Audit Course notes

Notes 10 Pages

Savitribai Phule Pune University

BE Computer Science

Contributed by

Darshan Salunke

Dr. D.Y. Patil Institute of Technology
Pimpri, Pune - 411 018

Department of Electrical Engineering

Subject: Audit Course

Name:___________________________________________
Class:___________________________________________
Roll No:_______________
Name of Audit Course Selected: Energy Storage Systems

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Energy Storage Technologies
I. Options for energy storage
Because solar energy supply is variable in time, energy storage is an important issue. Energy storage is
used to collect the energy generated by the solar conversion systems (thermal or photovoltaic) in order
to release it later on demand. This can be a situation when sufficient power is produced during the day,
and stored energy is used during the night. Also, when insolation conditions are ideal, the solar system
may produce enough power for the target application, but on dull days, direct energy supply from
collectors is diminished, and the energy from the storage is used to compensate the deficit. Energy storage
devices help to smooth out differences and minor fluctuations in energy supply caused by shading,
passing clouds, etc. Development of efficient and cost-effective energy storage is considered the main
bottleneck of the universal development of solar systems.
There are quite a few different technology options for energy storage, which are briefly outlined below:
1) Grid. For grid connected solar systems, the most natural and cost-effective way would be to store
energy in the grid. The main idea here is that the DC power from a solar facility (array or farm) is
converted to AC power and is fed to the grid and further on is used for on-site or off-site applications.
This way the grid acts as the medium that collects energy from different power-making facilities
(renewable or non-renewable) and redistributes it as necessary. Since a grid does not really represent
a separate system that is part of a solar plant, it will not be discussed further in this lesson.
2) Fluid. Fluid-based storages are typically used with solar thermal systems. Unlike grid which stores
electrical energy, fluids store thermal energy. Fluids, such as water, oil, molten salt or others act as a
medium for absorbing heat. The main idea is that the solar radiation heats the heat-transfer fluid
which is accumulated in the tank. The tank is insulated, so the hot fluid keeps its temperature for a
substantial period of time. When needed, the heated fluid is used in a heat-exchanger to produce
steam for the electric generator. This type of thermal energy storage was discussed in more detail in
Lesson 8.
3) Battery. A battery is an electrochemical device that stores chemical energy in internal components
and releases energy as electricity, which is generated through electrochemical reactions. Batteries are
reversible, i.e., can be charged and discharged, and the parameters of these processes are regulated
to avoid damage by overcharging or over-discharging. Battery life is expressed in number of charge-
discharge cycles. There are many different types of batteries, some of which will be discussed further
in this lesson.
4) Hydrogen. The idea behind hydrogen storage is that electricity generated by solar PV systems can be
used to electrolyze water - to split it to hydrogen and oxygen. Further, hydrogen gas is collected and
can be used as a fuel. One of highly efficient devices "converting" hydrogen back to electricity is
H
2
/O
2
fuel cell, which has zero carbon footprint during operation.
5) Compressed air. In this case, the electrical energy produced by a PV solar system is used to run
compressors to compress massive amounts of air and store it in underground, above-ground, or
underwater containers. Later on, when energy is needed, the air is de-compressed and is supplied to
a turbine to generate electricity. Compressed air energy systems (CAES) promise high efficiencies,
although this technology is not yet widely implemented.
6) Pumped storage hydropower. The available energy can be used to pump water into an elevated
reservoir for storage. When power is needed, the water can be discharged under gravity to run a
turbine, which is connected to a generator to produce electricity. The same as compressed air systems,
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the pumped storage technology has high energy return on investment, although it may require special
topographical conditions and water availability in order to be used.
All of the above options for energy storage should be employed with understanding the facility needs
and capacity. What energy storage is efficient for small residential systems may be insufficient or too
costly when scaled up to the utility-size systems. Determining capacity of energy storage for a particular
solar project is an important technical and economic issue. For example, if the capacity of the storage is
too large compared to the energy produced by the solar conversion facility, the total system cost will be
unnecessarily increased. On the contrary, if the capacity of the storage is too small, that leads to energy
dumping and overall unsatisfactory plant performance.
II.Battery storage
Batteries are commonly used to store electric energy generated by off-grid renewable energy systems
and also to mitigate the sharp fluctuations of power for on-grid systems. While there are many different
types of battery technologies, some are more applicable to utility scale energy storage than others.
Applicability to large systems depends on such factors as cost of materials, ability to scale up with no ill
effects or performance loss, and design and operation mode. Some well-known examples of battery types
used as stationary storage system for PV solar are listed in Table.
Table 1: Battery types used as stationary storage system for PV solar
Technology
(battery type)
Power subsystem
cost $/kW
Energy storage
subsystem cost $/kW
Charge-discharge
efficiency %
Cycles
Advanced lead-acid
400
330
80
2,000
Sodium/sulphur
350
350
75
3,000
Lead-acid with
carbon enhanced
electrodes
400
330
75
20,000
Zinc/bromine
400
400
70
3,000
Vanadium redox
400
600
65
5,000
Li-ion (large)
400
600
85
4,000
Flywheels (high
speed composite)
600
1,600
95
25,000
Super capacitors
500
10,000
95
25,000
Note: The costs in the table are based on standard assumptions for the applications and technologies
considered, and on expert opinion. They are meant to be used for comparative purposes. The actual costs
of any storage system depend on many factors and the assumptions and the means of calculating some
of the values are subjective and continue to be debated, even among experts in the field
For quite a while, lead-acid batteries have been the first choice for off-grid PV applications. This
lead-acid battery technology has been around since the 19th century and, historically, service providers
have more knowledge and tools to deal with those systems. But, despite their long existence and
widespread use, lead-acid batteries remain one of the lowest energy-to-weight and energy-to-volume
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battery designs, which means they are too big and heavy for the amount of energy they provide. This
technology is inexpensive and reliable, and it may be a while before it is replaced by more advanced
types on a wide scale.
A) Li-ion battery technology
Li-ion battery is one of the rapidly advancing technologies preferred for employment in conjunction
with solar systems due to high storage capacity, high charging rates, light weight, and relatively long
service life. However, the technology cost is still high and can be a limitation on the utility scale. Some
of the very attractive features of Li-ion batteries are high power output and high charge-discharge
efficiency. They can also withstand more charge-discharge cycles than lead-acid batteries. The principle
of operation of the Li-ion battery is discussed below.
A schematic representation of a generic Li-ion battery is given in Figure 1. Roughly, Li-ion cell
consists of three layers: electrode 1 (cathode) plate (usually lithium cobalt oxide), electrode 2 (anode)
plate (usually carbon), and a separator. The electrodes inside the battery are submerged in an electrolyte,
which provides for Li+ ion transfer between the anode and cathode. The electrolyte is typically a lithium
salt in an organic solvent.

Figure 1. Li-ion battery system and charge transfer processes
During the charging process, a DC current is used to withdraw Li
+
ions from the cathode and to
partially oxidize the cathode compound: LiCoO
2
→ Li
1-
xCoO
2
+ xLi
+
+ xe
–

The released Li
+
ions migrate through electrolyte towards the anode, where they become absorbed in the
porous carbon structure: xLi
+
+ xe
–
+ C
6
→ LixC
6

At the same time, electrons travel through the external circuit (electrolyte is not electron
conductive). During the battery discharge, the reverse process takes place. Li
+
ions spontaneously return
to the cathode, where electrochemical reduction occurs.
Limitations of the Li-ion batteries are rooted in the material properties. For example,
the LiCoO
2
⇔ Li
1-
x
CoO
2
conversion is only reversible with x < 0.5, which limits the depth of the
charge-discharge cycle. But with a wider variety of materials available, research is underway to develop
new generations of Li-ion batteries.

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Advantages
Limitations
1. Relatively high energy density and potential
of finding even better formulations.
1. Circuit protection needed to avoid damaging high
voltage / current.
2. No need for priming - new battery is ready
to operate.
2. Aging - battery gradually loses its capacity even if
not in use.
3. Low self-discharge (compared to other types
of batteries).
3. Toxic chemicals are subject to regulations.
4. Low maintenance.
4. High cost of materials and manufacturing process.
5. Capability to generate high current / power.
5. Technology is not fully mature; varying
components and chemicals.
B) Flow Batteries
Flow batteries, unlike solid-state batteries, have their chemical components dissolved in liquid
solutions, which can be pumped through the electrodes in a flow. If you are familiar with the concept of
fuel cell, it is something similar in principle of operation, although it is still a closed loop system. A flow
battery cell itself can be small, while the solutions can be contained in external storages. One of the
advantages of the flow batteries is almost instant replacement of the electrolyte liquid, thus eliminating
any gradient or concentration fluctuations at the electrodes. The main difference between the
conventional batteries and flow batteries is that the energy is typically stored in the liquid phase in flow
batteries. So, increasing the size of the storage tanks for the liquids allows easy scale-up of the battery to
match a specific application.
i) Zinc-bromine flow battery storage
Zinc-bromine battery is a type of hybrid flow battery. It uses zinc bromine as the working
solution, which is stored in two compartments, separated by a porous membrane. One compartment has
a negative zinc electrode and the other compartment has a positive bromide electrode. During charge,
supplied electricity (e.g., from a solar conversion system) is used to electroplate metallic zinc (Zn) on
the negative electrode, while bromine (Br
2
) is generated on the positive electrode. During discharge, the
opposite process occurs: Zn is dissolved to form Zn
2+
ions in solutions, and bromine is converted back
to bromide ions (Br
-
).
Here are the electrochemical reactions involved in this process:
Zn
2+
+ 2e
-
→ Zn(s) - Reduction of zinc during battery charging
2Br
-
→ Br
2
(aq) + 2e
-
- Oxidation of bromine during battery charging
The overall reaction is therefore: Zn
2+
+ 2Br
-
⇔ Zn(s) + Br
2
(aq)
This reaction proceeds to the right on charging and to the left on discharging. The standard
electrode potential for the overall reaction is 1.85 V, which is the maximum theoretical voltage that can
be expected from a single cell. The battery cells are stacked to increase the overall storage capacity of
the system.
The battery compartments are made of inert plastic. Unlike common batteries, which store
electrolyte within the reaction chamber, zinc-bromine batteries have solution storage in the external
tanks, from where it is circulated through the electrodes (flow battery type). The external bromide
solution storage also helps maintaining required concentration of bromide throughout the reaction cycle.
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This technology has been commercialized by ZBB EnerStore company, which engineered zinc-
bromine batteries into 50 kWh modules scalable up to bigger storage systems. Each module is a stand-
alone system that includes all necessary software and hardware. Some advantages of this technology
include high energy density (75-85 Wh/kg), stability, i.e., good resistance to performance degradation,
ability to operate at full output within a wide temperature range. Unlike most batteries, ZBB EnerStore
batteries use non-reacting electrodes (i.e., electrodes are not reactants, but simply are substrates for
reactions to take place), which helps minimize loss of performance from repeated cycling.
ii) Vanadium Redox Flow Batteries
This type of battery utilizes the multiple redox states of vanadium (V) in its charge-discharge
cycles. Vanadium is present in the dissolved form in the sulfuric acid medium, and because it is all-
vanadium system, this type of battery is not susceptible to performance loss due to cross contamination.
During charging, the following half-reactions occur in two separate compartments of the battery:
V
3+
+ e– → V
2+

VO
2+
+ H
2
O → VO
2
+
+2H
+
+ e
–

Electrons are supplied from the solar energy conversion system as DC current onto non-reacting
electrode dipped in the V
3+
solution. As a result V
3+
is reduced to V
2+
. At the same time in the other
compartment, vanadium (IV) species VO
2+
is oxidized to vanadium (V) species VO
2
+
, releasing the
electron. On discharging, these reactions are reversed.
The summary process is expressed through the following reaction:
VO
2+
+ V
3+
+ H
2
O ⇔ V
2+
+ VO
2
+
+ 2H
+

The total voltage generated by a single vanadium redox flow battery is around 1.25 V in ideal case.
The main benefits of the vanadium redox flow batteries are ability to go through "unlimited"
number of cycles; they have a long lifespan (>20 years), quick charging, and high efficiency of the
charge-discharge cycle (~80%). They are also more environmentally friendly in terms of component
toxicity than many other types of batteries.
The vanadium redox flow battery technology is potentially suitable for extra-large utility scale
applications. For example, the 200 MW VRB battery facility in Dalian, China, is expected to significantly
increase the stability of the electric grid by supplying power during peak hours and emergency black-
starts. Development of such a mega facility was enabled by its co-location with the VFB cell
manufacturing factory, which is tapping into local vanadium resources. The Dalian battery is expected
to become operational in 2020. Nearby wind power facilities have been forced to curtail electricity
production – this battery facility hopes to reduce curtailing significantly.
C) Compressed Air Storage
Compressed air storage technology may become an efficient solution of storing energy generated by
large solar plants. Air is used as the energy transfer medium. During the daytime, solar power is used to
heat and compress air in an airtight chamber. When energy is needed, that compressed air can be
expanded through a turbine or another expansion device to drive a generator to create electricity.
Compressed Air Energy Systems (CAES) have been in use in some conventional power plants, and they
are making a come-back as energy storage systems for renewable energy plants.
Traditionally CAES technology used underground geological formations, such as salt caverns, as
reservoirs for compressed air. While this approach was effective at some locations, it was not universal,
as geology in some areas may be just unsuitable. A newer approach with CAES is to use human-made
chambers - large pipes, such as those used for natural gas pipelines. While it involves more construction
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and installation, this type of artificial storage can be employed virtually anywhere and scaled up to the
required capacity by simply using longer pipes.
D) Pumped Hydro Energy Storage
Pumped-storage hydropower (PSH) is the type of storage technology that is based on storing
energy in the form of potential energy of water. It consists of two water reservoirs at placed at different
elevations connected by discharge channel. The available energy can be used to pump water to the upper
reservoir (recharge phase), and energy is released when water moves back down to the lower reservoir
through turbine (discharge).
Closed loop PSH storage does not need to be connected to an outside natural body of water, and
all the water is re-circulated.
This storage technology is not new. The first commercial systems employed for storage were
implemented in 1970s, and the design changed very little since then. According to U.S. DOE, pumped-
storage currently accounts for 95% of all utility-scale energy storage in the United States. However,
additional investments are considered in innovative pumped storage technologies to explore its potential
for storing non-dispatchable renewable power generated from utility scale wind and solar farms and
improving grid resiliency and reliability.
E) Hydrogen storage
Hydrogen (H
2
) is a common industrially used chemical and fuel, which can be obtained from
water by electrolysis or by reforming of natural gas. Electrolysis is of special interest in the energy storage
context, since it converts electric energy into something storable. The process of electrolysis involves
passing electric current through water or another aqueous solution, which initiates the electrochemical
reaction: H
2
O ⇔ H
2
+ 1/2O
2

The basic idea is that the electricity generated by solar PV systems during daytime can be used
to run electrolyzers to split water into hydrogen and oxygen gases. Hydrogen is collected and stored in
one or another form. When energy is needed, hydrogen can be used for combustion or for electrochemical
conversion (in a fuel cell) to recover energy as heat or electricity. Hydrogen provides a new form of
energy economy, which complies with the present-day environmental requirements. For instance,
hydrogen combustion does not result in any carbon emissions, and water and heat are the only products.
Electrochemical utilization of hydrogen in fuel cells is thermodynamically efficient and environmentally
benign. Fuel cells can be used for both stationary power generation and transportation. Unlike other
forms of energy storage, hydrogen can be transported and used at a different location.
There are a few advantages of the hydrogen energy storage in solar plants:
1. Hydrogen generation by electrolysis is a well-established technology. Hydrogen is used in multiple
branches of industry, so the procedures for its handling are well developed.
2. Water electrolysis uses low-voltage DC current, which is compatible with the output from the PV
cells.
3. Hydrogen can be stored with minimal losses.
4. Hydrogen can have multiple uses - electricity generation, heat and power generation.
5. Hydrogen is environmentally benign substance, its combustion does not produce harmful emissions,
it is volatile and is easily dissipated.
6. There are multiple ways of hydrogen transportation (pipelines, tanks, hydride compounds).
7. Existing infrastructure for natural gas can be adapted to hydrogen use.

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F) Flywheel Energy Storage Systems (FESS)
Flywheel energy storage systems (FESS) use electric energy input which is stored in the form of
kinetic energy. Kinetic energy can be described as “energy of motion,” in this case the motion of a
spinning mass, called a rotor. The rotor spins in a nearly frictionless enclosure. When short-term backup
power is required because utility power fluctuates or is lost, the inertia allows the rotor to continue
spinning and the resulting kinetic energy is converted to electricity. Most modern high-speed flywheel
energy storage systems consist of a massive rotating cylinder (a rim attached to a shaft) that is supported
on a stator – the stationary part of an electric generator – by magnetically levitated bearings. To maintain
efficiency, the flywheel system is operated in a vacuum to reduce drag. The flywheel is connected to a
motor-generator that interacts with the utility grid through advanced power electronics.
Some of the key advantages of flywheel energy storage are low maintenance, long life (some
flywheels are capable of well over 100,000 full depth of discharge cycles and the newest configurations
are capable of even more than that, greater than 175,000 full depth of discharge cycles), and negligible
environmental impact. Flywheels can bridge the gap between short-term ride-through power and long-
term energy storage with excellent cyclic and load following characteristics.
Typically, users of high-speed flywheels must choose between two types of rims: solid steel or
carbon composite. The choice of rim material will determine the system cost, weight, size, and
performance. Composite rims are both lighter and stronger than steel, which means that they can achieve
much higher rotational speeds. The amount of energy that can be stored in a flywheel is a function of the
square of the RPM making higher rotational speeds desirable. Currently, high-power flywheels are used
in many aerospace and UPS applications. Today 2 kW/6 kWh systems are being used in
telecommunications applications. For utility-scale storage a ‘flywheel farm’ approach can be used to
store megawatts of electricity for applications needing minutes of discharge duration.
How Flywheel Energy Storage Systems Work
Flywheel energy storage systems (FESS) employ kinetic energy stored in a rotating mass with
very low frictional losses. Electric energy input accelerates the mass to speed via an integrated motor-
generator. The energy is discharged by drawing down the kinetic energy using the same motor-generator.
The amount of energy that can be stored is proportional to the object’s moment of inertia times the square
of its angular velocity. To optimize the energy-to-mass ratio, the flywheel must spin at the maximum
possible speed. Rapidly rotating objects are subject to significant centrifugal forces however, while dense
materials can store more energy, they are also subject to higher centrifugal force and thus may be more
prone to failure at lower rotational speeds than low-density materials. Therefore, tensile strength is more
important than the density of the material. Low-speed flywheels are built with steel and rotate at rates up
to 10,000 PRM.
More advanced FESS achieve attractive energy density, high efficiency and low standby losses
(over periods of many minutes to several hours) by employing four key features: 1) rotating mass made
of fiber glass resins or polymer materials with a high strength-to-weight ratio, 2) a mass that operates in
a vacuum to minimize aerodynamic drag, 3) mass that rotates at high frequency, and 4) air or magnetic
suppression bearing technology to accommodate high rotational speed. Advanced FESS operate at a
rotational frequency in excess of 100,000 RPM with tip speeds in excess of 1000 m/s. FESS are best
used for high power, low energy applications that require many cycles.
Additionally, they have several advantages over chemical energy storage. They have high energy
density and substantial durability which allows them to be cycled frequently with no impact to
performance. They also have very fast response and ramp rates. In fact, they can go from full discharge
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to full charge within a few seconds or less. Flywheel energy storage systems (FESS) are increasingly
important to high power, relatively low energy applications. They are especially attractive for
applications requiring frequent cycling given that they incur limited life reduction if used extensively
(i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycle).
FESS are especially well-suited to several applications including electric service power quality
and reliability, ride-through while gen-sets start-up for longer term backup, area regulation, fast area
regulation and frequency response. FESS may also be valuable as a subsystem in hybrid vehicles that
stop and start frequently as a component of track-side or on-board regenerative braking systems.
G) Supercapacitors
Supercapacitors are a type of energy storage device that is superior to both batteries and regular
capacitors. They have a greater capacity for energy storage than traditional capacitors and can deliver it
at a higher power output in contrast to batteries. These characteristics, together with their long-term
stability and high cyclability, make supercapacitors an excellent energy storage device. These are
currently deployed in a variety of applications, either in conjunction with other energy storage devices
(mostly batteries) or as self-contained energy sources. Owing to their high conductivity and surface area,
porous carbons are being employed in the electrodes of commercial supercapacitors.
Types of supercapacitors
A supercapacitor's primary role is to accumulate energy via the spread of charged ions in the
electrolyte on the electrode surfaces. The first type of supercapacitor, the electrical double layer
capacitor, supports the reversible electrostatic buildup of ions on the surface of a porous electrode. This
category includes carbon compounds with a large surface area. Second, the pseudocapacitor category
includes the reversible redox reaction with electrolyte ions, which ions react with surface functional
moieties. Pseudocapacitive energy storage is made up of a few oxide compounds of transition metals like
manganese and ruthenium, conducting polymers and hetero-atom-doped carbon compounds.
The third form, a hybrid capacitor, is essentially a mixture of a faradaic battery-type electrode
and a non-faradaic electrical double layer capacitor-type electrode. The faradaic battery electrode is made
up of sulfides, transition metal oxides and phosphides, among other materials. Moreover, battery-type
electrodes are frequently confused with pseudocapacitive electrodes, despite the fact that their charge
storage mechanisms are fundamentally different. Pseudocapacitive charge storage is achieved using a
highly reversible surface redox reaction that does not require phase transition. On the contrary, battery-
type electrodes store charge via a reversible faradaic process in which the charged and discharged
electrodes undergo a phase transition.
Pros and cons of supercapacitors
As a novel kind of energy storage, the supercapacitor offers the following advantages:
1. Durable cycle life. Supercapacitor energy storage is a highly reversible technology.
2. Capable of delivering a high current. A supercapacitor has an extremely low equivalent series
resistance (ESR), which enables it to supply and absorb large amounts of current.
3. Extremely efficient. The supercapacitor is an extremely energy-efficient component. When charging
and discharging them, very little charge is lost.
4. Temperature range is extensive. The temperature range of a supercapacitor is far wider than that of a
battery, ranging from -40° C to 70° C.
5. State of charge is effortlessly monitored. Calculating the state of charge of a battery is critical for
battery system design, involving sophisticated data collecting and complex algorithms. In comparison,
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determining the state of charge of supercapacitors is quite straightforward, as the energy stored is
dependent on only capacitance and voltage, with capacitance being a constant factor.
6. Voltage range is extensive. The supercapacitor is capable of running at any voltage less than its
continuous operating voltage.
The key drawback of a supercapacitor is that its cell voltage is quite low. Multiple cells are
connected in series to achieve larger voltages, which may create further issues. However, such problems
can be resolved by load leveling and voltage balance technologies.
Applications of supercapacitors
Supercapacitors are used as standalone energy sources or in conjunction with other devices. The
following are some of the devices that are making use of supercapacitors:
• Portable devices. Supercapacitors are employed as an energy source in portable screwdrivers and
camera flashes, as they require only bursts of energy and speedy and continuous recharging.
• Memory backups. Supercapacitors with a rapid response time provide a temporary solution to a
momentary interruption of the memory's power source.
• Decoupling of energy and power requirements. Supercapacitors are employed to meet energy
requirements while a different system provides the primary source of energy. Decoupling of this type is
used in hybrid and electric automobiles.
• Regeneration devices. Supercapacitors can recover energy released by machines that perform repetitive
and steady movements. They are found in a variety of applications, including elevators and cranes, as
well as in the braking systems of electric or hybrid vehicles such as buses, trains, and delivery or garbage
trucks.
• Electrical energy distribution and storage. Supercapacitors are capable of compensating for short-
duration fluctuations in voltage signals in the distribution line and for bridging the gap between
production and consumption of electricity. Additionally, supercapacitors are utilized to restart power
systems that have failed or to provide energy until the original source is reinstated. Supercapacitors are
also employed as energy storage devices in renewable generation plants, most notably wind energy, due
to their low maintenance requirements.
Superconducting magnetic energy storage
Superconducting magnetic energy storage (SMES) is the only energy storage technology that
stores electric current. This flowing current generates a magnetic field, which is the means of energy
storage. The current continues to loop continuously until it is needed and discharged. The
superconducting coil must be super cooled to a temperature below the material's superconducting critical
temperature that is in the range of 4.5 – 80K (-269 to -193°C).[1] The direct current that flows through
the superconducting material experiences very little resistance so the only significant losses are
associated with keeping the coils cool.
The storage capacity of SMES is the product of the self-inductance of the coil and the square of
the current flowing through it. The maximum current that can flow through the superconductor is
dependent on the temperature, making the cooling system very important to the energy storage capacity.
The cooling systems usually use liquid nitrogen or helium to keep the materials in a superconductor state.
Applications of SMES
SMES is a specific technology with applications that can be applied to transmission networks on the
electrical grid. They have been commercially installed for several large industrial users.[2] Although
SMES systems are very expensive, they are extremely efficient, have almost instantaneous charge and
discharge, are easily scale-able, and have little environmental impact.
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