utra capacitor

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From Wikipedia, the free encyclopedia This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2010) Maxwell Technologies "MC" and "BC" series super capacitors (up to 3000 farad capacitance) ultracapacitor, is an electrochemical capacitor with relatively high energy density . Their energy density is typically hundreds of times greater than conventional electrolytic capacitors . [1] . A typical D-cell -sized electrolytic capacitor may have capacitance of up to tens of millifarads . The same size EDLC might reach several farads , an improvement of two orders of magnitude . As of 2011 EDLCs had a maximum working voltage of a few volts (standard electrolytics can work at hundreds of volts) and capacities of up to 5,000 farads. [2] The amount of energy stored per unit of mass is called specific energy , which is often measured in watt-hours per kilogram (Wh/kg) or megajoules per kilogram (MJ/kg). In 2010 the highest available EDLC specific

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Page 1: Utra Capacitor

From Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please help improve this article by

adding citations to reliable sources. Unsourced material may be challenged and removed.

(December 2010)

Maxwell Technologies "MC" and "BC" series super capacitors (up to 3000 farad capacitance)

ultracapacitor, is an electrochemical capacitor with relatively high energy density. Their energy

density is typically hundreds of times greater than conventional electrolytic capacitors.[1].

A typical D-cell-sized electrolytic capacitor may have capacitance of up to tens of millifarads.

The same size EDLC might reach several farads, an improvement of two orders of magnitude.

As of 2011 EDLCs had a maximum working voltage of a few volts (standard electrolytics can

work at hundreds of volts) and capacities of up to 5,000 farads.[2] The amount of energy stored

per unit of mass is called specific energy, which is often measured in watt-hours per kilogram

(W⋅h/kg) or megajoules per kilogram (MJ/kg). In 2010 the highest available EDLC specific

energy was 30 W⋅h/kg (0.1 MJ/kg).[3] Up to 85 W⋅h/kg has been achieved at room temperature in

the lab,[4] which is still lower than rapid-charging lithium-titanate batteries.[5]

Much research is being carried out to improve performance; for example an order of magnitude

energy density improvement was achieved in the laboratory in mid-2011.[6] Prices are dropping: a

3 kF capacitor that cost US$5,000 in 2000 cost $50 in 2011.

EDLCs are used for energy storage rather than as general-purpose circuit components. They have

a variety of commercial applications, notably in "energy smoothing" and momentary-load

devices. They have applications as energy-storage and KERS devices used in vehicles, and for

Page 2: Utra Capacitor

smaller applications like home solar energy systems where extremely fast charging is a valuable

feature

Concept

Comparison of construction diagrams of three capacitors. Left: "normal" capacitor, middle:

electrolytic, right: electric double-layer capacitor

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 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 the breakdown field strength 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 insulator, these capacitors use virtual plates that are in fact two layers of the same

substrate. Their electrochemical 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, and the porosity of the

Page 3: Utra Capacitor

material used, permits the packing of plates with much larger surface area into a given volume,

resulting in high capacitances in practical-sized packages.

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 electric double-layer capacitors rated for higher voltages must be made of matched series-

connected individual EDLCs, much like series-connected cells in higher-voltage batteries.

EDLCs have much higher power density than batteries. Power density combines the energy

density with the speed at which the energy can be delivered to the load. Batteries, which are

based on the movement of charge carriers in a liquid electrolyte, have [7] 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 of the electrodes.

So while existing EDLCs have energy densities that are perhaps 1/10 that of a conventional

battery, their power density is generally 10 to 100 times as great. This makes them most suited to

an intermediary role between electrochemical batteries and electrostatic capacitors, where neither

sustained energy release nor immediate power demands dominate one another.

History

General Electric engineers experimenting with devices using porous carbon electrodes first

observed the EDLC effect in 1957.[8] They believed that the energy was stored in the carbon

pores and the device exhibited "exceptionally high capacitance", although the mechanism was

unknown at that time.

General Electric did not immediately follow up on this work. In 1966 researchers at Standard Oil

of Ohio developed the modern version of the devices, after they accidentally re-discovered the

effect while working on experimental fuel cell designs.[9] 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 electric double-layer capacitors.

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Standard Oil did not commercialize their invention, licensing the technology to NEC, who

finally marketed the results as “super capacitors” in 1978, to provide backup power for

maintaining computer memory.[9] The market expanded slowly for a time, but starting around the

mid-1990s various advances in materials science and refinement of the existing systems led to

rapidly improving performance and an equally rapid reduction in cost.

The first trials of super capacitors in industrial applications were carried out for supporting the

energy supply to robots.

In 2005 aerospace systems and controls company Diehl Luftfahrt Elektronik GmbH chose super

capacitors to power emergency actuation systems for doors and evacuation slides in airliners,

including the new Airbus 380 jumbo jet.[10] In 2005, the ultra capacitor market was between US

$272 million and $400 million, depending on the source.

As of 2007 all solid state micrometer-scale electric double-layer capacitors based on advanced

superionic conductors had been for low-voltage electronics such as deep-sub-voltage

nanoelectronics and related technologies (the 22 nm technological node of CMOS and beyond).[11]

Comparisons

Super capacitors have several disadvantages and advantages relative to batteries:

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[4] as of 2010 compared to 30–40 W·h/kg for a lead acid

battery, 100-250 W·h/kg for a lithium-ion battery and about 1/1,000th the volumetric

energy density of gasoline.

Has the highest dielectric absorption of any type of capacitor.

High self-discharge – the rate is considerably higher than that of an electrochemical

battery.

Page 5: Utra Capacitor

Low maximum voltage – series connections are needed to obtain higher voltages, and

voltage balancing may be required.

Unlike practical batteries, the voltage across any capacitor, including EDLCs, drops

significantly as it discharges. Effective storage and recovery of energy requires complex

electronic control and switching equipment, with consequent energy loss. A detailed

paper on a multi-voltage 5.3 W EDLC power supply for medical equipment discusses

design principles in detail. It uses a total of 55 F of capacitance, charges in about 150

seconds, and runs for about 60 seconds. The circuit uses switch-mode voltage regulators

followed by linear regulators for clean and stable power, reducing efficiency to about

70%. The authors discuss the types of switching regulator available, buck, boost, and

buck-boost, and conclude that for the widely varying voltage across an EDLC buck-boost

is best, boost second-best, and buck unsuitable[12].

Very low internal resistance allows extremely rapid discharge when shorted, resulting in

a spark hazard similar to any other capacitor of similar voltage and capacitance (generally

much higher than electrochemical cells).

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.

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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[13]

Improved safety, no corrosive electrolyte and low toxicity of materials.

Simple charge methods—no full-charge detection is needed; no danger of overcharging.

When used in conjunction with rechargeable batteries, in some applications the EDLC

can supply energy for a short time, reducing battery cycling duty and extending life

Materials

In general, EDLCs improve storage density through the use of a nanoporous material, typically

activated charcoal, in place of the conventional insulating barrier. Activated charcoal is an

extremely porous, "spongy" form of carbon with an extraordinarily high specific surface area —

a common approximation is that 1 gram (a pencil-eraser-sized amount) has a surface area of

roughly 250 m2 — about the size of a tennis court. It is typically a powder made up of extremely

fine but very "rough" particles, which, in bulk, form a low-density heap with many holes. As the

surface area of even a thin layer of such a material is many times greater than a traditional

material like aluminum, many more charge carriers (ions or radicals from the electrolyte) can be

stored in a given volume. As carbon is not a good insulator (vs. the excellent insulators used in

conventional devices), in general EDLCs are limited to low potentials on the order of 2–3 V, and

thus must be "stacked" (connected in series), just as conventional battery cells must be, to supply

higher voltages.

Activated charcoal is not the "perfect" material for this application. The charge carriers are

actually (in effect) quite large—especially when surrounded by molecules—and are often larger

than the holes left in the charcoal, which are too small to accept them, limiting the storage.

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As of 2010 virtually all commercial super capacitors use powdered activated carbon made from

coconut shells.[citation needed] Higher performance devices are available, at a significant cost increase,

based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).

Research in EDLCs focuses on improved materials that offer higher usable surface areas.

Graphene has excellent surface area per unit of gravimetric or volumetric densities, is

highly conductive and can now be produced in various labs, but is not available in

production quantities. Specific energy density of 85.6 Wh/kg at room temperature and

136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current

density of 1 A/g have been observed. These energy density values are comparable to that

of the Nickel metal hydride battery. The device makes full utilization of the highest

intrinsic surface capacitance and specific surface area of single-layer graphene by

preparing curved graphene sheets that do not restack face-to-face. The curved shape

enables the formation of mesopores accessible to and wettable by environmentally benign

ionic liquids capable of operating at a voltage >4 V.[14]

Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the

polymer to sit in the tube and act as a dielectric.[15] Carbon nanotubes can store about the

same charge as charcoal (which is almost pure carbon) per unit surface area but

nanotubes can be arranged in a more regular pattern that exposes greater suitable surface

area.[16] The addition of carbon nanotubes in capacitors can greatly improve and enhance

the performance of electric double-layer capacitors. Due to the high surface area and high

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conductivity of single-wall carbon nanotubes, the addition of these nanotubes allows

optimization for these capacitors.[17] Multi-walled carbon nanotubes have a presence of

mesopores that allow for easy access of ions at the electrode/electrolyte interface. The

thin walls of a carbon nanotube allow for high capacitance in an electric double-layer

capacitor.[18] By adding multi-walled nanotubes to these capacitors, the resistance of the

electrodes can be decreased. The capacitor cells with multi-walled nanotube fibers had

higher electron and electrolyte-ion conductivities as compared to cells that did not have

these nanotubes. These nanotubes also improved the power capabilities of the capacitors.[19]

Ragone chart showing energy density vs.power density for various energy-storage devices

Some polymers (e.g. polyacenes and conducting polymers) have a redox (reduction-

oxidation) storage mechanism along with a high surface area.

Carbon aerogel provides extremely high surface area gravimetric densities of about 400–

1000 m²/g. The electrodes of aerogel super capacitors are a composite material usually

made of non-woven paper made from carbon fibers and coated with organic aerogel,

which then undergoes pyrolysis. The carbon fibers provide structural integrity and the

aerogel provides the required large surface area. Small aerogel super capacitors are being

used as backup electricity storage in microelectronics. Aerogel capacitors can only work

at a few volts; higher voltages ionize the carbon and damage the capacitor. Carbon

aerogel capacitors have achieved 325 J/g (90 W·h/kg) energy density and 20 W/g power

density.[20]

Solid activated carbon, also termed consolidated amorphous carbon (CAC). It can have a

surface area exceeding 2800 m2/g and may be cheaper to produce than aerogel carbon.[21]

Tunable nanoporous carbon exhibits systematic pore size control. H2 adsorption treatment

can be used to increase the energy density by as much as 75% over what was

commercially available as of 2005.[22][23]

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Mineral-based carbon is a nonactivated carbon, synthesised from metal or metalloid

carbides, e.g. SiC, TiC, Al4C3.[24] The synthesised nanostructured porous carbon, often

called Carbide Derived Carbon (CDC), has a surface area of about 400 m²/g to 2000 m²/g

with a specific capacitance of up to 100 F/mL (in organic electrolyte). As of 2006 this

material was used in a super capacitor with a volume of 135 mL and 200 g weight having

1.6 kF capacitance. The energy density is more than 47 kJ/L at 2.85 V and power density

of over 20 W/g.[25]

In August 2007 researchers combined a biodegradable paper battery with aligned carbon

nanotubes, designed to function as both a lithium-ion battery and a super capacitor (called

bacitor). The device employed an ionic liquid, essentially a liquid salt, as the electrolyte.

The paper sheets can be rolled, twisted, folded, or cut with no loss of integrity or

efficiency, or stacked, like ordinary paper (or a voltaic pile), to boost total output. 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.[26]

Other teams are experimenting with custom materials made of activated polypyrrole, and

nanotube-impregnated papers.

Properties

The properties of EDLCs are being improved as new research progresses.

Capacitance

The capacitance of EDLCs was up to several thousands of farads as of 2011.

Voltage

As of 2011 EDLCs rated up to a maximum working voltage of about 5 V were available.

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Specific energy

The specific energy of existing commercial EDLCs ranges from around 0.5 to 30 W·h/kg[27][28]

including lithium ion capacitors, known also as a "hybrid capacitor". Experimental electric

double-layer capacitors have demonstrated specific energies of 30 W·h/kg and have been shown

to be scalable to at least 136 W·h/kg.[29][30] For comparison, a conventional lead-acid battery

stores typically 30 to 40 W·h/kg and modern lithium-ion batteries about 160 W·h/kg. Gasoline

has a net calorific value (NCV) of around 12,000 W·h/kg; automobile applications operate at

about 20% tank-to-wheel efficiency, giving an effective specific energy of 2,400 W·h/kg.

Electrically driven automobiles run at a much higher efficiency. For example, the Tesla Roadster

runs at an average battery-to-wheel efficiency of 88%. This implies that the effective specific

energy of ultra capacitors in an automotive application could be close to 25 W·h/kg, 2 orders of

magnitude less than gasoline.

Power density

EDLCs have energy densities perhaps one tenth that of a rechargeable battery, but power

densities typically 10 to 100 times greater.

Self-discharge

An EDLC which is charged and stored loses its charge (self-discharge) much faster than a typical

electrolytic capacitor, and somewhat faster than a rechargeable battery.

Price

Research and development bring rapid improvements in price as well as physical properties.

Page 11: Utra Capacitor

Costs have fallen quickly, with cost per kilojoule dropping faster than cost per farad. By 2006 the

cost of super capacitors was 1 cent per farad and $2.85 per kilojoule and dropping. [31] A 3 kF

capacitor that was US$5,000 ten years before was $50 in 2011.[32]

Applications

Vehicles

Heavy and public transport

See also: Capa vehicle

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.[33][34] In the 2000s they attracted attention in the electric car industry, where their

ability to charge much faster than batteries makes them particularly suitable for regenerative

braking applications. New technology in development could potentially make EDLCs with high

enough energy density to be an attractive replacement for batteries in all-electric cars and plug-in

hybrids, as EDLCs charge quickly and are stable with respect to temperature.

China is experimenting with a new form of electric bus (capabus) that runs without powerlines

using large onboard EDLCs, which quickly recharge whenever the bus is at any bus stop (under

so-called electric umbrellas), and fully charge in the terminus. A few prototypes were being

tested in Shanghai in early 2005. In 2006, two commercial bus routes began to use electric

double-layer capacitor buses; one of them is route 11 in Shanghai.[35]

In 2001 and 2002 VAG, the public transport operator in Nuremberg, Germany tested an hybrid

bus that uses a diesel-electric battery drive system with electric double-layer capacitors.[36] Since

2003 Mannheim Stadtbahn in Mannheim, Germany has operated a light-rail vehicle (LRV) that

uses EDLCs to store braking energy.[37][38]

Other public transport manufacturers are developing EDLC technology, including mobile

storage[39] and a stationary trackside power supply.[40][41]

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A triple hybrid forklift truck uses fuel cells and batteries as primary energy storage and EDLCs

to supplement this energy storage solution.[42]

Automotive

Ultracapacitors are used in some concept prototype vehicles, in order to keep batteries within

resistive heating limits and extend battery life.[43][44] The ultrabattery combines a super capacitor

and a battery in one unit, creating an electric vehicle battery that lasts longer, costs less and is

more powerful than current plug-in hybrid electric vehicles (PHEVs).[45][46]

Motor racing

The FIA, the governing body for many motor racing events, proposed in the Power-Train

Regulation Framework for Formula 1 version 1.3 of 23 May 2007 that a new set of power train

regulations be issued that includes a hybrid drive of up to 200 kW input and output power using

"superbatteries" made with both batteries and super capacitors.[47]

The Toyota TS030 HYBRID LMP1 car uses a hybrid drivetrain with energy storage provided

through the use of super capacitors.[48]

Complementing batteries

When used in conjunction with rechargeable batteries in uninterruptible power supplies and

similar applications, the EDLC can handle short interruptions, requiring the batteries to be used

only during long interruptions, reducing the cycling duty and extending their life[49]

Low-power applications

EDLCs can be used to operate low-power equipment such as PC Cards, photographic flash,

flashlights, portable media players, and automated meter reading equipment.[50] They are

advantageous when extremely fast charging is required. In professional medical applications,

Page 13: Utra Capacitor

EDLCs have been used to power a handheld, laser-based breast cancer detector (55 F to provide

5.3 W at multiple voltages; charges in 150 seconds, runs for 60 seconds).[12]

In 2007 a cordless electric screwdriver that uses an EDLC for energy storage was produced.[51] It

charges in 90 seconds, retains 85% of the charge after 3 months, and holds enough charge for

about half the screws (22) a comparable screwdriver with a rechargeable battery will handle (37).

Two LED flashlights using EDLCs were released in 2009. They charge in 90 seconds.[52]

Alternative energy

The idea of replacing batteries with capacitors in conjunction with novel energy sources became

a conceptual umbrella of the Green Electricity (GEL) Initiative. [53] One successful GEL Initiative

concept was a muscle-driven autonomous solution that employs a multi-farad EDLC as 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.[54]

Page 14: Utra Capacitor

Ultra capacitors

Call them ultracapacitors. Or super capacitors. Whatever the name, they exhibit vastly greater

capacitance than conventional caps. Singly, you can buy radiallead board-mount devices rated

for 5 to 10 F at 2.5 V, flashlight-battery size units rated for 120 to 150 F at 5 V, and larger

single-capacitor cans good for 650 to 3000 F at 2.7 V. Note that all of those capacitance values

are in farads. Not so long ago, a couple of thousand microfarads were a lot of capacitance.

Need more? You can buy off-the-shelf modules spec’d for 20 to 500 F, with voltage ratings from

15 to 390 V. If you understand how to balance them in series/parallel combinations, you can

drive a bus with them—no, not two traces on a circuit board, but a passenger-hauling bus.

(Although not very far, as hybrid propulsion systems, chemical batteries, and fuel cells are still in

the picture. More on that shortly.)

What happened? In developing ultracaps, nobody discovered new laws of physics. In fact, the

theory behind them goes back to Helmholtz. Like all capacitors, ultracaps are still about storing

power in the form of an electrical charge between two “plates.” The capacitance is directly

related to the area of the plates and the permittivity of the material between the plates, and it’s

inversely related to the distance between them. After that, the story gets interesting.

Before we had ultracaps to provide astonishingly high values of capacitance, we had

electrolytics. ultra capacitors aren’t electrolytics, but understanding the older tech is helpful in

understanding the new tech.

Electrolytics are so named because one (or both) of the “plates” is a nonmetallic electrolyte on

top of a metallic backing. During manufacturing, a voltage drives a current from the anode metal

through a conductive bath to the cathode. That produces an insulating metal oxide on the surface

of the anode—the dielectric.

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One of the phenomena that happens inside electrolytics is the charge accumulation and charge

separation that occurs at the interface when any electrode is immersed into an electrolyte

solution. An accumulation of oppositely charged ions in the solution compensates for excess

charge on the electrode surface. The interface is called the Helmholtz layer.

To understand ultracaps, stop thinking about flat plates (or flat plates rolled up into tubes) with a

dielectric between them, much like peanut butter in a sandwich. In an ultracap,

charging/discharging takes place on the interfaces between porous carbon materials or porous

oxides of certain metals in an electrolyte.

The Helmholtz layers give rise to an effect called doublelayer capacitance. When a dc voltage is

applied across the porous carbon electrodes in an ultracap, compensating accumulations of

cations or anions develop in the solution around the charged electrodes. If no electron transfer

can occur across the interface, a “double layer” of separated charges (electrons or electron

deficiency at the metal side and cations or anions at the solution side of the interface boundary)

exists across the interface (Fig. 1).

The Helmholtz-region capacitance depends on the area of those porous carbon electrodes and the

size of the ions in solution. The capacitance per square centimeter of electrode double layers is

on the order of 10,000 times larger than those of ordinary dielectric capacitors. That’s because

the separation of charges in double layers is about 0.3 to 0.5 nm, instead of 10 to 100 nm in

electrolytics and 1000 nm in mica or polystyrene caps.

There’s a catch to this “double-layer” characteristic, though. The double-layer configuration

reduces the potential capacitance of a practical device because the ultracap consists of a pair of

electrodes, each with half the total mass. In addition, the ultra capacitor is effectively two

capacitors in series. Taken together, that means the ultracap achieves one quarter of the

theoretical capacitance based on electrode area and ion size.

If you want to read the theory behind ultra capacitors in more depth, check out an article from

the Electrochemistry Encyclopedia called “Electrochemical Capacitors, Their Nature, Function,

and Applications” (http://electrochem.cwru.edu/ed/encycl/artc03-elchem-cap.htm) by the late

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Brian E. Conway of the University of Ottawa’s chemistry department. Conway was an important

contributor to ultra capacitor research for several decades.

BATTERIES AND ULTRA CAPACITORS

The popular press likes to lump batteries and ultra capacitors together, obscuring a number of

important differences:

Batteries store watt-hours of energy. Capacitors store watts of power.

Batteries depend on chemical reactions with long time constants. They take a

relatively long time to charge, and they’re fussy about the profile of the current that

charges them. Conversely, capacitors are charged by applying a voltage across

their terminals, and their charge rate depends mostly on external resistance.

Batteries deliver power in the form of a more or less constant voltage over long

time periods. Capacitors discharge rapidly, and their output voltage decays

exponentially.

Batteries are good for only a limited number of charge/discharge cycles, and the

number of cycles depends on how deeply they are discharged. Capacitors,

especially ultracapacitors, can be charged and discharged repeatedly for tens of

millions of cycles. (This is an important way that ultracaps differ from electrolytics

—they aren’t cycle-limited by the electrode plating that accompanies electrolytics’

operation.)

Batteries are big and heavy. Capacitors are small and light.

Many of these differences can be heuristically illustrated in a Ragone plot (Fig. 2). Ragone plots

have more analytical uses, but essentially, they’re log-log graphs of energy density (in this case

in Wh/kg) on the Y axis versus power density (in W/kg) on the X axis. Because they’re log-log

plots, discharge time can be represented as straight-line diagonal parameters.

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The Ragone plot helps illustrate the differences among different kinds of battery chemistry,

clustered on the left, and capacitors on the right. Taken together as illustrated on the Ragone plot,

those characteristics make batteries and ultra capacitors complementary to each other, rather than

antagonists. In fact, that’s how they’re often used.

Super capacitor

The super capacitor, also known as ultra capacitor or double-layer capacitor, differs from a

regular capacitor in that it has a very high capacitance. A capacitor stores energy by means of a

static charge as opposed to an electrochemical reaction. Applying a voltage differential on the

positive and negative plates charges the capacitor. This is similar to the buildup of electrical

charge when walking on a carpet. Touching an object releases the energy through the finger.

We group capacitors into three family types and the most basic is the electrostatic capacitor,

with a dry separator. This capacitor has a very low capacitance and is used to filter signals and

tune radio frequencies. The size ranges from a few pico-farad (pf) to low microfarad (uF). The

next member is the electrolytic capacitor, which is used for power filtering, buffering and

coupling. Rated in microfarads (uF), this capacitor has several thousand times the storage

capacity of the electrostatic capacitor and uses a moist separator. The third type is the super

capacitor, rated in farads, which is again thousands of times higher than the electrolytic

capacitor. The super capacitor is ideal for energy storage that undergoes frequent charge and

discharge cycles at high current and short duration.

Faradis a unit of capacitance named after the English physicist Michael Faraday. One farad

stores one coulomb of electrical charge when applying one volt. One microfaradis one million

times smaller than a farad, and one pico-farad is again one million times smaller than the

microfarad.

Page 18: Utra Capacitor

Engineers at General Electric first experimented with the electric double-layer capacitor, which

led to the development of an early type of super capacitor in 1957. There were no known

commercial applications then. In 1966, Standard Oil rediscovered the effect of the double-layer

capacitor by accident while working on experimental fuel cell designs. The company did not

commercialize the invention but licensed it to NEC, which in 1978 marketed the technology as

“super capacitor” for computer memory backup. It was not until the 1990s that advances in

materials and manufacturing methods led to improved performance and lower cost.

The modern super capacitor is not a battery per se but crosses the boundary into battery

technology by using special electrodes and electrolyte. Several types of electrodes have been

tried and we focuse on the double-layer capacitor (DLC) concept. It is carbon-based, has an

organic electrolyte that is easy to manufacture and is the most common system in use today.

All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand

high volts, the super capacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible

but they would reduce the service life. To achieve higher voltages, several super capacitors are

connected in series. This has disadvantages. Serial connection reduces the total capacitance, and

strings of more than three capacitors require voltage balancing to prevent any cell from going

into over-voltage. This is similar to the protection circuit in lithium-ion batteries.

The specific energy of the super capacitor is low and ranges from 1 to 30Wh/kg. Although high

compared to a regular capacitor, 30Wh/kg is one-fifth that of a consumer Li-ion battery. The

discharge curve is another disadvantage. Whereas the electrochemical battery delivers a steady

voltage in the usable power band, the voltage of the super capacitor decreases on a linear scale

from full to zero voltage. This reduces the usable power spectrum and much of the stored energy

is left behind. Consider the following example.

Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. With

the linear discharge, the super capacitor reaches this voltage threshold within the first quarter of

the cycle and the remaining three-quarters of the energy reserve become unusable. A DC-to-DC

converter could utilize some of the residual energy, but this would add to the cost and introduce a

10 to 15 percent energy loss. A battery with a flat discharge curve, on the other hand, would

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deliver 90 to 95 percent of its energy reserve before reaching the voltage threshold. Table 1

compares the super capacitor with a typical Li-ion.

 

Function Super capacitor Lithium-ion (general)

Charge time

Cycle life

Cell voltage

Specific energy (Wh/kg)

Specific power (W/kg)

Cost per Wh

Service life (in vehicle)

Charge temperature

Discharge temperature

1–10 seconds

1 million or 30,000h

2.3 to 2.75V

5 (typical)

Up to 10,000

$20(typical)

10 to 15 years

–40 to 65°C (–40 to

149°F)

–40 to 65°C (–40 to

149°F)

10–60 minutes

500 and higher

3.6 to 3.7V

100–200

1,000 to 3,000

$2 (typical)

5 to 10 years

0 to 45°C (32°to 113°F)

–20 to 60°C (–4 to 140°F)

Table 1: Performance comparison between super capacitor and Li-ion

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Courtesy of Maxwell Technologies, Inc.

Rather than operating as a stand-alone energy storage device, super capacitors work well as low-

maintenance memory backup to bridge short power interruptions. Super capacitors have also

made critical inroads into electric powertrains. The virtue of ultra-rapid charging and delivery of

high current on demand makes the super capacitor an ideal candidate as a peak-load enhancer for

hybrid vehicles, as well as fuel cell applications.

The charge time of a super capacitor is about 10 seconds. The charge characteristic is similar to

an electrochemical battery and the charge current is, to a large extent, limited by the charger. The

initial charge can be made very fast, and the topping charge will take extra time. Provision must

be made to limit the initial current inrush when charging an empty super capacitor. The super

capacitor cannot go into overcharge and does not require full-charge detection; the current

simply stops flowing when the capacitor is full.

The super capacitor can be charged and discharged virtually an unlimited number of times.

Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by

cycling a super capacitor. Nor does age affect the device, as it would a battery. Under normal

conditions, a super capacitor fades from the original 100 percent capacity to 80 percent in 10

years. Applying higher voltages than specified shortens the life. The super capacitor functions

well at hot and cold temperatures.

The self-discharge of a super capacitor is substantially higher than that of an electrostatic

capacitor and somewhat higher than the electrochemical battery. The organic electrolyte

contributes to this. The stored energy of a super capacitor decreases from 100 to 50 percent in 30

to 40 days. A nickel-based battery self-discharges 10 to 15 percent per month. Li-ion discharges

only five percent per month.

Super capacitors are expensive in terms of cost per watt. Some design engineers argue that the

money for the super capacitor would better be spent on a larger battery. We need to realize that

the super capacitor and chemical battery are not in competition; rather they are different products

serving unique applications.Table 2 summarizes the advantages and limitations of the super

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capacitor.

 

Advantages

Virtually unlimited cycle life; can be cycled millions of time

High specific power; low resistance enables high load currents

Charges in seconds; no end-of-charge termination required

Simple charging; draws only what it needs; not subject to

overcharge

Safe; forgiving if abused

Excellent low-temperature charge and discharge performance

Limitations

Low specific energy; holds a fraction of a regular battery

Linear discharge voltage prevents using the full energy spectrum

High self-discharge; higher than most batteries

Low cell voltage; requires serial connections with voltage

balancing

High cost pe

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How An Ultra Capacitor Works

Ultra capacitors & Super Capacitors store electricity by physically separating positive and

negative charges— different from batteries which do so chemically. The charge they hold is like

the static electricity that can build up on a balloon, but is much greater thanks to the extremely

high surface area of their interior materials.

An advantage of the ultra capacitor is their

super fast rate of charge and discharge... which

is determined solely by their physical

properties. A battery relies on a slower

chemical reaction for energy.

A disadvantage of an ultra capacitor is that

currently they store a smaller amount of energy

than a battery does.

Ultracapacitors are very good at efficiently

capturing electricity from regenerative braking,

and can deliver power for acceleration just as

quickly. With no moving parts, they also have a

very long lifespan - 500,000 plus charge/recharge cycles.  ultra capacitors are currently used for

wind energy, solar energy, and hydro energy storage.

An ultra capacitor, also known as a double-layer capacitor, polarizes an electrolytic solution to

store energy electro statically. Though it is an electrochemical device, no chemical reactions are

involved in its energy storage mechanism. This mechanism is highly reversible, and allows the

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ultra capacitor to be charged and discharged hundreds of thousands of times.

Once the ultra capacitor is charged and energy stored, a load (the electric vehicle's motor) can

use this energy. The amount of energy stored is very large compared to a standard capacitor

because of the enormous surface area created by the porous carbon electrodes and the small

charge separation created by the dielectric separator.

Here is a very basic example of how an ultra capacitor works by using a circuit that uses a

dc motor.

 

TECHNICAL DESCRIPTION: An ultra capacitor can be viewed as two non reactive porous

plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the

collectors. In an individual ultra-capacitor cell, the applied potential on the positive electrode

attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts

the positive ions. A dielectric separator between the two electrodes prevents the charge from

moving between the two electrodes.

Electrical energy storage devices, such as capacitors, store electrical charge on an electrode.

Other devices, such as electrochemical cells or batteries, utilize the electrode to create, by

chemical reaction, an electrical charge at the electrodes. In both of these, the ability to store or

create electrical charge is a function of the surface area of the electrode. For example, in

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capacitors, greater electrode surface area increases the capacitance or energy storage capability

of the device.

As a storage device, the ultracapacitor, relies on the microscopic charge separation at an

electrochemical interface to store energy. Since the capacitance of these devices is proportional

to the active electrode area, increasing the electrode surface area will increase the capacitance,

hence increasing the amount of energy that can be stored. This achievement of high surface area

utilizes materials such as activated carbon or sintered metal powders. However, in both

situations, there is an intrinsic limit to the porosity of these materials, that is, there is an upper

limit to the amount of surface area that can be attained simply by making smaller and smaller

particles. An alternative method must be developed to increase the active electrode surface area

without increasing the size of the device. A much more highly efficient electrode for electrical

energy storage devices could be realized if the surface area could be significantly increased.

Energy Storage

Energy Storage: Solar Energy - Wind Energy - Hydro Energy

Using the ultra capacitor as an energy storage device has been making a lot of ground lately.

ultra capacitors have close to 100 percent efficiency and can be recycled up to 500,000 times.

The introduction of standard battery-sized ultra capacitors is a move that has the potential to

significantly improve market acceptance of ultra capacitors in a variety of applications and

hybrid electric vehicles (HEVs).

Large back up power users such like

manufacturers and utility providers

have been reluctant to move from

their traditional lead-acid batteries

because they are unfamiliar with the

new ultra capacitor technology.  All

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of this in spite of  of the significant advantages like greater reliability and efficiency.  The tide is

turning though... more and more companies are becoming aware.

It was recently noted by Miriam Nagel: "Environmental issues are now coming into play in the

selection of advanced energy storage technologies."  "Environmentally friendly technologies

such as flywheels and ultra capacitors – also called super capacitors – may soon get a lot more

consideration in the energy storage markets."

One of the issues that has slowed the ultra capacitor market is the high cost of integrating them

into new designs. It's just now becoming known that ultra capacitors can now be produced at

half the cost of its earlier earlier versions and the savings are likely to be passed on to original

equipment manufacturers (OEM).

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Reference

1. ̂ Garthwaite, Josie (12 July 2011). "How ultra capacitors work (and why they fall

short)". GigaOM's Earth2Tech. http://gigaom.com/cleantech/how-ultracapacitors-work-

and-why-they-fall-short/. Retrieved 13 July 2011.

2. ̂ 5000F, Nesscap Products

3. ̂ A 30 Wh/kg Super capacitor for Solar Energy and a New Battery. Jeol.com (3 October

2007). Retrieved on 13 September 2011.

4. ^ a b Graphene super capacitor breaks storage record. physicsworld.com. Retrieved on 13

September 2011.

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5. ̂ Note: all references to batteries in this article should be taken to refer to rechargeable,

not primary (aka disposable), batteries.

6. ̂ Chemistry World: New carbon material boosts super capacitors. Rsc.org. 13 May 2011.

Retrieved on 13 September 2011.

7. ̂ Garthwaite, Josie (12 July). "How ultra capacitors work (and why they fall short)".

Earth2Tech. GigaOM Network. http://gigaom.com/cleantech/how-ultracapacitors-work-

and-why-they-fall-short/. Retrieved 13 July 2011.

8. ̂ US 2800616, Becker, H.I., "Low voltage electrolytic capacitor", issued 1957-07-23

9. ^ a b The Charge of the Ultra – Capacitors. IEEE Spectrum, November 2007

10. ̂ Boostcap (of Maxwell Technologies)

11. ̂ Высокоёмкие конденсаторы для 0,5 вольтовой наноэлектроники будущего.

Nanometer.ru. 17 October 2007. Retrieved on 13 September 2011.

External links

Super Capacitor Seminar

Article on ultra capacitors at electronicdesign.com

Article on ultra capacitors at batteryuniversity.com

A new version of an old idea is threatening the battery industry (The Economist).

An Encyclopedia Article From the Yeager center at CWRU.

Ultracapacitors & Super capacitors Forum

Special Issue of Interface magazine on electrochemical capacitors

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Nanoflowers Improve Ultracapacitors: A novel design could boost energy storage

(Technology Review) and Can nanoscopic meadows drive electric cars forward? (New

Scientist)

If the cap fits... How super capacitors can help to solve power problems in portable

products.

A web that describes the development of solid-state and hybrid super capacitors from

CNR-ITAE (Messina) Italy