Flywheel in an all-electric propulsion system

JOHAN LUNDIN

Licentiate thesis

UURIE: 328-11LISSN: 0349-8352

Abstract

Energy storage is a crucial condition for both transportation purposes and forthe use of electricity. Flywheels can be used as actual energy storage but alsoas power handling device. Their high power capacity compared to other meansof storing electric energy makes them very convenient for smoothing powertransients. These occur frequently in vehicles but also in the electric grid. Inboth these areas there is a lot to gain by reducing the power transients andirregularities.

The research conducted at Uppsala university and described in this thesis isfocused on an all-electric propulsion system based on an electric flywheel withdouble stator windings. The flywheel is inserted in between the main energystorage (assumed to be a battery) and the traction motor in an electric vehicle.This system has been evaluated by simulations in a Matlab model, comparingtwo otherwise identical drivelines, one with and one without a flywheel.

The flywheel is shown to have several advantages for an all-electric propul-sion system for a vehicle. The maximum power from the battery decreasesmore than ten times as the flywheel absorbs and supplies all the high powerfluxes occuring at acceleration and braking. The battery delivers a low andalmost constant power to the flywheel. The amount of batteries needed de-creases whereas the battery lifetime and efficiency increases. Another benefitthe flywheel configuration brings is a higher energy efficiency and hence lessneed for cooling.

The model has also been used to evaluate the flywheel functionality foran electric grid application. The power from renewable intermittent energysources such as wave, wind and current power can be smoothened by the fly-wheel, making these energy sources more efficient and thereby competitivewith a remaining high power quality in the electric grid.

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List of papers

This thesis is based on the following papers, which are referred to in the textby their Roman numerals.

I J. Santiago, J. G. Oliveira, J. Lundin, A. Larsson, H. Bernhoff (2008)Losses in Axial-Flux Permanent-Magnet Coreless Flywheel EnergyStorage Systems. Published in proceedings of the 18th InternationalConference on Electrical Machines (ICEM-08), Vilamoura, Portugal,6-9 September, 2008, paper ID 910.

II J. Santiago, J. G. Oliveira, J. Lundin, J. Abrahamsson, A. Larsson, H.Bernhoff (2009) Design Parameters Calculation of a Novel Drivelinefor Electric Vehicles. Published in World Electric Vehicle Journal Vol.3.

III J. G. Oliveira, J. Lundin, J. Santiago, H. Bernhoff (2010) A DoubleWound Flywheel System under Standard Drive Cycles: Simulations andExperiments. Published in International Journal of Emerging ElectricPower Systems.

IV J. Abrahamsson, J. Santiago, J. G. Oliveira, J. Lundin, H. Bernhoff(2010) Prototype of electric driveline with magnetically levitateddouble wound motor. Published in proceedings of the 19th InternationalConference on Electrical machines (ICEM-19), Rome, Italy, 6-8September 2010.

V J. Lundin, H. Bernhoff (2010) Flywheel as Power Handling Devicein Electric and Hybrid Vehicles. Submitted to International Journal ofVehicular Technology, September 2010.

VI J. Lundin, J. G. Oliveira, C. Boström, K. Yuen, J. Kjellin, M. Rahm,H. Bernhoff, M. Leijon (2011) Dynamic stability of an electricity gen-eration system based on renewable energy. Accepted for publication inproceedings of the 21st International Conference on Electricity Distri-bution (CIRED 2011), Frankfurt, Germany, 6-10 June 2011.

VII J. G. Oliveira, J. Lundin, H. Bernhoff (2011) Power balance control inan AC/DC/AC converter for regenerative braking in a flywheel baseddrive line system. Submitted to International Journal of Vehicular Tech-nology, May 2011.

Reprints were made with permission from the publishers.

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1 The flywheel project at Uppsala university . . . . . . . . . . . . . . . . 111.2 History of flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.3 Energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.4 The electric grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.5 This thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1 Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Vehicle movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.1 Outer losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.2 Inner losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3 Conversion between kinetic and electric energy - The electricmachine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4 Drive cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1 General description of the model . . . . . . . . . . . . . . . . . . . . . . . 253.2 Part 1 of model - Calculation of power needed for vehicle . . . . 253.3 Part 2 of model - Simulation of power fluxes in driveline . . . . . 26

4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.1 Verification of concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2 Benefits of a flywheel electric driveline . . . . . . . . . . . . . . . . . . 294.3 Comparison between two models describing a part of the fly-

wheel electric driveline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.4 Flywheel as power handling device for renewable energy sources 33

5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 Sammanfattning på svenska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Nomenclature

E f [J = kgm2/s2] Energy in flywheelEv [J = kgm2/s2] Energy in vehicleJ [kgm2] Moment of inertaω [rad/s] Angular velocity (or rotational speed)m f [kg] Mass of flywheelmv [kg] Curb mass of vehiclemw [kg] Mass of wheelρ f [kg/m3] Density of rotating part of flywheelρair [1.204 kg/m3] Density of airr [m] Radiusro [m] Outer radiusri [m] Inner radiush [m] Height/thickness of the flywheelPtot [W] Total power required for vehiclePacc [W] Linear acceleration powerProt [W] Rotational acceleration powerPair [W] Air resistance powerPrr [W] Rolling resistance powerPs [W] Slope powerv [m/s] Speed of vehiclea [m/s2] Acceleration of vehiclen [-] Number of wheelsg [9.81 m/s2] Gravitational accelerationϕ [-] Slope of road in degreesAv [m2] Frontal area of vehicleCw [-] Air drag coefficientCrr [-] Rolling resistance coefficient

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I [A] CurrentU [V] VoltageR [Ω] ResistanceP [W] Electric powerε [V] Induced electromotive force (or back EMF)εrms [V] rms-value of back EMFΦB [Wb = Vs] Magnetic fluxt [s] TimeN [-] Number of coils exposed to the same magnetic flux variationB [T = Wb/m2] Magnetic fieldAc [m2] Area of the closed circuit in the magnetic field

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1. Introduction

Several of humanity’s great challenges are related to transportation and elec-tricity generation. Energy storage is vital for both these sectors. It is crucialfor vehicles to be able to go long distances non-stop, and for the electric gridto make it possible to supply, at every instant of time, exactly the amount ofelectricity demanded by millions of users connected to the grid.

The environmental problems associated with these sectors are unfortunatelysevere. The delicate conditions for life that all flora and fauna slowly andcontinuously adapt to are threatened by immense emissions of carbon thathas been out of the natural carbon cycle for millions of years [1], nitrogenthat the engine of a vehicle or the combustion chamber of a coal power planttransforms from stable and non-reactive N2, inaccessible to most natural pro-cesses, to the reactive forms N2O or NO2 [2] and a variety of numerous smallparticles created in the combustion process about whose impact on environ-ment and health we now little or nothing. Additionally oil, the fuel driving thetransportation sector, is running out [3].

Effective energy storage is a way to drastically reduce many of the nega-tive impacts of transportation and electricity generation. Flywheels can havea great importance in both these sectors, providing an opportunity to store en-ergy and thereby smoothen the power. Both the transportation and the energysectors are huge - not to say the least. Thus, there is a huge economic impactpotential and at the same time the possibility of making the world a little betterplace.

1.1 The flywheel project at Uppsala university"The flywheel research group" at the Division of electricity, Uppsala univer-sity, is conducting research on an all-electric propulsion system with an elec-tric flywheel as the central part. The main application is power smoothing invehicles. The flywheel is placed in between the battery, which is assumed tobe the main energy storage, and the traction motor in the driveline. The ideais that the sum of energy stored in the flywheel and the kinetic energy of thecar should be relatively constant. This arrangement smoothens the power con-sumed or produced by the traction motor and thus protect the battery from ex-periencing the short but large power transients typical for vehicles. The main

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components and connections of a flywheel all-electric propulsion system areshown in figure 1.1.

Another application for energy storage and power smoothing is the electricgrid where the flywheel would have the same power smoothing role as in avehicle.

Figure 1.1: A schematic overview of the flywheel all-electric propulsion system.

The idea of a flywheel as power handling device in a vehicle is not new, seefor example [4]. However, unlike flywheels previously studied, the flywheelexamined in this thesis is equipped with double stator windings, as seen infigure 1.2. This allows it to simultaneously and independently charge or dis-charge both the traction motor and the energy storage at two different powerand voltage levels. The two winding configuration also insulates the tractionmotor and the battery electrically from each other, acting like a protectionbarrier against voltage and current transients and overtones.

The project has so far resulted in numerous papers and two licentiate the-ses, [5, 6]. The results are intended to be commercialized within the spin-offcompany Electric Line which is the owner of a worldwide patent for the dou-ble stator winding flywheel [7].

1.2 History of flywheelThe history of flywheel goes back thousands of years. The potter’s wheel andthe spinning wheel are two examples where the flywheel, with its inertia, hasconverted a pulsating input power to a smooth output power. Flywheels canalso be used in the opposite way, by converting a smooth input to an irregularoutput. Hydro power generators are examples of this.

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Figure 1.2: A photo of the second prototype flywheel, described in paper II. The threephase configuration of the winding is visible since each phase have its own colour. Thedifference in number of turns for the both sets of windings can not be seen since thecolour is the same for both the high voltage windings and the low voltage windings.The upper rotor has been removed to show the stator windings.

In fact a mechanical flywheel is already used in almost every vehicle withinternal combustion engine, mounted on the outgoing engine shaft to sup-press the explosion impulses from the piston in each valve during combus-tion stroke. Another common flywheel application is as UPS (uninterruptiblepower supply) to prevent sensitive equipment or processes to be damaged bya power failure. For an overview of applications, see [8–11].

1.3 Energy storageThere are many different ways of storing energy, but few are suitable for mo-bile applications [12,13]. Basically the options for electric1 energy storage forvehicles available today are:

• Flywheels• Batteries• Ultracapacitors• Fuel cells

A comparison between the main advantages of these forms of energy storage,compared to each other one by one, is given in table 1.1.

The main advantage of the flywheel over the three other storages availableis its almost unlimited lifetime, in terms of cycles. It can also be produced ina harmless way by abundant materials easy to recycle. The flywheel does notproduce any hazardous gases, radiation or other potentially dangerous emis-sions.

1Electric energy storage is here defined as both input and output energy being in form of elec-tricity; the storage itself could be in any form

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Table 1.1: Comparison between four different ways of storing electric energy for mo-bile use. The advantages listed in the table are the advantages for the storage types inthe rows are compared to the storage types in the columns.

Flywheel Battery Ultracapacitor Fuel cell

Flywheel — -Power density -Energy storage -Power density-Lifetime -Cost -Efficiency-Environment -Lifetime -Materials used

Battery -Energy density - -Energy density -Efficiency-Storage time -Storage time

Ultra- -No moving -Power density — -Power densitycapacitor partsFuel cell -Energy density -Energy storage -Energy density —

Even though the energy density of a flywheel is lower than that of batter-ies - a complete flywheel system commercially available today would havean energy density of around 20 Wh/kg - there are no fundamental limits tosignificant increase. For example a new material with three times higher ten-sile strength would allow a three times higher rotational speed, increasing theenergy density for the complete flywheel system by nine times - and so eventhe best batteries would be left behind.

The drawback for the flywheel is mainly the self discharge time which is - atits very best - ten times higher than that of batteries, but more likely a thousandtimes higher [10]. For long time energy storage the flywheel is therefore not agood option at the time being. But low pressure operation (described in paperI) and magnetic bearings [14] can change the competitiveness for flywheels asan option even for long time energy storage.

Batteries are almost four times more energy efficient than fuel cells, com-paring the round-turn efficiency from electricity to electricity. The startingpoint would be the production of electricity in a power plant and the finalpoint would be electricity available for the vehicle’s traction motor. While bat-teries have a round-turn efficiency of about 80%, fuels cells have a round-turnefficiency of only about 20% [15].

1.4 The electric gridThe electric grid is being held at a specific electrical frequency (50 Hz inEurope) with only small deviations allowed. The voltage is also kept at somewell-defined constant levels (except for loss-related voltage drops), from400 kV line-to-line voltage in the transmission lines of the backbone gridto the one phase voltage of 230 V, available in the household sockets. The

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electrical frequency and voltage of the flywheel though is physically related,and thereby directly proportional, to the speed of the flywheel. It wouldbe impossible to vary the energy at the same time as keeping the electricalfrequency and the voltage constant, since the energy content of the flywheelis regulated by the speed. Thus the flywheel can not be directly connected tothe grid for energy storage. It has to have some power electronics connectedin between to adjust and control both frequency and voltage.

1.5 This thesisThis thesis focuses on the system design of a flywheel based all-electricpropulsion system for efficient energy use in vehicles (papers I-V and paperVII). It also deals with the positive effects flywheels can have on electric gridapplications (paper VI).

After this chapter, 1. Introduction, the theoretical background to this workis given in chapter 2. Theory. The theory from chapter 2 is used to createa simulation model presented in chapter 3. Method. Chapter 4. Results anddiscussion presents and discusses the results from the simulations made withthe model described in chapter 3 and chapter 5. Conclusions concludes thework presented in chapter 4. Some interesting fields of research for the futureare discussed in chapter 6. Future work. The papers constituting this thesisare summarized in chapter 7. Summary of papers. Finally the thesis is brieflyencapsulated in Swedish in chapter 8. Sammanfattning på svenska.

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2. Theory

This chapter presents the physics behind flywheels, vehicle motion and theconversion between kinetic and electric energy in the electric machine. Thechapter also gives a background to drive cycles.

2.1 FlywheelAn electric flywheel is basically an electric machine with some extra weighton the rotor to increase the moment of inertia. The amount of stored energydepends on the rotational speed of the flywheel as well as the distribution ofrotating mass (the moment of inertia), given by eq. 2.1. Eq. 2.1 is in fact ananalogy to the energy stored in a translationally moving object, for examplea car, given by eq. 2.5 on page 18. Moment of inertia in circular motion issomewhat the parallel to mass in linear motion.

E f =Jω2

2(2.1)

where E f [J] is the energy stored in the flywheel, ω [rad/s] the angular velocityand J [kgm2] the moment of inertia around the principle axis of rotation, givenby eq. 2.2.

J =∫ ro

ri

m f (x)xdx = 2πρ f

∫ ro

ri

h(r)r3dr (2.2)

where m f [kg] is the mass of the flywheel, ro [m] the outer radius and ri [m] theinner radius of the flywheel, ρ f [kg/m3] the density of the flywheel materialand h [m] the height/thickness of the flywheel, in this equation depending onthe radius.

If the height of the flywheel is constant, i.e. the flywheel has the shape of ahollow cylinder, the moment of inertia is given by eq. 2.3.

J =12

πρ f h(r4o− r4

i ) (2.3)

In the special case of a thin rim flywheel, the moment of inertia would thenbe given by eq. 2.4.

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J =m f r2

2(2.4)

where r is the radius of the flywheel.

A flywheel is basically dimensioned by its size and weight (what is oftenreferred to as its form factor) and regulated by its rotational speed. The ro-tating part of the second prototype flywheel (see figure 1.2 on page 13) has amoment of inertia of 0.364 kgm2.

2.2 Vehicle movementTo run a vehicle is physically to overcome a number of losses. Without lossesthe car would need no net energy. The energy, Ev [J], stored in a translationallymoving object, such as a car, is given by eq. 2.5.

Ev =mvv2

2(2.5)

where mv [kg] is the mass of the vehicle and v [m/s] the speed of the vehicle.

A vehicle, electric or conventional, travelling on a flat road has to overcometwo resistance losses to keep the desired speed. Figure 2.1 shows the relationbetween the two resistance losses for a standard car (see table 2.1) at differentspeeds. Note that this figure does not include any power required for accel-eration but shows the losses associated with cruising at a certain speed. Themost important loss at low speed is the rolling resistance loss, as seen in thefigure. At higher speed (above about 60 km/h) the air resistance loss is thepredominant power loss growing fast as it is proportional to the cube of thespeed of the vehicle1.

With these figures in mind the inefficiency of ordinary internal combustionengine vehicles is obvious, since they often consume even more fuel at lowspeed. Electric vehicles though follow the physically proper power consump-tion and therefore consume much less power at lower speed than at higherspeed. To exemplify the great difference between the two a standard ICE (in-ternal combustion engine) car with a gasoline2 consumption of 7 l/100 km ata constant speed of 80 km/h has a power consumption of 51 kW compared toaround 10 kW for an electric car with an efficiency of 70 %. The theoretical"ideal" power consumption is around 7 kW as seen in figure 2.1.

1However the energy needed to travel a certain distance is proportional to the square of thespeed - and of course directly to the distance.2Thermal energy content of gasoline is around 33 MJ/l.

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0 20 40 60 80 100 1200

5

10

15

20

Speed [km/h]

Pow

er [

kW]

Figure 2.1: Power needed to overcome the two main resistance losses for a vehicletravelling on a flat road. The dashed line shows the rolling resistance power loss andthe dotted line the air resistance power loss. The total power loss is shown by the solidline. For low speeds the rolling resistance losses are the predominant while for highspeeds the air resistance account for most of the losses.

Table 2.1: Data for vehicle used in simulations. Data from [16].

Curb mass of vehiclea) mv 1181 kgMass of wheel mw 25 kg eachAir drag coefficient Cw 0.321Rolling resistance coefficient Crr 0.015Frontal area of vehicle A 2.1 m2

Tyre outer radius ro 0.309 ma) There are slightly different definitions of curb mass but in this thesis it is defined as the totalmass of vehicle including fluids, standard equipment and a 75 kg driver, i.e. everything neededto propel the vehicle, but no extra load.

Yet, the energy efficiency of the electric car heavily depends on the powersource. For example [17] claims that a battery electric vehicle is 3.56 timesmore efficient than a standard ICE vehicle. This is for the US electricity gen-eration mix, consisting of 48% coal-power.

The greatest advantage of electric vehicles over standard ICE vehicles oc-curs in urban driving, with many accelerations, brakings, starts and stops. Inurban areas the reduction of emissions is also most valuable since the localhealth effects of emissions can be more important than the global environ-mental effects [18, 19].

The physical laws governing the losses during motion of an electric vehiclecan be divided in two main groups; outer losses that define how much poweris needed in the wheels of the vehicle and inner losses that determine howmuch power the energy storage will have to supply and where the losses areoccurring and therefore have to be taken care of. In addition to these losses,

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energy is used to run auxiliary devices such as air condition, lights et.c. whichadds to the total power consumption.

2.2.1 Outer lossesThe predominant outer losses are acceleration loss, slope resistance loss, airresistance loss and rolling resistance loss. Standard drive cycles assume flatand straight roads so the slope resistance loss will not be active in the model(it will be set to zero). However, slope resistance loss is implemented in themodel to enable the use in a real drive pattern. The total power required topropel the vehicle, Ptot [W], is then given by eq. 2.6.

Ptot = Pacc +Prot +Ps +Pair +Prr (2.6)

where Pacc [W] is the linear acceleration power, Prot [W] the rotational accel-eration power, Ps [W] the slope power, Pair [W] the air resistance power andPrr [W] the rolling resistance power.

These five terms can be evolved according to eq. 2.7-2.11.

Pacc = mv ·v ·a (2.7)

Prot = mw ·v ·a ·n4

(1+

r2i

r2o

)(2.8)

Ps = m ·v ·g ·sinϕ (2.9)

Pair =12·Av ·v3 ·ρair ·Cw (2.10)

Prr = m ·v ·Crr ·g (2.11)

where mv [kg] is the mass of the vehicle, v [m/s] the speed of the vehicle, a[m/s2] the acceleration of the vehicle, mw [kg] the mass of each wheel, n [-]the number of wheels, g [9.81 m/s2] the gravitational acceleration, ϕ [-] theslope of the road in degrees, Av [m2] the frontal area of the vehicle, Cw [-] theair drag coefficient and Crr [-] the rolling resistance coefficient.

The first three terms in eq. 2.6, Pacc, Prot and Ps, are not really losses butrather the conversion rate between two "reusable" forms of energy, for exam-ple from chemical energy in the battery to kinetic energy of the vehicle or

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from kinetic to potential energy of the vehicle. Therefore these three termscan also be negative, i.e. power is gained. The two last terms though, Pair andPrr, are losses in the meaning that energy is converted to some kind of uselessenergy, for example heat, wind turbulence and wearing of tyres.

2.2.2 Inner lossesFor an electric vehicle inner losses consist mainly of electrical losses whichoccur in all electric parts where a current is passing through and causes resis-tive losses. Electrical losses are basically governed by Ohm’s law (eq. 2.12),which states that a load connected to two points causes a current flowingthrough the resistance which is directly proportional to the voltage differencebetween the points and inversely proportional to the resistance in the load.The load is not necessarily a useful load. All cables, diodes and other currentconducting apparatus have resistances which causes losses and a continuousvoltage drop over them. Since the electrical power is described by eq. 2.13these losses are proportional to the square of the current, according to eq.2.14.

The bearing loss in the flywheel would also be part of inner losses, butmechanical. In the model it is set to a constant value.

I =UR

(2.12)

P = U · I (2.13)

P = I2 ·R (2.14)

where I [A] is current, U [V] voltage, R [Ω] resistance and P [W] electricpower.

2.3 Conversion between kinetic and electric energy -The electric machineAn electric machine converts energy from electric to kinetic (motor) or theother way around (generator). The principle for both directions of energy con-version is the same, and thus governed by the same laws.

Faraday’s law (eq. 2.15) states that the induced electromotive force ε [V],also called back emf, in a closed circuit is proportional to the time rate ofchange of the magnetic flux ΦB [Wb = Vs] through the circuit.

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ε =−dΦB

dt(2.15)

The rms-value of the back emf (εrms) is given by eq. 2.16:

εrms =NωBAc√

2(2.16)

where N [-] is the number of turns of identical coils exposed to the samemagnetic flux variation, ω [rad/s] is the angular velocity, B [T] is the magneticfield and Ac [m2] is the area of the closed circuit in the magnetic field.

An electric motor can run in four basic different modes, consisting of thecombination of the two directions of rotation (forward and backward) and thetwo possible ways to change the rotational direction in time (acceleration ordeceleration). The four modes, seen in table 2.2, can be controlled by twoparameters. The phase shift between voltage and current decides whether themotor is accelerating or braking, and the sequence of phases decides the di-rection of rotation.

Table 2.2: Four modes of operation of an electric motor with possible control.

Acceleration Deceleration

Forward U and I in phase 180 phase shift between U and Iphase sequence a, b, c phase sequence a, b, c

Backward U and I in phase 180 phase shift between U and Iphase sequence a, c, b phase sequence a, c, b

2.4 Drive cyclesDrive cycles are used to have a common reference point in various contexts.Some are standardized and used for compulsory testing before a new vehicleis allowed to be put on the market. The tested entities are often fuel consump-tion and emissions of some chemical compounds. The outcome of these testsprovide the basis for decision making for the buyer of the vehicle, and also thebasis for a classification as an "environmentally friendly vehicle" which canhave great impact on the economical conditions such as motor vehicle taxesand taxable benefit.

A certain drive cycle is often given per time unit, either as acceleration (forexample NEDC - the New European Drive Cycle [20], see figure 2.2) or asspeed (for example the North American FTP 75 [21], see figure 2.3). Practi-

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cally the vehicle is tested with a driver that drives the vehicle on a dynamome-ter in the lab and tries to follow the drive cycle as accurately as possible.

However, it is not evident that standard drive cycles reflects reality in aproper way. There are two main reasons for this. The drive cycles differ toomuch from real-world driving behaviour and the tested vehicles are new incontrary to the average vehicle in traffic. Both these matters significantly af-fects the fuel consumption and the emissions measured in a lab in comparisonto real-world data. [22] discusses this problem thoroughly.

0 200 400 600 800 1000 12000

20

40

60

80

100

120

Time [s]

Spee

d [k

m/h

]

Figure 2.2: Speed vs time in the European drive cycle NEDC. The total time for thedrive cycle is 1180 s.

0 200 400 600 800 1000 1200 14000

20

40

60

80

100

Time [s]

Spee

d [k

m/h

]

Figure 2.3: Speed vs time in the American drive cycle FTP 75. The total time for thedrive cycle is 1372 s.

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3. Method

The results in this thesis are achieved with a model, implemented in Matlab,and based on the theory described in the previous chapter. The model con-sists of two parts, following each other in time. The simulation is divided intoarbitrary small time steps (typically tenths of a second) and in each of themall required calculations are made and the results added to an array. The timesteps are equally small in both parts. The model is described in more detail inpaper V.

3.1 General description of the modelIn the first part, the power needed in the traction motor is calculated. Thevehicle is considered "a black box" in the sense that only what is happeningoutside the vehicle is relevant, i.e. the outer losses.

In the second part the power calculated in the first part is used to simulatethe power fluxes in different parts of the driveline and also the resulting speedof the flywheel. In this part the system simulated is moved into the vehicle,i.e. the inner losses.

There are multiple options for the choice of drive cycle, control strategy,configuration of driveline and loss function for every part of the driveline.The model can be used to test, evaluate and compare all possible interestingcombinations of these variables.

3.2 Part 1 of model - Calculation of power needed forvehicleThe first part of the model calculates the power needed to propel a vehicleaccording to a certain drive cycle. For this purpose, eq. 2.6-2.11 from thetheory chapter are used.

The main input data to part 1 of the model is:• Vehicle data such as front area and mass (see table 2.1 on page 19)• Natural constants/values such as gravitational constant and air density• Drive cycle data (time and speed/acceleration)

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The main output data from part 1 of the model is:• Power needed in the traction motor

3.3 Part 2 of model - Simulation of power fluxes indrivelineIn the second part of the model the simulation of power fluxes in the drivelinestarts over from the beginning of the drive cycle with focus on power fluxes in-side the vehicle. For these simulations equations used from the theory chapterare eq. 2.1, 2.2, 2.5, 2.13 and 2.16.

The main input data to part 2 of the model is:• Power (from part 1)• Control strategy• Configuration of driveline

The control strategy defines how much power the flywheel shall demandfrom the battery. In these simulations the battery is assumed to deliver a con-stant amount of power which equals the average power needed for the com-plete drive cycle. This control strategy is the ideal for the battery though verydifficult to achieve in real life as it requires the average power for the com-plete drive cycle which of course is not known in advance, except for in thelab. However, there are ways of controlling the battery power quite close toconstant power with modest and slow power changes.

The configuration of driveline refers to which parts the driveline is built upwith. Two drivelines are simulated, one with a flywheel (figure 3.1) and onewithout a flywheel (figure 3.2).

Figure 3.1: Model of the driveline with a flywheel. Positive power direction is fromthe battery to the traction motor.

The main output data from part 2 of the model is:• Power flux in every part of the driveline• Power loss in every part of the driveline• Energy stored in battery, flywheel and vehicle

26

Figure 3.2: Model of the driveline without a flywheel. Positive power direction is fromthe battery to the traction motor.

In the simulations of this thesis the power loss in the driveline componentsare assumed to be directly proportional to the input power, calculated as apercentage of the input power.

27

4. Results and discussion

In this section the main results from the papers included in the thesis are pre-sented and discussed.

4.1 Verification of conceptThe concept of an electric driveline with a double-wound two voltage levelflywheel is presented and verified in paper II. Paper III shows, in both simula-tions and experiments, that the flywheel smoothens the input power comparedto the output power with robust controlling and a low level of harmonic con-tent of both voltage and current. Paper I presents an axial-flux electric machinewith low idle losses and a reduction of air resistance losses for a flywheel ro-tating in a low pressure chamber.

4.2 Benefits of a flywheel electric drivelinePaper V demonstrates the benefits of introducing a flywheel into an electricdriveline. The comparison is made between two drivelines, one with a fly-wheel and one without a flywheel (see figures 3.1 and 3.2 on page 26), for twodifferent drive cycles, the NEDC and the FTP 75 (see figures 2.2 and 2.3 onpage 23).

The maximum power flux from the battery, and thereby also the peak cur-rents, decreases by 91% for both drive cycles in the flywheel driveline, from47.8 kW and 208 A to 4.49 kW and 19.5 A for the NEDC and from 42.1 kWand 183 A to 3.63 kW and 15.8 A for the FTP 75 (assuming a battery voltageof 230 V DC). The resistive losses in the battery decreases by 70%, from 404to 119 Wh for the NEDC and from 454 to 138 Wh for the FTP 75. The totallosses in the complete driveline are lower in the flywheel driveline, despite thelosses in the components introduced in the flywheel driveline. The decreasein total losses is 18% (133 Wh) for the NEDC and 24% (201 Wh) for the FTP75.

The number of partial charge/discharge cycles decreases from 36 (NEDC)and 118 (FTP 75) without a flywheel to only 1 with a flywheel. The effec-tive flywheel storage capacity (the range of the energy content in the flywheelbetween the minimum and maximum value) is 648 Wh (NEDC) and 312 Wh

29

(FTP 75), giving a total flywheel storage capacity of 864 Wh for the NEDCand 416 Wh for the FTP 751. The power flux in the battery and the flywheel re-spectively in a flywheel driveline for the two drive cycles simulated are shownin figures 4.1 and 4.2. The energy stored in the battery and the flywheel re-spectively for the same flywheel driveline for the two drive cycles simulatedare shown in figures 4.3 and 4.4.

0 200 400 600 800 1000 1200

−10

0

10

20

30

40

50

Time [s]

Pow

er [k

W]

Figure 4.1: Power flux in the battery with flywheel (dashed line) and without flywheel(solid line) for a vehicle performing a NEDC drive cycle. Positive power means thatpower is taken out from the battery. From paper V.

0 200 400 600 800 1000 1200−10

0

10

20

30

40

Time [s]

Pow

er [k

W]

Figure 4.2: Power flux in the battery with flywheel (dashed line) and without flywheel(solid line) for a vehicle performing a FTP 75 drive cycle. Positive values means thatpower is taken out from the battery. From paper V.

Paper IV presents how the energy density of the complete flywheel drive-line is higher than for the same driveline without a driveline. This is due to theshift from high power density batteries (100 Wh/kg) to high energy densitybatteries (150 Wh/kg). Paper IV also gives an example of what this differencein energy density could mean in real-life. The flywheel in this system is as-sumed to weigh 35 kg with an energy storage capacity of 0.5 kWh which is

1The flywheel is assumed to use 75% of its total energy stored, which implies a practical speedrange of half the total speed range (from half maximum speed to maximum speed).

30

0 200 400 600 800 1000 12000.5

1

1.5

2

2.5

Time [s]

Ene

rgy

stor

ed [

kWh]

20

40

60

80

100

120

Spee

d of

veh

icle

[km

/h]

Figure 4.3: Energy in flywheel (solid line) and battery (dashed line) on the left axisand speed of the vehicle on the right axis for a vehicle performing a NEDC drivecycle. From paper V.

0 200 400 600 800 1000 1200 14000.5

1

1.5

2

Time [s]

Ene

rgy

stor

ed [

kWh]

20

40

60

80

100

Spee

d of

veh

icle

[km

/h]

Figure 4.4: Energy in flywheel (solid line) and battery (dashed line) on the left axisand speed of the vehicle on the right axis for a vehicle performing a FTP 75 drivecycle. From paper V.

reasonable for a car. Given a total amount of energy of 20 kWh to be stored inthe car the use of a flywheel together with high energy density batteries wouldreduce the total weight of the propulsion system by 20% (45 kg) compared toa system with high power batteries, capable of supplying the power neededbut with a lower energy density. The energy density of the combined systemwould be 25% higher (113 compared to 90 Wh/kg). These figures are basedon a FTP 75 drive cycle.

The benefits are summarised in table 4.1.

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Table 4.1: Comparison between an all-electric propulsion system with and without aflywheel. From paper V and paper IV

.

No flywheel Flywheel Change

Peak power NEDC 47.8 kW 4.49 kW -91%FTP 75 42.1 kW 3.63 kW -91%

Peak current NEDC 208 A 19.5 A -91%FTP 75 183 A 15.8 A -91%

Battery losses NEDC 404 Wh 119 Wh -70%FTP 75 454 Wh 138 Wh -70%

Total losses NEDC 727 Wh 594 Wh -18%FTP 75 827 Wh 626 Wh -24%

Number of partial charge/ NEDC 36 1 -97%discharge cycles FTP 75 118 1 -99%Total flywheel storage NEDC – 864 –capacity needed FTP 75 – 416 –Total weight FTP 75 222 kg 178 kg -20%Useful energy density FTP 75 90 Wh/kg 113 Wh/kg +25%

4.3 Comparison between two models describing a partof the flywheel electric drivelineIn paper VII two models for simulating the high voltage side of the drivelinehave been compared for a short (5 s) braking drive cycle (see to the left infigure 1.1 on page 12 and to the left in figure 3.1 on page 26). The first model,implemented in Simulink, simulates every component on a high level of detail.The other model, implemented in Matlab by the author, was described earlierin the thesis. This model does not go into the detailed component level but hasthe advantage of being able to simulate drive cycles of several hours for thecomplete driveline.

Results show very good agreement between the two models, giving concor-dance correlation coefficients (rc) [23] between 0.956 and 0.989 for the threesimulated variables, voltage level in capacitor (figure 4.5), speed of tractionmotor (figure 4.6) and speed of flywheel (figure 4.7). However, the Matlabmodel consequently simulates the capacitor voltage in the first AC/DC/ACconverter to be higher and the speeds of the traction motor and the flywheel tobe lower than for the Simulink model. The discrepancies are probably due tothe different levels of complexity of the two models.

Paper VII also indicates that regenerative braking can be performed withvery high efficiency (92%) utilising the flywheel’s great power capacity.

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5250260270280290300310

Time [s]

DC

vol

tage

[V

]

Figure 4.5: DC voltage in AC/DC/AC converter simulated with Matlab model (solidline) and Simulink model (dashed line). The concordance correlation coefficient rc is0.956. From paper VII.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5300

400

500

600

Time [s]

Rot

atio

nal s

peed

[rp

m]

Figure 4.6: Rotational speed of traction motor in AC/DC/AC converter simulated withMatlab model (solid line) and Simulink model (dashed line). The concordance corre-lation coefficient rc is 0.989. From paper VII.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 54700

4800

4900

5000

Time [s]

Rot

atio

nal s

peed

[rp

m]

Figure 4.7: Rotational speed of flywheel in AC/DC/AC converter simulated with Mat-lab model (solid line) and Simulink model (dashed line). The concordance correlationcoefficient rc is 0.970. From paper VII.

4.4 Flywheel as power handling device for renewableenergy sourcesThe potential role of a flywheel as power handling device in the electric gridhas been investigated in paper VI. Simulations show a great increase in thesmoothness of the power output from intermittent energy sources such aswave, wind and current power (see figure 4.8). However, the flywheel sizeneeded to achieve such an enhancement is considerable. The benefits for a lin-

33

ear wave power generator are greater than for wind and current power whichalready generate a sinusoidally shaped voltage.

Figure 4.8: Photos of wave, wind and current power generators developed at the Divi-sion of electricity, Uppsala university. a) one of the wave energy converters installedat the Lysekil research site. b) A 12 kW vertical axis wind turbine in Marsta outsideUppsala. c) Laboratory marine current generator. From paper VI.

The peak power from a wave power plant is reduced by a factor of tenusing even a quite small flywheel (50 kJ effective energy storage), as shownin figure 4.9 and 4.10. The average power from the wave power generator inthis simulation (VI) is 1.28 kW . Thus this flywheel has an average storagetime of 39 seconds while a 500 kJ flywheel has an average storage time of390 s. However the plateau of the red line in figure 4.10 after approximately6 minutes of simulation indicates that the 50 kJ flywheel is too small to havethe ability to control the output power. Calculations show that a flywheel witha storage capacity of 104 kJ would be the smallest possible to maintain thepower outtake controllability.

0 5 10 15 20 25 30

5

10

15

20

Time [min]

Pow

er [

kW]

Figure 4.9: Power flux from a wave power plant with no flywheel (black line), with a50 kJ flywheel (red line) and with a 500 kJ flywheel (blue line). From paper VI.

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0 5 10 15 20 25 300.5

1

1.5

2

2.5

Time [min]

Pow

er [

kW]

Figure 4.10: Detail of figure 4.9. Power flux from a wave power plant with a 50 kJflywheel (red line) and with a 500 kJ flywheel (blue line). The average power flux isshown by the black line. From paper VI.

35

5. Conclusion

The papers presented in this thesis show that the concept of introducing a fly-wheel in to a conventional electric driveline has many potential advantages.The amount of batteries needed decreases significantly which means a lot interms of cost but also weight and volume. The battery losses decrease mak-ing cooling systems cheaper, lighter and smaller. The propulsion system ishighly likely to be more energy efficient with a flywheel than without one.The weight, volume and cost introduced to the vehicle by the flywheel is by farcompensated by the advantages the flywheel brings to the system. To concludethe flywheel based all-electric propulsion system have a set of advantages thatcan be summarized as follows:

• Battery can be optimized for energy density since the flywheel care for thepower handling. This leads to a smaller amount of batteries needed• Battery efficiency increases since their charge and discharge power de-

creases• Battery lifetime is longer since the number of charge and discharge cycles

decreases drastically, especially in urban driving• Battery can be optimized for discharging since charging normally does not

occur during driving. Charging between driving occasions are made in asmooth and controlled way• High efficiency leads to low energy loss and less need for cooling• Vehicle performance enhances due to higher power capacity• Regenerative braking is efficiently accomplished by the flywheel• Fast charging is possible however limited by the size of the flywheel• Operation with flywheel as sole energy source is possible• Allows a robust all-electric propulsion system with few mechanical parts• Can be used with any kind of fuel, both conventional and future, as long as

the main energy transmission is electric

Two models describing the high voltage/high peak power side of the driv-eline, one complex and detailed Simulink model and one much more unem-bellished in Matlab have been implemented. The two models are matched al-lowing smooth and seamless transition between detailed small-scale and longlarge-scale simulations.

Flywheels can also be beneficial for stationary use, i.e. connected to theelectric grid to smoothen the power output from an intermittent energy source.

37

However, the size and cost of the flywheel needed to achieve this benefit is tobe taken into account.

38

6. Future work

For the author, future work will consist of further development of the Matlabmodel of the driveline in several ways:• Enhance the possibility of numerous configurations of the driveline for si-

multaneous simulation and comparison• Refine the simulation of the losses in all parts of the driveline• Introduce the possibility of numerous control systems of the driveline for

simultaneous simulation and comparison• Analyze the model statistically to find which parts of the driveline to focus

on for highest rate of optimization

The future work also includes the performance of various simulations:• Quantify the benefits for the battery introducing a flywheel into the electric

driveline, shown in this thesis• Simulate different configurations and compare them• Simulate different control strategies and compare them• Compare the simulation results with experimental data from the bench test

set-up

For the flywheel research group as a whole the future work consists of final-ising the current test set-up based on the third flywheel prototype, described inpaper IV, perform measurements during realistic drive conditions and eventu-ally place the driveline in a vehicle.

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7. Summary of papers

Paper ILosses in axial-flux permanent-magnet coreless flywheel energy storagesystemsThis reviewed conference paper describes the design and construction of asmall-scale axial-flux permanent-magnet motor. Mechanical and electricallosses are investigated by a spin-down test, with a emphasis on eddy currentlosses in the windings which are the main losses in a coreless flywheel.Measurements show good agreement with theory.

The author participated in the construction and measurements for the paperand contributed to the writing.

Published in proceedings of the 18th International conference on electricalmachines, ICEM-08, in Vilamoura, Portugal, 6-9 September 2008.

Paper IIDesign parameters calculation of a novel driveline for electric vehiclesThis paper presents the electric driveline based on a flywheel with two voltagelevels and discusses the basic features and advantages of the driveline. Mag-netic bearings, motor control and the main design parameters and equivalentcircuit of a two voltage level machine are discussed. Finally results from testsmade on a scaled test bench set-up are presented.

The author’s contribution to the paper was the drive cycle simulations andthe associated writing.

Published in World Electric Vehicle Journal Vol. 3. Presented as poster byauthor in the 24th International battery, hybrid and fuel cell electric vehiclesymposium and exhibition, EVS-24, in Stavanger, Norway, 13-16 May 2009.

Paper IIIA double wound flywheel system under standard drive cycles: Simula-tions and experimentsIn this paper the functionality of the electric driveline is investigated. Dif-ferent drive cycles are applied and the response of the speed control system

41

is simulated and measured in the experimental set-up. The results prove thefunctionality of the system.

The author contributed to the paper by making the drive cycle simulationsand some writing.

Published in International Journal of Emerging Electric Power Systems,Vol. 11, Issue 4, 2010.

Paper IVPrototype of electric driveline with magnetically levitated double woundmotorThis reviewed conference paper describes the complete bench test set-up ofa flywheel-based electric driveline under construction. The driveline consistsof lead-acid batteries, a DC/AC converter, a double wound, magnetically lev-itated electric machine, an AC/DC/AC converter and two motors. The formeracting as the vehicle’s motor and the latter working as a load. The paper alsoquantifies the benefits of introducing a flywheel in to an electric driveline.

The author’s contribution to the paper were the drive cycle simulations andsome of the analysis and writing.

Published in proceedings of the 19th International conference on electricalmachines, ICEM-10, in Rome, Italy, 6-8 September 2010.

Paper VFlywheel as power handling device in electric and hybrid vehiclesIn this paper the flywheel-based all-electric propulsion system is studied froma system perspective. Simulations are made where two drivelines are com-pared - one with a flywheel and one without a flywheel. The simulations showthe advantages of the flywheel based driveline, especially from a battery per-spective. The maximum power fluxes as well as the maximum battery currentsare decreased by more than an order of magnitude. The number of partialcharge and discharge cycles decreases to one as the batteries are not exposedto the fluctuating powers at the driving wheels.

The author performed all the simulations and analysis for this paper andmade most of the writing and preparation.

Submitted to International Journal of Vehicular Technology, September2010.

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Paper VIDynamic stability of an electricity generation system based on renewableenergyThis reviewed conference paper presents the flywheel as a power handlingdevice in renewable electricity production. Renewable power sources tend tobe intermittent with stochastic variations in power production. A flywheel canbe connected to one or several production units to smooth the output powerto the grid. Simulations show that the power quality could greatly improvewith a flywheel as the energy to be stored would be small. For longer storagetimes and larger amounts of energy the benefit of a flywheel would be lesssignificant.

The author made most of the simulations and the main part of the writingof the paper.

Accepted to 21st International Conference on Electricity Distribution,CIRED 2011, in Frankfurt, Germany, 6-10 June 2011. Presented by author,orally and as poster.

Paper VIIPower balance control in an AC/DC/AC converter for regenerative brak-ing in a flywheel based drivelineThis paper is focused on the function of regenerative braking in the proposeddriveline. Two models describing the driveline between the traction motor andthe flywheel, one quite simple in Matlab and one more detailed in Simulink,are simulating a drive cycle consisting of 5 s of braking, i.e. power is transmit-ted from the traction motor to the flywheel. Exactly the same power is takenout from the traction motor as is put in to the flywheel, leaving the capacitorin the AC/DC/AC converter to supply power for the losses. The one way ef-ficiency of the regenerative braking system is around 92%. The two differentmodels show a very good concordance, simulating the capacitor voltage, theflywheel speed and the wheel machine speed.

The author made some of the simulations and contributed to the analysis ofthe results as well as the writing of the paper.

Submitted to International Journal of Vehicular Technology, May 2011.

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8. Sammanfattning på svenska

Några av mänsklighetens största utmaningar finns inom transport- och ener-gisektorn. Utmaningarna består framför allt av miljöproblem förknippade medutsläpp av fossil koldioxid, kväve och andra mer eller mindre okända ämnensom bildas vid förbränning. Effektiv energilagring skulle dock avsevärt kunnaminska många av dessa problem.

Författarens forskning är inriktad på systemaspekter på energilagring inomtransportsektorn, närmare bestämt på ett svänghjul som effekthanterare i etteldrivet fordon. Ett stort problem för dagens batterier, som utgör den hu-vudsakliga energilagringen i elfordon, är de stora effektflödena som måstehanteras vid både acceleration och inbromsning. Lösningen är att introduceraett svänghjul i drivlinan som tar hand om dessa stora effektvariationer ochlåter batterierna få betydligt gynnsammare förutsättningar med en låg och rel-ativt konstant belastning på mindre än en tiondel av den maxeffekt de skullebehöva leverera utan svänghjul.

Ett svänghjulssystem med en vikt av ca 35 kg och en energilagringska-pacitet på ca 0,5 kWh är rimligt för en vanlig personbil. Den viktökningenkompenseras mer än väl av att batterierna kan göras mycket energitätareoch därmed väga mindre per lagrad kWh. För en personbil med enenergilagringskapacitet på 20 kWh (som räcker till 15 mils landsvägskörningeller minst det dubbla i stadskörning) är den totala vikten för batterier ochsvänghjul och svänghjulssystem 20% lägre än motsvarande vikt för barabatterier utan svänghjul. Energitätheten är samtidigt 25% högre för drivlinanmed svänghjul.

Dessutom minskar förlusterna i batterierna med ca 70% vilket inte baraminskar energiförbrukningen utan också minskar kylbehovet av batterierna.De totala förlusterna för hela drivlinan minskar med ca 20%. Antalet upp- ochurladdningscykler minskar drastiskt, från flera i minuten till endast en långurladdning. Detta, tillsammans med de i övrigt gynnsamma förhållandena förbatterierna, leder till att batteriernas livslängd ökar.

Kontentan av att introducera ett svänghjul i den elektriska drivlinan är alltsåatt mängden batterier kan reduceras avsevärt samt att de håller längre. Dettamedför lägre kostnad, mindre vikt och volym samt därmed också mindremiljöpåverkan. Dessutom minskar de totala förlusterna i systemet vilket min-skar energiförbrukningen per mil och kylbehovet.

Dessa resultat har uppnåtts genom ett av författaren konstrueratmodellerings- och simuleringsprogram i Matlab där två elektriska drivlinor,

45

en med och en utan svänghjul, har modellerats, simulerats och jämförts förtvå olika standardiserade körcykler.

Samma funktion som för elfordon kan svänghjulet ha också på elnätet föratt jämna ut effekten från förnybara elproduktionsanläggningar såsom våg-,vind- eller strömkraftverk, vilket också har visats i simuleringar. Därmed kanockså dessa anläggningar användas effektivare och bli mer konkurrenskraftigasamtidigt som den höga elkvaliteten på elnätet kan upprätthållas.

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9. Acknowledgements

I would first like to thank my supervisor Hans and my co-supervisor Matsfor taking the initiative to this exciting project and for guiding me and mycolleagues through it.

Thanks to my project co-workers, Janaína, Juan, Johan and Magnus. Youare always willing to help and encourage my work - but also to discuss lifeover a cup of wiener melange!

I would also like to thank all lovely friends and colleagues at the Divisionof electricity that make all time at work a pleasure... well, almost all time!Magnus and Daniel are especially thanked for nice lunches and discussionsabout things quite far away from working matters.

Thanks to Gunnel, Elin, Thomas, Ulf, Ingrid and Maria who guide methrough the administrative work.

Thanks to all of you introducing me to and helping me in the mysteriousworld of LaTeX. I owe you lifetime support to MS Office 2007!

Thanks to Hans, Janaína and my father Tomas who proof-read, questionedand commented the thesis. Your comments were appreciated and valuable.

Finally, thanks to my beloved family - Maja, Albin, Pelle, Tea and Anna -who certainly make me feel alive 24/7!

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References

[1] Intergovernmental Panel on Climate Change. Third assessment report - climatechange 2001 - the scientific basis. Technical report, GRID-Arendal, 2003.

[2] Mark A. Sutton, Clare M. Howard, Jan Willem Erisman, Gilles Billen, AlbertBleeker, Peringe Grennfelt, Hans van Grinsven, and Bruna Grizzetti, editors.The European Nitrogen Assessment. Cambridge University Press, 2011.

[3] Kjell Aleklett, Mikael Höök, Kristoffer Jakobsson, Michael Lardelli, SimonSnowden, and Bengt Söderbergh. The peak of the oil age - analyzing the worldoil production reference scenario in world energy outlook 2008. Energy Policy,38:1398–1414, 2010.

[4] Paul P. Acarnley, Barrie C. Mecrow, James S. Burdess, J. Neville Fawcett,James G. Kelly, and Philip G. Dickinson. Design principles for a flywheel en-ergy store for road vehicles. IEEE Transactions on Industry Applications, 32(6),1996.

[5] Juan de Santiago Ochoa. AFPM motor/generator flywheel for electric powerstabilization. Licentiate thesis, Uppsala university, 2009.

[6] Janaína Goncalves de Oliveira. Power control systems for PM synchronousflywheel alternators. Licentiate thesis, Uppsala university, 2009.

[7] Mats Leijon, Hans Bernhoff, and Björn Bolund. U.S. patent 7768176, 2007.

[8] Björn Bolund, Hans Bernhoff, and Mats Leijon. Flywheel energy and powerstorage systems. Renewable and Sustainable Energy Reviews, 11:235–258,2007.

[9] C.S. Hearn, M.M. Flynn, M.C. Lewis, R.C. Thompson, B.T. Murphy, and R.G.Longoria. Low cost flywheel energy storage for a fuel cell powered transit bus.In Vehicle Power and Propulsion Conference, Sept 9-12, 2007. VPPC 2007.IEEE, 2007.

[10] D. Cross and J. Hilton. High speed flywheel based hybrid systems for low carbonvehicles. Technical report, Flybrid Systems LLP.

[11] O. Briat, J.M. Vinassa, W.Lajnef, S. Azzopardi, and E. Woirgard. Principle, de-sign and experimental validation of a flywheel-battery hybrid source for heavy-duty electric vehicles. IET Electric Power Applications, 1:665–674, 2007.

[12] Juan Dixon. Energy storage for electric vehicles. In IEEE International Confer-ence on Industrial Technology (ICIT), 2010.

49

[13] Andrew F. Burke. Batteries and ultracapacitors for electric, hybrid, and fuel cellvehicles. Proceedings of the IEEE, 95(4), April 2007.

[14] Johan Abrahamsson and Hans Bernhoff. Magnetic bearings in kinetic energystorage systems. submitted to Journal of Electrical Systems, 2011.

[15] J. van Mierlo, G. Maggetto, and P. Lataire. Which energy source for road trans-port in the future? A comparison of battery, hybrid and fuel cell vehicles. EnergyConversion and Management, 47:2748–2760, 2006.

[16] Concawe/EUCar. Tank-to-wheels report, version 3. Technical report, October2008.

[17] Jeremy Hackney and Richard de Neufville. Life cycle model of alternative fuelvehicles: emissions, energy, and cost trade-offs. Transportation Research PartA, 35:243–266, 2001.

[18] Bengt Johansson and Max Åhman. A comparison of technologies for carbon-neutral passenger transport. Transportation Research Part D, pages 175–196,2002.

[19] Georgios Fontaras, Panayotis Pistikopoulos, and Zissis Samaras. Experimen-tal evaluation of hybrid vehicle fuel economy and pollutant emissions overreal-world simulation driving cycles. Atmospheric Environment, 42:4023–4035,2008.

[20] European Council. Directive 70/220/eec, 1970.

[21] U.S. Environmental Protection Agency. http://www.epa.gov/nvfel/methods/ftpcol.txt.

[22] Karl Ropkins, Joe Beebe, Hu Li, Basil Daham, James Tate, Margaret Bell, andGordon Andrews. Real-world vehicle exhaust emissions monitoring: Reviewand critical discussion. Critical Reviews in Environmental Science and Technol-ogy, 39:79–152, 2009.

[23] Lawrence I-Kuei Lin. A concordance correlation coefficient to evaluate repro-ducibility. Biometrics, 45(1):255–268, Mars 1989.

[24] Mehrdad Ehsani, Yimin Gao, Sebastien E. Gay, and Ali Emadi. Modern Elec-tric, Hybrid Electric, and Fuel Cell Vehicles. CRC Press, 2005.

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