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A Review of Flywheel Energy Storage System Technologies and Their Applications

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Energy storage systems (ESS) provide a means for improving the efficiency of electrical systems when there are imbalances between supply and demand. Additionally, they are a key element for improving the stability and quality of electrical networks. They add flexibility into the electrical system by mitigating the supply intermittency, recently made worse by an increased penetration of renewable generation. One energy storage technology now arousing great interest is the flywheel energy storage systems (FESS), since this technology can offer many advantages as an energy storage solution over the alternatives. Flywheels have attributes of a high cycle life, long operational life, high round-trip efficiency, high power density, low environmental impact, and can store megajoule (MJ) levels of energy with no upper limit when configured in banks. This paper presents a critical review of FESS in regards to its main components and applications, an approach not captured in earlier reviews. Additionally, earlier reviews do not include the most recent literature in this fast-moving field. A description of the flywheel structure and its main components is provided, and different types of electric machines, power electronics converter topologies, and bearing systems for use in flywheel storage systems are discussed. The main applications of FESS are explained and commercially available flywheel prototypes for each application are described. The paper concludes with recommendations for future research.
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applied
sciences
Review
A Review of Flywheel Energy Storage System
Technologies and Their Applications
Mustafa E. Amiryar * and Keith R. Pullen *
School of Mathematics, Computer Science and Engineering, University of London, London EC1V 0HB, UK
*Correspondence: mustafa.amiryar.2@city.ac.uk (M.E.A); k.pullen@city.ac.uk (K.R.P.);
Tel.: +44-(0)20-7040-3475 (K.R.P.)
Academic Editor: Frede Blaabjerg
Received: 10 December 2016; Accepted: 9 March 2017; Published: 16 March 2017
Abstract:
Energy storage systems (ESS) provide a means for improving the efficiency of electrical
systems when there are imbalances between supply and demand. Additionally, they are a key
element for improving the stability and quality of electrical networks. They add flexibility into
the electrical system by mitigating the supply intermittency, recently made worse by an increased
penetration of renewable generation. One energy storage technology now arousing great interest
is the flywheel energy storage systems (FESS), since this technology can offer many advantages as
an energy storage solution over the alternatives. Flywheels have attributes of a high cycle life, long
operational life, high round-trip efficiency, high power density, low environmental impact, and can
store megajoule (MJ) levels of energy with no upper limit when configured in banks. This paper
presents a critical review of FESS in regards to its main components and applications, an approach not
captured in earlier reviews. Additionally, earlier reviews do not include the most recent literature in
this fast-moving field. A description of the flywheel structure and its main components is provided,
and different types of electric machines, power electronics converter topologies, and bearing systems
for use in flywheel storage systems are discussed. The main applications of FESS are explained and
commercially available flywheel prototypes for each application are described. The paper concludes
with recommendations for future research.
Keywords:
energy storage systems (ESS); flywheel energy storage systems (FESS); power electronics
converters; power quality improvement
1. Introduction
Energy storage systems (ESS) can be used to balance electrical energy supply and demand.
The process
involves converting and storing electrical energy from an available source into another
form of energy, which can be converted back into electrical energy when needed. The forms of energy
storage conversion can be chemical, mechanical, thermal, or magnetic [
1
,
2
]. ESS enable electricity
to be produced when it is needed and stored when the generation exceeds the demand. Storage is
beneficial when there is a low demand, low generation cost, or when the available energy sources
are intermittent. At the same time, stored energy can be consumed at times of high demand, high
generation cost, or when no alternative generation is available [14].
Energy demand continues to increase, as demanded by the households and industries with
high growth rates in BRIC and developing countries. This has led to increases in energy prices
and traditional energy generation methods are less able to adapt, exacerbating the issues due to
market deregulation, power quality problems, and pressures to limit carbon dioxide emissions [
2
,
3
].
Renewable energy sources (RES) and potential distributed generation (DG) are considered as
supplements or replacements for traditional generation methods [
3
]; however, there are major
challenges associated with energy supply coming from renewables, due to their intermittent nature
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Appl. Sci. 2017,7, 286 2 of 21
across a range of timescales [
4
]. At a time when RES are supplying energy, there may be low
demand, but when the energy is demanded, it may exceed RES energy production [
3
]. Also, there
are monthly, seasonal, and annual fluctuations in RES supply, as their availability is always subject to
weather conditions. On the other hand, the energy demand differs from time to time, which does not
necessarily match the intermittences of RES, thus creating reliability problems [
3
,
4
]. Therefore, ESS
are a vital necessity to aggregate traditional generating plants in order to meet an excessive demand,
and supplement intermittent RES for their integration into the electrical network [5].
As a counterpart to today’s electrical network, there is a high demand for reliable, cost-effective,
long lasting, and environmentally sound energy storage systems to support a variety of energy
storage applications. With advances in materials technology, bearings, and power electronics, the
technology of flywheels for energy storage has significantly developed [
6
,
7
]. Flywheels with the
main attributes of high energy efficiency, and high power and energy density, compete with other
storage technologies in electrical energy storage applications, as well as in transportation, military
services, and space satellites [
8
]. With storage capabilities of up to 500 MJ and power ranges from
kW to GW, they perform a variety of important energy storage applications in a power system [
8
,
9
].
The most common applications of flywheels in electrical energy storage are for uninterruptible power
supplies (UPS) and power quality improvement [
10
12
]. For these applications, the electrochemical
battery is highly mismatched and suffers from an insufficient cycle life, since the number of cycles
per day is usually too high [
13
]. The authors note that this is not necessarily true for some UPS with
highly reliable grids, so storage is seldom called upon. Particularly for power quality improvement,
electrical disturbances are frequent but short, with the vast majority of them lasting for less than
5 s.
Such disturbances are effectively managed by flywheels and offer an improvement over batteries
considering the instantaneous response time and longer life cycle of the former. Even with one cycle
a day, an electrochemical battery is unlikely to last for even 10 years under these circumstances
(3650 cycles).
This can only be achieved if the depth of discharge is kept low and the battery is carefully
managed, both electrically and thermally. It also requires specifying an energy storage capacity two
to five times the required capacity, to reduce the depth of discharge, thus leading to a higher cost.
Supercapacitors have been tested for these types of applications; however, with more or less the same
capital cost as flywheels [
1
], their operational lifetime is relatively low (reaching up to 12 years) [
3
].
To make
more use of such a system and minimise its capacity in order to reduce the cost, it is more
useful for the storage system to be used many times a day, to allow for the time shifting of demand
and to feed into the grid at times of high demand. Interest in this new paradigm of how energy is used
will be greatly enhanced once Time of Use (ToU) tariffs are in place.
A number of reviews of flywheel storage systems have been presented by several papers in
the literature. A comparison of energy storage technologies is made in [
14
], where a numerical and
graphical review demonstrates the improvements and problems associated with FESS. A comparative
analysis of energy storage technologies for high power applications is carried out in [
15
] and a survey
of FESS for power system applications is provided in [
16
]. The control of high speed FESS in space
applications is discussed in [
17
]. FESS is briefly reviewed in [
18
] and an overview of some previous
projects is presented in [
9
]; however, such sources offer a scarcity of information. The authors of [
19
]
focus on the developments of motor-generator (MG) for FESS, where the common electrical machines
used with flywheels, along with their control, is reported in [
20
]. A review and simulation of FESS for
an isolated wind power system is presented in [
10
]. This review takes a different approach from earlier
work and particularly picks up on very recent literature in what is a rapidly developing subject.
This paper focuses on the description and applications of FESS, providing an overview of some
commercial projects for each application. Many of the above papers have provided reviews of
FESS,
but what is missing
in the literature is a comprehensive review of FESS with a description
of the applications which are commercially available. Following the introduction, a description of
FESS is presented. The main components of FESS, including the rotor, electrical machine, bearings,
and flywheel
containment are discussed in detail in Section 2. A flywheel’s main characteristics
Appl. Sci. 2017,7, 286 3 of 21
are stated in Section 3and its applications are described in Section 4. The paper concludes with
recommendations for future research in Section 5.
2. Description of Flywheel Energy Storage System
2.1. Background
The flywheel as a means of energy storage has existed for thousands of years as one of the earliest
mechanical energy storage systems. For example, the potter’s wheel was used as a rotatory object
using the flywheel effect to maintain its energy under its own inertia [
21
]. Flywheel applications were
performed by similar rotary objects, such as the water wheel, lathe, hand mills, and other rotary objects
operated by people and animals. These spinning wheels from the middle ages do not differ from those
used in the 19th or even 20th centuries. In the 18th century, the two major developments were metals
replacing wood in machine constructions and the use of flywheels in steam engines. Developments
in cast iron and the production of iron resulted in the production of flywheels in one complete piece,
with greater moment of inertia for the same space [
21
]. The word ‘flywheel’ appeared at the beginning
of the industrial revolution (namely in 1784). At the time, flywheels were used on steam engine boats
and trains and as energy accumulators in factories [
22
]. In the middle of 19th century, as a result of the
developments in cast iron and cast steel, very large flywheels with curved spokes were built.
The first
three-wheeled vehicle was built by Benz in 1885 and can be named as an example [
21
]. Over time,
several shapes and designs have been implemented, but major developments came in the early 20th
century, when rotor shapes and rotational stresses were thoroughly analysed, and flywheels were
considered as potential energy storage systems [
23
]. An early example of a flywheel system used
in transport was the Gyrobus, powered by a 1500 kg flywheel, produced in Switzerland during the
1950s [
24
]. In the 1960s and 1970s, FESS were proposed for electric vehicles, stationary power back up,
and space missions [
9
,
10
]. In the following years, fibre composite rotors were built and tested. In the
1980s, relatively low-speed magnetic bearings started to appear [25].
Despite major developments during their early stages, the utilization of flywheels has not
been significant and has declined with the development of the electric grid. However, due to
the recent improvements in materials, magnetic bearings, power electronics, and the introduction
of high speed electric machines, FESS have been established as a solid option for energy storage
applications [79,26,27].
A flywheel stores energy that is based on the rotating mass principle. It is a mechanical
storage device which emulates the storage of electrical energy by converting it to mechanical energy.
The energy
in a flywheel is stored in the form of rotational kinetic energy. The input energy to the FESS
is usually drawn from an electrical source coming from the grid or any other source of electrical energy.
The flywheel speeds up as it stores energy and slows down when it is discharging, to deliver the
accumulated energy. The rotating flywheel is driven by an electrical motor-generator (MG) performing
the interchange of electrical energy to mechanical energy, and vice versa [
28
,
29
]. The flywheel and MG
are coaxially connected, indicating that controlling the MG enables control of the flywheel [3033].
2.2. Structure and Components of FESS
FESS consist of a spinning rotor, MG, bearings, a power electronics interface, and containment or
housing, which are discussed in detail in the following subsections. A typical flywheel system suitable
for ground-based power is schematically shown in Figure 1.
Appl. Sci. 2017,7, 286 4 of 21
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Figure 1. Structure and components of a flywheel.
2.2.1. Flywheel Rotor
The stored energy in a flywheel is determined by the rotor shape and material. It is
linearly proportional to the moment of inertia and the square of its angular velocity, as shown in
Equation (1) [27,34]:
=1
2ω (1)
where E is the stored kinetic energy, I is the moment of inertia, and ω is the angular velocity. The
useful energy of a flywheel within a speed range of minimum speed (ωmin) and maximum speed
(ωmax) can be obtained by:
=1
2(ω−ω)=1
21ω
ω (2)
Typically, an electrically driven flywheel normally operates between (ωmin) and (ωmax), to avoid
too great a voltage variation and to limit the maximum MG torque for a given power rating. The
moment of inertia is a function of the mass of the rotor and the rotor shape factor. Flywheels are often
built as solid or hollow cylinders, ranging from short and disc-type, to long and drum-type [28,35].
For a solid cylinder or disc-type flywheel, the moment of inertia is given by:
=1
2 (3)
where m is the rotor mass and r is the outer radius. For a hollow cylinder flywheel of outer radius b
and inner radius a, as shown in Figure 2, the moment of inertia is:
=1
2(−) (4)
For a flywheel with length h and mass density ρ, the moment of inertia is determined by:
=1
2πρℎ(−) (5)
Thus:
=1
4πρℎω(−) (6)
Figure 1. Structure and components of a flywheel.
2.2.1. Flywheel Rotor
The stored energy in a flywheel is determined by the rotor shape and material. It is
linearly proportional to the moment of inertia and the square of its angular velocity, as shown in
Equation (1) [27,34]:
E=1
2Iω2(1)
where Eis the stored kinetic energy, Iis the moment of inertia, and
ω
is the angular velocity. The
useful energy of a flywheel within a speed range of minimum speed (
ωmin
) and maximum speed
(ωmax) can be obtained by:
E=1
2I(ωmax2ωmin2) = 1
2Iωmax21
ωmin2
ωmax2(2)
Typically, an electrically driven flywheel normally operates between (
ωmin
) and (
ωmax
), to avoid
too great a voltage variation and to limit the maximum MG torque for a given power rating. The
moment of inertia is a function of the mass of the rotor and the rotor shape factor. Flywheels are often
built as solid or hollow cylinders, ranging from short and disc-type, to long and drum-type [
28
,
35
].
For a solid cylinder or disc-type flywheel, the moment of inertia is given by:
I=1
2mr2(3)
where mis the rotor mass and ris the outer radius. For a hollow cylinder flywheel of outer radius b
and inner radius a, as shown in Figure 2, the moment of inertia is:
I=1
2mb2a2(4)
For a flywheel with length hand mass density ρ, the moment of inertia is determined by:
I=1
2πρhb4a4(5)
Thus:
E=1
4πρhω2b4a4(6)
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Figure 2. Hollow cylinder flywheel.
The maximum speed limit at which a flywheel may operate is determined by the strength of the
rotor material, called tensile strength σ [18,23]. A suitable safety margin must be maintained, to
keep the stress experienced by the rotor below the strength of the rotor material. The maximum stress
of a thin rotating ring is given by:
σ
ω (7)
where σ is the maximum stress and ρ is the density of the flywheel material. More complex
equations are available for different rotor geometries, but the maximum stress is always proportional
to ρ, and the square of peripheral speed, equal to rω. The effect of rotor geometries can be
accommodated by introducing a shape factor K. The maximum specific energy and energy density
are then given by:
=σ
ρJ/kg (8)
=σ [J/m
3
] (9)
Equations (8) and (9) indicate that the specific energy (energy per mass unit) and energy density
(energy per volume unit) of the flywheel are dependent on its shape, expressed as shape factor K.
The shape of a flywheel is an important factor for determining the flywheel speed limit, and hence,
the maximum energy that can be stored. The shape factor K is a measurement of flywheel material
utilisation [18]. Figure 3 shows the values of K for the most common types of flywheel geometries.
Figure 3
.
Different flywheel cross sections [18].
According to Equation (1), the stored energy of a flywheel can be optimised by either increasing
the spinning speed (ω) or increasing the moment of inertia (I). This allows two choices for FESS: low
speed FESS (typically up to 10,000 rpm) and high speed FESS (up to 100,000 rpm) [18,23]. Low speed
Figure 2. Hollow cylinder flywheel.
The maximum speed limit at which a flywheel may operate is determined by the strength of the
rotor material, called tensile strength σ[18,23]. A suitable safety margin must be maintained, to keep
the stress experienced by the rotor below the strength of the rotor material. The maximum stress of a
thin rotating ring is given by:
σmax =ρr2ω2(7)
where
σ
is the maximum stress and
ρ
is the density of the flywheel material. More complex equations
are available for different rotor geometries, but the maximum stress is always proportional to
ρ
,
and the
square of peripheral speed, equal to r
ω
. The effect of rotor geometries can be accommodated
by introducing a shape factor K. The maximum specific energy and energy density are then given by:
E
m=Kσmax
ρ[J/kg](8)
E
V=Kσmax hJ/m3i(9)
Equations (8) and (9) indicate that the specific energy (energy per mass unit) and energy density
(energy per volume unit) of the flywheel are dependent on its shape, expressed as shape factor K.
The shape of a flywheel is an important factor for determining the flywheel speed limit, and hence,
the maximum energy that can be stored. The shape factor Kis a measurement of flywheel material
utilisation [18]. Figure 3shows the values of Kfor the most common types of flywheel geometries.
Appl. Sci. 2017, 7, 286 5 of 20
Figure 2. Hollow cylinder flywheel.
The maximum speed limit at which a flywheel may operate is determined by the strength of the
rotor material, called tensile strength σ [18,23]. A suitable safety margin must be maintained, to
keep the stress experienced by the rotor below the strength of the rotor material. The maximum stress
of a thin rotating ring is given by:
σ
ω (7)
where σ is the maximum stress and ρ is the density of the flywheel material. More complex
equations are available for different rotor geometries, but the maximum stress is always proportional
to ρ, and the square of peripheral speed, equal to rω. The effect of rotor geometries can be
accommodated by introducing a shape factor K. The maximum specific energy and energy density
are then given by:
=σ
ρJ/kg (8)
=σ [J/m
3
] (9)
Equations (8) and (9) indicate that the specific energy (energy per mass unit) and energy density
(energy per volume unit) of the flywheel are dependent on its shape, expressed as shape factor K.
The shape of a flywheel is an important factor for determining the flywheel speed limit, and hence,
the maximum energy that can be stored. The shape factor K is a measurement of flywheel material
utilisation [18]. Figure 3 shows the values of K for the most common types of flywheel geometries.
Figure 3
.
Different flywheel cross sections [18].
According to Equation (1), the stored energy of a flywheel can be optimised by either increasing
the spinning speed (ω) or increasing the moment of inertia (I). This allows two choices for FESS: low
speed FESS (typically up to 10,000 rpm) and high speed FESS (up to 100,000 rpm) [18,23]. Low speed
Figure 3. Different flywheel cross sections [18].
According to Equation (1), the stored energy of a flywheel can be optimised by either increasing
the spinning speed (
ω
) or increasing the moment of inertia (I). This allows two choices for FESS: low
Appl. Sci. 2017,7, 286 6 of 21
speed FESS (typically up to 10,000 rpm) and high speed FESS (up to 100,000 rpm) [
18
,
23
]. Low speed
flywheels are usually made of heavier metallic material and are supported by either mechanical or
magnetic bearings. High speed flywheels generally use lighter but strong composite materials and
typically require magnetic bearings. The price of high speed flywheels can be up to five times higher
than the cost of low speed flywheels, according to [
18
,
23
]. The authors note that the cost of a flywheel
system is governed by the design of the whole system, not the rotor, although this core element may
dictate the design of the other elements in the system, hence the total cost. A new class of intermediate
speed flywheels, benefiting from the low cost of steel materials but a sufficiently high energy density,
is also being developed, based on the use of laminated steel. This has the potential to offer low cost,
but also compact, options [36].
2.2.2. Electric Machine
As explained previously, the electrical machine or integrated MG is coupled to the flywheel, to
enable the energy conversion and charging process of the flywheel. The machine, acting as a motor,
charges the flywheel by accelerating it and drawing electrical energy from the source. The stored
energy on the flywheel is extracted by the same machine, acting as a generator, and hence, the flywheel
is slowed down during discharge. Common electrical machines used in FESS are the induction machine
(IM), permanent magnet machine (PM), and variable reluctant machine (VRM) [18,23].
An IM is used for high power applications due to its ruggedness, higher torque, and low cost [
18
].
The speed limitations, complex control, and higher maintenance requirements are the main problems
with IMs [
16
]. The squirrel cage type can be a less expensive option for slow response applications [
20
].
A doubly fed induction machine (DFIM) has recently been used in FESS applications, due to its
flexible control and lower power conversion rating, allowing mitigated power electronics sizing [
16
,
20
].
An IM is widely
used in wind turbine applications to enable the power smoothing of wind generation
systems [16,20].
A VRM is very robust, and has low idling losses and a wide speed range. It has a simpler control
mechanism than IMs, when it comes to high speed operations [
16
]. On the downside, it has a low
power factor and low power density, as well as high torque ripples [
16
]. Both switched reluctance and
synchronous reluctance types are applied in high speed FESS applications [20].
A PM is the most commonly used machine for FESS because of its higher efficiency, high
power density, and low rotor losses [
18
]. It is widely used in high speed applications due to
the speed limitations of IMs, and the torque ripple, vibration, and noise of VRMs. The problem
with a PM is its idling losses due to stator eddy current losses, its high price, and its low tensile
strength [
18
].
A brushless
dc machine (BLDCM), permanent magnet synchronous machine (PMSM),
and Halbach array machine (HAM), are the main types of PM machines used in FESS applications [
20
].
A comparison
between IMs, VRMs, and PMs is presented in Table 1. Regarding the quantification of
specific power, the authors in [
18
] appear to have misreported the published values. In our experience,
depending on the rotor speed, the specific power of an IM and VRM would be around half of that of
the PM, and not the large difference shown in [
18
]. In fact, further analysis of the original literature
reports that the specific power density of PM synchronous machines is approximately 1.2 kW/kg [
37
].
Table 1. Comparison of electrical machines suitable for use in FESS [18,19].
Machine Asynchronous Variable Reluctance Permanent Magnet Synchronous
Power High Medium and low Medium and low
Specific power
Rotor losses Medium (~0.7 kW/kg)
Copper and iron Medium (~0.7 kW/kg)
Iron due to slots High (~1.2 kW/kg)
Very low
Spinning losses Removable by annulling flux Removable by annulling flux Non-removable, static flux
Efficiency High (93.4%) High (93%) Very high (95.5%)
Control Vector control Synchronous: Vector Control.
Switched: DSP Sinusoidal: Vector control.
Trapezoidal: DSP
Appl. Sci. 2017,7, 286 7 of 21
Table 1. Cont.
Machine Asynchronous Variable Reluctance Permanent Magnet Synchronous
Size 1.8 L/kW 2.6 L/kW 2.3 L/kW
Tensile strength Medium Medium Low
Torque ripple Medium (7.3%) High (24%) Medium (10%)
Maximum/base speed Medium (>3) High (>4) Low (<2)
Demagnetization No No Yes
Cost Low (22 /kW) Low (24 /kW) Low (38 /kW)
Advantages
Low cost Robustness of
temperature overheat Low loss, high efficiency
Simple manufacture Overcurrent capability High power density
Technology-matured Excitation coil can repeat
adjustment High load density
Adjustable power factor Lower loss at starting torque High torque density
No demagnetization Easy to dissipate heat Small volume, light quality
High energy storage Lower loss, higher efficiency low rotor resistance loss
No running loss High power density No field winding loss
Flexible shape and size
Simple control mode
High reliability
Disadvantages
High slip ratio of rotor Complex structure poor robustness of temperature
Limited speed Difficult to manufacture Demagnetisation
Larger volume Low power factor High cost
Low power to quality ratio Torque ripple,
vibration and noise Materials fragile
High losses, low efficiency More outlet from machine Difficult air gap flux-
Difficult to regulate speed field adjustment
To take advantage of both PMs and VRMs, hybrid PM reluctance machines have recently
been developed. Other unconventional machine types for FESS are presented in [
20
] and the latest
developments of MG for FESS are discussed in [19].
2.2.3. Power Electronics
The energy conversion in a FESS is accomplished by the electrical machine and a bi-directional
power converter. The power electronic converter topologies that can be used for FESS applications
are DC-AC, AC-AC, and AC-DC-AC, or a combination of these. The switching devices of the power
converters are selected, based on their operational characteristics and application. These include a
bipolar junction transistor (BJT), metal oxide semiconductor field effect transistor (MOSFET), insulated
gate bipolar transistor (IGBT), and thyristor (SCR, GTO, MCT) [
38
]. The commonly used switches are
a silicon controlled rectifier (SCR), gate turn off thyristor (GTO), and IGBT. SCR and GTO have been
traditionally used for variable frequency power converters. However, IGBT has been greatly adopted
in recent years, due to its higher power capability and higher switching frequencies [9].
The widely used configuration of power converters in FESS is the back-to-back (BTB) or
AC-DC-AC configuration, connected to a DC link capacitor. The converters in the BTB topology
are three-phase bridged semiconductor switches, often controlled by the pulse width modulation
(PWM) technique [
18
]. The PWM uses rectangular pulses and modulates the width of these pulses to
produce a variable waveform. The pulses are applied to the power electronics converter to produce a
sinusoidal AC current from a DC input [
9
,
39
]. The grid side converter maintains the DC link voltage,
where the machine side converter is used to control the operation of the MG and the flywheel.
A DC to AC converter, usually known as an inverter, produces an AC output of a desired
magnitude and frequency from a fixed or variable DC input. Variable output waveforms are achieved
by either varying the DC input or adjusting the gain of the inverter [
40
]. The widely used industrial
Appl. Sci. 2017,7, 286 8 of 21
applications of inverters for FESS are variable-speed ac motor drives, renewable energy, transportation,
and uninterruptible power supplies (UPS) [40,41]. In wind power applications, an inverter is used to
connect the FESS directly to the grid or to the DC link of the wind generator [
42
,
43
]. In high power and
high voltage applications, the two level DC-AC inverters are limited to operating at a high frequency,
due to switching losses and device rating constraints [
44
]. Multilevel inverters, with an additional
DC link capacitor, can be used to generate higher voltages with a less transient dv/dtand reduced
THD [
45
]. The common multilevel converter topologies in industry are diode-clamped converters or
neutral-point-clamped (NPC) converters, cascaded H-bridge (CHB) converters, and flying capacitor
(FC) converters. A three-level twelve-pulse NPC converter topology for FESS is proposed in [
46
].
Two NPC converters are connected in BTB combination to connect the FESS to the PCC between the
electrical grid and wind generation.
An AC to AC or matrix converter (MC) is an array of nine bidirectional switches arranged in a way
to allow the connection of any of the output phases of the converter to any of the input phases. MCs are
used to modify the amplitude, frequency, and phases of the waveforms between the two asynchronous
ac systems [
44
]. During its early stages, the AC-AC power converter was termed a Forced Commutated
Cycloconverter, as it mainly relied on forced commutated thyristors [
45
].
These converters
became
a viable alternative to BTB converters after the emergence of BJTs. A comparison of MCs and BTB
converters is presented in [
47
]. MCs are compact in size and have a lower weight, since there are
no large capacitors or inductors required for energy storage. However, the lack of an energy storing
capacitor limits their maximum voltage transfer ratio to 86%. This limit can be exceeded, but with
the cost of unwanted low frequency components in the input and output waveforms [
45
]. AC-AC
converters are used with FESS in dynamic voltage restorer applications to mitigate voltage sags [
48
,
49
].
The choice of the matrix converter improves the reliability of the system and increases the power
density [48].
A cascaded DC-DC and DC-AC converter configuration can also be used for FESS applications
connected to a DC microgrid. A three-phase full bridge circuit in series with a bidirectional Buck-boost
converter is proposed in [
50
]. This combination will be used in more applications with the emergence
of DC MG in the future.
In cases where the discharging speed of the FESS is low, a DC-DC boost converter is connected
at the DC link between the BTB converters, to regulate the output voltage. The boost converter can
be bypassed by a switch during the charging [
51
]. In [
52
], a Z-source inverter topology is used as an
alternative to a boost converter. In this configuration, inverter output waveform distortion is reduced
and the reliability of the system is greatly improved, because of the short circuit which can exist across
any phase leg of the inverter.
A higher switching frequency for the inverter and rectifier reduces current ripples and increases
control bandwidth, but switching losses will be increased. In addition, fast switching of the power
converter reduces higher order harmonics to produce an improved sinewave. Harmonics can be
further reduced by the introduction of AC filters on the converter AC side. Filters can reduce current
ripples, winding deterioration, and losses [53,54].
2.2.4. Bearings
Bearings are required to keep the rotor in place with very low friction, yet provide a support
mechanism for the flywheel [
3
]. The bearing system can be mechanical or magnetic, depending
on the weight, lifecycle life, and lower losses [
7
]. Gas bearings cannot be used due to the vacuum
within the enclosure. Traditionally mechanical ball bearings have been used, but these have a higher
friction compared to magnetic bearings and also require higher maintenance as a result of lubricant
deterioration [
7
]. These difficulties may be mitigated by using a hybrid system of magnetic and
mechanical bearings. A magnetic bearing has no friction losses and does not require any lubrication
but, if active, requires power to energize it. It stabilizes the flywheel by supporting its weight using
permanent magnets [
9
]. Permanent (passive) magnetic bearings (PMB), active magnetic bearings
Appl. Sci. 2017,7, 286 9 of 21
(AMB), and superconducting magnetic bearings (SMB) are the main types of magnetic bearing
systems [
16
]. A PMB has high stiffness, low cost, and low losses, due to lack of a current. However,
it has
limitations in providing stability and is usually considered as an auxiliary bearing system [
16
,
18
].
An AMB is operated by the magnetic field produced from current carrying coils controlling the rotor
position. It positions the rotor through a feedback system by applying variable forces which are
determined based on the deviation of the rotor position, due to external forces. An AMB has a high
cost, a complicated control system, and consumes energy to operate, which in turn, adds to system
losses [
16
]. FESS standby loss is affected by the AMB mass. As a result, increasing the rotational speed
and AMB mass add to the AMB iron and copper loss [
35
]. To ensure a good efficiency of the overall
system, a compromise between speed and losses has to be made. An SMB provides a high speed,
frictionless, long life, compact, and stable operation. It is the best magnetic bearing for a high speed
operation as it can stabilize the flywheel without electricity or a positioning system, according to [
9
,
16
].
However, an SMB requires a cryogenic cooling system as it operates at a very low temperature; but
recently, it has been improved by using high temperature superconductors (HTS). The main drawback
of an SMB system is its very high cost [
7
,
9
,
16
]. The parasitic losses of mechanical bearings are about
5% of the total storage capacity per hour unless hybrid systems are used. This factor is about 1% for
electromagnetic bearings [
14
] and can be further lowered to 0.1% by using HTS bearings [
55
].
The use
of hybrid bearings will reduce the losses and complexity of the control system and also provide a
stable and cost effective solution [
56
]. A compact flywheel energy storage system assisted by hybrid
mechanical-magnetic bearings is proposed in [
57
]. The magnetic levitation in the vertical orientation is
maintained by the magnetic bearing, while the translational and rotational levitation is assisted by
mechanical bearing. In [
58
], a combination of SMB and PMB have been analysed to reduce the cost of
the cooling system. The position of the flywheel rotor is controlled by the PMB and the SMB is mainly
used to suppress the rotor vibrations. However, the capability of PMB in restraining the system during
high speeds still remains in question.
2.2.5. Housing
The housing has two purposes: to provide an environment for low gas drag and for the
containment of the rotor in the event of a failure. The aerodynamic drag loss in an FESS increases with
the cube of the rotational speed, if the system is operated in atmospheric pressure [
23
]. These losses
are reduced by mounting the flywheel in a vacuum enclosure to improve the system performance
and safety. The housing or enclosure is the stationary part of the flywheel and is usually made of
a thick steel or other high strength material, such as composites. The container holds the rotor in a
vacuum to control rotor aerodynamic drag losses by maintaining the low pressure inside the device,
thus withstanding failures as a result of any possible rotor failures [
28
,
29
,
59
]. Operating the system
in such a low pressure requires a vacuum pump and an efficient cooling system to handle the heat
generated from MG and some other parts of FESS [
23
]. When the power into and out of the flywheel is
via an electric machine, there are no rotary seals, so leakage can be very small. This means that the
vacuum pump does not need to operate frequently, or can be eliminated with a sufficient sealing of
the housing. The operation of the vacuum pump depends on the rotor type. Composite rotors have a
very high tip speed requiring lower (harder) vacuum pressures and outgas, due to the nature of the
polymer resin matrix materials in contrast to steel. An alternative approach [
23
] is to use a gas mixture
of helium and air, which reduces both the aerodynamic drag loss and the system cooling requirements.
In the event of rotor failure, composite rotors tend to break into numerous small fragments and
its energy is dissipated by friction as the fragments rotate inside the casing. As this happens, pressure
is built up inside the casing and the end plates of the casing. If air enters during failure, then a much
stronger dust type explosion must also be contained, leading to the need for stronger containment.
Single piece steel rotors can burst into several fragments which will be difficult for the enclosure to
withstand, so require very large containment systems. This issue can be mitigated by making the rotor
from a stack of thinner discs, as explained in the section on rotors. This is because the catastrophic
Appl. Sci. 2017,7, 286 10 of 21
failure would release a fraction of the energy contained in the flywheel rotor. The enclosure design
for high-speed FESS will contribute to half of the flywheel weight, whereas this factor would be two
and half times larger for low speed FESS, according to [18]. The authors hold a different view on this
multiplier, as do most flywheel manufacturers who adopt a range of stances to justify safety including
placing flywheels in bankers for large composite rotors. Published literature on what is required to
contain both composite and steel rotors is not readily available and no officially adopted standards
currently exist, to the author’s knowledge. One recent publication has proposed a standard [
60
], which
is a useful step forward.
3. FESS Characteristics
The main characteristics of flywheels are a high cycle life (hundreds of thousands), long calendar
life (more than 20 years), fast response, high round trip efficiency, high charge and discharge rates, high
power density, high energy density [
7
,
9
], and low environmental impacts [
2
,
3
,
9
,
28
,
29
,
61
]. The state of
charge can be easily measured from the rotational speed and is not affected by life or temperature [
9
].
On the downside, flywheel self-discharge at a much higher rate than other storage mediums and
flywheel rotors can be hazardous, if not designed safely.
Flywheels have a long life time and very low operational and maintenance requirements.
The cycle
life is also high, compared to many other energy storage systems, as flywheels do not require
long charge-discharge cycles. It can be charged and discharged rapidly, based on the application and
functionality, and is not affected by the depth of discharge (DoD). The life time is estimated to be more
than 20 years and a charge-discharge cycle life in excess of hundreds of thousands, with no deterioration
in its performance [
3
,
28
,
29
]. The technology is capable of transferring large amounts of power in
seconds, with a high roundtrip energy efficiency in the range of 90%–95% [
1
,
2
,
18
].
It can deliver
its
stored energy and recharge quickly, in a matter of seconds. It is an environmentally friendly technology
and there are no emissions as a result of its operation, since the material used is not hazardous to the
environment [
18
,
29
]. The power and energy ratings of flywheels are independent and each can be
optimized, based on the application of the energy storage. The power rating of a flywheel depends on
the size of MG and associated power electronics, where its energy rating is determined by the size and
speed of the rotor [3,8].
There are a wide range of applications of flywheels in high power output for short periods;
however, in addition to the rotor material, a longer storage period requires developing new rotor
designs (e.g., larger diameter rotors and/or rotor laminations), to allow longer storage durations [
8
].
The size of FESS is an important characteristic which puts them in competition with other energy
storage technologies. Flywheels can have power densities up to five to ten times that of batteries. Due
to their relatively lower volume requirements and longer working life, they can replace batteries in
certain applications, including in transportation and space vehicles [8].
4. FESS Applications
Flywheel applications range from large scale at the electrical grid level, to small scale at the
customer level [
8
,
9
]. A high power and capacity is reached by arranging flywheels in banks, rather
than by using large machines [
62
]. The best and most suitable applications of flywheels fall in the
areas of high power for a short duration (e.g., 100 s of kW/10 s of seconds) [
6
], when frequent
charge-discharge cycles are involved [
8
]. The most common applications are power quality such as
frequency and voltage regulation [
2
,
63
], pulsed power applications for the military [
61
], attitude control
in space craft [
61
], UPS [
18
], load levelling [
2
], hybrid and electric vehicles [
18
,
61
], and energy storage
applications [
61
]. As part of energy storage applications, flywheels perform storage applications
both at the grid, as well as at the customer level. A brief description of some common applications
associated with flywheel energy storage systems will now be given.
Appl. Sci. 2017,7, 286 11 of 21
4.1. Power Quality
As part of the power quality requirements, the system frequency and voltage need to be
maintained to an acceptable level and deviations should be avoided. When the loads are added
or subtracted from the grid, the system voltage and frequency will also be increased or reduced.
Energy storage systems, especially those which are fast performing like flywheels, can quickly add
or take power from the grid, to keep the system voltage and frequency within range [
64
]. Flywheels
provide ride-through applications for interruptions of up to 15 s long and provide a means of switching
between power sources without any service interruptions [
2
]. Flywheels can operate for up to tens of
minutes for reactive power support, spinning reserve, and voltage regulation, to supply reliable electric
power and improve the power quality in applications such as communication facilities and computer
server centres [
2
]. As long as run down losses are kept low, durations can be extended to several hours,
without losing excessive amounts of energy. In North Western Australia, a flywheel system has been
integrated into a town’s power supply to support the increased power demand during the tourist
season [
65
]. Coral Bay, a wind energy operated power station, consisted of seven 320 kW low-load
diesel generators with three 200 kW wind turbines. In 2007, the integration of a 500 kW flywheel
virtual generator into the system allowed the wind turbines to provide up to 95% of Coral Bay’s supply
at peak times. The reported data shows that for nearly 900 h per year, 90% of the power station’s total
supply comes from wind generation. In addition, while maintaining the grid standards and improving
the power quality, 80% of this total power is wind-generated for one-third of the year [
65
]. Another
flywheel-based stabilisation system has been planned for the Marsabit wind farm, a remote community
served by an isolated microgrid in northern Kenya. A 500 kW flywheel-based system will be integrated
into the existing two 275 kW wind turbines and diesel generators. The PowerStore flywheel to be
installed by ABB will stabilise the grid connection to maximise renewable energy penetration [
66
].
PowerStore is a flywheel-based stabilising generator which is mainly used for improving the power
quality. It enables the integration and control of renewable wind and solar energy in the electrical grid.
Acting like a static synchronous compensator (STATCOM), it combines an 18 MWs (Megawatt second)
low-speed flywheel with solid state converters that absorb or inject full energy in 1 millisecond [
67
].
The range of models from 500 kW to 1.5 MW allows the configuration of either a grid support mode
for MW scale grids, or as a virtual generator for use in smaller isolated grids.
4.2. Frequency Regulation
Frequency fluctuations occur as a result of variations between the loads and supply, where one
exceeds the other. When demand exceeds supply, the generating plants are slowed down by the extra
load, thus decreasing the system frequency. On the other hand, the generators accelerate and the
frequency increases whenever the generation exceeds the demanded loads [
68
,
69
]. The frequency
fluctuates every second, as demand varies and generators turn on and off. In order to avoid this,
frequency regulation is applied, which demands the generators to hold capacity in reserve, to maintain
the stability of generation and consumption. This ramping up and down of the generators not only
increases the fuel cost and emissions, but also takes a minute or longer for some generating power
plants to respond [68,70].
A frequency regulation service is a very cyclic application requiring thousands of charge-discharge
cycles in a year. Any storage device in this application will be very rarely “resting”, due to the fact
that it includes constant charging and discharging at variable rates, from very slow to rapid and deep
cycling [
70
]. Today, grid operators look for ‘fast-acting regulators’ including flywheels and batteries to
respond to the frequency regulation issue. Because of their fast response and frequent charge-discharge
capabilities, flywheels are likely to dominate over batteries in this application. Similarly, the capability
of flywheels to switch from full output to full absorption in seconds, puts them on a par with the
immediate energy produced by gas fired power plants. Flywheel energy storage systems can deliver
twice as much frequency regulation for each megawatt of power that they produce, while cutting
carbon emissions in half [
68
,
71
]. The earliest, but shortest lifespan of a flywheel system reported
Appl. Sci. 2017,7, 286 12 of 21
for frequency regulation using renewables, was installed in Shimane, Japan, in 2003. This 200 kW
Urenco Power Technology Flywheel was installed by Fuji electric to reduce system fluctuations due
to wind generation. The diesel generators supporting the 1.8 MW wind turbines were operating at
a higher efficiency when the flywheel system was integrated. The system was decommissioned in
2004, as Urenco abandoned power quality operated flywheels and removed all previously installed
units [
65
]. The largest power rated FESS for frequency regulation is the Joint European Torus (JET)
Fusion Flywheel of the European Atomic Energy Community (EURATOM) located in Oxfordshire,
UK [
72
]. JET uses the grid for powering electromagnets, and for generating fields to confine and
ohmically heat hydrogen plasma. Experiments typically last 20 s and draw 1000 MW—a large fraction
of the local Didcot Power Station’s capacity [
73
]. There are two large flywheels capable of supplying
up to 400 MW for 30 s and an additional 300 MW power is pulled from the grid to combine with the
FESS, in order to satisfy the peak consumption of the JET pulse. The JET flywheels are charged up
from the grid for several minutes, then quickly discharged into the required loads. Hence, distribution
network congestion is avoided by reducing the demand on the grid during experiments [
74
].
A 20 MW
flywheel-based facility provides frequency regulations services to New York Independent System
Operator (NYISO) in Stephentown, New York [
29
]. The facility is built and operated by Beacon Power
and comprises 200 flywheels, each with a storage capacity of 100 kW. The tests during early trials
showed that 1 MW of fast response flywheel storage could produce up to 30 MW of regulation service;
two to three times better than an average Independent System Operator (ISO) generator. A second
20 MW frequency-regulation facility in the Hazle Township of Pennsylvania is commissioned by
Spindle Grid Regulation, LLC. This zero emission facility is designed for 20 year-life and at least
100,000 full-depth discharge cycles. It is comprised of 200 Beacon Power’s 100 kW (25 kWh) flywheels
connected in parallel, which can respond in less than 2 s [75].
4.3. Voltage Sag Control
Voltage sag problems are created due to load unbalance or faults in the power grid, causing a
decrease in voltage magnitude. Voltage sags due to unbalanced loads occur when large amounts of
power for a short period of time is absorbed by the load, which will decrease the voltage and cause
voltage drop problems [69,76].
Voltage sag has become one of the major power quality problems affecting sensitive loads such as
modem industrial manufacturing like semiconductor production, food processing and paper making,
sensitive microprocessors, and high frequency power electronic devices [
76
]. Further drawbacks of
voltage sag in three-phase power networks are increased line losses, neutral conductor overloads,
and extra
rotating losses in drives [
69
]. About 92% of the power quality problems are as a result of
voltage sag and 80% of these occasions last for only 20–50 ms [76].
Traditionally, voltage sag has been compensated for by generation reserves adding power to the
system, when demanded. The recent approach is to engage energy storage systems in mitigating the
voltage problems in power networks. The energy storage system is used to store the energy in times
when excessive power is required, in order to keep the grid voltage fixed. This reduces the cost and
eliminates the need for oversized generating units [69].
FESS, with their excellent characteristics, can be viable alternatives to other storage systems for
this application. Particularly, a fast response, high power density, and frequent charge-discharge cycle
capability, are the best attributes of flywheels for voltage compensation applications [
69
].
A 10 MJ
flywheel energy storage system for high quality electric power and reliable power supply from the
distribution network, was tested in the year 2000. It was able to keep the voltage in the distribution
network within 98%–102% and had the capability of supplying 10 kW of power for
15 min [9]. In 2005
,
a flywheel-based grid stabilising generator (PowerStore) commenced operation in Flores Island,
Portugal. It has been used to perform a frequency and voltage ride through to safeguard conventional
grids and allows the integration of renewable generation from wind and solar sources. The PowerStore
with a 500 kW rated power and 60 s duration capability is still operational [65].
Appl. Sci. 2017,7, 286 13 of 21
4.4. UPS
A short term (seconds to minutes) energy storage device with control electronics is referred to as
uninterruptible power supply (UPS). A UPS is one of the existing markets and the most successful
application for high power flywheels to supply power for occasions which usually don’t last longer
than 15 s. More than 80% of the power outages last for less than a second [
8
] and 97% of them last for
less than 3 s [
10
]; however, this causes voltage and frequency problems, as well as power interruptions.
In these applications, the UPS, as a backup storage, bridges the gap between the loss of the grid and
the start of backup sources during an interruption.
The most developed and widely used storage medium in UPS applications is batteries. FESS can
be used as a substitute for batteries or in combination with batteries in UPS systems [
12
].
In cases
with only flywheels as a backup storage, sufficient power is provided by the flywheel to run the
system, until the power source is restored or a standby power source comes online. Depending
on the level of power required, 10–15 s of back up support is enough to meet the demand loads,
without transferring to the generator set power [
8
,
10
]. This is the case when power outages last less
than 15 s. Meanwhile,
in diesel-rotary
UPS with diesel generators for long-term outages, the diesel
engines commonly start and accept 100% load within 3–4 s and flywheels are best suited to bridge the
power, until the generators are fully operated and synchronized. As a result, either scenario can be
accomplished, with flywheels acting as energy storage systems for UPS applications.
In addition, flywheels are used in combination with batteries in UPS systems requiring longer
durations. A flywheel can deal with shorter interruptions, while batteries can be saved for longer
outages. This will save the battery from frequent charge-discharge, which will further increase its
lifetime [
12
]. Usually, flywheels and batteries are combined for applications requiring a mix and
match between energy density and cost, which cannot be otherwise achieved with one of these storage
systems [
71
]. Many manufacturers around the world have developed flywheel systems for UPS.
To name a few, one of the earliest flywheels for on-site power applications was built in Munchen,
Germany, in 1973. It was rated for 155 MW power and 0.93 power factor, for a pulsed duration of 9.7 s.
In the following years, this flywheel was complemented with two more flywheels to utilise a flywheel
generator system for high energy fusion experiments. The system was fully commissioned in 1987 and
the total rated power and pulsed duration capability of the system was increased to 387 MW and 12 s,
respectively [
65
]. A hybrid microgrid supplying heat and electricity to an industrial/airport facility in
an island in Alaska was commissioned in 1999. Originally consisting of a 225 kW wind turbine, and
two 150 kW diesel generators, the system was upgraded with a 160 kW FESS by Beacon technology in
2014. In addition to efficiency improvements, it has also provided fuel savings of up to 30% [
65
]. Piller
GmbH has installed a FESS in a combined heat and power station to supply a semiconductor facility
in Dresden, Germany. The integrated 5 MW flywheel subsystem can supplement the 30 MW plant
with 5 s storage [
7
]. A battery-free UPS system has been announced by VYCON to protect a light-out
data centre located in Texas, US. The system will involve multiple 750 kVA double conversion UPS
modules, paired with an 8 MW (300 kW power rating per unit) FESS. Deploying approximately 1MW
of clean energy, a similar flywheel system has also been planned to protect EasyStreet Online Service’s
data centre in California, US [
77
]. Similarly, the control centre of Austin Energy is protected by a
4.8 MW flywheel UPS by VYCON. Austin Energy, one of the largest electric utilities in US, supplies
approximately 400,000 customers and of a population of about one million. The attributes associated
with a battery-free flywheel system are reduced downtime, no battery maintenance requirement,
and savings on cooling of the environment for batteries.
4.5. Transportation
In transportation, flywheels are used in hybrid and electric vehicles to store energy, for use when
harsh acceleration is required or to assist with uphill climbs. In hybrid vehicles, the constant power
is provided by the internal combustion engines to keep the vehicle running at a constant optimum
speed, reducing fuel consumption, air and noise pollution, and extending the engine life by reducing
Appl. Sci. 2017,7, 286 14 of 21
maintenance requirements [
7
,
8
]. At the same time, energy from regenerative braking during vehicle
slowdown is stored in flywheels, which will be supplied back to provide a boost during acceleration or
climbing hills [
7
,
8
,
59
]. The only competitors to flywheels in hybrid vehicle applications are chemical
batteries and ultra-capacitors. However, ultra-capacitors suffer from a low energy density and higher
cost. Flywheels rank better than batteries based on their longer life time, higher power density, higher
efficiency, and frequent charge-discharge capability [
8
]. Furthermore, flywheels are developed for
rail applications, both for hybrid and electric systems. They also find a place in gas turbine trains
for the same purpose. The desired speed and maximum weight of the train determines the power
and energy requirements. It is estimated that 30% of the braking energy could be recovered by this
system, due to receptivity issues [
8
]. In electrical vehicles with chemical batteries as their source of
propulsion, flywheels are considered to cope well with a fluctuating power consumption. This will
prolong the lifetime of the battery as its charge-discharge cycles become more regular [
10
]. In train
energy recovery systems, flywheels are installed at stations or substations to recover energy through
regenerative braking, and supply it back into the system for traction purposes. Flywheels are well
suited for this application due to the high rate of charge-discharge cycles needed. In addition, it allows
voltage sag control for transmission and distribution lines, without increasing the line capacity of the
railway.
A number of
flywheels for trackside energy recovery systems have been demonstrated by
URENCO and Calnetix [
78
]. In April 2014, VYCON Inc. installed a FESS for the Los Angeles Country
Metropolitan Transportation Authority (LA Metro) Red line (MRL), to recover the braking energy
from trains. MRL provides rail subway service connecting downtown to San Fernando Valley through
six-car trains with AC or DC traction systems [
79
]. VYCON’s flywheel, known as Metro’s Wayside
Energy Storage Substation (WESS), can recover 66% of the braking train energy [
80
]. The collected
data, after six months of operation, showed 20% energy savings (approximately 541 MWh), which is
enough to power 100 average homes in California [
79
]. A total of 190 metro systems operating in 9477
stations and approximately 11,800 km of track has been reported globally [
13
]. The introduction of
energy storage into rail transit for braking energy recovery can potentially reduce 10% of the electricity
consumption, while achieving cost savings of $90,000 per station [
81
]. Flywheels are also used in
roller coaster launch systems to accumulate the energy during downhill movements and then rapidly
accelerate the train to reach uphill positions, using electromagnetic, hydraulic, and friction wheel
propulsion [
82
]. The Incredible Hulk roller coaster at an adventure theme park in Orlando, Florida, uses
several 4500 kg flywheels to propel the system. The flywheels charge continuously at about
200 kW
and then discharge at 8 MW, to launch the train [
13
]. Since the late 2000s, the use of flywheel hybrid
storage systems in motorsports has seen major developments, beginning with Formula 1 and followed
by the highest class of World Endurance Championship (WEC) [
13
]. Williams Hybrid Power (WHP),
part of Williams Group of companies, pioneered the use of flywheel energy storage in motorsport.
WHP’s electric flywheel was used in Porsche Motorsport on their 2010 911 GT3 R Hybrid endurance
racing car. This car competed in several endurance races in 2010, including the 24 h Nürburgring
race, where it led the race by two laps until 22nd h, before retiring due to an engine-related failure-an
unrelated problem to the hybrid system. The following year, the GT3 R secured first position in the
VLN race at the Nordschleiefe [
83
]. Porsche hybrid’s latest version, the 918 RSR hybrid concept sports
car with electric flywheel energy storage, was announced at the 2010 Detroit Motorshow. In March
2012, WHP was announced as the hybrid energy storage supplier for Audi R18 e-tron Quattro. WHP’s
entirely new design flywheel (150 kW power, 45,000 rpm speed) for Audi made history by becoming
the first hybrid car to win Le Mans, the most demanding race in the world, in 2010, 2013, and 2014 [
84
].
In public transport, city buses are an ideal application for electric flywheel hybridisation, due to their
higher mass and frequent start-stop nature. The technology can save fuel and reduce greenhouse gas
emissions by up to 30% [
83
]. WHP started developing flywheel energy storage for use in buses for the
Go-Ahead Group in March 2012. It also developed a kinetic energy recovery system (KERS) for GKN
Gyrodrive in April 2014. The GKN has recently demonstrated a design for use in city buses [85].
Appl. Sci. 2017,7, 286 15 of 21
4.6. Spacecraft
Flywheels find applications in space vehicles where the primary source of energy is the sun, and
where the energy needs to be stored for the periods when the satellite is in darkness [
7
,
8
]. FES for
the international space station (ISS) was discussed in 1961 and was first proposed in the 1970s [
7
].
For the past decade, the NASA Glenn Research Centre (GRC) has been interested in developing
flywheels for space vehicles. Initially, designs used battery storage, but now, FES are being considered
in combination with or to replace batteries [
7
,
8
]. The combined functionality of batteries and flywheels
will improve the efficiency, and reduce the spacecraft mass and cost [
7
]. The proposed flywheel system
for NASA has a composite rotor and magnetic bearings, capable of storing an excess of 15 MJ and
peak power of 4.1 kW, with a net efficiency of 93.7%. Based on the estimates by NASA, replacing
space station batteries with flywheels will result in more than US$200 million savings [
7
,
8
]. It has been
reported that a flywheel system would be significantly smaller and offer a better weight reduction
than the use of NiH
2
battery devices for use on EOS-AMI-type spacecraft. It has been shown that the
flywheel offers a 35% reduction in mass, 55% reduction in volume, and a 6.7% area reduction for solar
array [
86
]. FESS is the only storage system that can accomplish dual functions, by providing satellites
with renewable energy storage in conjunction with attitude control [24,25].
4.7. Renewables
Flywheels can assist in the penetration of wind and solar energy in power systems by improving
system stability. The fast response characteristics of flywheels make them suitable in applications
involving RES for grid frequency balancing. Power oscillations due to solar and wind sources are
compensated for by storing the energy during sunny or windy periods, and are supplied back when
demanded [
9
,
10
]. Flywheels can be used to rectify the wind oscillations and improve the system
frequency; whereas, in solar systems, they can be integrated with batteries to improve the system
output and elongate the battery’s operational lifetime [9].
The authors in [
18
] indicate that the formation of a hybrid system by the addition of wind
turbines and photovoltaic panels could not result in fuel savings, as expected. This is because diesel
generators, even unloaded, will consume up to 40% fuel. Diesel generators should only be started
when demanded and shut down most of the time. Therefore, flywheel energy storage systems can
reduce frequent start/shut-down cycles of the diesel generators; thus reducing fuel consumption and
bridging the power fluctuations [
18
]. The current authors see a great benefit of flywheels backing up
solar PV, since they can cope with the high cycles due to the cloud passing, yet provide ride through,
as long as standing losses are kept low. There has been a wide range of flywheel systems developed
for the penetration of renewable energy systems. For example, ABB’s PowerStore, Urenco Power,
Beacon Power, and VYCON technology, have all provided flywheel-based systems for wind and solar
applications. On a larger scale, the world’s first high penetration solar PV diesel power stations were
installed in 2010, supplying the towns of Nullagine and Marble Bar in Western Australia. A FESS
is operated as a UPS system, to allow maximum solar power injection during sunshine and ramp
up diesel generators when the sun is obscured. This enables a saving of 405,000 litres of fuel and
1100 metric tons of greenhouse gas emissions each year. Moreover, the integration of flywheels in
the system has helped the PV system to supply 60% of the average daytime energy for both towns,
generating 1 GWh of renewable energy per year [65].
4.8. Military
In the military, a recent trend has been towards the inclusion of electricity in military applications,
such as in ships and other ground vehicles, as well as for weapons, navigation, communications, and
their associated intelligent systems. This use of electric energy at different rates and different power
levels requires energy storage to respond rapidly and reliably to this variable energy demand [
8
].
Hybrid electric power is essential for future combat vehicles, based on their planned electrically
Appl. Sci. 2017,7, 286 16 of 21
powered applications. Flywheels appear as an appropriate energy storage technology for these
applications. They are combined with supercapacitors to provide power for high speed systems
requiring power in less than 10 µs.
Flywheels are also likely to find applications in the launching of aircraft from carriers. Currently,
these systems are driven by steam accumulators to store the energy; however, flywheels could replace
these accumulators to reduce the size of the power generating systems that would otherwise be sized
for the peak power load [
8
]. A FESS is integrated into a microgrid serving the US Marine Corp in
California, to provide energy storage applications throughout the entire distributed generation at
the base [
65
]. The purpose of the project is to provide energy security to military facilities using
renewable energy. It is a network of interconnected smaller microgrids that are nested into a 1.1 MW
bigger-scale microgrid, that include solar PV systems, diesel generators, batteries, and 60 kW, 120 kWh
FESS [
87
]. The flywheel storage is intended to decrease the dependency on diesel generators by about
40% and provide peak shaving applications by mainly supplying high power loads such as elevators.
In addition to extending the lifespan of the batteries, the FESS is estimated to work for 50,000 cycles
and have a lifespan of 25 years [87].
5. Recommendations for Future Research
Although the flywheel is one of the earliest forms of energy storage, compact, reliable, low
maintenance flywheels have only become available relatively recently. The numbers produced have
been small, and the use of more exotic materials and their processing, such as carbon fibre composites,
have kept the cost at about five times higher than steel flywheels [
10
,
18
]. New, innovative designs
based on steel overcome the concern about safety for highly stressed rotors, which can now operate at
much higher tip speeds than was considered safe for monolithic steel rotors [
88
]. Steel has the benefit
that the material and processing routes are well established and understood to the supply base which
is already there, for low cost manufacturing at the all critical batch scales of 10 s to 1000 s. Steel is easily
recyclable in comparison to batteries, although recycling will not need to be done for decades given an
effective infinite calendar life and cycle life of many 10 s of thousands.
Another concern relates to the charge holding ability of flywheels, since the losses in currently
available flywheels are high. These losses are mechanical (drag, bearing, friction), electrical (hysteresis,
eddy current, copper), and power converter-related (switching and conduction) [
89
]. If flywheel losses
can be kept to around 10%–20% run down per 24 h, given the available technologies and the lower
speeds of the steel flywheel, the possibility for these applications is certainly promising. The vacuum
needed for this is not unduly high and will be held by means of a hermetically sealed system, only
requiring occasional re-pumping. The majority of the weight can be levitated on passive magnetic
bearings, with inevitable losses in the lightly loaded ball bearing system being well within the 20%
run down loss allowance. The remaining loss is the electromagnetic drag in the generator and this
depends on the design. Since a flywheel for this duty is likely to have an electrical machine of kW
rating similar to the kWh rating, numerically, the electromagnetic drag of a well-designed system can
also be kept within the loss budget.
6. Conclusions
This paper has presented a critical review of FESS with reference to its main components and
applications. The structure and components of the flywheel are introduced and the main types for
electric machines, power electronics, and bearing systems for flywheel storage systems are described
in detail. The main applications of FESS in power quality improvement, uninterruptible power supply,
transportation, renewable energy systems, and energy storage are explained, and some commercially
available flywheel storage prototypes, along with their operation under each application, are also
mentioned. FESS offer the unique characteristics of a very high cycle and calendar life, and are the
best technology for applications which demand these requirements. A high power capability, instant
response, and ease of recycling are additional key advantages. Given the demand for ESS is expanding
Appl. Sci. 2017,7, 286 17 of 21
substantially, and that FESS has these unique attributes, the future for FESS remains very bright, even
in a time when the cost of Li-ion and other chemistry battery technology continues to reduce. Future
work will include the detailed modelling and analysis of a flywheel system for backup power and grid
support applications.
Acknowledgments:
This work has been funded by European Union’s Erasmus Mundus under INTACT project,
the City University Graduate School and the City Future Fund.
Conflicts of Interest: The authors declare no conflict of interest.
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... Among several typical energy storage devices, flywheel has not only the high efficiency, high instantaneous power, but also fast response and long cycle life, becoming one of the energy storage technologies with promising future [5][6][7]. For that reason, an EC-BERS is proposed in this paper. ...
Chapter
Full-text available
The electromagnetic coupling braking energy recovery system (EC-BERS) was proposed with the advantages of zero friction, none impact of electromagnetic coupler and higher power and efficiency of flywheel. It is potentially cost-effective between a wheel and a small flywheel. The proposed EC-BERS, which requires only a motor and a converter, can capture more of the mechanical energy, and the rest needs to be processed by the converter. In this paper, the electromagnetic coupler model was established based on Simulink software. Then a coupler test platform was built to verify the effectiveness of the model. Finally, energy conversion process of EC-BERS under deceleration and cruise state was simulated. The results show that most of the energy between wheels and flywheels is transferred as mechanical energy, and the battery had a low participation in this state.
... Batter bank, supercapacitor energy storage, pumped hydro energy storage, hydrogen energy storage, compressed air energy storage, flywheel energy storage, liquid air energy storage, stacked blocks etc. These technologies are discussed as follows [23][24][25][26][27]: ...
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... Depending on the application, different geometric structures with different types of materials, such as steel, composite, and nanotechnology, are provided for the flywheel, each of which has different mechanical stress [23,24]. The geometric structure of the FESS is shown in Figure 10. ...
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... c. Flywheel Energy Storage [22]: Flywheels have the attributes of high cycle and long operational life, high round-trip efficiency, high power density, and low environmental impact. A FES consists of a spinning rotor, motor, generator, bearings, a power electronics interface, and containment. ...
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... The flywheel speeds up as it stores energy by bringing a mass into rotation around an axis and slows down when discharging to deliver the accumulated energy. The fast response characteristics of FES makes them suitable in applications involving solar and wind resource for grid frequency balancing [171]. However, FES systems are still not considered a mature technology because they are expensive compared to other ESSs [143]. ...
Thesis
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Solar energy represents a promising solution to meet future energy demands in an era of depleting fossil fuel sources. However, solar energy faces two main challenges: grid instability and intermittency. To overcome these drawbacks, a large number of combinations of solar technologies have been studied over the years. In this thesis, we propose to focus on the hybridization of PV and CSP technologies into one compact system. Therefore, the objective of this thesis is to go beyond general descriptions and assumptions to study the annual energy production of two compact hybrid systems, one-sun and high-temperature plants. To answer these questions, a detailed electrical, thermal, and optical model is developed to analyze the dynamic output characteristics of the hybrid plants, based on realistic input parameters of a large-scale solar tower plant in Targassonne, France. We demonstrate the superiority of the two compact hybrid plants over stand-alone technologies. The addition of a thermal energy storage system in the compact plants has the advantage of making the energy production independent of the solar resource, which allows for better control of the plant and longer production time. We also investigate the extent to which weather conditions and demand profiles are likely to affect the capacity of compact PV-CSP hybrid systems.
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Chapter
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Book
As industry power demands become increasingly sensitive, power quality distortion becomes a critical issue. The recent increase in nonlinear loads drawing non-sinusoidal currents has seen the introduction of various tools to manage the clean delivery of power. Power demands of medical facilities, data storage and information systems, emergency equipment, etc. require uninterrupted, high quality power. Uninterruptible power supplies (UPS) and active filters provide this delivery. The first to treat these power management tools together in a comprehensive discussion, Uninterruptible Power Supplies and Active Filters compares the similarities of UPS, active filters, and unified power quality conditioners. The book features a description of low-cost and reduced-parts configurations presented for the first time in any publication, along with a presentation of advanced digital controllers. These configurations are vital as industries seek to reduce the cost of power management in their operations. As this field of power management technology continues to grow, industry and academia will come to rely upon the comprehensive treatment found within this book. Industrial engineers in power quality, circuits and devices, and aerospace engineers as well as graduate students will find this a complete and insightful resource for studying and applying the tools of this rapidly developing field.
Book
Power electronics is an area of extremely important and rapidly changing technology. Technological advancements in the area contribute to performance improvement and cost reduction, with applications proliferating in industrial, commercial, residential, military and aerospace environments. This book is meant to help engineers operating in all these areas to stay up-to-date on the most recent advances in the field, as well as to be a vehicle for clarifying increasingly complex theories and mathematics. This book will be a cost-effective and convenient way for engineers to get up-to-speed on the latest trends in power electronics. BENEFIT TO THE READER: The reader will obtain the same level of informative instruction as they would if attending an IEEE course or a training session, but without ever leaving the office or living room! The author is in an excellent position to offer this instruction, as he teaches many such courses, and is also the author of a key college textbook in the area with Prentice Hall. * Self-learning advanced tutorial, falling between a traditional textbook and a professional reference. * Almost every page features either a detailed figure or a bulleted chart, accompanied by clear descriptive explanatory text. * The companion CD will include figures as PowerPoint presentations, so the reader can follow along on his or her home computer.
Book
Power electronics, which is a rapidly growing area in terms of research and applications, uses modern electronics technology to convert electric power from one form to another, such as ac-dc, dc-dc, dc-ac, and ac-ac with a variable output magnitude and frequency. Power electronics has many applications in our every day life such as air-conditioners, electric cars, sub-way trains, motor drives, renewable energy sources and power supplies for computers. This book covers all aspects of switching devices, converter circuit topologies, control techniques, analytical methods and some examples of their applications.
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The demonstration of kinetic energy storage system (KESS) which supports line voltage, regenerates braking energy and provides uninterrupted supplies at stations on urban railways is discussed. Three 100 kW KESS units which are connected in parallel are installed at Northfields substation for trials. These three KESS units absorb braking energy and reduce power requirements during acceleration. The KESS stores kinetic energy in rotor which consists of carbon-glass composite cylinder. These units forms integrated UPT Trackside Energy Management System (TEMS) when configured with appropriate traction power interface and control logic.
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Energy storage technology has great potential to improve electric power grids, to enable growth in renewable electricity generation, and to provide alternatives to oil-derived fuels in the nation's transportation sector. In the electric power system, the promise of this technology lies in its potential to increase grid efficiency and reliability-optimizing power flows and supporting variable power supplies from wind and solar generation. In transportation, vehicles powered by batteries or other electric technologies have the potential to displace vehicles burning gasoline and diesel fuel, reducing associated emissions and demand for oil. Federal policy makers have become increasingly interested in promoting energy storage technology as a key enabler of broad electric power and transportation sector objectives. The Storage Technology for Renewable and Green Energy Act of 2011 (S. 1845), introduced on November 10, 2011, and the Federal Energy Regulatory Commission's Order 755, Frequency Regulation Compensation in the Organized Wholesale Power Markets, are just two recent initiatives intended to promote energy storage deployment in the United States. Numerous private companies and national laboratories, many with federal support, are engaged in storage research and development efforts across a very wide range of technologies and applications. This report attempts to summarize the current state of knowledge regarding energy storage technologies for both electric power grid and electric vehicle applications. It is intended to serve as a reference for policymakers interested in understanding the range of technologies and applications associated with energy storage, comparing them, when possible, in a structured way to highlight key characteristics relevant to widespread use. While the emphasis is on technology (including key performance metrics such as cost and efficiency), this report also addresses the significant policy, market, and other non-technical factors that may impede storage adoption. It considers eight major categories of storage technology: pumped hydro, compressed air, batteries, capacitors, superconducting magnetic energy storage, flywheels, thermal storage, and hydrogen. Energy storage technologies for electric applications have achieved various levels of technical and economic maturity in the marketplace. For grid storage, challenges include roundtrip efficiencies that range from under 30% to over 90%. Efficiency losses represent a tradeoff between the increased cost of electricity cycled through storage, and the increased value of greater dispatchability and other services to the grid. The capital cost of many grid storage technologies is also very high relative to conventional alternatives, such as gas-fired power plants, which can be constructed quickly and are perceived as a low risk investment by both regulated utilities and independent power producers. The existing market structures in the electric sector also may undervalue the many services that electricity storage can provide. For transportation storage, the current primary challenges are the limited availability and high costs of both battery-electric and hydrogen-fueled vehicles. Additional challenges are new infrastructure requirements, particularly for hydrogen, which requires new distribution and fueling infrastructure, while battery electric vehicles are limited by range and charging times, especially when compared to conventional gasoline vehicles. Substantial research and development activities are underway in the United States and elsewhere to improve the economic and technical performance of electricity storage options. Changes to market structures and policies may also be critical components of achieving competitiveness for electricity storage devices. Removing non-technical barriers may be as important as technology improvements in increasing adoption of energy storage to improve grid and vehicle performance.
Conference Paper
Microgrids are an attractive option in remote areas with elevated renewable resources. However, with or without grid connection, microgrids often results in weak grids. Hence, microgrids are much affected by the power variations and require energy storage systems to smooth them out. Flywheel based energy storage systems (FESSs) are gaining momentum in microgrids as, despite the limited amount of stored energy, they allow to interchange high power and have long useful lifetime. In addition, the state-of-charge is very simple to estimate as it only consists on measuring the spinning speed. In FESSs, the flywheel is attached to an electrical machine, which is connected to the grid through a back-to-back converter. Controlling the three phase converter as a virtual synchronous machine allows to overcome instability issues of the PLL-based control in a weak grids. For his reason, his paper proposes to control the grid-side converter of the FESS as a virtual synchronous machine. Small signal analysis is applied to the equations of the virual synchronous machine. The DC-link voltage is regulated by the machine-side converter. Simulation results are provided to illustrate the proposed concepts.