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hybrid vehicles super capacitors
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SEMINAR REPORT
ON
SUPERCAPACITORS FOR HYBRID
ENERGYSTORAGE APPLICATIONS
SUBMITTED BY
V.Naveen Kumar
09711A0249
EEE
CONTENTS
ABSTRACT
INTRODUCTION
REPORT
CONCLUSION
ABSTRACT
Super capacitor also known as electric double-layer
capacitor (EDLC), super condenser, pseudo capacitor, electrochemical
double layer capacitor, or ultracapacitors, is an electrochemical capacitor
with relatively high energy density. Compared to conventional electrolytic
capacitors the energy density is typically on the order of hundreds of times
greater. In comparison with conventional batteries or fuel cells, EDLCs also
have a much higher power density.
In this article the use of super capacitors likes hybrid
power supply for various applications is presented. The main application is
in the field of automation. The specific Power of the super capacitors and its
high lifetime (1 million of Cycles) makes it very attractive for the startup of
the automobiles. Unfortunately, the specific energy of this component is
very low. For that this technology is associated with battery to supply the
starter alternator.
INTRODUCTION
This paper offers a concise review on the use of a super
capacitor in various energy storage applications. Super capacitor is also
known as Electric/electrochemical double layer capacitor (EDLC) is a
unique electrical storage device, which can store much more energy than
conventional capacitors and offer much higher power density than batteries.
Electric double-layer capacitor would have a capacitance
of several farads, an improvement of about two or three orders of magnitude
in capacitance, but usually at a lower working voltage. Larger, commercial
electric double layer capacitors have capacities as high as 5,000 farads.
These particularities make them very attractive for some applications as
vehicle, electric grid, UPS, etc. So, this component can used with battery to
supply the high power needed for the vehicle starting-up and acceleration,
what can reduce the maximum power given by the battery and improves the
lifetime of this last one.
These super capacitors fill up the gap between the batteries
and the conventional capacitor, allowing applications for various power and
energy requirements i.e., back up power sources for electronic devices,
engine start or acceleration for hybrid vehicles.
This paper deals with the the construction and working of
super capacitors and its application in various electronics energy storage
applications and hybrid power supply for the vehicles. For that the battery is
used us energy tank and supercapacitors to ensure the phases which need
high power (startup, acceleration etc.).
REPORT
Super capacitors also known as Electric double-layer
capacitors, or electrochemical double layer capacitors (EDLCs), or
ultracapacitors, are electrochemical capacitors that have an unusually high
energy density when compared to common capacitors, typically on the order
of thousands of times greater than a high capacity electrolytic capacitor. For
instance, a typical electrolytic capacitor will have a capacitance in the range
of tens of millifarads. The same size super capacitor would have a
capacitance of several farads, an improvement of about two or three orders
of magnitude in capacitance but usually at a lower working voltage. Larger,
commercial electric doublelayer capacitors have capacities as high as
5,000farads.
In a conventional capacitor, energy is stored by the
removal of charge carriers, typically electrons, from one metal plate
depositing them on another. This charge separation creates a potential
between the two plates, which can be harnessed in an external circuit. The
total energy stored in this fashion increases with both the amount of charge
stored and the Potential between the plates. The amount of charge stored per
unit voltage is essentially a function of the size, the distance, and the
material properties of the plates and the material in between the plates (the
dielectric), while the potential between the plates is limited by breakdown of
the dielectric. The dielectric controls the capacitor's voltage. Optimizing the
material leads to higher energy density for a given size of capacitor.
EDLCs do not have a conventional dielectric. Rather than
two separate plates separated by an intervening substance, these capacitors
use "plates" that are in fact two layers of the same substrate, and their
electrical properties, the so-called "electrical double layer", result in the
effective separation of charge despite the vanishingly thin (on the order of
nanometers) physical separation of the layers. The lack of need for a bulky
layer of dielectric permits the packing of plates with much larger surface
area into a given size, resulting in high capacitances in practical-sized
packages.
Super capacitor technology is based the electric double
layer phenomenon that has been understood for over a hundred years.
However, it has only been exploited by commercial applications for about
ten years. As in a conventional capacitor, in an ultracapacitor two conductors
and a dielectric generate an electric field where energy is stored. The double
layer is created at a solid electrode-solution interface - it is, then, essentially
a charge separation that occurs at the interface between the solid and the
electrolyte. Two charge layers are formed, with an excess of electrons on
one side and an excess of positive ions on the other side. The polar
molecules that reside in between form the dielectric. In most ultracapacitors,
the electrode is carbon combined with an electrolyte. The layers that form
the capacitor plate's boundaries, as well as the small space between them,
create a very high capacitance. In addition, the structure of the carbon
electrode, which is typically porous, increases the effective surface area to
about 2000 m2/g
In general, electric double-layer capacitors improve storage
density through the use of a nanoporous material, typically activated
charcoal, in place of the conventional insulating barrier. Activated charcoal
is a powder made up of extremely small and very "rough" particles, which in
bulk form a low-density volume of particles with holes between them that
resembles a sponge. The overall surface area of even a thin layer of such a
material is many times greater than a traditional material like aluminum,
allowing many more charge carriers (ions or radicals from the electrolyte) to
be stored in any given volume. The downside is that the charcoal is taking
the place of the improved insulators used in conventional devices, so in
general electric double-layer capacitors use low potentials on the order of 2
to 3 V.
Super capacitor is a double layer capacitor; the
energy is stored by charge transfer at the boundary between electrode and
electrolyte. The amount of stored energy is function of the available
electrode and electrolyte surface, the size of the ions, and the level of the
electrolyte decomposition voltage.
Super capacitors are constituted of two electrodes, a
separator and an electrolyte. The two electrodes, made of activated carbon
provide a high surface area part, defining so energy density of the
component. On the electrodes, current collectors with a high conducting part
assure the interface between the electrodes and the connections of the
supercapacitor. The two electrodes are separated by a membrane, which
allows the mobility of charged ions and forbids no electronic contact. The
electrolyte supplies and conducts the ions from one electrode to the other.
Usually super capacitors are divided into two types:
double-layer capacitors and electrochemical capacitors. The former depends
on the mechanism of double layers, which is result of the separation of
charges at interface between the electrode surface of active carbon or carbon
fiber and electrolytic solution. Its capacitance is proportional to the specific
surface areas of electrode material. The latter depends on fast faraday redox
reaction. The electrochemical capacitors include metal oxide supercapacitors
and conductive polymer supercapacitors. They all make use of the high
reversible redox reaction occurring on electrodes surface or inside them to
produce the capacitance concerning with electrode potential. Capacitance of
them depends mainly on the utilization of active material of electrode.
When metal oxides/ metal oxide and carbon
composite/conducting polymer and carbon composite are used as electrodes
for the construction of EDLCs, the charge storage mechanism includes both
double layer capacitance and pseudo capacitance which result in higher
capacitance output and the EDLCs are termed as supercapacitors (SCs). One
major disadvantage of carbon based EDLC is the lower specific stored
energy.
DIFFERENCES BETWEEN
SUPERCAPACITORS AND BATTERY
Charge Cycles:
Ultracapacitors can be charged and discharged hundreds of
thousands (and millions) of cycles without losing performance. A battery
is only good for a limited amount of charge and discharge cycles. You
probably notice this now with your cell phone or if you have a cordless
phone at the house. The longer you have and more you use the less
effective the battery holds the charge.
Charging Time:
As we know, batteries rely on chemical reactions and take
more time to charge unlike ultracapacitors which charge and discharge
very quickly.
• Size / Weight:
Batteries are larger and heavier where ultracapacitors tend
to be smaller and lighter.
Energy Density:
Typically ultracapacitors hold one fifth to one tenth the
energy of an electrochemical battery. This will be changing though as the
development of ultracapacitors continue.
Energy Release:
Batteries release energy on a slower longer period of time
while capacitors release stored energy very quickly. For an electric
vehicle, this quick burst will give the energy needed for passing other
cars or going up a hill
ADVANTAGES
Long life, with little degradation over hundreds of thousands of
charge cycles. Due to the capacitor's high number of charge-discharge
cycles (millions or more compared to 200 to 1000 for most
commercially available rechargeable batteries) it will last for the
entire lifetime of most devices, which makes the device
environmentally friendly. Rechargeable batteries wear out typically
over a few years, and their highly reactive chemical electrolytes
present a disposal and safety hazard. Battery lifetime can be optimised
by charging only under favorable conditions, at an ideal rate and, for
some chemistries, as infrequently as possible. EDLCs can help in
conjunction with batteries by acting as a charge conditioner, storing
energy from other sources for load balancing purposes and then using
any excess energy to charge the batteries at a suitable time.
Low cost per cycle.
Good reversibility.
Very high rates of charge and discharge.
Extremely low internal resistance (ESR) and consequent high cycle
efficiency (95% or more) and extremely low heating levels.
High output power.
High specific power. According to ITS (Institute of Transportation
Studies, Davis, California) test results, the specific power of electric
double-layer capacitors can exceed 6 kW/kg at 95% efficiency.
Improved safety, no corrosive electrolyte and low toxicity of
materials.
Simple charge methods—no full-charge detection is needed; no
danger of overcharging.
DISADVANTAGES
The amount of energy stored per unit weight is generally lower than
that of an electrochemical battery (3–5 W·h/kg for a standard
ultracapacitor, although 85 W.h/kg has been achieved in the lab as
compared to 30-40 W·h/kg for a lead acid battery), and about
1/1,000th the volumetric energy density of gasoline.
Typical of any capacitor, the voltage varies with the energy stored.
Effective storage and recovery of energy requires complex electronic
control and switching equipment, with consequent energy loss.
Has the highest dielectric absorption of any type of capacitor.
High self-discharge - the rate is considerably higher than that of an
electrochemical battery.
Cells hold low voltages - serial connections are needed to obtain
higher voltages. Voltage balancing is required if capacitors are
connected in series.
Very low internal resistance allows extremely rapid discharge when
shorted, resulting in a shock hazard similar to any other capacitor of
similar voltage and capacitance (generally much higher than
electrochemical cells).
APPLICATIONS
Applications of super capacitors in the field of consumer
electronics and vehicles etc.
Consumer electronics
Automotive application
Consumer electronics
Super capacitors can be used in PC Cards, flash
photography devices in digital cameras, flashlights, portable media players,
solar power calculator, electronic toy, internet equipments and in automated
meter reading, particularly where extremely fast charging is desirable.
In 2007, a cordless electric screwdriver that uses an EDLC for energy
storage was produced. It charges in 90 seconds, retains 85% of the charge
after 3 months, and holds enough charge for about half the screws a
comparable screwdriver with a rechargeable battery will handle. Two LED
flashlights using EDLCs were released in 2009. They charge in 90 seconds.
Cordless electric screwdriver
AUTOMOTIVE APPLICATION
In the automotive domain the supercapacitor applications
are classified in three categories:
Onboard electrical systems:
Electromagnetic valve control, catalysts preheating, brake
actuators, steering.
Micro hybrid:
Integrated starter-generator, electro-hydraulic or mechanical
braking.
Mild hybrid:
Energy storage for the traction assistance.
Strong hybrid:
Energy storage for the traction.
TOPOLOGY OF A SERIES HYBRID SUPPLY FOR
AUTOMOTIVE APPLICATIONS.
Enhanced starting of automobile engines is another
attractive application for double-layer capacitors. Today the energy required
to crank an internal combustion engine, is stored in batteries, either Pb or Ni.
Because of their high internal resistance, which limits the initial peak
current, they have to be oversized. The fast battery discharging and the cold
environmental temperature affect heavily its properties.
The use of supercapacitors like power sources allows us to
reduce considerably the size of the battery which will be used just like an
energy source. On the other hand, the use of the converters ensures a good
control of the starting-up dynamics (control of the current). Two topologies
can be distinguished for the power supply which depends to the association
BATTERY CON 1DC/DC
SUPER CAPACITORS
CON 2DC/AC
STARTR ALTERNATOR
of the two storage components (battery and supercapacitors): in series or
parallel.
For the series topology the two storage systems are putted
in series. In this case, the supercondensator module is charged by the battery
through a chopper (Boost). The starting-up is only assured by the
suspercapacitors through a chopper in the case of the starter and through the
inverter in the case of the starter-alternator. This topology allows us to
reduce the size of the coil which is sized for a current of 20A.
Mild and strong hybrid vehicles
These environmentally friendly drives are based on the combination of an
internal combustion engine with an electric power train. The double-layer
capacitors absorb the kinetic energy from braking and release it later to
accelerate the vehicle. In addition, they cover the energy requirements of
auxiliary electrical power equipment. The duration and magnitude of typical
acceleration and braking events determines the size of the double-layer
capacitor bank. The double-layer capacitors can be also a device to improve
the lifetime of a storage system as they present a high number of
charge/discharge cycles, withstand wide temperature ranges, require little
maintenance, and be placed more optimally for vehicle ergonomics.
Fuel cell vehicles
In the future the combustion engine mechanical energy obtained from the
fuel combustion could be replaced electrical engine supplied by electricity
produced by a fuel cell. The promise of fuel cell technology has had a recent
resurgence due to new advancements not in fuel cells, but in the double-
layer capacitors. Indeed, high power energy storage is required in all types
of fuel cell applications and double-layer capacitors are ideally suited to
provide it. These improvements open up opportunities for the development
of new power train and subsystem architectures utilizing both double-layer
capacitors and fuel cells which can improve performance, efficiency, and
cleanliness in electric and hybrid vehicle technology.
In collaboration with the Paul Scherrer Institute, the Volkswagen group and
other partners, a fuel cell vehicle has been built up with BOOSTCAPs .The
fuel cell, which acts as a primary power source, is sized for the continuous
load requirement. The super capacitor bank, which acts as the secondary
power source, is sized for peak load leveling events such as fuel starting,
acceleration and braking. These short duration events are experienced many
thousands of times throughout the life of the vehicle and require relatively
little energy but substantial power.
GREEN TECHNOLOGY SUPER CAPACITORS
Activated carbon used is unsustainable and expensive.
Biochar is viewed as a green solution to the activated carbon currently
used in super capacitor electrodes. Unlike activated carbon, biochar is the
byproduct of the pyrolysis process used to produce biofuels and it is
nontoxic and will not pollute the soil when it is tossed out. Biochar costs
almost half as much as activated carbon, and is more sustainable because
it reuses the waste from biofuel production, a process with sustainable
intentions to begin with.
CONCLUSION
In this paper the use of super capacitor for various
energy storage applications is described. They would have a capacitance of
several farads, an improvement of about two or three orders of magnitude in
capacitance, but usually at a lower working voltage. The specific Power of
the super capacitors and its lifetime (1 million of Cycles) is very high. These
peculiarities make it very attractive for various energy storage applications
and the startup of the automobiles etc. The power density of super capacitors
makes them very interesting for the applications which need high power
during short time. The use of this component technology allows reducing the
battery size and optimizing the lifetime of the supply.