Flywheel in an all-electric propulsion system
JOHAN LUNDIN
Licentiate thesis
UURIE: 328-11L
ISSN: 0349-8352
Abstract
Energy storage is a crucial condition for both transportation purposes and for
the use of electricity. Flywheels can be used as actual energy storage but also
as power handling device. Their high power capacity compared to other means
of storing electric energy makes them very convenient for smoothing power
transients. These occur frequently in vehicles but also in the electric grid. In
both these areas there is a lot to gain by reducing the power transients and
irregularities.
The research conducted at Uppsala university and described in this thesis is
focused on an all-electric propulsion system based on an electric flywheel with
double stator windings. The flywheel is inserted in between the main energy
storage (assumed to be a battery) and the traction motor in an electric vehicle.
This system has been evaluated by simulations in a Matlab model, comparing
two otherwise identical drivelines, one with and one without a flywheel.
The flywheel is shown to have several advantages for an all-electric propulsion system for a vehicle. The maximum power from the battery decreases
more than ten times as the flywheel absorbs and supplies all the high power
fluxes occuring at acceleration and braking. The battery delivers a low and
almost constant power to the flywheel. The amount of batteries needed decreases whereas the battery lifetime and efficiency increases. Another benefit
the flywheel configuration brings is a higher energy efficiency and hence less
need for cooling.
The model has also been used to evaluate the flywheel functionality for
an electric grid application. The power from renewable intermittent energy
sources such as wave, wind and current power can be smoothened by the flywheel, making these energy sources more efficient and thereby competitive
with a remaining high power quality in the electric grid.
to instant coffee
List of papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals.
I
II
III
IV
V
VI
VII
J. Santiago, J. G. Oliveira, J. Lundin, A. Larsson, H. Bernhoff (2008)
Losses in Axial-Flux Permanent-Magnet Coreless Flywheel Energy
Storage Systems. Published in proceedings of the 18th International
Conference on Electrical Machines (ICEM-08), Vilamoura, Portugal,
6-9 September, 2008, paper ID 910.
J. Santiago, J. G. Oliveira, J. Lundin, J. Abrahamsson, A. Larsson, H.
Bernhoff (2009) Design Parameters Calculation of a Novel Driveline
for Electric Vehicles. Published in World Electric Vehicle Journal Vol.
3.
J. G. Oliveira, J. Lundin, J. Santiago, H. Bernhoff (2010) A Double
Wound Flywheel System under Standard Drive Cycles: Simulations and
Experiments. Published in International Journal of Emerging Electric
Power Systems.
J. Abrahamsson, J. Santiago, J. G. Oliveira, J. Lundin, H. Bernhoff
(2010) Prototype of electric driveline with magnetically levitated
double wound motor. Published in proceedings of the 19th International
Conference on Electrical machines (ICEM-19), Rome, Italy, 6-8
September 2010.
J. Lundin, H. Bernhoff (2010) Flywheel as Power Handling Device
in Electric and Hybrid Vehicles. Submitted to International Journal of
Vehicular Technology, September 2010.
J. Lundin, J. G. Oliveira, C. Boström, K. Yuen, J. Kjellin, M. Rahm,
H. Bernhoff, M. Leijon (2011) Dynamic stability of an electricity generation system based on renewable energy. Accepted for publication in
proceedings of the 21st International Conference on Electricity Distribution (CIRED 2011), Frankfurt, Germany, 6-10 June 2011.
J. G. Oliveira, J. Lundin, H. Bernhoff (2011) Power balance control in
an AC/DC/AC converter for regenerative braking in a flywheel based
drive line system. Submitted to International Journal of Vehicular Technology, May 2011.
Reprints were made with permission from the publishers.
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 The flywheel project at Uppsala university . . . . . . . . . . . . . . . .
1.2 History of flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 The electric grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 This thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Vehicle movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Outer losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Inner losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Conversion between kinetic and electric energy - The electric
machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Drive cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1 General description of the model . . . . . . . . . . . . . . . . . . . . . . .
3.2 Part 1 of model - Calculation of power needed for vehicle . . . .
3.3 Part 2 of model - Simulation of power fluxes in driveline . . . . .
4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Verification of concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Benefits of a flywheel electric driveline . . . . . . . . . . . . . . . . . .
4.3 Comparison between two models describing a part of the flywheel electric driveline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Flywheel as power handling device for renewable energy sources
5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Sammanfattning på svenska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
11
12
13
14
15
17
17
18
20
21
21
22
25
25
25
26
29
29
29
31
33
37
39
41
45
47
49
Nomenclature
Ef
Ev
J
ω
mf
mv
mw
ρf
ρair
r
ro
ri
h
Ptot
Pacc
Prot
Pair
Prr
Ps
v
a
n
g
ϕ
Av
Cw
Crr
[J = kgm2 /s2 ]
[J = kgm2 /s2 ]
[kgm2 ]
[rad/s]
[kg]
[kg]
[kg]
[kg/m3 ]
[1.204 kg/m3 ]
[m]
[m]
[m]
[m]
[W]
[W]
[W]
[W]
[W]
[W]
[m/s]
[m/s2 ]
[-]
[9.81 m/s2 ]
[-]
[m2 ]
[-]
[-]
Energy in flywheel
Energy in vehicle
Moment of inerta
Angular velocity (or rotational speed)
Mass of flywheel
Curb mass of vehicle
Mass of wheel
Density of rotating part of flywheel
Density of air
Radius
Outer radius
Inner radius
Height/thickness of the flywheel
Total power required for vehicle
Linear acceleration power
Rotational acceleration power
Air resistance power
Rolling resistance power
Slope power
Speed of vehicle
Acceleration of vehicle
Number of wheels
Gravitational acceleration
Slope of road in degrees
Frontal area of vehicle
Air drag coefficient
Rolling resistance coefficient
9
I
U
R
P
ε
εrms
ΦB
t
N
B
Ac
10
[A]
[V]
[Ω]
[W]
[V]
[V]
[Wb = Vs]
[s]
[-]
[T = Wb/m2 ]
[m2 ]
Current
Voltage
Resistance
Electric power
Induced electromotive force (or back EMF)
rms-value of back EMF
Magnetic flux
Time
Number of coils exposed to the same magnetic flux variation
Magnetic field
Area of the closed circuit in the magnetic field
1. Introduction
Several of humanity’s great challenges are related to transportation and electricity generation. Energy storage is vital for both these sectors. It is crucial
for vehicles to be able to go long distances non-stop, and for the electric grid
to make it possible to supply, at every instant of time, exactly the amount of
electricity demanded by millions of users connected to the grid.
The environmental problems associated with these sectors are unfortunately
severe. The delicate conditions for life that all flora and fauna slowly and
continuously adapt to are threatened by immense emissions of carbon that
has been out of the natural carbon cycle for millions of years [1], nitrogen
that the engine of a vehicle or the combustion chamber of a coal power plant
transforms from stable and non-reactive N2 , inaccessible to most natural processes, to the reactive forms N2 O or NO2 [2] and a variety of numerous small
particles created in the combustion process about whose impact on environment and health we now little or nothing. Additionally oil, the fuel driving the
transportation sector, is running out [3].
Effective energy storage is a way to drastically reduce many of the negative impacts of transportation and electricity generation. Flywheels can have
a great importance in both these sectors, providing an opportunity to store energy and thereby smoothen the power. Both the transportation and the energy
sectors are huge - not to say the least. Thus, there is a huge economic impact
potential and at the same time the possibility of making the world a little better
place.
1.1
The flywheel project at Uppsala university
"The flywheel research group" at the Division of electricity, Uppsala university, is conducting research on an all-electric propulsion system with an electric flywheel as the central part. The main application is power smoothing in
vehicles. The flywheel is placed in between the battery, which is assumed to
be the main energy storage, and the traction motor in the driveline. The idea
is that the sum of energy stored in the flywheel and the kinetic energy of the
car should be relatively constant. This arrangement smoothens the power consumed or produced by the traction motor and thus protect the battery from experiencing the short but large power transients typical for vehicles. The main
11
components and connections of a flywheel all-electric propulsion system are
shown in figure 1.1.
Another application for energy storage and power smoothing is the electric
grid where the flywheel would have the same power smoothing role as in a
vehicle.
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, see
for example [4]. However, unlike flywheels previously studied, the flywheel
examined in this thesis is equipped with double stator windings, as seen in
figure 1.2. This allows it to simultaneously and independently charge or discharge both the traction motor and the energy storage at two different power
and voltage levels. The two winding configuration also insulates the traction
motor and the battery electrically from each other, acting like a protection
barrier against voltage and current transients and overtones.
The project has so far resulted in numerous papers and two licentiate theses, [5, 6]. The results are intended to be commercialized within the spin-off
company Electric Line which is the owner of a worldwide patent for the double stator winding flywheel [7].
1.2
History of flywheel
The history of flywheel goes back thousands of years. The potter’s wheel and
the spinning wheel are two examples where the flywheel, with its inertia, has
converted a pulsating input power to a smooth output power. Flywheels can
also be used in the opposite way, by converting a smooth input to an irregular
output. Hydro power generators are examples of this.
12
Figure 1.2: A photo of the second prototype flywheel, described in paper II. The three
phase configuration of the winding is visible since each phase have its own colour. The
difference in number of turns for the both sets of windings can not be seen since the
colour 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 with
internal combustion engine, mounted on the outgoing engine shaft to suppress the explosion impulses from the piston in each valve during combustion stroke. Another common flywheel application is as UPS (uninterruptible
power supply) to prevent sensitive equipment or processes to be damaged by
a power failure. For an overview of applications, see [8–11].
1.3
Energy storage
There are many different ways of storing energy, but few are suitable for mobile applications [12,13]. Basically the options for electric1 energy storage for
vehicles 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 available
is its almost unlimited lifetime, in terms of cycles. It can also be produced in
a harmless way by abundant materials easy to recycle. The flywheel does not
produce any hazardous gases, radiation or other potentially dangerous emissions.
1 Electric
energy storage is here defined as both input and output energy being in form of electricity; the storage itself could be in any form
13
Table 1.1: Comparison between four different ways of storing electric energy for mobile use. The advantages listed in the table are the advantages for the storage types in
the rows are compared to the storage types in the columns.
Flywheel
Battery
Ultracapacitor
Fuel cell
Flywheel
—
Battery
-Energy density
-Storage time
-No moving
parts
-Energy density
-Power density
-Lifetime
-Environment
-
-Power density
-Efficiency
-Materials used
-Efficiency
-Power density
-Energy storage
-Cost
-Lifetime
-Energy density
-Storage time
—
-Energy storage
-Energy density
—
Ultracapacitor
Fuel cell
-Power density
Even though the energy density of a flywheel is lower than that of batteries - a complete flywheel system commercially available today would have
an energy density of around 20 Wh/kg - there are no fundamental limits to
significant increase. For example a new material with three times higher tensile strength would allow a three times higher rotational speed, increasing the
energy density for the complete flywheel system by nine times - and so even
the best batteries would be left behind.
The drawback for the flywheel is mainly the self discharge time which is - at
its very best - ten times higher than that of batteries, but more likely a thousand
times higher [10]. For long time energy storage the flywheel is therefore not a
good option at the time being. But low pressure operation (described in paper
I) and magnetic bearings [14] can change the competitiveness for flywheels as
an option even for long time energy storage.
Batteries are almost four times more energy efficient than fuel cells, comparing the round-turn efficiency from electricity to electricity. The starting
point would be the production of electricity in a power plant and the final
point would be electricity available for the vehicle’s traction motor. While batteries have a round-turn efficiency of about 80%, fuels cells have a round-turn
efficiency of only about 20% [15].
1.4
The electric grid
The electric grid is being held at a specific electrical frequency (50 Hz in
Europe) with only small deviations allowed. The voltage is also kept at some
well-defined constant levels (except for loss-related voltage drops), from
400 kV line-to-line voltage in the transmission lines of the backbone grid
to the one phase voltage of 230 V, available in the household sockets. The
14
electrical frequency and voltage of the flywheel though is physically related,
and thereby directly proportional, to the speed of the flywheel. It would
be impossible to vary the energy at the same time as keeping the electrical
frequency and the voltage constant, since the energy content of the flywheel
is regulated by the speed. Thus the flywheel can not be directly connected to
the grid for energy storage. It has to have some power electronics connected
in between to adjust and control both frequency and voltage.
1.5
This thesis
This thesis focuses on the system design of a flywheel based all-electric
propulsion system for efficient energy use in vehicles (papers I-V and paper
VII). It also deals with the positive effects flywheels can have on electric grid
applications (paper VI).
After this chapter, 1. Introduction, the theoretical background to this work
is given in chapter 2. Theory. The theory from chapter 2 is used to create
a simulation model presented in chapter 3. Method. Chapter 4. Results and
discussion presents and discusses the results from the simulations made with
the model described in chapter 3 and chapter 5. Conclusions concludes the
work presented in chapter 4. Some interesting fields of research for the future
are discussed in chapter 6. Future work. The papers constituting this thesis
are summarized in chapter 7. Summary of papers. Finally the thesis is briefly
encapsulated in Swedish in chapter 8. Sammanfattning på svenska.
15
2. Theory
This chapter presents the physics behind flywheels, vehicle motion and the
conversion between kinetic and electric energy in the electric machine. The
chapter also gives a background to drive cycles.
2.1
Flywheel
An electric flywheel is basically an electric machine with some extra weight
on the rotor to increase the moment of inertia. The amount of stored energy
depends on the rotational speed of the flywheel as well as the distribution of
rotating mass (the moment of inertia), given by eq. 2.1. Eq. 2.1 is in fact an
analogy to the energy stored in a translationally moving object, for example
a car, given by eq. 2.5 on page 18. Moment of inertia in circular motion is
somewhat the parallel to mass in linear motion.
Ef =
Jω 2
2
(2.1)
where E f [J] is the energy stored in the flywheel, ω [rad/s] the angular velocity
and J [kgm2 ] the moment of inertia around the principle axis of rotation, given
by eq. 2.2.
Z ro
J=
ri
Z ro
m f (x)xdx = 2πρ f
h(r)r3 dr
(2.2)
ri
where m f [kg] is the mass of the flywheel, ro [m] the outer radius and ri [m] the
inner radius of the flywheel, ρ f [kg/m3 ] the density of the flywheel material
and h [m] the height/thickness of the flywheel, in this equation depending on
the radius.
If the height of the flywheel is constant, i.e. the flywheel has the shape of a
hollow cylinder, the moment of inertia is given by eq. 2.3.
1
J = πρ f h(ro4 − ri4 )
2
(2.3)
In the special case of a thin rim flywheel, the moment of inertia would then
be given by eq. 2.4.
17
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 often
referred to as its form factor) and regulated by its rotational speed. The rotating part of the second prototype flywheel (see figure 1.2 on page 13) has a
moment of inertia of 0.364 kgm2 .
2.2
Vehicle movement
To run a vehicle is physically to overcome a number of losses. Without losses
the car would need no net energy. The energy, Ev [J], stored in a translationally
moving object, such as a car, is given by eq. 2.5.
Ev =
mv v2
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 overcome
two resistance losses to keep the desired speed. Figure 2.1 shows the relation
between the two resistance losses for a standard car (see table 2.1) at different
speeds. Note that this figure does not include any power required for acceleration but shows the losses associated with cruising at a certain speed. The
most important loss at low speed is the rolling resistance loss, as seen in the
figure. At higher speed (above about 60 km/h) the air resistance loss is the
predominant power loss growing fast as it is proportional to the cube of the
speed of the vehicle1 .
With these figures in mind the inefficiency of ordinary internal combustion
engine vehicles is obvious, since they often consume even more fuel at low
speed. Electric vehicles though follow the physically proper power consumption and therefore consume much less power at lower speed than at higher
speed. To exemplify the great difference between the two a standard ICE (internal combustion engine) car with a gasoline2 consumption of 7 l/100 km at
a constant speed of 80 km/h has a power consumption of 51 kW compared to
around 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.
1 However
the energy needed to travel a certain distance is proportional to the square of the
speed - and of course directly to the distance.
2 Thermal energy content of gasoline is around 33 MJ/l.
18
Power [kW]
20
15
10
5
0
0
20
40
60
Speed [km/h]
80
100
120
Figure 2.1: Power needed to overcome the two main resistance losses for a vehicle
travelling on a flat road. The dashed line shows the rolling resistance power loss and
the dotted line the air resistance power loss. The total power loss is shown by the solid
line. For low speeds the rolling resistance losses are the predominant while for high
speeds 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)
Mass of wheel
Air drag coefficient
Rolling resistance coefficient
Frontal area of vehicle
Tyre outer radius
mv
mw
Cw
Crr
A
ro
1181 kg
25 kg each
0.321
0.015
2.1 m2
0.309 m
a) There are slightly different definitions of curb mass but in this thesis it is defined as the total
mass of vehicle including fluids, standard equipment and a 75 kg driver, i.e. everything needed
to propel the vehicle, but no extra load.
Yet, the energy efficiency of the electric car heavily depends on the power
source. For example [17] claims that a battery electric vehicle is 3.56 times
more efficient than a standard ICE vehicle. This is for the US electricity generation mix, consisting of 48% coal-power.
The greatest advantage of electric vehicles over standard ICE vehicles occurs in urban driving, with many accelerations, brakings, starts and stops. In
urban areas the reduction of emissions is also most valuable since the local
health effects of emissions can be more important than the global environmental effects [18, 19].
The physical laws governing the losses during motion of an electric vehicle
can be divided in two main groups; outer losses that define how much power
is needed in the wheels of the vehicle and inner losses that determine how
much power the energy storage will have to supply and where the losses are
occurring and therefore have to be taken care of. In addition to these losses,
19
energy is used to run auxiliary devices such as air condition, lights et.c. which
adds to the total power consumption.
2.2.1
Outer losses
The predominant outer losses are acceleration loss, slope resistance loss, air
resistance loss and rolling resistance loss. Standard drive cycles assume flat
and 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 the
model to enable the use in a real drive pattern. The total power required to
propel 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 acceleration power, Ps [W] the slope power, Pair [W] the air resistance power and
Prr [W] the rolling resistance power.
These five terms can be evolved according to eq. 2.7-2.11.
Pacc = mv · v · a
Prot
ri2
n
1+ 2
= mw · v · a ·
4
ro
Ps = m · v · g · sinϕ
Pair =
1
· Av · v3 · ρair · Cw
2
Prr = m · v · Crr · g
(2.7)
(2.8)
(2.9)
(2.10)
(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 [-] the
slope of the road in degrees, Av [m2 ] the frontal area of the vehicle, Cw [-] the
air drag coefficient and Crr [-] the rolling resistance coefficient.
The first three terms in eq. 2.6, Pacc , Prot and Ps , are not really losses but
rather the conversion rate between two "reusable" forms of energy, for example from chemical energy in the battery to kinetic energy of the vehicle or
20
from kinetic to potential energy of the vehicle. Therefore these three terms
can also be negative, i.e. power is gained. The two last terms though, Pair and
Prr , are losses in the meaning that energy is converted to some kind of useless
energy, for example heat, wind turbulence and wearing of tyres.
2.2.2
Inner losses
For an electric vehicle inner losses consist mainly of electrical losses which
occur in all electric parts where a current is passing through and causes resistive 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 flowing
through the resistance which is directly proportional to the voltage difference
between the points and inversely proportional to the resistance in the load.
The load is not necessarily a useful load. All cables, diodes and other current
conducting apparatus have resistances which causes losses and a continuous
voltage drop over them. Since the electrical power is described by eq. 2.13
these 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, but
mechanical. In the model it is set to a constant value.
I=
U
R
(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] electric
power.
2.3 Conversion between kinetic and electric energy The electric machine
An electric machine converts energy from electric to kinetic (motor) or the
other way around (generator). The principle for both directions of energy conversion 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 of
change of the magnetic flux ΦB [Wb = Vs] through the circuit.
21
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 same
magnetic flux variation, ω [rad/s] is the angular velocity, B [T] is the magnetic
field 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 the
combination of the two directions of rotation (forward and backward) and the
two possible ways to change the rotational direction in time (acceleration or
deceleration). The four modes, seen in table 2.2, can be controlled by two
parameters. The phase shift between voltage and current decides whether the
motor is accelerating or braking, and the sequence of phases decides the direction of rotation.
Table 2.2: Four modes of operation of an electric motor with possible control.
Forward
Backward
2.4
Acceleration
Deceleration
U and I in phase
phase sequence a, b, c
U and I in phase
phase sequence a, c, b
180◦ phase shift between U and I
phase sequence a, b, c
180◦ phase shift between U and I
phase sequence a, c, b
Drive cycles
Drive cycles are used to have a common reference point in various contexts.
Some are standardized and used for compulsory testing before a new vehicle
is allowed to be put on the market. The tested entities are often fuel consumption and emissions of some chemical compounds. The outcome of these tests
provide the basis for decision making for the buyer of the vehicle, and also the
basis for a classification as an "environmentally friendly vehicle" which can
have great impact on the economical conditions such as motor vehicle taxes
and taxable benefit.
A certain drive cycle is often given per time unit, either as acceleration (for
example NEDC - the New European Drive Cycle [20], see figure 2.2) or as
speed (for example the North American FTP 75 [21], see figure 2.3). Practi22
cally the vehicle is tested with a driver that drives the vehicle on a dynamometer 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 a
proper way. There are two main reasons for this. The drive cycles differ too
much from real-world driving behaviour and the tested vehicles are new in
contrary to the average vehicle in traffic. Both these matters significantly affects the fuel consumption and the emissions measured in a lab in comparison
to real-world data. [22] discusses this problem thoroughly.
120
Speed [km/h]
100
80
60
40
20
0
0
200
400
600
Time [s]
800
1000
1200
Figure 2.2: Speed vs time in the European drive cycle NEDC. The total time for the
drive cycle is 1180 s.
Speed [km/h]
100
80
60
40
20
0
0
200
400
600
800
Time [s]
1000
1200
1400
Figure 2.3: Speed vs time in the American drive cycle FTP 75. The total time for the
drive cycle is 1372 s.
23
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 consists of two parts, following each other in time. The simulation is divided into
arbitrary small time steps (typically tenths of a second) and in each of them
all required calculations are made and the results added to an array. The time
steps are equally small in both parts. The model is described in more detail in
paper V.
3.1
General description of the model
In the first part, the power needed in the traction motor is calculated. The
vehicle is considered "a black box" in the sense that only what is happening
outside the vehicle is relevant, i.e. the outer losses.
In the second part the power calculated in the first part is used to simulate
the power fluxes in different parts of the driveline and also the resulting speed
of 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 interesting
combinations of these variables.
3.2 Part 1 of model - Calculation of power needed for
vehicle
The first part of the model calculates the power needed to propel a vehicle
according to a certain drive cycle. For this purpose, eq. 2.6-2.11 from the
theory 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)
25
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 in
driveline
In the second part of the model the simulation of power fluxes in the driveline
starts over from the beginning of the drive cycle with focus on power fluxes inside the vehicle. For these simulations equations used from the theory chapter
are 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 demand
from the battery. In these simulations the battery is assumed to deliver a constant amount of power which equals the average power needed for the complete drive cycle. This control strategy is the ideal for the battery though very
difficult to achieve in real life as it requires the average power for the complete drive cycle which of course is not known in advance, except for in the
lab. However, there are ways of controlling the battery power quite close to
constant power with modest and slow power changes.
The configuration of driveline refers to which parts the driveline is built up
with. Two drivelines are simulated, one with a flywheel (figure 3.1) and one
without a flywheel (figure 3.2).
Figure 3.1: Model of the driveline with a flywheel. Positive power direction is from
the 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 from
the battery to the traction motor.
In the simulations of this thesis the power loss in the driveline components
are assumed to be directly proportional to the input power, calculated as a
percentage of the input power.
27
4. Results and discussion
In this section the main results from the papers included in the thesis are presented and discussed.
4.1
Verification of concept
The concept of an electric driveline with a double-wound two voltage level
flywheel is presented and verified in paper II. Paper III shows, in both simulations and experiments, that the flywheel smoothens the input power compared
to the output power with robust controlling and a low level of harmonic content of both voltage and current. Paper I presents an axial-flux electric machine
with low idle losses and a reduction of air resistance losses for a flywheel rotating in a low pressure chamber.
4.2
Benefits of a flywheel electric driveline
Paper V demonstrates the benefits of introducing a flywheel into an electric
driveline. The comparison is made between two drivelines, one with a flywheel and one without a flywheel (see figures 3.1 and 3.2 on page 26), for two
different drive cycles, the NEDC and the FTP 75 (see figures 2.2 and 2.3 on
page 23).
The maximum power flux from the battery, and thereby also the peak currents, decreases by 91% for both drive cycles in the flywheel driveline, from
47.8 kW and 208 A to 4.49 kW and 19.5 A for the NEDC and from 42.1 kW
and 183 A to 3.63 kW and 15.8 A for the FTP 75 (assuming a battery voltage
of 230 V DC). The resistive losses in the battery decreases by 70%, from 404
to 119 Wh for the NEDC and from 454 to 138 Wh for the FTP 75. The total
losses in the complete driveline are lower in the flywheel driveline, despite the
losses in the components introduced in the flywheel driveline. The decrease
in total losses is 18% (133 Wh) for the NEDC and 24% (201 Wh) for the FTP
75.
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 effective flywheel storage capacity (the range of the energy content in the flywheel
between 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 NEDC
and 416 Wh for the FTP 751 . The power flux in the battery and the flywheel respectively in a flywheel driveline for the two drive cycles simulated are shown
in figures 4.1 and 4.2. The energy stored in the battery and the flywheel respectively for the same flywheel driveline for the two drive cycles simulated
are shown in figures 4.3 and 4.4.
50
Power [kW]
40
30
20
10
0
−10
0
200
400
600
Time [s]
800
1000
1200
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 that
power is taken out from the battery. From paper V.
Power [kW]
40
30
20
10
0
−10
0
200
400
600
800
Time [s]
1000
1200
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 that
power is taken out from the battery. From paper V.
Paper IV presents how the energy density of the complete flywheel driveline is higher than for the same driveline without a driveline. This is due to the
shift from high power density batteries (100 Wh/kg) to high energy density
batteries (150 Wh/kg). Paper IV also gives an example of what this difference
in energy density could mean in real-life. The flywheel in this system is assumed to weigh 35 kg with an energy storage capacity of 0.5 kWh which is
1 The
flywheel is assumed to use 75% of its total energy stored, which implies a practical speed
range of half the total speed range (from half maximum speed to maximum speed).
30
120
100
2
80
1.5
60
40
1
20
0.5
0
200
400
600
Time [s]
800
1000
Speed of vehicle [km/h]
Energy stored [kWh]
2.5
1200
Figure 4.3: Energy in flywheel (solid line) and battery (dashed line) on the left axis
and speed of the vehicle on the right axis for a vehicle performing a NEDC drive
cycle. From paper V.
80
1.5
60
40
1
20
0.5
0
200
400
600
800
Time [s]
1000
1200
Speed of vehicle [km/h]
Energy stored [kWh]
100
2
1400
Figure 4.4: Energy in flywheel (solid line) and battery (dashed line) on the left axis
and speed of the vehicle on the right axis for a vehicle performing a FTP 75 drive
cycle. From paper V.
reasonable for a car. Given a total amount of energy of 20 kWh to be stored in
the car the use of a flywheel together with high energy density batteries would
reduce the total weight of the propulsion system by 20% (45 kg) compared to
a system with high power batteries, capable of supplying the power needed
but with a lower energy density. The energy density of the combined system
would be 25% higher (113 compared to 90 Wh/kg). These figures are based
on a FTP 75 drive cycle.
The benefits are summarised in table 4.1.
31
Table 4.1: Comparison between an all-electric propulsion system with and without a
flywheel. From paper V and paper IV
Peak power
Peak current
Battery losses
. Total losses
Number of partial charge/
discharge cycles
Total flywheel storage
capacity needed
Total weight
Useful energy density
NEDC
FTP 75
NEDC
FTP 75
NEDC
FTP 75
NEDC
FTP 75
NEDC
FTP 75
NEDC
FTP 75
FTP 75
FTP 75
No flywheel
Flywheel
Change
47.8 kW
42.1 kW
208 A
183 A
404 Wh
454 Wh
727 Wh
827 Wh
36
118
–
–
222 kg
90 Wh/kg
4.49 kW
3.63 kW
19.5 A
15.8 A
119 Wh
138 Wh
594 Wh
626 Wh
1
1
864
416
178 kg
113 Wh/kg
-91%
-91%
-91%
-91%
-70%
-70%
-18%
-24%
-97%
-99%
–
–
-20%
+25%
4.3 Comparison between two models describing a part
of the flywheel electric driveline
In paper VII two models for simulating the high voltage side of the driveline
have been compared for a short (5 s) braking drive cycle (see to the left in
figure 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 earlier
in the thesis. This model does not go into the detailed component level but has
the advantage of being able to simulate drive cycles of several hours for the
complete driveline.
Results show very good agreement between the two models, giving concordance correlation coefficients (rc ) [23] between 0.956 and 0.989 for the three
simulated variables, voltage level in capacitor (figure 4.5), speed of traction
motor (figure 4.6) and speed of flywheel (figure 4.7). However, the Matlab
model consequently simulates the capacitor voltage in the first AC/DC/AC
converter to be higher and the speeds of the traction motor and the flywheel to
be lower than for the Simulink model. The discrepancies are probably due to
the different levels of complexity of the two models.
Paper VII also indicates that regenerative braking can be performed with
very high efficiency (92%) utilising the flywheel’s great power capacity.
32
DC voltage [V]
310
300
290
280
270
260
250
0
0.5
1
1.5
2
2.5
Time [s]
3
3.5
4
4.5
5
Rotational speed [rpm]
Figure 4.5: DC voltage in AC/DC/AC converter simulated with Matlab model (solid
line) and Simulink model (dashed line). The concordance correlation coefficient rc is
0.956. From paper VII.
600
500
400
300
0
0.5
1
1.5
2
2.5
Time [s]
3
3.5
4
4.5
5
Rotational speed [rpm]
Figure 4.6: Rotational speed of traction motor in AC/DC/AC converter simulated with
Matlab model (solid line) and Simulink model (dashed line). The concordance correlation coefficient rc is 0.989. From paper VII.
5000
4900
4800
4700
0
0.5
1
1.5
2
2.5
Time [s]
3
3.5
4
4.5
5
Figure 4.7: Rotational speed of flywheel in AC/DC/AC converter simulated with Matlab model (solid line) and Simulink model (dashed line). The concordance correlation
coefficient rc is 0.970. From paper VII.
4.4 Flywheel as power handling device for renewable
energy sources
The potential role of a flywheel as power handling device in the electric grid
has been investigated in paper VI. Simulations show a great increase in the
smoothness of the power output from intermittent energy sources such as
wave, wind and current power (see figure 4.8). However, the flywheel size
needed to achieve such an enhancement is considerable. The benefits for a lin33
ear wave power generator are greater than for wind and current power which
already generate a sinusoidally shaped voltage.
Figure 4.8: Photos of wave, wind and current power generators developed at the Division of electricity, Uppsala university. a) one of the wave energy converters installed
at the Lysekil research site. b) A 12 kW vertical axis wind turbine in Marsta outside
Uppsala. c) Laboratory marine current generator. From paper VI.
The peak power from a wave power plant is reduced by a factor of ten
using even a quite small flywheel (50 kJ effective energy storage), as shown
in figure 4.9 and 4.10. The average power from the wave power generator in
this simulation (VI) is 1.28 kW . Thus this flywheel has an average storage
time of 39 seconds while a 500 kJ flywheel has an average storage time of
390 s. However the plateau of the red line in figure 4.10 after approximately
6 minutes of simulation indicates that the 50 kJ flywheel is too small to have
the ability to control the output power. Calculations show that a flywheel with
a storage capacity of 104 kJ would be the smallest possible to maintain the
power outtake controllability.
20
Power [kW]
15
10
5
0
5
10
15
Time [min]
20
25
30
Figure 4.9: Power flux from a wave power plant with no flywheel (black line), with a
50 kJ flywheel (red line) and with a 500 kJ flywheel (blue line). From paper VI.
34
Power [kW]
2.5
2
1.5
1
0.5
0
5
10
15
Time [min]
20
25
30
Figure 4.10: Detail of figure 4.9. Power flux from a wave power plant with a 50 kJ
flywheel (red line) and with a 500 kJ flywheel (blue line). The average power flux is
shown by the black line. From paper VI.
35
5. Conclusion
The papers presented in this thesis show that the concept of introducing a flywheel in to a conventional electric driveline has many potential advantages.
The amount of batteries needed decreases significantly which means a lot in
terms of cost but also weight and volume. The battery losses decrease making cooling systems cheaper, lighter and smaller. The propulsion system is
highly 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 far
compensated by the advantages the flywheel brings to the system. To conclude
the flywheel based all-electric propulsion system have a set of advantages that
can be summarized as follows:
• Battery can be optimized for energy density since the flywheel care for the
power handling. This leads to a smaller amount of batteries needed
• Battery efficiency increases since their charge and discharge power decreases
• 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 a
smooth 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 driveline, one complex and detailed Simulink model and one much more unembellished in Matlab have been implemented. The two models are matched allowing smooth and seamless transition between detailed small-scale and long
large-scale simulations.
Flywheels can also be beneficial for stationary use, i.e. connected to the
electric 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 to
be taken into account.
38
6. Future work
For the author, future work will consist of further development of the Matlab
model of the driveline in several ways:
• Enhance the possibility of numerous configurations of the driveline for simultaneous 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 finalising the current test set-up based on the third flywheel prototype, described in
paper IV, perform measurements during realistic drive conditions and eventually place the driveline in a vehicle.
39
7. Summary of papers
Paper I
Losses in axial-flux permanent-magnet coreless flywheel energy storage
systems
This reviewed conference paper describes the design and construction of a
small-scale axial-flux permanent-magnet motor. Mechanical and electrical
losses are investigated by a spin-down test, with a emphasis on eddy current
losses 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 paper
and contributed to the writing.
Published in proceedings of the 18th International conference on electrical
machines, ICEM-08, in Vilamoura, Portugal, 6-9 September 2008.
Paper II
Design parameters calculation of a novel driveline for electric vehicles
This paper presents the electric driveline based on a flywheel with two voltage
levels and discusses the basic features and advantages of the driveline. Magnetic bearings, motor control and the main design parameters and equivalent
circuit of a two voltage level machine are discussed. Finally results from tests
made on a scaled test bench set-up are presented.
The author’s contribution to the paper was the drive cycle simulations and
the associated writing.
Published in World Electric Vehicle Journal Vol. 3. Presented as poster by
author in the 24th International battery, hybrid and fuel cell electric vehicle
symposium and exhibition, EVS-24, in Stavanger, Norway, 13-16 May 2009.
Paper III
A double wound flywheel system under standard drive cycles: Simulations and experiments
In this paper the functionality of the electric driveline is investigated. Different 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 the
functionality of the system.
The author contributed to the paper by making the drive cycle simulations
and some writing.
Published in International Journal of Emerging Electric Power Systems,
Vol. 11, Issue 4, 2010.
Paper IV
Prototype of electric driveline with magnetically levitated double wound
motor
This reviewed conference paper describes the complete bench test set-up of
a flywheel-based electric driveline under construction. The driveline consists
of lead-acid batteries, a DC/AC converter, a double wound, magnetically levitated electric machine, an AC/DC/AC converter and two motors. The former
acting as the vehicle’s motor and the latter working as a load. The paper also
quantifies the benefits of introducing a flywheel in to an electric driveline.
The author’s contribution to the paper were the drive cycle simulations and
some of the analysis and writing.
Published in proceedings of the 19th International conference on electrical
machines, ICEM-10, in Rome, Italy, 6-8 September 2010.
Paper V
Flywheel as power handling device in electric and hybrid vehicles
In this paper the flywheel-based all-electric propulsion system is studied from
a system perspective. Simulations are made where two drivelines are compared - one with a flywheel and one without a flywheel. The simulations show
the advantages of the flywheel based driveline, especially from a battery perspective. The maximum power fluxes as well as the maximum battery currents
are decreased by more than an order of magnitude. The number of partial
charge and discharge cycles decreases to one as the batteries are not exposed
to the fluctuating powers at the driving wheels.
The author performed all the simulations and analysis for this paper and
made most of the writing and preparation.
Submitted to International Journal of Vehicular Technology, September
2010.
42
Paper VI
Dynamic stability of an electricity generation system based on renewable
energy
This reviewed conference paper presents the flywheel as a power handling
device in renewable electricity production. Renewable power sources tend to
be intermittent with stochastic variations in power production. A flywheel can
be connected to one or several production units to smooth the output power
to the grid. Simulations show that the power quality could greatly improve
with a flywheel as the energy to be stored would be small. For longer storage
times and larger amounts of energy the benefit of a flywheel would be less
significant.
The author made most of the simulations and the main part of the writing
of 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 VII
Power balance control in an AC/DC/AC converter for regenerative braking in a flywheel based driveline
This paper is focused on the function of regenerative braking in the proposed
driveline. Two models describing the driveline between the traction motor and
the 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 transmitted from the traction motor to the flywheel. Exactly the same power is taken
out from the traction motor as is put in to the flywheel, leaving the capacitor
in the AC/DC/AC converter to supply power for the losses. The one way efficiency of the regenerative braking system is around 92%. The two different
models show a very good concordance, simulating the capacitor voltage, the
flywheel speed and the wheel machine speed.
The author made some of the simulations and contributed to the analysis of
the results as well as the writing of the paper.
Submitted to International Journal of Vehicular Technology, May 2011.
43
8. Sammanfattning på svenska
Några av mänsklighetens största utmaningar finns inom transport- och energisektorn. Utmaningarna består framför allt av miljöproblem förknippade med
utsläpp av fossil koldioxid, kväve och andra mer eller mindre okända ämnen
som bildas vid förbränning. Effektiv energilagring skulle dock avsevärt kunna
minska många av dessa problem.
Författarens forskning är inriktad på systemaspekter på energilagring inom
transportsektorn, närmare bestämt på ett svänghjul som effekthanterare i ett
eldrivet fordon. Ett stort problem för dagens batterier, som utgör den huvudsakliga energilagringen i elfordon, är de stora effektflödena som måste
hanteras vid både acceleration och inbromsning. Lösningen är att introducera
ett svänghjul i drivlinan som tar hand om dessa stora effektvariationer och
låter batterierna få betydligt gynnsammare förutsättningar med en låg och relativt konstant belastning på mindre än en tiondel av den maxeffekt de skulle
behöva leverera utan svänghjul.
Ett svänghjulssystem med en vikt av ca 35 kg och en energilagringskapacitet på ca 0,5 kWh är rimligt för en vanlig personbil. Den viktökningen
kompenseras mer än väl av att batterierna kan göras mycket energitätare
och därmed väga mindre per lagrad kWh. För en personbil med en
energilagringskapacitet på 20 kWh (som räcker till 15 mils landsvägskörning
eller minst det dubbla i stadskörning) är den totala vikten för batterier och
svänghjul och svänghjulssystem 20% lägre än motsvarande vikt för bara
batterier utan svänghjul. Energitätheten är samtidigt 25% högre för drivlinan
med svänghjul.
Dessutom minskar förlusterna i batterierna med ca 70% vilket inte bara
minskar energiförbrukningen utan också minskar kylbehovet av batterierna.
De totala förlusterna för hela drivlinan minskar med ca 20%. Antalet upp- och
urladdningscykler minskar drastiskt, från flera i minuten till endast en lång
urladdning. Detta, tillsammans med de i övrigt gynnsamma förhållandena för
batterierna, leder till att batteriernas livslängd ökar.
Kontentan av att introducera ett svänghjul i den elektriska drivlinan är alltså
att mängden batterier kan reduceras avsevärt samt att de håller längre. Detta
medför lägre kostnad, mindre vikt och volym samt därmed också mindre
miljöpåverkan. Dessutom minskar de totala förlusterna i systemet vilket minskar energiförbrukningen per mil och kylbehovet.
Dessa resultat har uppnåtts genom ett av författaren konstruerat
modellerings- och simuleringsprogram i Matlab där två elektriska drivlinor,
45
en med och en utan svänghjul, har modellerats, simulerats och jämförts för
två olika standardiserade körcykler.
Samma funktion som för elfordon kan svänghjulet ha också på elnätet för
att jämna ut effekten från förnybara elproduktionsanläggningar såsom våg-,
vind- eller strömkraftverk, vilket också har visats i simuleringar. Därmed kan
också dessa anläggningar användas effektivare och bli mer konkurrenskraftiga
samtidigt som den höga elkvaliteten på elnätet kan upprätthållas.
46
9. Acknowledgements
I would first like to thank my supervisor Hans and my co-supervisor Mats
for taking the initiative to this exciting project and for guiding me and my
colleagues through it.
Thanks to my project co-workers, Janaína, Juan, Johan and Magnus. You
are always willing to help and encourage my work - but also to discuss life
over a cup of wiener melange!
I would also like to thank all lovely friends and colleagues at the Division
of electricity that make all time at work a pleasure... well, almost all time!
Magnus and Daniel are especially thanked for nice lunches and discussions
about things quite far away from working matters.
Thanks to Gunnel, Elin, Thomas, Ulf, Ingrid and Maria who guide me
through the administrative work.
Thanks to all of you introducing me to and helping me in the mysterious
world of LaTeX. I owe you lifetime support to MS Office 2007!
Thanks to Hans, Janaína and my father Tomas who proof-read, questioned
and 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!
47
References
[1] Intergovernmental Panel on Climate Change. Third assessment report - climate
change 2001 - the scientific basis. Technical report, GRID-Arendal, 2003.
[2] Mark A. Sutton, Clare M. Howard, Jan Willem Erisman, Gilles Billen, Albert
Bleeker, 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, Simon
Snowden, and Bengt Söderbergh. The peak of the oil age - analyzing the world
oil 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 energy store for road vehicles. IEEE Transactions on Industry Applications, 32(6),
1996.
[5] Juan de Santiago Ochoa. AFPM motor/generator flywheel for electric power
stabilization. Licentiate thesis, Uppsala university, 2009.
[6] Janaína Goncalves de Oliveira. Power control systems for PM synchronous
flywheel 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 power
storage 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 carbon
vehicles. Technical report, Flybrid Systems LLP.
[11] O. Briat, J.M. Vinassa, W.Lajnef, S. Azzopardi, and E. Woirgard. Principle, design and experimental validation of a flywheel-battery hybrid source for heavyduty electric vehicles. IET Electric Power Applications, 1:665–674, 2007.
[12] Juan Dixon. Energy storage for electric vehicles. In IEEE International Conference on Industrial Technology (ICIT), 2010.
49
[13] Andrew F. Burke. Batteries and ultracapacitors for electric, hybrid, and fuel cell
vehicles. Proceedings of the IEEE, 95(4), April 2007.
[14] Johan Abrahamsson and Hans Bernhoff. Magnetic bearings in kinetic energy
storage systems. submitted to Journal of Electrical Systems, 2011.
[15] J. van Mierlo, G. Maggetto, and P. Lataire. Which energy source for road transport in the future? A comparison of battery, hybrid and fuel cell vehicles. Energy
Conversion and Management, 47:2748–2760, 2006.
[16] Concawe/EUCar. Tank-to-wheels report, version 3. Technical report, October
2008.
[17] Jeremy Hackney and Richard de Neufville. Life cycle model of alternative fuel
vehicles: emissions, energy, and cost trade-offs. Transportation Research Part
A, 35:243–266, 2001.
[18] Bengt Johansson and Max Åhman. A comparison of technologies for carbonneutral passenger transport. Transportation Research Part D, pages 175–196,
2002.
[19] Georgios Fontaras, Panayotis Pistikopoulos, and Zissis Samaras. Experimental evaluation of hybrid vehicle fuel economy and pollutant emissions over
real-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, and
Gordon Andrews. Real-world vehicle exhaust emissions monitoring: Review
and critical discussion. Critical Reviews in Environmental Science and Technology, 39:79–152, 2009.
[23] Lawrence I-Kuei Lin. A concordance correlation coefficient to evaluate reproducibility. Biometrics, 45(1):255–268, Mars 1989.
[24] Mehrdad Ehsani, Yimin Gao, Sebastien E. Gay, and Ali Emadi. Modern Electric, Hybrid Electric, and Fuel Cell Vehicles. CRC Press, 2005.
50