppsala University

Marine Ecology and Environmental Questions

All energy systems have an effect on the environment. This is valid even for energy generation from renewable sources. In these cases, it is especially the local environment that is affected. The effect is double-sided, as the installations on the one hand have an effect on animals and nature to a greater or lesser extent. On the other hand, the installation itself is affected by and has to be adapted to the local conditions.

Hydropower is one example of renewable energy production confronted with this problem. The dams and level control affect chiefly waterborne animals, salmon being the classical example. Even wind power can have various negative effects, e.g. when some bird species suffer fiercely in certain areas.

Wave power is largely untested as a source of energy production. It is therefore important to learn at an early stage about the possible negative effects of wave power plants on marine organisms and make adjustments accordingly. This is why the research installation consists of 30 extra buoys (only anchored to buoy foundations), besides the 10 with generators.

Thus, potential area effects, which are connected to large wave power projects, are simulated better. Indications of negative effects and their causes can be addressed at an early stage and lessons can be learnt about how possible problems can be avoided.

Wave power projects, unlike sea-based wind power plants are not dependent on biologically sensitive seabed banks for their location. Nevertheless, they can be expected to have a local effect, especially on smaller seabed based organisms. Bigger wave power plants may also have effects on fish, marine mammals like seals and, in Swedish waters, on porpoises. In other seas, other species, like whales, should be studied in more detail.

These studies are done in cooperation with the Department for Ecology and Evolution at Uppsala University , department of Animal Ecology (www.ebc.uu.se/zooeko/index.shtml). Uppsala University also has at its disposal the Klubban Biological Station (www.ebc.uu.se/klubban/Address.htm) at Fiskebäckskil. The vicinity to Kristineberg Marine Research Station (www.kmf.gu.se) has opened up possibilities of cooperation in the project.

Biological/ecological sub-projects will, among others, consist of:

• Effects on the seabed fauna, mainly marine invertebrates
• Effects on fish living on the seabed (benthic) as well as on pelagic fish (living in the open water)
• Effects on seabirds
• Effects on marine mammals (maybe the local harbour seal can be used as an indication)
• Positive effects such as fouling and artificial reefs - see more below

The knowledge gained from these studies can, if needed, lead to adaptations and adjustments of the generator and buoy technologies. It could also give indications, in a larger perspective, on the most suitable locations for larger wave power parks and how the spacing design should look like.

A combined biological and technical problem for example, is fouling. Solid objects located at sea are quickly fouled by algae, mussels and barnacles (a well-known problem for boat-owners). It is therefore reasonable to assume that buoys and the upper part of the rope connecting the buoy with the generator will be fouled. The generator on the seabed though, will not be affected as fouling is dependent on sunlight and mainly occurs near the surface.

The technical aspect of this is to calculate and understand how the system will be affected by the extra mass due to the biofouling of buoys. There will probably also be some effect on the buoy's mobility in the water. Regular removal of algae, mussels etc, means higher maintenance costs and will therefore lead to a higher energy price. The technical/economical question is then whether the fouling should be taken care of or whether it would be better to compensate for the fouling from the start, e.g. through a certain over-dimensioning of the components.

From a biological point of view, fouling can only be a good thing. It leads to an increased local biodiversity, which in later stages might lead to an influx of small animals, e.g. smaller crawfish, which in its turn would attract predators and in the end would result in more fish in the area. Several of the concrete foundations for both generators and buoys will be cast in special designs that could be beneficial to certain organisms like crabs and lobsters.

Acceptance and General Questions

The general public's attitude to wave power is an important matter. Big-scale eco-friendly energy production requires the use of large areas in a way that limits other activities or makes them altogether impossible (think of hydropower or wind power). This goes for big-scale wave power plants as well. However, the most suitable location for commercial wave power plants is further offshore and will therefore not spoil the view from the coast, e.g. wide, open horizons. A buoy sticks out at most one meter above the surface and is consequently only visible at close range. Commercial shipping is not possible through a wave power plant, which is why they will not be located close to fairways etc. Smaller leisure boats will probably be able to navigate through a wave power area and there should be no objections to leisure fishing activities.

Links to cooperation partners

• Västra Orust Energitjänst (www.voe.se)

Project Publications

M. Rahm, C. Boström, O. Svensson, M. Grabbe, F. Bülow, M. Leijon, "Offshore underwater substation for wave energy converter arrays", Renewable Power Generation, IET , vol.4, no.6, pp.602-612, November 2010
doi: 10.1049/iet-rpg.2009.0180

Jens Engström, M. Eriksson, J. Isberg, and M. Leijon, "Wave energy converter with enhanced amplitude response at frequencies coinciding with Swedish west coast sea states by use of a supplementary submerged body",
JOURNAL OF APPLIED PHYSICS 106, 064512, 2009,
Available at: http://link.aip.org/link/?JAP/106/064512

Rahm, M., Boström, C., Svensson, O., Grabbe, M., Bülow, F. & Leijon, M. "Laboratory experimental verification of a marine substation." In Proceedings of the 8th European wave and tidal energy conference, EWTEC09, Uppsala, Sweden, pp. 51–58.

Svensson, O., Boström, C. , Rahm, M., & Leijon, M., "Description of the control and measurement system used in the Low Voltage Marine Substation at the Lysekil research site." Proceedings of the 8th European wave and tidal energy conference, EWTEC09, Uppsala, Sweden, pp. 44–50.

O. Langhamer, D. Wilhelmsson and J. Engström, "Development of Invertebrate Assemblages and Fish on Offshore Wave Power", Proceedings of the 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2009),
May 31 to June 5, 2009, Honolulu, Hawaii.

A. Savin, O. Svensson, E. Strömstedt, C. Boström and M. Leijon, "Determining the service life of a steel wire under a working load in the Wave Energy Converter (WEC)", Proceedings of the 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2009), May 31 to June 5, 2009, Honolulu, Hawaii.

S. Tyrberg, H. Gravråkmo and M. Leijon, "Tracking a Wave Power Buoy Using a Network Camera: System Analysis and First Results", Proceedings of the 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2009),
May 31 to June 5, 2009, Honolulu, Hawaii.

Bostrom, C.; Waters, R.; Lejerskog, E.; Svensson, O.; Stalberg, M.; Stromstedt, E.; Leijon, M. "Study of a Wave Energy Converter Connected to a Nonlinear Load"
IEEE Journal of Oceanic Engineering, vol. 34, issue 2, pp. 123-127, 2009.,
DOI: 10.1109/JOE.2009.2015021

C. Boström, E. Lejerskog, M. Stålberg, K. Thorburn and M. Leijon. "Experimental results of rectification and filtration from an offshore wave energy system", Renewable Energy, Vol.34 No.5 pp.1381-1387, 2009.

Leijon, M.; Waters, R.; Rahm, M.; Svensson, O.; Bostrom, C.; Stromstedt, E.; Engstrom, J.; Tyrberg, S.; Savin, A.; Gravrakmo, H.; Bernhoff, H.; Sundberg, J.; Isberg, J.; Agren, O.; Danielsson, O.; Eriksson, M.; Lejerskog, E.; Bolund, B.; Gustafsson, S.; Thorburn, K.;, "Catch the wave to electricity", IEEE Power and Energy Magazine, Volume 7,  Issue 1,  January-February 2009 Page(s):50 - 54, doi: 10.1109/MPE.2008.930658

R. Waters, J. Engström, J. Isberg and M. Leijon, "Wave climate off the Swedish west coast", Renewable Energy 34 (2009), pp. 1600-1606, doi:10.1016/j.renene.2008.11.016

Jan Isberg, Mikael Eriksson, Mats Leijon, "Transport of energy in polychromatic fluid gravity waves", Journal of Engineering Mathematics doi: 10.1007/s10665-008-9243-1

Mats Leijon, Cecilia Boström, Oskar Danielsson, Stefan Gustafsson, Kalle Haikonen, Olivia Langhamer, Erland Strömstedt, Magnus Stålberg, Jan Sundberg, Olle Svensson, Simon Tyrberg and Rafael Waters, "Wave Energy from the North Sea: Experiences from the Lysekil Research Site", Surveys in Geophysics, Volume 29 2008 :221–240, DOI 10.1007/s10712-008-9047-x

Stålberg, M., Waters, R., Danielsson, O. & Leijon, M. "Influence of generator damping on peak power and variance of power for a direct drive wave energy converter". Journal of Offshore Mechanics and Arctic Engineering vol 130, Iss. 3, 2008.

C. Boström, E. Lejerskog, S. Tyrberg, O. Svensson, R. Waters, A. Savin, B. Bolund, M. Eriksson and M. Leijon "Experimental results from an offshore wave energy converter", Proceedings of the 27th International Conference on Offshore Mechanics and Arctic Engineering OMAE 2008, June 15-20, 2008, Estoril, Portugal, OMAE2008-57415.

Mats Leijon, Annika Skoglund,Rafael Waters, Alf Rehn and Marcus Lindahl, "On the Physics and Economics of Renewable Electric Energy Sources – part I utilization", The 2nd WSEAS/IASME International Conference on ENERGY PLANNING, ENERGY SAVING, ENVIRONMENTAL EDUCATION (EPESE'08)Corfu, Greece, October 26-28, 2008

Annika Skoglund, Mats Leijon, Alf Rehn, Marcus Lindahl and Rafael Waters, "On the Physics and Economics of Renewable Electric Energy Sources –part II Engineering", The 2nd WSEAS/IASME International Conference on ENERGY PLANNING, ENERGY SAVING, ENVIRONMENTAL EDUCATION, (EPESE'08)Corfu, Greece, October 26-28, 2008

Simon Tyrberg, Magnus Stålberg, Cecilia Boström, Rafael Waters, Olle Svensson, Erland Strömstedt, Andrej Savin, Jens Engström, Oliva Langhamer, Halvar Gravråkmo, Kalle Haikonen, Jenny Tedelid, Jan Sundberg & Mats Leijon, "The Lysekil Wave Power Project: Status Update", WREC X Glasgow, july 21-25, 2008

Oskar Danielsson, Karin Thorburn, Mats Leijon, Chapter: 6.2:  Direct Drive - Linear Generators, Book: Ocean Wave Energy - Current Status and Future Perspectives, Editor: João Cruz, Springer, Tyskland, 2008

Oskar Danielsson and Mats Leijon, "Flux Distribution in Linear Permanent-Magnet Synchronous Machines Including Longitudinal End Effects", IEEE Trans. on Magnetics, Volume 43,  Issue 7,  July 2007 pp 3197 – 3201

M.Eriksson, R.Waters, O.Svensson, J.Isberg and M.Leijon, "Wave power absorption: Experiments in open sea and simulation", JOURNAL OF APPLIED PHYSICS 102,084910 (2007) Abstract | Full Text: | PDF (674 kB)

Urban Henfridsson, Viktoria Neimane, Kerstin Strand, Robert Kapper, Hans Bernhoff, Oskar Danielsson, Mats Leijon, Jan Sundberg, Karin Thorburn, Ellerth Ericsson and Karl Bergman, "Wave energy potential in the Baltic Sea and the Danish part of the North Sea, with reflections on the Skagerrak", Renewable Energy, Volume 32, Issue 12, October 2007, Pages 2069-2084
SummaryPlus| Full Text + Links | PDF (886 K)

Karin Thorburn and Mats Leijon, "Farm size comparison with analytical model of linear generator wave energy converters", Ocean Engineering, Volume 34, Issues 5-6, April 2007, Pages 908-916

R. Waters, M. Stålberg, O. Danielsson, O. Svensson, S. Gustafsson, E. Strömstedt, M. Eriksson, J. Sundberg, and M. Leijon, "Experimental results from sea trials of an offshore wave energy system", Appl. Phys. Lett. 90, 034105 (2007), Full Text:   HTML  Sectioned HTML | PDF (129 kB)

R. Waters, O. Danielsson and M. Leijon, "Measuring air gap width of permanent magnet linear generators using search coil sensor", J. Appl. Phys. 101, 024518 (2007), Full Text:   HTML | Sectioned HTML | PDF (225 kB)

J. Engström, R. Waters, M. Stålberg, E. Strömstedt, M. Eriksson, J. Isberg, U. Henfridsson, K. Bergman, J. Asmussen, and M. Leijon, "Offshore experiments on a direct-driven Wave Energy Converter", Proceedings of the 7th European Wave and Tidal Energy Conference, 11-13 September 2007, Porto, Portugal.

Magnus Stålberg, Rafael Waters, Oskar Danielsson, Prof. Mats Leijon, "Influence of Generator Damping on Peak Power and Variance of Power for a Direct Drive Wave Energy Coverter", Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering OMAE 2007, June 10-15, 2007, San Diego, California, USA

Hans Bernhoff, Elisabeth Sjöstedt and Mats Leijon, "Wave energy resources in sheltered sea areas: A case study of the Baltic Sea", Renewable Energy, Volume 31, Issue 13, October 2006, Pages 2164-2170 SummaryPlus | Full Text + Links | PDF (286 K)

O. Danielsson, M. Eriksson, M. Leijon, "Study of a longitudinal flux permanent magnet linear generator for wave energy converters", International Journal of Energy Research, vol 30, Issue 14, Nov 2006 (p 1130-1145) Abstract| References | Full Text

Eriksson, M.; Isberg, J.; Leijon, M., "Theory and Experiment on an Elastically Moored Cylindrical Buoy", IEEE Journal of Oceanic Engineering, Volume 31,  Issue 4, Oct. 2006 Page(s):959 - 963, Abstract | Full Text: PDF (449 KB)

M. Leijon, O. Danielsson, M. Eriksson, K. Thorburn, H. Bernhoff, J. Isberg, J. Sundberg, I. Ivanova, E. Sjöstedt, O. Ågren, "An electrical approach to wave energy conversion", Renewable Energy, Volume 31, Issue 9, July 2006, Pages 1309-1319
SummaryPlus | Full Text + Links | PDF (286 K)

K. Nilsson, O. Danielsson, and M. Leijon, "Electromagnetic forces in the air gap of a permanent magnet linear generator at no load", JOURNAL OF APPLIED PHYSICS 99, 034505, 2006

Karin Thorburn, Karl-Erik Karlsson, Arne Wolfbrandt, Mikael Eriksson and Mats Leijon
"Time stepping finite element analysis of a variable speed synchronous generator with rectifier", Applied Energy, Volume 83, Issue 4, April 2006 , Pages 371-386 SummaryPlus | Full Text + Links | PDF (436 K)

Anna Wolfbrandt, "Automated Design of a Linear Generator for Wave Energy Converters - A Simplified Model", IEEE Transactions on Magnetics, Volume 42, No7, July 2006

K. Thorburn and M. Leijon, "Analytical and circuit simulations of linear generators in farm", 2005/06 IEEE PES T&D Conference and Exposition, Dallas, USA, 21-24 May 2006.

K. Thorburn, K. Nilsson, O. Danielsson and M. Leijon, "Generators and electrical systems for direct drive energy conversion", MAREC 2006 at WMTC 2006, London, UK, 6-10 March 2006.

Danielsson, O.; Leijon, M.; Sjostedt, E. " Detailed Study of the Magnetic Circuit in a Longitudinal Flux Permanent-Magnet Synchronous Linear Generator" , IEEE Trans on Magnetics Vol 41 Issue 9 Sept 2005 Page(s):  2490- 2495.

Eriksson M., J. Isberg and M. Leijon, "Hydrodynamic modelling of a direct drive wave energy converter",International Journal of Engineering Science, Volume 43,
Issues 17-18, November 2005 , pp. 1377-1387, SummaryPlus | Full Text + Links | PDF (382 K)

I.A. Ivanova, H. Bernhoff, O. Ågren and M. Leijon
"Simulated generator for wave energy extraction in deep water"
Ocean Engineering, Volume 32, Issues 14-15, pp. 1664-1678, October 2005
SummaryPlus | Full Text + Links | PDF (409 K)

Ivanova, I.A., Agren, O., Bernhoff, H., Leijon, M., " Simulation of Wave-Energy Converter With Octagonal Linear Generator", IEEE Journal of Oceanic Engineering Volume 30,  Issue 3,  July 2005 Page(s):619 – 629

Leijon, M., Bernhoff, H., Ågren, O., Isberg, J., Sundberg, J., Berg, M., Karlsson, K.E., Wolfbrandt, A., "Multiphysics Simulation of Wave Energy to Electric Energy Conversion by Permanent Magnet Linear Generator", IEEE Transactions on Energy Conversion, Volume: 20 , Issue: 1 , March 2005, pp. 219 – 224

O. Danielsson, M. Leijon, K. Thorburn, M. Eriksson, H. Bernhoff, "A Direct Drive Wave Energy Converter – Simulations and Experiments" Proceedings of OMAE 2005: 24th International Conference on Mechanics and Arctic Engineering, Halkidiki, Greece, 12-17 June 2005

S. Gustafsson, O. Svensson, J. Sundberg, H. Bernhoff, M. Leijon, O. Danielsson, M. Eriksson, K. Thorburn, K. Strand, U. Henfridsson, E. Ericsson, K. Bergman, "Experiments at Islandsberg on the west coast of sweden in preparation of the construction of a pilot wave power plant" , Presented at the 6th EWTEC conference in Glasgow , 28th of August to 3rd of September 2005

M. Stålberg , R. Waters, M. Eriksson, O. Danielsson, K. Thorburn, H. Bernhoff, M. Leijon, " Full-Scale Testing of PM Linear Generator for Point Absorber WEC" , Presented at the 6th EWTEC conference in Glasgow, 28th of August to 3rd of September 2005

Bernhoff. H , Leijon. M, "Conversion of wave energy to electricity", The Scandinavian Shipping Gazette, The Scandinavian Yearbook of Maritime Technology 2004, October 1, 2004

Thorburn K., Bernhoff H., and Leijon M, "Wave energy transmission system
concepts for linear generator arrays" Ocean Engineering, 31(11-12), pp 1339 – 1349, August 2004.

B. Bolund, E. Segergren, A. Solum, R. Perers, L. Lundström, A. Lindblom, K. Thorburn, M. Eriksson, K. Nilsson, I. Ivanova, O. Danielsson, S. Eriksson, H. Bengtsson, E. Sjöstedt, J. Isberg, J. Sundberg, H. Bernhoff, K.-E. Karlsson, A. Wolfbrandt, O. Ågren, and M. Leijon, "Rotating and linear synchronous generators for renewable electric energy conversion – an update of the ongoing research projects at Uppsala University" Nordic Workshop on Power and Industrial Electronics, NORPIE 2004, Trondheim, Norway, 14–16 June 2004.

O. Danielsson, K. Thorburn, E. Sjöstedt, and M. Leijon, "Simulated response of a linear generator wave energy converter" ISOPE-2004, Toulon, France, 23–28 May 2004.

M. Eriksson, K. Thorburn, H. Bernhoff, and M. Leijon, "Dynamics of a linear generator for wave energy conversion" 23rd International Conference on Offshore Mechanics and Arctic Engineering, Vancouver, Canada, 20–25 June 2004.

Ivanova I., Ågren O., Bernhoff H., Leijon M., "Simulation of a 100 kW permanent magnet octagonal linear generator for ocean wave energy comversion and utilization", Scientific Technical Review Journal (Nauchno-Tekhnicheskie Vedomosti), Vol.1. pp. 239 – 244, Saint-Petersburg, Russia, 2004

A. Ivanova, O. Ågren, H. Bernhoff, M. Leijon, "Simulation of cogging in a 100kW permanent magnet octagonal linear generator for ocean wave conversion", Int. Symp. on underwater technology, Taipei, Taiwan, 20-23 april, 2004

Mats Leijon, Hans Bernhoff, Marcus Berg, Olov Ågren, "Economical considerations of renewable electric energy production -especially development of wave energy" Renewable Energy Volume 28, Issue 8, July 2003, Pages 1201-1209
SummaryPlus | Full Text + Links | PDF (245 K)

O. Danielsson, K. Thorburn, M. Eriksson, M. Leijon "Permanent magnet fixation concepts for linear generator" Fifth European wave energy conference 17-19 sept, 2003 PDF (472k)

Ivanova, O. Ågren, H. Bernhoff, M. Leijon "Simulation of a 100kW permanent magnet octagonal linear generator for ocean wave conversion"
Fifth European wave energy conference 17-19 sept, 2003 PDF (371k)

Dissertations
Magnus Rahm , "Ocean Wave Energy: Underwater Substation System for Wave Energy Converters", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214; 711, Abstract + Fulltex

Olivia Langhamer, "Wave energy conversion and the marine environment: Colonization patterns and habitat dynamics", Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214; 663, Abstract

Rafael Waters, ”Energy from Ocean Waves. Full Scale Experimental Verification of a Wave Energy Converter”, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 580, ISBN 978-91-554-7354-9, Abstract

Mikael Eriksson, "Modelling and Experimental Verification of Direct Drive Wave Energy Conversion. Buoy-Generator Dynamics", Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 287, ISBN 978-91-554-6850-7

Oskar Danielsson, ”Wave Energy Conversion, Linear Synchronous Permanent Magnet Generator”, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 232, ISBN 91-554-6683-4

Karin Thorburn, ”Electric Energy Conversion Systems: Wave Energy and Hydropower”, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 202, ISBN 91-554-6617-6

Licentiate thesis
Simon Tyrberg, ”Studying Buoy Motion for Wave Power”,
UURIE 313-09L, Uppsala 2009 PDF

Cecilia Boström, ”Electrical System of a Wave Power Plant”,
UURIE 308-09L, Uppsala 2009