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Design of a unique direct driven PM generator adapted for a telecom tower wind turbine
Renewable Energy 44 (2012) 453e456
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical note
Design of a unique direct driven PM generator adapted for a telecom tower wind turbine S. Eriksson*, H. Bernhoff, M. Bergkvist Swedish Centre for Renewable Electric Energy Conversion, Division for Electricity, Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2011 Accepted 20 January 2012 Available online 14 February 2012
A vertical axis wind turbine has been designed to electrify a novel kind of telecommunication tower. This paper presents the design of a generator for this purpose. The generator is a permanent magnet generator rated at 10 kW. It has an unusually large diameter to fit on the outside of the telecommunication tower. The generator has been designed by using a two-dimensional FEM model. Simulations show that the generator has high efficiency through the whole operational interval. Furthermore, the generator has a high overload capability enabling electric control of the turbine. The generator has been built and the design shown feasible. Preliminary experimental results show that the induced voltage is lower than expected from simulations indicating insufficient modelling of three-dimensional effects, which are particularly large in a generator with these unusual dimensions. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Vertical axis Wind turbine Telecommunication Permanent magnet Generator
1. Introduction The global demand on telecommunication access even in rural areas is increasing. For developing countries the crucial electricity supply for telecom towers is usually solved with diesel generators, which gives emissions as well as demands on fuel transportation. Here, a solution with a self-standing, integrated system is presented. A novel telecom tower is electrified from wind power supplemented with some kind of energy storage, for instance batteries or hydrogen. This paper focuses on design of the generator in the wind turbine. Alternative designs of using wind turbines for power supply for telecom applications have been presented based on free-standing wind devices [1e3]. However, there are several reasons why a wind turbine integrated with the telecom tower can be an interesting power supply solution; it limits the use of land, the wind turbine reaches higher wind speeds and an already existing tower is used. In 2007, the Swedish telecom company Ericsson launched a new base station tower called the Tower Tube, for which a concrete tower substitutes the traditional lattice tower. The main advantage is its low environmental impact and the reduced need of land. The concrete tower has both antennas and radio base station in the top of the tower, which has an outer radius of 1.8 m. The idea was raised * Corresponding author. Tel.: þ46 184715823; fax: þ46 184715810. E-mail address: [email protected] (S. Eriksson). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.01.090
to have a wind turbine on the tower placed below the antennas to avoid the risk of electromagnetic interference [4]. For a thick telecom tower like this one the obvious choice of wind turbine is a three-dimensional turbine which is not dependent of the wind direction. Therefore, the choice falls on a vertical axis wind turbine [5e7]. There are apparent reasons for the suitability of this configuration over a traditional horizontal axis wind turbine, mainly to avoid yawing and the unbalanced loading of the tower for a two-dimensional turbine. Furthermore, for this thick tower, less blade area will risk being shadowed by the tower when the blades are placed on support arms. The turbine has fixed blades, eliminating a pitching system. The turbine is controlled electrically through the strong generator, which must have a high overload capacity. Generators for wind turbines with full variable speed require a high efficiency for the whole operational regime [8e10]. Furthermore, a directly driven generator with a fixed blade turbine must have high generator torque capability for the whole operational regime, in order to safely stop the turbine in all wind speeds. Additionally, the generator design is focused on simplicity and on having an almost maintenance free design. This paper presents the design and simulations of the unique wind power generator adapted for the Tower Tube. A preliminary measurement of no load voltage is also discussed. Turbine design and aerodynamic effects from the large tower wake, although important research topics, are not included in this paper.
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S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
2. Method
Table 1 Wind turbine and tower characteristics.
2.1. Simulations Electromagnetic simulations using the finite element method (FEM) are performed to design the generator and to simulate the generator’s behaviour at different loading conditions. In the simulations the electromagnetic field inside the generator is assumed to be axi-symmetrical and is therefore modelled in two dimensions. Three-dimensional effects such as end region fields are taken into account by introducing coil end impedances in the circuit equations of the windings. The permanent magnets are modelled by surface current sources. The electromagnetic model is described by a combined field and circuit equation model. The field equation, Eq. (1), originates from Maxwell’s equations.
s
vAz 1 vV ¼ s VAz V, m0 mr vt vz
(1)
where s is the conductivity, m0 and mr are vacuum and relative permeability respectively, Az is the axial magnetic potential and vV=vz is the applied potential. The circuit equations, Eq. (2) are described by
Ia þ Ib þ Ic ¼ 0 vIa vIb Ub Rs Ib Lend s vt vt vIc end vIb end ¼ Uc þ Rs Ic þ Ls Ub Rs Ib Ls vt vt
Uab ¼ Ua þ Rs Ia þ Lend s Ucb
(2)
where Ia, Ib, Ic are the conductor currents, Uab and Ucb are the terminal line voltages, Ua, Ub, Uc are the terminal phase voltages obtained from solving the field equation, Rs is the cable resistance describe the coil end inductance. and Lend s The generator parts are, after the generator geometry is set, assigned different material properties such as conductivity, permeability, density, sheet thickness etc. The electromagnetic model is solved in the finite element environment ACE [11]. The mesh is finer close to critical parts such as the airgap and coarser in areas like the yoke of the stator. Only one pole has to be modelled since the generator is symmetric. Simulations can be performed either in a stationary mode where the results are given for a fixed rotor position or in a dynamic mode including the time-dependence and thereby giving more accurate results. The simulations have been verified by comparison with experimental results for generators similar to the one studied here [12e14]. Stationary simulations are performed to design the generator and set the geometry. Dynamic simulations of the generator are performed on the set design. The generator is modelled to be connected to a resistive load, which gives a good estimate when studying general behaviour and to find the maximum torque. However, in reality the generator is connected to a rectifier bridge consisting of diodes. Modelling the efficiency with a resistive load is a simplification but will give rather accurate results. However, the copper losses could be expected to be slightly higher during diode rectification due to current peaks. The wind turbine will be run at its highest aerodynamic efficiency by adjusting the rotational speed to the wind speed up to a rotational speed of 89 rpm, where the rotational speed is held constant until 10 kW is reached. The aerodynamic efficiency is not assumed to decrease substantially when the rotational speed is held constant. A theoretical aerodynamic efficiency from fluid mechanical simulations is used [15]. The electric efficiency of the generator has been calculated by simulating the generator with
Rated power (kW) Rated rotational speed (rpm) Rated wind speed (m/s) Swept area (m2) Hub height (m) Tower outer diameter (m) Number of blades Blade length (m) Maximum blade speed (m/s) Aerodynamic control Blade profile
10 89 12 40 30 1.8 4 5 40 Passive stall NACA0021
a varying rotational speed and a varying load sweeping through a wide range of values to cover the whole operational interval [16]. 2.2. Design objectives The objective was to design a generator adapted to the telecom tower and the turbine, see Table 1. Furthermore, all parts of the wind turbine, except power cables, should be placed outside the tower, in order for the elevator transporting the antennas to go up and down through the tower if needed. The turbine has according to simulations a rather low aerodynamic efficiency due to the unusually large tower wake. The most important generator design objectives were:
A generator design geometrically adapted to the telecom tower. A design adapted to the turbine speed and torque. A design adapted for diode rectification. Low speed (i.e. direct drive). High overload capacity to ensure safe operation. High efficiency at varying speed and load. High reliability and low need for maintenance. Low cost.
3. Results 3.1. Generator design A directly driven, outer-rotor generator is chosen, in order not to interfere with the inside of the tower. The turbine blades are
Fig. 1. The magnetic flux density (T) in a 2D cross section of one generator pole for dynamic FEM simulations at rated speed and load.
S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
455
Table 2 Generator geometric characteristics. Inner diameter (m) Height (mm) Height to diameter ratio Airgap (mm) Cable area (mm2) No of cables per slot Steel thickness (mm) No of poles Slots per pole and phase
1.8 162 0.09 15.5 16 4 0.35 102 1
directly mounted on the generator rotor and the stator holding the cables is at stand-still on the outside of the tower. A permanent magnet rotor is chosen to avoid electrification of the rotor. A cable wound stator is chosen. The wind turbine is placed on the outside of the tower, apart from electrical cables and the electrical system on the bottom of the tower. The generator characteristics were derived by dynamic simulations using the electromagnetic model described in Section 2.1. Fig. 1 shows the generator geometry and the magnetic flux density. The stator winding consists of PVC insulated circular cables, which are easy to wind. The circular shape gives an evenly distributed electric field in the cables and hence makes better use of the insulation material [17]. Furthermore, the PVC is persistent to mechanical wear and the cables are maintenance free. The rotor of the generator is equipped with surface-mounted, rectangular, highenergy magnets made of Neodymium-Iron-Boron. The stator consists of steel laminations with a thickness of 0.35 mm to minimise iron losses. A large airgap between rotor and stator is chosen for the first prototype since the diameter is large, to reduce manufacturing tolerances. The large airgap reduces cogging effects and thereby enables the number of slots per pole and phase to be one, which simplifies the winding substantially. Additionally, the width of the magnets has been adjusted in order to minimize cogging. This generator design is unique for several reasons. It has an unusually large diameter for its low power rating and a very low height to diameter ratio. Consequently the generator has a large number of poles. Furthermore, it has an unusually high overload capacity to enable full electrical control and a low internal voltage drop of only 9 V at full loading. The maximum torque is 4.6 kNm, which is more than four times the rated torque. Geometric characteristics and results from dynamic simulations are shown in Tables 2 and 3 respectively. Fig. 2 shows the generator during manufacturing.
Fig. 2. Winding of the stator cable showing the stator mounted on a tower section.
The efficiency curve shown in Fig. 3 is achieved. The efficiency is optimized for a power of 7.7 kW, which represents a wind speed of 10.9 m/s. 3.3. Experimental results Preliminary experimental results, measuring the no load voltage at 51 rpm, showed that only approximately 84% of the from simulations expected voltage was reached, i.e. the generator gives a lower output voltage than expected. The tower tube with the installed wind turbine is shown in Fig. 4. 4. Discussion The generator design requirements have been achieved. The electrical efficiency of the generator is high throughout the whole operational regime. The maximum torque is more than four times the nominal torque which is enough to provide safe stopping of the turbine in all operational cases. The use of cables and permanent magnets minimizes the need for maintenance, which is important for this application. Preliminary measurements indicate that the voltage is about 84% of what was expected from two-dimensional FEM simulations. The difference between experimental results and simulations are
3.2. Generator efficiency simulations
95
Table 3 Generator electrical characteristics at rated speed and load. Power (kW) No load LeL voltage (V) rms Load LeL voltage (V) rms Current (A) rms Rotational speed (rpm) Electrical frequency (Hz) Torque (kNm) Cogging (%) Iron losses (kW) Copper losses (kW) Electrical efficiency (%)
10.0 219 210 27.4 89 76 1.1 2.9 0.23 0.37 94.3
Efficiency (%)
The generator has been simulated as being driven by a turbine operating at optimum tip speed ratio as described in Section 2.1.
94 93 92 91 2
4 6 Power (kW)
8
10
Fig. 3. Electrical efficiency of the generator as a function of power for a turbine operating at optimum tip speed ratio.
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S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
these unusual dimensions and the feasibility of the concept. Simulations should be improved concerning predicting threedimensional behaviour even though a two-dimensional model might still be used. Acknowledgements Dr. Arne Wolfbrandt and Doc. Urban Lundin are acknowledged for assistance with electromagnetic FEM simulations. Acknowledgements are given to Vertical Wind Communications AB, Ericsson and Swedish Energy Authority. The Swedish Energy Authority, Vinnova and Statkraft are acknowledged for contributions to the Swedish Centre for Renewable Electric Energy Conversion (CFE). References
Fig. 4. The telecom tower with the wind turbine installed.
probably caused by the simulations being performed with a twodimensional simulation method. The two-dimensional model is taking three-dimensional effects into account but probably not modelling them correctly. This generator, which has a very small height to diameter ratio, is very sensitive to correct modelling of end effects. Unfortunately the generator is currently not available for further research. However, the simulation model can still be improved. Simulations are made for an inner-rotor machine even though an outer-rotor machine was built. However, due to the large diameter the curvature per pole in the airgap is very small (see Fig. 1) so this is not expected to have an impact on the results. 5. Conclusions A novel generator with a unique geometry has been designed and successfully constructed for a wind turbine electrifying a telecom tower. Initial tests show that the generator works. However, test results differ somewhat from simulations, indicating a need for improved modelling. Even though the experimental voltage was lower than expected from simulations the first step was to demonstrate a generator of
[1] Hashimoto S, Yachi T, Tani TA. A new stand-alone hybrid power system with wind generator and photovoltaic modules for a radio base station, Telecom. Energy Conf., 2004. INTELEC 2004. 26th An. Intern. (2004) 254e259. [2] Hopkins W. Wind turbines for professional telecommunication applications: the 15 year running of a wind turbine for a microwave relay station in Southern France and subsequent developments, Telecom. Energy Conf. INTELEC ’95., 17th Intern. (1995) 741e746. [3] Kwasinski A, Krein PT. Telecom power planning for natural and man-made disasters, Telecom. Energy Conf. INTELEC 2007. 29th Intern. (2007) 216e222. [4] Bülow F, Kjellin J, Bergkvist M, Eriksson S, Bernhoff H, Ström P. Adapting a VAWT with PM generator to telecom applications. EWEC 2010 e Europ. Wind Energy Conf. & Exhib., Warsaw, Poland (2010). [5] Darrieus GJM. Turbine having its rotating shaft transverse to the flow of the current, U.S. Patent 1.835.018, December (1931). [6] Eriksson S, Bernhoff H, Leijon M. Evaluation of different turbine concepts for wind power. Renewable Sustainable Energy Rev 2008;12(5):1419e34. [7] Kjellin J, Bülow F, Eriksson S, Goude A, Deglaire P, Leijon M, et al. Power coefficient measurements on a 12 kW straight bladed vertical axis wind turbine. Renewable Energy 2011;36(11):3050e3. [8] Eriksson S. Direct Driven Generators for Vertical Axis Wind Turbines, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology vol. 547, ISBN 978-91-554-7264-1 (2008). [9] Lampola P. Directly driven, low-speed permanent-magnet generators for wind power applications, Ph.D. dissertation, Dept. Electr. Eng., Helsinki University of Technology (2000). [10] Grauers A. Design of Direct Driven Permanent Magnet Generators for Wind Turbines, Ph.D. dissertation, Dept. Electr. Power Eng., Chalmers University of Technology (1996). [11] Anon. 1. Ace, modified version 3.1, ABB common platform for field analysis and simulations. ABB Corporate Research Centre. ABB AB, Corporate Research, pp. 721 78 Västerås, Sweden. [12] Eriksson S, Solum A, Bernhoff H, Leijon M. Simulations and experiments on a 12 kW direct driven PM synchronous generator for wind power. Renewable Energy 2008;33(4):674e81. [13] Eriksson S, Bernhoff H, Leijon M. FEM simulations and experiments of different loading conditions for a 12 kW direct driven PM synchronous generator for wind power. Int J Emerg Electr Power Syst 2009;10(1) [Article 3]. [14] Eriksson S, Bernhoff H, Leijon M. A 225 kW direct driven PM generator adapted to a vertical axis wind turbine, vol. 2011, Article ID 239061, 7 pages, Adv. in Power Electr. (2011). [15] Deglaire P. Analytical aerodynamic simulation tools for Vertical Axis Wind Turbines, Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651e6214, (2010). [16] Eriksson S, Bernhoff H. Loss evaluation and design optimisation for direct driven permanent magnet synchronous generators for wind power. Appl Energy 2011;88:265e71. [17] Leijon M, Dahlgren M, Walfridsson L, Ming L, Jaksts A. Windformer-a recent development in the electrical insulation systems of generators and transformers. IEEE Electr Insul Mag 2001;17(3):10e5.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical note
Design of a unique direct driven PM generator adapted for a telecom tower wind turbine S. Eriksson*, H. Bernhoff, M. Bergkvist Swedish Centre for Renewable Electric Energy Conversion, Division for Electricity, Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2011 Accepted 20 January 2012 Available online 14 February 2012
A vertical axis wind turbine has been designed to electrify a novel kind of telecommunication tower. This paper presents the design of a generator for this purpose. The generator is a permanent magnet generator rated at 10 kW. It has an unusually large diameter to fit on the outside of the telecommunication tower. The generator has been designed by using a two-dimensional FEM model. Simulations show that the generator has high efficiency through the whole operational interval. Furthermore, the generator has a high overload capability enabling electric control of the turbine. The generator has been built and the design shown feasible. Preliminary experimental results show that the induced voltage is lower than expected from simulations indicating insufficient modelling of three-dimensional effects, which are particularly large in a generator with these unusual dimensions. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Vertical axis Wind turbine Telecommunication Permanent magnet Generator
1. Introduction The global demand on telecommunication access even in rural areas is increasing. For developing countries the crucial electricity supply for telecom towers is usually solved with diesel generators, which gives emissions as well as demands on fuel transportation. Here, a solution with a self-standing, integrated system is presented. A novel telecom tower is electrified from wind power supplemented with some kind of energy storage, for instance batteries or hydrogen. This paper focuses on design of the generator in the wind turbine. Alternative designs of using wind turbines for power supply for telecom applications have been presented based on free-standing wind devices [1e3]. However, there are several reasons why a wind turbine integrated with the telecom tower can be an interesting power supply solution; it limits the use of land, the wind turbine reaches higher wind speeds and an already existing tower is used. In 2007, the Swedish telecom company Ericsson launched a new base station tower called the Tower Tube, for which a concrete tower substitutes the traditional lattice tower. The main advantage is its low environmental impact and the reduced need of land. The concrete tower has both antennas and radio base station in the top of the tower, which has an outer radius of 1.8 m. The idea was raised * Corresponding author. Tel.: þ46 184715823; fax: þ46 184715810. E-mail address: [email protected] (S. Eriksson). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.01.090
to have a wind turbine on the tower placed below the antennas to avoid the risk of electromagnetic interference [4]. For a thick telecom tower like this one the obvious choice of wind turbine is a three-dimensional turbine which is not dependent of the wind direction. Therefore, the choice falls on a vertical axis wind turbine [5e7]. There are apparent reasons for the suitability of this configuration over a traditional horizontal axis wind turbine, mainly to avoid yawing and the unbalanced loading of the tower for a two-dimensional turbine. Furthermore, for this thick tower, less blade area will risk being shadowed by the tower when the blades are placed on support arms. The turbine has fixed blades, eliminating a pitching system. The turbine is controlled electrically through the strong generator, which must have a high overload capacity. Generators for wind turbines with full variable speed require a high efficiency for the whole operational regime [8e10]. Furthermore, a directly driven generator with a fixed blade turbine must have high generator torque capability for the whole operational regime, in order to safely stop the turbine in all wind speeds. Additionally, the generator design is focused on simplicity and on having an almost maintenance free design. This paper presents the design and simulations of the unique wind power generator adapted for the Tower Tube. A preliminary measurement of no load voltage is also discussed. Turbine design and aerodynamic effects from the large tower wake, although important research topics, are not included in this paper.
454
S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
2. Method
Table 1 Wind turbine and tower characteristics.
2.1. Simulations Electromagnetic simulations using the finite element method (FEM) are performed to design the generator and to simulate the generator’s behaviour at different loading conditions. In the simulations the electromagnetic field inside the generator is assumed to be axi-symmetrical and is therefore modelled in two dimensions. Three-dimensional effects such as end region fields are taken into account by introducing coil end impedances in the circuit equations of the windings. The permanent magnets are modelled by surface current sources. The electromagnetic model is described by a combined field and circuit equation model. The field equation, Eq. (1), originates from Maxwell’s equations.
s
vAz 1 vV ¼ s VAz V, m0 mr vt vz
(1)
where s is the conductivity, m0 and mr are vacuum and relative permeability respectively, Az is the axial magnetic potential and vV=vz is the applied potential. The circuit equations, Eq. (2) are described by
Ia þ Ib þ Ic ¼ 0 vIa vIb Ub Rs Ib Lend s vt vt vIc end vIb end ¼ Uc þ Rs Ic þ Ls Ub Rs Ib Ls vt vt
Uab ¼ Ua þ Rs Ia þ Lend s Ucb
(2)
where Ia, Ib, Ic are the conductor currents, Uab and Ucb are the terminal line voltages, Ua, Ub, Uc are the terminal phase voltages obtained from solving the field equation, Rs is the cable resistance describe the coil end inductance. and Lend s The generator parts are, after the generator geometry is set, assigned different material properties such as conductivity, permeability, density, sheet thickness etc. The electromagnetic model is solved in the finite element environment ACE [11]. The mesh is finer close to critical parts such as the airgap and coarser in areas like the yoke of the stator. Only one pole has to be modelled since the generator is symmetric. Simulations can be performed either in a stationary mode where the results are given for a fixed rotor position or in a dynamic mode including the time-dependence and thereby giving more accurate results. The simulations have been verified by comparison with experimental results for generators similar to the one studied here [12e14]. Stationary simulations are performed to design the generator and set the geometry. Dynamic simulations of the generator are performed on the set design. The generator is modelled to be connected to a resistive load, which gives a good estimate when studying general behaviour and to find the maximum torque. However, in reality the generator is connected to a rectifier bridge consisting of diodes. Modelling the efficiency with a resistive load is a simplification but will give rather accurate results. However, the copper losses could be expected to be slightly higher during diode rectification due to current peaks. The wind turbine will be run at its highest aerodynamic efficiency by adjusting the rotational speed to the wind speed up to a rotational speed of 89 rpm, where the rotational speed is held constant until 10 kW is reached. The aerodynamic efficiency is not assumed to decrease substantially when the rotational speed is held constant. A theoretical aerodynamic efficiency from fluid mechanical simulations is used [15]. The electric efficiency of the generator has been calculated by simulating the generator with
Rated power (kW) Rated rotational speed (rpm) Rated wind speed (m/s) Swept area (m2) Hub height (m) Tower outer diameter (m) Number of blades Blade length (m) Maximum blade speed (m/s) Aerodynamic control Blade profile
10 89 12 40 30 1.8 4 5 40 Passive stall NACA0021
a varying rotational speed and a varying load sweeping through a wide range of values to cover the whole operational interval [16]. 2.2. Design objectives The objective was to design a generator adapted to the telecom tower and the turbine, see Table 1. Furthermore, all parts of the wind turbine, except power cables, should be placed outside the tower, in order for the elevator transporting the antennas to go up and down through the tower if needed. The turbine has according to simulations a rather low aerodynamic efficiency due to the unusually large tower wake. The most important generator design objectives were:
A generator design geometrically adapted to the telecom tower. A design adapted to the turbine speed and torque. A design adapted for diode rectification. Low speed (i.e. direct drive). High overload capacity to ensure safe operation. High efficiency at varying speed and load. High reliability and low need for maintenance. Low cost.
3. Results 3.1. Generator design A directly driven, outer-rotor generator is chosen, in order not to interfere with the inside of the tower. The turbine blades are
Fig. 1. The magnetic flux density (T) in a 2D cross section of one generator pole for dynamic FEM simulations at rated speed and load.
S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
455
Table 2 Generator geometric characteristics. Inner diameter (m) Height (mm) Height to diameter ratio Airgap (mm) Cable area (mm2) No of cables per slot Steel thickness (mm) No of poles Slots per pole and phase
1.8 162 0.09 15.5 16 4 0.35 102 1
directly mounted on the generator rotor and the stator holding the cables is at stand-still on the outside of the tower. A permanent magnet rotor is chosen to avoid electrification of the rotor. A cable wound stator is chosen. The wind turbine is placed on the outside of the tower, apart from electrical cables and the electrical system on the bottom of the tower. The generator characteristics were derived by dynamic simulations using the electromagnetic model described in Section 2.1. Fig. 1 shows the generator geometry and the magnetic flux density. The stator winding consists of PVC insulated circular cables, which are easy to wind. The circular shape gives an evenly distributed electric field in the cables and hence makes better use of the insulation material [17]. Furthermore, the PVC is persistent to mechanical wear and the cables are maintenance free. The rotor of the generator is equipped with surface-mounted, rectangular, highenergy magnets made of Neodymium-Iron-Boron. The stator consists of steel laminations with a thickness of 0.35 mm to minimise iron losses. A large airgap between rotor and stator is chosen for the first prototype since the diameter is large, to reduce manufacturing tolerances. The large airgap reduces cogging effects and thereby enables the number of slots per pole and phase to be one, which simplifies the winding substantially. Additionally, the width of the magnets has been adjusted in order to minimize cogging. This generator design is unique for several reasons. It has an unusually large diameter for its low power rating and a very low height to diameter ratio. Consequently the generator has a large number of poles. Furthermore, it has an unusually high overload capacity to enable full electrical control and a low internal voltage drop of only 9 V at full loading. The maximum torque is 4.6 kNm, which is more than four times the rated torque. Geometric characteristics and results from dynamic simulations are shown in Tables 2 and 3 respectively. Fig. 2 shows the generator during manufacturing.
Fig. 2. Winding of the stator cable showing the stator mounted on a tower section.
The efficiency curve shown in Fig. 3 is achieved. The efficiency is optimized for a power of 7.7 kW, which represents a wind speed of 10.9 m/s. 3.3. Experimental results Preliminary experimental results, measuring the no load voltage at 51 rpm, showed that only approximately 84% of the from simulations expected voltage was reached, i.e. the generator gives a lower output voltage than expected. The tower tube with the installed wind turbine is shown in Fig. 4. 4. Discussion The generator design requirements have been achieved. The electrical efficiency of the generator is high throughout the whole operational regime. The maximum torque is more than four times the nominal torque which is enough to provide safe stopping of the turbine in all operational cases. The use of cables and permanent magnets minimizes the need for maintenance, which is important for this application. Preliminary measurements indicate that the voltage is about 84% of what was expected from two-dimensional FEM simulations. The difference between experimental results and simulations are
3.2. Generator efficiency simulations
95
Table 3 Generator electrical characteristics at rated speed and load. Power (kW) No load LeL voltage (V) rms Load LeL voltage (V) rms Current (A) rms Rotational speed (rpm) Electrical frequency (Hz) Torque (kNm) Cogging (%) Iron losses (kW) Copper losses (kW) Electrical efficiency (%)
10.0 219 210 27.4 89 76 1.1 2.9 0.23 0.37 94.3
Efficiency (%)
The generator has been simulated as being driven by a turbine operating at optimum tip speed ratio as described in Section 2.1.
94 93 92 91 2
4 6 Power (kW)
8
10
Fig. 3. Electrical efficiency of the generator as a function of power for a turbine operating at optimum tip speed ratio.
456
S. Eriksson et al. / Renewable Energy 44 (2012) 453e456
these unusual dimensions and the feasibility of the concept. Simulations should be improved concerning predicting threedimensional behaviour even though a two-dimensional model might still be used. Acknowledgements Dr. Arne Wolfbrandt and Doc. Urban Lundin are acknowledged for assistance with electromagnetic FEM simulations. Acknowledgements are given to Vertical Wind Communications AB, Ericsson and Swedish Energy Authority. The Swedish Energy Authority, Vinnova and Statkraft are acknowledged for contributions to the Swedish Centre for Renewable Electric Energy Conversion (CFE). References
Fig. 4. The telecom tower with the wind turbine installed.
probably caused by the simulations being performed with a twodimensional simulation method. The two-dimensional model is taking three-dimensional effects into account but probably not modelling them correctly. This generator, which has a very small height to diameter ratio, is very sensitive to correct modelling of end effects. Unfortunately the generator is currently not available for further research. However, the simulation model can still be improved. Simulations are made for an inner-rotor machine even though an outer-rotor machine was built. However, due to the large diameter the curvature per pole in the airgap is very small (see Fig. 1) so this is not expected to have an impact on the results. 5. Conclusions A novel generator with a unique geometry has been designed and successfully constructed for a wind turbine electrifying a telecom tower. Initial tests show that the generator works. However, test results differ somewhat from simulations, indicating a need for improved modelling. Even though the experimental voltage was lower than expected from simulations the first step was to demonstrate a generator of
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