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LW1DSE > TECH 20.01.18 23:13l 255 Lines 15044 Bytes #999 (0) @ WW
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Ί * Supercapacitors * Ί
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Supercapacitors, also known as ultracapacitors or electrochemical
double layer capacitors (EDLC), 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 D-cell sized electrolytic capacitor will
have a storage capacity measured in microfarads, while the same size super-
capacitor would store several farads, an improvement of about 10,000 times.
Larger commercial supercapacitors have capacities as high as 3,000 farads.
Supercapacitors have a variety of commercial applications, notably in
"energy smoothing" and momentary-load devices. Some of the earliest uses were
motor startup capacitors for large engines in tanks and submarines, and as the
cost has fallen they have started to appear on diesel trucks and railroad
locomotives. More recently they have become a topic of some interest in the
green energy world, where their ability to quickly soak up energy makes them
particularly suitable for regenerative braking applications, whereas batteries
have difficulty in this application due to slow charging times. If the LEES or
EEStor devices can be commercialized, they will make an excellent replacement
for batteries in all-electric cars and plug-in hybrids, as they combine quick
charging, temperature stability and excellent safety properties.
Contents
1) Concept
2) History
3) Technology advantages
4) Transportation applications
4.1) Formula 1 Racing Application
5) Technology
1) Concept
In a conventional capacitor, energy is stored by the removal of charge
carriers, typically electrons, from one metal plate and 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 is a combination of the number of charges stored and the potential
between the plates. The former is essentially a function of size and the
material properties of the plates, while the latter is limited by the
dielectric breakdown between the plates. Various materials can be inserted
between the plates to allow higher voltages to be stored, leading to higher
energy densities for any given size.
In contrast with traditional capacitors, supercapacitors don't have a
conventional dielectric, as such. They are based on a structure that contains
an electrical double layer. In a double layer, the effective thickness of the
"dielectric" is on the order of nanometers (exceedingly thin) and that,
combined with the very large surface area, is responsible for their
extraordinarily high capacitances in practical sizes.
In an electrical double layer, each layer by itself is quite conductive,
but the physics at the interface where the layers are effectively in contact
means that no significant current can flow between the layers. However, the
double layer can withstand only a low voltage, which means that supercaps
rated for higher voltages must be made of matched series-connected individual
supercapacitors, much like series-connected cells in higher-voltage batteries.
In general, supercapacitors improve storage density through the use of a
nanoporous material in place of the conventional insulating barrier, typically
activated charcoal. Activated charcoal is a powder made up of extremely small
and very "rough" particles, in bulk they 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 electrons 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 supercaps
use low potentials on the order of 2 to 3 V. Activated charcoal isn't the
"perfect" material for this application. Free electrons are actually (in
effect) quite large, often larger than the holes left in the charcoal, which
are too small to accept them, limiting the storage. Recent research in
supercapacitors has generally focused on improved materials that offer even
higher usable surface areas. Experimental devices developed at MIT replace the
charcoal with carbon nanotubes, which have similar charge storage capability
as charcoal (which is almost pure carbon) but are mechanically arranged in a
much more regular pattern that exposes a much greater suitable surface area.
Other teams are experimenting with custom materials made of activated
polypyrrole, and even nanotube-impregnated papers.
A completely different approach is being pioneered by EEStor, who claim
to have developed a dramatically improved insulator based on barium titanate
that improves the permissivity of the insulator by several orders of
magnitude, improving energy density not through electron capacity but via much
higher potentials. EEStor claims that their capacitors can operate at
extremely high voltages, on the order of several thousand volts.
In terms of energy density, existing commercial supercapacitors range
around 0.5 to 10 Wh/kg, with the standardized cells available from Maxwell
Technologies rated at 6 Wh/kg. Experimental supercapacitors from the MIT LEES
project have demonstrated densities of 30 Wh/kg and appear to be scalable to
60 Wh/kg in the short term, while EEStor claims their examples will offer
capacities on the order of 200 to 300 Wh/kg. For comparison, a conventional
lead-acid battery is typically 30 to 40 Wh/kg, modern lithium-ion batteries
are about 120 Wh/kg, and in an automobile applications gasoline has a net
calorific value (NCV) of around 12,000 Wh/kg.
Additionally, supercapacitors offer much higher power density than
batteries. Power density combines the energy density with the speed that the
energy can be drawn out of the device. Batteries, which are based on the
movement of charge carriers in a liquid electrolyte, have relatively slow
charge and discharge times. Capacitors, on the other hand, can be charged or
discharged at a rate that is typically limited by current heating the
electrodes. So while existing supercapacitors have energy densities that are
perhaps 1/10 th that of a conventional battery, their power density is
generally ten to one-hundred times as great.
2) History
The supercapacitor effect was first noticed in 1957 by General Electric
engineers experimenting with devices using porous carbon electrode. It was
believed that the energy was stored in the carbon pores and it exhibited
"exceptionally high capacitance, although the mechanism was unknown at that
time.
General Electric didn't immediately follow up on this work, and it was
Standard Oil of Ohio that eventually developed the modern version of the
devices in 1966 after accidentally re-discovering the effect while working on
experimental fuel cell designs. Their cell design used two layers of activated
charcoal separated by a thin porous insulator, and this basic mechanical
design remains the basis of most supercapacitors to this day. Standard Oil
also failed to commercialize their invention, licensing the technology to NEC,
who finally marketed the results as "supercapacitors" in 1978, to provide
backup power for maintaining computer memory. The market expanded slowly for
a time, but starting around the mid-1990s various advances in materials
science and simple development of the existing systems led to rapidly
improving performance and an equally rapid reduction in cost. In 2005, the
ultracapacitor market was between US $272 million and $400 million, depending
on the source. It is rapidly growing, especially in the automotive sector.
Recently, all solid state micron-sized supercapacitors based on advanced
superionic conductors had been recognized as critical electron component of
future sub-voltage and deep-sub-voltage nanoelectronics and related
technologies (22 nm technological node of CMOS and beyond).
3) Technology advantages
Due to the capacitor's high number of charge-discharge cycles (millions
or more compared to 200-1000 for most commercially available rechargeable
batteries) there were no disposable parts during the whole operating life of
the device, which makes the device environmentally friendly. Batteries wear
out on the order of a few years, and their highly reactive chemical
electrolytes represent a serious disposal and safety hazard. This can be
improved by only charging under favorable conditions, charging at an ideal
rate and as rarely as possible. Supercapacitors can help in this regard,
acting as a charge conditioner, storing energy from other sources for load
balancing purposes and then using any excess energy to charge the batteries
only at opportune times.
Other advantages of supercapacitors compared with rechargeable batteries
are extremely low internal resistance or ESR (Equivalent series resistance),
high efficiency (up to 97-98%), high output power, extremely low heating
levels, and improved safety. According to ITS (Institute of Transportation
Studies, Davis, CA) test results, the specific power of supercapacitors can
exceed 6 kW/kg at 95% efficiency. The idea of replacing batteries with
capacitors in conjunction with novel alternative energy sources became a
conceptual umbrella of the Green Electricity (GEL) Initiative, introduced by
Dr. Alexander Bell. One particular successful implementation of the GEL
Initiative concept was a muscle-driven autonomous solution which employs a
multi-farad supercapacitor (hecto- and kilofarad range capacitors are now
available) as an intermediate energy storage to power a variety of portable
electrical and electronic devices such as MP3 players, AM/FM radios,
flashlights, cell phones, and emergency kits. As the energy density of
supercapacitors is bridging the gap with batteries, it is hoped that in the
near future the automotive industry will start to deploy ultracapacitors as a
replacement for chemical batteries.
4) Transportation applications
China is experimenting with a new form of electric bus (capabus) that
runs without powerlines using power stored in large onboard supercapacitors,
which are quickly recharged whenever the electric bus stops at any bus stop
(under so-called electric umbrellas), and fully charged in the terminus. A
few prototypes were being tested in Shanghai in early 2005. In 2006, two
commercial bus routes began to use supercapacitor buses; one of them is route
11 in Shanghai.
In 2001 and 2002, VAG, the public transport operator in Nuremberg,
Germany tested a bus which used a diesel-electric drive system with supercaps.
Since 2003 Mannheim Stadtbahn in Mannheim, Germany has ope.......
(light-rail vehicle) which uses supercapacitors.
Other companies from the public transport manufacturing sector are
developing supercapacitor technology: The Transportation Systems division of
Siemens AG is developing a mobile energy storage based on double-layer
capacitors called Sibac Energy Storage and also Sitras SES, a stationary
version integrated into the trackside power supply. The company Cegelec is
also developing a supercapacitor-based energy storage system.
Proton Power Systems has created the world's first triple hybrid
Forklift Truck, which uses fuel cells and battery as primary energy storage
and supercapacitors to supplement this overall energy efficient storage
solution.
5) Formula 1 Racing Application
The FIA proposed on May 23, in the Power-Train Regulation Framework for
Formula 1, version 1.3, that a new set of power train regulations be issued
that includes a hybrid drive of up to 200 kW input and output power,
involving both batteries and supercapacitors.
6) Technology
Carbon nanotubes and certain conductive polymers, or carbon aerogels,
are practical for supercapacitors: Carbon nanotubes have excellent
nanoporosity properties, allowing tiny spaces for the polymer to sit in the
tube and act as a dielectric. MIT's Laboratory of Electromagnetic and
Electronic Systems (LEES) is researching using carbon nanotubes.
Some polymers (eg. polyacenes) have a redox (reduction-oxidation)
storage mechanism along with a high surface area. Supercapacitors are also
being made of carbon aerogel. This is a unique material providing extremely
high surface area of about 400-1000 mύ/g. The electrodes of aerogel
supercapacitors are usually made of non-woven paper made from carbon fibers
and coated with organic aerogel, which then undergoes pyrolysis. The paper is
a composite material where the carbon fibers provide structural integrity and
the aerogel provides the required large surface. Small aerogel supercapacitors
are being used as backup electricity storage in microelectronics, but
applications for electric vehicles are expected.
In August 2007, a research team at RPI (Rensselaer Polytechnic
Institute) developed a paper battery with aligned carbon nanotubes, designed
to function as both a lithium-ion battery and a supercapacitor (called
bacitor), using an ionic liquid, essentially a liquid salt, as the
electrolyte. The sheets can be rolled, twisted, folded, or cut into numerous
shapes with no loss of integrity or efficiency, or stacked, like printer
paper (or a Voltaic pile), to boost total output. As well, they can be made
in a variety of sizes, from postage stamp to broadsheet. Their light weight
and low cost make them attractive for portable electronics, aircraft,
automobiles, and toys (such as model aircraft, while their ability to use
electrolytes in blood make them potentially useful for medical devices such
as pacemakers. In addition, they are biodegradable.
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Ί Compilled from various sources in the Intenet. Translatted to ASCII by Ί
Ί LW1DSE Osvaldo F. Zappacosta. Banfield (1832), Buenos Aires, Argentina. Ί
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Ί January 15, 2008 Ί
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