2. Please cite this article as: A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai, Review of energy storage services, applications, limitations, and benefits. Energy Reports (2020),
https://doi.org/10.1016/j.egyr.2020.07.028.
2 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
7. Economic aspects of electrical energy storage .............................................................................................................................................................. 13
8. The impact of electrical energy storage on the global environment........................................................................................................................... 13
9. Challenges and prospects of energy storage technologies ............................................................................................................................................ 15
10. Contribution of the present study for decision making ................................................................................................................................................ 16
11. Conclusions.......................................................................................................................................................................................................................... 16
Declaration of competing interest.................................................................................................................................................................................... 17
Acknowledgment................................................................................................................................................................................................................ 17
References ........................................................................................................................................................................................................................... 17
Further reading................................................................................................................................................................................................................... 20
1. Introduction
The need for energy emerged as soon as human beings learned
to cook food, although people were unknowingly benefitting from
solar energy to protect their bodies from coldness and drying
clothes in the sun etc. The first planned utilization of energy was
from wood and fire. However, increasing awareness of nature
for taking advantage of energy, various sources of energy were
identified and put to versatile uses. People also acquainted to
change forms of energy and storing it for the times when sources
were not available, for example, solar energy at night, though
the ways of conserving energy were very basic like storing wood
under shelter and other safe places. However, increased popula-
tions and energy usage versatility added other sources like coal,
steam, water, wind, and petroleum. The invention of electricity
changed the whole scenario of energy. The olden sources of en-
ergy were replaced partially by the production and consumption
of electricity. Some modern sources of energy like nuclear and re-
newable resources have been identified in the twentieth century.
Presently, an energy mix is prevailing and being used in different
parts of the globe. The demands for energy are increasing rapidly
due to an increase in populations, economic development in
developing countries, enhancement in per capita consumption,
change in lifestyle, and supply at more remote places as stored
energy. The world’s primary energy consumption was 149,634
and 157,064 Terawatt-hours (TWh) in 2015 and 2018 respectively
(Ritchie and Roser, 2019). According to their estimate, the re-
gional consumptions were 69,615, 32,936, 23,859, 10,822, 10,494,
8164, and 5367 TWh for Asia Pacific, North America, Europe, CIS,
Middle East, South and Central America, and Africa respectively.
Thus, the biggest consumers of energy were Asia Pacific and North
America while Africa used the least quantum of energy in 2018.
The Gulf Cooperation Council (GCC) countries are although low
populated, but are high consumer of energy, even in comparison
to some of the developed countries (Al-Badi and AlMubarak,
2019). The consumption of electricity in the GCC region has
grown from just 51 TWh in 1990 to almost 536 TWh in 2015
whereas the per capita use has been recorded as one of the high-
est rates. It is estimated that the GCC countries will be consuming
1094 TWh by 2025 (Almulla, 2014). Such a pattern is mostly
due to rapid economic development and significant change in
the lifestyle. The electricity systems necessarily require smooth,
balanced, reliable and quality supply (maintaining constant volt-
age and frequency) to the customers without any breaks and
potential damage to electrical appliances. The strong variations
always exist in demand of electricity at different times. Hence,
there could be certain times when the energy production will be
more than demand and vice versa. Just to quote an instance, the
peak demand of GCC countries in summer is twice the off-peak
summertime requirement due to the running of air condition-
ers and is thrice of winter peak times (Al-Badi and AlMubarak,
2019). For balancing and matching the demand and supply, the
storage of energy is a necessity. The present trends indicate that
the need for energy storage will increase with high production
and demand, necessitating the energy storage for many days or
weeks or even months in the future. According to estimates,
requirements for storing energy will become triple of the present
values by 2030 while the stationary energy could dominate in
quantities of electricity supply through grids (IRENA, 2017). The
energy storage techniques and devices have been changed and
modernized simultaneously along with increasing production and
demand. The devices conventionally were magnets, batteries, dry
cells, and capacitors. However, besides changes in the olden de-
vices, some recent energy storage technologies and systems like
flow batteries, super capacitors, Flywheel Energy Storage (FES),
Superconducting magnetic energy storage (SMES), Pumped hydro
storage (PHS), Compressed Air Energy Storage (CAES), Thermal
Energy Storage (TES), and Hybrid electrical energy storage (HES)
were developed for sustainable and renewable usage (Frack-
owiak and Béguin, 2001; Doetsch, 2014; Haisheng et al. 2009; Luo
et al., 2015; Silva & Hendrick. 2016; Stanley, 2012; UCS, 2006).
However, energy storage mechanisms also face many challenges
as well (Mohd et al., 2008) because none is complete in all
respects due to one or more limitations like storage capacity
and form, string time, special structural or implementation re-
quirements, energy releasing efficiency, and operation time (Yae,
et al., 2016). In addition, there are cost, and environmental as-
pects like CO2 emissions (IEA, 2019) associated with the energy
storage technologies, which must be identified and considered
when planning and deciding the selection of technologies for
installation in the grid systems of an area. The aspects identified
above need to be elaborated through a systematic study from
the literature so that valuable research work of earlier authors
is gathered, understood well, and arranged in a good array to
clarify the study areas, which can contribute to support and ease
the decision makers and practitioners for selection of best energy
storage devices and mechanisms for their particular grid systems.
Considering the high importance and problems of electric energy
storage, some aspects of this subject are being discussed and
highlighted with support from the literature review.
2. A dynamism in the forms and sources of energy
The types and uses of energy had been dynamically changing
in history because Beltran (2018) regarded energy as a living,
evolving, and reactive system, which remained an integral part of
civilizations and their development. The sun was the only source
of heat and light while wood, straw and dried dung were also
burnt. The horses and other animals, wind, and water were used
for transportation, working in the fields, grinding grains, pumping
water, and driving the simple machines in very earlier times.
Later, the power of steam was harnessed which dated back to
ancient Alexandria. The steam engines remained in use till the
17th and 18th centuries. Simultaneously, coal was also used for
heating and production of steam from water. By the late 1800s,
petroleum was introduced as a fuel and is still in wider use.
Thomas Alva Edison installed the first electric light plant in the
city of New York in 1880. (UCS, 2006). The invention of elec-
tricity revolutionized energy usage and consequently, industrial
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A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx 3
revolutions happened on the globe. Currently, electricity is the
dominating form of energy all over the world. The introduction of
nuclear energy started in the 1950s and was increasing rapidly,
but the Chernobyl accident in Russia (1986) and some later inci-
dents in India and other countries discouraged its spreading due
to safety concerns and social pressure (King, 2019).
The modern biofuels, wind, and solar are finding their way
again while geothermal and marine technologies are new addi-
tions in the field of energy. Advances in technology, alternative
energy sources, costs of energy and pressures of social issues
associated with energy production are the driving forces behind
the above changes, but the static fact is the consistent increase
of energy utilization during the global history (Ritchie and Roser,
2019). Ritchie and Roser (2018) reported that the total global
energy consumption in 2018 was 160,228 TWh while different
energy production sources contributing to this huge production
are oil, coal, gas, hydropower, wind, solar, nuclear, and other
renewals. According to them, the biggest sources are Oil, Coal,
and Gas contributing energy (TWh) as 54,220 (33.84%), 43,869
(27.38%), and 38,489 (24.02%), respectively. Thus, these three
major sources are meeting 85.24% of global energy requirements.
The respective shares of Hydropower, Wind, and Solar were 6.89%
(11,034 TWh), 2.09% (3342 TWh), and 0.96% (1539 TWh). The
contribution from nuclear resource was 4.43% (7109 TWh) and
Other Renewals 0.38% (626 TWh). Due to CO2 emissions during
electricity generation from fossil fuels, demand is increasing to
shift gradually to renewal sources, but it is not possible in the
short-term because demand of electricity may go thrice by 2040.
According to estimates of World Energy Council (2019), global
emission of CO2 might stabilize by 2030 and reductions could be
expected afterwards. These days an energy mix (electricity, the
solar, wind, and nuclear) is being consumed in various countries
of the world. However, all the other forms contributed only less
than 1% of the total energy utilization (BP Statistical Review,
2019; Ritchie and Roser, 2019).
3. Global status of the consumption of energy
The energy consumption has increased tremendously after the
industrial revolutions due to an increase in population, inven-
tion of new techniques and machines, economic development,
accessing remote and far flanged areas, and big changes in the
lifestyle. According to estimates, energy use was doubling in each
decade in earlier times (UCS, 2006). Simultaneously, a significant
increase also took place in the production of energy, especially
electricity. Among other drivers of increasing demand for energy
are selling the electricity even below the actual cost in GCC and
some other countries, wastage due to usage and building designs,
and lower efficiency of generation and delivery equipment (Al-
Badi and AlMubarak, 2019). Nevertheless, production could not
match demands in so many developing countries. According to
estimates, the world’s primary energy consumption in 2015 re-
mained as 146,000 terawatt-hours (TWh), 25 times higher than
the year 1800 (Ritchie and Roser, 2018). As the data values are
not mostly same when reported by different sources, in another
report (BP Statistical Review, 2019), the global energy consump-
tion was 136,129 TWh in 2008 and 161,250 TWh in 2018. There
has been a 2.9% increase in consumption for the 10 years. World
Energy Council (2019) while finding scenarios and exploring in-
novative pathways to 2040, contemplate that the globe will be
entering in a new energy era promising enough, clean, and sus-
tainable energy for all communities with increasing uses and
users. About 10% increase is presumed in demand of energy by
2040. However, there will be more emphasis on renewal sources
considering environmental protection, but fossil fuels (especially
gas replacing major part of coal) will remain dominating although
decreasing as source of electricity generation. The energy con-
sumption is highly variable in different countries of the world, not
necessarily proportional to the populations but also many other
factors; economic development, lifestyle, and climate. The top
ten high consuming countries in the descending order are China,
USA, India, Russia, Japan, Canada, Germany, South Korea, and
Brazil (Table 1). It is very clear that these ten countries swallow
66% of energy utilization of the world. Only China consumes
23.9% while USA takes 16.6%, thus these two countries share
40.5% of the word’s energy consumption. If India and Russia are
added too, the energy dissipation of the four biggest countries
rise to 51.5%, which means that the whole of the rest world
consumes even a bit lesser (0.5%) than 50% (Table 1). The per
capita consumption of electricity is also highly variable in dif-
ferent countries. The values range from 52921.73 KWh (Iceland)
to 8.32 KWh (Liberia). This rate for GCC countries ranged from
5340 KWh to 17,610 KWh in 2010, compared to 3378 KWh and
2728 KWh, the respective means for the Middle East and the
globe (IRENA, 2012). The countries ranking in the top ten list
during 2015 are Iceland (52921.73), Norway (25018.59), Kuwait
(18818.11), Bahrain (18491.19), UAE (18213.33), British Virgin
Island (18035.13), Qatar (15784.42), Canada (14501.59), Finland
(14328.50), and Sweden (12589.75). At the bottom of the list
are Burundi, SierraLeone, Guinea-Bissau, Chad, and Liberia in the
descending order. The per capita consumptions of Saudi Arabia
and Oman were 10248 and 5987 KWh, respectively while the
other four countries of GCC have already been included in the
top ten list above (CIA, 2019).
4. The need and urgency for the storage and services rendered
by energy storage
The demand of energy does not remain uniform in 24 h in a
day and the entire year, rather it drastically varies within a day
and during various seasons of the year. Thus, peak and off-peak
demands arise within a day and the seasons due to individual
needs and climatic effects. These phenomena necessitate storing
of energy.
4.1. The need for storing energy
The electrical energy when produced in excess over demand
must be stored otherwise it cannot be used later and the cost of
production for that part will go waste. Thus, it will increase the
cost per unit of electricity. Moreover, when electricity is being
produced from renewal sources like wind and solar, the storing
of excess energy is highly necessary because solar energy at night
and wind energy will not be available at certain times. Certainly,
the generation may surpass the total demand of electricity during
off-peak hours and give rise to an urgent need for storing excess
electricity. EPA (2019) elaborated that the storage of electricity
can keep a balance between supply (generation) and demand
(consumer use), avoid electric fluctuations, reduce brownouts
during peak demand, decrease environmental pollution and in-
crease Electric Grid Efficiency. The energy storage can stabilize
grid power and make the grid system more efficient. Storing
electricity is a key mechanism for supplying electricity reliably,
increasing security and economic value and decreasing carbon
dioxide emissions (Mathew, 2012; Revankar, 2019). Electricity is
not easy to store, and special devices and mechanisms are re-
quired for this purpose that are being improved and innovated by
researchers and technologists. Consequently, the present global
capacity for energy storage is continuously increasing rapidly
(World Energy Council, 2019). To quote an instance, according to
estimates of IEA (2019), there has been a 100% increase in the
storing capacity comparing the year 2017 with 2018. The market
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4 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
Table 1
Consumption of energy of top ten countries of the world during 2008–2018.
Country Consumption of energy (TWh) of top ten consuming countries
(based 2018)
Growth rate% per annum World’s share in
2018
2008 2010 2015 2018 2008–17 2018
China 25,935 28,982 35,006 38,076 4.3 3.9 23.9
USA 26,272 25,853 25,737 26,760 −0.4 3.5 16.6
India 5559 6269 8025 9409 5.2 7.9 5.8
Russia 7874 7780 7850 8385 0.3 3.8 5.2
Japan 6024 5873 5268 5292 −1.4 −0.2 3.3
Canada 3745 3629 3943 4001 0.7 0.2 2.5
Germany 3908 3815 3756 3768 0.1 −3.0 2.3
South Korea 2803 3035 3315 3501 2.3 1.3 2.2
Brazil 2791 3059 3442 3466 2.5 1.3 2.1
Iran 2396 2477 2896 3326 3.2 5.0 2.1
Total of top ten
countries
87,307 90,772 99,237 105,984 – – 66.0
World 136,129 140,723 151,724 161,250 1.5 2.9 100.0
for storage devices and systems’ is growing accordingly. Surplus
generation of electricity can also be exchanged with neighboring
grid zones through increased interconnection capacity. Thus, the
excess of production of one grid can be shared with other grid
where the demand is increasing (Metz, 2016). However, due to
almost same On peak and Off-peak hours in the adjoining areas,
the usefulness of this option is limited. The energy storagebattery
system is used for supplying energy in remote areas. This form
of energy has been started to use in vehicles, many of which
are now running on stored electricity. Whittingham, (2012). The
fossil fuels highly affect the global environment as CO2 emis-
sions; hence, there is a huge emphasis to have more generation
from renewable sources. Resultantly, the need for increasing the
capacity of energy storage will enhance too much because of
an intermittent supply from renewable resources, which cannot
meet the demand at odd and peak times. Therefore, renewable
installations must be paired with energy storage devices and
systems in the coming future (Wilson, 2018; IRENA, 2017). The
major need for storing stationary energy, other than electricity,
is to meet portable forms required for so many broad uses in
the present era. For example, using smartphones, iPhones, and
iPods, we are so accustomed to, would have not possible without
modern energy storage. Lithium-ion (Li-ion) batteries are provid-
ing energy storage for the operation of modern phone devices.
The energy storage is also vital high-tech manufacturing where
the essentiality is having uninterrupted power sources with con-
sistent frequency. (Fletcher, 2011). Energy storage is also vital
for essential services providers like the telephone industry and
healthcare sector which rely mainly upon energy storage (in the
form of large batteries for backup in case of power failure).
4.2. Services rendered by storage of energy
The storage of energy renders many direct and ancillary ser-
vices to the generation, supply system of energy, and facilitate
the customers who are the end-users of energy. The capacity, na-
ture, and quality of different services depend upon the strength,
versatility, technological innovations, and automation of the grid
system (generation, storage capacity, and transmission features),
and location, customer demands, and regulatory constraints. The
Energy Generation is the first system benefited from energy stor-
age services by deferring peak capacity running of plants, en-
ergy stored reserves for on-peak supply, frequency regulation,
flexibility, time-shifting of production, and using more renewal
resources (NC State University, 2018; Poullikkas, 2013). The fluc-
tuations of generation, especially from renewal resources, can
be controlled. A good energy storage system removes the need
of installing a broad transmission system for transmitting elec-
tricity to other places. Such a system is deemed necessary in
the absence of enough storing system (Chen et al., 2008; Rahul
and Apt, 2008). Energy storage can help to control new chal-
lenges emerging from integrating intermittent renewable energy
from wind and solar PV and diminishing imbalance of power
supply, promoting the distributed generation, and relieving the
grid congestion. Many other services rendered by energy stor-
age are Electric Service Reliability, Black Start Capability, Voltage
Support and Control, Power Quality, Renewable Energy Capacity
Firming, Backup Power, Time-of-Use Shifting, and Management of
Demand, Supply, Peak Limiting, Distribution, and Power Quality
(Günter, 2015; Ibrahim and Adrian, 2013; NC State University,
2018; Zakerin and Syri, 2015). Large Scale Energy Time-Shift
service to the grid system is possible if large scale storage facil-
ities along with energy discharge capacities are simultaneously
available within generation plants. The most important devices
and systems for energy storage are PHS, CAES, and big banks
of storage batteries. The availability of such devices enables the
grid system to charge the capacity of electric supply in off-peaks
and discharge during on-peaks, thus avoiding problems emerging
during full peak periods. Resultantly, flexibility is possible in
running the generation plants according to needs. The storage
system, mostly comprising of modern batteries, strengthens the
running of the grids system and increases the reliability and
self-sustainability during emergent situations. The short-term de-
viations of generation and loads can be managed easily if some
energy reserves are built in the system. Black start capability, on
the generation side of the grid, provides reserved energy to the
grid to restart after the occurrence of a major power shut down
due to any reason. This facility is required to provide electricity
for restarting the generation resources again (Castillo and Gayme,
2014; Eyer and Corey, 2010; Kirby, 2007; Kirschen and Rebours,
2005; Pearre and Swan, 2014; Tweed, 2013). The most recent
technologies of energy storage support supplying electricity and
operation of plants on a ‘Just-in-time’ basis, ensuring high power
quality and reliability for the benefit of ratepayers. Because of
more intermittent renewable generation (for example in Mas-
sachusetts, USA), maintaining a perfect balance in the delivery
system is becoming more challenging because of various reasons
like the severe weather conditions and uncertainty of demands.
The energy storage is emerging as a great help to coping with
sudden power shuts and gaining self-reliance on the grids. There-
fore, new energy-storing technologies are becoming an integral
part of modern smart grids and ensuring quality energy supply
of the world in the future (Mass.gov., 2015). It is imperative for
the practitioner and decision maker to be aware of installing and
using the recent techniques and devices of energy storage for
getting maximum utility of these in handling smart grid problems
of today and the near future.
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5. Energy storage techniques
Populations of even olden times knew the mechanisms of
storing energy for the delayed use. Of course, these were of clas-
sical types and methods like protecting wood from wetting for
burning at night and during the rainy season. However, the ways
and devices remained changing and improving through different
eras in consequence to the development of technology. The first
systematic device was the battery which is the still most used
technique for the storage of energy storage because their output
is more than 90%. Volta’s cell was the first invented battery in
1800. This primitive battery was structured from zinc and copper
discs, which were alternating each other, but a cord was separat-
ing these apart. A brine solution was being used as an electrolyte.
Later, Daniel’s cell was transformed from the Volta cell in 1836.
In Daniel’s cell, two electrolytes were used. Leclanche cell was
formed in 1866, which comprised of a zinc anode and a carbon
cathode. The significantly small-sized dry cells, presently in use,
were invented in 1948. An alkaline electrolyte, a zinc anode, and
a manganese oxide cathode form the structure of these cells.
Current rechargeable cells, also known as secondary batteries,
were evolved in the mid-1980s which remained changing over
time from lead-acid to Ni–Cd, Li-ion (Li–O2 and Li–S), and finally
NiMH (nickel-metal hydride). The Sony corporation launched Li-
ion batteries (LIB) in 1991 and have reconstructed some other
portable devices subsequently. The Ni–Cd, Li–O2, and Li–S are the
batteries which can store higher quantum of energy, therefore,
are still in extensive use (Bruce et al., 2011; Rao et al., 1977;
Whittingham, 2004; Whittingham, 2012; YaoKummer, 1967). It is
worth to mention that the ultimate conclusion is that the energy
storage capacity through electrochemical systems are limited by
constraints of chemistry. Therefore, the capacities have to be
increased using couples with very low equivalent weights (Abra-
ham, 2015). Now, the world has entered the digital technologies,
the energy storage devices have been modernized accordingly.
The capacitor is another widely used device for storing en-
ergy as a surface charge which was developed sometimes after
the batteries. This device needs large amounts of surface, which
is in direct relation to the capacity of a capacitor that can be
stored and released repeatedly with the least damage to the
composed materials. The efforts for the best caused the develop-
ment of supercapacitors. Supercapacitors are a hybrid of battery
and capacitor, the surfaces of which are charged while Faradaic
reactions are majorly occurring in the inner material. This type of
capacitor can complement or replace batteries for storing elec-
trical energy when high power delivery or uptake is required
(Simon and Gogotsi, 2008; Whittingham, 2012). According to
Bruce et al. (2011), very recent energy storage materials and
devices are of two types; Lithium-ion battery or electric double
layer of porous carbon. An example of carbon-based materials
is ‘graphene’, the use of which for energy storage has grown
tremendously. The graphene varies in terms of morphology, di-
mensions, and a few layers. The electrochemical features depend
on synthesized procedures like mechanical exfoliation, liquid-
phase exfoliation, and reduction of graphene oxide (Novoselov
et al., 2004; Rinaldo et al., 2015, Zhang et al. 2015). The most
advanced polymer materials, constituting Li-ion batteries are cov-
ering diverse mechanisms; movable electronic devices, vehicles
run by electricity, and smart grids, which require power watt
hours on the lower side and to megawatt-hours on the higher side
(Isah, 2018). Nanostructured carbons are highly porous and have
a large surface area that can maximize electrode performance
by functional groups; oxidative groups (carboxylate, ketone), or
hydroxyl groups or nitrogen. Increased capacity, appropriately
to the electrolyte, and electrochemical activities were observed
due to such surface modifications Wu et al. (2017). The carbon
nanofibers (NFs) can be formed from a polymer precursor, which
is having various fiber morphologies (hollow, porous surface fiber
and ribbon). The process of Electrospinning is useful during the
formation of polymer NFs whereas fiber diameter, fiber align-
ment, and shape of the fibrous material can also be controlled
during manufacturing (Merlet et al., 2012). Vatamanu, Borodin,
and Smith (2010) developed a multistep method, which proved
useful and effective in the preparation of carbon nanofibers (N-
CNFs)/polymer composite film grown on silicon. In addition to
wind and solar energy, electricity is largely generated in power
stations of various sizes where petroleum-based fuel is mostly
used. However, there is a wide difference in demand and gen-
eration of electric power while storing electricity at any scale
is not possible. For the storing purpose, electricity should be
transformed into the forms, which are storable and recoverable
as electricity at the time of demand Chen (2009). Besides stor-
age devices as batteries, flywheel compressed air and pumped
hydro storage, electricity can be stored through various systems
along with transmission system as ancillary services (Luo et al.,
2015; World Nuclear Association, 2019). The major systems are;
Renewable energy grid-connected system, Grid auxiliary service
system, and Distributed and microgrid system (HNAC, 2019). The
collection of all the methods and systems utilized for storing
electricity in a larger quantity associated with the grid system
is called Grid Energy Storage or large-scale energy storage (Mo-
hamad et al., 2018). PHS (Pumped hydro storage) is the bulk
mechanism of energy storage capacity sharing almost 96% of the
global amplitude. The large electrochemical storing is also there
in the form of batteries and flywheels but needs to be developed
further (TAWAKI, 2018).
6. The potential technologies of storing stationary energy and
electrical energy in various devices and grid system
The Electrical Energy Storage (EES) technologies consist of
conversion of electrical energy to a form in which it can be
stored in various devices and materials and transforming again
into electrical energy at the time of higher demands Chen (2009).
EES can prove highly useful to the grid systems due to multi-
ple advantages and functions. The usefulness of ESS is visible
through meeting high demands, managing delivery of energy,
controlling the sporadic supply and generation of electricity, in-
creasing power trustworthiness, matching load requirements of
customers, cognizance of grid systems, and decreasing electrical
energy import when demands are high (Luo et al., 2015). An
electricity grid is a network of electrical power comprised of
power generation plant(s), substations, transmission and distri-
bution lines, transformers, and the consumers of electricity. This
network is interconnecting the units of generation, transmission,
and distribution and supplying the electricity from the generation
units to the units of distribution. There are two types of Electrical
Grid Systems; Regional Grid, and the National Grid (Circuit Globe,
2019; Student Energy, 2019). The energy storage technologies and
devices can be classified on various bases. The categorization of
EES technologies may be functions-based, time of response or
storing periods (Baker, 2008) as shown in Fig. 1.
However, the most common are the forms and modes in
which the energy is stored in the electrical network (Bakers,
2008; Evans et al., 2012; Zhao et al. 2015). The mechanisms
and storing devices may be Mechanical (Pumped hydroelectric
storage, Compressed air energy storage, and Flywheels), Thermal
(Sensible heat storage and Latent heat storage), Thermochem-
ical (Solar fuels), Chemical (Hydrogen storage with fuel cells),
Electrochemical (Conventional rechargeable batteries and flow
batteries), and Electrical (Capacitors, super capacitors, and SMES).
Luo et al. (2015) represented this classification diagrammatically
(Fig. 2).
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Fig. 1. Positioning of Energy Storage Technologies.
Source: Akhil et al. 2013.
Fig. 2. Classification of energy storage systems.
Source: Magyar Silakhori 2019.
The features of ESS devices and systems are relative to the type
of energy production, storage duration, and power delivery to
the grid governed by the following theoretical framework (Rugolo
and Aziz, 2012).
ST(t) + PR(t) = SU(t)
ST (t) is the storage power of the ESS as a function of time,
it is positive when discharging the power and negative when
charging. PR (t) is a power production profile by an installed
generation system, which is either supplied to ESS or delivered
to the grid or lost by dissipation in the network. The SU (t) is the
supply profile power that ESS delivers to the grid as a function
of time. Aligning to the subject and scope of the present article,
only electrochemical and electrical approaches/devices/systems
are being majorly concentrated in the subsequent sub-sections.
6.1. The flow batteries
Flow batteries are replacing conventional batteries, which are
comprised of two electrolytes in a liquid state (Fig. 2, Zipp, 2017),
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A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx 7
Fig. 3. Structure of a flow battery source (After Zipp, 2017).
in contrast to solid compounds in standard batteries that has
limited energy storage capacity. Various types of electrolytes are
used in a flow battery; bromine as a central element with zinc
(ZnBr), sodium (NaBr), vanadium (VBr) and many more as the
anode while a recent addition is of sodium polysulfide. Flow
batteries have relatively higher capacities of energy storage and
subsequent release (15 MWh–120 MWh; storage efficiency about
75%). Other good features are fast recharge, long life (about a
decade), full discharging possibility, non-toxic materials in the
structure, and operational functions at low temperatures.
Mobility of mechanical parts is the main disadvantage of the
system, the major limitation of commercial adaptation of flow
batteries (Chen et al., 2008; Vazquez et al. 2010, Zipp, 2017). The
placement of batteries in a grid system has been represented in
Fig. 3 (Liu et al., 2010).
The capacity imbalances have been observed in flow batteries
because of mixing of electrolyte (active component, vanadium)
diffusing from both sides across the membrane which ultimately
results in an irreversible loss of capacity and efficiency (Chen
et al., 2018; Hwang et al., 2018; Jung et al., 2018; Lee et al., 2019;
Strużyńska-Pirona, 2017). The vanadium ion is having various
oxidation numbers from 2 to 5. In the flow batteries, this chemical
characteristic of vanadium is utilized. VO2+
, VO2+
, V3+
, and V2+
are various forms of Vanadium changing into one another during
reactions. The solution of V3+
is put into the tank of negative
electrolyte, whereas the solution of VO2+
is poured into the
positive electrolyte tank. When the VRFB (Vanadium Redox Flow
Battery) is got charged, the oxidation number increases from +1
to +3, resulting in c energy storage. When the VRFB is discharged,
a negative electrolyte is oxidized and the positive electrolyte is
reduced, just releasing the energy. The charge–discharge chemi-
cal reactions for taking place in this process are expressed as Eqs.
(1), (2), and (3) (Kim, 2019):
Negative electrode: V2+
↔ V3+
+ e−
E0
= −0.255 V (1)
Positive electrode: VO+
2 + e−
+ 2H+
↔ VO2+
H2O E0
= +1.004 V (2)
Overall reaction: VO+
2 + V2+
+ 2H+
↔ VO2+
+ V3+
+ H2O E0
= 1 : 259 V (3)
The state-of-the-art of Li-ion batteries is discussed and the chal-
lenges of developing ultrahigh energy density rechargeable bat-
teries are identified. Examples of ultra-high energy density bat-
tery chemical couples include Li/O2, Li/S, Li/metal halide and
Li/metal oxide systems. Future research and technology devel-
opments must be strengthened to not only increase the storage
capacity of solid-state batteries and liquid electrolyte batteries
(the flow batteries) but also to structure biodegradable batteries
to address environmental challenges. Low cost, long cycle-life,
large-scale energy storage, and biodegradable batteries must be
the ultimate target (Abraham, 2015) (see Fig. 4).
6.2. Superconducting magnetic energy storage (SMES)
Another technology is ‘Superconducting magnetic energy stor-
age (SMES)’, which is characterized as instantaneous and highly
efficient (about 95% for a charge–discharge cycle). The SMEs
consists of 3 MW units (Anzano et al., 1989; Boom and Peter-
son, 1972). The systems possess the capability of discharging
the energy storage near to totality in a shorter time, usually
lesser than 100 ms, as compared to the batteries. The flow of
direct current in a coil of superconducting material creates a
magnetic field that stores energy. However, the system must be
cooled continuously. It is the best suitable to provide constant
and instant power supply as well as regulating network stability
with very high-power output within a short time (DTI Report,
2004; Bueno and Carta, 2006). SMES systems can stabilize the grid
networks, providing power quality to the consumers, although
the systems are costly. The structure of SMES comprises modular
DG building blocks connected to the network. An electric power
generation plant, and a conversion and storage unit are the com-
ponents of a commercial DG facility. The conversion and storage
plants consist of an electrolyzer, fuel cell, and tanks capable of
controlling rapid variations of electricity generation and sudden
demands of consumers. The shock-absorbing role is conspicuous
to provide quality power supply and make this system superior
over others (Louie and Strunz, 2007). The resistance losses in
SMES after charging are almost zero because of the supercon-
ducting coil [Xue, Cheng, and Sutanto, 2005]. A special cooling
mechanism, cryogenic cooling installed as a part of the SMES for
cooling the coil to keep its temperature below the critical value.
The SMES can release a higher quantum of energy into the grid
system within a fraction of a second (milliseconds) during the
discharging mode. Niobium–titanium (NbTi), a superconductor
material, is used to make fine coils whereas liquid helium coolant
or superfluid helium coolant at 4.2 K are used for cooling the
system in the SMES design. SMES works on the basic principle
of charging of the coil with the electric supply and keeping the
temperature of the system within critical values. The storing of
energy is permanent without any loss of charge which can be
got released when required (Moghadasi et al., 2010). This is good
merit of SMES energy storing system making it highly useful. The
working of the system has been diagrammatically represented in
Fig. 5.
The SWOT Analysis of SMES indicated that this technology
has strengths; high power capacity, stability, and quality, fast
response time, high storage efficiency, flexible and reliable, com-
plete charge and discharge, no moving parts, and no environ-
mental hazard. The storage system has opportunities and po-
tentials like large energy storage, unique application and trans-
mission characteristics, innovating room temperature super con-
ductors, further R & D improvement, reduced costs, and enhanc-
ing power capacities of present grids. However, presently it has
weaknesses of high cooling demand, expensive raw materials,
complicated design, temperature sensitivity, costly in operation,
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8 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
Fig. 4. A diagram showing the role of batteries in the Grid System source (After Luo et al., 2015).
Fig. 5. Diagram of superconducting magnetic energy storage system source (Pavlos Nikolaidis, 2017).
and economical only for short cyclic periods. This device has
threats like low temperature and high magnetic fields. Validation
and commercialization are still required (Ali et al.; Shaw, 2016).
6.3. Flywheel Energy Storage (FES)
FES devices are comprising of various types of flywheels (mas-
sive or composite), a motor-generator, and magnetic brackets set
inside a housing case (Ruddell, Schönnenbeck, and Jones, 1996).
These are having very high cycling capacity with cycle values
of 10,000 to 100,000 (Fig. 6 Pavlos Nikolaidis, 2017). The high-
capacity flywheels with lesser friction losses (200 KW of a 200
tons flywheel) are required for the electrical power systems. The
efficiency depends upon the energy storage time e.g. an average
efficiency of 85% may decrease to 78% and 45% after 5 h, and
24 h (full one day) respectively. Hence, flywheels are inefficient
to store electrical energy on a long-term basis but can be used in
combination with other devices. The FES is made up of carbon-
fiber and can be of low speed (6 × 103 rpm) or high speed
(∼105 rpm) (Pena-Alzola et al., 2011). However, the high-speed
FES would be incurring high costs (Díaz-González et al., 2013).
The flywheel systems are usually operating in the high vacuum
and possessing characteristics; no friction loss, small wind resis-
tance, long life, no harmful effects on the environment, and needs
negligible maintenance. The FES can help to control the frequency
of the grid system and ensure the quality of electricity being
delivered. It is mostly employed with renewal energy generation
where electricity fluctuations are high and frequent. The low
energy density and the higher cost for ensuring the security of
the system are the major shortcomings. Presently, its main use
is in supplementing the battery system (Ding and Zhi, 2016). Due
to better characteristics, the Global Flywheel Energy Storage (FES)
Systems Market is anticipated to grow at a higher rate. The use of
FES in the automobile industry will be increasing with time and
improvement of the technology. The advantages of FES are many;
high power and energy density, long life time and lesser periodic
maintenance, short recharge time, no sensitivity to temperature,
85%–90% efficiency, reliable, high charging and discharging rate,
no degradation of energy during storage, high power output, large
energy storage capacity, and non-energy polluting. The major
disadvantages and limitation could be; low specific energy, short
discharge time, complexity of structure, mechanical stress and
fatigue, safety concerns due to high speed of rotor and possibility
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Fig. 6. Schematic diagram of flywheel energy storage system source (Pavlos Nikolaidis; 2017).
of breaking, and high cost (Amiryar and Pullen, 2017; Cooper,
2016). Flywheels are a less mature technology as compared with
batteries while the current cost is too high making them un-
competitive in the market. However, the cost of the system can
be kept lesser by using small capacity flywheels. The flywheel
energy storage market could grow (estimated volume in 2025
by Market, 2019 is $479.3) due to two major factors; industrial
development and growing population causing significant increase
in global demand for power energy which often creates frequent
demand-supply gap of energy in developing nations, leading to
requirements for power backups.
6.4. Pumped hydro energy storage (PHES)
Two water bodies (natural or artificial) located/ constructed at
higher and lower elevations are the key points of the pumping
hydro storage system for storing energy. The water is pushed
into the higher elevation water body using extra electricity dur-
ing off-peak while during on-peak hours, the water from the
upper reservoir is routed through pipes down a level into a
hydroelectric generator, which lastly gets stored in the lower
water pool. The running of generators produces the electricity
again. The water is pumped back into the upper water body
during off-peak periods (Mears and Epri-Doe, 2003; Mohd et al.,
2008). Thus, motor/generator and reversible pump-turbine are
the main components of PHES, in addition to two large water
reservoirs. This system can startup in a few minutes and be kept
running associated with the volume of stored water in the upper
water reservoir. The problems of this mechanism are dependence
upon the geographical location and climatic conditions of the
plant area. The conversion efficiency ranges from 65% to 80%
based upon climatic conditions and on equipment characteristics
(Mears and Epri-Doe, 2003). Generally, 4 KWh are needed to
generate 3 KWh whereas the energy storage capacity depends
on the height of the waterfall and the volume of water. The
rough calculations have indicated that a mass of one-ton water
falling 100 m could generate 0.272 kWh. The energy storage in
this system can prolong for longer periods. Jung (2010) reported
that PHS is one of the oldest methods for storing energy; the
first plants were built in Switzerland and Italy in 1890. Mostly,
the existing conditions of topography and hydrology of the area
are used, if available, otherwise artificial water reservoirs (one
upper and the other at a lower level) are constructed (Fig. 7). The
operating cost per energy unit has been reported as the cheapest
in the PHS (ARUP, 2014; Nadeem et al., 2019). However, the
construction of reservoirs and other infrastructures claim very
high investment costs.
PHES is mature and an established technology for the storage
of electricity. It can readily make available electricity during peak-
ing power demand without requiring ramp-up time. The energy
storagein the PHES system can also help as a ‘black start’ source
in case of a power shut down. However, the major limitations
for PHES are related to environmental considerations and the
higher costs of establishing the technology. The geographical
and topographical feasibilities are further concerns. Due to more
anticipated advantages, PHES shares about 90% share of the global
energy storage capacity. In 2017, there were approximately 270
PHES stations in the world generating 127 GW power capacity;
the United States has 40 PHES projects having a cumulative
power capacity of 22 GW while European Union (EU) is operating
approximately 160 PHES stations with an overall capacity of
47.44 GW (Pure PHES, 24.91 GW and mixed 22.53 GW). Rest of
the world has 70 PHES plants (Fortune Business Insights, 2019;
Kougias and Szabó, 2017; Yang and Jackson, 2011).
6.5. Thermal energy storage (TES)
Thermal energy storage (TES) has been adopted broadly in
which such materials are used that can be preserved at high or
low temperatures in insulated captivity. The energy stored as
heat is recovered back on reverting the cold or hot material to
normal conditions which are used again for the generation of
electricity using heat engine devices. Energy input in this system
is electrical resistance during heating or cooling; therefore, the
overall efficiency of TES varies from 30 to 60%, which looks
low. The heat cycle is comparatively more efficient (70%–90%),
depending upon the environment. The TES systems could be of
two types; low-temperature or high-temperature in comparison
to room temperature. Ibrahim and Adrian (2013) classified TES
as industrial cooling (below −18 ◦
C), building cooling (at 0–
12 ◦
C), building heating (at 25–50 ◦
C) and industrial heat storage
(higher than 175 ◦
C). Various materials are used in the TES
system for storing energy upon which the storage capacity of
the device depends. Sharma et al. (2016) identified polyethylene
glycol (PEG) material having a significant potential for using as
a TES material because of its stable melting temperature (in the
range of 55–60 ◦
C with a deviation of 6.5%). Nevertheless, an
increase in the number of thermal cycles can cause a gradual
decrease in the latent heat of fusion. During the peaks of nu-
merous thermal cycles of FT-IR, recordable changes could not
be seen confirming the stability of polyethylene glycol (PEG)
composition. However, the research work on the suitability of
PEG continued. The thermochemical tests and techno-economic
analysis showed the reliability of PEG 6000. A research team
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10 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
Fig. 7. Schematic diagram of pumped hydro storage plant.
Source: Pavlos Nikolaidis; 2017.
(Su, Darkwa, and Kokogiannakis, 2017) developed and tested
MF-3 (a microencapsulated phase change material) for storing
solar energy in a hot water storing system. They recorded the
highest energy storage capacity of 126 kJ/kg with an efficiency
of 97.4% in comparison to some additional materials. The higher
energy storage density indicated the thermal effectiveness of MF-
3 Although this material requires a relatively smaller physical size
than the water-based system, its energy storage value was still
about double of many storage units in use currently. The metal-
lurgical slags also performed best in vertical TES with axial flow
direction (Krüger et al., 2019). Zanganeh et al. (2014) designed a
packed bed of rocks as a thermal energy storage (TES) system in
which air was used as the heat transferring carrier. A pilot-scale
TES unit (6.5 MWh capacity) was built and tested in Morocco
and found useful. Subsequently, an industrial-scale (100 MWh)
TES unit was designed for a solar power plant using a simulation
model. Thermal Energy Storage is technique well suited to energy
management in buildings. It may help to control the cost and
provide comfort conditions in the indoor environments as well
as decreasing greenhouse emissions. The recent increase in the
demands of heating and cooling buildings can be met and man-
aged through TES effectively (Figure 8), which can also reduce the
fluctuations of electricity and match the changing requirements
at various times and seasons (Parameshwaran et al., 2012). TES
has low thermal inertia. This technology supports to achieve a
good control of the indoor temperature even if the heat pump is
turned off for some time. The various adaptable options are based
on most known technologies of TES; steam accumulators, molten
salts (MS), and phase change materials (PCM). The combined
system based on PCM-MS has a clear advantage when storage
hours are 6 or more, while for lesser than 6 h, steam accumulators
are the best option (Arteconi et al., 2013; Prieto et al., 2017). This
system can sustain as low carbon/high performance for electricity
supply in buildings. However, research is anticipated to find
efficient, stable, and less costly storing materials. Nevertheless,
TES has limited storing capacity for storing energy.
6.6. Compressed Air Energy Storage (CAES)
In the CAES system, air is pressurized into an underground
reservoir using electric power during off-peak. The compressed
air is released during on-peak which drives the turbine/generator
unit to produce electricity again. CAES is the only technology
(in addition to pumped-hydro) having the capability of commer-
cial adaptability in the very-large deliverable system to store
energy (single unit sizes of 100 MW or more) for the use of
customers (Ibrahim et al., 2008b). The energy density for CAES
is about 12 kWh/m3 (Multon et al., 2003) with an approximate
efficiency of 70% (Robyns, 2005). This system absorbs 0.7–0.8
KWh of electricity during off-peak hours for compressing air to
release one KWh into the network again during peak hours. This
technology is being adopted by many companies in Europe for
storing electricity in the grid system. The CAES system stores
energy as intermolecular gas, which is compressed into a reser-
voir (Fig. 8), then releasing again for rotating the turbine and
generator to reproduce electricity (Khamis et al., 2010). CAES
system can replace (partially or fully) PHS systems due to good
characteristics of bigger capacity, long life and lesser cost per KW
(Molina, 2017). The exothermic and endothermic processes are
taking place during compression and expansion of the air and
exchange of heat. Three designs of the CAES systems are available
which are; Isothermal storage, Adiabatic storage, and Diabatic
storage (Cheng and Choobineh, 2017). Isothermal and Adiabatic
systems are appropriate for lesser power requirements whereas
Diabatic storage systems suit commercial CAES systems because
of higher density and flexibility of storage and regeneration. The
simultaneous coupling of turbines of high and low pressures
and electrical generators has done to produce electricity (Chen
et al., 2018). The fossil fuel combustion and CO2 emission are
a limitation in the classical design of the CAES system which
can cause environmental problems. However, this problem has
been controlled in advanced Adiabatic CAES systems because
of the capability to produce electricity without fossil fuel, as
there is no combustion process (Bullough et al., 2004). Thus, not
only the efficiency of the plant can be increased to 70% but the
modified system has been proved as optimal for medium and
smaller implementations [He et al. 2017]. Advance research is
there to further improve the CAES system. For example, Saadat,
Shirazi, and Li, (2015) prepared a model, which captures excess
air power before the generation of electricity so that electrical
components can controlled to meet demand rather than gener-
ation and afterwards storage. High-pressure dual chamber and
liquid-compressed air storage vessel are used to store energy (see
Fig. 9).
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Fig. 8. Schematic presentation of heating and cooling arrangement of TES.
Source: IRENA & ETSAP (2013).
Fig. 9. Schematic diagram of compressed air storage plant source: Pavlos Nikolaidis; (2017).
6.7. Hybrid energy storage systems (HESS)
When an energy storage system is developed by integrating
more than one device and established in one grid network, the
system is called Hybrid Energy Storage System (HESS). Resul-
tantly, advantages of each technology in the integrated system
add up to meet specified needs, facing hard conditions, and
enhancing the performance and efficiency of the system. The
process of devising a super energy storage device by hybridizing
together two or more storage systems having complementary
characteristics are defined as a HESS. The major objectives are
coping with real-time harsh working environments that a single
device is unable to do. Hybrid EES helps as well to add many
desirable technicalities like the density of energy, rating of power,
operation temperature, discharge rate, life cycle, and cost (Zohuri,
2018). The environmental and economic factors and usage types
help in deciding e the size and amalgamation of HESS systems
(Martinez et al., 2013). An improved cycle efficiency is targeted
by pooling up the sole benefits of individual devices or systems.
Mostly, in HESS systems, a slow response system is hybridized
with fast response systems for achieving higher and improved
characteristics (Jamahori and Rahman, 2017).
For example, when super capacitors and batteries are com-
bined, it becomes possible to increase the storage capacity and
making rates of charge or discharge faster (Hall and Bain, 2008).
One project using the above-mentioned theme along with a
demonstration unit was installed in the UK. The combination of
battery–supercapacitor for hybrid electrical vehicle applications
was completed in 2012 as another instance (Fairweather et al.,
2013). A back-up system for renewable energy power generation
was designed by the researchers in Japan through a combina-
tion of SMES systems with a hydrogen fuel cell system (AEA,
2010; Hamajima et al., 2012; Nadeem et al., 2019). Similarly, a
new constant-pressure CAES system was coupled with PHS for
addressing the current problem found in the conventional CAES
systems (Kim, Shin, and Favrat, 2011). The cavern volume should
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12 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
remain as the smallest so that the construction cost and utiliza-
tion of the space should remain as the minimum. For better and
consistent efficiency, the operating pressure must remain limited
in a CAES system. This combination was found useful in control-
ling CAES problems. The operation characteristics of the hybrid
system like the height of the storage cavern and heat transfer
between two media (air, water) were found suitable. Solar Fuels
(solar hydrogen, carbon-basedfuels, and solar chemical heat pipe)
are the technology of recent origin which is still in stages of
research and development (Chen et al., 2008; Steinfeld, 2002;
Styring, 2012). The energy can be stored in these fuels and can
be subsequently generated when the demand emerges. Hydrogen
energy storage systems is another technology under development
and commercialization. The technology consists of two separate
processes; energy storage and electricity production. Hydrogen is
produced in a water electrolysis unit, and electricity is produced
in the generation unit (Díaz-González et al., 2012). The role and
utilization of HESS have been identified in various sectors. In
electrified transport sector, hybridization of batteries and super
capacitors proved effective when utilized in the electrified pow-
ered vehicles. Many researchers in fuel cell-powered vehicles
have proposed the HESS of fuel cell with batteries and/or super
capacitors. Energy smoothing and grid integration is the most
practical by using battery–super capacitor in case of wind energy
systems. It has been widely proposed to support PV plants with
battery–super capacitor or fuel cell–battery hybrids. The hybrid
wind-PV renewable energy systems can be well supported by fuel
cell–battery combinations. Thus, utilization of HESS might be con-
sidered a favorable solution for various applications in the future.
However, for demonstrating HESS feasibility and functionalities,
further research and development must be conducted (Hemmati
and Saboori, 2016).
7. Economic aspects of electrical energy storage
Although energy storage ensures a consistent supply of elec-
tricity in the regular grid network, remote places not covered
in the delivery system, and so many utility and entertainment
devices, but a significant cost of storing must also be paid. Zakerin
and Syri (2015) emphasized that consistent, updated cost data
and a holistic cost analysis framework is required for techno-
economic and cost–benefit analysis of electricity storage systems.
The life cycle cost analysis will require updated information for
the cost elements. The identified cost elements include capital
costs, operational and maintenance costs, and replacement costs
as well as safe disposal of borne out devices. Capital cost is
the first and mostly huge expenditure incurred on creating in-
frastructures for installation of energy storing systems followed
by various storage devices and equipment with subsequent op-
erational, maintenance, and replacement costs. For the sake of
comparison, energy storage costs are mostly calculated and ex-
pressed kWh, per kW and kWh per cycle. The device and system
efficiencies are considered as well to obtain the cost per output of
energy (Kondoh et al., 2000). Besides, other factors must also be
considered to decide the feasibility of a storage system or device;
some of which are ease of load leveling, storage and regeneration
time; quality, consistency, and reliability of discharged energy
and specific conditions of the area concerning storage demand.
Although the capital cost of lead-acid batteries is low, even then,
it may not be considered a cheap device because of relatively
short life (ESA, 2019; Kondoh et al., 2000). No agreed parameters
can be found for making comparisons for the cost of energy
storage devices. A few authors used the Levelized Cost of Storage
(LCOS), which can be computed by using the following equation
(Belderbos et al., 2016).
LCOS =
∑
(Capitalt + O&Mt + fuelt ) × (1 + r)t
∑
MWht × (1 + r)t
Capitalt = Total capital expenditures in year t
O&Mt = Fixed operation and maintenance costs in year t
fuelt = Charging cost in year t
MWht = The amount of electricity discharged in MWh in year t,
measure for the capacity factor
(1 + r)t
= The discount factor for year t
Luo et al. (2015) reported the costs of different ESS using
the research work of previous authors. These are presented in
Table 1. It is clear from these data that different energy storage
technologies are significantly varying in Power capital cost, En-
ergy capital cost, and Operating and Maintenance cost, depending
upon peculiar characteristics of the devices and systems, size
and material of devices, as well as energy storage capacity and
duration (Chen et al., 2008; Evans et al., 2012; Farret and Simões,
2006). PHS has been regarded as the cheapest energy-saving
system by Nadeem et al. (2019). A wise decision can be made
for the selection of alternatives, depending upon the nature and
quantity of energy storage demand, local conditions, the quality
of energy storage, and requirements of regeneration of electricity.
The cost incurred on storage of energy is paid back in the forms of
charging customers for released electricity, revenue increase from
more energy production, income increase from enhanced ser-
vices, reduction in demand charges, decrease in reliability-related
financial losses, overcoming quality-related financial losses, and
increased revenue from Renewable Energy Sources (James et al.,
2004; Mohd et al., 2008) (see Table 2).
Rodrigues et al. (2017) compared the feasibility of batteries
and PHS energy-saving devices in the Island of Terceira. They
concluded that PHS is the best device for storing energy in com-
parison with batteries. For the sake of cost comparison, they
considered the costs of all equipment used in the PHS. The lifes-
pan assumed was 10 years for the batteries and 50 years for
the PHS. PHS investment costs, according to their estimates, vary
from 190 e/kWh to 340 e/kWh. Cho et al. (2015) reported battery
costs from 1000 to 3000 USD/kW for NaS batteries and 175–
4000 USD/kW for Li-ion batteries. Present costs range from 350
e to 440 e per kWh for NaS batteries and 700 to 1400 e/kWh
for the Li-ion batteries. The fixed and variable costs for PHS
were estimated as 3.8 e/kW-Year and 0.38 e/kWh whereas for
batteries these were 0.34 e/kW-Year and 0.51 e/kWh. Thus,
batteries are low costing but do not meet some other require-
ments like, total life and capacity of storing energy. Mongird
et al. (2019) evaluated cost and performance parameters of six
battery energy storage technologies (BESS) (lithium-ion batteries,
lead-acid batteries, redox flow batteries, sodium–sulfur batteries,
sodium metal halide batteries and zinc–hybrid cathode batter-
ies) and four non-BESS storage technologies (pumped storage
hydropower, flywheels, compressed air energy storage, and ultra-
capacitors). Data for combustion turbines are also presented. Cost
information was procured for the most recent year for which data
were available based on an extensive literature review, conver-
sations with vendors and stakeholders, and summaries of actual
costs provided from specific projects at sites across the United
States. Detailed cost and performance estimates were presented
for 2018 and projected out to 2025 (see Tables 3 and 4).
8. The impact of electrical energy storage on the global envi-
ronment
The major sources of electrical production all over the world
are Fossil fuels (oil and gas). However, these can pollute the envi-
ronment through CO2 emissions. For example, in Gulf Arab states,
99% production of electrical energy is from oil and gas which
is even 99.4% in Oman. The use of gas for energy production in
Oman can increase by 28% by 2040 (Al-Sarihi and Bello, 2019). Al-
Badi and AlMubarak (2019) predicted from their estimations that
13. Please cite this article as: A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai, Review of energy storage services, applications, limitations, and benefits. Energy Reports (2020),
https://doi.org/10.1016/j.egyr.2020.07.028.
A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx 13
Table 2
Comparison of various costs of different energy storage technology — Recompiled using data of
(Luo et al., 2015) reported from various authors.
Energy Storage
technologies
Cost of Power
capital ($/kW)
Cost of Energy
capital ($/kWh)
Cost of Operation
and maintenance
($)
SMES 200–489 1000–72000 0.001–18.5
FES 250–350 1000–14000 0.004–20
PHS 2500–4300 5–100 0.004–3.0
TES 200–400 20–60 Not available
CAES 400–1000 2–120 0.003–25
Batteries
Lead Acid
Li-ion
NaS
NiCd
VRB
ZnBr
PSB
300–600
1200–4000
350–3000
500–1500
600–1500
400–2500
700–2500
200–400
600–3800
300–500
800–2400
150–1000
150–1000
150–1000
50
−
80
20
70
−
−
Capacitors 200–400 500–1000 0.005–13.0
Supercapacitors 100–450 300–2000 0.005–6.0
Table 3
Summary of compiled 2018 findings and 2025 predictions for cost and parameter ranges by technology type — BESS.(a).
Source: (Mongird et al., 2019).
the CO2 emission from natural gas in GCC will reach 453 × 106
tons whereas the CO2 emission from oil will reach 203 × 106
tons by the year 2025 when these are used for electrical energy
production. However, in contrast, the electrical energy storage
has a dual picture because its impacts on the environment may
be useful as well as harmful. The Deep decarbonization of elec-
tricity production is a characteristic associated with renewable
energy generation and subsequent storage of energy. Maryam
et al. (2019) reported curtailed CO2 emissions in California and
Texas by 72% for renewables and 90% for storage of energy.
Sisternes et al. (2019) also found that storing energy favors decar-
bonization during electricity production. Sternberg and Bardow
(2015) claimed that a comparison of impacts, avoided by using
1 MWh of energy storage rather than generating it newly, can
be used as the basis of studying the environmental effect. After
collecting data for many countries like the United States, Brazil,
Japan, Germany, and United Kingdom, they reported the least
impact on the environment was from heat pumps with hot water
storage and battery electric vehicles. The benefit values for the
environment were intermediate numerically in various electrical
energy storage systems: PHS, CAES, and redox flow batteries. Ben-
efits to the environment are the lowest when the surplus power is
used to produce hydrogen. The electrical energy storage systems
revealed the lowest CO2 mitigation costs. Rydh (1999) deter-
mined that the environmental impact of the vanadium battery
was lower than for the lead-acid battery. The positive impacts of
energy storage in heat devices were seen. The possible decrease
in the quantum of electricity consumed and saved could help to
meet the requirements of other residential customers (Qureshi
et al., 2011). The results of Ibrahim and Rosen (2001) indicated
that cold thermal energy storage (TES) can successfully match the
demand of society for more efficient and environmentally useful
electricity storage.
The study of Bonte et al. (2013) revealed that pumping ground-
water from an aquifer, mixing in the aquifer, and subsequent
injection in a well when working at low temperatures (<25 ◦
C)
can cause an increase in arsenic concentration. However, during
the working of the system at 60 ◦
C, precipitation of carbonate,
mobilization of dissolved oxygen, K and Li, and desorption of trace
metals like Arsenic (As) could occur. The disposal problem of used
14. Please cite this article as: A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai, Review of energy storage services, applications, limitations, and benefits. Energy Reports (2020),
https://doi.org/10.1016/j.egyr.2020.07.028.
14 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
Table 4
Summary of compiled 2018 findings and 2025 predictions for cost and parameter by technology type – non-BESS.
Source: (Mongird et al., 2019).
material in energy storage devices can also appear, especially
when these are not recyclable. These may create a pollution
problem in different regions and countries country (Faure, 2003).
This dimension must not be overlooked because populations may
protest such materials, especially nanostructures and heavy met-
als like Ni, Li, and Cd in modern devices. EPA (2019) pointed out
that the electricity storage may have potentially negative effects
on the environment like emissions, CO2 releases, and disposal of
the material of devices. For example, inappropriate disposal and
recycling of raw materials of batteries such as lithium and lead
can present environmental hazards. Florin and Dominish (2017)
anticipated that Lithium-ion batteries are expected to continue
deployment at the higher rates even in near future but have
associated short and longer-term adverse effects on the environ-
ment as well as human rights which may appear during mining
(such as lithium, cobalt, and graphite) and implementation. The
probable harmful effects are from fire risk, waste management,
and recycling systems too, which have not much established yet.
The mining of raw materials and battery production is attached
to social problems as well, like poor working conditions and
health risks. The scanty data and lesser awareness of stakeholders
are making the front-end of the supply chain highly complex.
The harmful impacts are dependent on the nature, type, and
efficacy of energy storage devices as well as disposal and recy-
cling procedures. The impacts can be managed by making the
storage systems more efficient and disposal of residual material
appropriately. The energy storage is most often presented as
a ’green technology’ decreasing greenhouse gas emissions. But
energy storage may prove a dirty secret as well because of causing
more fossil-fuel use and increased carbon emissions. However,
energy storage can make the grid more flexible and reduce
emissions If employed strategically, nevertheless, generally, has
not been done so (Roberts, 2019). Current planning and decision-
making to deploy energy storage technologies must manage
these impacts. The very effective strategy to decrease greenhouse
gas emissions lies in the increased electricity production from
renewable resources, as recommended in the Paris Agreement
(Al-Badi and AlMubarak, 2019). For example, the capacity of
Oman for renewable energy was just 1 megawatt in 2014 which
was increased to 8 megawatts at the end of 2018 but still must
be enhanced to achieve the nationally determined target of 2%
reduction in the greenhouse gas emissions.
9. Challenges and prospects of energy storage technologies
The innovations and development of energy storage devices
and systems also have simultaneously associated with many chal-
lenges, which must be addressed as well for commercial, broad
spread, and long-term adaptations of recent inventions in this
field. A few constraints and challenges are faced globally when
energy storage devices are used, and storage systems are in
operation for storing the surplus of generated energy. It has
been reported that none of the devices and systems release back
100% quantity of the energy that was stored for the later usage
which means that some wastage must occur during the storing
and releasing process. The values reported varying from 10%–
30% in various devices and systems as well as specific conditions
attached to the area (Chen et al., 2008; Ibrahim and Adrian, 2013;
Mears and Epri-Doe, 2003; Pena-Alzola et al., 2011; Vazquez
et al. 2010). A lot of money is also incurred in the implemen-
tation, running, and replacement of the energy storage systems,
which in certain cases is quite high (Cho et al., 2015; Díaz-
González et al., 2013). Some devices of the energy storage can
cause environmental problems which start from the mining of
material for manufacturing and persist to disposal after availing
full life (EPA, 2019; Faure, 2003; Florin and Dominish, 2017).
Therefore, research is required to develop devices not only with
higher efficiencies but also must be cheaper and have minimum
environmental problems, especially the disposal of used devices
after completing the life cycle. Standards must be developed to
assess the environmental impacts of various devices and systems
and regulations must be implemented to control these (Al-Badi
and AlMubarak, 2019; Arab Future Energy Index, 2015).
The current energy production is dominated by generation
from fossil fuel which is not only costly but also nonrenewal as
well, therefore it cannot be sustained indefinitely. Moreover, elec-
tricity production from fossil fuel plants is necessarily associated
with CO2 emissions which cause heavy environmental pollution.
15. Please cite this article as: A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai, Review of energy storage services, applications, limitations, and benefits. Energy Reports (2020),
https://doi.org/10.1016/j.egyr.2020.07.028.
A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx 15
The optimum management of energy storage system (ESS) for
efficient power supply is a challenge in modern electric grids.
The integration of renewable energy sources and energy storage
systems (ESS) to minimize the share of fossil fuel plants is gaining
increasing interest and popularity (Faisal et al. 2018). Therefore,
a very loud voice is there to gradually decrease dependency on
oil and gas utilization for electricity production, but alternatively,
renewal sources like wind and solar must be developed and
their share be increased gradually, as recommended in the Paris
Agreement (Al-Badi and AlMubarak, 2019) to achieve the ulti-
mate target of 100%. Awareness of the energy storage impacts
should be created among all the stakeholders including customers
(Al-Sarihi and Bello, 2019). With the installation of modern and
more efficient devices of energy storage, the fossil fuel operated
power plants can become more flexible and successful to manage
rapid changes in demands of customers because now most of
these could be equipped with reliable back-up power in the form
of stored energy. However, the total energy storage capacity at
present is low, for example that of the European energy system
is just 5% of total generation capacity, which is sole as PHS in-
stalled majorly in the mountainous areas. Therefore, the electrical
storage capacity must be enhanced to keep pace with modern
developments (European Commission, 2019).
10. Contribution of the present study for decision making
The planners, policy makers, and the practitioners often face
problems to select the most appropriate device or the combina-
tion of two or more devices/systems to store energy for the grids
or static forms. Therefore, they need some broad guidelines. The
outcomes of the present study might help in this regard. For this
purpose, significant basis of decision making must be highlighted
here. The first one is the objective of storing energy which could
be deferring of some quantum of electricity for the peak hours,
balancing and stabilizing delivery, facing sudden and routine shut
downs, for using in the remote areas, utilizing in the traveling
and transport sector or usage in various machines, portable de-
vices and buildings. Some more factors considered to decide the
feasibility of a storage system or device are storage capacity, easy
load leveling, time required for storage and regeneration, lifetime
of device, and quality, consistency, and reliability of discharged
energy (Fig. 1). The specific geographical and climatic conditions
of the area must also be considered. Moreover, cost comparison
(capital and running cost, cost of replacement and maintenance,
and cost of the disposal of borne-out material) is highly im-
portant as well. The environmental concerns are becoming very
important due to complaints and protests of people which must
be considered while making an implementation decision (ESA,
2019; Kondoh et al., 2000). Batteries like Lead-acid, Ni–Cd, Li-
ion (Li–O2 and Li–S), NiMH (nickel-metal hydride), and the flow
batteries have very high capacities and can supply energy even
to run heavy vehicles and grids of electricity networks (Fletcher,
2011). These can also be hybridized with other devices to increase
storage capacities and improve other characteristics. Although,
batteries are mostly low cost, but their shorter lifecycle and
consequent frequent replacement make these costly. Another
problem with batteries is their environmental impact during min-
ing of material, shifting and installing, and disposal of borne-out
material. Supercapacitors may indicate high efficiency, medium
capacity of storage, longer life cycle and low environment impact
but are of higher cost. These can be used to store energy in
the low to medium range electrical systems. The hybridization
of batteries and Supercapacitors proves useful to increase the
storing capacity and decreasing the cost. Flywheel have high
density energy, low storage capacity, high efficiency and longer
life cycle. These can be used in storing energy with low range
electric grids unless their capacity is not increased by making
hybrids with other suitable devices or systems (Ammar et al.,
2013; Bueno and Carta, 2006; DTI Report, 2004).
CAES is storing energy by compressing air and considered one
of the most efficient and economic attractive system that enables
load management successful with large electrical grids. However,
it is usually possible where huge storage reservoirs already ex-
ist, such as underground caverns, hard-rock mines, or natural
aquifers. It may cause low environmental impacts and having
a longer life cycle, but its operational costs are high. Pumped
hydro energy storage (PHES) uses the potential energy of water
transferred between two reservoirs located at different altitudes.
It is also a mature technology but expensive during installation
and require suitable sites for construction of reservoirs. SMES is
the best suitable device to provide constant and instant power
supply as well as regulating grid stability with very high-power
output within a short time and can provide power quality to
the consumers, although the systems are costly. Hydrogen is an
immature technology but can attract huge interest in future if
aspects like generation, storage and utilization in fuel cells are
developed (Ibrahim et al., 2008a; Wenxing, & Lu, 2013; Zhaobin,
Guiping, Yunhua, Wu, & Cao, 2013; Zhao, 2016).
11. Conclusions
The human beings and the energy have been integral parts
of each other and could not be separated at any stage of the
history because they need food to eat and energy to cook and
protect from hard and unfavorable surrounding environment.
The humans learned to store energy for difficult times when
direct sources (Sun, air, and wind) were not available, although
the storing process was just putting firewood under shade to
protect from rain, dew, and moisture. However, with the contin-
uous research and development, energy storage forms, mecha-
nisms, and devices remained changing and have reached to the
present systems, techniques, and processes. The major theme
and need of storing energy are ensuring its availability when
direct sources cannot be captured, or renewal resources are pro-
ducing/generating/reforming energy at almost fixed rates while
the demands are fluctuating simultaneously. Thus, energy storage
makes it possible to supply energy at peak times and storing it at
off-peak times (Baker, 2008; Beltran, 2018; Chen et al., 2008).
The device used for storing energy from olden times is the
battery, which has been changing so much and appearing in so
many forms (Bruce et al., 2011). The first invented battery was
Volta’s cell in 1800. Most of the modifications have occurred in
this device of energy storage but it is still very useful and mostly
adopted mechanism of energy storage (Beltran, 2018). At present,
batteries like Lead-acid, Ni–Cd, Li-ion (Li–O2 and Li–S), NiMH
(nickel-metal hydride), and finally the flow batteries are in use.
The batteries in today’s use have very high capacities and can
supply energy even to run heavy vehicles and grids of electricity
networks (Fletcher, 2011). Capacitors and Supercapacitors are
also used to store energy for electrically run utility devices (Hall
and Bains, 2008). The most advanced polymer materials, consti-
tuting Li-ion batteries, are being utilized in portable electrical
devices, electrically driven vehicles, and stationary grids which
may require charges from 10watt hours to many megawatt-hours
(Isah, 2018).
Nanostructured carbons are highly porous and have a large
surface area that can maximize electrode performance by the
addition of functional groups like oxidative groups (carboxylate,
ketone, or hydroxyl groups or nitrogen). Increased storage ca-
pacity, electrolyte efficiency, and electrochemical reactions were
observed due to such surface modifications (Wu et al. 2017).
More advanced mechanisms and systems of storing energy are:
16. Please cite this article as: A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai, Review of energy storage services, applications, limitations, and benefits. Energy Reports (2020),
https://doi.org/10.1016/j.egyr.2020.07.028.
16 A.Z. AL Shaqsi, K. Sopian and A. Al-Hinai / Energy Reports xxx (xxxx) xxx
Superconducting magnetic energy storage (SMES), Flywheel En-
ergy Storage (FES), Pumped hydro storage (PHS), Thermal Energy
Storage (TES), Compressed Air Energy Storage (CAES), and Hybrid
Electrical Energy Storage (HES). Each of the systems/technologies
has advantages and constraints but these can be used to match
the requirements of the location and the delivery network as well
as the demand of customers (Bueno and Carta, 2006; Chen et al.,
2008; Evans et al., 2012; Luo et al., 2015; Wu et al. 2017; Zhao
et al. 2015).
For obtaining high efficiency and meeting the objectives, the
decision makers and practitioners must select specific storage
technologies. The size of grid networks, customer demands, stor-
ing capacity of devices, their advantages and limitations, cost,
lifetime, and impacts on the environment must be considered
during selection decision. The sources of power production; re-
newable or fossil fuels, must also be accounted. The various types
and sizes of batteries are required for storing static energy to
run vehicles/transports, machines and equipment, and entertain-
ment and communication devices. For low power energy storage,
lithium-ion batteries could be more suitable. When the electrical
systems are smaller using renewable resources (up to few kWh)
and located in isolated areas, the most suited device is lead-acid
battery which may be a good compromise between electrical per-
formance and cost. Energy can be successfully saved using a lead
acid battery when the electrical systems comprise of few hundred
kWh. For energy saving in the electrical systems of many MW,
large compressed air and flow batteries are the suitable devices
which can save higher quantum of energy. Flywheels and super-
capacitors are the most appropriate when power-quality savings
are required because they possess the desirable characteristics
like energy discharging speed and cycling ability. Other technolo-
gies like PHS and SMES are considerable for meeting the storage
of higher quantities from energy intermittent resources while TES
is best suited for heating and cooling of buildings. (Florin and
Dominish, 2017; Jamahori and Rahman, 2017; San Martín et al.,
2011; Hemmati and Saboori, 2016; Rodrigues et al., 2017; Zohuri,
2018). Various energy storage technologies also differ in their cost
(Capital, running and maintenance, labor, and replacement after
some intervals) but a wise decision can be made to implement the
best-suited mechanism or a combination that matches most of
the requirements and demands of a peculiar situation. The storing
techniques and devices can also affect the environment positively
as well as negatively. The positive impacts may be the decreased
impact on global warming and a lesser effect emerging from the
use of fossil fuels. However, CO2 emissions and disposal of devices
material may also emerge as a constraint to the environment
if not deployed and managed appropriately. The environmental
and social impacts emerging during mining of raw materials and
disposing of after completion life of storage devices must be
considered as well. Therefore, a strong need and priority of good
management and disposal processes are highly important. Recy-
clable materials must be used in making energy storage devices
(ESA, 2019; Evans et al., 2012; Farret and Simões, 2006; Kondoh
et al., 2000; Luo et al., 2015). There are some constraints and
challenges during the processes of energy storage. None of the
devices and systems returns 100% quantum of the stored energy,
meaning that there must be wastage (10%–30%). Research must
be conducted, and devices should be developed with higher effi-
ciencies. A few building codes should be implemented. Standards
must be developed to assess the environmental impacts of vari-
ous devices and systems and regulations must be implemented to
control these (Al-Badi and AlMubarak, 2019; Arab Future Energy
Index, 2015). Efforts should be concerted to increase renewable
resources of energy production for decreasing emissions and en-
vironmental impact. Awareness of the energy storage impacts
should be created among all the stakeholders including customers
(Al-Sarihi and Bello, 2019).
Declaration of competing interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgment
The authors would like to acknowledge Solar Energy Research
Institute (SERI), Universiti Kebangsaan Malaysia (GUP-2018-127),
for the provision of the grant.
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