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  • 1.
    Möllerström, Erik
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS). Uppsala University, Uppsala, Sweden.
    Noise, eigenfrequencies and turbulence behavior of a 200 kW H-rotor vertical axis wind turbine2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Vertical-axis wind turbines (VAWTs) have with time been outrivaled by the today more common and economically feasible horizontal-axis wind turbines (HAWTs). However, VAWTs have several advantages which still make them interesting, for example, the VAWTs can have the drive train at ground level and it has been argued that they have lower noise emission. Other proposed advantages are suitability for both up-scaling and floating offshore platforms.

    The work within this thesis is made in collaboration between Halmstad University and Uppsala University. A 200-kW semi-guy-wired VAWT H-rotor, owned by Uppsala University but situated in Falkenberg close to Halmstad, has been the main subject of the research although most results can be generalized to suit a typical H-rotor.

    This thesis has three main topics regarding VAWTs: (1) how the wind energy extraction is influenced by turbulence, (2) aerodynamical noise generation and (3) eigenfrequencies of the semi-guy-wired tower.

    The influence from turbulence on the wind energy extraction is studied by evaluating logged operational data and examining how the power curve and the tip-speed ratio for maximum Cp is impacted by turbulence. The work has showed that the T1-turbine has a good ability to extract wind energy at turbulent conditions, indicating an advantage in energy extraction at turbulent sites for VAWTs compared to HAWTs.The noise characteristics are studied experimentally, and models of the two most likely aerodynamic noise mechanisms are applied. Here, inflow-turbulence noise is deemed as the prevailing noise source rather than turbulent-boundary-layer trailing-edge noise (TBL-TE) which is the most important noise mechanism for HAWTs. The overall noise emission has also been measured and proven low compared to similar sized HAWTs.

    The eigenfrequencies of a semi-guy-wired tower are also studied. Analytical expressions describing the first-mode eigenfrequency of both tower and guy wire has been derived and verified by experiments and simulations.

  • 2.
    Möllerström, Erik
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Vertical Axis Wind Turbines: Tower Dynamics and Noise2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Vertical axis wind turbines (VAWTs) have with time been outrivaled by the today common and economically feasible horizontal axis wind turbines (HAWTs). However, VAWTs have several advantages such as the possibility to put the drive train at ground level, lower noise emissions and better scaling behavior which still make them interesting for research.

    The work within this thesis is made in collaboration between the Department of Construction and Energy Engineering at Halmstad University and the Division for Electricity at Uppsala University. A 200 kW VAWT owned by the latter and situated close to Falkenberg in the southwest of Sweden has been the main subject of the research even if most learnings has been generalized to fit a typical vertical turbine. This particular turbine has a wooden tower which is semi-guy-wired, i.e. the tower is both firmly attached to the ground and supported by guy-wires.

    This thesis has two main topics both regarding VAWTs: eigenfrequency of the tower and the noise generated from the turbine. The eigenfrequency of a semi-guy-wired tower is studied and an analytical expression describing this is produced and verified by experiments and simulations. The eigenfrequency of the wire itself and how it is affected by wind load are also studied.  The noise characteristics of VAWTs have been investigated, both theoretically and by noise measurement campaigns. Both noise emission and frequency distribution of VAWTs has been studied.

    The work has resulted in analytical expressions for tower and wire eigenfrequency of a semi-guy-wired tower as well as recommendations for designing future towers for VAWTs. The noise emission of VAWTs has been studied and proven low compared to HAWTs. The noise frequency distribution of the 200 kW VAWT differs significantly from that of a similar size HAWTs with for example lower levels for frequencies below 3000 Hz.

  • 3.
    Möllerström, Erik
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Whatever became of the vertical axis wind turbine?2019In: Modern power systems, ISSN 0260-7840, Vol. 39, no 4Article in journal (Other (popular science, discussion, etc.))
    Abstract [en]

    Alongside the story of today’s commercially successful, propeller-type, horizontal axis wind turbine (HAWT), there is the lesser known story of the vertical axis wind turbine (VAWT). Once seen as a competitor in setting the standard for wind turbine design, utility-scale VAWTs have become a more and more unusual sight. However, utility-scale VAWT prototypes do from time to time still appear as attempts are made to establish vertical axis technology in a market totally dominated by horizontal axis machines. This is a retrospective survey of utility-scale VAWT projects, with turbines of 100 kW or more.

  • 4.
    Möllerström, Erik
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Wind Turbines from the Swedish Wind Energy Program and the Subsequent Commercialization Attempts – A Historical Review2019In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 4, article id 690Article, review/survey (Refereed)
    Abstract [en]

    This paper summarizes wind turbines of Swedish origin, 50 kW and above. Both the large governmental-funded prototypes from the early 1980s and following attempts to build commercial turbines are covered. After the 1973 oil crisis, a development program for wind turbine technology was initiated in Sweden, culminating in the early 1980s with the 2 and 3-MW machines at Maglarp and Näsudden. However, government interest declined, and Sweden soon lost its position as one of the leading countries regarding wind turbine development. Nevertheless, several attempts to build commercial wind turbines in Sweden were made in the following decades. Most attempts have, like the earlier prototypes, used a two-bladed rotor, which has become synonymous with the Swedish wind turbine development line. The current ongoing Swedish endeavors primarily focus on the niche-concept of vertical axis wind turbines (VAWTs), which is a demonstration of how far from the broad commercial market of Sweden has moved. Thus far, none of the Swedish attempts have been commercially successful, and unlike countries like Denmark or Germany, Sweden currently has no large wind turbine producer. Suggested reasons include early government interventions focusing on two-bladed prototypes and political disinterest, with wind power grants cut in half by 1985, and the domestic industry not being favored in government policies for deploying wind power.

  • 5.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Division for Electricity, Department of Engineering Sciences, Uppsala University, Uppsala, Sweden.
    Bolin, Karl
    Marcus Wallenberg Laboratory, Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bååth, L.B.
    Halmstad University, School of Business, Engineering and Science, Mechanical Engineering and Industrial Design (MTEK).
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Noise directivity from a vertical axis wind turbine2015In: Proceedings of the 6th International Conference on Wind Turbine Noise 2015, New Brighton: INCE/Europe , 2015Conference paper (Other academic)
    Abstract [en]

    The directivity of wind turbine noise is of widespread interest and has received attention over an extended period. The reason for this is partly that noise optimization procedures can be implemented in order to increase the efficiency of the power output. Although directivity of horizontal axis wind turbines (HAWTs) are well understood, the directivity pattern around vertical axis wind turbines (VAWTs) are less well investigated.This paper presents measurements of directivity on a 200 kW VAWT, a so called H-rotor, with variable speed and a tower height of 40 m. The turbine, which is situated in a flat agricultural landscape close to Falkenberg at the Swedish west coast, is one of the world’s largest today operational VAWTs. Experimental results as well as a theoretical model based on the turbulent-boundary-layer trailing-edge (TBL-TE) noise suggest higher noise levels upwind of the rotor and the lowest noise levels in the perpendicular directions. Moreover, modulation analysis of the measurements indicates the presence of modulations but the results are inconclusive.

  • 6.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Uppsala University, Uppsala, Sweden.
    Eriksson, Sandra
    Uppsala University, Uppsala, Sweden.
    Goude, Anders
    Uppsala University, Uppsala, Sweden.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Turbulence influence on optimum tip speed ratio for a 200 kW vertical axis wind turbine2016In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 753, article id 032048Article in journal (Refereed)
    Abstract [en]

    The influence of turbulence intensity (TI) on the tip speed ratio for maximum power coefficient, here called λCp-max, is studied for a 200 kW VAWT H-rotor using logged data from a 14 month period with the H-rotor operating in wind speeds up to 9 m/s. The TI - λCp-max relation is examined by dividing 10 min mean values in different turbulence intensity ranges and producing multiple CP(λ) curves. A clear positive relation between TI and λCp-max is shown and is further strengthened as possible secondary effects are examined and deemed non-essential. The established relation makes it possible to tune the control strategy to enhance the total efficiency of the turbine.

  • 7.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Gipe, Paul
    Wind-works.org, Bakersfield, CA, USA.
    Beurskens, Jos
    SET Analysis, Schagen, the Netherlands.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    A historical review of vertical axis wind turbines rated 100 kW and above2019In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 105, p. 1-13Article, review/survey (Refereed)
    Abstract [en]

    This paper summarizes and introduces all vertical axis wind turbine (VAWT) projects where 100 kW or larger turbines have been installed. The basis for the review is both existing literature and personal correspondence with people once involved in the different developments. By citing the most relevant work for each project, the paper will also work as an information hub, making information on these projects more accessible.

    Since the 1970s, there have been several VAWT projects with installed turbines of significant size, either as attempts to commercialize VAWTs, or as university led research projects, or as a combination of the two. Most have involved Darrieus turbines built in North America during the 1980s. However, H-rotors, which have always been a favored concept in Europe, have seen a revival during the 2010s.

    The reason VAWTs have never fully challenged the success of the horizontal axis wind turbine (HAWT) is too broad a question to answer here. However, the reasons some VAWT projects have failed are addressed in this paper. Besides the fact that many of the prototypes had terminal failures, most of the installed medium or large-scale VAWTs have to some extent had problems with metal fatigue and durability. Additionally, a lack of long-term interest from governmental or private funders, as well as the introduction of reliable HAWTs, was a recurring theme from those involved in VAWT development, regarding the reason VAWTs so far have failed to succeed. © 2018 The Author(s)

  • 8.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Uppsala University, Uppsala, Sweden.
    Larsson, Sebastian
    Halmstad University, School of Business and Engineering (SET).
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bååth, Lars
    Halmstad University, School of Business, Engineering and Science, Mechanical Engineering and Industrial Design (MTEK).
    Noise Propagation from a Vertical Axis Wind Turbine2014In: Proceedings of 43rd International Congress on Noise Control Engineering: Internoise 2014 / [ed] John Davy, Charles Don, Terry McMinn, Liz Dowsett, Norm Broner & Marion Burgess, Brisbane, QLD: Australian Acoustical Society , 2014Conference paper (Refereed)
    Abstract [en]

    Initial noise measurements were performed on a 200kW vertical axis wind turbine (VAWT) and results were compared to that of a Vestas V27, a similar size horizontal axis wind turbine (HAWT). Multiple recording units were placed in line downwind of the turbine to investigate noise propagation. The frequency distribution of the noise were analyzed indicating that the VAWT has lower relative levels for frequencies under 3000 Hz, especially within 600-1200 Hz. Furthermore, VAWT noise seems to occur more around the same frequencies as the natural background noise, increasing masking probability. Results from propagation measurements seemed to indicate that noise declines more rapidly with distance for the VAWT then for the reference HAWT, possibly explained by the lower levels at low frequencies. Further investigation is needed to establish these differences and the 200 kW VAWT creates an opportunity doing so utilizing arguably the largest operational VAWT existing today.

  • 9.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Comparison of inflow-turbulence and trailing-edge noise models with measurements of a 200-kW vertical axis wind turbine2019In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1222, article id 012028Article in journal (Refereed)
    Abstract [en]

    Models of inflow-turbulence noise and turbulent-boundary-layer trailing-edge noise are compared to earlier measurements of a 200-kW vertical axis wind turbine so that conclusions regarding the origin of the aerodynamic noise can be drawn. The measurement campaigns, which aimed at establishing the noise emission value and locating the aerodynamic noise sources with a microphone array, are here both compared to further modified versions of the trailing-edge and inflow-turbulence models respectively. Unlike the case for horizontal axis wind turbine, inflow-turbulence noise is deemed as the prevailing noise mechanism. Reducing the self-induced turbulence could then be an effective way of lowering the noise levels for vertical axis wind turbines. Also, looking at the directivity of the inflow-turbulence noise model which indicate most noise in the cross-wind directions, a deviation from the standard downwind measurement position for measuring noise emission is suggested for the VAWT case.

  • 10.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Uppsala University, Uppsala, Sweden.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Goude, Anders
    Uppsala University, Uppsala, Sweden.
    Eriksson, Sandra
    Uppsala University, Uppsala, Sweden.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bernhoff, Hans
    Uppsala University, Uppsala, Sweden.
    Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine2016In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 19, no 11, p. 1963-1973Article in journal (Refereed)
    Abstract [en]

    The relation between power performance and turbulence intensity for a VAWT H-rotor is studied using logged data from a 14 month (discontinuous) period with the H-rotor operating in wind speeds up to 9 m/s. The turbine, designed originally for a nominal power of 200 kW, operated during this period mostly in a restricted mode due to mechanical concerns, reaching power levels up to about 80 kW. Two different approaches are used for presenting results, one that can be compared to power curves consistent with the International Electrotechnical Commission (IEC) standard and one that allows isolating the effect of turbulence from the cubic variation of power with wind speed. Accounting for this effect, the turbine still shows slightly higher efficiency at higher turbulence, proposing that the H-rotor is well suited for wind sites with turbulent winds. The operational data are also used to create a Cp(λ) curve, showing slightly lower Cp compared with a curve simulated by a double multiple streamtube model. Copyright © 2016 John Wiley & Sons, Ltd.

  • 11.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Division for Electricity, Department of Engineering Sciences, Uppsala University, PO Box 534, SE - 751 21 Uppsala, SwedenDivision for Electricity, Department of Engineering Sciences, Uppsala University, Uppsala, Sweden.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bernhoff, Hans
    Division for Electricity, Department of Engineering Sciences, Uppsala University, PO Box 534, SE - 751 21 Uppsala, SwedenDivision for Electricity, Department of Engineering Sciences, Uppsala University, Uppsala, Sweden.
    Avoidance of resonances in a semi-guy-wired vertical axis wind turbine2014Conference paper (Refereed)
    Abstract [en]

    Resonance analysis for a vertical axis wind turbine is performed. The turbine may be described as semi-guy wired, being bolted stiffly to the ground as well as supported by guy wires. The influence of the first mode eigen frequency of the guy wires and how it is affected by wind load is examined. Using beam theory, an analytical model for calculating the first mode eigen frequency of the guy wire for different wind loads is derived. The analytical model is verified with FEM-simulations and then used to assemble a diagram showing how to combine the wire size, inclination angle and pre-tension for an eigen frequency range over the 3P load for nominal rotational speed and for a certain effective spring force acting on the tower. This diagram, here called an EA-T diagram, may be used as a quick tool for comparing wire setups and a similar diagram can be used for other guy wired structures.

  • 12.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bernhoff, Hans
    Uppsala Universitet.
    Eigen Frequencies of A Vertical Axis Wind Turbine Tower Made of Laminated Wood and the Effect Upon Attaching Guy Wires2014In: Wind Engineering: The International Journal of Wind Power, ISSN 0309-524X, E-ISSN 2048-402X, Vol. 38, no 3, p. 277-290Article in journal (Refereed)
    Abstract [en]

    Eigen frequencies of a vertical axis wind turbine tower made out of laminated wood which are both bolted to the ground and supported by guy wires are studied and compared. Using beam theory, an analytical model taking the guy wires into account for calculating the first mode eigen frequency of the tower has been derived. The analytical model is then evaluated by comparing with FEM-simulations and measurements performed on the actual tower. The model is found to be reasonably accurate keeping in mind that the estimated masses and second moments of area are somewhat rough. Furthermore the model can be used to give an indication of the magnitude of change in eigen frequency when modifying a tower or guy wire property.

  • 13.
    Möllerström, Erik
    et al.
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik. Uppsala University, Uppsala, Sweden.
    Ottermo, Fredric
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, Biological and Environmental Systems (BLESS), Energiteknik.
    Bernhoff, Hans
    Uppsala University, Uppsala, Sweden.
    Noise Emission of a 200 kW Vertical Axis Wind Turbine2016In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 9, no 1, article id 19Article in journal (Refereed)
    Abstract [en]

    The noise emission from a vertical axis wind turbine (VAWT) has been investigated. A noisemeasurement campaign on a 200 kW straight-bladed VAWT has been conducted, and the result hasbeen compared to a semi-empirical model for turbulent-boundary-layer trailing edge (TBL-TE) noise.The noise emission from the wind turbine was measured, at wind speed 8 m/s, 10 m above ground, to96.2 dBA. At this wind speed, the turbine was stalling as it was run at a tip speed lower than optimaldue to constructional constraints. The noise emission at a wind speed of 6 m/s, 10 m above groundwas measured while operating at optimum tip speed and was found to be 94.1 dBA. A comparisonwith similar size horizontal axis wind turbines (HAWTs) indicates a noise emission at the absolutebottom of the range. Furthermore, it is clear from the analysis that the turbulent-boundary-layertrailing-edge noise, as modeled here, is much lower than the measured levels, which suggests thatother mechanisms are likely to be important, such as inflow turbulence.

  • 14.
    Ottermo, Fredric
    et al.
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Möllerström, Erik
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS). Uppsala University, Uppsala, Sweden.
    Nordborg, Anders
    Sound View Instruments, Borrby, Sweden.
    Hylander, Jonny
    Halmstad University, School of Business, Engineering and Science, The Rydberg Laboratory for Applied Sciences (RLAS).
    Bernhoff, Hans
    Uppsala University, Uppsala, Sweden.
    Location of aerodynamic noise sources from a 200 kW vertical-axis wind turbine2017In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 400, p. 154-166Article in journal (Refereed)
    Abstract [en]

    Noise levels emitted from a 200 kW H-rotor vertical-axis wind turbine have been measured using a microphone array at four different positions, each at a hub-height distance from the tower. The microphone array, comprising 48 microphones in a spiral pattern, allows for directional mapping of the noise sources in the range of 500 Hz to 4 kHz. The produced images indicate that most of the noise is generated in a narrow azimuth-angle range, compatible with the location where increased turbulence is known to be present in the flow, as a result of the previous passage of a blade and its support arms. It is also shown that a semi-empirical model for inflow-turbulence noise seems to produce noise levels of the correct order of magnitude, based on the amount of turbulence that could be expected from power extraction considerations.

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