Supercapacitors hailed as potential answer to greener public transportation

Oct 03, 2012

(Phys.org)—Imagine a bus that uses electrical storage that costs much less than traditional batteries, can fully charge each time the bus stops, and has enough power to easily get to the next stop. The supercapacitor, once used to power robots and as a backup power source for computer memory, is now being hailed as such an energy source that could be the key to greener public transportation.

In an article written by well-known Philip Ball for the Quarterly feature of MRS Bulletin, researcher Yury Gogotsi posits that the has the potential to become a big player in the global search for reliable . This is particularly true for transportation, based on experience with the use of supercapacitors in Germany.

Supercapacitors are allowing trams in Mannheim, Germany, to use 30 per cent less energy than their equivalents in other cities. In a recent 24-hour speed race at Le Mans, Toyota put their faith in a hybrid TS030 car that used "supercaps" for energy-capture during braking. In China, supercapacitor technology has been embraced so fervently over just the past four years that tens of thousands of supercap buses are now on the roads.

So what are supercapacitors and just what do they bring to the party? Gogotsi, professor of at Drexel University in Philadelphia, explains supercapacitors as power-storage devices that can supply onboard electrical power in hybrid vehicles. Whereas batteries store energy in chemical form-in substances that can react to release electrical energy-capacitors store it by simply piling up on two electrodes. The larger the electrodes and the closer they are, the more energy can be stored.

Unlike batteries, supercapacitors can be charged and discharged in seconds and can withstand many hundreds of thousands of such charging cycles. This is ideal for energy-saving applications that capitalize on transient opportunities for recharging, such as energy capture during braking, and other actions that require power to be delivered in short bursts. They can help with acceleration, restart engines that cut out, drive air conditioning, and power automatic windows and passenger doors. In some aircraft, they are entrusted with powering emergency actuation systems for doors and evacuation slides.

Supercapacitor technology is now deployed on Spanish and French trains and hybrid buses all over the world, on construction equipment such as cranes, and on garbage-collection trucks in the US. On buses, it can reduce carbon-dioxide emissions by around 30%. The Munich-based heavy-vehicle manufacturer MAN estimates that their supercapacitor-fitted coaches each save around $4,500 a year on fuel costs.

The take-up of the technology looks set to expand, as both energy-saving and low-emission technologies become more necessary and as the technical capabilities of supercapacitors improve.

"There is no single perfect energy-storage solution, no 'one size fits all,'" said Gogotsi. "A 'battery of the future' may well be a battery-supercapacitor hybrid which combines the long lifetime, fast charging, and high power of a supercapacitor with the high energy density of a battery."

With a huge variety of potential uses, supercapacitors are one of the few electronic components that have had a steadily growing market over recent years with rapid growth widely expected.

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More information: A copy of the article can be found at journals.cambridge.org/MRSEQ201209

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Eikka
2 / 5 (4) Oct 03, 2012
Unlike batteries, supercapacitors can be charged and discharged in seconds and can withstand many hundreds of thousands of such charging cycles.


Unfortunately, they will only withstand one accidental short circuit.

For tiny amounts of energy, up to a few dozen kilojoules, they're perfectly fine if somewhat hazardous, but once you have a megajoule capacitor system on-board you're practically carrying a bomb. Safe - as long as nothing breaks the microscopically thin insulation layers.

In a city bus for example, you need about 4 MJ per km to drive. Storing that amount of energy in one capacitor though is playing with your luck, because of what happens when you release 4 MJ of energy in an instant should something go wrong. You can compare it to dynamite at 7.5 MJ/kg, so you got the equivalent of half a kilo of dynamite there.
Sonhouse
2 / 5 (1) Oct 03, 2012
Unlike batteries, supercapacitors can be charged and discharged in seconds and can withstand many hundreds of thousands of such charging cycles.


Unfortunately, they will only withstand one accidental short circuit.

For tiny amounts of energy, up to a few dozen kilojoules, they're perfectly fine if somewhat hazardous, but once you have a megajoule capacitor system on-board you're practically carrying a bomb. Safe - as long as nothing breaks the microscopically thin insulation layers.

In a city bus for example, you need about 4 MJ per km to drive. Storing that amount of energy in one capacitor though is playing with your luck, because of what happens when you release 4 MJ of energy in an instant should something go wrong. You can compare it to dynamite at 7.5 MJ/kg, so you got the equivalent of half a kilo of dynamite there.
Well sure but look at the number of car fires that happen because of crashes igniting the gas tank and the car exploding. You are projecting.
dschlink
not rated yet Oct 03, 2012
Gasoline is typically 47 MJ/kg, superconductors around 0.56. That's two orders of magnitude less.
VendicarD
5 / 5 (2) Oct 03, 2012
They will withstand exactly zero internal short circuits.

"Unfortunately, they will only withstand one accidental short circuit." - Eikka

However it is entirely possible to develop supercaps that are safe, through the addition of circuitry that isolates banks of supercaps so that exceptionally large inflows of energy are halted as they occur preventing a short in one supercap from catastrophically dumping the energy of the entire bank.

Inside each supercap, isolation between elements can be made quite easily by the simple addition of passive choke coils. The device will still discharge rapidly even with the addition of choke coils but the rate at which the energy is dumped can be made orders of magnitude longer and can be distributed over a larger volume with the addition of resistors placed in parallel across each choke coil.

In this configuration, a current pulse which can not pass through the coil because of it's exceptionally high will pass through the resistor, CONT.
VendicarD
5 / 5 (2) Oct 03, 2012
heating it and dissipating the energy over a larger volume than otherwise.

Extendng the protection further can be as simple as putting another supercap in parallel with the resistor. The energy not lost to resistance will then charge that supercap, which will then discharge even more slowly than the resistor alone.

Ultimately all of the energy gets dumped, but not in an explosive manner.
geokstr
1 / 5 (6) Oct 03, 2012
They've already got buses that recharge continuously and have been in use for a hundred years. They are known as trolleys.
Eikka
2.3 / 5 (3) Oct 03, 2012
Well sure but look at the number of car fires that happen because of crashes igniting the gas tank and the car exploding.


Exactly zero, because gasoline doesn't detonate. It just burns.

through the addition of circuitry that isolates banks of supercaps so that exceptionally large inflows of energy are halted as they occur preventing a short in one supercap from catastrophically dumping the energy of the entire bank.


But if you physically crush one or many capacitor cells, it still releases its energy in an explosive way, which can and probably will do the same thing to adjacent cells, and you got a right mess in your hands there.

Consider that a small .22 pistol round has about 200 Joules of kinetic energy, and it punches through steel. Suddenly releasing a kilojoule of energy or more inside a capacitor cell is a bit difficult to contain.
Eikka
2 / 5 (4) Oct 03, 2012
The difference to gasoline, or regular batteries is that gasoline needs oxygen to react, and batteries stop working when you sever the connections and are generally much slower to react. Capacitors have the inherent capability of immediate internal discharge of energy by simply breaking the microns thick dielectric layer.

And as a bonus point, a terrorist placing a small explosive under a big capacitor bank in a car or a bus will suddenly have a far bigger bomb.
Eikka
2.3 / 5 (3) Oct 03, 2012
Here's a graphic example of what the sudden release of 1 kJ of energy means: http://www.youtub...-V2-kr9Q

And then you realize there's at least ten thousand times that in a capacitor bank big enough to drive a bus.
Husky
5 / 5 (1) Oct 03, 2012
well, this discussion begs for a youtube or mythbuster video
VendicarD
5 / 5 (1) Oct 03, 2012
Not a problem with the proper circuitry - described above - to prevent catastrophic failure.

"And then you realize there's at least ten thousand times that in a capacitor bank big enough to drive a bus." - Eikka
VendicarD
not rated yet Oct 03, 2012
True, but when properly designed destructive internal discharge safety can be guaranteed.

It isn't rocket science. A series of chokes, resistors and capacitors can do it easily.

"Capacitors have the inherent capability of immediate internal discharge of energy by simply breaking the microns thick dielectric layer." - Eikka
VendicarD
not rated yet Oct 03, 2012
Here is a low turn choke in action.

http://www.youtub...=related
Skepticus
1 / 5 (3) Oct 04, 2012
Being roasted alive for minutes by a fuel fire is being preferable to being blown to bits in microseconds by a supercap catastrophic failure? I know what my choice is..:-)
unknownorgin
1 / 5 (1) Oct 04, 2012
Exploding capacitors are just not a problem with proper componet and circut design. A capacitor below a certain energy capacity can be shorted repeatedly without damage so it is a simple matter of connectng smaller capacitors in parralel and connecting a simple fuse to each capacitor so if one fails its fuse will disconnect it from the main buss. An inductive choke will only protect a circut for a millisecond or less because when its magnetic field reaches saturation it no longer can generate a counter voltage so it becomes a low resistance conductor and also having an inductor in series with a motor control circut power supply can do some really ugly things to the controller.
antialias_physorg
not rated yet Oct 04, 2012
[q9In a city bus for example, you need about 4 MJ per km to drive. Storing that amount of energy in one capacitor though is playing with your luck, because of what happens when you release 4 MJ of energy in an instant should something go wrong.

Well, since these have been on the road by the thousands in China (and by the dozens in other countries) and we don't hear about mass electric-death I'd say that they are quite safe.

If you're really paranoid then you could hook them up to a piece of copper or a small box of salt. Then hook that up to something like what is alread in your car for your airbag. In the event of a crash the energy would just be dumped into the copper/salt and be used to heat/melt it way before the deformation due to the crash compromises the integrity of the capacitor.
jerryd
not rated yet Oct 04, 2012

UC's are way overpriced and store little power, Current Lithium batteries like A123's cost less than 1% and hold 100's or x's more power/lb of UC's. UC's are mostly hype and will have little to no use in a cost effective EV.
antialias_physorg
5 / 5 (2) Oct 05, 2012
UC's are mostly hype and will have little to no use in a cost effective EV.

Apples and oranges.

UCs have very different characteristics from batteries. They can charge/discharge very fast. They last many more cycles than batteries do.
On the down side their energy density is low.
They also do not provide a constant voltage during discharge which makes the electronics that need to feed a constant voltage to the motor rather complicated.

So they are suited for short-term, dynamic scenarios (e.g. a bus hopping from one stop to another or reclaiming/feeding in brake energy) - and not so much for long term/stationary operations like cruising down the highway.
Jeddy_Mctedder
1 / 5 (2) Oct 07, 2012
is this an informative article or an advertisement for a company that sells these things?
SteveL
not rated yet Oct 07, 2012
Public transportation buses could use these in cities. The capacitors could be charged at the bus stops in just a minute while picking up passengers through an inductive pickup under the bus stop pad. Even if the coils have a 50% efficiency there is still a significant amount of energy that could be recharged into the bus capacitors in the short period of time to discharge and pick up passengers.
Eikka
1 / 5 (1) Oct 10, 2012
Exploding capacitors are just not a problem with proper componet and circut design. A capacitor below a certain energy capacity can be shorted repeatedly without damage so it is a simple matter of connectng smaller capacitors in parralel


That's just the point though. You lose energy density by splitting the capacitors into such small units that they can't explode. All the number of fuses, chokes, resistors and other materials that you have to put in between the cells increases the size, weight, cost and probability of faults in the circuit.

People laugh at how Tesla spot-welded 7800 battery cells together into a 500 pound monster of a battery. That isn't even half of the story if you want to do similiar capacity -safely- with capacitors.

In the event of a crash the energy would just be dumped into the copper/salt


The amount of energy would cause the salt to vaporize and explode instantly, so you're really talking about a torpedo on wheels.
antialias_physorg
not rated yet Oct 10, 2012
The amount of energy would cause the salt to vaporize and explode instantly,

If you use copper you would need just 2-3kg. If you dumped a full charge of 4MJ into that it would 'merely' melt (if you use salt you'd need about twice as much).

Now one may argue that molten copper/salt itself isn't great to have around in a crash - but if we're talking a crash where protected parts of a BUS (like the motor block) are entering the passenger area then we're talking crash scenarios that are already deadly in any case.

But I would hazard that such a part can be fitted on a bus where the chances of catastrophic electrocution to passengers is minimal. So I wouldn't feel any unsafer riding one of those than one of the old kind.