B.Bolund, E.Segergren, A.Solum, R.Perers, L.Lundström, A. Lindblom, K.Thorburn, M.Eriksson, K.Nilsson, I.Ivanova,
O.Danielsson, S. Eriksson, H. Bengtsson, E.Sjöstedt, J.Isberg, J.Sundberg, H.Bernhoff, K-E Karlsson, A.Wolfbrandt, O.Ågren and M.Leijon
Department of Engineering Sciences
Uppsala University
Box 534, 751 21 Uppsala, SWEDEN
Phone (+46) 471-5817 Fax (+46) 471-5810 E-Mail: bjorn.bolund@hvi.uu.se WWW: http://www.el.angstrom.uu.se
Abstract - The discussion regarding renewable energy has gone on for several years. The many ideas and opinions that are presented in this field reflect the great impact future energy production has on people all over the world. This paper describes the new direction of the division of Electricity at
Uppsala University after the admission of the new professor,
Mats Leijon, in February 2001.
Full electromagnetic dynamics can be used in order to improve performance of existing electromagnetic conversion systems and to adapt new technology to the renewable power in nature. These ideas are adopted in wind power, wave power, water-current power, bio-fuelled plants as well as in conventional hydropower, i.e. in every different area were the division is active. This paper is a coarse description of the different activities at the division and aims to highlight their link to each other. Theoretical and experimental results from the different PhD projects are briefly introduced and summarized.
Keywords - Water Current Power, Bio-fuelled plants, Wave
Energy, Wind Power, Hydropower, Pulsed Power
I. INTRODUCTION
The global energy consumption is increasing continuously. The situation of today, when environmental considerations limit the use of fossil fuel and new hydropower dams, leads to a higher demand for renewable energy alternatives and more efficient use of already utilized energy sources. Most of the work at the division for Electricity at Uppsala University aims to make electromagnetic power conversion more efficient. Higher efficiency means more efficient use of already utilized energy sources as well possibility to use power sources earlier considered as uninteresting. To also succeed in implementing new technologies, it is essential that economic and ecologic considerations be thoroughly investigated, in parallel, with the technology development. Hopefully these ideas can be used for renewable energy sources e.g. wind and water.
Although exceptions exist, the by far most common way to produce electricity is to let a turbine, propelled by force of a moving fluid, operate a rotating generator. The fluid can be vapour from boiling water as in Bio-fuelled plants, moving water or air. Different pressure, velocity, and density of the fluids mean different rotational speed and torque form the turbine, thus different working conditions for the generator. An ordinary steam turbine rotates about 10 000 rpm while a water current turbine might reach 10 rpm. That rotating motion is, usually, via a gearbox adjusted to the optimal speed of the generator and in most cases a transformer is used to adjust the generator voltage to the grid. The principal idea behind all projects at the division is however the other way round, to adjust the electromagnetic power converter directly to the power source (which has been done in hydro power stations in nearly a decade). By excluding unnecessary stages, such as gearboxes and transformers, less losses and less need of maintenance can be expected, leading to an economically more viable solution.
Still the power output needs to be handled, but in this case voltage conversion and electric overloads can be dealt with electrically by using power electronics. The most obvious example of adjustment of the electromagnetic power converter to the power source is however not a rotating generator, it is the linear wave power generator. A buoy, coupled to the rotor
(piston), can directly make use of the vertical motion of the wave. The opposite is to use complex mechanics to adjust the movement of the wave into rotation suitable for a conventional generator.
In addition to the projects mentioned above other groups of researchers, who do not study energy conversion from renewable energy sources, are active within the division.
Especially the diamond project and the pulsed power project have to be mentioned. The diamond project aims to study diamonds as semiconductors and in the pulsed power project high-energy pulses are produced for various pulsed power applications.
II. RESEARCH PROJECTS
A. Water Current Power
Water currents are caused by interaction between the gravitational fields of the moon and the sun with the Earth’s oceans (tidal currents), differences in salinity, difference in temperature and/or the Coriolis effect (the Gulf Stream) [1]. The high density of water means that water currents are an enormous source of energy, which could be utilized with virtually no environmental impact, that remains unexploited. A few projects in the field have resulted in experimental facilities and in 2003 the 350-kilowatt prototype in Hammerfest, Norway, was the first current power station to deliver power to the grid [2].
From a power conversion point of view the low velocities of water currents are the most important difficulty to overcome.
Although very powerful, water currents only rarely exceed velocities of 1 m/s [3]. A phenomenon similar to boiling, called cavitation, occur when the partial pressure a fluid fall below the vapour pressure, further complicates the problem. Cavitation
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cause fast abrasion of the turbine blades and has to be avoided, which is easiest done by lowering the turbine speed [4]. The research on water current generators at the division of
Electricity at Uppsala University has, so far, been focusing on theoretical simulation and design of low speed generators suitable for water current conditions. In future an experimental generator will be constructed in order to verify the theoretical results.
B. Bio-fuelled plants
A deep understanding of the underlying generator physics has led to advanced computer programs enabling full field simulation of generators. To ensure the validity of these simulations and to improve the model, the result from simulations must be compared to measurements on existing generators. A well-calibrated simulation-tool might also be used to simulate faults and various types of malfunction. In order to determine the accuracy of an existing generator simulation program measurements from the biomass fueled combined heat and power (CHP) plant in Eskilstuna is compared with simulations of an identical machine.
The biomass fueled combined heat and power (CHP) plant in
Eskilstuna, Fig. 1, has a rating of 38.7 MW electric and- 71
MW thermal energy. The boiler is of the bubbling fluidized bed type, with a thermal rating of 110 MW and steam data of 139
Bar at 540°C. One high pressure and one low pressure steam turbine is mounted on either side of the generator, which is a
3000 rpm two pole turbo Powerformer. The cooling water from the generator is used for heating the surrounding dens populated area, in what is called a CHP-arrangement (combined heat and power) [5].
Since cooling of turbo generators is costly, especially cooling of the rotor, research is directed towards the possibility of replacing the electromagnets in the rotor with permanent magnets [6]. Moreover the potential of generating power at higher frequencies than 50 or 60 Hz is addressed. In that case a rectifier and a DC-AC converter is needed which will introduce losses. On the other hand the gearbox, normally mounted between turbine and generator, and its associated losses can be removed. The losses, generator dimensions, and performance of such higher frequency generators are investigated.
C. Wave Energy
A system for conversion of energy from ocean waves to electrical energy is studied in the wave energy project. The concept uses a point absorber on the ocean surface, connected with a rope to a three phase synchronous linear generator with permanent magnets placed on the seabed, Fig. 2. A watertight enclosure is used to protect the equipment from the difficult surroundings under the water surface. The wave power units must handle the pressure and the salinity and survive storms to reduce the need for maintenance. No gearbox is needed as the buoy is directly coupled to the piston, which reduces maintenance and provides more reliable working conditions.
The vertical piston motion induces an EMF with variable amplitude and variable frequency in the stator winding. An important issue with such system is to obtain a smooth power output. The power fluctuations can be reduced by connecting several units in a farm, Fig. 3.
The generator is simulated in a finite element environment where the generator equations and external circuit equations are solved simultaneously. The tool is used in stationary and timestepping simulations. Our main conclusions so far concern magnet mounting, financial frames, DC voltage impact and wave climate impact on power production [7-13].
A first prototype has been designed and built. The first experiments in the laboratory show that the system works in principal. Ocean experiments are planned for the near future.
Fig. 1. Schematic of a bio-fuelled CHP plant.
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Fig. 2. Wave energy conversion unit.

Fig. 3. Several units are connected in a farm.
D. Wind power
The research on wind power is focused on a new concept including a VAWT (Vertical Axis Wind Turbine) and an improved generator based on the Powerformer technology [14].
An important issue is to make wind power more economically viable by increasing the efficiency. The studied VAWT is called
H-rotor and has straight vertical blades, Fig. 4. Some of the advantages with this wind turbine are that the generator can be placed on the ground, that it is independent of wind directions and that the wing tip speed is lower than for conventional propeller type wind turbines, thus less noise. As generator a directly driven PM (permanent magnet) generator, which needs neither gearbox nor a transformer, is considered. This, combined with the fact that it is placed on the ground, increases the efficiency of the transformation, makes the construction cheaper, and lowers the need of maintenance.
One research project, among others, is to analytically evaluate and improve the overload capacity of the generator. This would make it possible to utilize winds, which are considered to be to strong for conventional wind power plants. The energy of the wind is proportional to the cube of the wind speed, which means that a small increase in the wind speed yields a large increase in the power output. The theoretical results will later be verified experimentally.
E. Hydropower
The installed hydropower capacity in Sweden peaked in the
1960s [15], Fig. 5. Several of the Swedish hydropower generators are constructed at the first half of the 1900s. Future activity is likely to be mainly confined to modernization and refurbishment of existing plants since construction of new hydropower plants has been stopped on account of environmental and political considerations. At Uppsala
University some projects regarding the potential upgrading of the Swedish hydropower [16-18] have been conducted. These projects have shown that the electricity production could be increased with a couple of TWh/year a normal year. The upgrade potential is found both in turbines and generators. The research in our group is naturally focused on the generator. A majority of all large synchronous generators are built from electrical equivalent circuit theory developed in 1929 by Park
[19]. The equivalent circuit theory has its limitations especially when it comes to abnormal situations. An ongoing project deals with full physics simulation of an old in service hydropower generator along with experimental verifications. FEM (Finite
Element Method) computer calculations gives options for better and more generalized modeling. These simulations may offer the owner a complementary way to test different conditions and a base for future investments and operating decisions.
Fig. 4. The H-rotor in focus for the wind power research.
In the field of hydropower technology there have only been a few publications about the electromechanical coupling between the turbine and the generator. Today’s methods used for design are only simplified stationary simulations. In an ongoing research project at Luleå Technical University [20] it has been found that additional electromechanical research is necessary for the development of better dynamical models for hydropower generators. At the group of Electricity at Uppsala University
FEM computer calculations is used to solve the field equations in the generator. With this method the electromechanical loads at transient course can be calculated from the electromagnetic field equations. The aim is to develop new electromechanical models for the forces in the generator and give insight in how geometrical deviations in generators affects the electromechanical loads in a generator.
F. Pulsed power
Transmission line transformers (TLT) are used in various pulse power technologies. A step-up TLT is made of a coaxial cable where the inner conductor acts as the secondary winding and the screen as the primary winding. The cable insulation is a limiting property of the TLT since it must withstand the full output voltage. Modern high-voltage cables are equipped with a
GWh/year
100000
80000
60000
40000
20000
0
1950 1960 1970 1980
Hydro Nuclear Thermal
Fig. 5. The historic production of electricity in Sweden [15]
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resistive layer (semicon) on the inner conductor and on the inside of the outer conductor, which increases their dielectric performance substantially. Thus, using this kind of cable in a
TLT, the voltage levels of the TLT can be increased. Electric circuit simulations of the TLT were made in order to facilitate construction. This type of transformer was constructed and tested in [21]. The coupling factor for the single layer transformer is 0.8. The high coupling factor is achieved due to the coaxial structure.
The transformer was tested in an existing pulse conditioning system, and the operating principle for that system is illustrated in Fig. 6. The primary energy storage is discharged through the primary winding of the transformer by closing the first switch.
When the second switch is opened the current is abruptly interrupted and the magnetically stored energy in the transformer is discharged into the load. The opening switch is based on electrically exploding copper wires. Fig. 7A shows the high voltage results when the opening switch was equipped with
18 and 37 copper wires, respectively, and Fig. 7B shows an electric circuit simulation. The electric circuit model of the transformer consists of distributed capacitance, resistance, and inductance in order to resemble a transmission line. The high voltage cable with PEX insulation proved to withstand very high stress (85 MV/m). This type of transformer is useful in applications where weight is an important factor. The simple design ensures low cost manufacture. Fig. 6 and Fig. 7 are from
[21].
Fig. 6. Simplified electric circuit of the pulse transformer in the pulse conditioning system.
Fig. 7A. Measured load voltage of the transformer for two different numbers of wires in the opening switch.
Fig. 7B. Simulated load voltage of the transformer for two different numbers of wires in the opening switch.
III. ACKNOWLEDGMENT
We acknowledge the financial support from and collaboration with the Swedish research council, Swedish energy agency,
FOI-FMV, Eskilstuna Energi och Miljö, Ångpanne-föreningen research foundation, the J Gust Richert foundation, Vargön
Alloys AB, Göteborg Energi, Draka Kabel, / Swedish Energy
Authorities / CF - Environmental Fund, Helge Ax-Johnsson
Research Foundation, the Vargön Smältverk foundation,
Graninge Energi AB, Carl Tryggers found for scientific research,
IV.
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