Discussing the different types of EESS , the cost/KWHr of , the limitations , the advanatges and disadvantages , and a comparison to summarize.

1. Introduction
Energy storage is the capture of energy produced at one time for use at a later time. A device that
stores energy is sometimes called an accumulator. Energy comes in multiple forms including
radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature,
latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to
store to more conveniently or economically storable forms. Bulk energy storage is currently
dominated by hydroelectric dams, both conventional as well as pumped.
Some technologies provide short-term energy storage, while others can endure for much longer.
A wind-up clock stores potential energy (in this case mechanical, in the spring tension), a
rechargeable battery stores readily convertible chemical energy to operate a mobile phone, and a
hydroelectric dam stores energy in a reservoir as gravitational potential energy. Fossil fuels such
as coal and gasoline store ancient energy derived from sunlight by organisms that later died,
became buried and over time were then converted into these fuels. Food (which is made by the
same process as fossil fuels) is a form of energy stored in chemical form.
Energy storage fundamentally improves the way we generate, deliver, and consume
electricity. Energy storage helps during emergencies like power outages from storms,
equipment failures, accidents or even terrorist attacks. But the game-changing nature
of energy storage is its ability to balance power supply and demand instantaneously - within
milliseconds - which makes power networks more resilient, efficient, and cleaner than ever
before.
Energy storage systems currently make up approximately 2% of global generation
capacity.
That percentage is growing significantly, especially with the advent of more renewable
energy. Pumped hydroelectric power (represent nearly 99 % of
world-wide installed electrical storage capacity) has played an important part of our
electricity grid since the 1930s. Yet today, electricity from wind, solar and other 'intermittent'
sources have created urgent needs for additional energy storage.
World-wide demand for grid-scale energy storage is estimated to reach over 185.4 GWh by 2017
– which is approximately the amount of electricity New York City consumes in 17
days.

2. Capabilities Provided by EES (Electric Energy Storage)
Electric energy storage units can be used to provide a number of benefits to a power system. These are
discussed below:
Peak reduction and load flattening: Energy storage is often used to store power during times of low
usage and to output that power to meet power needs when demand is high. Figure 1 shows the daily
usage of a small community in the western US with HR, 7.5 MW storage units to flatten daily load
cycle. Peak demand that must be supplied by
generation and T&D delivery to the town is
reduced by 10%.
Making renewable energy sources such as
wind and solar generation into dispatchable
sources of power: Both wind and solar
generation are somewhat erratic in their
expected output depending on the weather.
Solar generation definitely will not provide any
power at night. Without electric energy storage,
the power they generate must be consumed at
the instant generated. Otherwise it is lost: they
are non-dispatchable sources of power. A
properly sized and configured electric energy
storage system can accept power from the solar
and wind generation when they produce it, save
it for later use, and then release it upon
command (dispatch it as requested) when
needed, making the energy produced by the
renewable energy generation dispatchable power.
Reliability backup: Electric energy storage can be used to provide power when the grid is unable to
supply it due to equipment outages, storms, or other reasons. Millions of homes and businesses
throughout the US have Un-interruptible Power Supplies (UPS) that provide backup power for
computers, telephone switching equipment, and similar high-value electric demands in the event of a
utility or grid outage. These provide the power required to operate the equipment their owners wish
to operate for at least as long as it takes to shut down smoothly or while backup generation is started.
Millions of these units are on the size of a very large book and provide backup to only one personal
computer. Others provide backup power for large digital equipment installations and entire buildings
in some cases.

3. Spinning reserve: Regional power systems are required to keep significant amounts of generators
running, but not producing power. This is so that they can take over, quite literally in the blink of an
eye, in the event of a sudden emergency such the failure of a major power plant. This is quite
expensive. One or more large power plants have to be operated, which uses fuel and personnel and
creates wear and a need for maintenance and service on the equipment.
Electric energy storage can be used to defer the need to keep that machinery running. The utility still
needs the extra generation capability but can now operate it as “cold standby.” It is not started or
running but is available to start in a short amount of time – a period less than that over which the
energy storage unit can provide the required power.
Improved efficiency of generation: Almost all generators are most efficient at a certain design point,
a certain amount of outputted power. Some generators are also just much more efficient than others.
A power system is normally operated with only the most efficient generators available operating at
any one time. That means that in daily cycles like that shown in Figure 1, the least efficient
generators are left until last (peak time) to be run. A curve like that shown also means that they have
to be run to “follow load” – they are throttled up or back and so do not always run at their most
efficient level. For these reasons, the mix of generators and the way they are operated is much more
efficient when there is energy storage as compared to when there is not.
Regulation and Stabilization: Energy storage can be used to inject, or draw out, power from the grid
to even out fluctuations, and/or to keep voltage and frequency at a highly regulated level. Generally
electric energy storage units designed for these purposes are „all about the control and power
electronics”: the focus of design is mostly on the characteristics of the control system and the
ultimate performance and value of the device is a function of those characteristics.
Reduced emissions: In many cases, not only efficiency but emissions of generators vary one from the
other. Again, the best units are run most often. By flattening the load curve, energy storage permits
the utility to run its least emitting units more and its most emitting units less.

4. Benefits and Applications of Energy Storage
Arbitrage: involves buying power cheap, storing it, and then selling it at a higher price sometime
later. The power bought to charge a storage unit (the yellow shaded area in Figure 1) might be
purchased at night, when power can be purchased on the wholesale grid for 4¢/kWHr, and sold
during the peak demand period of mid-day when power is selling for 12¢/kWHr. Even allowing for
the inevitable electrical losses in the system and the need to pay for the storage unit and its operation
and maintenance, this may provide a steady daily profit to the owner/operator.
Avoidance of congestion charges: Electric energy storage that is connected to the right places in a
transmission grid can mitigate or eliminate congestion and congestion charges, which occur when
demand for power transmission exceeds capability. A power transmission line might be limited to
300 MW, but there could be a peak demand for an hour of 325 MW on the other end of it. If the line
is congested and additional fees would be charged for the line's use in order to reduce demand back
to 300 MW. A 25 MW storage unit could avoid the need for any reduction or cost increases.
Deferral of capital additions to utility systems: In many utility systems there is a slow but steady
growth of local peak demand as communities and neighborhoods gradually expand and fill out. At
times, new or expanded facilities to deliver power into these areas must be added, requiring the
utility to invest considerable money to pay for them. Suppose that a community has a 75 MW peak
day demand like that diagrammed by the base (black) line in Figure 1, and is growing at about 2%, or
1.5 MW per year, and that the substation is limited in capability to serve no more than 75 MW. The
only recourse available to the utility is to upgrade the substation, and typically such upgrades are
available only in relatively large steps – perhaps here the next lowest option for this substation is 90
MW capability. Installing 15 MW of additional capacity to handle the growth the first year it
becomes an issue, when peak demand is only 76.5 MW, means the new capacity is only utilized by
10%. Even in the second year, it is used only to a 20% utilization.
Temporarily adding 7.5 MW energy storage means that the utility can defer this expensive addition
of this 15 MW upgrade for 5 years, at which point when it does the new upgrade it will be utilized by
60% in the following year. The utility saves the carrying charges on the upgrade for five years. The
storage can then be moved to another location at the end of the five year period or kept at the
substation to be used again (if the growth is expected to continue, this same problem will reoccur in
year 11, even with the substation capability upgrade). Whether there is a business case for the use of
the energy storage unit in this application is something that needs to be determined through detailed
study. Often there is not. But there are cases where the deferral is cost justifiable.
Rate Reduction: With electric energy storage, the power system serving the community shown in
Figure 1 will be more efficient in regards to equipment utilization and electric losses, and the power
generation required will be more efficient over the course of a day, a month, or a year (see
discussions above). Even allowing for the cost of the storage system, the total cost may be lower than
in the case with no energy storage. In cases where the cost savings is not taken as profit through
arbitrage, it may result in a net savings for the utility and/or its customers.

5. Reliability Augmentation: Electrical energy storage can provide improved reliability (see above).
Whether there is a business case for a utility or an energy consumer to use it for that purpose depends
on a host of factors that need to be analyzed for the specific situation and need. Among them are,
how good (or bad) is reliability now? Is there even a need for improvement? Numerous demand-side
(utility customer) issues and factors including peak demand, energy needs and load curve shape need
to be included, along with the value of uninterrupted, or less interrupted, service to the particularly
energy consumer. A host of utility system issues have to be considered including the cost of fitting
the equipment to the site and controlling and monitoring it so it will perform as needed.
Making Renewable Energy Dispatchable:
This benefit was listed under the basic capabilities of energy storage but it is perhaps the most
valuable benefit energy storage can provide and almost certainly the application that will lead to very
widespread use of electric energy storage in the 21st century.
Renewable energy from solar, wind, and other technologies makes good sense from so many
perspectives. But except for a few niche technologies that are difficult to site and fit to systems (solar
tower generation), renewable generation systems are not dispatchable sources of power. Energy
storage makes them so. Renewable energy systems also contribute to existing reliability and
regulation problems for widespread power grids. Energy storage provides the means to mitigate
those, too.
Many of the “fundamental concepts” and mainstream ways electric energy storage is used in power
systems have not yet been determined or set in the power industry. There may never be a “typical”
way electric energy storage is used for this type of application. One reason is that the storage does
not have to be located at or operated in conjunction with the renewable generation in order to
provide this benefit. For example, the owner of a 50 MW wind plant could install energy storage at
the plant or at a site electrically convenient to it. A study of past wind and weather cycles, and the
plant's design and expected reliability, and the region's grid load and operation might determine
that 110 MW hours of storage with a 65 MW peak output capability would give the owner a 99.98%
probability of meeting peak commitments if contracted – better than for a coal plant. Installed and
operated in conjunction with the wind farm, this would make the farm's output dispatchable power.
But a farmer 80 miles away could also install storage, and what is actually a fairly simple buying
control system to operate it, sufficient to allow her to buy non-dispatchable power from the grid
when it is a bargain (when wind farms are producing lots of power) and store it for use when she
needs it to run her business or power her home. Again, a study would be needed to determine the
characteristics of the storage – how much energy it would store, what peak load it could serve, how
fast it could recharge, etc. That would need to include a comprehensive look at a number of factors,
but the unit that would do the job for the farmer could be determined and once installed, she would
get power when she needed it but buy cheap non-dispatchable power from the grid when she could.
The interesting point here is that both of these alternatives are very realistic: current technologies can
do either well, and also permit a range of choices between these two extremes. The storage required
to make all the wind farms‟ output into dispatchable power could be added in one large system at the
wind farm site, or dispersed as dozens, perhaps hundreds of smaller installations at customer sites.
Characteristics would vary: probably more net storage capability would need for the dispersed
scenario, but that would provide more benefits, too (in addition to having dispatchable power all the
time, the farmer would have power, for a while, if the utility system was experiencing an outage of
service).

6. Furthermore, a range of options between these two extremes are both technically feasible and
sometimes economically feasible. Storage could be installed at key waypoints in the regional
transmission grid. There is no need to store wind energy at the wind farm. Decision makers can
transport it to convenient central locations and keep it there until needed. It could be stored in
numerous smaller but still “utility size” locations at substations, etc. Each option would have
different characteristics, different initial and operating costs, and different benefits to different
stakeholders.
In some sense, all these potential electric energy storage options compete against one another:
someone is going to see economic benefits from the electric energy storage, either the wind farm
owners who can sell the output of their plant for much more when it is dispatchable, or the farmer
who will buy cheaper power to operate her business. And only so much is needed. Potentially, a
power system could have more storage installed on it than needed, causing a glut of storage
capability and reducing the margin between the cost of dispatchable and non-dispatchable power.
Currently, however, there is no reason to be concerned about this.
Parts of an Energy Storage System
An energy storage system consists of
four main components as shown in
Figure 2. The first and the element that
sets the basic system storage capability
limits is the energy storage medium
itself: the battery, or flywheel,
compressed air reservoir or the lake or
reservoir that can be filled with water in
pumped hydro systems.
Second is the charging mechanism,
which takes power from the utility
system and converts it into the form that
can be put into and stored by the storage
medium. For example in flywheel
systems this is basically an electric
motor that can rev up the flywheel to
very high speeds. In a thermal storage
system, it is a set of heating elements.

7. Third is the equipment that takes energy from the storage and converts it back to electricity and
inputs it out onto the grid for an energy consumer's demand. In a flywheel, this is just the
aforesaid motor (its field is reversed to turn it into a generator that produces electric power while
gradually slowing the momentum of the rapidly spinning flywheel). For thermal storage it is a
heat exchanger and steam generator. Regardless, it is this component that shapes what the unit
looks like to the system and electric demands as far as the quality and quantity of power
provided, voltage regulation and other aspects it provides.
Finally there is the control system, which consists of two sub-systems. First, there is the
equipment needed to monitor and control the unit itself: for example with a flywheel, signals
need to be sent and equipment activated to turn the motor generator into a motor, to permit
power flow to the motor so it accelerates the flywheel, storing energy. This must include sensors
that can determine when the unit is “full” (the flywheel is spinning at its maximum allowed rate)
and shut off this process. Similarly it must control the discharge cycle, too. And typically the
equipment acts as a diagnostic and protection system, monitoring the unit, setting alarms if there
is any anomaly in condition or operation, and activating protective equipment such as fuses,
breakers, brakes, etc., in the event of a contingency.
The second part of the control system is the electrical energy storage system control – the system
that determines how, when, and why the unit performs. This may be a simple UPS control
system (when you sense power from the grid has stopped, you provide power from the battery
backup system). Or it could be a very complex computer algorithm that takes a number of
factors, including weather (the forecast is for record temperatures tomorrow – demand will likely
be high), utility load (its peak season, demand is already high), generation on line (the big nuke
unit south of here is down for emergency inspection and thus electric prices are 12% higher than
normal) and perhaps many other factors (status of renewable
energy and other storage owned) into account to determine when it should charge, how much it
should charge, and when it should release energy.

8. Different Types of Energy Storage Systems
 Mechanical storage systems
The most common mechanical storage systems are pumped hydroelectric power plants
(pumped hydro storage, PHS), compressed air energy storage (CAES) and flywheel
energy storage (FES).
1) Pumped hydro storage (PHS)
The main advantage of this technology is that it is readily available. It uses the
power of water, a highly concentrated renewable energy source. This technology
is currently the most used for high-power applications (a few tens of GWh or 100
of MW).
Pumped storage sub-transmission stations will be essential for the storage of electrical
energy. The principle is generally well known: during periods when demand is low, these
stations use electricity to pump the water from the lower reservoir to the upper reservoir.When
demand is very high, the water flows out of the upper reservoir and activates the turbines to
generate high-value electricity for peak hours.
Pumped hydroelectric systems have a conversion efficiency, from the point of view of a
power network, of about 65–80%, depending on equipment characteristics
Considering the cycle efficiency, 4kWh are needed to generate three. The storage capacity
depends on two parameters: the height of the waterfall and the volume of water.
A mass of 1 ton falling 100m generates 0.272kWh. The main shortcoming of this technology is
the need for a site with different water elevations.

9. 2) Compressed air energy storage (CAES)
CAES relies on relatively mature
technology with several high-power projects
in place.
A power plant with a standard gas turbine
uses nearly two-thirds of the available
power to compress the combustion air. It
therefore seems possible, by separating the
processes in time, to use electrical power
during off-peak hours (storage hours) in
order to compress the air, and then to
produce, during peak hours (retrieval
hours), three times the power for the
same fuel consumption by expanding the air
in a combustion chamber before feeding
it into the turbines. Residual heat from the
smoke is recovered and used to heat the air.
Compressed air energy storage is achieved
at high pressures (40–70 bars), at near
ambient temperatures. This means less
volume and a smaller storage reservoir. Large
caverns made of high-quality rock deep in the ground, ancient salt mines, or underground

10. natural gas storage caves are the best options for compressed air storage, as they benefit
from geostatic pressure, which facilitates the containment of the air mass.
A large number of studies have shown that the air could be compressed and stored in
underground, high pressure piping (20–100 bars). This method would eliminate the
geological criteria and make the system easier to operate.
The energy density for this type of system is in the order of 12 kWh/m3 ,while the
estimated efficiency is around 70% .Let us note that to release 1 kWh into the network,
0.7–0.8 kWh of electricity needs to be absorbed during off-peak hours to compress the air,
as well as 1.22kWh of natural gas during peak hours (retrieval). To improve efficiency and
reduce operation costs, air leaks (self-discharge) must be kept to an absolute minimum.
The first storage station using an underground compressed air reservoir has been in
operation since November 1978 in Huntorf, near Bremen, Germany . In 1991, an American
installation in Macintosh, Alabama, began to deliver 100MW of power for
226 h. The ambient air is compressed and stored at a pressure between 40 and 70 bars in a
2,555,000-m3 cavern, 700m deep in the ground . During summer, the system generates
energy 10 h per day on weekdays. The company using this application partially recharges
the cavern weekday nights and full recharge is done on weekends. The system is in use
1770 h per year .
3) Flywheel energy storage (FES)
Flywheel energy accumulators are comprised
of a massive or composite flywheel coupled
with a motor generator and special brackets
(often magnetic), set inside a housing at very
low pressure to reduce self-discharge losses.
They have a great cycling capacity
(a few 10,000 to a few 100,000 cycles)
determined by fatigue design.
To store energy in an electrical power system,
high-capacity flywheels are needed.
Friction losses of a 200-tons flywheel are
estimated at about 200 kW. Using this
hypothesis and instantaneous efficiency of
85%, the overall efficiency would drop to 78%
after 5 h, and 45% after one day. Long-term
storage with this type of apparatus is therefore
not
foreseeable.
Kinetic energy storage could also be used for the distribution of electricity in urban
areas through large capacity buffer batteries, comparable to water reservoirs, aiming
to maximize the efficiency of the production units. For example, large installations

11. made up of forty 25 kW–25kWh systems are capable of storing 1MW that can be released within
1 h.
 Thermal energy storage (TES)
There are two types of TES systems, depending on whether they use sensible or latent
heat.
Latent-fusion-heat TES makes use of the liquid–solid transition of a material at constant
temperature. During accumulation, the bulk material will shift from the solid state to liquid
and, during retrieval, will transfer back to solid. The heat transfers between the thermal
accumulator and the exterior environment are made through a heat-transfer fluid. The energy
is stored at a given temperature, the higher the heat the higher the concentration; the fusion
enthalpy grows with the fusion temperature of the bulk material used.
Despite its highly corrosive nature, sodium hydroxide is considered to be a good storage fluid.
It has a high fusion temperature, an adequate thermal conductivity coefficient, high-temperature
stability, and a very low steam pressure. Between 120 and 360 1C, it has a specific thermal
storage capacity (mass or volume) of 744 MJ/t, or 1332MJ/m3 .
Setting up a sodium-hydroxide, latent-heat accumulation systems in electric boilers
could help limit demand for electrical power in industrial processes where the needs for
steam are not continuous and vary in intensity.
Sensible heat thermal storage is achieved by heating a bulk material (sodium, molten
salt, pressurized water, etc.) that does not change states during the accumulation phase; the
heat is then recovered to produce water vapor, which drives a turbo-alternator system.
The use of molten salt in the The´mis station in France has made it possible to store heat
economically and simplify the regulation of the solar panel . This system was
designed to store 40,000kWh of thermal energy, equivalent to almost 1 day of average
sunlight, in 550 tones of fused electrolyte .
Using water as storage fluid involves high temperatures, above 200 1C, making it
impossible to store the water is a confined groundwater basin because irreparable damage

12. to the ground would ensue. Very large volume watertight cisterns set in rock are needed .
During off-peak hours, the hot water for storage can be obtained from a thermal plant,
for example, condensation of the high-pressure steam from the boiler , or by
tapping, at lower temperature, from the turbine outlets. Generating extra electricity during
peak hours can be achieved by heating the water supply when retrieving stored energy and
simultaneously reducing turbine outlet. A 5% overpower is obtained by an increase in
steam output through the turbine.
A new technology that has unfortunately not yet been applied is high-temperature,
sensible heat storage with turbine . It consists in heating a refractory material to
1400 1C by electric resistances (high efficiency) during storage and retrieving the
accumulated energy by injecting the air heated by the refractory material into a
combined-cycle turbine . The estimated efficiency of such a system is in the area of 60%.
The system can store very large quantities of energy without major hazards and is not
subject to geological constraints. Losses due to self-discharge are relatively small,
especially for very large systems. For example , a thermal storage reservoir designed for
1000kWh, would only be 20m in diameter and 20m high, for a volume of 5000m3. The
estimated investment costs are considered to be among the lowest, a good reason why this
concept should be developed.
 Electrochemical storage systems
1. Lead acid battery (LA)
Lead acid batteries are the world’s most widely used battery type and have been commercially
deployed since about 1890. Lead acid battery systems are used in both mobile and stationary
applications. Their typical applications are emergency power supply systems, stand-alone
systems with PV, battery systems for mitigation of output fluctuations from wind power and as
starter batteries in vehicles. In the past, early in the “electrification age” (1910 to 1945), many
lead acid batteries were used for storage in grids. Stationary lead acid batteries have to meet far
higher product quality standards than starter batteries. Typical service life is 6 to 15 years with a
cycle life of 1500 cycles at 80 % depth of discharge, and they achieve cycle efficiency levels of
around 80 % to 90 %. Lead acid batteries offer a mature and well-researched technology at low
cost. There are many types of lead acid batteries available, e.g. vented and sealed housing
versions (called valve regulated lead acid batteries, VRLA). Costs for stationary batteries are
currently far higher than for starter batteries. Mass production of lead acid batteries for stationary systems
may lead to a price reduction.
One disadvantage of lead acid batteries is usable capacity decrease when high power is
discharged. For example, if a battery is discharged in one hour, only about 50 % to 70 % of the
rated capacity is available. Other drawbacks are lower energy density and the use of lead, a
hazardous material prohibited or restricted in various jurisdictions.

13. Advantages are a favorable cost/performance ratio, easy recyclability and a simple charging
technology. Current R&D on lead acid batteries is trying to improve their behavior for micro-
hybrid electric vehicles
2. Lithium ion battery (Li-ion)
Lithium is the lightest metal with the highest potential due to its very reactive behavior, which, in
theory, makes it very fitting as a compound for batteries. Just as the lead-acid and most other
batteries the Lithium-Ion battery by definition uses chemical reactions to release electricity.
Although all are called lithium-ion batteries, there’s a variety of types with slightly different
chemical compounds. The construction looks somehow similar to a capacitor, using three
different layers curled up in order to minimize space. The first layer acts as the anode and is
made of a lithium compound; the second one is the cathode and is usually made of graphite.
Between anode and cathode is the third layer – the separator that, as suggested by the name,
separates them while allowing lithium-ions to pass through. The separator can be made of
various compounds allowing different characteristics and with that, different benefits and flaws.
In addition, the three layers are submerged in an organic solvent – the electrolyte, allowing the
ions to move between the anode and the cathode.
In the charging process, the lithium ions pass through the microporous separator into spaces
between the graphite (though not compounded), receiving an electron from the ex-ternal power
source.
During the discharge process the lithium atoms located between the graphite release its electrons
again that migrates over the external circuit to the anode providing a current. The lithium ions
move back to the anode as well, parallel to their released electrons.
Because lithium is a very reactive compound and can burst into flames, safety measures have to
be included, such as onboard control chips to manage the temperature and prevent a complete
discharge.
Lithium-ion batteries would be suitable for storing large amounts of energy if it weren’t for the
costs. The rather expensive processing and the safety measures make them too ex-pensive for
commercial use besides small electronic devices like smart phones and laptops. Even for small

14. decentralized systems, competitors like lead-acid batteries are more cost-effective right now,
although that will change as they become cheaper.
However, lithium-based batteries have an incredibly huge potential. IBM is currently working on
a project called Battery 500. This project’s goal is to develop a battery using lithium and the air
of the atmosphere as components (both the two lightest elements suitable for this purpose),
capable of storing enough energy to power an electric car for 500 miles (~804 kilometers).
Commercial use is targeted somewhere between 2020 and 2030 as there are still a lot of
obstacles to overcome.
3. Sodium Battery
The liquid sodium sulfur battery is yet an-other type
of battery in development, but already operational in
some countries like Japan. About 250 Megawatts
(MW) of sodium battery power have been installed
there.5 Sodium batteries have the advantage of a
relatively high density with up to 240 Wh/kg, a long
life span of 10 – 15 years and high efficiency (75 – 90
percent); but, they need to be operated at high
temperatures (350° C/623° K) to get the sodium
liquid, which not only makes it more difficult and
expensive to operate but also more dangerous as the
liquid sodium reacts easily with the water in the
atmosphere. Since the Nippon Tokusyu Tōgyō
Kabushiki-gaisha Co. LTD (NGK) and the Tokyo
Electric Power Co. LTD (TEPCO) began shipping out
sodium batteries in 2002, three incidents resulting in
fires have occurred, setting the development back.
4. Energy storage using flow batteries (FBES)
Flow batteries are a two-electrolyte system in which the chemical compounds used for
energy storage are in liquid state, in solution with the electrolyte. They overcome the
limitations of standard electrochemical accumulators (lead–acid or nickel–cadmium for
example) in which the electrochemical reactions create solid compounds that are stored
directly on the electrodes on which they form. This is therefore a limited-mass system,
which obviously limits the capacity of standard batteries.
Various types of electrolyte have been developed using bromine as a central element:
with zinc (ZnBr), sodium (NaBr), vanadium (VBr) and, more recently, sodium polysulfide.
The electrochemical reaction through a membrane in the cell can be reversed
(charge–discharge). By using large reservoirs and coupling a large number of cells, large
quantities of energy can be stored and then released by pumping electrolyte into the
reservoirs.
The best example of flow battery was developed in 2003 by Regenesys Technologies,

15. England, with a storage capacity of 15MW–120MWh. It has since been upgraded to an
electrochemical system based entirely on vanadium. The overall electricity storage
efficiency is about 75%
 Chemical storage systems
1) Fuel cells—Hydrogen energy storage
Fuel cells are a means of restoring spent energy to produce hydrogen through water
electrolysis. The storage system proposed includes three key components: electrolysis
which consumes off-peak electricity to produce hydrogen, the fuel cell which uses that
hydrogen and oxygen from air to generate peak-hour electricity, and a hydrogen buffer
tank to ensure adequate resources in
periods of need.
Oxidation-reduction between
hydrogen and oxygen is a
particularly simple reaction
which occurs within a structure
(elementary electrochemical cell)
made up of two
electrodes (anode–cathode) separated
by electrolyte, a medium for the
transfer of charge as ions .
There are many types of fuel cells,
such as: Alkaline Fuel Cell (AFC),
Polymer Exchange

16. Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Phosphoric Acid
Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC).
The basic differences between these types
of batteries are the electrolyte used, their
operating temperature, their design, and
their field of application. Moreover, each
type has specific fuel requirements. Fuel
cells can be used in decentralized
production (particularly low-power
stations—
residential, emergency, etc.), spontaneous
supply related or not to the network, mid-
power cogeneration (a few 100 kW), and
centralized electricity production without
heat
upgrading. They can also represent a
solution for isolated areas where the
installation
of power lines is too difficult or expensive
(mountain locations, etc.).
There are several hydrogen storage modes,
such as: compressed, liquefied, metal
hydride, etc. For station applications,
pressurized tanks with a volume anywhere between
10 -
2 and 10,000m3 are the simplest solution to date. Currently available commercial
cylinders can stand pressures up to 350 bars.
Combining an electrolyzer and a fuel cell for electrical energy storage is a low-efficiency
solution (at best 70% for the electrolyzer and 50% for the fuel cell, and 35% for the
combination). As well, the investment costs are prohibitive and life expectancy is very
limited, especially for power network applications.
2) Synthetic natural gas (SNG)
 Electrical storage systems
1) Superconducting magnetic energy storage (SMES)
Superconducting magnetic energy storage is achieved by inducing DC current into a coil
made of superconducting cables of nearly zero resistance, generally made of niobiumtitane
(NbTi) filaments that operate at very low temperature (-270 1C). The current
increases when charging and decreases during discharge and has to be converted for AC or
DC voltage applications.

17. ARTICLE IN PRESS
One advantage of this storage system is its great instantaneous efficiency, near 95%
for a charge–discharge cycle . Moreover, these systems are capable of discharging the
near totality of the stored energy, as opposed to batteries. They are very useful for
applications requiring continuous operation with a great number of complete charge–
discharge cycles. The fast response time (under 100 ms) of these systems makes them ideal
for regulating network stability (load leveling). Their major shortcoming is the
refrigeration system which, while not a problem in itself is quite costly and makes
operation more complicated.
Massive storage projects (5000–10,000MWh) require very large coils (several 100m in
diameter) that generate enormous electromagnetic forces. They have to be installed
underground to limit infrastructure costs.
2) Energy storage in super capacitors
These components have both the characteristics of
capacitors and electrochemical batteries, except that there
is no chemical reaction, which greatly increases cycling
capacity. Energy storage in super capacitors is done in the
form of an electric field between two electrodes. This is the
same principle as capacitors except that the insulating
material is replaced by electrolyte ionic conductor in which
ion movement is made
along a conducting electrode with a very large specific
surface (carbon percolants grains or polymer conductors) .

18. The energy/volume obtained is superior to that of capacitors (5 Wh/kg or even 15 Wh/kg), at
very high cost but with better discharge time constancy due to the slow
displacement of ions in the electrolyte (power of 800–2000 W/kg). The direct consequence
is that the maximum operational voltage is limited to a few volts per element (2.5–3 V,
modules up to 1500 F). Serial connection, as opposed to capacitors, is required to reach
normal voltages in power applications and form modules with 50–100kW of storage
capacity.
Super capacitors generally are very durable, that is to say 8–10 years, 95% efficiency,
and 5% per day self-discharge, which means that the stored energy must be used
quickly.
Applications on EES Power Systems
 Energy Storage for the Electricity Grid
Energy storage is not a new concept in itself. It has been an integral component of electricity generation,
transmission and distribution systems for well over a century. Traditionally, energy storage needs have
been met by the physical storage of fuels for fossil-fuelled power plants, and by the use of generated
power in pumped hydro storage schemes.
Recently the power landscape has shifted towards greater use of renewable energy in the form of wind
and solar. Although this type of power generation is more sustainable, it makes delivering reliable power
on demand a major challenge.
Wind and solar power installations generate power only intermittently and with a highly variable
output. When the wind is blowing or the sun is shining, excess power should be stored and made
available during suboptimal generating conditions or during peak demand. This requirement has led to
greater demand for alternative energy storage facilities to support the grid.
Such fundamental changes in the architecture and controllability of the grid calls for smart, efficient
power transmission and distribution networks, to make the power supply smoother and more
predictable. These require the storage of energy at appropriate times and locations, both to balance
generation with consumption and to maintain grid stability.
The energy stored in the batteries can also be used in times of peak demand, when more electricity is
required. This means less chance of the grid becoming overloaded and disrupting the power supply.

19.  Energy Storage off the Grid
For people living in areas that cannot access electricity grids, or for those who would prefer to be self-
sufficient, battery energy storage is the best way to go. They can be easily connected to freestanding
solar panels or small wind turbines to provide a reliable electricity supply. They can also be used with
diesel generators.
 Energy Storage for Rooftop Solar Panels
Rooftop solar panels are becoming popular in towns and cities, but changes to tariffs have meant that
solar panels now make less money for the owners. The electricity generated is fed into the grid for very
little return. If a home owner installs their own energy storage, they will be able to use more of the
power generated by their rooftop panels, and save more money on power bills.
 Energy Storage for Hybrid Electric Vehicles (HEVs)
Energy storage can also help reduce our reliance on petrol-driven cars. Hybrid electric vehicles combine
a small petrol engine with battery-driven electric motor, massively reducing the amount of fuel required
for day-to-day driving. Some HEVs also use regenerative braking, in which the electric motor also works
as a generator. When braking, some of the wheels’ kinetic energy is transformed into electrical energy
and stored in the battery. This energy is then available to use when the car accelerates again.
 Energy Storage for Trains
Electric trains can use energy storage batteries to reduce the amount of electricity they take from the
grid. This makes the trains more energy efficient, and also saves money by cutting peak power demand
from the network. Trains can also use regenerative braking to recharge the batteries, in much the same
way as HEVs.