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Ottermo F.,Uppsala University | Ottermo F.,Halmstad University | Bernhoff H.,Uppsala University | Bernhoff H.,Swedish Center for Renewable Electrical Energy Conversion
Wind Energy | Year: 2014

The scaling behaviour of a straight-bladed vertical axis wind turbine is considered. A scaling scheme is described that, in the presence of a wind shear profile, aims at leaving the material stresses of the scaled construction unchanged. On the basis of a recent 200kW three-bladed H-rotor design, a structural upper size of the turbine is proposed, this size being the scale at which the gravitational force starts to become important. As gravity has a much worse scaling behaviour than the aerodynamic and centrifugal forces, the construction work will become increasingly more difficult above this scale. The upper size is estimated to be around 30MW. Copyright © 2013 John Wiley & Sons, Ltd. Copyright © 2013 John Wiley & Sons, Ltd. Source


Ekstrom R.,Swedish Center for Renewable Electrical Energy Conversion | Leijon M.,Swedish Center for Renewable Electrical Energy Conversion | Leijon M.,University of Southampton
International Journal of Marine Energy | Year: 2014

To grid-connect an offshore wave power farm, an intermediate marine substation is suggested. As a part of the Uppsala University wave power project, a marine substation has been designed, assembled and deployed at sea. The substation is capable of connecting up to seven wave energy converters (WECs), and to transfer the power to the onshore 1 kV-grid. In this article, the control procedure for grid connection of the WECs is described step-by-step, and the practical implementations are presented. The system is designed with autonomous control and will connect or disconnect the WECs, depending on the sea state. Fault handling is taken into account, and grid power quality such as harmonic distortion and flicker are considered in the design. Experimental results are presented to verify the system functionalities. © 2014 Elsevier Ltd. All rights reserved. Source


Langhamer O.,UU | Langhamer O.,Swedish Center for Renewable Electrical Energy Conversion | Haikonen K.,Swedish Center for Renewable Electrical Energy Conversion | Sundberg J.,Swedish Center for Renewable Electrical Energy Conversion
Renewable and Sustainable Energy Reviews | Year: 2010

Generating electricity from waves is predicted to be a new source of renewable energy conversion expanding significantly, with a global potential in the range of wind and hydropower. Several wave power techniques are on the merge of commercialisation, and thus evoke questions of environmental concern. Conservation matters are to some extent valid independent of technique but we mainly focus on point absorbing linear generators. By giving examples from the Lysekil project, run by Uppsala University and situated on the Swedish west coast, we demonstrate ongoing and future environmental studies to be performed along with technical research and development. We describe general environmental aspects generated by wave power projects; issues also likely to appear in Environmental Impact Assessment studies. Colonisation patterns and biofouling are discussed with particular reference to changes of the seabed and alterations due to new substrates. A purposeful artificial reef design to specially cater for economically important or threatened species is also discussed. Questions related to fish, fishery and marine mammals are other examples of topics where, e.g. no-take zones, marine bioacoustics and electromagnetic fields are important areas. In this review we point out areas in which studies likely will be needed, as ventures out in the oceans also will give ample opportunities for marine environmental research in general and in areas not previously studied. Marine environmental and ecological aspects appear to be unavoidable for application processes and in post-deployment studies concerning renewable energy extraction. Still, all large-scale renewable energy conversion will cause some impact mainly by being area demanding. An early incorporation of multidisciplinary and high quality research might be a key for new ocean-based techniques. © 2009 Elsevier Ltd. All rights reserved. Source


Kurupath V.,Swedish Center for Renewable Electrical Energy Conversion | Ekstrom R.,Swedish Center for Renewable Electrical Energy Conversion | Leijon M.,Swedish Center for Renewable Electrical Energy Conversion
Energies | Year: 2013

This article proposes a simple and reliable damping strategy for wave power farm operation of small-scale point-absorber converters. The strategy is based on passive rectification onto a constant DC-link, making it very suitable for grid integration of the farm. A complete model of the system has been developed in Matlab Simulink, and uses real site data as input. The optimal constant DC-voltage is evaluated as a function of the significant wave height and energy period of the waves. The total energy output of the WEC is derived for one year of experimental site data. The energy output is compared for two cases, one where the optimal DC-voltage is determined and held constant at half-hour basis throughout the year, and one where a selected value of the DC-voltage is kept constant throughout the year regardless of sea state. Source


Rahm M.,Swedish Center for Renewable Electrical Energy Conversion | Svensson O.,Swedish Center for Renewable Electrical Energy Conversion | Bostrom C.,Swedish Center for Renewable Electrical Energy Conversion | Waters R.,Swedish Center for Renewable Electrical Energy Conversion | Leijon M.,Swedish Center for Renewable Electrical Energy Conversion
IET Renewable Power Generation | Year: 2012

Wave energy comes in pulses and is unsuitable for direct conversion and transmission to the grid. One method to smooth the power is to deploy arrays of wave energy converters (WECs), the geometrical layout and damping optimisation of which many have studied analytically and numerically, but very few by experiments at sea. In this study, the standard deviation of electrical power as function of various parameters is investigated. Two offshore experiments have been conducted. During the longer run, three WECs were operated in linear damping during 19.7 days. It is shown that the standard deviation reduces with the number of WECs in the array up to three WECs. The reduction compared to single WEC operation was found here to be 30 and 80 with two and three WECs, respectively, as a mean for an arbitrary array member. It is found that in sea states above ∼2kW/m, the standard deviation is independent of sea state parameters. This is contradictory to a previous study on the same device. The results are, however, in accordance with numerical results of the SEAREV device but show larger reduction in standard deviation with number of WECs. This could be because of suboptimal damping conditions. © 2012 The Institution of Engineering and Technology. Source

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