1
Research on Energy Storage
Derek Lusby, Lu Wang, Tyler McGraw, Edward Palmer, Nathan Peck, and Jake Woods
Abstract—While generation of energy is a very important task,
storing the energy is just as paramount. There have been
numerous methods of storing energy throughout history, and it is
a subject that is still heavily researched to this day. In order to
understand the necessity of energy storage and why there is such
a focus on advancement of its technology, various methods must
be observed along with the benefits and issues with each.
I. HISTORY
Energy storage is a vital technology that is vital to
technological advancement both past and present. It allowed
for portable electronics as early as the late 19th century, such
as electric cars and flashlights. Grid balancing techniques rely
on energy storage to be as efficient as possible. In the future, it
could help stabilize intermittent power sources like wind and
solar by providing a steady stored output.
Almost all energy storage is some form of potential energy.
Storage devices, to give some examples, can be mechanical,
gravitational, electrical, or thermal in nature. The only widely
used kinetic energy storage is flywheels, which can convert
electrical potential to rotational kinetic and back again.
Another factor to consider when looking at energy storage
is the length that the energy is intended to be stored. Different
systems produce different losses that make them suitable only
for a certain timeframe of storage. Capacitors are intended to
store electrical energy for a much shorter time than lithium-ion
batteries. Each system is suited for a different purpose.
Early developments in energy storage were almost
exclusively mechanical or gravitational potential energy
storage. Reservoirs and dams, notably, were used to store and
release water that could supply drinking water or be used to
power water wheels [1].
The first practical advance in electrical energy storage was
the Leyden jar. It was developed independently by two
different European scientists in 1745-1746: Ewald Georg von
Kleist and Pieter van Musschenbroek [3]. It stores static
electricity between two electrodes on the inside and outside of
a glass jar, which is filled with a liquid. It is considered to be
the earliest useful form of capacitor and battery. Both stored
voltage and capacitance could be changed by changing the
properties of the jar. Leyden jars allowed scientists obtain
enough charge to perform experiments, leading to other
developments.
Pumped-storage hydroelectricity was used as early as the
1909 s in Switzerland [1]. It pumps water into a heightened
reservoir to run through turbines later. Pumping losses actually
make these facilities net consumers of power. These losses
were lessened by the implementation of reversible turbinegenerator assemblies that act as both a pump and turbine in the
1920s [2]. Because the pumps are only run during cheaper offpeak energy hours though, it remains profitable when more
expensive power is sold during peak hours. It is also important
as a grid-balancing measure so that power isn't wasted in large
amounts.
Compressed air storage works on a similar concept as
pumped-storage hydroelectricity. Cheap electricity is used to
compress air, which is used later during peak hours. Paris had
a system working as early as 1870, which generated 2.2 MW
to be used with specific pneumatic machines. The first utility
scale project was built in Germany in 1978, which was 290
MW scale [4]. The compressed air is often used to run
turbines, which produce electricity. It is stored in tanks or
sometimes in abandoned mines, which provide ample
enclosed space.
Ice storage air conditioning operates much like the other "offpeak" electricity technologies. It creates ice to be used in lieu
of air conditioning later during peak hours. It is unusual in that
it is a more widely available storage technique as it can be
installed in businesses and homes. It isn't as generally useful
though, because it can only be used as air conditioning.
Flywheels are rotors accelerated by usually electrical means
that store energy as rotational kinetic energy. It slows down as
it converts energy back to electricity. Modern flywheels date
back to the 1950s when smaller systems were made bigger and
more efficient for larger scale energy storage [5]. The rotor
itself exists in a vacuum and has magnetic bearings to reduce
friction losses as much as possible.
Batteries are important to several technologies, including
portable electronics. Practical batteries were developed in
1836 when the Daniels cell introduced two (wet) electrolytes.
The modern dry cell batteries are smaller, more efficient, and
were developed in 1949 [1]. Rechargeable batteries were
developed in the 1970s and have been improved until the
present day.
II. STATE OF THE ART DESIGNS
With continued research over the years, new, state of the art
designs have been developed in order to improve upon
older, historical methods of energy storage.
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A. Lithium Poly-Sulfide Flow Battery
The Lithium Poly-Sulfide flow battery differs from
traditional redox flow batteries. Today’s redox based flow
batteries have two streams of high cost liquids that are
pumped through the system and pass through a high cost
membrane where the streams undergo chemical reactions to
wither produce or store energy. The Lithium Poly-Sulfide flow
battery differs in two main ways. The Lithium Poly-Sulfide
flow battery only uses one stream of relatively cheap Lithium
Poly-Sulfides, which are dissolved in an organic solution to
conserve the life of the battery. The Lithium Poly-Sulfide
battery also does not use an expensive membrane. Instead the
Lithium anode is coated with a barrier that protects the raw
Lithium but still allows electrons to freely pass. During the
discharge cycle, Lithium Ions are absorbed by the Lithium
Poly-Sulfide molecules. During the charging cycle, the
Lithium ions are lost back into the organic solution.
Fig. 1.
Common Redox Flow battery design and state of the art Lithium
Poly-Sulfide flow battery design
B. Hydrogen Storage
The hydrogen energy storage uses a Hydrogen generator to
produce Hydrogen, which can be stored as a fuel for later use.
The Hydrogen generator uses a battery-like design to produce
Hydrogen from water through the process of electrolysis. An
anode and cathode are submerged in water and when an
electrical current is passed between the two, protons are able
to pass through a Proton Exchange Membrane (PEM) where
the protons combine with electrons to produce stable
Hydrogen. This stable Hydrogen can then be stored in large
tanks and are converted to energy when demand is high.
Fig. 2. Diagram of Hydrogen Generator
C. Pumped Water Storage
During the off peak hours, water is pumped up a hill into a
reservoir for storage. These pumps are powered using excess
off-peak energy, which is cheap and available. During times of
high-energy demand, the water is released from the reservoir
where gravity pulls it down through turbines generating
electricity. This system as a whole has losses associated, but
since the high-demand energy can be sold at a premium, a
profit can be made.
D. Compressed Air Storage
Compressed air storage uses the same principle as pumped
water storage. During low energy demand, excess energy is
used to run pumps, which compress air into large underground
reservoirs. When the energy is in high demand, a release valve
is opened allowing the compressed air to shoot through airturbines, which recovers the stored energy in the form of
electrical energy, which can be sold at a premium price.
E. Flywheel Energy Storage
Flywheels work in the same way as electrical motors and
generators work. The flywheel is supported by magnets and
enclosed in a vacuum. During off-peak hours, excess energy is
used to spin the flywheel to a very high speed just like an
electrical motor. Then when the energy is in demand, the
flywheel acts like an electrical generator and converts the
kinetic energy of the flywheel into electrical energy, which
slows the flywheel back down. This generated energy is then
sold at a premium.
III. BENEFITS AND VALUES
Each method of energy storage has certain benefits that make
it a viable method.
A. Batteries
Batteries have the most well known benefits compared to
other types of electrical energy storage. They are the most
widely used storage due to their size and weight. From AAs to
car batteries, individuals have the ability to manually change
them and apply them to other applications. The capacity of
different batteries changes, thus they can be designed to fit
different products. Battery systems also are more reliable and
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run for a longer timespan compared to other methods such as
flywheels and ultra-capacitors. Depending on the requirements
of a particular system, batteries can be combined easily to
apply for different uses.
B. Flow Batteries
Flow Batteries are a special type of storage that has been
developed recently. The greatest benefit this type of battery
provides is the increased utility of renewable energy storage.
These batteries are efficient enough to be used during periods
of peak demand for systems that are more dependent on
renewable energy sources. Due to the lack of solid-solid phase
transitions, flow batteries of long cycle lives. Flow batteries
have the potential to be almost instantly recharged by
replacing the electrolyte liquid while simultaneously
recovering the spent material for re-energization. Some
models of flow batteries offer very good tolerance to
overcharging and over discharging. The power and energy are
separated in the flow batteries with makes designing these for
application very flexible. The design can be tailored
specifically to the load or generating asset.
C. Hydrogen Storage
Hydrogen energy storage is a method of converting
electricity to hydrogen using electrolysis. This method has
much higher energy storage capacity compared to batteries or
pumped hydro and CAES systems. Electrolytic hydrogen can
be used for the production of synthetic liquid fuels from
biomass; this significantly increases the efficiency of biomass
utilization. Hydrogen has the ability to hold 120MJ/kg; this
means that a small amount is needed to hold a significant
amount of energy. Because hydrogen is a stable element,
energy can be stored for a longer period of time compared to
other mediums. Hydrogen fuel cells have a fast response time
making them able to correct rapid fluctuations in electricity
demand or supply, regulating the frequency.
is also the most cost efficient after taking capital costs and
geographic features into consideration. The graph below
shows the amount of power consumed (green) compared to the
amount of power generated (red).
Fig. 4.
Pumped hydroelectric power graph
If a large body of water or a large variation in height is
available, pumped storage has a relatively low energy density.
Compared to thermal plants and their reactions to sudden
changes in electrical demand, hydro storage can respond to
load changes within seconds.
E. Compressed Air Storage
Compressed Air Energy Storage is a method that reuses
underground tunnels and caverns as way to store electrical
energy. If a salt cavern can be utilized, this method of storage
obtains much higher flexibility due to the lack in pressure
losses within the storage. CAES methods can generate three
times the output of the same natural gas methods because
CAES lacks the compression stage. The compression stage
uses up 2/3 of the turbine capacity. This also reduces the gas
consumption and reduces CO2 emissions by 40 to 60%.
F. Flywheel Energy Storage
Flywheel energy storage is a method that uses kinetic energy
to store electricity. When compared to other methods of
storing electricity, flywheels have very long lifetimes that
required little to no maintenance. Full-cycle lifetimes for
flywheels have been quoted at ranges from 105 up to 107
cycles of use. Flywheels also have high energy densities
ranging from 360-500 kJ/kg and have large maximum power
outputs. Flywheel energy efficiencies are also usually around
90%, which makes them one of the most efficient methods of
storing energy. The capacity range of typical flywheels ranges
from 3kWh to 133kWh.
Fig. 3.
Discharge time versus storage capacity graph
D. Pumped Water Storage
Pumped-storage hydroelectricity is mainly utilized to
balance out electricity during peak hours of use. This method
G. Superconducting Magnetic
Superconducting magnetic Energy Storage systems store
energy in the magnetic field created by the flow of direct
current in a superconducting coil. These systems have a round
–trip efficiency greater than 95%. The delay time during
charge and discharge is very short. Power is available almost
instantly and very high power output can be provided for a
short duration. Not only do these systems provide stability to
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the grid, it has next to no environmental issues. SMES does
not produce harmful chemicals, does not require radical
changes to the landscape, and it is silent in operation.
H. Double-layered Capacitors
Double Layer Capacitors are a new type of electrochemical
capacitors called super capacitors. These capacitors have the
highest available capacitance values per unit volume and the
greatest energy density of all capacitors. They have
capacitance values of 10,000 times that of electrolytic
capacitors. Super capacitors have power densities that are
from 10-100x greater than conventional batteries. Because of
this high density, the charge and discharge cycles are much
faster than batteries as well. Superconductors have greater
than 1000 cycles of charge and discharge, which means that
they will last for the entire lifetime of most devices. This
makes superconductors environmentally friendly. The internal
resistance of the capacitors is low and consequently makes the
cycle efficiency very high (95% or more). These capacitors
use non-corrosive electrolytes and low material toxicity,
which makes them, have improved safety.
Fig. 5.
Energy density versus power density graph
IV. CONCERNS AND PROBLEMS
Each method of storage also has a set of issues that keep it
from performing ideally.
A. Pumped Water Storage
Pumped hydroelectric storage a relatively high efficiency
method of energy storage, yet the maximum return is only in
the range of 70 to 85% [23]. Aside from efficiency issues, this
method requires a mountainous area, as it gravity to generate
hydroelectric power from the reservoir water. This limits the
use of the method to very specific geographic locations [23].
As this method requires mountains, electrical equipment must
be installed in a mountainous region, such as transmission
lines that must run from possibly isolated areas to the main
grid. Not only does this make installation and sustentation
difficult and inconvenient, it can also damage the environment
[23].
B. Compressed Air Storage
Compressed air energy storage has a comparable efficiency
to pumped hydroelectric, so the return is still less than unity.
This method also requires an underground cavern to store the
air. While these are more common than the mountainous areas
required by pumped hydroelectric, this may still limit its use in
some areas. While the gas is in the storage process, it must be
compressed. This compression causes the gas to heat. The
amount of gas that can be stored is limited by this
phenomenon, as temperatures could rise to dangerous levels if
too much gas is stored [23]. What gas is stored underground is
also subject to energy loss through heat transfer to the walls of
the cavern, thus limiting its long-term storage capabilities
[23].
C. Batteries
Each different type of battery carries unique concerns and
issues. Lead acid batteries, commonly seen in automobiles,
have a low energy density; thus they require large size to store
large amounts of energy [23]. Building sizable batteries to
store large amounts of energy is neither cost effective nor
space effective. Sodium-sulfur (NaS) batteries require that the
sodium and sulfur are kept molten and separated from one
another [23]. This method of storage is sizeable, and not
feasible in every situation. In addition, the battery may suffer
critical damage if it is completely drained and left cold [23].
Lithium-ion batteries are very efficient, sometimes nearly 90%
efficient, but also very expensive due to safety considerations.
The lithium salt solution used in the batteries is flammable;
thus extra measures must be taken to ensure that the batteries
will not catch on fire [23].
D. Flywheel Energy Storage
Flywheel storage is still a very developmental technology.
Key advances may eliminate issue that face flywheels,
however this in not currently the case. Flywheels are
expensive to build and require an incredible amount of
precision in order to operate properly [24]. As energy is stored
through the spinning of the wheel, they must be built strong
enough to withstand heavy rotation while not breaking or
seeing a drop-off in performance [23]. This necessary integrity
limits the effectiveness of the flywheel as a long-term method
of energy storage [23].
E. Ice Storage
Ice storage is an effective way to reduce energy costs and
demand during times of stress on cooling systems [25].
However, this method still faces its share of issues. The major
issue of ice storage arises from the nature of the method itself:
it is only saving energy originally intended for cooling
systems. This method is supplemental, and not viable for
large-scale storage, as not all energy stored and supplied goes
to the cooling system. Thus, while effective at reducing costs,
ice storage is not the answer for the problem of effective,
efficient energy storage [25].
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V. RESEARCH AND DEVELOPMENT
There is great interest and focus on developing better, more
efficient forms of energy storage.
A. Liquid Lead Batteries
Batteries have always been a widely used storage technique
for the energy grid. Researchers at the Massachusetts Institute
of Technology in Cambridge are developing a new battery,
which is believed to be more competitive for large-scale use
on the energy grid. Most batteries are made of solid electrodes
and possibly a solid electrolyte, but this new development uses
liquid electrodes and electrolytes. The latest model from the
team at Cambridge uses a liquid lithium negative electrode, a
liquid lithium salt electrolyte and a liquid lead-antimony
positive electrode instead of the previously used magnesiumantimony electrode [26]. According to [26], “When the battery
discharges, lithium atoms in the negative electrode give up an
electron and travel through the electrolyte to the leadantimony electrode. Charging pushes them back in the
opposite direction, and the flow of current is enough to keep
the metals liquefied.”
The latest version of the molten battery could run at 450°C
over the previous 700°C which the magnesium-antimony
version required to run. This not only lowers the energy
needed to sustain the liquid battery but it will also help to
decrease the rate of corrosion. After 1,800 hours of operation
no sign of corrosion could be seen and the lead-antimony
battery was able to maintain 94% of its capacity after 450
complete cycles. Estimations predict the battery will keep 85%
of its charge capacity after a decade of complete cycles [26].
Cost effectiveness is the main concern for this new molten
metal battery. Currently a large-scale lead-antimony battery is
estimated to cost upwards of $500 per kilowatt-hour of
electricity produced, but the Cambridge team’s goal is to
lower the cost to around $100 per kilowatt-hour. This would
allow the battery to become an alternative in energy grid
storage. However, this is just the beginning of the Cambridge
team’s research as they continue to refine the battery’s
chemistry, exploring more ways to develop a longer lasting
and more efficient battery [26].
B. Graphene Aerogel in Super-Capacitors
Another energy storage development undergoing research is
a graphene aerogel and its ability to improve the power grid.
Lawrence Livermore National Laboratory researchers believe
that graphene aerogel could help improve energy storage by
smoothing out power fluctuations in the grid. Graphene
aerogel is currently one of the lightest materials in the world
and may be especially useful in the super-capacitors.
According to [27], “Graphene aerogel-based super-capacitor
electrodes could be particularly useful… because they feature
high surface area, good electrical conductivity, chemical
inertness and long-term cycling stability.” Graphene aerogels
also support enhanced pore size distribution and density
control over currently used carbon-based super-capacitor
electrodes. LLNL’s Patrick Campbell states in [27], “Our
materials can potentially improve on the performance of these
commercial super-capacitors by more than 100 percent.” This
new development is still being heavily tested but could lead to
improved super-capacitors, in turn improved batteries, and an
increase in the capability of energy storage [27].
C. Molten Salt Storage
Molten salt storage is another form of energy storage that is
undergoing research and continual develop. Most current
means of energy storage are not viable since the costs heavily
outweigh the profit, so more unique ways of storage energy
have been developed. Molten salt uses solar energy to heat
sodium into a liquefied state that is then stored until necessary,
such as peak or nighttime hours. Once more energy is needed
the liquid sodium is pumped into a steam generator and the
molten salt gives off enough heat to boil the water, creating
steam. The steam is then directed through a path to a turbine,
which it spins to generate electricity. This provides a new way
to store thermal energy for a later time when electricity is
needed. It is also an environmentally safe way to store great
amounts of energy as well as a cheaper method than that of
batteries while still keeping a reasonable efficiency [28].
The mixture of salt used varies, but the more common ones
contain sodium nitrate, potassium nitrate, and calcium nitrate.
The average temperature that the salt melts also varies by
depending on the concentration of compounds, though the
average temperature is around 131°C and is kept around
288°C in the storage tanks. The molten salt is kept here until it
is finally reheated by a solar collector to around 566°C and
finally sent to another storage tank where it is kept up to a
week before the salt is used to generate energy though a steam
generator and the cycle is continued [29].
VI. POWER ELECTRONICS FOR GRID-SCALE ENERGY
STORAGE
Generally, power electronics is the process of using
semiconductor switching devices to control and convert
electrical power flow from one form to another to meet a
specific need. They are widely used in the flexible alternating
current transmission system (FACTS) and used as distributed
energy interfaces. In 2005, approximately 30% of all electric
power generated utilizes power electronics somewhere
between the point of generation and its end use. By 2030, it is
expected that perhaps as much as 80% of all electric power
will use power electronics somewhere between generation and
consumption [30].
A. Grid Interfacing Technology for Batteries
Power electronics is expected to play an integral role in
interfacing battery storage to the grid. Bidirectional battery
chargers and Solid State Transformers (SSTs) are two primary
examples of making this role a reality. An advanced research
project in US entitled “Gallium nitride switch technology for
bi-directional battery to grid charger applications” (20102014) has achieved higher frequency (100kHz), 2 times faster
charging (6.6kW), 2 times higher efficiency (>95%), and 10
times smaller (>120W/in3), Fig. 6 shows a demo of this
project [31].
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Fig. 6 A demo of Bidirectional battery charger
The elements of an SST are a dual active bridge including a
physically smaller high-frequency transformer and two
converters (rectifier and inverter per given direction). An
assembly diagram of the SST is found in Fig. 7 [32]. SST
centers on FACTS features, namely reactive power
compensation and harmonic filtering. Depending on the SST
design used, advantages can include bidirectional power flow
(four-quadrant operation), reactive power compensation,
harmonic isolation, voltage sag compensation, fault isolation,
a common dc link, or energy storage integration. Another
benefit of the SST is communication capability between
utilities, end users, various other switching devices in the
network, and other SSTs.
is flexible and has lower space requirements due to the
reduced size and fewer passive components resulting in a
smaller footprint. Standard sizes are available and include 25,
35, and 50 Mvar. The open rack modular system configuration
enables a transformerless grid connection up to 36 kV and 100
Mvar [33].
The NPC topology for an n-level, multilevel converter is
illustrated in Fig. 8. This topology creates the smallest
converter ac voltage steps, has a small rate of rise in voltage,
and generates lower harmonics and low switching losses. The
total required capacitance is shared by the modules instead of
being concentrated, as in other traditional power converter
designs [34].
Fig. 8 NPC topology for STACOM
Fig. 7 Diagram of Example SST configuration
SST development has been specifically targeted at the
distribution level. They operate at 20 kVA, 20 kVA, 300 kVA,
500kVA and 1 MVA, respectively, and range between 7.2 and
15 kV on the distribution side, which are compared in [32]
showing their unique benefits and limitations. The SST has
great potential of becoming the standard for interfacing
various power system technologies to the grid, including
battery storage systems.
B. Utility Scale Power Electronics Utilizing Energy Source
Reactive power compensator is very important for the
dynamic voltage support, dynamic reactive power
compensation, power system stability, increased power
transfer capability, renewable energy and energy storage
integration, and the overall enhanced power quality. Static
synchronous compensator (STATCOM) systems, which are
designed with self-commutated voltage sourced converter
(VSC) technology, have become a more widely accepted,
advanced technology, solution for reactive power
compensation applications because of faster response and
improved operational characteristics. The widely used
STACOM types are neutral point clamped (NPC) topology
developed by Siemens and the modular multilevel converter
(MMC) topology used by ABB. The design of the SATCOM
ABB has recently developed and tested an STATCOM
solution referred to as DynaPeaQ [35], which is MMC with
energy storage, as shown as Fig. 9. Currently, the amount of
power that can be delivered by the energy storage system is
about 20MWfor tens of minutes. But the technology permits
up to 50 MW for periods of 60 min. There are four application
areas where DynaPeaQ is expected to find widespread use,
such as renewable generation grid connection, backup power,
emergency and short-time power, intermittent loads of a
railway [36].
Fig. 9 MMC topology for STATCOM
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C. Battery Storage Applications in Microgrids
The fundamental microgrid requirements include the
capability of operating in islanding and/or on-grid modes with
high stability, mode switching with minimum load disruption
and shedding during transitions, and after a transition,
stabilizing in a certain amount of time. The medium-voltage
direct current (MVDC) architecture (Fig. 10) has often been
referred to as a type of microgrid upon first view [36].
Fig. 12. Current-sourced-based grid supporting
Fig. 13. Voltage-source-based grid supporting
Fig. 10 MVDC Microgrid Example
Energy storage systems act as a buffer, either absorbing excess
generation, or discharging energy to meet minimum load
requirements. Energy storage can smooth renewable energy
output and frequency deviations, thereby preventing voltage
instability. Most importantly, energy storage can supply power
during outages that last for extended periods of time.
Power electronic converters operate as three modes in
Microgrid: grid feeding, grid forming, and grid supporting.
Grid-feeding (Fig. 11b) power converters are mainly designed
to deliver power to an energized grid. These units are
modeled, simply, as a current source with high impedance in
parallel with the source. Grid-forming power converters (Fig.
11a) are represented by a voltage-controlled source and low
series impedance. Grid-supporting converters (Fig.12 13) are
used to regulate their output current/voltage to keep the value
of the grid frequency and voltage amplitude close to their
rated values. Its main objective is to deliver proper values of
active and reactive power to contribute to the regulation of the
grid frequency and voltage.
D. Conclusions
Power electronics do in fact have a key role to play in grid
scale energy storage applications. Any regulation of nonlinear
output and any coupling of battery storage to the grid are
performed by the use of power electronic systems.
Bidirectional dc chargers, FACTS devices with integrated
energy storage, battery storage within microgrids, and EVCSs
are a number of examples that show power electronics being
interfaced to battery storage. Successful realizations were
expounded upon, exemplifying the utilization of power
electronics in each. Within the transmission, microgrid, and
distribution layers of the current and future grid, power
electronics technologies are integral to the implementation of
new equipment developments including battery storage
systems.
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