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Publications (10 of 96) Show all publications
Rossander, M., Fjellstedt, C. & Bernhoff, H. (2018). Multiple Vertical Axis Wind Turbines with Passive Rectification to a Common DC-link. Renewable energy, 127, 1101-1110
Open this publication in new window or tab >>Multiple Vertical Axis Wind Turbines with Passive Rectification to a Common DC-link
2018 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 127, p. 1101-1110Article in journal (Refereed) Published
Abstract [en]

Wind turbines are commonly placed in wind farms, usually operating as separate units. Possible benefits could be found by allowing turbines to share a common DC-link. Diode rectifiers are a robust and cost effective way to rectify variable speed wind turbines, with loss of direct control of the generator. This paper studies the electromechanical interactions between four passively rectified vertical axis wind turbines connected to a common DC-link. Two different load approaches for the DC-link are compared using simulations in terms of performance and stability: a power source and a voltage source. The optimal torque (or optimal power) control is implemented for the two loads approaches. In addition, three-phase and dual stator winding (six-phase) generators are compared. The results show that all suggested solutions work with similar performance. However, the power load requires a large DC-link capacitance to achieve stability. More generatorphases improve the system with passive rectification in most cases. The simulations suggest that the common DC-link systems are expected to have a few percent lower energy capture due to the lack of individual turbine control. On the other hand, there is a significant reduction in peak power and a potential for smoother output power.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-331784 (URN)10.1016/j.renene.2018.05.013 (DOI)000437077300099 ()
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2018-09-20Bibliographically approved
Hedlund, M., Abrahamsson, J., Pérez-Loya, J. J., Lundin, J. & Bernhoff, H. (2017). Eddy Currents in a Passive Magnetic Axial Thrust Bearing for a Flywheel Energy Storage System. International journal of applied electromagnetics and mechanics, 54(3), 389-404
Open this publication in new window or tab >>Eddy Currents in a Passive Magnetic Axial Thrust Bearing for a Flywheel Energy Storage System
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2017 (English)In: International journal of applied electromagnetics and mechanics, ISSN 1383-5416, E-ISSN 1875-8800, Vol. 54, no 3, p. 389-404Article in journal (Refereed) Published
Abstract [en]

Two types of passive magnetic lift bearings were evaluated in terms of thrust force and eddy current losses. The first type of bearings were based on two sets of segmented Halbach arrays mounted in repulsive mode, and the second type was based on ring-magnets. The eddy-currents studied arose in the bearing due to manufacturing variations of magnetic remanence, and due to non-radial magnetization. Both a 3D time-dependent and a quasi-stationary Finite-Element Method (FEM) formulation were used, and the simulated results were compared with lift-force measurements from experiment. The losses were found (by FEM) to be in the order of 25 W at a rotational speed of 30000 rpm while lifting a 45 kg rotor with a stiffness of 359 N/mm.

Keywords
Flywheel energy storage; magnetic bearings; FEM; eddy-currents
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-317880 (URN)10.3233/JAE-16015 (DOI)000405742300008 ()
Funder
Swedish Energy Agency
Available from: 2017-03-21 Created: 2017-03-21 Last updated: 2017-10-24Bibliographically approved
Ottermo, F., Möllerström, E., Nordborg, A., Hylander, J. & Bernhoff, H. (2017). Location of aerodynamic noise sources from a 200 kW vertical-axis wind turbine. Journal of Sound and Vibration, 400, 154-166
Open this publication in new window or tab >>Location of aerodynamic noise sources from a 200 kW vertical-axis wind turbine
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2017 (English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 400, p. 154-166Article in journal (Refereed) Published
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.

National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-316384 (URN)10.1016/j.jsv.2017.03.033 (DOI)000402355100011 ()
Funder
Stiftelsen Olle Engkvist Byggmästare
Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2017-08-08Bibliographically approved
Hedlund, M., Kamf, T., Santiago, J. d., Abrahamsson, J. & Bernhoff, H. (2017). Reluctance Machine for a Hollow Cylinder Flywheel. Energies, 10(3), Article ID 316.
Open this publication in new window or tab >>Reluctance Machine for a Hollow Cylinder Flywheel
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2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 3, article id 316Article in journal (Refereed) Published
Abstract [en]

A hollow cylinder flywheel rotor with a novel outer rotor switched reluctance machine (SRM) mounted on the interior rim is presented, with measurements, numerical analysis and analytical models. Practical experiences from the construction process are also discussed. The flywheel rotor does not have a shaft and spokes and is predicted to store 181 Wh/kg at ultimate tensile strength (UTS) according to simulations. The novel SRM is an axial flux machine, chosen due to its robustness and tolerance for high strain. The computed maximum tip speed of the motor at UTS is 1050 m/s . A small-scale proof-of-concept electric machine prototype has been constructed, and the machine inductance has been estimated from measurements of voltage and current and compared against results from analytical models and finite element analysis (FEA). The prototype measurements were used to simulate operation during maximal speed for a comparison towards other high-speed electric machines, in terms of tip speed and power. The mechanical design of the flywheel was performed with an analytical formulation assuming planar stress in concentric shells of orthotropic (unidirectionally circumferentially wound) carbon composites. The analytical approach was verified with 3D FEA in terms of stress and strain.

Keywords
flywheel energy storage, hollow cylinder flywheel, reluctance machine, high-speed machines
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-317875 (URN)10.3390/en10030316 (DOI)000398736700056 ()
Available from: 2017-03-21 Created: 2017-03-21 Last updated: 2017-11-29Bibliographically approved
Möllerström, E., Fredric, O., Hylander, J. & Bernhoff, H. (2016). Noise Emission of a 200 kW Vertical Axis Wind Turbine. Energies, 9(1), Article ID 19.
Open this publication in new window or tab >>Noise Emission of a 200 kW Vertical Axis Wind Turbine
2016 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 9, no 1, article id 19Article in journal (Refereed) Published
Abstract [en]

The noise emission from a vertical axis wind turbine (VAWT) has been investigated. A noise measurement campaign on a 200 kW straight-bladed VAWT has been conducted, and the result has been 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, to 96.2 dBA. At this wind speed, the turbine was stalling as it was run at a tip speed lower than optimal due to constructional constraints. The noise emission at a wind speed of 6 m/s, 10 m above ground was measured while operating at optimum tip speed and was found to be 94.1 dBA. A comparison with similar size horizontal axis wind turbines (HAWTs) indicates a noise emission at the absolute bottom of the range. Furthermore, it is clear from the analysis that the turbulent-boundary-layer trailing-edge noise, as modeled here, is much lower than the measured levels, which suggests that other mechanisms are likely to be important, such as inflow turbulence.

Keywords
vertical axis wind turbine (VAWT); H-rotor; noise; noise emission; sound power level
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-242263 (URN)10.3390/en9010019 (DOI)000369501500013 ()
Funder
StandUp
Available from: 2015-01-19 Created: 2015-01-22 Last updated: 2017-12-05Bibliographically approved
Möllerström, E., Ottermo, F., Goude, A., Eriksson, S. S., Hylander, J. & Bernhoff, H. (2016). Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine. Wind Energy, 19(11), 1963-1973
Open this publication in new window or tab >>Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine
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2016 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 19, no 11, p. 1963-1973Article in journal (Refereed) Published
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 fora nominal power of 200 kW, operated during this period mostly in a restricted mode due to mechanical concerns, reachingpower levels up to about 80 kW. Two different approaches are used for presenting results, one that can be compared topower curves consistent with the International Electrotechnical Commission (IEC) standard and one that allows isolatingthe effect of turbulence from the cubic variation of power with wind speed. Accounting for this effect, the turbine stillshows slightly higher efficiency at higher turbulence, proposing that the H-rotor is well suited for wind sites with turbulentwinds. The operational data are also used to create a Cp(λ) curve, showing slightly lower Cp compared with a curvesimulated by a double multiple streamtube model.

Keywords
VAWT; H-rotor; turbulence intensity; power coefficient curve
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-277352 (URN)10.1002/we.1962 (DOI)000386149700001 ()
Funder
StandUpStandUp for Wind
Available from: 2016-02-19 Created: 2016-02-19 Last updated: 2017-11-28
de Santiago, J. & Bernhoff, H. (2015). Calculation of Tooth Ripple Losses in Solid Poles. Electric power components and systems, 43(3), 245-251
Open this publication in new window or tab >>Calculation of Tooth Ripple Losses in Solid Poles
2015 (English)In: Electric power components and systems, ISSN 1532-5008, E-ISSN 1532-5016, Vol. 43, no 3, p. 245-251Article in journal (Refereed) Published
Abstract [en]

Tooth ripple losses in solid salient poles are calculated with analytical and semi-empirical methods. A numerical method based on the finite element method is presented in this article. The distribution of the eddy currents induced by the tooth ripple is obtained with this new method. The traditional analytic approach is based on some assumptions on the eddy current losses distribution that are finally verified with the Finite Element Method simulations presented. Analytic solutions of tooth ripple losses are only applicable to distributed windings with a homogeneous slot pitch while the method presented is applicable both to distributed and concentrated windings.

Keywords
electric machines, solid-rotor, magnetic losses, finite element method, synchronous machines, eddy currents
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-242369 (URN)10.1080/15325008.2014.981898 (DOI)000347289500001 ()
Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved
Rossander, M., Dyachuk, E., Apelfröjd, S., Trolin, K., Goude, A., Bernhoff, H. & Eriksson, S. (2015). Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells. Energies, 8(6), 5973-5996
Open this publication in new window or tab >>Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells
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2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 6, p. 5973-5996Article in journal (Refereed) Published
Abstract [en]

Unique blade force measurements on an open site straight-bladed vertical axis wind turbine have been performed. This paper presents a method for measuring the tangential and normal forces on a 12-kW vertical axis wind turbine prototype with a three-bladed H-rotor. Four single-axis load cells were installed in-between the hub and the support arms on one of the blades. The experimental setup, the measurement principle, together with the necessary control and measurement system are described. The maximum errors of the forces and accompanying weather data that can be obtained with the system are carefully estimated. Measured forces from the four load cells are presented, as well as the normal and tangential forces derived from them and a comparison with theoretical data. The measured torque and bending moment are also provided. The influence of the load cells on the turbine dynamics has also been evaluated. For the aerodynamic normal force, the system provides periodic data in agreement with simulations. Unexpected mechanical oscillations are present in the tangential force, introduced by the turbine dynamics. The measurement errors are of an acceptable size and often depend on the measured variable. Equations are presented for the calculation of measurement errors.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260148 (URN)10.3390/en8065973 (DOI)000357489700067 ()
Funder
StandUpStandUp for Wind
Available from: 2015-08-17 Created: 2015-08-17 Last updated: 2017-12-04
Hedlund, M., Lundin, J., de Santiago, J., Abrahamsson, J. & Bernhoff, H. (2015). Flywheel Energy Storage for Automotive Applications. Energies, 8(10), 10636-10663
Open this publication in new window or tab >>Flywheel Energy Storage for Automotive Applications
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2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 10, p. 10636-10663Article, review/survey (Refereed) Published
Abstract [en]

A review of flywheel energy storage technology was made, with a special focus on the progress in automotive applications. We found that there are at least 26 university research groups and 27 companies contributing to flywheel technology development. Flywheels are seen to excel in high-power applications, placing them closer in functionality to supercapacitors than to batteries. Examples of flywheels optimized for vehicular applications were found with a specific power of 5.5 kW/kg and a specific energy of 3.5 Wh/kg. Another flywheel system had 3.15 kW/kg and 6.4 Wh/kg, which can be compared to a state-of-the-art supercapacitor vehicular system with 1.7 kW/kg and 2.3 Wh/kg, respectively. Flywheel energy storage is reaching maturity, with 500 flywheel power buffer systems being deployed for London buses (resulting in fuel savings of over 20%), 400 flywheels in operation for grid frequency regulation and many hundreds more installed for uninterruptible power supply (UPS) applications. The industry estimates the mass-production cost of a specific consumer-car flywheel system to be 2000 USD. For regular cars, this system has been shown to save 35% fuel in the U.S. Federal Test Procedure (FTP) drive cycle.

Keywords
flywheel; kinetic energy storage; energy storage
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-268572 (URN)10.3390/en81010636 (DOI)000364230500004 ()
Funder
StandUp
Available from: 2015-12-08 Created: 2015-12-08 Last updated: 2017-12-01Bibliographically approved
Dyachuk, E., Rossander, M., Goude, A. & Bernhoff, H. (2015). Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine. Energies, 8(8)
Open this publication in new window or tab >>Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 8Article in journal (Refereed) Published
Abstract [en]

The knowledge of unsteady forces is necessary when designing vertical axis wind turbines (VAWTs). Measurement data for turbines operating at an open site are still very limited. The data obtained from wind tunnels or towing tanks can be used, but have limited applicability when designing large-scale VAWTs. This study presents experimental data on the normal forces of a 12-kW straight-bladed VAWT operated at an open site north of Uppsala, Sweden. The normal forces are measured with four single-axis load cells. The data are obtained for a wide range of tip speed ratios: from 1.7 to 4.6. The behavior of the normal forces is analyzed. The presented data can be used in validations of aerodynamic models and the mechanical design for VAWTs.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260569 (URN)10.3390/en8088482 (DOI)000360586600052 ()
Available from: 2015-08-20 Created: 2015-08-20 Last updated: 2017-12-04Bibliographically approved
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