| 1. |
- Möllerström, Erik, 1984-, et al.
(författare)
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Avoidance of resonances in a semi-guy-wired vertical axis wind turbine
- 2014
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Ingår i: Proceedings of EWEA 2014 - Annual Event (Conference), Barcelona, 10-13 March 2014.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Möllerström, Erik, et al.
(författare)
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Eigen Frequencies of A Vertical Axis Wind Turbine Tower Made of Laminated Wood and the Effect Upon Attaching Guy Wires
- 2014
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Ingår i: Wind Engineering. - Brentwood, United Kingdom : Multi-Science Publishing. - 0309-524X .- 2048-402X. ; 38:3, s. 277-290
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Möllerström, Erik
(författare)
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Noise, eigenfrequencies and turbulence behavior of a 200 kW H-rotor vertical axis wind turbine
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 4. |
- Möllerström, Erik, 1984-, et al.
(författare)
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Noise Emission of a 200 kW Vertical Axis Wind Turbine
- 2016
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Ingår i: Energies. - Basel : MDPI. - 1996-1073. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Möllerström, Erik, 1984-, et al.
(författare)
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Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine
- 2016
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Ingår i: Wind Energy. - Chichester : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 19:11, s. 1963-1973
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Möllerström, Erik
(författare)
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Vertical Axis Wind Turbines : Tower Dynamics and Noise
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Ottermo, Fredric, et al.
(författare)
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An upper size of vertical axis wind turbines
- 2014
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Ingår i: Wind Energy. - Chichester : Wiley. - 1095-4244 .- 1099-1824. ; 17:10, s. 1623-1629
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Tidskriftsartikel (refereegranskat)abstract
- 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 200 kW 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 30 MW.
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| 8. |
- Ottermo, Fredric, 1975-, et al.
(författare)
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Location of aerodynamic noise sources from a 200 kW vertical-axis wind turbine
- 2017
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Ingår i: Journal of Sound and Vibration. - London : Elsevier. - 0022-460X .- 1095-8568. ; 400, s. 154-166
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 9. |
- Ottermo, Fredric, et al.
(författare)
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Parking Strategies for Vertical Axis Wind Turbines
- 2012
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Ingår i: ISRN Renewable Energy. - New York : Hindawi Limited. - 2090-746X. ; 2012, s. 904269-
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Tidskriftsartikel (refereegranskat)abstract
- Strategies for parking a vertical axis wind turbine at storm load are considered. It is proposed that if a directly driven permanent magnet synchronous generator is used, an elegant choice is to short-circuit the generator at storm,since this makes the turbine efficiently damped. Nondamped braking is found to be especially problematic for the case of two blades where torsional oscillations may imply thrust force oscillations within a range of frequencies.
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| 10. |
- Ottermo, Fredric, 1975-, et al.
(författare)
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Resonances and Aerodynamic Damping of a Vertical Axis Wind Turbine
- 2012
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Ingår i: Wind Engineering. - Essex, UK : Multi-Science Publishing. - 0309-524X .- 2048-402X. ; 3:36, s. 297-304
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of a straight-bladed vertical axis wind turbine is investigated with respect to oscillations due to the elasticity of struts and shaft connecting to the hub. In particular, for the three-bladed turbine, a concept is proposed for dimensioning the turbine to maximize the size of the resonance free rpm range for operation. The effect of aerodynamic damping on the struts is also considered. The damping of these types of oscillations for a typical turbine is found to be good.
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