Review Article

Energy & Environment

2023, Vol. 34(4) 1094–1141
PERFORMANCE EVALUATION © The Author(s) 2022
Article reuse guidelines:
OF ADVANCED ENERGY sagepub.com/journals-permissions
DOI: 10.1177/0958305X221074729
STORAGE SYSTEMS: A journals.sagepub.com/home/eae

REVIEW

Gulam Smdani1, Muhammad Remanul Islam2 ,

Ahmad Naim Ahmad Yahaya3
and Sairul Izwan Bin Safie2

Abstract
Energy systems are progressive and revolutionary for their alternative resources, technical devel-
opments, demands, effectiveness and environmental effects. The recently published research’s goal
is to assess and evaluate the systems that are already in operation and those that will be in the
future. Energy can be stored as electrical energy such as supercapacitors (SCs) and superconduct-
ing magnetic energy storage (SMES) etc., mechanical energy such as pumped hydro energy storage
(PHES), compressed air energy storage (CAES) and flywheel energy storage (FES) etc., chemical
energy, electrochemical energy such as batteries and fuel cells etc., and thermal energy.
Performance of these energy storage systems (ESSs) have been evaluated in terms of energy dens-
ity, power density, power ratings, capacitance, discharge-time, energy-efficiency, life-time and cyc-
ling-times, and costs. Supercapacitors provide highest power density (>10,0000 W/l), while
hydrogen fuel cells provide highest energy density (500-3000Wh/l) among other EESs. Batteries
also provide high energy density(200-500Wh/l). The energy efficiency is found highest in SMES sys-
tem (95-98%), and lowest in TES system (30-50%). Moreover, batteries and supercapacitors have
the cycle efficiency above 90%. PHES and CAES seem to be the most cost-effective energy storage
systems reviewed in this analysis in terms of $/kWh. In addition, power-based capital cost of super-
capacitors is lower (100-300$/kW) compared to energy-based capital cost of supercapacitors
(300-2000$/kWh). In comparison with power-based capital costs, the energy-based capital cost
of batteries is lower, which is 150-400$/kWh for Lead-acid battery, and <300$/kWh for Li-ion

1
Malaysian Institute of Chemical and Bioengineering Technology, Malacca, Malaysia
2
Malaysian Institute of Industrial Technology, Universiti Kuala Lumpur, Johor, Malaysia
3
Institute of Postgraduate Studies, Kuala Lumpur, Malaysia
Corresponding author:
Muhammad Remanul Islam, Malaysian Institute of Industrial Technology, Universiti Kuala Lumpur, Jln Persiaran Sinaran Ilmu,
Bandar Seri Alam, 81750 Masai, Johor, Malaysia.
Email: muhammad.remanul@unikl.edu.my
Smdani et al. 1095

battery. This essay may help researchers in choosing the advanced energy storage technologies for
relevant purposes.

Keywords
energy storage systems, energy density, long duration energy storage, energy storage device,
batteries and supercapacitors

Introduction
Energy storage systems (ESSs) are critical for capturing the energy from a variety of sources and
transforming it into the energy forms required for uses in a variety of industries, including utility,
manufacturing, construction, and transportation.1 Common energy sources such as fossil fuels are
used to meet consumer demand because they are easily storable while not in use.2 However, fossil
fuel combustion for power generation accounts for more than 40% of overall greenhouse gas
(GHG) emissions. The need to minimize GHG emissions has become more apparent as a result
of global climate change. As a result, most scientists have placed a high priority on reducing the
amount of electricity generated from fossil fuels.3 Renewable energy sources, such as solar and
wind, must be captured and stored when they are available.4 ESSs can bring a number of benefits
to energy systems, including greater renewable energy integration and improved economic profit-
ability. Energy storage system is also crucial for electrical system, which allows for load balancing
and peak shaving, energy management, mitigating energy fluctuations, and improved power per-
formance and consistency.5,6,7 Energy storage methods have emerged for centuries and have
experienced continuous evolution to achieve their current stage of growth, which are mature for
many storage kinds. Energy storage methods have become a critical component in attaining
energy sustainability objectives, such as energy and economic.8 On an energy micro-grid with
renewable energy resources and energy storages, Eshraghi et al. built an energy management
program with both time-of-use (TOU) and real-time-price cost structures. They claim a 28% reduc-
tion in operating expenses and pollution emissions.9 Energy storage systems come in a variety of
shapes and sizes, and they can be classified in a variety of ways. For example, a ‘Ragone plot’ is the
analysis of electrochemical energy storage systems based on the specific power and specific energy
which guide to identify the pros and cons of each energy storage methods for application.10 The
Figure 1 assists in determining the best energy storage option for a given application or necessity.
The energy generated in the unit volume or mass of the storage medium is called storage energy
density, while the power transmission rate per unit volume or mass is called power density.
When production of energy is not possible for an extended period of time, a storage device with
a high energy density and capacity to hold large quantities of energy is needed.11 A high power
density device is required when the energy discharge duration is small, such as for devices with
charge/discharge swings over short intervals. Energy storage methods also can be categorized
according to the storage duration. Short duration energy storage (SDES) refers to energy storage
for a few hours to a few days, and long duration energy storage (LDES) refers to energy storage
for a few months.12 A long duration thermal energy storage system, for example, stores thermal
energy in the underground throughout the summer for application in the winter.13 Other energy
storage systems are also used to store energy for short and/or long time for supplying uninterrupted
energy from renewable sources.
The total capacity of energy storage is currently growing drastically around the world. As per the
China Energy Storage Alliance (CNESA) worldwide operational energy project database 2020.Q1,
1096 Energy & Environment 34(4)

Figure 1. Ragone plot for supercapattery along with other electrochemical energy storage systems.10

Figure 2. Total operating energy storage facility capacity around the world (MW).14
Smdani et al. 1097

global operational energy storage project capacity reached 186.1 GW at the end of September 2020,
increased 2.2 percent from 2019. Figure 2 shows that pumped hydro storage systems contribute for
91.9% (171.03 GW) among all existing windows energy storage technologies (ESTs), followed by
electrochemical ESTs at 5.9%. (10.98 GW). Lithium-ion batteries had the highest energy generat-
ing capacity of electrochemical ESTs, accounting for 89% (9.88 GW).14 Wind and solar energy
continued to be the most popular renewable energy sources (RESs) for new capacity increase in
2020. However, changes in energy availability from such sources are common, due to factors
such as inadequate wind or sunlight. As a result, it is required to manage the power fluctuation
of a power system that includes a high number of renewable energy sources such as wind and
solar.15 By supplying voltage support, flattening output oscillations, regulating the power capacity
of the system, and harmonizing supply and demand, energy storage is a critical means of mitigating
such elevated intermittency or power quality issues. Moreover, energy storage systems provide
smooth energy flow, good control over fluctuation, power quality enhancement, energy transmis-
sion network development and ultimate reduction in costs.16 Furthermore, as the amount of
power or energy produced by RESs grows, it becomes increasingly vital to examine methods or
procedures for selecting the suitable form of ESTs for RESs grid integration applications. The rela-
tionship between ESTs and their applications is interdependent; understanding the technical prop-
erties of each EST, as well as their possible use in RESs, is critical for technology acceptance.17
Furthermore, while manufacturers and engineers provide a wide range of ESTs, the comprehensive-
ness of these technologies are examined and synthesized in a logical manner. However, energy
storage systems present numerous obstacles because none of them are perfect in every way due
to limits such as storage capacity and shape, discharge period, specific architectural or implemen-
tation criteria, energy release efficiency, and cycle life.18, 19 In addition, there are economic and
environmental issues of energy storage methods, such as GHG emissions, that must be detected
and addressed when designing and determining which technologies to deploy in an area’s grid
systems.20 The above factors must be clarified through a comprehensive survey so that important
research work from previous authors can be assembled, comprehended, and sorted in a useful
manner to make clear the study areas, which can help policymakers and professionals pick the
best energy storage technologies and mechanisms for their specific power grid.21 Given the rele-
vance and challenges of electric energy storage, various facets of the issue are examined and
emphasized with the help of a literature study.
Energy storage systems (ESS) is a method of turning electrical energy into a storable state and
then transferring the stored energy back to electricity when required.22 ESS have been available for
a long period of time, with Pumped Hydro Energy Storage (PHES) being the oldest and most preva-
lent type. Compressed air energy storage (CAES), flywheel energy storage (FES), thermal energy
storage (TES), super-capacitor (SC), and battery are all examples of ESS. All of these ESS tech-
nologies have benefits and drawbacks, and they are used based on the ESS’s intended usage as
well as other factors such as economics, geography, performance, and whether high energy or
high power supplies are required.23 According to Nadeem et al., ESSs can successfully reduce
the intermittencies generated by RESs by tracing the renewable inconsistent production trend
and charging and discharging actual power correspondingly, thus enhancing power supply and
dependability.24 Moreover, they have the ability to store the extra energy produced by RESs, limit-
ing energy wastage. Electric vehicles (EVs), whose major source of power is EES, i.e. batteries,
have recently become a very attractive mode of transportation.25 Furthermore, according to
Poullikkas et al., 26 EES offers a wide range of grid scale uses, including peak shaving, load shift-
ing, backup power, and uninterruptible power supply (UPS) etc. Many countries have recently
worked to develop large-scale storage systems to take advantage of the ESSs. However, the
1098 Energy & Environment 34(4)

development of effective, economical, and sustainable energy storage technology is critical for
quick and utility-scale implementation of renewable sources like solar, wind, tide, and wave.
In fact, the scarcity of electricity storage may be the “ball-and-chain” that prevents intermittent
renewables from breaking free and reaching the implementation levels predicted by mid-
century.27 Indeed, according to Bloomberg Technology News, Germany had to throw away
4% of its wind energy in 2015, China had to discard 17% of its renewable energy, and
California had to discard 300,000 MWh of renewable energy in the first half of 2017, all
caused by a lack of adequate electricity storage ability.28, 29 Rapid expansion of renewable
energy production might potentially be an unreasonable financial burden without enough electri-
city storage capacity. California, which has made investments a lot of money in solar energy over
the last couple of years, has not only been obligated to curtail solar power generation due to power
gluts, but has also had to pay high prices to other states, up to $25/MWh, only to get them to hold
its surplus electricity, a practice known as ’negative pricing’ 30 As a result, it is an appropriate
time to evaluate and review recent findings in this topic, which will assist future research and
spread the use and adoption of EES in real applications.
In this study, the state-of-the-art development energy storage strategies are examined from a per-
formance perspective, with an emphasis on characteristics that can help enhance the efficiency and
development of energy storage devices instead of categories and concepts. Various factors includ-
ing specific capacitance, energy and power density, operation duration, total capital costs, reliability
and lifecycles are frequently examined when evaluating the technological performance of different
energy storage devices. The development of energy storage systems in terms of these parameters are
evaluated in this work.

Energy storage systems (ESSs)

The first law of thermodynamics asserts that the amount of energy in a closed system is constant and
that energy cannot be generated or destroyed. It can only be changed from one state to the next.
Almost all energy storage techniques are based on this basic principle. The highest quantity of elec-
trical work that a storage system can harvest is provided by 31
G = H − TS (1)
G represents Gibbs free energy, H represents enthalpy, T represents temperature, and S represents
entropy. In other words, G is the highest amount of energy that can be used to perform mechanical
or electrical work.
Potential, EPOT, and/or kinetic, EKIN, energies of pumped-hydro energy storage (PHES) can be
expressed32:
EPOT = fd (2)
1
EKIN = mv2 (3)
2
Here, f stands for force, d stands for distance, m stands for mass, and v stands for velocity.
For flywheel energy storage (FES), the kinetic energy (EKIN) can be expressed as
1
EKIN = Iω2 (4)
2
where I is the moment of inertia and ω is the spinning system’s angular velocity. The moment of
Smdani et al. 1099

inertia can be defined as

1 1
I = mr 2 = ρlπr4 (5)
2 2
Here, m is the mass of the body, r is the radius, ρ is the density, l is the length. It can be seen from
equation (5) that high-density materials with a vast radius assist in energy storage. For mechanical
energy storage in FES, Equation (5) can alternatively be written as
σmS
EKIN = (6)
ρ
The maximum stress is denoted by σ m , while the shape factor is denoted by S. It is conceivable to
reach storage capacities larger than 200 kJ/kg of flywheel mass using current flywheels comprised
of reinforced carbon fibers.
The gas law (PV = nRT) is used to store energy in compressed air energy storage (CAES), and
the total work, w, is determined by PdV integrated across the incremental volume change, dV.
Thermal energy storage (TES) is determined by the quantity of heat, q, that could be stored in a
medium or material with a specified volume, V, material density, ρ, and specific heat, CP, resulting
in a temperature increase of ΔT. This can be formulated in the following way:
q = ρCP VΔT (7)
In batteries, charge is held at the electrodes, whereas in capacitors and supercapacitors, charge is
stored at the electrochemical interface. In fuel cells, charge is also stored in the chemical bonds
of molecules.
Energy storage provides a variety of direct and supplementary services to the production, trans-
mission, and distribution of energy, as well as assisting energy end-users. The grid system’s power,
flexibility, technical innovations, and automation, as well as region, consumer expectations, and
regulatory limits, influence the capacity, type, and quality of various services.33, 34 The Energy
Generation was the first entity to get profit from energy storage services by postponing
maximum capacity plant operations, energy stored reserves for on-peak provision, frequency
control, adaptability, production time-shifting, and the use of additional renewable resources.
Variations in generation, particularly from renewable resources, can be managed. An effective
energy storage system eliminates the necessity for a large transmission line to send electricity to
other locations. Electrochemical energy storage, mechanical energy storage, thermal energy
storage (TES), superconducting magnetic energy storage (SMES) and fuel cell energy storage
are some of the most common types of energy storage systems. Capacitors, supercapacitors
(SCs) and batteries lies in the electrochemical energy storage systems. In addition, pumped
hydro energy storage (PHES), compressed-air energy storage (CAES) and flywheel energy
storage (FES) are categorized in the mechanical energy storage systems.35, 36 Other types of
energy storage, such as biological energy storage, are not discussed in this work since they have
not been the subject of considerable storage research.

Performance of energy storage systems

Energy density
Energy storage system can be divided into two types based on their energy density, such as high
energy density and low energy density. Energy storage devices having large power ratings with
1100 Energy & Environment 34(4)

Table 1. Power density, Energy density, Power ratings and energy efficiency of different energy storage
systems.

Power Energy density Power rating Energy Technology

EESS density (W/l) (Wh/l) (MW) Efficiency (%) maturity Ref.
1,38
Pumped hydro 0.5-1.5 0.5-1.5 100–5000 65-87 Commercialized/
(PHES) mature
39, 40
Compressed air 0.5-2 3-6 Up to 300 80-89 Commercializing
(CAES) and maturing
41, 42
Flywheel (FES) 1000–2000 20-80 0.1–20 90-95 Commoditising,
Proven
43, 44
Pb-acid battery 10–400 50-80 0–40 75-80 Completely
commercialized/
mature
44
Ni-Cd battery 80–600 60-150 0–40 85-90 Completely
commercialized/
mature
Na-S battery ∼140–180 150-250 <34 80-90 Commoditising, 45, 46

Proven
47
Li-ion battery 1500–10,000 200-500 1–100 85-90 Commoditising,
Proven
48
Capacitor >100,000 2–10 0–0.05 92-95 Commoditising,
Proven
49
Supercapacitors >100,000 2.5-15 0–0.3 90-95 Commoditising,
Proven
50
Superconducting 1000–4000 0.2-2.5 0.1–10 95-98 Commoditising,
Magnetic (SMES) Proven
49
Hydrogen fuel cell >500 500–3000 <50 90-95 Commoditising,
Proven
1, 39
TES - 80-120 0-5 30-50 Commoditising,
Proven

low energy density are suitable for power quality or uninterruptible power supply (UPS). Power
quality and dependability includes energy storage systems, like supercapacitors, superconducting
magnetic energy storage system (SMES), flywheels, and batteries. On the other hand, high
energy density energy storage devices are developed for energy management. Pumped hydroelec-
tric storage (PHES), compressed air energy storage system (CAES), thermal energy storage system
(TES), substantial batteries and fuel cells are used in energy management. Table 1 shows the energy
densities of various energy storage systems. Figure 3 represents a comparative analysis of power
density and energy density of various energy storage systems. The energy density of the mechanical
energy storage devices is low. For example, PHES devices have energy density of 0.5−1.5 Wh/l
only.1 Gravimetric potential energy is used to store the energy in the PHES system. The stored
energy (E) can be calculated as follows.37:
E = mgh (8)
Here, m is the mass of water, g is the acceleration force due to gravity, and h is the head. From
equation (8), it can be seen that greater value of m is required for the production of electricity
through PHES technology. This may be one of the reasons of low energy density of PHES
Smdani et al. 1101

Figure 3. Comparative analysis of power density and energy density of various energy storage systems.

systems. In addition, CAES systems have also low energy density which value is 3−6 Wh/l.39 On
the other hand, flywheel (FES) devices have little bit higher energy density (20−80 Wh/l) compared
to PHES and CAES systems.41 FES system holds energy in the rotational motion of a rotating mass
by activating a flywheel through a motor/generator combination during off-peak periods and deli-
vering the electricity through the rotational inertia of the flywheel during peak times. The energy
density of FES system is calculated using the following formula37:
σ
Energy density = (9)
ρ

where σ is the material’s ultimate strength and ρ is the density of the substance. The high strength fly-
wheel materials with low mass density increases the energy density of the FES systems. Furthermore,
hydrogen fuel cells have very high energy density ranging from 500 to 3000 Wh/l. Because liquefied
hydrogen has a high specific weight (71 g/l), it offers the highest energy density per unit volume of any
fuel or energy source. For example, 1 kg of hydrogen has the same amount of energy as 2.1 kg of
natural gas or 2.8 kg of oil.51 This feature of it, which makes hydrogen the fuel utilized in spaceship
propulsion and electricity generation.52 Again, thermal energy storage (TES) systems also provide high
energy density ranging from 80 Wh/l to 120 Wh/l. Generally, the energy density of electrochemical
energy storage (EES) devices is higher than that of mechanical energy storage technologies, though
capacitors have comparatively lower energy density than battery technologies. Supercapacitors and
superconducting magnetic energy storage systems provide energy density of 2.5−15 Wh/l and 0.2
−2.5 wh/l, respectively. Among all the energy storage devices, Pb-A, Ni-Cd, Na-S, NaNiCl2, and
Li-ion-based EES systems have a higher energy density that allow them to supply more energy
over a longer period of time.53 Moreover, the energy density of Li-ion batteries is higher than other
conventional rechargeable batteries.
While batteries are the most common EES technology, supercapacitors have fast charging cap-
abilities that complement those of batteries, and their use has grown significantly in the last decade.
1102 Energy & Environment 34(4)

The energy density, E (Whkg−1) of supercapacitors can be expressed as follows.54:

t2
E = ∫ IVdt (10)
t1

Here, I = Current density (Ag–1),

V = Voltage (V),
t1 = starting time (s) of discharging, and
t2 = ending time (s) of discharging.

The energy-storage methods in the two kinds of devices are based on basically distinct mechan-
isms, which results in the two types of systems having differing charge-storage qualities. Redox
reactions deposit large quantities of energy (200 Whkg–1) in batteries, resulting in sluggish char-
ging. However, supercapacitors conserve significantly less energy (5 Whkg–1) and do so very
quickly through the creation of electrical double layers.55 Supercapacitors have achieved the
fast-charging ability for the pseudocapacitance characteristic of electrode materials. Battery-like
redox processes are involved in pseudocapacitive compounds at rates similar to double-layer depos-
ition in capacitive compounds. The production of hybrid composites energy storage systems with
ionic electrolyte was focused on the incorporation of pseudocapacitive compounds into EES
devices. Hybrid EES system was developed with the goal of performing at similar power ratings
of electrical double-layer capacitors (EDLC) devices but with considerably increased energy dens-
ities due to the use of redox-active substances. An EDLC is made up of two activated carbon
(AC)-based electrodes (Figure 4a). On the other hand, hybrid devices consist of two electrodes,
one of which is a redox compound and the other of which is a AC-based electrical double-layer
electrode (Figure 4b). Figure 5 shows the Ragone plot of the hybrid EES devices with activated
carbon (AC)-based electrodes in H2SO4 and KOH electrolytes.56 The energy density and power
density in measured 6.0 Whkg−1 and 4.0 kWkg−1, respectively, at a 10 s time constant, compared
to 1.5 Whkg−1 and 1.1 kWkg−1 for commercially available activated carbon AC for hybrid EES
systems as shown in Figure 6. Compared to the aqueous electrolytes, hybrid composites electrodes
with non-aqueous electrolytes provides higher energy density. Furthermore, hybrid EES devices

Figure 4. (a) An EDLC with two activated carbon (AC)-based electrodes; (b) hybrid supercapacitors devices
consist of two electrodes, one of which is a redox compound and the other of which is a AC-based electrical
double-layer electrode.
Smdani et al. 1103

Figure 5. Ragone plot of supercapacitors with activated carbon (AC)-based electrodes in H2SO4 and KOH
electrolytes 56.

Figure 6. Comparison of specific energy of different electrode materials of EDLCs, Li-ion capacitors, and
aqueous and non-aqueous pseudocapacitors 61.

including non-aqueous electrolytes have demonstrated improved energy densities compared to the
symmetric EDLC devices.57 Lithium-ion capacitors (LICs) are well-known hybrid EES technolo-
gies that combine Li-ion and activated-carbon materials in electrode of EES devices. The very first
reported LIC anodes were large LICs based on Li4Ti5O12, which perform redox reactions at 1.55 V
through Li/Li+ mechanism. Energy density of LICs based on Li4Ti5O12 have found up to 40 Whkg–1
which is 3–4 times greater than those of typical EDLCs.58 Li4Ti5O12 is a very effective electrode
material which can sustain over than 10000 cycle without serious deterioration because it only
suffers a minor volume change and has a tiny potential window. Nowadays, graphite is often
used as a Faradic anode instead of Li4Ti5O12 because graphite has a larger specific capacity and
1104 Energy & Environment 34(4)

smaller potential compared to Li4Ti5O12. Graphite-based LICs hybrid EES devices provided energy
densities up to 40−60 Whkg–1. However, the slow mobility of Li+ in graphite, the emergence of an
electrode-electrolyte interphase, and the necessity for pre-lithiation of the graphite electrode restrict
the use of the graphite-based LICs. To overcome these limitations, non-aqueous pseudocapacitive
compounds have been introduced in the EES devices which improve energy density as well as
power density. At high power rates, models of hybrid devices based on the fundamental pseudocapa-
citive compounds T-Nb2O5 and TiO2(B) have achieved energy densities comparable to those of
Li4Ti5O12 -based LICs.59, 60 Furthermore, thick Nb2O5 electrodes (with mass loadings >10 mgcm–2)
have shown that fundamental pseudocapacitive elements can give superior efficiency at commercially
viable mass loading condition. Due to the paucity of recognized materials and the complexity of
nano-structuring, favorable pseudocapacitive components with enhanced voltages vs Li/Li+ still pose
a barrier for hybrid devices in the development into high energy-density systems.61
It can be seen in Table 1 that the energy density of supercapacitors is very low. Between the
energy density of supercapacitors (<20 Whkg−1) and the energy density of batteries (30–
200 Whkg−1), there is still a significant disparity.62 In the realm of supercapacitors, improving
energy density is currently a research focus and difficulty. Improved production method and tech-
nology are effective ways to increase supercapacitor storage capacity, but finding novel electrolyte
and electrode compounds with greater corresponding electrochemical properties is critical and chal-
lenging in the long run. Supercapacitors are bulkier because they have a lower energy density.
Enhancing the specific surface area of electrode materials or expanding the operating voltage
window, or both, can improve supercapacitor energy densities. More research is being conducted
to produce novel materials with a large surface area and appropriate organic electrolytes that can
withstand a wider voltage range.63 Researchers have studied to find other ways also to improve
the energy density of supercapacitors. Another convenient way to improve the energy density of
supercapacitor is to make supercapattery device which is the combination of a supercapacitor
and a battery. In supercapattery, the mechanism of pseudocapacitor is avoided, and the mechanism
of electric double layer capacitor (EDLC) and battery are used. Table 2 shows energy density and
capacity retention of various supercapattery devices. It is clear from figure that the supercapattery
provides high energy density with high power density while other electrochemical energy storage
devices give either higher energy density or power density. Supercapacitors’ energy densities could
become equivalent to batteries if these gaps are resolved appropriately.

Power density
The fast growth of science and engineering has resulted in a massive increase in energy demand.
The power density (W/kg or W/litre) is calculated by dividing the total output power by the
storage system’s volume. The whole volume of the energy storage system, including the energy
storage component, peripherals and associated structures, and the inverter scheme, is the volume
of the storage device.39 Table 1 shows the energy density of different kinds of energy storage
systems. Among all types of energy storage systems, capacitors and supercapacitors have high
power density (≈100,000 W/l). Medium energy density is found in FES, batteries, hydrogen fuel
cell and SMES. However, PHES and CAES have very low power density (<2 W/l). The power
densities of FES, Batteries, supercapacitors and SMES, on the other hand, are quite high,
making them ideal for power quality applications requiring significant discharge currents and
quick responses.
Among all types of energy storage systems, supercapacitors have high power density
(≈100,000 W/l). On the other hand, batteries have relatively less power density (50−200 W/kg).73
 Smdani et al.

Table 2. Energy density, power density and capacity retention of various supercapattery devices.

Power Energy Specific Capacity

Operating density (W/ density (Wh/ capacity retention
Materials Synthesis method Electrolyte voltage (V) Kg) Kg) (mAhg–1) (cycles) Ref…
64
MWCNT-Co3O4-Ag Hydrothermal route KOH 0-0.5 297.5 16.5 83.88 93.1% (3000)
65
MWCNT-Co3O4-Au Hydrothermal route KOH 0-0.5 302 18.80 108 91.9 % (3500)
66
CoFe2O4-rGO// Nano-casting and KOH 0-1.6 953.0 77.2 - 96% (5000)
CuCo2O4-rGO co-precipitation
process
67
Co3O4 Sol-gel process. KOH 0-0.5 742 40 620 nearly 100%
(5000)
68
CoNiSi In-situ polymerization Ni(OH)2 -08-0.6 63.3 20 254 82% (1000)
69
MnCo2O4 Bottom-up KOH 0-1.8 318.9 33.8 152.7 ∼85%, (1000)
70
MnFe2O4 In-situ polymerization KOH 0-1.0 290 27 824 F/g 94% (1000)
71
Ni-Co LDH//rGO Hydro-thermal KOH 0-0.5 1499 188 2682 F/g 82% (5000)
72
NiO-ln2O3 Hydro-thermal KOH -0.2-0.6 1752 26.2 766.65 C/g 79% (5000)
∗
(Ni-Co LDH) = nickel-cobalt layered double hydroxide, rGO = reduced graphene oxide.
1105
1106 Energy & Environment 34(4)

The power density of electrochemical energy storage devices can be expressed as.74:
1
Pdensity, bat = EF(1 − EF).Vr2 . (11)
Wt. or Vol.
Where EF stands for energy pulse efficiency in the charging process, R stands for resistance, Vr stands
for voltage at the point of resistance, and Wt and Vol stand for weight and volume, respectively.
Equation (11) becomes Equation (12), if the capacitor’s power pulse is 3/4 Vr.
9 1
Pdensity, EC = (1 − EF).Vr2 . (12)
16 Wt. or Vol.
The power capacities of batteries and suercapacitors (ECs) should be evaluated on a standardized basis,
that is, at the similar energy efficiency. Batteries and EDLCs are frequently described in terms of their
matched impedance power (PMI = V20/4R). In this context, PMI equates to a power with a 50% effi-
ciency, and so is of little practical use in most cases. The ESS’s pulse efficiency is especially important
in industries with large power demands, such as manufacturing and transportation (cars). Equation (12)
can be used to calculate an EC’s usable power capability in certain situations. For these reasons, high-
power gadgets require very low resistance.
Researchers have been trying to improve the power density of energy storage devices. Iqbal et al.
have produced a supercapattery to analyse the combined effect of supercapacitors and batteries.64
They used MWCNT-Co3O4-Ag as electrode materials and KOH as electrolyte materials. They have
recorded the power density of 297.5 Wkg−1 at 0.2 Ag−1 current density. The high power density
may found for the good dispersion and intercalation of Co3O4 into MWCNTs. Then, the incorpor-
ation of high conductive Ag nanoparticles with MWCNT-Co3O4 boast the electrical performance of
the supercapattery. Further, Subramaniam et al. followed the same methodology and used Au nano-
particles with Co3O4@MWCNTs instead of Ag to make supercapattery.65 They found better elec-
trochemical performance of Co3O4/Au@MWCNTs composite electrodes. It provided the higher
electrical conductivity due to the distinctive shape and a rise in the amount of active sites which
results in a power density of 302 Wkg−1. Devi et al. investigated the performance of Co3O4
using the sol-gel process and employed it as an electrode material for supercapattery. The
Co3O4-based electrode provided high power density for the faradic reaction, which causes more
charge to be stored. Obrezkov et al. have analysed the performance of potassium (K) ion batteries
and identified that K ion batteries provide high power density.75 They have used
poly(N-phenyl-5,10-dihydrophenazine) (p-DPPZ) as cathode materials in potassium ion batteries.
Results showed that the p-DPPZ//K batteries provide power density of >104 Wkg−1, which look
identical to supercapacitors. Table 2 represents the power density of various supercapattery devices.

Capacitance
In recent decades, scientists have begun to think more deeply about what has to be accomplished to
regulate energy utilization and conserve natural resources. While much research is being conducted
on renewable energy, as the world’s population continues to rise, focus must also be paid in upgrad-
ing or designing new energy storage technologies. Most of the renewable sources cannot provide
energies continuously because there is peak time for energy generation from renewable sources. For
this reason, the storage media must be capable of storing massive amounts of energy during peak
hours until they are needed during periods when no energy is generated.76 Scientists are increas-
ingly interested in renewable energy storage elements with high specific capacitance.
Smdani et al. 1107

The most common energy storage systems used by electrochemical principles of transforming
electrical and chemical energies include supercapacitors (SCs), batteries and fuel cells.
Electrochemical energy storage systems have gained a lot of interest because of their distinct char-
acteristics, such as higher capacitance (Csp), a higher life expectancy, and improved capacity reten-
tion as depicted in Table 2. The Csp of the electrochemical energy storage system (EESS) largely
depends on the electrode and electrolyte materials. The capacity of electrode of EESS can be cal-
culated by the equation (13).77:
zF
C= (13)
3.6M
Here, M = Molecular weight,
Z = Number of transferred electrons,
F = Faradic constant.

According to the equation (13), a lower molecular weight and more electrons in unit mass can
provide a larger capacity of the electrodes. Lithium-based electrode has a high theoretical capacity
of 3850 mAhg−1 for their low molecular weight. The molecular weight of the metal oxides is small
in size. For this reason, MnO, FeO, CoO, NiO, and CuO have a high theoretical capacity of 756.0,
746.6, 715.7, 717.6, 674.3, and 674.3 mAhg–1, respectively. Materials having a significant amount
of transferred electrons also can have a high specific capacitance when using the same component
combination. For example, Cu2O has a capacity of 372 mAhg–1 which is lower than the capacity
(674 mAhg–1) of CuO.78 As electrode materials with enhanced specific capacities, lighter ions
are preferred in EESS.
Carbon nanoparticles are advantageous in electrochemical devices because of their low molecu-
lar weight, relatively inexpensive, and low toxicity. In lithium ion batteries, graphite and graphene
are frequently employed as electrode material, whereas active carbons, CNTs, and graphene are uti-
lized as electrode materials in supercapacitors. Researchers concentrated on the construction of sp2
and sp3 hybrid carbon, functionalization of carbon molecules, and diverse structures due to the fact
that they only included carbon elements. Graphene has high specific capacitance (550 Fg−1) for
their large specific surface area (SSA) of about 2600 m2g−1. However, because to the tendency
of layer stacking or agglomeration, it has been difficult to achieve these levels. To avoid the
agglomeration, many methods have been used, including heteroatom doping and the fabrication
of three-dimensional (3D) networks. Zhu et al. 79 used nanofibers to generate N-doped porous
carbon (NPCs) structure and utilized as electrode martials. They have found the specific capacitance
of 316 Fg−1 at a current density of 1 Ag−1. Results also showed that the porous structure of carbon
accelerated the double layer capacitance and redox reaction directed to the pseudocapacitance at
low scan rates. Both the double layer capacitance and redox reaction pseudocapacitance influence
to the electrochemical capacitance of N-doped carbon electrodes. However, at high scanning rates,
porous structure got precedence over electrochemical activity. Jiang et al. 80 have studied the impact
of pores in nitrogen-doped holey graphene (NHG). They have investigated the specific capacitance
of NHG-based electrode in a three-electrode system and found a specific capacitance of 343 Fg-1 at
a current density of 0.3 Ag−1. The NHG’s exceptional performance was achieved for the material’s
porous structure, which resulted in accelerated electron diffusion due to the pores’ greater electro-
lyte accommodation as shown in Figure 7. Sun et al.studied the low-cost hydrothermal process
using hydrazine. Results showed that N-doped graphene nanosheets (NGS) provides a specific cap-
acitance of 326 Fg−1 at a current density of 0.2 Ag−1 in a three-electrode system. However, this
method has some safety risk due to the difficulties of preserving hydrazine. Kumar et al. 81 also
1108 Energy & Environment 34(4)

Figure 7. TEM images of nitrogen-doped holey graphene and nitrogen-doped graphene used in
supercapacitors (a, c), and SEM images of nitrogen-doped holey graphene and nitrogen-doped graphene used
in supercapacitors (b, d) 83.

investigated the use of ammonium hydroxide instead of hydrazine to synthesis nitrogen-doped gra-
phene (NG) in a low-cost hydrothermal process. When tested in a three-electrode setup, they were
succeeded of creating NG electrodes with a specific capacitance of 459 Fg−1 at a current density of
1 mAcm−2. The NG also produced energy densities ranging from 25.8 to 63.5 W hkg−1 with power
densities ranging from 0.7 to 15.5 kWkg−1. Gopalsamy et al.82 introduced co-doping method to
improve the specific capacitance of graphene for high performance electrochemical energy
storage devices. They have used nitrogen and sulfur to produce co-doped graphene nanoribbons
(NS-GNRs) which resulted a specific capacitance of 442 Fg−1 at a current density of 0.5 Ag−1.
Table 3 describes the specific capacitance of various EESSs.
Supercapacitors were constructed with carbon-based materials because of their enormous
surface area. Metal oxides, on the other hand, have a high specific capacitance and low resistance,
enabling it possible to make a supercapacitor with high capacitance. Transition metal-based elec-
trodes are gaining a lot of interest nowadays. Nickel appears as a possible candidate of electrode
materials among other metals for its outstanding conductive nature, non-toxicity, availability,
low cost, and other factors. The synthesis of high surface area electrodes using nickel can be accom-
plished through a variety of processes, including sol–gel, electrodeposition, hydrothermal, sol-
vothermal, and others, which result in a shorter ion diffusion length and improved electrolyte
penetration. Various morphology of nickel including nanoparticles, micro particles, nanowires
and thin films are used to produce electrodes of EESS. Because different morphology of nickels
provides different specific capacitance of electrodes. For example, Cheng et al. used three sizes
of nickel particles to produce electrodes, including nanospheres, nanobelts and nanoparticles.94
 Smdani et al.

Table 3. Specific capacitance of various EESSs.

Materials Morphology Method Electrolyte Potential (V) Csp (F/g) Ref..

79
N-doped porous graphene Nanofiber-like Carbonization KOH 0-1.0 316 (1 Ag-1)
80
N-doped holey graphene In-plane hole Ball milling KOH -0.2-0.8 343 (0.3 Ag-1)
81
N-graphene Honeycomb lattice Hydrothermal H2SO4 -0.2-0.8 459 (1 mA.cm-2)
83
N-graphene Nanosheets One-step hydrothermal reaction KOH -1.2-0 326 (0.2 Ag-1)
82
NS- graphene Nanoribbons Solvothermal Na2SO4 0-1.8 442 (0.5 Ag-1)
84
Ni@Al Hexagonal Hydrothermal KOH 0–0.6 1146 (2 mVs-1)
85
Ni–Mn Lamellar Co-precipitation KOH 0–0.5 1268 (1 Ag-1)
78
Graphene Ni–Mn LDH/NF Flower-like Hydrothermal KOH 0–0.45 1538 (1 Ag-1)
85
GNS/Ni–Al LDH/NF Agglomerated sheets In situ- crystallization KOH 0–0.47 781.5
86
RuO2 Amorphous and porous Electrodeposition H2SO4 650 (0.349 mg.cm-2)
87
RuO2/MWCNT Nanoparticles Solvothermal H2SO4 0-1.0 900
88
RuO2.xH2O/ rGO/CNT Nanoparticles Anodic Deposition HCl 0-1.0 973 (25 mVs−1)
89
HRGO-RuO2 Nanoparticles Sol-gel method PVA-H2SO4 0-1.0 418 (1 Ag-1)
90
PPy − GO − CDs Ultra-small In-situ polymerization H2SO4 0-1.0 576 (0.5 Ag-1)
91
PANI/rGO 3D structure Steamed water H2SO4 -0.2-0.8 1182 (1 Ag-1)
92
PMTA/CNTs/rGO In-situ polymerization KOH -0.3-0.7 616 (1 Ag-1)
93
PANI/CNTs/GNS 3D structure In-situ polymerization KOH -0.8-0.4 1035 (1 mVs−1)
1109
1110 Energy & Environment 34(4)

The electrodes made of NiO based nanospheres, nanobelts and nanoparticles provided the specific
capacitance of 45.2, 556.2 and 609.5 Fg−1, respectively. Yu et al. 95 have studied the effect of
chestnut-like porous nanosphere NiO on the capacitance of electrodes. They have created chestnut-
like porous nanosphere NiO. The produced electrodes exhibited a capacitance of 982 Fg−1 in 6 M
KOH at a current density of 1 Ag−1 and 92.6% of capacitance retention after 10,000 cycles. This
high performance can be attributed to the distinctive and organized core-shell structure and greater
specific surface area of chestnut-like porous NiO, which can deliver an extensive porous surface
area for the electrode-electrolyte contact, hence speeding up electrochemical reactions.
Moreover, manganese or ruthenium oxides-based electrodes also provide high capacitance for
their pseudocapacitive characteristics. Fang et al. 96 produced a thing film of ruthenium oxides
(RuO2) deposited in metal substrates. At 200°C, they were able to create a very porous RuO2
film electrode with a specific capacitance of 593 Fg−1. Kim et al. 97 have further improved the spe-
cific capacitance of RuO2 by incorporating carbon nanotubes (CNTs) to form three-dimensional
(3D) network structure. Results showed that the specific capacitance of RuO2/CNTs electrode
increased to 1170 Fg−1 for the 3D interlinked structure. To improve the pseudocapacitance,
RuO2 was transformed into a hydrated amorphous and porous structure and a tiny size. Because
this structure gives a high surface area and creates conduction channels for protons to quickly
access even the interior part of the RuO2. They showed that carboxylated carbon nanotubes can
increase the degree of dispersion of RuO2 which can prevent the agglomeration of RuO2, resulting
higher surface area as well as higher specific capacitance. Shi et al. 98 followed the same procedure
substituting RuO2 with the MnO2. The advantages of using MnO2 in electrodes are high
mass-loading and high charging and discharging rate. Results showed that MnO2/
N-AC-MWCNTs (N-doped activated carbon coated MWCNTs) electrodes provided 311.7 Fg−1
at a mass loading of 20 mgcm−2.
In most circumstances, conductive polymers (CPs) enhance the specific capacitance of elec-
trochemical energy storage devices by allowing faradaic redox processes to occur, resulting in
greater pseudocapacitance. Polymers have a wide range of applications for its excellent electrical,
mechanical, thermal and optical properties.99,100,101,102,103,104 CPs are able to utilize the entire
material into the charging process while the only surfaces are involved in the charging process
in case of other ordinary electrode materials. During the redox reaction in CPs, electrons from
the electrolyte transport into and out of the polymeric chains. For this reason, CPs-based electro-
des provide higher specific capacitance to the EESS. For example, Yan et al. have investigated the
specific capacitance of polyaniline (PANI)-based electrodes.93 Figure 8 shows the pseudocapaci-
tance behavior of PANI and hybrid electric double layer capacitance (EDLC)/pseudocapacitance
behavior of PANI-based composites. They compared the specific capacitance of four types of
electrodes, such as PANI, PANI/CNTs, PANI/graphene Nano sheets (GNS) and PANI/CNTs/
GNS electrodes and recorded the capacitance of 115, 780, 1046, and 1035 Fg−1, respectively.
The synergistic impact between GNS and PANI is responsible for the increased specific capaci-
tance. Furthermore, the small nanometer-sized PANI can have improved electrode/electrolyte
interface areas, resulting in high electroactive regions and short diffusion lengths, ensuring
that PANI is fully utilized. It was observed that the PANI/CNTs/GNS composite electrodes
showed higher value of specific capacitance compared to PANI/CNTs electrodes, but lower
than that PANI/GNS electrodes. This may happen for the formation of more agglomeration of
PANI in PANI/CNTs/GNS composite as shown in Figure 9. PANI was also studied by Zhang
et al. 91, who used a simple, enclosed vessel, steamed water regulation procedure to generate
PANI/rGO films as depicted in Figure 10. When evaluated in a three-electrode system with a
current density of 1 Ag−1, the PANI/rGO films generated had an amazing capacitance of
Smdani et al. 1111

Figure 8. (a) CV curves at scan rates of 10 mVs−1 of pure PANI, CNT/PANI, GNS/PANI and GNS/CNT/
PANI composite electrode materials of supercapacitors, and (b) CV curves at different scan rates of 1, 10, 20
and 50mVs−1 of GNS/CNT/PANI composite electrode materials of supercapacitors 93.

1182 Fg−1. Zhang et al.90 also investigated how polypyrrole (PPy) may be incorporated into a
graphene oxide/carbon dots (GO/CDs) composite material. The purpose of the PPy was to
make electron transmission easier in order to lower internal resistance. Results showed that the
PPy-GO-CDs composite electrodes exhibit a specific capacitance of 576 Fg−1 at a current
density of 0.5 Ag−1 in a three-electrode system.

Power ratings and discharge duration

Discharge duration of energy storage devices is an important parameter for industrialization. High
discharge time is preferable for uninterrupted energy supply using energy storage devices. The
1112 Energy & Environment 34(4)

Figure 9. (a) TEM image of pure PANI and (b–d) SEM images of CNT/PANI, GNS/PANI and GNS/CNT/
PANI composite electrode materials of supercapacitors 93.

discharge time duration is largely dependent on the self-discharge mechanism of energy storage
devices. The energy storage devices can supply energy for long time if the self-discharge ratio is
low. Table 4 shows the comparison of discharge duration of different energy storage systems.
PHES, CAES and fuel cells have discharge duration more than 12 h. PHES power ratings are nor-
mally in the hundreds or even thousands of megawatts scale, while energy storage capability is
dependent to the altitude difference between the bottom and top reservoirs as well as the amount
of water stored. A PHES can often store enough energy to run for several hours, and because
energy losses are minimal, such a system can store enormous amounts of energy over
months.105 CAES is another widely established technology that can store enormous amounts of
energy and supply high power. Diabatic-CAES has certain noteworthy technical features, such
as a high power rating (100–300 MW) and a big storage ability with a lengthy discharge
time.106 However, FES typically possess a power rating in the hundreds of kW level, with
modular components exceeding several MW, and a discharge period of seconds to minutes. The
features of the motor-generator unit and the related power electronics govern the power rating of
a FES system.107 The lengthy discharge time of these devices is possible for their low self-discharge
ratio. These devices can supply energy for very long time. Moreover, molten salt, lead-acid (LA)
batteries, flow batteries and Li-ion based small-scale batteries have a moderate self-discharge pro-
portion, which allows them a discharge time of up to 12 h.25. On the other hand, supercapacitors
have a much more self-discharge proportion, that is why their discharge time is usually lower
Smdani et al. 1113

Figure 10. The preparation scheme of rGO/PANI hybrid films for supercapacitors by steamed water
regulation techniques 91.

Table 4. Discharge duration of different energy storage systems (ESSs).

ESSs Discharge time Application Ref

112
Pumped hydro energy storage (PHES) 4-16 h Electric energy time shift
113, 114
Compressed air energy storage (CAES) 2-30 h Arbitrage, Spinning reserve, Black start
applications
115
Flywheel energy storage (FES) Few minutes Renewable energy capacity increasing
13
Molten salt Few hours Time shift for renewable energy
Li-ion battery 1 min – 8 h Management of periodicity 49

Lead-acid (LA) 1 min – 8 h Control of frequency 116

117
Flow battery Few hours Renewable energy capacity increasing
118
Supercapacitors seconds Control of frequency
24
SMES seconds Reactive power support
118
Hydrogen fuel cell Minutes to weeks Time shift for renewable energy
109
TES 1-8 h Load shifting and electricity generation
for heat engine cycles

than one hour.108 Another device that can store power virtually directly is superconducting mag-
netic energy storage (SMES). SMES typically possess a power rating of hundreds of kW to only
few MW with a short discharge time (<1 min). SMES generates a dynamic electric field, or a magnetic
1114 Energy & Environment 34(4)

Figure 11. Relation between power ratings and discharge time of energy storage systems.

field, by passing a current across a superconducting coil, rather of collecting charges and causing a
static electric field. There is no resistance in the superconducting state of the coil material, and
current can flow through it endlessly with absolutely little losses.109 Moreover, the power capacity
of TES normally ranges from a few hundred kW to several megawatts.110 The power ratings of dif-
ferent energy storage devices are given in Table 1. Figure 11 demonstrates the graphical representation
of discharge time of the different energy storage systems in terms of power rating. The EES systems
can be categorized into three groups according to the power ratings and discharge time.111.

1. Power quality: Because flywheels, batteries, SMES, capacitors, and supercapacitors have a
quick response time, they can be used for power quality issues such as immediate voltage
drop, and back-up UPS. For this type of service, the normal power rating is less than 1 MW.
2. Energy management: PHES and CAES are designed for applications with discharge frequencies
ranging from hourly to daily at scales greater than 100 MW. Devices can be employed for
large-scale generating energy management, such as capacity levelling, ramping/load tracking,
and frequency regulation. Intermediate energy management with a capacity of 10–100 MW
can benefit from big batteries, flow batteries, fuel cells and FES.
3. Bridging power: Batteries, flow batteries, hydrogen fuel cells, and Lead-Air batteries not only
have a quick response (∼1 s), but also a lengthy discharge period (hours), making them better
adapted for bridging power. Large power ratings are required for these operations range from
100 kW to 10 MW.

Long-duration energy storage (LDES). Energy storage devices, that can supply electrical energy at least
10 hours continuously are termed as long duration energy storage (LDES). Usually, LDES devices are
used where backup of continuous energy supply for long time due to the blackout of power grid or
other severe incident that affected the power system. Till now, gas or diesel generators are used to
supply power as backup power, which emit pollutants and are restricted by fuel supply. Another
important application of LDES systems is the integration of large numbers of variable renewable
energy (VRE). Intensely decarbonized energy systems are essential for achieving our aims of mitigat-
ing the worst impacts of climate change and running life in a much more sustainable manner. Several
Smdani et al. 1115

studies have found that LDES is required when renewable uptake exceeds 80%. Furthermore, the cap-
ability of the utilization of renewable energy is largely depends on the ability of the energy storage
devices. Only 10-15% of the energy production capacity form the existing renewable energy gener-
ation system are used for the lacking of high-performance energy storage system.119 Considering
the possible requirements of supplying energy during catastrophic weather conditions, it is very
important to figure out the inexpensive way of storing large scale energy.
LDES systems can be constructed in at least two ways to take benefits of reduced-cost wind and
solar while also advancing decarburization aims: (1) as grid-tied, independent systems, and (2) as
integrated systems having solar and/or wind behind a shared assembly station. In case of grid-tied
systems, dynamic resources have typically been buffered by natural gas-fired peaker plants,
although other techniques involve transmission extension, demand-side control, and electricity
storage.120,121 Modeling studies have shown that storing electricity for up to 8 h can boost the
amount of annual energy generated by wind and solar which can be used on a major local grid.
Numerous studies have shown that increasing storage length beyond 10 h reduces curtailment
and encourages the use of variable resources like wind and solar, with a decreasing minimal
effect.122 More modelling work is required to precisely evaluate the effect of LDES on wind and
solar penetration at the local scale. This research should entail realistic management of transmission
power flow limitations, network steadiness, contingency prerequisites, expenses of curtailed elec-
tricity, charge capacity limits, and other factors required to reflect the actual complexity of trans-
mitting electricity within a huge power grid. It’s also worth noting that, while longer-duration
resources may be able to support shorter-duration uses like daily cycling, which could promote
the creation of longer-duration energy storage systems.123

Application of LDES. A variety of energy storage systems are able to deliver energy supply for long
time duration, such as thermal energy storage process, mechanical energy storage process like
pumped hydro storage (PHES) and electrochemical energy storage process like lithium-ion
(Li-ion) batteries. PHES and Li-ion batteries are the two most common types of electricity
storage devices on the power system nowadays. PHES system can supply energy at a
maximum rate with a time duration of 6−10 h.124 However, PHES expansions have been
restricted due to issues related to the authorization and huge investment for large-project. On
the other hand, Li-ion batteries are highly efficient to utilize in project of all sizes, including
kW to GW and different time duration. Cost effectiveness, fast charge/discharge process,
improved power transmission system and long durability of energy supply (≤ 10 h) have made
Li-ion batteries implementable.125
Major energy storage projects are mainly concentrating on batteries, which accounted for 88
percent of new deployments to energy storage worldwide in 2016. Figure 12 shows the typical rela-
tionship between storage duration and size scale of various storage technologies.41 It can be seen in
Figure 12 that capacitors can store from very low energy (∼1Wh) to very high energy (several
TWh), and these stored energies can be used for long duration.
Batteries are portable energy storage devices with excellent round-trip charge discharge effi-
ciency (around 90%) and lower price. In general, batteries can be advantageous to variable renew-
able energy systems (VRE). The investment cost and operating cost of energy storage system is
related to the usage scenario (Figure 13). The usage of more batteries reduces the capital cost as
well as operating cost per kWh. LDES systems can be constructed in 2 different ways to take advan-
tages of the cheap renewable energy sources while also advancing decarbonization objectives: (1)
as grid-tied, independent systems, and (2) as combined systems with solar and/or wind renewable
energy sources.120,126 Modeling studies have shown that storing electrical energy for back up
1116 Energy & Environment 34(4)

Figure 12. The typical relationship between storage duration and size scale of various storage technologies 39.

127
Figure 13. The relation between investment cost and operating cost of energy storage system (ESS) .

supply up to 8 h can boost the amount of seasonal energy generated by wind and solar which can be
used on a huge regional network. Researchers have found that LDES has storage duration greater
than about 10 h. It has been also noticed that higher storage periods provide lower curtailment,
which results the good usage of intermittent assets like wind and solar with a minor effect. More
modelling and analysis are necessary to appropriately evaluate the impact of LDES on the realistic
renewable power stimulation at the regional grid, high scalability, risk response specifications,
trade-offs of curtailed energy, capacity leverage limits and other characteristics required to describe
the entire complexity of power distribution in a massive energy system.
Figure 14 shows the relation between the energy storage duration and annual electricity production
percentage from solar and wind. In Figure 14, the colored zone represents conventional assumptions
for renewable curtailment, transmission and grid flexibility. The arrows to the left from colored zone
represents less curtailment, less transmission and less grid flexibility while arrows to the right from
colored means high value of these parameters. It can be seen from Figure 14 that the necessity of
storage systems is increased with the usages percentage of annual generation of energy from renewable
Smdani et al. 1117

Figure 14. The relation between the energy storage duration and annual electricity production percentage
from solar and wind 119.

sources. Only little storage is required to use 20% to 50% energy of the total capacity of wind and/or
solar projects.121 Moreover, annual energy usage from renewable sources can be reached between
∼50% to 80% of annual production with the use of storage duration about 10 h. Extended storage dura-
tions, ranging from 10 h to few hundreds hours, are expected to be required to obtain around 70 percent
to 90 percent of annual energy from wind or solar.128 Figure 14 shows that 100% of the generated
energy from wind and/or solar cane be used if storage duration potentially reaches to the seasonal
region. However, building a power grid using different renewable sources is less cost-effective and
risky compared to conventional power generation technologies.

Efficiency
Pumped hydroelectric storage (PHES) is a type of hydro-electric power that can transfer water to the
overhead tank from the storage tank and reserve potential energy from altitude variations in water
levels. PHES varies from conventional hydro-power in that it can hold potential energy from alti-
tude variations in water levels. PHES is extremely durable and adaptable, and it may be employed
for energy up-down regulation as well as rate stabilizing. This is the most popular type of energy
storage, accounting for roughly 3% of the total worldwide installed power generation capacity and
97% of overall storage capacity. There are around 300 PHES plant in the world having total power
generation capacity of 120 GW, which is increasing at a rate of about 5 GW per year. One of the
main reasons for becoming quite desirable of PHES is its relatively high efficiency (65−85%).129
Sabihuddin et al.130 have analyzed the performances of PHES and reported that the efficiency of
PHES can be about 65−87%. Moreover, data provided by the international electrochemical com-
mission (IEC) showed that the efficiency of PHES is ranges from 70 to 85 percent. The PHES has
some advantages include its high efficiency, long lifespan and virtually limitless cycle stability.
The main disadvantages of PHES are the reliance on geographical conditions and the extensive
use of land.
Compressed air energy storage (CAES) has been around since the nineteenth century and has
been employed in a variety of industrial applications. Because air is readily available, it is employed
as a storage medium. Compressing air and storing it in an underground excavation or above the land
in containers or pipes is done with electricity. In a modified gas turbine, compressed air is blended
1118 Energy & Environment 34(4)

with natural gas before burning, and expanded while required. The air must be preheated before
entering the turbine if the heat generated during compression are allowed to release, resulting
cooling the air. This method is known as diabatic CAES, and it gives lower efficiencies of less
than 50%.131 The benefit of CAES is its huge capacity; the cons are its poor round-trip efficiency
and regional limitations. Pre-heating of air stage can be eliminated if the heat generated during com-
pression can be stored using molten salts, which increases the overall efficiency of the CAES.132
According to studies, typical first-generation CAES systems have a low efficiency of 42−54%
due to higher heat losses to the environment during compression and the need for heat when the
temperature of the turbine falls due to decompressed air.109 Advanced adiabatic and isothermal
CAES systems have been devised to overcome the key challenges limiting total efficiency.
CAES systems of the second generation take advantage of the heat created during the compres-
sion, which is transported and preserved in heat storage facilities. Advanced adiabatic CAES
(AA-CAES) uses low or zero fuel or auxiliary energy to heat the air while expanding, improv-
ing overall efficiency to a potential 70% and removing pollutants.133 Isothermal CAES incor-
porates heat dissipation from by-products during compression to keep a steady temperature and
eliminate the cost of building a thermal storage. This can be accomplished by slowly but stead-
ily compressing the air and enabling the temperature to adjust to the ambient environment. In
terms of increased efficiency, these systems appear to be promising, with estimates ranging
from 70 to 80%.134,135
Flywheel energy systems (FES) have various features that make them an excellent energy
storage technology for short-duration or medium-duration operations; some sophisticated designs
can store up to 133 kWh. A FES system is good for the environment, does not depreciate much
after multiple charge/discharge cycles, ease of maintenance, has a long lifespan, and has a
greater energy efficiency of up to 90%. Smith et al. have showed that FESS has an average
cycle trip efficiency of 80-85 percent. The main disadvantages of FES systems, on the other
hand, are their high cost and poor energy density, which limit their applicability to small-scale
and extremely specialized needs, such as space applications.136
Electrochemical energy storage (EES) systems are important energy storage technology for their
extremely high efficiency. Batteries and capacitors may preserve electricity electrochemically.
Batteries are well-established energy storage technologies that have high cycle efficiency and
cycle life time. Lithium-ion (Li-ion) battery, sodium-sulphur (NaS) battery, lead acid (Pb-acid)
battery, lead-carbon battery and flow batteries are among the various varieties of batteries.
Lead-acid batteries have the energy efficiency of 75-85%. Furthermore, as compared to other bat-
teries, nickel-cadmium batteries have somewhat lower overall energy efficiency (60−70%). Among
these batteries, Li-ion batteries have the highest efficiency (about 85%-95%) and largest life
time.137 On the other hand, electrostatic capacitors, electrolytic capacitors, and electrochemical
capacitors are the three types of capacitors that store and transfer energy electrochemically.
Electrochemical capacitors, also known as supercapacitors, exhibit the highest performance
among the three forms of capacitors. Supercapacitors have excellent storage efficiency (>95%)
and can be cycled even thousands times without degrading their energy storage capability.
Supercapacitors are prone to self-discharge, and their working voltages rarely exceed the current
where the electrolyte performs chemical reactions, notwithstanding their great efficiency and
cycle life time. They can be applied in conjunction with batteries for greater voltage applications.
Energy efficiency of various ESSs are shown in Table 1 while cycle efficiency of various ESSs are
shown in Table 5. These two table gives a deep understanding of efficiencies of different energy
storage devices.
Smdani et al. 1119

Table 5. Efficiency and life cycle performances of various energy storage systems.

Storage name Cycle efficiency (%) Cycle life time Ref

Pumped hydro 65–85 10 – 3 × 10

4 4 129

65–87 104 – 6 × 104 130

70–85 >0.5 × 104 138

Compressed air (CAES) >50 >104 138

60–90 104 –3 × 104 130

Flywheel (FES) 80-90 2 × 104 –107 138

80-85 >2 × 104 139

Lead-acid battery 70-80 ∼103-1.8× 103 39

Li-ion battery 85-98 500–104 138

90-97 1000–104 140

141
95 4000
85-95 250–104 142

Capacitors 60-70 5 × 104 143

Supercapacitor 90-100 <106 144

85-98 104 – 105 138

90-97 >105 39

65-99 104 – 106 145

SMES 95-97 >105 146

Hydrogen fuel cell 45–66 1000-2× 104 147

148
TES 30–60 -

The storage efficiencies of TES systems based on sensible (hot water) heat storage technologies
ranging from 50 to 90%, relying on the storage medium’s specific heat and insulation technique.149
The efficiency of TES system is higher in phase-change material (PCM) based TES system com-
pared to sensible heat storage based TES system. For PCM storage, the efficiency of TES system is
found in the range of 75-90%. In TES systems, PCMs with high latent heat and thermal conduct-
ivity are employed. PCM has a high density, yet low volume variations during phase transition. As a
result, the PCM storage volume in the TES system is lowered, which helps to increase the TES
system’s efficiency.150 Thermochemical material (TCM) based thermal energy storage systems
have the maximum efficiency ranging from 75 to 100%. Thermo-chemical materials (TCM)
employed in the TES system use a reversible endothermic/exothermic reaction mechanism to
absorb and emit heat.151

Life time and cycling times

Life time and cycling times are two important parameters those should be considered for the
deployment of high-performance energy storage systems. The total installation costs of ESSs is
influenced by the life time and cycling times of the ESS devices. Short lifetime and less cycle
numbers raise operational and maintenance costs. Table 6 shows the life time and cycling times
of various energy storage systems. These two parameters are somehow correlated with the operat-
ing activity of the energy storage devices. Mechanical storage technologies, such as PHES, CAES,
and flywheels, typically have long cycling times (10,000 or more) due to its excellent mechanical
features. Mechanical energy storage devices are based on traditional mechanical engineering, and
the service life is mostly defined by the mechanical equipment’ service life. Electrochemical energy
1120 Energy & Environment 34(4)

Table 6. Life time and cycling times of various energy storage systems.

ESS Life time (Years) Cycling times (Cycles) Ref

39, 129, 158, 159
PHES 40-60 10,000–30,000
>40
>30
39, 129, 160
CAES 20-40 8000–12,000
30
>20
39, 158
FES 15 >20000
50 >21000
>15
39, 129, 161, 162
Lead-acid battery 5-15 500–1000
13 200–1800
39, 163, 164
Li-ion battery 5-15 1000–10,000
14-16 Up to 20000
Capacitors ∼5 >50000 39, 165

∼1-10 5000
39, 59, 139
Supercapacitors 10-30 >100,000
10-12 >50000
39, 59
SMES >20 >100000
30 >20000
39, 139, 166
Hydrogen fuel cell 5-15 >1000
20 >20000
39, 167
TES 10-20 -
5-15

storage systems (EESSs), including capacitors, supercapacitors, batteries and SMES cycling times
that are often greater than 20,000. However, the life time of the electrochemical energy storage
devices are lower than that of the mechanical energy storage devices. PHES is primarily designed
to meet longer-term needs, such as bridging extended intervals of low sun while also experiencing
low wind. According to Immendoerfer et al., Pumped hydro energy storage (PHES) has a life dur-
ation of 50 to 150 years with nearly no performance depreciation.152 For the basic instance, an
80-year timeframe has been selected. Other researchers showed that PHES plants have the
average life time of 40-60 years that is maximum among all energy storage systems. Pumped
hydro storage has a long lifespan (about more than 50 years) and does not suffer from cyclic defi-
ciency.153 Large-CAES systems have a life expectancy of 40 years and a 71% energy efficiency.154
Moreover, Large-CAES can provide around 8000–12,000 cycles during its service life. On the
other hand, small-CAES systems have a life duration of about 23 years. Scientists have recorded
30000 cycles in case of small-CAES system.155 Simply said, the service life of CAES is determined
by its mechanical condition, which indicates it is difficult to fatigue. Kondoh et al.129 reported that a
flywheel plant provided service for 12 years continuously from 1984 and functioned more than
21,000 cycles. The charging and draining of flywheels is accomplished by accelerating inertial
masses (rotors). The rotor is the most important part of a flywheel. The FES plant’s lifetimes are
limited (about 100,000 cycles) because flywheel components are easily worn down in continuous
operation. As a result, the fundamental issue with flywheels is to reduce and eventually eliminate all
component frictions. However, conventional batteries’ cycle capacities are lower than those of
Smdani et al. 1121

other EES systems, owing to electrodes deformation and electrolytes deterioration over time for the
chemical reaction. Lead-acid batteries are very common electrochemical energy storage devices
which have only 3-10 years’ life-time. Moreover, nickel-cadmium batteries have longer life time
compared to lead-acid batteries. Nickel cadmium batteries sustains for 10-15 years. Lithium-ion
batteries also have the life-time around 10-15 years. The life of a metal-air battery is only a few
hundred cycles thus it clearly needs to be improved. SMES system also have long service life
ranging from 20 to 30 years. SMES system provides very high cycling times which can reach to
more than 100000 cycles.39 SMES technologies are increasingly employed for short-term energy
storage due to the energy demands of refrigeration and the expensive price of superconducting
wire. On the other hand, hydrogen fuel cell exhibits service life only about 5 to 15 years and life
cycles around 1000 to 20000 cycles.39 The usage of less-than-pure grade hydrogen causes
damage to specific types of fuel cell membranes, reducing the lifetime of the fuel cell system.
Studies have been undertaken on how to enhance the performance and efficacy of hydrogen fuel
cells in order to increase their life time and cycles.156 Furthermore, the average life span of TES
system is in between 10 to 20 years. The life duration of 20 years of TES system means that depre-
ciation of TES is about 5% per year.157

Cost
As prices continue to drop and prospects in consumer, automotive, and utility applications become
clearer, the energy storage business has grown worldwide. As the industry is growing at a break-
neck pace, it’s becoming progressively crucial to understand how technological developments
compare in terms of price and performance. Despite the great level of interest, few complete and
in-depth evaluations of storage prices and performance are widely available. The utilization of
energy storage technology for numerous applications has a variety of financial benefits and con-
straints. The expense of an energy storage system is frequently determined by the application.
Carnegie et al. analyzed energy storage technology’s applications and evaluated energy storage
device’s prices for certain uses.168 Furthermore, the investment of an energy storage system for
a certain application varies significantly depending on location, building method, and capacity,
and the cost efficiency is dependent on the market price of the energy source, such as fossil
fuels. Zakeri et al.106 have studied the relation between the investment cost and geological features
for high volume energy storage systems using CAES technologies. They found that CAES costs $1/
kWh for salt cavern (solution mined) while CAES costs $30/kWh for hard rock (excavated and
existing mines). As a result, economic evaluations comparing a broad range of energy technologies
frequently contain a degree of uncertainty that must be considered. Table 7 shows anticipated
capital costs for several energy storage systems. Due to lacking of technological advancements
and cost savings, the prices of a number of energy storage systems which have not achieved a
stable phase of development at the date of publication are predicted to be reduced. PHES and bat-
teries, on the other hand, have attained maturity and have had less cost volatility during the last two
decades. As a result, their current prices are unlikely to differ significantly from the figures in
Table 8. When choosing the most appropriate energy storage system for operations where
energy must be stored and released frequently with a high energy rate, the cost per unit power
output becomes a critical element. Similarly, when storing energy for extended periods of time,
the cost of unit energy stored becomes a significant consideration. When higher energy supplies
are necessary, both the flywheels and capacitors are the less expensive energy storage devices
among other energy storage systems. These systems are frequently utilized in electricity distribution
1122 Energy & Environment 34(4)

Table 7. Capital cost estimation for a variety of energy storage systems.

Power-based Capital cost Energy-based Capital cost

Storage system ($/kW) ($/kWh) Estimation year Ref
39
PHES 600–2000 5–100 2009
169
500–4600 <300 2014
129, 168
CAES 400–800 2-50 2009-2014
500–1500 1-30
144
FES 300–1000 300–6000 2019
39
250–350 1000–5000 2009
170
Lead-acid battery 175-600 150-400 2011
39, 169
Li-ion battery 300–3500 100–2500 2009-2019
1200–4000 <300
39
Capacitors 200-400 500-1000 2009
39, 144
Supercapacitor 100-300 300-2000 2009
144
130–515 10000 2019
39, 168
SMES 200–300 1000–10,000 2009
129
Hydrogen fuel cell 500–10,000 2009
39, 129
TES 200–300 3-60 2009

subsystems. Both PHES and CAES provides the lowest per unit capital costs for energy storage,
which are commonly used in generation subsystems.
The cost and efficiency aspects of the technologies collected in this study are summarized in the
Table 9. The capital costs of Li-ion batteries are decreasing day by day as the installation of station-
ary batteries are increasing. For example, over 500 MW of stationary lithium-ion batteries had been
installed globally by 2015, which increased to 1629 MW by 2018.190 Consequently, the price of
batteries had reduced by 80% in between 2010 to 2017, and reached to about $200/kWh. It is
expected that the price of battery will drop to around $96/kWh by 2025.191 An assessment of
Li-ion based electric vehicle (EV) battery pack cost was performed to compare the DC battery
cost for energy storage with estimated costs for EV battery packs as shown in Table 9. Results
showed that the average price of DC battery for grid-scale energy storage is 10% higher than the
price of an EV battery pack unit of energy.
PHES units are looked after for their capacity to deliver huge electricity and associated services
to the grid at a cheap cost per kilowatt hour. PHES is a well-known technology that has been around
for over decades. PHES has a fast synchronization time, a fast response time, and the ability to
operate as both a load and a generator.199 Despite these advantages, implementations of the tech-
nology have stagnated in last several years in the potential markets due to high cumulative capital
costs demanding financing in the billions of dollars, uncertainty about future growth, and environ-
mental concerns arising from the technology’s features. Regardless of the lack of current installa-
tions, PHES accounts for more than 97% of all energy storage system built. Between 2018 and
2023, PHES capability is predicted to grow by 26 GW globally.200 According to the
International Renewable Energy Agency (IRENA), the cost of electromechanical equipment
lowers with rising generation of power.11 It was found that the costing of a 48 MW system is
$485/kW while a 4MW system costing $570/kW. At around 50 MW, there seems to become an
inflection point. From 4.3 MW to 48 MW, the $/kW dropped by 15%, and from 48 MW to
500 MW, it dropped by 50% as described in Figure 15. The unit power cost for electromechanical
equipment in this study is about a third of the $835/kW, showing the difficulty in determining a
 Smdani et al.

Table 8. Summary of costs of various energy storage systems in terms of different parameters.

Costing parameters PHES CAES FES Li-ion battery Ultracapacitors

Capital cost ($/kW) 1500-4700, 1500-2000, 1300, 1105, 1481, 2400, 600, 1050 209, 209-343, 273, 285, 241, 400
2230 1050-1400 300
Power Conversion System Added in capital cost - Added in capital 288 350
($/kW) cost
Total Project Cost ($/kW) 2638 1669 2880 1876 930
System RTE (%) 80, 82, 70-87 65, 70, 73 70-80, 81, 85-90 77–85, 83, 80–90, 90–98, 95, 96, 98
Cycles at 80% DoD 15,000 10,000 200,000 3500 1 million
Life (Years) >25 25 20 10 16
MRL 9 8 8 9 9
TRL 8 7 7 8 8
171,172,173,174,175 171, 172, 175, 176 177,178,179,180,181 182,183,184,185,186 187,188,189
Ref
∗
RTE = round-tip efficiency, MRL = manufacturing readiness level, TRL = technology readiness level, DoD = depth of discharge.
1123
1124 Energy & Environment 34(4)

Table 9. The costs of the Lithium-ion batteries for electric vehicle (EV), 2016–2018.

Cost ($/kWh) Component Estimation Year Ref

192
250–300 Pack 2018
193
200 Pack 2018
194
209 Pack 2017
195
236 Pack 2017
196
190 Pack 2018
196
250 Pack 2016
197
227 Pack 2016
198
200-250 Pack 2016

190
Figure 15. The cost ($/kW) of electromechanical equipment for PHES plants versus power capacity .

single cost value for each category. Moreover, compared to combined cycle gas turbines, pumped
hydro energy storage systems have the least investment risk in terms of price per kilowatt - hour
(kW-h) of electricity generated and minimum levelized cost of supplied energy.201 However, the
PHES technology is linked with high capital costs, restrictions on site location, and a lengthy instal-
lation time, leaving it only suited for large-scale implementation.202 It’s also worth noting that a
PHES system is a marginal consumer of energy because it requires more electricity to lift the
water up the slope than is produced during the falling of water; thus, the advantage of PHES
emerges from storing produced electricity while demand is less and releasing it while demand is
high.203
CAES works by loading a cavern with compressed air throughout low-cost energy periods, then
heating and expanding the air before passing it through a turbine to produce electricity when the
energy is required. CAES facilities may additionally feature a combustion process, where air and
natural gas are mixed together, and ignited before entering the turbine. This final component
aids in increasing the pressure and temperature of the air taken from the storage, allowing the
CAES system to “boost” productivity.204 Machinery, building, setup, design, and other expendi-
tures associated with the grid-level storage system are included in the capital cost of CAES projects.
Studies showed that the cost of the 110 MW McIntosh project was $1310/kW.205 The cost of a
270 MW plant in the Iowa Storage Park was increased to $1481/kW due to the site limitations.
Smdani et al. 1125

Siemens forecasts that a 150 MW/48-hour CAES system using the SXT-800 engine will cost
between $1050 and $1400/kW. The key to lowering CAES costs is to lower the costs of each sub-
system, particularly the TES and gas storage units. Heat storage and cold storage are significant
components of the studies to minimize the system’s building costs. It is desirable to have a minimal-
cost heat storage and cold storage functioning fluid. The light-heat energy storage should be used
for the high-temperature TES method. The system’s cost is reduced by utilizing mature storage
technologies. Mine helps to reduce construction costs by utilizing subsurface salt caverns and
the cave’s unique geological features. It also helps to minimize the cost of CAES installation by
utilizing sophisticated power electronics technologies.206
Flywheel energy storage (FES) system is ideal for operations that only need to be used for a
limited period of time or as a backup power supply that can make a connection between the grid
and heavier backup suppliers. FES systems can also be used for industrial-scale quick power
surges and renewable stabilization. Long life spans, quick response times, and high RTE value
are all advantages of FES technology. FES devices can operate for long time without maintenance
or little maintenance, also maintenance cost is very low. The cost of a 15-min Beacon Power FES
20 MW plant was estimated $2400/kW. The cost of 2.7 MW FES plant was calculated near to $600/
kW.174 According to information obtained from Kinetic Traction, a flywheel manufacturer, the cost
for a 333 kW, 1.5 kWh system is $600/kW without installation costs. Helix Power built a 1 MW
FES project that cost $1000/kW, plus $50/kW, for installation.179 Plotting the $/kW vs.
energy-to-power (E/P) ratio to determine the $/kW value for any specified E/P ratio is a superior
methodology as shown in Figure 16. When the E/P ratio is greater than 0.25, extension of the
straight-line fit yields the $/kW. At an E/P ratio of 0.01, the $/kW was $672. High-speed FES
system is usually limited by its heavy cost, which is often five times that of low-speed FES
system. The sorts of materials, size and shape, as well as the kinds of bearings and electrical
machinery accessible, are all affected by the requisite speeds. The growing industries, on the
other hand, provide great composites in terms of increased strength and reduced weight, as well
as cutting-edge electronics for improved FES system control, enabling them a true substitute for
chemical battery storage approaches.207

Figure 16. Graphical representation of Capital Cost ($/kW) versus energy-to-power (E/P) ratio (h) for
Flywheel energy storage (FES) system.190
1126 Energy & Environment 34(4)

Ultracapacitors are used in conjunction with battery systems to give and receive pulse power,
and their ramp rates are exceedingly quick. The charge is held in the electrode’s double layer
and may thus be released instantly when required. Ultracapacitors can be made up of several
cells/modules to rise to the desired capacity range of a project. Ioxus energy estimated the cost
of $160/kW for 250 kW DC capacitor.189 Ultracapacitors are not cost-competitive with battery
technologies due to their poor specific energy and energy density. They are, nevertheless, more
competitive in terms of $/kW power due to their high specific power and power density. A
1000 kW/7.43 kWh system from Maxwell costs $241,000, while a 1000 kW/12.39 kWh system
costs $401,000. This equates to $32,565/kWh for the 7.43 kWh plant and $32,365/kWh for the
12.39 kWh plant, with the $/kW rising from $241/kW to $4001/kW for constant rated power as
when the energy rises from 7.43 kWh to 12.39 kWh. While the Ioxus system’s energy content
was not revealed, its $160/kW assessment is on the same scale as the Maxwell capacitor expenses.
Also because overall power rating is fixed at 1000 kW, the price rises with energy, and the per
energy falls only minimally as energy grows.188
The only exhaust generated by hydrogen fuel cells is water vapor, which is formed by a non-
combustive electrochemical interaction among hydrogen and oxygen from surrounding atmos-
phere. Hydrogen fuel cells have a thermodynamic energy conversion efficacy that is typically
greater than the Carnot efficiency threshold of traditional internal combustion engines, making
them advantageous in terms of energy consumption and fuel economy. An electric or hybrid electric
drive vehicle receives the electrical energy produced by the cell reaction. The inadequacy of hydro-
gen fuel technology, as well as the expensive expense (500–10,000 $/kW) of installing it, is a major
barrier that the automotive industry must overcome as part of its commercialization strategy.208
A variety of TES systems exist, each with varying levels of technology maturity because thermal
energy can be preserved in sensible, latent, or chemical form. Cost of the TES system is low for the
availability of the various TES technology. Capital cost is an important factor for the implementa-
tion of TES systems. The cost of materials, the cost of components that transfer heat into and out of
the material, and the cost of integrating the TES into the building all contribute to the capital cost of
the TES. Because a portion of the cost of thermal energy storage is directly tied to the price of the
storage material, maximising the adequate storage capacity for a specific mass of material is critical
to lowering the capital cost. The maximal energy density and the power density determine the
amount of available energy.209 Energy-based capital cost of TES is 3-60 $/kWh while power-based
capital cost of TES is 200-300 $/kW. These important aspects of TES systems must be tuned both at
the material and system stages because there is a direct relationship between material qualities, heat
exchanger structure, and operational parameters.210

Challenges and opportunities of energy storage technologies

Advancement and progress of the energy storage systems have brought with them a slew of new
issues that must be resolved in order for new ideas in this field to become commercially viable,
widely used, and long-lasting. When energy storage systems are deployed and operated for
storing the excess of produced energy, there are a few limits and issues that must be addressed inter-
nationally. According to reports, no energy storage system can return 100% of its energy that was
stored for future use, implying that some loss must happen during the storing and discharging
process. The documented values range from 10% to 30% in diverse devices and systems, as
well as specific variables associated with the location.211, 212 The installation, operation, and main-
tenance of energy storage systems all cost a lot of money, which can be extremely high in some
cases.213 Some energy storage systems can create environmental concerns, which begin with the
Table 10. The current global situation for energy storage projects.

How to use the Energy Cycle

Smdani et al.

Bibliographical Energy storage Location, Energy storage efficiency efficeincy

data system Proposed Method Date system Criteria used Objective function (%) (%) Ref
25, 219,220,221
Hossain et al., PHES Raccoon Mountain Tennessee, A technique for Electric Energy Time • Extensive and 65-87 71–85,
2020 Pumped United holding and Shift, Electric developed 70-80,
Storage Plant. States, discharging the Supply Capacity, operational 75, 78,
Ludington 1978. gravitational Electric Supply experience 87, 80
Pumped Michigan, potential energy of Reserve Capacity • Low
Storage United lifted water —Spinning, Load self-discharge
States, Following • Massive storage
1973. (Tertiary capacity for
Balancing) extended
periods of time
• Excellent
efficiency
25, 133,
Energy Storage CAES Toronto A-CAES Ontario, The energy is stored load management, • Large storage 80-89 70–89,
222,223,224
Association, Facility, Angas Canada, using compressed spinning reserve, capacity 85, 71,
2017 A-CAES Project 2015. air’s elastic load following, • Quick startup 84, 88
South potential. and intermediate • Excellent
Australia, power generation maturity level
Australia, • Long lifespan
2020
109, 225, 226
Amirante et al., FES 20 MW Flywheel New York, Kinetic energy is Frequency • High power 90-95 90–95,
2017 Energy Storage USA, stored regulations, and density 89,
Plant 2014. mechanically in the voltage regulation irrespective of 80-85,
rotational motion stored energy 92,
of a spinning mass, • Quick charge/ 90-93
such as a disc or a discharge cycle
cylinder. • Minimal
maintenance
requirements
• High durability

(continued)
1127
Table 10. Continued.
1128

How to use the Energy Cycle

Bibliographical Energy storage Location, Energy storage efficiency efficeincy
data system Proposed Method Date system Criteria used Objective function (%) (%) Ref
25, 226, 227
IRENA, 2019 Li-ion battery Tesla 100 MW South Electrical energy is Frequencyregulation, • Very long life 85-90 90-100,
/129 MWh Australia, stored in the form capacity firming span 85
Li-ion battery Australia, of chemical • Portable
storage project 2018. energy. • Quick response
at Hornsdale Germany, time
Wind Farm, 2017. • Operational
STEAG’s 90 Arizona, flexibility
MW /120 MWh United • Lightweight
battery storage States,
project, Green 2016.
Mountain
Energy Storage
- NextEra
25, 219
Energy Storage Lead-acid Kaheawa Wind Hawaii, Chemical energy is Electric Supply • Quick response 75-80 70–90,
Exchange, Power Project United used to store Reserve Capacity time 63, 85,
2020 II-Younicos States, electrical energy. —Spinning, • Cost-effective 85–90
2012 Frequency • High efficiency
Regulation,
Ramping,
Renewables
Capacity Firming,
Renewables
Energy Time Shift
25, 219, 228
González et al., Supercapacitors Red Hook New York, Electrostatic fields Electric Energy Time • Quick response 90-95 84–95,
2016 (Brooklyn, NY) United are used to store Shift, Microgrid time 86, 95,
- NY Prize States, electrical energy. Capability, • Long cycle life 90,
Microgrid 2015. Renewables and effectiveness 85–98
United Energy Time Shift • Suitable battery
States, substitute
2015. • Conveniently
utilized in strings

(continued)
Energy & Environment 34(4)
Table 10. Continued.
Smdani et al.

How to use the Energy Cycle

Bibliographical Energy storage Location, Energy storage efficiency efficeincy
data system Proposed Method Date system Criteria used Objective function (%) (%) Ref
25, 133, 224,
Hossain et al., SMES Distributed SMES Wisconsin, The electromagnetic Grid and voltage High cycle life 95-98 95–98,
229
2020 and Power USA, field induced by a stabilization • Good power 97, 95
Quality 2000-2003 current flowing density
Industrial through a • Long cycle life
Voltage superconducting
Regulator conductor is used
to store electric
energy.
25, 230, 231
Kopp et al., Hydrogen fuel Energiepark Mainz Mainz, H2 produced via Renewables • High energy 90-95 20–50,
2017 cell Germany, water electrolysis Capacity, Firming, density 32, 59,
2015 and stored in high Renewables • Reduced 45–66
pressure Energy Time Shift, Reduced self
container. Later, Transportation • Environmentally
stored H2 is used Services friendly
for electricity • Minimal
production. maintenance
39, 219, 232,
EASE, 2016 TES Riverside Public California, Uses thermal energy Electric Bill Relatively low cost 30-50 30–60
233
Utilities 5 MW United to store electrical Management, • Good energy
Ice Energy States, energy; transforms Electric Energy density
Project 2016 thermal energy Time Shift, • Wide range of
into electrical Electric Supply applications
energy. Reserve Capacity • Universal
- Non-Spinning availability of
storage medium
like ground,
water etc.
1129
1130 Energy & Environment 34(4)

mining of materials for production and continue through disposal after the item has served its
purpose.214 As a result, research is needed to produce systems that are not only more efficient,
but also less expensive and have fewer environmental issues, particularly when it comes to dispos-
ing of obsolete gadgets at the end of their useful lives.215
The utilization of the combination of renewable energy sources and energy storage systems
(ESS) to reduce the use of fossil fuel-based electricity production is growing global popularity
and attention.216 Table 10 gives an overall idea about the current energy storage systems.
Therefore, there is a growing pressure to steadily reduce consumption of fossil fuel for electricity
generation, but alternatively, renewable energy sources such as wind and solar must be improved
and their proportion progressively expanded, as prescribed in the Paris Agreement, to reach the
ultimate goal of 100%.217 With the introduction of modern energy storage technologies, fossil fuel-
based electricity production plants may become more dynamic and economical in managing fre-
quent fluctuations in consumer needs since they will have steady backup power in terms of
stored energy.218 However, overall energy storage facility is currently modest; for example, the
European energy system’s storage capacity is only 5% of total electricity production, with PHES
constructed primarily in mountainous locations. As a result, to keep up with new innovations,
the electrical storage capacity must be increased.

Conclusion
The known energy storage systems such as pumped hydro, compressed air, flywheel, thermochem-
ical, batteries, supercapacitors, fuel cell and superconducting magnetic energy storage, are given an
overview and detailed evaluation. This study discusses energy storage categories, performances,
applications, current advancements and future research. Important performance factors like as
energy density, power density, power ratings, capacitance, energy efficiency, discharge duration,
life time, cycling times and costs are presented, allowing alternative energy storage devices to be
tailored to specific utilizations.
Electrochemical energy storage systems, like as batteries and supercapacitors, are clearly the
prominent ESTs to utilize when high energy density, power density and ratings, long duration
energy storage, extended discharge time, fast response time, and high energy efficiency are
required. Such ESTs have promising utility mostly in renewable energy industry and in the
power system as a whole, including energy conservation and bridging electricity. Among all
EESSs, Li-ion batteries are a more advantageous choice for grid-scale ESSs like as RES distribution
and transmission due to its high energy density (200-500 Wh/l) and power density (1500–
10,000 W/l), lighter weight, thinner and smaller sizes, high cycle efficiency (around 90%) and
fast response. However, the price of the storage device must be minimized for Li-ion batteries to
be widely absorbed in the renewable energy industry. Development in cell technologies will
lower the cost of Li-ion batteries. For different storage purposes, batteries are frequently compared
to supercapacitors, and it is predicted that harnessing their properties through integration could
assist overcome future electrochemical energy storage difficulties. Flow batteries appear to
become the most appropriate candidates for massive electrical energy storage employing batteries,
however pricing and electrolyte improvement remain problems.
Other methods of electrical energy storage, including FES and SMES systems, which are uti-
lized for relatively small storage time and frequent usage, have attracted less interest though FES
and SMES need lower power-based capital costs. Moreover, FES and SMES have very high
power density, 1000-2000W/l, and 1000-4000 W/l, respectively. Reduced mechanical, electrical,
and power dissipation are critical requirements for flywheels. Furthermore, materials engineering
Smdani et al. 1131

research is being conducted to enhance the strength of severely stressed rotors operating at high
rpm. Energy storage systems such as PHES and CAES are established, cost-effective, and depend-
able, and are utilized for scalable storage with numerous energy efficiencies, 65-87% and 80-89%,
respectively. However, more researches are required to boost their round-trip effectiveness.
Developments in turbine structure are required in PHES devices to increase performance. In
CAES system, compression operation methods are required to improve for better performance of
CAES devices. Though TES systems have high energy density (80-120 Wh/l), there is less study
on thermochemical heat storage. Adsorption techniques for TES system are generally not finan-
cially sustainable. To further improvement of this technique, more study on materials is required
to prevent adsorbent instabilities, as well as process optimization. Furthermore, storage devices
with a high efficiency, short discharge rate, increased power density, and long cycle life, such as
PHES, FES, SCs, and SMES, are better suited to power quality and regulatory operations such min-
imizing the effects of wind velocity random variation. Researchers have been trying to solve the
technical and economic barriers, and upgrading the performance of the ESSs gradually.

ORCID iD
Muhammad Remanul Islam https://orcid.org/0000-0002-1714-6642

References
1. Khan N, Dilshad S, Khalid R, et al. Review of Energy Storage and Transportation of Energy. Energy
Storage. 2019; e49.
2. Siala K, Chowdhury AK, Dang TD, et al. Solar energy and regional coordination as a feasible alternative
to large hydropower in Southeast Asia. Nature Communications 2021; 12: 1.
3. Eshraghi A, Salehi G, Heibati S, et al. An assessment of the effect of different energy storage technolo-
gies on solar power generators for different power sale scenarios: The case of Iran. Sustainable Energy
Technologies and Assessments 2019; 34: 62–67.
4. Cherp A, Vinichenko V, Tosun J, et al. National growth dynamics of wind and solar power compared to
the growth required for global climate targets. Nature Energy 2021; 6(7): 742–754.
5. Jayachandran M, Reddy CR, Padmanaban S, et al. Operational planning steps in smart electric power
delivery system. Scientific Reports 11(17250).
6. Kittner N, Lill F and Kammen DM. Energy storage deployment and innovation for the clean energy tran-
sition. Nature Energy 2017; 2(9): 17125.
7. Mancò G, Guelpa E and Verda V. Optimal Integration of Renewable Sources and Latent Heat Storages
for Residential Application. Energies 2021; 14: 5528.
8. Eshraghi A, Salehi G, Heibati S, et al. Developing operation of combined cooling, heat, and power
system based on energy hub in a micro-energy grid: The application of energy storages. Energy &
Environment 2019; 30(8): 1356–1379.
9. Eshraghi A, Salehi G, Heibati S, et al. An enhanced operation model for energy storage system of a typical
combined cool, heat and power based on demand response program: The application of mixed integer linear
programming. Building Services Engineering Research and Technology 2018; 40(1): 47–74.
10. Iqbal MZ, Faisal MM and Ali SR. Integration of supercapacitors and batteries towards high-performance
hybrid energy storage devices. International Journal of Energy Research 2020.
11. Amiryar M and Pullen K. A Review of Flywheel Energy Storage System Technologies and Their
Applications. Applied Sciences 2017; 7(3): 286.
12. Guerra OJ. Beyond short-duration energy storage. Nature Energy 2021; 6(5): 460–461.
13. Sarbu I and Sebarchievici C. A Comprehensive Review of Thermal Energy Storage. Sustainability 2018;
10: 191.
14. China Energy Storage Alliance (CNESA). CNESA Global Energy Storage Market Analysis—2020.Q3
(Summary). 2020.
1132 Energy & Environment 34(4)

15. Impram S, Varbak Nese S and Oral B. Challenges of renewable energy penetration on power system
flexibility: A survey. Energy Strategy Reviews 2020; 31: 100539.
16. Owusu PA and Asumadu-Sarkodie S. A review of renewable energy sources, sustainability issues and
climate change mitigation. Cogent Engineering 2016; 3: 1.
17. Palizban O and Kauhaniemi K. Energy storage systems in modern grids—Matrix of technologies and
applications. Journal of Energy Storage 2016; 6: 248–259.
18. Riaz A, Sarker MR, Saad MHM, et al. Review on Comparison of Different Energy Storage Technologies
Used in Micro-Energy Harvesting, WSNs, Low-Cost Microelectronic Devices: Challenges and
Recommendations. Sensors 2021; 21(15): 5041.
19. Mohd A, Ortjohann E, Schmelter A, et al. Challenges in integrating distributed Energy storage systems
into future smart grid 2008; 2008, IEEE International Symposium on Industrial Electronics.
20. Gielen D, Boshell F, Saygin D, et al. The role of renewable energy in the global energy transformation.
Energy Strategy Reviews 2019; 24: 38–50.
21. Dodón A, Quintero V, Chen Austin M, et al. Bio-Inspired Electricity Storage Alternatives to Support
Massive Demand-Side Energy Generation: A Review of Applications at Building Scale. Biomimetics
2021; 6(3): 51.
22. Georgious R, Refaat R, Garcia J, et al. Review on Energy Storage Systems in Microgrids. Electronics
2021; 10: 2134.
23. Sani SB, Celvakumaran P, Ramachandaramurthy VK, et al. Energy storage system policies: Way
forward and opportunities for emerging economies. Journal of Energy Storage 2020; 32: 101902.
24. Nadeem F, Hussain SMS, Tiwari PK, et al. Comparative Review of Energy Storage Systems, Their
Roles, and Impacts on Future Power Systems. IEEE Access 2019; 7: 4555–4585.
25. Hossain E, Faruque HMR, Sunny MSH, et al. A Comprehensive Review on Energy Storage Systems:
Types, Comparison, Current Scenario, Applications, Barriers, and Potential Solutions. Policies, and
Future Prospects. Energies 2020; 13(14): 3651.
26. Poullikkas A. A comparative overview of large-scale battery systems for electricity storage. Renewable
and Sustainable Energy Reviews 2013; 27: 778–788.
27. Gür TM. Perspective—Low-Carbon Electricity Is Great: What about “Less-Carbon”? Journal of The
Electrochemical Society 2017; 164(14): F1587–F1590.
28. www.bloomberg.com/news/articles/2016-12-15/world-energy-hits-a-turning-pointsolar-that-s-cheaper-
than wind, (accessed Nov 10, 2017).
29. Bloomberg Technology News, July 31, 2017, https://www.bloomberg.com/news/articles/2017-07-31/
alphabet-wants-to-fixrenewable-energy-s-storage-problem-with-salt.
30. Penn I. California invested heavily in solar power. Now there’s so much that other states are sometimes
paid to take it. Los Angeles Times. JUNE 2017; 22. https://www.latimes.com/projects/la-fi-electricity-
solar/
31. Dimian AC, Bildea CS and Kiss AA. Generalised Computational Methods in Thermodynamics.
Integrated Design and Simulation of Chemical Processes 2014; 35: 157–200.
32. Khalid M, Arshid N and Grace N. Advances in Supercapacitor and Supercapattery: Innovations in
Energy Storage. Cambridge. US: Elsevier, 2020.
33. Bistline J, Blanford G, Mai T, et al. Modeling variable renewable energy and storage in the power sector.
Energy Policy 2021; 156: 112424.
34. Akrami A, Doostizadeh M and Aminifar F. Power system flexibility: an overview of emergence to evo-
lution. J. Mod. Power Syst. Clean Energy 2019; 7: 987–1007.
35. Tan KM, Babu TS, Ramachandaramurthy VK, et al. Empowering smart grid: A comprehensive review
of energy storage technology and application with renewable energy integration. Journal of Energy
Storage 2021; 39: 102591.
36. Johnson SC, Papageorgiou DJ, Harper MR, et al. The economic and reliability impacts of grid-
scale storage in a high penetration renewable energy system. Advances in Applied Energy 2021;
3: 100052.
Smdani et al. 1133

37. Hameer S and van Niekerk JL. A review of large-scale electrical energy storage. International Journal of
Energy Research 2015; 39(9): 1179–1195.
38. Fujihara T, Imano H and Oshima K. Development of pump turbine for seawater pumped-storage power
plant. Hitachi Rev 1998; 47: 199–202.
39. Chen H, Cong TN, Yang W, et al. Progress in electrical energy storage system: A critical review.
Progress in Natural Science 2009; 19(3): 291–312.
40. Succar S and Williams RH. Compressed air energy storage: theory, resources, and applications for wind
power. Princeton Environmental Institute. Energy Anal Group 2008.
41. Aneke M and Wang M. Energy storage technologies and real life applications – A state of the art review.
Applied Energy 2016; 179: 350–377.
42. Ries G and Neumueller H.-W. Comparison of energy storage in flywheels and SMES. Phys C Supercond
2001; 357–360: 1306–1310.
43. Das CK, Bass O, Kothapalli G, et al. Overview of energy storage systems in distribution networks:
Placement, sizing, operation, and power quality. Renewable and Sustainable Energy Reviews 2018;
91: 1205–1230.
44. McDowall J. Integrating energy storage with wind power in weak electricity grids. Journal of Power
Sources 2006; 162(2): 959–964.
45. Rohit AK and Rangnekar S. An overview of energy storage and its importance in Indian renewable
energy sector: Part II – energy storage applications, benefits and market potential. Journal of Energy
Storage 2017; 13: 447–456.
46. Ben Elghali S, Outbib R and Benbouzid M. Selecting and optimal sizing of hybridized energy storage
systems for tidal energy integration into power grid. Journal of Modern Power Systems and Clean
Energy 2018.
47. Du Pasquier A, Plitz I, Menocal S, et al. A comparative study of Li-ion battery, supercapacitor and non-
aqueous asymmetric hybrid devices for automotive applications. Journal of Power Sources 2003;
115(1): 171–178.
48. Haidl P, Buchroithner A, Schweighofer B, et al. Lifetime Analysis of Energy Storage Systems for
Sustainable Transportation. Sustainability 2019; 11: 6731.
49. Ribeiro PF, Johnson BK, Crow ML, et al. Energy storage systems for advanced power applications.
Proceedings of the IEEE 2001; 89(12): 1744–1756.
50. Arepalli S, Fireman H, Huffman C, et al. Carbon-nanotube-based electrochemical double-layer capacitor
technologies for spaceflight applications. JOM 2005; 57(12): 26–31.
51. Felseghi R.-A, Carcadea E, Raboaca MS, et al. Hydrogen Fuel Cell Technology for the Sustainable
Future of Stationary Applications. Energies 2019; 12(23): 4593.
52. Stolten D. Hydrogen and Fuel Cells, Fundamentals, Technologies and Applications (Hydrogen Energy).
Weinheim, Germany: WILEY-VCH Verlag GmbH&Co. KgaA, 2010.
53. Luta DN. An energy management system for a hybrid reversible fuel cell/supercapacitor in a 100%
renewable power system. PhD thesis. Cape Peninsula University of Technology. Cape Town. South
Africa, 2019.
54. Zhang F, Zhang T, Yang X, et al. A high-performance supercapacitor-battery hybrid energy storage
device based on graphene-enhanced electrode materials with ultrahigh energy density. Energy &
Environmental Science 2013; 6(5): 1623.
55. Long JW, Bélanger D, Brousse T, et al. Asymmetric electrochemical capacitors—Stretching the limits of
aqueous electrolytes. MRS Bulletin 2011; 36(07): 513–522.
56. Ra EJ, Raymundo-Piñero E, Lee YH, et al. High power supercapacitors using polyacrylonitrile-based
carbon nanofiber paper. Carbon 2009; 47(13): 2984–2992.
57. Lukatskaya MR, Dunn B and Gogotsi Y. Multidimensional materials and device architectures for future
hybrid energy storage. Nature Communications 2016; 7: 12647.
58. Naoi K, Ishimoto S, Isobe Y, et al. High-rate nano-crystalline Li4Ti5O12 attached on carbon nano-fibers
for hybrid supercapacitors. Journal of Power Sources 2010; 195(18): 6250–6254.
1134 Energy & Environment 34(4)

59. Zhao Y, Zhang H, Liu A, et al. Fabrication of nanoarchitectured TiO 2 (B)@C/rGO electrode for 4 V
quasi-solid-state nanohybrid supercapacitors. Electrochimica Acta 2017; 258: 343–352.
60. Kong L, Zhang C, Zhang S, et al. High-power and high-energy asymmetric supercapacitors based on Li
+-intercalation into a T-Nb2O5/graphene pseudocapacitive electrode. J. Mater. Chem. A 2014; 2(42):
17962–17970.
61. Choi C, Ashby DS, Butts DM, et al. Achieving high energy density and high power density with pseu-
docapacitive materials. Nature Reviews Materials 2019; 5: 5–19.
62. Tie D, Huang S, Wang J, et al. Hybrid Energy Storage Devices: Advanced Electrode Materials and
Matching Principles. Energy Storage Materials 2018; 21: 22–40.
63. Huang S, Zhu X, Sarkar S, et al. Challenges and opportunities for supercapacitors. APL Materials 2019;
7(10): 100901.
64. Iqbal J, Numan A, Rafique S, et al. High performance supercapattery incorporating ternary nanocompo-
site of multiwalled carbon nanotubes decorated with Co 3 O 4 nanograins and silver nanoparticles as
electrode material. Electrochimica Acta 2018; 278: 72–82.
65. Subramaniam RT, Iqbal J, Li L, et al. Density Functional Theory Simulation of Cobalt Oxide
Aggregation and Facile Synthesis of Cobalt oxide, Gold and Multiwalled Carbon Nanotubes
based Ternary Composite for a High Performance Supercapattery. New Journal of Chemistry
2019; 43: 13183.
66. Rahmanifar MS, Hemmati M, Noori A, et al. Asymmetric supercapacitors: An alternative to activated
carbon negative electrodes based on earth abundant elements. Materials Today Energy 2019; 12: 26–36.
67. Devi S, Athika V, Duraisamy M, et al. Facile sol-gel derived nanostructured spinel Co3O4 as electrode
material for high-performance supercapattery and lithium-ion storage. Journal of Energy Storage 2019;
25: 100815.
68. Zhang Y, Wang C, Jiang H, et al. Cobalt-nickel silicate hydroxide on amorphous carbon derived from
bamboo leaves for hybrid supercapacitors. Chemical Engineering Journal 2019; 375: 121938.
69. Saravanakumar B, Wang X, Zhang W, et al. Holey Two Dimensional Manganese Cobalt Oxide
Nanosheets as a High-performance Electrode for Supercapattery. Chemical Engineering Journal 2019.
70. Geng L, Yan F, Dong C, et al. Design and Regulation of Novel MnFe2O4@C Nanowires as High
Performance Electrode for Supercapacitor. Nanomaterials 2019; 9(5): 777.
71. Chen H, Hu L, Chen M, et al. Nickel-Cobalt Layered Double Hydroxide Nanosheets for
High-performance Supercapacitor Electrode Materials. Advanced Functional Materials 2013; 24(7):
934–942.
72. Padmanathan N, Shao H, McNulty D, et al. Hierarchical NiO–In2O3 microflower (3D)/ nanorod (1D)
hetero-architecture as a supercapattery electrode with excellent cyclic stability. Journal of Materials
Chemistry A 2016; 4(13): 4820–4830.
73. Zhang Y, Feng H, Wu X, et al. Progress of electrochemical capacitor electrode materials: A review.
International Journal of Hydrogen Energy 2009; 34(11): 4889–4899.
74. Zhao J and Burke AF. Review on supercapacitors: Technologies and performance evaluation. Journal of
Energy Chemistry 2021; 59: 276–291.
75. Obrezkov FA, Ramezankhani V, Zhidkov I, et al. High-Energy and High-Power-Density Potassium Ion
Batteries Using Dihydrophenazine-Based Polymer as Active Cathode Material. The Journal of Physical
Chemistry Letters 2019; 5440–5445.
76. Miller EE, Hua Y and Tezel FH. Materials for energy storage: Review of electrode materials and
methods of increasing capacitance for supercapacitors. Journal of Energy Storage 2018; 20: 30–40.
77. Raghavendra KVG, Vinoth R, Zeb K, et al. An intuitive review of supercapacitors with recent progress
and novel device applications. Journal of Energy Storage 2020; 31: 101652.
78. Chen K and Xue D. Materials chemistry toward electrochemical energy storage. Journal of Materials
Chemistry A 2016; 4(20): 7522–7537.
79. Zhu D, Cheng K, Wang Y, et al. Nitrogen-doped porous carbons with nanofiber-like structure derived
from poly (aniline-co-p-phenylenediamine) for supercapacitors. Electrochimica Acta 2017; 224: 17–24.
Smdani et al. 1135

80. Jiang Z, Jiang Z and Chen W. The role of holes in improving the performance of nitrogen-doped holey
graphene as an active electrode material for supercapacitor and oxygen reduction reaction. Journal of
Power Sources 2014; 251: 55–65.
81. Kumar MP, Kesavan T, Kalita G, et al. On the large capacitance of nitrogen doped graphene derived by a
facile route. RSC Adv 2014; 4(73): 38689–38697.
82. Gopalsamy K, Balamurugan J, Thanh TD, et al. Fabrication of nitrogen and sulfur co-doped graphene
nanoribbons with porous architecture for high-performance supercapacitors. Chemical Engineering
Journal 2017; 312: 180–190.
83. Sun L, Wang L, Tian C, et al. Nitrogen-doped graphene with high nitrogen level via a one-step hydro-
thermal reaction of graphene oxide with urea for superior capacitive energy storage. RSC Advances 2012;
2(10): 4498.
84. Liu X, Wang C, Dou Y, et al. A NiAl layered double hydroxide@carbon nanoparticles hybrid electrode
for high-performance asymmetric supercapacitors. J. Mater. Chem. A 2014; 2(6): 1682–1685.
85. Gao Z, Wang J, Li Z, et al. Graphene Nanosheet/Ni2+/Al3+Layered Double-Hydroxide Composite as a
Novel Electrode for a Supercapacitor. Chemistry of Materials 2011; 23(15): 3509–3516.
86. Patake VD, Lokhande CD and Joo OS. Electrodeposited ruthenium oxide thin films for supercapacitor:
Effect of surface treatments. Applied Surface Science 2009; 255: 4192–4196.
87. Park JH, Ko JM and Ok Park O. Carbon Nanotube/RuO[sub 2] Nanocomposite Electrodes for
Supercapacitors. Journal of The Electrochemical Society 2003; 150(7): A864.
88. Hu C-C, Wang C-W, Chang K-H, et al. Anodic composite deposition of RuO2/reduced graphene oxide/
carbon nanotube for advanced supercapacitors. Nanotechnology 2015; 26(27): 274004.
89. Amir FZ, Pham VH, Mullinax DW, et al. Enhanced performance of HRGO-RuO 2 solid state flexible
supercapacitors fabricated by electrophoretic deposition. Carbon 2016; 107: 338–343.
90. Zhang X, Wang J, Liu J, et al. Design and preparation of a ternary composite of graphene oxide/carbon
dots/polypyrrole for supercapacitor application: Importance and unique role of carbon dots. Carbon
2017; 115: 134–146.
91. Zhang L, Huang D, Hu N, et al. Three-dimensional structures of graphene/polyaniline hybrid films con-
structed by steamed water for high-performance supercapacitors. Journal of Power Sources 2017; 342:
1–8.
92. Jena RK, Yue CY, Sk MM, et al. A novel high performance poly (2-methyl thioaniline) based composite
electrode for supercapacitors application. Carbon 2017; 115: 175–187.
93. Yan J, Wei T, Fan Z, et al. Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as
electrode material for supercapacitors. Journal of Power Sources 2010; 195(9): 3041–3045.
94. Cheng G, Yan Y and Chen R. From Ni-based nanoprecursors to NiO nanostructures: morphology-
controlled synthesis and structure-dependent electrochemical behavior. New Journal of Chemistry
2015; 39(1): 676–682.
95. Yu F, Zhu L, You T, et al. Preparation of chestnut-like porous NiO nanospheres as electrodes for super-
capacitors. RSC Advances 2015; 5(116): 96165–96169.
96. Fang QL, Evans DA, Roberson SL, et al. Ruthenium Oxide Film Electrodes Prepared at Low Temperatures
for Electrochemical Capacitors. Journal of The Electrochemical Society 2001; 148(8): A833.
97. Kim I-H, Kim J-H and Kim K-B. Electrochemical Characterization of Electrochemically Prepared
Ruthenium Oxide/Carbon Nanotube Electrode for Supercapacitor Application. Electrochemical and
Solid-State Letters 2005; 8(7): A369.
98. Shi K and Zhitomirsky I. Asymmetric Supercapacitors Based on Activated-Carbon-Coated Carbon
Nanotubes. ChemElectroChem 2015; 2(3): 396–403.
99. Faiza W, Firouzi A, Islam MR, et al. Degradation analysis of epoxy resin composites reinforced with
bioprotein: Effects of hydrolysis using papain and bromelain. Polymer Composites 2021; 42(6):
2717–2727.
100. Sumdani MG, Islam MR and Yahaya ANA. Effects of variation of steric repulsion between multiwall
carbon nanotubes and anionic surfactant in epoxy nanocomposites. Journal of Applied Polymer
Science 2018; 135(48): 46883.
1136 Energy & Environment 34(4)

101. Hani F, Firouzi A, Islam MR, et al. Mechanical and thermal properties of fishbone-based epoxy compo-
sites: The effects of thermal treatment. Polymer Composites 2020; 42(3): 1224–1234.
102. Sumdani MG, Islam MR, Yahaya ANA, et al. Acid-Based Surfactant-Aided Dispersion of Multi-Walled
Carbon Nanotubes in Epoxy-Based Nanocomposites. Polymer Engineering & Science 2018; 59: 80–87.
103. Syamimi NF, Islam MR, Sumdani MG, et al. Mechanical and thermal properties of snail shell particles-
reinforced bisphenol-A bio-composites. Polym. Bull 2020; 77: 2573–2589.
104. Sumdani MG, Islam MR and Yahaya ANA. The effects of anionic surfactant on the mechanical, thermal,
structure and morphological properties of epoxy–MWCNT composites. Polymer Bulletin 2019; 76:
5919–5938.
105. Gallo AB, Simões-Moreira JR, Costa HKM, et al. Energy storage in the energy transition context: A
technology review. Renewable and Sustainable Energy Reviews, 2016; 65: 800–822.
106. Zakeri B and Syri S. Electrical energy storage systems: A comparative life cycle cost analysis.
Renewable and Sustainable Energy Reviews 2015; 42: 569–596.
107. Ferreira HL, Garde R, Fulli G, et al. Characterisation of electrical energy storage technologies. Energy
2013; 53: 288–298.
108. Khodaparastan M and Mohamed A. Flywheel vs. Supercapacitor as Wayside Energy Storage for Electric
Rail Transit Systems. Inventions 2019; 4(4): 62.
109. Luo X, Wang J, Dooner M, et al. Overview of current development in electrical energy storage technolo-
gies and the application potential in power system operation. Applied Energy 2015; 137: 511–536.
110. Trausel F, de Jong A-J and Cuypers R. A Review on the Properties of Salt Hydrates for Thermochemical
Storage. Energy Procedia 2014; 48: 447–452.
111. Behabtu HA, Messagie M, Coosemans T, et al. A Review of Energy Storage Technologies’ Application
Potentials in Renewable Energy Sources Grid Integration. Sustainability 2020; 12(24): 10511.
112. N’Tsoukpoe KE, Liu H, Le Pierrès N, et al. A review on long-term sorption solar energy storage.
Renewable and Sustainable Energy Reviews 2009; 13(9): 2385–2396.
113. Yu Q, Wang Q, Tan X, et al. A review of compressed-air energy storage. Journal of Renewable and
Sustainable Energy 2019; 11(4): 042702.
114. Steen E and Torestam M. Compressed air energy storage: Process review and case study of small scale
compressed air energy storage aimed at residential buildings. Degree project in technology. KTH Royal
Institute of Technology. 2018.
115. Olabi AG, Wilberforce T, Abdelkareem MA, et al. Critical Review of Flywheel Energy Storage System.
Energies 2021; 14: 2159.
116. Banguero E, Correcher A, Pérez-Navarro Á, et al. A Review on Battery Charging and Discharging
Control Strategies: Application to Renewable Energy Systems. Energies 2018; 11(4): 1021.
117. Sánchez-Díez E, Ventosa E, Guarnieri M, et al. Redox flow batteries: Status and perspective towards
sustainable stationary energy storage. Journal of Power Sources 2021; 481: 228804.
118. Kharel S and Shabani B. Hydrogen as a Long-Term Large-Scale Energy Storage Solution to Support
Renewables. Energies 2018; 11: 2825.
119. Albertus P, Manser JS and Litzelman S. Long-Duration Electricity Storage Applications. Economics,
and Technologies. Joule 2020; 4: 21–32.
120. Shaner MR, Davis SJ, Lewis NS, et al. Geophysical constraints onthe reliability of solar and wind power
in the United States. Energy Environ. Sci 2018; 11: 914–925.
121. Denholm P and Margolis R. Energy storage requirements for achieving 2016; 50, % solar photovoltaic
energy penetration in California (National Renewable Energy Laboratory (NREL)).
122. Ziegler MS, Mueller JM, Pereira GD, et al. Storage requirements and costs of shaping renewable energy
Toward grid decarbonization. Joule 2019; 3: 2134–2153.
123. Jones R. System perspectives on long duration storage, ARPA-E. Evolved Energy Research 2017.
https://docs.wixstatic.com/ugd/a35761_773bbc576d8b4b02ae2c02ad93c6166f.pdf
124. U.S. Energy Information Administration. Energy storage and renewables beyond wind,hydro, solar
make up 4% of U.S. power capacity. Today in Energy. 2017. https://www.eia. gov/todayinenergy/
detail.php?id=31372.
Smdani et al. 1137

125. Grid N. National Grid develops innovative solution for an island community’s unique energy challenges.
2017. https://news.nationalgridus.com/2017/11/national-griddevelops-innovative-solution-islandcommunitys-
unique-energy-challenges/
126. Denholm P and Hand M. Grid flexibility and storage required to achieve very high penetration of variable
renewable electricity. Energy Policy 2011; 39: 1817–1830.
127. Abdulgalil MA, Khalid M and Alismail F. Optimal Sizing of Battery Energy Storage for a
Grid-Connected Microgrid Subjected to Wind Uncertainties. Energies 2019; 12(12): 2412.
128. Denholm P and Mai T. Timescales of energy storage needed for reducing renewable energy curtailment.
Renew. Energy 2019; 130: 388–399.
129. Beaudin M, Zareipour H, Schellenberglabe A, et al. Energy storage for mitigating the variability of
renewable electricity sources: An updated review. Energy for Sustainable Development 2010; 14(4):
302–314.
130. Sabihuddin S, Kiprakis A and Mueller M. A Numerical and Graphical Review of Energy Storage
Technologies. Energies 2014; 8(1): 172–216.
131. King M, Jain A, Bhakar R, et al. Overview of current compressed air energy storage projects and analysis
of the potential underground storage capacity in India and the UK. Renewable and Sustainable Energy
Reviews 2021; 139: 110705.
132. He W and Wang J. Optimal selection of air expansion machine in Compressed Air Energy Storage: A
review. Renewable and Sustainable Energy Reviews 2018; 87: 77–95.
133. Nikolaidis P and Poullikkas A. A comparative review of electrical energy storage systems for better sus-
tainability. Journal of Power Technologies 2017; 97(3): 220–245.
134. Kim Y-M, Lee J-H, Kim S-J, et al. Potential and Evolution of Compressed Air Energy Storage: Energy
and Exergy Analyses. Entropy 2012; 14(8): 1501–1521.
135. Bullough C, Gatzen C, Jakiel C, et al. Advanced adiabatic compressed air energy storage for the integra-
tion of wind energy, in: Proceedings of the European Wind Energy Conference. EWEC 2012; 22: 25.
136. Arabkoohsar A. Mechanical Energy Storage Technologies. Chapter Five - Flywheel energy storage.
Elsevier Inc. Academic Press 2020.
137. Koohi-Fayegh S and Rosen MA. A review of energy storage types, applications and recent develop-
ments. Journal of Energy Storage 2020; 27: 101047.
138. International Electrochemical Commission (IEC). White paper report: electrical energy storage. 2011.
http://www.iec.ch/whitepaper/pdf/iecWP-energystorageLR-en.pdf. (Accessed in May 2019).
139. Smith SC, Sen PK and Kroposki B. Advancement of energy storage devices and applications in electrical
power system 2008; 2008, IEEE Power and Energy Society General Meeting - Conversion and Delivery
of Electrical Energy in the 21st Century.
140. Kwade A, Haselrieder W, Leithoff R, et al. Current status and challenges for automotive battery produc-
tion technologies. Nature Energy 2018; 3(4): 290–300.
141. Ge S, Leng Y, Liu T, et al. A new approach to both high safety and high performance of lithium-ion
batteries. Science Advances 2020; 6: 9. eaay7633.
142. Ziadi Y and Qjidaa H. A High Efficiency Li-Ion Battery LDO-Based Charger for Portable Application.
Active and Passive Electronic Components 2015; 2015: 1–9.
143. Miller RJ. Capacitors for power grid storage - multi-hour bulk energy storage using capacitors. JME. Inc.
and Case Western Reserve University. In: Trans-Atlantic Workshop on Storage Technologies for Power
Grids 2010.
144. Meishner F and Sauer DU. Wayside energy recovery systems in DC urban railway grids.
eTransportation 2019; 1: 100001.
145. Christen T and Carlen MW. Theory of Ragone plots. Journal of Power Sources 2000; 91(2): 210–216.
146. Shoenung SM. Characteristics and technologies for long- vs. short-term energy storage: a study by the
DOE energy storage systems program. Technical report. SAND 2001: 2001–0765. Sandia National
Laboratories. United States Department of Energy.
147. Schaber C, Mazza P and Hammerschlag R. Utility-Scale Storage of Renewable Energy. The Electricity
Journal 2004; 17(6): 21–29.
1138 Energy & Environment 34(4)

148. Kyriakopoulos GL and Arabatzis G. Electrical energy storage systems in electricity generation: Energy
policies, innovative technologies, and regulatory regimes. Renewable and Sustainable Energy Reviews
2016; 56: 1044–1067.
149. Hauer A. Storage Technology Issues and Opportunities, International Low-Carbon Energy Technology
Platform. In Proceedings of the Strategic and Cross-Cutting Workshop “Energy Storage—Issues and
Opportunities”, Paris, France, 15 February 2011.
150. Kasaeian A, Bahrami L, Pourfayaz F, et al. Experimental studies on the applications of PCMs and
nano-PCMs in buildings: A critical review. Energy and Buildings 2017; 154, 96–112.
151. Kerskes H, Mette B, Bertsch F, et al. Chemical energy storage using reversible solid/gas-reactions
(CWS) – results of the research project. Energy Procedia 2012; 30: 294–304.
152. Immendoerfer A, Tietze I, Hottenroth H, et al. Life-cycle impacts of pumped hydropower storage and
battery storage. Int J Energy Environ Eng 2017; 8: 231–245.
153. Acar C. A comprehensive evaluation of energy storage options for better sustainability. International
Journal of Energy Research 2018.
154. Denholm P and Kulcinski GL. Life cycle energy requirements and greenhouse gas emissions from large
scale energy storage systems. Energy Conversion and Management 2004; 45(13–14): 2153–2172.
155. Venkataramani G, Parankusam P, Ramalingam V, et al. A review on compressed air energy storage – A
pathway for smart grid and polygeneration. Renewable and Sustainable Energy Reviews 2016; 62:
895–907.
156. Hosseini SE and Butler B. An overview of development and challenges in hydrogen powered vehicles.
International Journal of Green Energy 2019; 1–25.
157. Nienborg B, Gschwander S, Munz G, et al. Life Cycle Assessment of thermal energy storage materials
and components. Energy Procedia 2018; 155: 111–120.
158. Kondoh J, Ishii I, Yamaguchi H, et al. Electrical energy storage systems for energy networks. Energy
Conversion and Management 2000; 41(17): 1863–1874.
159. IRENA (International Renewable Energy Agency). Renewable Energy Technologies: Cost Analysis
Series (Hydropower); Working Paper, Volume 1: Power Sector Issue 3/5; IRENA: Abu Dhabi, UAE,
2012.
160. Farret FA and Simões MG. Integration of alternative sources of energy. John Wiley & Sons Inc 2006:
262–300.
161. Hadjipaschalis I, Poullikkas A and Efthimiou V. Overview of current and future energy storage tech-
nologies for electric power applications. Renewable and Sustainable Energy Reviews 2009; 13(6–7):
1513–1522.
162. Díaz-González F, Sumper A, Gomis-Bellmunt O, et al. A review of energy storage technologies for wind
power applications. Renewable and Sustainable Energy Reviews 2012; 16(4): 2154–2171.
163. Rydh CJ and Sandén BA. Energy analysis of batteries in photovoltaic systems. Part II: Energy return factors
and overall battery efficiencies. Energy Conversion and Management 2005; 46(11–12): 1980–2000.
164. Review of electrical energy storage technologies and systems and of their potential for the UK. Dti
Report. DG/DTI/00055/00/00. UK Department of Trade and Industry; 2004.
165. Capacitors age and capacitors have an end of life: a white paper from the experts in business-critical con-
tinuity. Emerson network power. n.d. http://www.emersonnetworkpower.com/documentation/en-us/
brands/liebert/documents/white%20papers/sl-24630.pdf (accessed 27.05.14).
166. Smith W. The role of fuel cells in energy storage. Journal of Power Sources 2000; 86(1–2): 74–83.
167. Andrepont SJ. Energy storage – thermal energy storage coupled with turbine inlet cooling. In: 14th
annual electric power conf. & exhibition. n.d. http://www.turbineinletcooling.org/resources/papers/
Andrepont_2012EP.pdf (accessed 17.06.14).
168. Carnegie R, Gotham D, Nderitu D, et al. Utility Scale Energy Storage Systems Benefits, Applications,
and Technologies. State Utility Forecasting Group 2013.
169. Schmidt O, Hawkes A, Gambhir A, et al. The future cost of electrical energy storage based on experience
rates. Nature Energy 2017; 2(8): 17110.
Smdani et al. 1139

170. Schoenung S. Energy storage systems cost update: a study for the DOE energy storage systems program.
Technical report. Sandia National Laboratories 2011.
171. May GJ, Davidson A and Monahov B. Lead batteries for utility energy storage: A review. Journal of
Energy Storage 2018; 15: 145–157.
172. Aquino T, Roling M, Baker C, et al. Battery Energy Storage Technology Assessment; Prepared for the
Platte River Power authority by HDR. HDR: Omaha, NE. USA, 2017.
173. Shan R, O’Connor P, Oak Ridge ORNL, et al. Personal Communication. 2018.
174. Manwaring M. National Hydropower Association Washington, DC. USA: Personal Communication,
2018.
175. Black & Veatch. Cost and Performance Data for Power Generation Technologies; Prepared for the
National Renewable Energy Laboratory. Overland Park, KS, USA: Black & Veatch, 2012.
176. Bailie R. Siemens, Munich, Germany. Personal Correspondence, 3 August 2018.
177. Siemens. Roundtrip Efficiency Characterization for Dresser-Rand SMARTCAES Compressed Air
Energy Storage Systems; Presentation. Munich, Germany: Siemens, 2018.
178. Manuel W. Energy Storage Study 2014. California Energy Commission Report, Turlock Irrigation
District. 17 September 2014; California. Available online: https://www.energy.ca.gov/assessments/
ab2514_reports/Turlock_Irrigation_District/2014-10-28_Turlock_Irrigation_District_Energy_Storage_
Study.pdf (accessed on 27 July 2018).
179. Power H. Regenerative Braking Using Flywheels. In Proceedings of the NY-BEST Energy Storage
Technology Conference, Syracuse, NY. USA 20 October 2016.
180. Goodwin A. Kinetic Traction, Los Angeles, CA, USA: Personal correspondence, 9 July 2018.
181. Lazarewicz M. Helix Power, Somerville, MA, USA. Personal Communication, 2018.
182. Morris C. Tesla is Getting Close to the Magic Battery Cost Number that will Turbocharge Demand.
Evannex. 19 June 2018. Available online: https://evannex.com/blogs/news/tesla-is-getting-close-to-the-
magic-batterycost-number-that-will-turbocharge-demand (accessed on 26 July 2018).
183. Damato G. Energy Storage Cost Analysis: Executive Summary of 2017 Methods and Results; Report
3002012046. Palo Alto, CA. USA: Electric Power Research Institute, 2017.
184. Curry C. Lithium-Ion Battery Costs and Market. Bloomberg New Energy Finance. 5 July 2017.
Available online: https://data.bloomberglp.com/bnef/sites/14/2017/07/BNEF-Lithium-ion-battery-costs-
andmarket. pdf (accessed on 11 July 2018).
185. Watanabe C. Why Battery Cost Could Put the Brakes on Electric Car Sales. Bloomberg. 28 November
2017. Available online: https://www.bloomberg.com/news/articles/2017-11-28/electric-cars-need-cheaper-
batteriesbefore-taking-over-the-road (accessed on 26 July 2018).
186. Greenspon A. The Energy Storage Landscape: Feasibility of Alternatives to Lithium Based Batteries.
Cambridge, MA. USA: Harvard Energy Journal Club, 2017.
187. Sahay K and Dwivedi B. Supercapacitors Energy Storage System for Power Quality Improvement: An
Overview. J. Electr. Syst 2009; 5: 4.
188. Garcia A, Technologies M, Diego S, et al. Personal Correspondence. 18 June 2018.
189. Colton J, Ioxus O and USA NY. Personal Correspondence. 11 July 2018.
190. Mongird K, Viswanathan V, Balducci P, et al. An Evaluation of Energy Storage Cost and Performance
Characteristics. Energies 2020; 13(13): 3307.
191. EASE (European Association for Energy Storage). Energy Storage Technologies. 2016. Available
online: http://ease-storage.eu/energy-storage/technologies/ (accessed on 20 July 2018).
192. Evertiq. Power Battery Prices Decrease as Global e-Car Market Expands. 9 July 2018. Available online:
https://evertiq.com/news/44460 (accessed on 7 August 2018).
193. Posawatz T. Mine is Bigger: Why Larger EV Batteries are not the Answer. 23 July 2018. Available online:
https://www.forbes.com/sites/tonyposawatz/2018/07/23/mine-is-bigger-why-bigger-biggerev-batteries-
are-not-the-answer/#681f215f1837 (accessed on 8 August 2018).
194. Chediak M. Electric Car Batteries Drop Closer to a Cost Tipping Point. 5 December 2017. Available
online: https://www.industryweek.com/energy/electric-car-batteries-drop-closer-cost-tipping-point (accessed
on 6 August 2018).
1140 Energy & Environment 34(4)

195. Eckert M. Analysis Shows Continued Industry-Wide Decline in Electric Vehicle Battery Costs. 21
February 2018. Available online: https://mackinstitute.wharton.upenn.edu/2018/electric-vehicle-battery-
costs-decline/ (accessed on 6 August 2018).
196. Safari M. Battery electric vehicles: Looking behind to move forward. Energy Policy 2018; 115:
54–65.
197. Lambert F. Electric Vehicle Battery Cost Dropped 80% in 6 Years down to $227/kWH—Tesla Claims to
be Below $190/kWH, Electrek. 30 January 2017. Available online: https://electrek.co/2017/01/30/
electric-vehiclebattery-cost-dropped-80-6-years-227kwh-tesla-190kwh/ (accessed on 6 August 2018).
198. Lacey S. Stem CTO: Lithium-Ion Battery Prices Fell 70% in the Last 18 Months, GreentechMedia. 28
June 2016. Available online: https://www.greentechmedia.com/articles/read/stem-cto-weve-seen-battery-
pricesfall-70-in-the-last-18-months#gs.UwTIT0g (accessed on 6 August 2018).
199. Balducci P, Mongird K, Wu D, et al. Shell Energy North America Hydro Battery System: Market
Assessment 1; PNNL-27162. Richland, WA. USA: Pacific Northwest National Laboratory, 2018.
200. CAISO (California Independent System Operator). 2017. “Impacts of renewable energy on grid opera-
tions.” Accessed on: https://www.caiso.com/documents/curtailmentfastfacts.pdf (December 13, 2018).
201. Rastler D. Electric energy storage technology options: a white paper primer on applications, costs, and
benefits. Palo Alto. CA: EPRI, 2010; 1020676.
202. Dell RM and Rand DAJ. Clean energy. Cambridge: Royal Society of Chemistry, 2004.
203. Evans A, Strezov V and Evans TJ. Assessment of utility energy storage options for increased renewable
energy penetration. Renewable and Sustainable Energy Reviews 2012; 16(6): 4141–4147.
204. Breeze P. Compressed Air Energy Storage. In Power System Energy Storage Technologies. Cambridge,
MA. USA: Academic Press, 2018: 23–31. ISBN: 9780128129029.
205. Dugan K. Huntsman Corp. breaks ground on $64 million McIntosh expansion. Alabama Business. 2013.
Available online: https://www.al.com/business/index.ssf/2013/05/huntsman_corp_breaks_ground_on.
html (accessed on 14 December 2018).
206. Chen L, Zheng T, Mei S, et al. Review and prospect of compressed air energy storage system. Journal of
Modern Power Systems and Clean Energy 2016; 4(4), 529–541.
207. Mahmoud M, Ramadan M, Olabi A-G, et al. A review of mechanical energy storage systems combined
with wind and solar applications. Energy Conversion and Management 2020; 210: 112670.
208. Ahmadi P and Kjeang E. Realistic simulation of fuel economy and life cycle metrics for hydrogen fuel
cell vehicles. International Journal of Energy Research 2016; 41(5): 714–727.
209. Woods J, Mahvi A, Goyal A, et al. Rate capability and Ragone plots for phase change thermal energy
storage. Nature Energy 2021; 6(3): 295–302.
210. Odukomaiya A, Woods J, James N, et al. Addressing energy storage needs at lower cost via on-site
thermal energy storage in buildings. Energy Environ. Sci 2021; 14: 5315–5329.
211. Ibrahim H and Adrian I. Techno-economic analysis of different energy storage technologies. In: Zobaa
AF (Ed.), Energy Storage. London. UK: IntechOpen Limited, 2013.
212. Pena-Alzola R, Sebastian R, Quesada J, et al. Review of flywheel based energy storage systems 2011;
2011, International Conference on Power Engineering, Energy and Electrical Drives. IEEE, 1–6.
213. Cho J, Jeong S and Kim Y. Commercial and research battery technologies for electrical energy storage
applications. Progress in Energy and Combustion Science 2015; 48: 84–101.
214. Florin N and Dominish E. Sustainability Evaluation of Energy Storage Technologies. Report prepared by
the. Institute of Sustainable Futures for the Australian Council of Learned Academies. University of
Technology Sydney 2017.
215. Al-Badi A and AlMubarak I. Growing energy demand in the GCC countries. Arab Journal of Basic and
Applied Sciences 2019; 26(1): 488–496.
216. Abdallah L and El-Shennawy T. Reducing Carbon Dioxide Emissions from Electricity Sector Using
Smart Electric Grid Applications. Journal of Engineering 2013; 2013: 1–8.
217. Al-Sarihi A and Bello H. Challenges to Maximizing Renewables in Oman’S Energy Mix. The Arab Gulf
States Institute in Washington 2019.
Smdani et al. 1141

218. Kalair A, Abas N, Saleem MS, et al. Role of energy storage systems in energy transition from fossil fuels
to renewables. Energy Storage 2020; 3(1): 1–27.
219. Exchange ES. Available online: . https://energystorageexchange.org/projects (accessed on 14 February
2020).
220. Krishan O and Suhag S. An updated review of energy storage systems: Classification and applications in
distributed generation power systems incorporating renewable energy resources. International Journal of
Energy Research 2018; 43: 6171–6210.
221. Gür TM. Review of electrical energy storage technologies, materials and systems: challenges and pro-
spects for large-scale grid storage. Energy & Environmental Science 2018; 11: 2696–2767.
222. Hydrostor. Available online: https://www.hydrostor.ca/ (accessed on 14 February 2020).
223. Energy Storage Association. Compressed Air Energy Storage (CAES). 2017:1–3.
224. Amirante R, Cassone E, Distaso E, et al. Overview on recent developments in energy storage:
Mechanical, electrochemical and hydrogen technologies. Energy Conversion and Management 2017;
132:, 372–387.
225. Arseneaux J. 20 MW Flywheel Energy Storage Plant New York, US: Beacon Power, 2014.
226. IRENA. Innovation landscape brief: Utility-scale batteries, International Renewable Energy Agency,
Abu Dhabi, 2019.
227. El Kharbachi A, Zavorotynska O, Latroche M, et al. Exploits, advances and challenges benefiting
beyond Li-ion battery technologies. Journal of Alloys and Compounds 2019; 817, 153261.
228. González A, Goikolea E, Barrena JA, et al. Review on supercapacitors: Technologies and materials.
Renewable and Sustainable Energy Reviews 2016; 58: 1189–1206.
229. IEA. Technology Roadmap - Energy Storage, IEA, Paris https://www.iea.org/reports/technology-
roadmap-energy-storage, 2014.
230. Kopp M, Coleman D, Stiller C, et al. Energiepark Mainz: Technical and economic analysis of the world-
wide largest Power-to-Gas plant with PEM electrolysis. International Journal of Hydrogen Energy 2017;
42(19): 13311–13320.
231. Taylor P, Bolton R, Stone D, et al. Pathways for energy storage in the UK. Technical report. Centre for
low carbon futures. Published 2012: 27t. h March 2012.
232. European Associaton for Storage of Energy. Pumped heat electrical storage. 2016:2.
233. Castillo A and Gayme DF. Grid-scale energy storage applications in renewable energy integration: A
survey. Energy Conversion and Management 2014; 87: 885–894.