energies
Review
Modern Supercapacitors Technologies and Their Applicability
in Mature Electrical Engineering Applications
Kasun Subasinghage 1, *, Kosala Gunawardane 2 , Nisitha Padmawansa 2 , Nihal Kularatna 3
and Mehdi Moradian 2
1
2
3
*
Department of Materials and Mechanical Technology, Faculty of Technology,
the University of Sri Jayewardenepura, Homagama 10206, Sri Lanka
Department of Electrical and Electronic Engineering, Auckland University of Technology, WS Building,
34 St Paul Street, Auckland 1142, New Zealand
School of Engineering, the University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
Correspondence: kasun.weranga@fot.sjp.ac.lk
Abstract: Supercapacitors can store a million times more energy per unit mass or volume compared
to electrolytic capacitors. Due to their low internal resistance, they are capable of driving or absorbing
pulsative high currents. Over the last quarter, century supercapacitor (SC) manufacturers have
developed several families of mass-scale devices with high-power density and a longer cycle life
that helped the end-users to improve their energy storage systems and products. Today, there are
three common device families, namely, (i) symmetrical double-layer capacitors (EDLCs), (ii) hybrid
capacitors with a lithium electrode, and (iii) battery capacitors based on pseudo capacitance concepts.
This review paper compares these families and provides an overview of several state-of-the-art
applications in electric vehicles (EVs), microgrids, and consumer electronics.
Citation: Subasinghage, K.;
Gunawardane, K.; Padmawansa, N.;
Keywords: supercapacitors; energy storage devices; electric double-layer capacitors; hybrid capacitors;
pseudocapacitors
Kularatna, N.; Moradian, M. Modern
Supercapacitors Technologies and
Their Applicability in Mature
Electrical Engineering Applications.
Energies 2022, 15, 7752. https://
doi.org/10.3390/en15207752
Academic Editors: Sumanta Sahoo
and Santosh K. Tiwari
Received: 13 September 2022
Accepted: 17 October 2022
Published: 20 October 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affiliations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
1. Introduction
Capacitors are used for various functionalities in electrical circuits, such as smoothing,
buffering, filtering, bypassing, and DC blocking. Normally, capacitors have a relatively
small capacitance in the range of picofarads to microfarads, and their energy densities are
small. Batteries, on the other hand, have been the normal solution for portable energy
storage because they offer significantly higher energy density. By using electrodes with
greater surface area and thinner dielectric materials, SCs can provide capacitances a million
times greater than conventional capacitors. Thus, SCs are considered energy-storing
devices that bridge the gap between conventional capacitors and batteries and suit the everincreasing demand for reliable power in industrial, energy, and computing applications.
SCs are also governed by the same fundamental equations applicable to conventional
capacitors. Table 1 compares the major differences between capacitors, SCs, and batteries,
and Figure 1 depicts the Ragone plot of various energy storage devices [1–4]. In the Ragone
plot, x-axis represents the power density (W/kg), and the y-axis represents the energy
density (Wh/kg) [4]. Parameters, such as internal losses and leakages, determine the
boundaries of each energy storage device in the Ragone plot. Straight lines of the plot
represent the time which is the ratio between energy and power.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Energies 2022, 15, 7752. https://doi.org/10.3390/en15207752
https://www.mdpi.com/journal/energies
Energies 2022, 15, 7752
2 of 15
Table 1. Comparison of characteristics of supercapacitors versus batteries and conventional capacitors [1–3,5–10].
Characteristics
Power density (W/kg)
Energy density (Wh/kg)
Equivalent series resistance
Cycle time
Charge time (s)
time (s)
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15, x FOR PEER
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Typical lifetime (years)
Operating temperature range (◦ C)
Capacitors
Supercapacitors
Batteries
>106
<0.1
Typically, in mΩ range
106
−
3
10 to 10−6
10−3 to 10−6
30
−40 to +125
60,000
1 to 73
Typically, in mΩ range
50,000 to 1,100,000
0.3 to 60
0.1 to 1800
30
−40 to +70
<3000
10 to 250
Fractional Ω to few Ω
500 to 18,000
3600 to 18,000
600 to 10,800
5–20
−20 to +65
2 of 15
Figure 1.
1. Ragone
Ragone plot
energy-storing
technologies
[4]. [4].
Figure
plotofofvarious
various
energy-storing
technologies
Moreover, supercapacitors have further attractive features compared to batteries.
supercapacitors
have further
attractive
compared
batteries.
The Moreover,
internal resistance
of supercapacitors
is almost
constantfeatures
during the
chargingtoand
The
internal
resistance
of
supercapacitors
is
almost
constant
during
the
charging
discharging processes and their cycle life is very high. Their charging and dischargingand discharging
their
life is
very
charging
and discharging
limits areprocesses
not critical,and
unlike
forcycle
batteries,
and
theyhigh.
poseTheir
zero thermal
runaway
risk over a limits
wide
range
of temperatures.
They are also
they do not
contain
are
not
critical,
unlike for batteries,
andenvironmentally
they pose zerofriendly
thermalasrunaway
risk
over a wide
heavy
metals.
range of temperatures. They are also environmentally friendly as they do not contain
heavy metals.
2. Supercapacitor Types
The three common basic types of SC families are symmetrical electric double-layer
Table 1. Comparison of characteristics of supercapacitors versus batteries and conventional capaccapacitors, hybrid capacitors, and pseudocapacitors. The structures of these three families
itors [1–3,5–10].
are displayed in Figure 2, and three corresponding commercial examples are displayed in
families are summarized in Table Batteries
2 [5,11,12].
Characteristics Figure 3. The basic characteristics
Capacitors of the three SCSupercapacitors
Power density (W/kg)
Energy density (Wh/kg)
Equivalent series resistance
Cycle time
Charge time (s)
Discharge time (s)
Typical lifetime (years)
Operating temperature range (°C)
>106
<0.1
Typically, in mΩ range
106
10−3 to 10−6
10−3 to 10−6
30
−40 to +125
60,000
1 to 73
Typically, in mΩ range
50,000 to 1,100,000
0.3 to 60
0.1 to 1800
30
−40 to +70
<3000
10 to 250
Fractional Ω to few Ω
500 to 18,000
3600 to 18,000
600 to 10,800
5–20
−20 to +65
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3 of 315
(a)
(b)
(c)
Figure
2. Electro-chemistry
of three
commercial
SC families:
(a) Electric
doubledouble
layer capacitor,
(b)
Figure
2. Electro-chemistry
of three
commercial
SC families:
(a) Electric
layer capacitor,
hybrid
capacitor,
(c)
pseudocapacitor
[11,12].
(b) hybrid capacitor, (c) pseudocapacitor [11,12].
Hermann
von Helmholtz,
a German physicist,
the EDLC
concept
Table
2. Comparison
of electric double-layer
capacitorsfirst
and introduced
other supercapacitor
types
[1–3]. in
1853, and then it was commercialized by Standard Oil of Ohio (SOHIO) Research Center,
USA, inElectric
1961 [13].
EDLCs
Double
Layerare composed of two carbon-based electrodes and a dielectric
Parameter
Hybrid Supercapacitor
Pseudocapacitor
Capacitors
separating them,
as shown in Figure 2a. As the potential difference is applied across the
electrodes,
the charges in the material move to electrodes of opposite potentials [14]. The
Non-faradic/electrostatic,
Faradic, reversible
Storage mechanism
electrical
charge
stored
at the next
Both
and non-faradic
electrical
double
layer
formed
tofaradaic
a large-area
electrode and an electrolyte is effecredox reaction
tivelymetal/electrolyte
used, and henceinterface
the name EDLCs is used. Their larger area plates and shorter disMaximum Specific Power tance between plates can give a higher effective capacitance [5].
10,000
5000
4000
(W/kg)
The next SC family is the hybrid supercapacitors that merge the chemistry of a batSpecific energy (Wh/kg)
1–20
7–12
20–60
physics of an SC in a single structure
[15]. Due to their higher
specific energy
Life (number of cycles) tery with the
50,000–1,000,000
40,000–50,000
15,000–20,000
compared to EDLCs and good power
density,
there has been a strong demand for these
Metal
oxide/carbon-based
materials
materials,
conducting
Metal oxides, conducting
devicesCarbon-based
in recent years
[14,16]. As shown
in Figure
2b, in hybrid supercapacitors,
one elecMaterial
activated
carbon,
polymer/carbon-based
(e.g., NiO,[5].
trode is(e.g.,
activated
carbon,
and the other
is similar to an electrode inpolymers
Li-ion batteries
nanotubes)
materials (e.g.,
Ni(OH)2power
/rGO, in applications,
MgO, PANI)
These SCscarbon
can provide
reliable ride-through
or backup
such as data
PANI/rGO)
storage systems,
servers, utility meters, and
controllers for automated systems
[14].
Temperature range (◦ C)
–40 to 70 ◦ C
–20 to 60 ◦ C
–20 to 50 ◦ C
The third type is battery-capacitors, based on the concept of pseudocapacitance [13].
Although these have an energy density similar to lead-acid batteries they suffer from a
Hermann
vondensity
Helmholtz,
a German
physicist,
first
introduced
the EDLC
relatively
low power
and lack
of cycling
stability
compared
to EDLCs
[17].concept
Figure in
and
it wasof
commercialized
by Standard
Oil of Ohio (SOHIO)
2c 1853,
depicts
thethen
structure
the electro-chemistry
of a pseudocapacitor
[5]. Research Center,
USA, in 1961 [13]. EDLCs are composed of two carbon-based electrodes and a dielectric
separating them, as shown in Figure 2a. As the potential difference is applied across the
electrodes, the charges in the material move to electrodes of opposite potentials [14]. The
electrical double layer formed next to a large-area electrode and an electrolyte is effectively
used, and hence the name EDLCs is used. Their larger area plates and shorter distance
between plates can give a higher effective capacitance [5].
The next SC family is the hybrid supercapacitors that merge the chemistry of a battery
with the physics of an SC in a single structure [15]. Due to their higher specific energy
compared to EDLCs and good power density, there has been a strong demand for these
devices in recent years [14,16]. As shown in Figure 2b, in hybrid supercapacitors, one
electrode is activated carbon, and the other is similar to an electrode in Li-ion batteries [5].
These SCs can provide reliable ride-through or backup power in applications, such as data
storage systems, servers, utility meters, and controllers for automated systems [14].
Energies 2022, 15, 7752
4 of 15
The third type is battery-capacitors, based on the concept of pseudocapacitance [13].
Although these have an energy density similar to lead-acid batteries they suffer
from a
4 of 15
relatively low power density and lack of cycling stability compared to EDLCs [17]. Figure 2c
depicts the structure of the electro-chemistry of a pseudocapacitor [5].
OR PEER REVIEW
er
anism
fic Power
(Wh/kg)
f cycles)
l
nge (°C)
(a)
(b)
(c)
Figure 3. CommercialFigure
supercapacitor
families
from SAMWHA:
electric (a)
double-layer
capacitors,
3. Commercial
supercapacitor
families from(a)
SAMWHA:
electric double-layer
capacitors,
capacitors, (c) pseudocapacitors
[18].
(b) hybrid capacitors,(b)
(c)hybrid
pseudocapacitors
[18].
3. Supercapacitor Equivalent Circuit Models
Table 2. Comparison of electric double-layer capacitors and other supercapacitor types [1–3].
In the current literature, different types of circuit models are used to emulate the
behavior
Electric Double Layer of SCs when considering the steady state and the transient response. Figure 4
Hybridcircuit
Supercapacitor
Pseudocapacitor
represents the SC equivalent
models of ELDCs [19,20].
Capacitors
Figure 4a shows a basic simplified model of the SC where C is the capacitance, Rs
Non-faradic/electrostatic,
electrical
represents
the equivalent resistance of the SC, and Rp isFaradic,
the parallel
resistance
that interprets
reversible
redox
recharge stored at the metal/electrolyte
Both
faradaic
and
non-faradic
the self-discharging phenomena [19,21,22]. This model is able only to adequately represent
action
interfacethe dynamic behavior of an SC for several seconds, which limits its applicability.
Figure 4b shows an equivalent model developed by Zubieta and Boner for a wide
10,000 variety of power electronic applications
5000
4000(immediate branch,
[23]. The three RC branches
delayed branch, and long-term branch) represent the SC characteristics on a distinct time
1–20
7–12
20–60
scale. The parallel resistor (Rp ) represents the self-discharging properties of the SC [23].
50,000-1,000,000 Each branch has different
40,000–50,000
15,000-20,000
time constants to create a model that is more easily measurMetal
oxide/carbon-based
mate- is composed of a voltage-dependent
able during various periods. The immediate branch
Metal oxides, conducting polCarbon-based materialsnonlinear
(e.g., activated
rials,
polymer/carcapacitor
(CV ),conducting
a constant capacitor
(CO ), and a constant resistor (Ri ). This branch
ymers (e.g., NiO, MgO,
is used to capture the
transient behavior
the SC over periods of seconds. The delayed
carbon, carbon nanotubes)
bon-based
materialsof(e.g.,
PANI)
branch can emulate Ni(OH)
the behavior
of the SC in the range of minutes,
and it is composed of a
2/rGO, PANI/rGO)
constant capacitor (Cd ) and a constant resistor (Rd ). Finally, to capture the behavior of the
–40 to 70 °C
–20 to 60 °C
–20 to 50 °C
SC in long time scales (typically higher than 10 min), the long-term branch is used, which
consists of a constant capacitor (Cj ) and a constant resistor (Rj ).
3. Supercapacitor Equivalent
Circuit
Models
In [24], the
authors
present a method to extract the parameters for the model in
Figure 4b. As shown in Figure 4c, a model combined with a series resistor (Rs ), a bulk
In the current literature, different types of circuit models are used to emulate the becapacitor (C), and n number of parallel RC networks represents a dynamic model of SC
havior of SCs whenusing
considering
the steady
statespectroscopy
and the transient
response.
Figure
electrochemical
impedance
in the frequency
domain
[25]. 4 represents the SC equivalent
of transmission
ELDCs [19,20].
As circuit
shown inmodels
Figure 4d,
line modules are used to emulate the distributed
capacitance
and
electrolyte
resistance
determined
by theC
porous
electrodes
[26]. In R
general,
Figure 4a shows a basic simplified model of the SC where
is the
capacitance,
s
a
higher
number
of
RC
networks
increases
the
fidelity
of
the
model
with
the
cost
of high
represents the equivalent resistance of the SC, and Rp is the parallel resistance that intercomputational power.
prets the self-discharging phenomena [19,21,22]. This model is able only to adequately
represent the dynamic behavior of an SC for several seconds, which limits its applicability.
Figure 4b shows an equivalent model developed by Zubieta and Boner for a wide
variety of power electronic applications [23]. The three RC branches (immediate branch,
delayed branch, and long-term branch) represent the SC characteristics on a distinct time
Energies 2022, 15, 7752
4b. As shown in Figure 4c, a model combined with a series resistor (Rs), a bulk capacitor
(C), and n number of parallel RC networks represents a dynamic model of SC using electrochemical impedance spectroscopy in the frequency domain [25].
As shown in Figure 4d, transmission line modules are used to emulate the distributed
capacitance and electrolyte resistance determined by the porous electrodes [26]. In gen5 of 15
eral, a higher number of RC networks increases the fidelity of the model with the cost of
high computational power.
Figure
Supercapacitorequivalent
equivalentcircuit
circuitmodels
models
[19,23,24,26].
Simplified
model,
(b) ZuFigure 4.
4. Supercapacitor
[19,23,24,26].
(a):(a):
Simplified
model,
(b) Zubieta
and Boner
developed
model,
(c) modified
version
of Zubieta
and Boner
developed
model,
(d)
bieta
and Boner
developed
model,
(c) modified
version
of Zubieta
and Boner
developed
model,
Transmission
line
model.
(d)
Transmission
line
model.
Limitations and
and Future
FutureTrends
Trendsof
ofSupercapacitor
Supercapacitor
4.4. Limitations
Oneof
ofthe
themajor
majordrawbacks
drawbacksof
ofSCs
SCsisistheir
theirlow
lowvoltage
voltagerating,
rating,which
whichisistypically
typicallyless
less
One
than
4
V
for
a
single
cell.
Due
to
this
limitation,
SC
modules
(SCMs)
have
been
introduced
than 4 V for a single cell. Due to this limitation, SC modules (SCMs) have been introduced
byconnecting
connectingaalarge
largenumber
numberof
ofSCs
SCsin
inseries
seriesto
toincrease
increasethe
thevoltage
voltageratings
ratingsup
upto
toseveral
several
by
hundred
volts
[27].
However,
these
SCMs
have
higher
total
equivalent
resistance
due to
to
hundred volts [27]. However, these SCMs have higher total equivalent resistance due
the
series
of
connected
SCs.
the series of connected SCs.
Normally,SCs
SCshave
haveaa20%
20%tolerance
tolerancein
intheir
theirrated
ratedcapacitance.
capacitance. As
As aa result,
result, in
in an
an SC
SC
Normally,
bank,the
thetotal
totalvoltage
voltage
will
equally
distributed
among
the individual
SC [5].
cellsThis
[5].
bank,
will
notnot
be be
equally
distributed
among
the individual
SC cells
This could
an overcharging
issue
inSC
some
cells
and potentially
cause damage.
could
createcreate
an overcharging
issue in
some
cellsSC
and
potentially
cause damage.
Active
Active
and
passive
voltage
balancing
techniques
are
used
to
mitigate
this
issue by moniand passive voltage balancing techniques are used to mitigate this issue by monitoring
and
toring and the
controlling
charging
of each
SC cell.
Another
is to
hybridthat
SC
controlling
chargingthe
of each
SC cell.
Another
solution
is to solution
use hybrid
SCuse
modules
modules athat
a high(typically
voltage rating
(typically
to 100 the
V) while
keeping
the
maintain
highmaintain
voltage rating
up to 100
V) whileupkeeping
internal
resistance
internal
resistance
and[16].
the The
size concept
minimum
[16]. The
SC modules
is of
to
and
the size
minimum
of hybrid
SCconcept
modulesofishybrid
to combine
an anode
anode of acapacitor
Titanium
electrolyte
capacitor
with a cathode
of electrochemical
acombine
Titaniumanelectrolyte
with
a cathode
of electrochemical
capacitors.
In [28], the
capacitors.
Indemonstrated
[28], the authors
have demonstrated
hybrid
module
sethas
up rate
at 30 J,
authors
have
a hybrid
SC module seta up
rate SC
at 30
J, which
a working
which
has
a
working
voltage
of
100
V
and
the
equivalent
internal
resistance
is
0.05
Ω.
voltage of 100 V and the equivalent internal resistance is 0.05 Ω.
Although the
the energy
energy density
density of
of SCs
SCs isis relatively
relatively low
low compared
compared to
to batteries,
batteries, there
thereisis
Although
much
muchongoing
ongoing research
researchto
toenhance
enhance this.
this. In
In [16],
[16], the
the authors
authors fabricated
fabricated SCs
SCs with nitrogenincorporated,
incorporated, thiol-functionalized,
thiol-functionalized, reduced
reduced graphene
graphene oxide
oxide scrolls
scrolls (NTGS)
(NTGS) powder
powder and
and
determined
determinedan
anenergy
energydensity
densityof
of206
206Wh/kg
Wh/kg and
and aa power
power density
densityof
of32
32kW/h.
kW/h. Moreover,
Moreover,
the NTGS cell has cycling stability for 20,000 cycles. Another team from University College
London and the Chinese Academy of Sciences developed an SC using graphene laminate
films and concentrating on the space between layers [29]. They tested their design and
it achieved an energy density of 88.1 Wh/L and retained 97.8% of its energy capacity
even after 5000 cycles [29]. Likewise, many researchers and SC manufacturers are in the
process of introducing high-energy density SCs to the market. These developments grow
the market potential for SCs. It is predicted that the SC market size will grow from USD
472 million in 2022 to USD 912 million by 2027 at an average compound annual growth
rate of 14.1% [30].
5. Supercapacitor Applications
5.1. Electrical Vehicle Applications
15
EV manufacturers use new techniques to enhance the performance and driving6 ofrange
of their products. The unique qualities of SCs, such as higher power density and long-life
cycles, are used in EVs
to capture regenerative energy more efficiently and deliver a quick
5. Supercapacitor Applications
power boost with minimal
power
in the DC bus, which reduces stress on the
5.1. Electrical
Vehiclefluctuations
Applications
EV manufacturers use new techniques to enhance the performance and driving range
battery pack.
Energies 2022, 15, 7752
5.1.1. SC and
of their products. The unique qualities of SCs, such as higher power density and long-life
cycles, are used in EVs to capture regenerative energy more efficiently and deliver a quick
Batterypower
Packboost
Integrated
EVpower
Systems
with minimal
fluctuations in the DC bus, which reduces stress on the
battery pack.
Figure 5 shows a generalized diagram of how SCs are integrated with batteries to
power the drive train
ofSCanandEV.
When
accelerator
pedal is pressed, the main control
5.1.1.
Battery
Pack the
Integrated
EV Systems
Figure 5 to
shows
a generalized
diagram of how
SCs are
integrated
batteries
to
system commands the motor
generate
the required
torque
and
speed.with
When
braking,
power the drive train of an EV. When the accelerator pedal is pressed, the main control
the motor goes into generating
mode, and the power flow is reversed to charge the battery
system commands the motor to generate the required torque and speed. When braking,
and the SC, as shown.
the motor goes into generating mode, and the power flow is reversed to charge the battery
and the SC, as shown.
Driving Power + Engine start/stop
Regenerative Braking
Battery
Controlling
Interface
DC
AC
Inverter
Supercapacitors
Motor
Wheels
DC to DC
Converter
Figure 5. Power
in an electric vehicle.
Figure 5. Power management
in anmanagement
electric vehicle.
Depending on the power requirement and the type of batteries used, three configura-
are available
to interface theand
SC with
battery
and the DC
link of
an EV.configuThese
Depending on tions
the power
requirement
thethetype
ofpack
batteries
used,
three
configurations
are
passive,
semi-active,
and
active,
depicted
in
Figure
6
[31].
rations are available to interface the SC with the battery pack and the DC link of an EV.
In the passive configuration, as shown in Figure 6a, the battery and the SC are conThese configurations
are directly
passive,
semi-active,
active,
depicted
in Figureis 6the[31].
nected
in parallel
to the DCand
link [32].
Although
this configuration
simplest
and most economical method due to the absence of any intermediate DC-to-DC converters,
it has some disadvantages [33]. For example, the terminal voltage always follows the
discharging characteristics of the battery, which limits the usage of SC voltage. Similarly,
the battery requires to supply both load and the SC soon after the SC discharges, which
creates additional stresses on the battery [33].
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Energies 2022, 15, 7752
7 of 15
Figure
6. Clarification
of SC +of
battery
configurations
used in EVs:used
(a) Passive
configuration,
(b)configuration
semiFigure
6. Clarification
SC + battery
configurations
in EVs:
(a) Passive
active configuration, and (c) active configuration [31].
active configuration, and (c) active configuration [31].
To overcome the issues in passive configuration, the semi-active configuration disthe passive
in Figure the
6a, battery
the battery
and the SC
played In
in Figure
6b has configuration,
been developed. as
In shown
this configuration,
is directly
connected
to the DCin
link,
while the
SC isDC
connected
to the
DC link through
a DC-to-DC is the
nected directly
parallel
to the
link [32].
Although
this configuration
converter. Since the battery is directly connected to the DC link, it can maintain the DC
and most economical method due to the absence of any intermediate DC-to-DC
link voltage, while SCs can be operated to deal with large current fluctuations [34]. This
ers, it has
some preferred
disadvantages
[33]. For
example,
voltage always fo
approach
is typically
when a smaller
SC pack
is usedthe
dueterminal
to its cost-effectiveness
discharging
of the
battery,
whichoflimits
the usage
of link,
SC voltage.
S
and
compactnesscharacteristics
[34]. Moreover, with
the direct
connection
the battery
to the DC
it
is the
possible
to have
situations
large
currents
drawn
battery
battery
requires
to where
supply
both
loadmay
andbethe
SC from
soonthe
after
theinstead
SC discharg
ofcreates
the SC due
to
the
converter
delays
[35].
additional stresses on the battery [33].
Figure 6c shows the active configuration. This is the most recent development in the SC
To overcome the issues in passive configuration, the semi-active configur
battery hybrid configurations used in EVs. Here, both the SC and the battery are connected
played
in Figure
6b has been
developed.
In this configuration,
the abattery
parallel to the
DC link through
separate
DC-to-DC converters.
Using this method,
wider is dir
nected voltage
to the DC
link,
while
thebeSC
is connected
to the DCa constant
link through
operating
range
of the
SC can
achieved
while maintaining
currenta DC-to
flow
through
the
battery.
The
active
configuration
can
be
further
modified
to
achieve
a
verter. Since the battery is directly connected to the DC link, it can maintain
th
series connection. More details are available in [36]. Even regarding the advantages of this
voltage, while SCs can be operated to deal with large current fluctuations [34]
novel configuration, there are still some technical challenges that need to be addressed.
proach
is typically
preferred
when a input
smaller
SC pack
usedlevel
dueofto
its cost-effe
For
example,
this topology
requires multiple
variables
and is
a high
control
and compactness
[34].control
Moreover,
architecture
to give precise
[37]. with the direct connection of the battery to the
These
configurations
beneficialwhere
in regenerative
braking may
(RB) and
engine startit is
possible
to have are
situations
large currents
be drawn
from the b
stop
function
in
EVs
and
hybrid
EVs
where
a
higher
amount
of
power
is
required
over
a
stead of the SC due to the converter delays [35].
short period.
Figure 6c shows the active configuration. This is the most recent developm
5.1.2.
Breaking
SC Regenerative
battery hybrid
configurations used in EVs. Here, both the SC and the battery
nected
parallel
DCEVs,
linkthe
through
DC-to-DC
Using this
In modern
EVsto
orthe
hybrid
kinetic separate
energy is converted
to converters.
electrical energy
during
the
braking
by
driving
the
motor
as
a
generator.
As
shown
in
Figure
5,
reverse
a wider operating voltage range of the SC can be achieved while maintaining a
current flow through the battery. The active configuration can be further mo
achieve a series connection. More details are available in [36]. Even regardin
vantages of this novel configuration, there are still some technical challenges tha
be addressed. For example, this topology requires multiple input variables an
Energies 2022, 15, 7752
8 of 15
power flow is directed to the SC and the battery. This RB method extends the driving range
of an EV or improves the fuel economy of a hybrid EV [38]. Nevertheless, directing the
rapid energy recovery during RB into the battery itself can cause stress on the battery, and
its long-term performance will degrade [39]. SCs are used as an alternative method to
capture the regenerative energy, which is ideal for work during rapid high-power variation
situations. With the use of the configurations discussed in Section 5.1.1, all or part of the
regenerative energy can be diverted into SCs, depending on the requirement. Most of the
major car manufacturers use SCs to capture the energy from the RB. Toyota Supra and
FCHV, Mazda 6 and 3, Hyundai Tucson, and Chevrolet Equinox are a few examples [40]. A
project demonstrated that the deployment of the RB into the trolley bus system in Ningbo,
China, by using a 1500 V SC pack, improved the driving range from 4 to 10 km [41].
There have been many studies to further investigate the best approaches to harvest
most of the RB energy while not compromising the safety-braking technologies, such as
anti-locked brake systems (ABS). In [42], a team of researchers investigated an optimization method to identify the required braking power to capture the maximum amount of
regenerative energy whilst not compromising safety. They formulated an optimal control
framework using Pontryagin’s maximum theorem.
A constant current control technique [39] is proposed to capture the regenerative
energy using a passive series configuration where the SCs alone absorb the regenerative
energy and reduce the stress on the battery.
A simulation-based study was performed to identify the energy-recapturing efficiency
of LiFePO4 batteries vs. SCs [43]. Results show that LiFePO4 batteries have an efficiency of
approximately 67% when recovering the regenerative energy and the SCs achieved 97%
efficiency in capturing it.
A novel approach to control the regenerative power flow between the battery and
the SC is proposed in [44], where the controller is programmed either to direct the current
to the battery or the SC during the RB based on the pre-defined current threshold values.
Afterwards, there is current sharing between the battery and the SC within their respective
current limits to keep the charge balance between the battery and the SC. One drawback of
this concept is that the entire control is based on pre-defined current threshold values, and
this could change from one system to another. Moreover, as the batteries age, the current
rating changes, and the threshold values should be updated.
5.1.3. Start and Stop Systems
Typically, when restarting EVs or hybrid EVs, a high current is drawn over a short
period. Drawing this current from a conventional battery will cause significant stress and
will reduce its life span. Due to the low ESR of SCs compared to batteries, they can be used
to supply this burst of power to start the vehicle [45]. This extends the life of the battery
by lowering the stress on it and provides a faster and smoother starting capability [45,46].
Moreover, this method is attractive as the SCs are maintenance-free and typically last the
entire life of the vehicle [46].
Maxwell Technologies’ Continental Automotive Systems’ Maxwell-powered voltage
stabilization system (VSS) is one of the well-established automotive applications where
SCs are used for the engine start-stop function. This technique used in Cadillac ATS, CTS
sedans and ATS coupés [46]. As shown in Figure 7a, six 2000F ultracapacitors (UCs) are
connected parallel with a 30 Ah battery to assist the battery during the start and stop [47].
As shown in the performance curves in Figure 7b, most of the starting current is supplied
by the UCs, alleviating stress on the battery.
Energies 2022, 15, x FOR PEER REVIEW
9 of 1
Energies
2022,
7752PEER REVIEW
Energies
2022,
15,15,
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(a)
of 15
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(a)
(b)
(b)
Figure 7. Maxwell-powered voltage stabilization system (VSS) (a) capacitor and battery setup, (b
Figure 7. Maxwell-powered voltage stabilization system (VSS) (a) capacitor and battery setup, (b)
performances
characteristics
[47]. stabilization system (VSS) (a) capacitor and battery setup,
Figure
7. Maxwell-powered
voltage
performances characteristics [47].
(b) performances characteristics [47].
Skeleton
Technologies’,
SkelStart
Engine
Start Module
inisFigure
8) is anothe
Skeleton
Technologies’,
SkelStart
Engine
Start
Module
(shown
in(shown
Figure
8)
Skeleton
Technologies’,
SkelStart
Engine
Start
Module
(shown
in
Figure
8) another
is another
SC
automotive
application
primarily
used
in
trucks
to
assist
the
start-stop
function
[48
SCSC
automotive
application
primarily
used
in in
trucks
to to
assist
thethe
start-stop
function
[48].
automotive
application
primarily
used
trucks
assist
start-stop
function
[48].
The
V SkelStart
Engine
Start Module
with capacitance
a rated capacitance
of 1280F can
deliver 764
The
1212
V12SkelStart
Engine
Start
The
V SkelStart
Engine
StartModule
Modulewith
witha arated
rated capacitanceof
of1280F
1280Fcan
candeliver
deliver 7644
7644 A
A0.4
in s0.4
s achieve
and
achieve
a
peak
power
is
62.9
kW.
Due
to
its
wide
operating
temperatur
A in
in
0.4
sand
and
achieve
a
peak
power
is
62.9
kW.
Due
to
its
wide
operating
temperature
a peak power is 62.9 kW. Due to its wide operating temperature range
◦
◦
range
(−40
°C
to
+65
°C),
canused
be can
used
in used
a wider
of environments.
The
range
(−40
°C
to
+65
°C),module
thiscan
module
in range
aofwider
range ofThe
environments.
Th
(−40 C to +65 C), thisthis
module
be
in abe
wider
range
environments.
module
module
is
installed
between
the
battery
and
the
starter
motor
of
the
vehicle.
While
the
module
is
installed
between
the
battery
and
the
starter
motor
of
the
vehicle.
While
th
is installed between the battery and the starter motor of the vehicle. While the Skelstart
Skelstart
module
drives
the
starter
motor,
the
battery
powers
up
the
other
electrical
loads,
module
drives
the starter
the battery
powers
up thepowers
other electrical
as loads
Skelstart
module
drivesmotor,
the starter
motor,
the battery
up the loads,
other such
electrical
such
as fans.
light
fans.
Itfound
was
thatmodule
this
module
can reduce
fuel
consumption
light
It
was
that this
can reduce
fuel
consumption
2%by
to 2%
6% to
andby 2% t
such
as light
fans.
It found
was
found
that
this
module
can
reduce
fuelbyconsumption
6%increase
and increase
the battery
up to
to 2 times
[48].
battery
lifebattery
uplife
to 1.5
to 1.5
2up
times
[48].
6% andthe
increase
the
life
to 1.5
to 2 times [48].
Figure 8. SkelStart module installation [48].
Figure
SkelStart
module
installation
8.8.SkelStart
module
installation
[48]. [48].
5.2.Figure
Microgrid
Applications
have become popular due to the distributed power generation.
5.2.Recently,
Microgridmicrogrids
Applications
5.2.of
Microgrid
Applications
Most
the distributed
power generators, such as solar and wind in microgrids, depend
Recently, microgrids have become popular due to the distributed power generation.
highly on
weather microgrids
conditions and
arebecome
treated popular
as intermittent
sources. Therefore,
Recently,
have
due topower
the distributed
power generation
Most of the distributed power generators, such as solar and wind in microgrids, depend
microgrids
require
energy storage
togenerators,
compensate such
for power
fluctuations.
Although
batMost
of
the
distributed
power
as
solar
and
wind
in
microgrids,
highly on weather conditions and are treated as intermittent power sources. Therefore, depend
teries
are
used
for
this
purpose,
their
performance
degrades,
and
their
lifetime
becomes
highly onrequire
weather
conditions
are treated
as intermittent
sources.
Therefore
microgrids
energy
storage toand
compensate
for power
fluctuations.power
Although
batteries
shorter
due to the
rapid charging
and discharge
currents [31,49].
SCs are
the perfect solumicrogrids
require
energy
to compensate
fortheir
power
fluctuations.
Although bat
are
used for this
purpose,
theirstorage
performance
degrades, and
lifetime
becomes shorter
tion to stabilize power fluctuations and reduce the stress on batteries. Therefore, hybrid
due
to the
and discharge
[31,49].degrades,
SCs are theand
perfect
to become
teries
are rapid
used charging
for this purpose,
theircurrents
performance
theirsolution
lifetime
energy storage (HES) systems are typically used in microgrids to integrate both batteries
stabilize
power
fluctuations
and reduce
the
stress on batteries.
Therefore,
hybrid
energy
shorter
due
to
the
rapid
charging
and
discharge
currents
[31,49].
SCs
are
the
perfect
solu
and
SCs [31,49,50].
storage
(HES)
systems
are
typically
used
in
microgrids
to
integrate
both
batteries
and
tion
to stabilize power fluctuations and reduce the stress on batteries. Therefore, hybrid
Researchers
SCs
[31,49,50]. focus on implementing new strategies to integrate SCs in HES systems.,
energy
(HES)
are typically
used in microgrids
to integrate
both batterie
Three mainstorage
topologies
havesystems
been proposed
in the literature
to control HES
systems [51]:
and SCs [31,49,50].
Researchers focus on implementing new strategies to integrate SCs in HES systems
Three main topologies have been proposed in the literature to control HES systems [51
Energies 2022, 15, 7752
10 of 15
Energies 2022, 15, x FOR PEER REVIEW
10 of 15
Researchers focus on implementing new strategies to integrate SCs in HES systems.,
Passive
topology,
semi-active
topology,inand
topology.
InHES
passive
topology,
Three
main
topologies
have been proposed
the active
literature
to control
systems
[51]: as
Passive
topology,
anddirectly
active topology.
In the
passive
topology,
shown this
showntopology,
in Figuresemi-active
9a, batteries
and SCs
connect to
inverter
[52]. as
Although
intopology
Figure 9a,isbatteries
and SCs directly
connect
to the inverter
[52].
Although
this topology
a cost-effective
and simple
solution,
it brings
some
limitations,
such as less
iscontrollability
a cost-effective in
and
simple
solution,
it
brings
some
limitations,
such
as
less
controllability
sharing power between the battery and the SC, vulnerability of the batintery
sharing
power power
between
the batteryand
andthe
therequirement
SC, vulnerability
of thevoltage
batterymatching
to suddento the
to sudden
variations,
for exact
power
variations,
for exact
voltage
matching totopology
the DC bus
voltage
[53]. as
DC bus
voltageand
[53].the
Torequirement
overcome these
issues,
a semi-active
was
introduced,
To overcome these issues, a semi-active topology was introduced, as depicted in Figure 9b,
depicted in Figure 9b, which has some flexibility in the power flow control [54]. In this
which has some flexibility in the power flow control [54]. In this configuration, while
configuration, while the battery is directly connected to the inverter, the SC is connected
the battery is directly connected to the inverter, the SC is connected through a DC-to-DC
through a DC-to-DC converter. Hence the battery maintains the DC link voltage, and the
converter. Hence the battery maintains the DC link voltage, and the SCs compensate for
SCs
compensate
for theThe
sudden
main drawback
topology
the
sudden
fluctuations.
mainfluctuations.
drawback ofThe
semi-active
topologyofissemi-active
that it requires
is
that
it
requires
expensive
high
current
rated
DC-to-DC
converters
to
meet
high
expensive high current rated DC-to-DC converters to meet high power demands. Figure 9cpower
demands.
Figure
9c shows
the both
active
where
both
the
SC and the
battery
are conshows
the active
topology
where
thetopology
SC and the
battery
are
connected
to the
inverter
nected
to
the
inverter
via
separate
DC-to-DC
converters
[55].
Although
active
control
via separate DC-to-DC converters [55]. Although active control topology offers a high topology
offers a highinlevel
controllability
in voltage
both power
sharing
and
voltage
control, it
level
of controllability
bothofpower
sharing and
control,
it is the
most
expensive
approach
of the
three methods.
is the most
expensive
approach of the three methods.
Figure
9. 9.
Supercapacitor
+ battery
hybrid
topologies
used inused
microgrids
(a) passive
semi- (b)
Figure
Supercapacitor
+ battery
hybrid
topologies
in microgrids
(a)topology,
passive(b)
topology,
active
topology,
and (c) active
topology
[51].
semi-active
topology,
and (c)
active topology
[51].
So far, there have been few real-world applications of SCs in microgrids as most of
the HES technologies are at the research level. In 2016, Duke Energy installed a 1.2 MW
HES system using Maxwell Technologies’ fast response ultra-capacitors in North Carolina
in the United States. In 2018, the Fraunhofer Institute for Solar Energy Systems energized
Energies 2022, 15, x FOR PEER REVIEW
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Energies 2022, 15, 7752
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a HES system utilizing renewable energy in Borkum Island in Germany. This microgrid
So far, there have been few real-world applications of SCs in microgrids as most of
HES system
consists of 1 MW/10 kWh of Maxwell ultracapacitors and 500 kW/500 kW
the HES technologies are at the research level. In 2016, Duke Energy installed a 1.2 MW
of Li-ion
batteries
[56]. Moreover,
in 2019,
a hydrogen-supercapacitor-based
HES system
using Maxwell
Technologies’
fast response
ultra-capacitors in North Carolinarural m
crogrid
pilot
project
was
successfully
executed
in
the
Malaysian
mountainside
in the United States. In 2018, the Fraunhofer Institute for Solar
Energy Systems
energizedvillage o
“Orang
consists
of 4.7energy
kW (peak
power)
solar
PV, a 1.1This
kWmicrogrid
fuel cell, and 1
a HESAsli”.
systemThis
utilizing
renewable
in Borkum
Island
in Germany.
HES
system
consists
of
1
MW/10
kWh
of
Maxwell
ultracapacitors
and
500
kW/500
kWh
of will b
kWh supercapacitors [57]. With continuous successes at the research level,
there
Li-ion
batteries [56].
Moreover,of
in SCs
2019,in
a hydrogen-supercapacitor-based
rural microgrid
more
real-world
applications
microgrids in the near future.
pilot project was successfully executed in the Malaysian mountainside village of “Orang
Asli”. This consists of 4.7 kW (peak power) solar PV, a 1.1 kW fuel cell, and 10 kWh
5.3.supercapacitors
Portable Applications
[57]. With continuous successes at the research level, there will be more
real-world
applications
SCs in microgrids
in the
near future. power, and most traditiona
In general,
cameraofflashes
require high
instantaneous
devices use batteries to fulfill this requirement, which makes the overall design bulkie
5.3. Portable Applications
heavier, and more expensive. In modern camera flash systems, the batteries are supple
In general, camera flashes require high instantaneous power, and most traditional
mented by SCs to achieve a longer lifetime, less complexity, high flash intensity, and hig
devices use batteries to fulfill this requirement, which makes the overall design bulkier,
stability.
commercialized
brands,camera
such as
Dual
1080Pthe
Dash
Cam,
heavier,Some
and more
expensive. In modern
flash
systems,
batteries
areAvaett
supple-D60 (N
battery
included),
SC aWIFI
GPS
dashless
camQ6S4 already
useintensity,
this technology.
mented
by SCs to and
achieve
longer
lifetime,
complexity,
high flash
and high
stability.
commercialized
brands,
such as
Dual 1080P
Cam,
AvaettThis
D60 (No
FigureSome
10 depicts
a dual-cell
SC-based
camera
flashDash
system
[58,59].
circuit is ac
battery
included),
and
SC
WIFI
GPS
dash
camQ6S4
already
use
this
technology.
tivated through a series of external commands, such as activation of the charge pum
Figure
10 depicts
a dual-cell
SC-basedflash
camera
flash
system [58,59].
This current
circuit islimiting
circuit to
charge
the SCs,
LED enabling,
torch
selection,
and flash
activated through a series of external commands, such as activation of the charge pump
Once
the charge pump circuit is enabled, the SCs are charged, and when the flashligh
circuit to charge the SCs, LED enabling, flash torch selection, and flash current limiting.
drive
is the
enabled,
SCs
release
high instantaneous
power
The balanc
Once
charge the
pump
circuit
is enabled,
the SCs are charged,
and into
whenthe
theflashlight.
flashlight drive
ingisresistors
thehigh
voltage
on eachpower
cell isinto
equal
half of the
voltage t
enabled, ensure
the SCs that
release
instantaneous
the to
flashlight.
The output
balancing
resistors
that the condition
voltage on each
cell is equal
to halfflashlight
of the output
voltage
to prevent
prevent
anensure
overvoltage
on either
cell. Some
driver
chip
manufacturer
an overvoltage
condition on
either cell.
Someof
flashlight
driver
chip manufacturers
often
use active balancing
circuits
instead
resistors
to minimize
the lossesoften
and reduc
use active balancing circuits instead of resistors to minimize the losses and reduce the
the self-discharge [59]. The current switch solid-state circuit ensures that the right amoun
self-discharge [59]. The current switch solid-state circuit ensures that the right amount of
of light
is provided
provided
high-quality
photography.
Additionally,
a current lim
light is
forfor
high-quality
photography.
Additionally,
it providesitaprovides
current limiting
iting
function,
overload
protection,
and aand
steady
and consistent
function,
overload
protection,
and a steady
consistent
range of therange
flash. of the flash.
LED Enable, Turns Current Switch ON
Flash Torch Select, Select Current
Enable
Charge Pump
Circuit
R1
DC Power Supply
Balancing
Resistors
Dual Cell
SuperCaps
Flash
R2
Figure 10. Camera Flash SC-based block diagram [58,59].
Figure 10. Camera Flash SC-based block diagram [58,59].
Cordless screwdrivers are another application where SCs are used to give powe
boost function along with a fast-charging capability. For example, the 5.4 V cordless Flash
Cell screwdriver from Coleman shown in Figure 11a can charge quickly (typically in 90 s
and drive 22 screws per single charge compared to the 37 screws capability of the equiv
Energies 2022, 15, 7752
12 of 15
Cordless screwdrivers are another application where SCs are used to give power boost
function along with a fast-charging capability. For example, the 5.4 V cordless FlashCell
12 in
of 90
15s) and
screwdriver from Coleman shown in Figure 11a can charge quickly (typically
drive 22 screws per single charge compared to the 37 screws capability of the equivalent
battery-powered screwdriver, shown in Figure 11b, which takes hours to recharge.
R PEER REVIEW
(a)
(b)
Figure 11. Cordless screwdrivers:
(a) Coleman
5.4 (a)
V Flash-Cell
driven screwdriver,
Figure 11. Cordless
screwdrivers:
Coleman 5.4ultracapacitor
V Flash-Cell ultracapacitor
driven screwdriver,
(b) SKIL 2336-02 iXO 3.6
V Lithium-ion
driven screwdriver.
(b) SKIL
2336-02 iXO battery
3.6 V Lithium-ion
battery driven screwdriver.
In summary, supercapacitors
are cutting-edge
energydevices
storage devices
that aoffer a
In summary, supercapacitors
are cutting-edge
energy storage
that offer
wide range of desirable features, including high capacitance, high power density, and
wide range of desirable
features, including high capacitance, high power density, and exextended cycle times, and they are now being used in a large variety of applications, such
tended cycle times, as
and
they
are now
used
in a large
of applications,
such of
EVs,
microgrids,
andbeing
portable
products.
Table 3variety
summarizes
three practical examples
of supercapacitors
with3their
voltage ratings
andpractical
capacity. examples
as EVs, microgrids, applications
and portable
products. Table
summarizes
three
of applications of supercapacitors with their voltage ratings and capacity.
Table 3. Summary of supercapacitor applications [47,56–58].
Table 3. Summary of supercapacitor applications [47,56–58].
Application
Supercapacitor
Stack Size
Voltage Level
of the Stack
Supercapacitor Voltage Level of
Regenerative breaking
(e.g., trolley bus system inStack Size30,000 F the Stack
1500 V
Electric vehicles
Ningbo,
China
Regenerative breaking
Start-stop function
30,000 F
1500 V
(e.g., trolley bus system in
Ningbo,
(e.g.,
SkeletonChina
Technologies,
1280 F
12 V
SkelStart Engine Start Module)
Start-stop function
Hybrid
energy
storage
(e.g., Skeleton Technologies,
SkelStart
En1280 F
12 V
(HES) systems
gine Start Module)
Microgrids
(e.g., Duke Energy HES system in
1.2 MW
Not specified
Northsystems
Carolina in the
Hybrid energy storage (HES)
United
States)
(e.g., Duke Energy HES system
in North
1.2 MW
Not specified
Typically,
Camera
Flash
systems
5.4 V
Carolina
in the United States)
Portable
products
0.4-to-1 F
Screwdrivers
Typically,
Camera Flash systems
5.4 V 5.4 V
(e.g., Coleman
0.4-to-1Not
F specified
ultracapacitor-driven screwdriver)
Application
Electric vehicles
Microgrids
Portable products
Screwdrivers
(e.g., Coleman ultracapacitor-driven screw6. Conclusions
driver)
Not specified
5.4 V
It is evident that SC technology has matured over the past decade into a variety of
SC families, which achieve energy densities above 10,000 F with a longer cycle life and
6. Conclusions
many other attractive characteristics complementary to battery energy storage. With the
exponentially growing electrical vehicle market, the attractive properties of SCs have a
It is evident that
SC technology has matured over the past decade into a variety of
wide application scope as power boost mechanisms. In addition, SCs as hybrid energy
SC families, which achieve energy densities above 10,000 F with a longer cycle life and
many other attractive characteristics complementary to battery energy storage. With the
exponentially growing electrical vehicle market, the attractive properties of SCs have a
wide application scope as power boost mechanisms. In addition, SCs as hybrid energy
Energies 2022, 15, 7752
13 of 15
stores in microgrids support the growing distributed energy applications. Furthermore, an
SC is a reliable energy source for power boost requirements in some portable devices, such
as camera flashes, screwdrivers, etc. The application scope will continue to grow in other
domains due to the fast-evolving SC technological advancements.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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