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Capacitors to Replace Batteries?
Posted by
CowboyNeal
on Fri Jun 09, 2006 06:42 AM
from the going-and-going-and-going dept.
from the going-and-going-and-going dept.
An anonymous reader writes "MIT's Joel Schindall plans to use old technology in a new way with nanotubes.
'We made the connection that perhaps we could take an old product, a capacitor, and use a new technology, nanotechnology, to make that old product in a new way.'
Capacitors contain energy as an electric field of charged particles created by two metal electrodes, and capacitors charge faster and last longer than normal batteries, but the problem is that storage capacity is proportional to the surface area of the battery's electrodes.
MIT researchers solved this by covering the electrodes with millions of nanotubes.
'It's better for the environment, because it allows the user to not worry about replacing his battery,' he says. 'It can be discharged and charged hundreds of thousands of times, essentially lasting longer than the life of the equipment with which it is associated.'"
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Oh great (Score:5, Funny)
Re:Oh great (Score:5, Funny)
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Re:Oh great (Score:5, Funny)
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Obligatory, with apologies (Score:3, Funny)
Re:Oh great (Score:4, Funny)
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Re:Oh great (Score:5, Funny)
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Re:Oh great (Score:5, Funny)
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Re:Oh great (Score:5, Informative)
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Not sure how this works (Score:3, Interesting)
Re:Not sure how this works (Score:5, Interesting)
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Re:Not sure how this works (Score:5, Informative)
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Re:Not sure how this works (Score:4, Informative)
So the nanotubes from one electrode are not immersed in dielectric (insulator), they are immersed in the other electrode.
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Re:Not sure how this works (Score:5, Interesting)
In electrolytic capacitors, one electrode is formed by a conducting liquid, and an oxide layer on the metallic conductor acts as the insulator. The nanotube version may use something like this.
On another note, every time someone proposes to replace batteries with capacitors, I wonder how they make up for the huge variation of voltage that a capacitor delivers. Basically, the voltage of a capacitor is proportional to the amount of charge stored, whereas a battery provides more or less constant voltage. The capacitor-battery would require a circuit (something like a switching power supply) to be able to provide constant voltage. That, in turn, would take up space and waste some energy.
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Re:Not sure how this works (Score:5, Informative)
There are some very efficient (90%+) DC/DC converters available right now. Some will even automatically switch from step-up to step-down mode on-the-fly. Many battery powered devices already use these ICs to supply the multiple voltages needed, e.g. 1.5V and 3.3V logic, and 10-14V for a white LED backlight in phones and digital cameras So designing these devices to use a nanotube capacitor wouldn't necessarily require a more complex or less efficient power supply. So I think we can solve the voltage issue if they can build the capacitors.
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Re:Not sure how this works (Score:4, Informative)
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Riverworld anyone? (Score:5, Interesting)
Re:Riverworld anyone? (Score:5, Insightful)
Well - its a bit of a no-brainer to any EE kind of guy. No wasteful energy conversion process, etc etc.
Everyone's been waiting for the materials technology to catch up to the rather obvious idea that's all
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Re:Riverworld anyone? (Score:5, Funny)
It was a sad, sad time to be Hello Kitty.
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Fascinating (Score:4, Interesting)
Are these capacitors only likely to be suitable for for small scale charges/discharges? Mobile phones? laptops? cars themselves?
More questions than insights, I'm afraid, but I find it fascinating
Re:Fascinating (Score:3, Informative)
time to market (Score:3, Interesting)
hydrogen fuel cells would still be great for larger things like cars.
could these be produced in a way to fit in existing devices as soon as possible? I'f this really is safer for the environment, I'd love to see these asap, especially as most batteries are standard sizes already, even inside a laptop battery there are often (always?) muliple standard sized cells.
I hope they're easilly recyclable too, for when they do finally fail.
A good electric Car. (Score:5, Interesting)
Re:A good electric Car. (Score:5, Insightful)
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Re:A good electric Car. (Score:5, Insightful)
Unlikely at best. The problem is that the rate of energy transfer for chemical storage (that is, fuels, like gasoline) is really, really high. While you could in principle build a station which could recharge your batteries in the same amount of time it takes to gas up your car, it wouldn't be something you'd want to be near.
Why?
When you put gasoline in your car, you are moving power at a rate of about 5 MW. That's the entire output of a small power plant. Liquid fuels, gasoline in particular, are a very dense way to store and transport energy. Electrical wires aren't very good for that in comparison, even with superconductive cables. Think of it this way, even if we could transfer energy from a station to your car with 99.9% efficiency (which is well and far beyond anything we can do in the forseable future), that's 500 W of power that needs to be dissipated at the conversion site between the station and your car. That's going to be too hot to hold like a fueling nozzle for gasoline cars. If we use 48V to move 5MW (48V is gaining traction as a new standard for power transfer), that's 100,000 A of current. Even if we use an insane voltage level like 5 kV, prone to arcing and causing nasty things like fires and death, that's still 1,000 A of current. Not small. If this power is transferred by direct contact, you get immediate electromigration at the contacts, arcing problems when starting and stopping the current (ever wonder why power transmission towers are so tall?). If it's transferred by induction, then the EM fields will be enough to cause cancer (ok, I don't know that one for sure, but it's going to be as if 1000 microwave ovens are all operating right there at your car, something I don't want to be near).
Building an electrical system that can move megawatts of power is not something that will ever happen on the consumer level.
What about improving the efficiency of cars? We can make cars at best an order of magnitude more energy efficient. That isn't going to solve the problems alone.
Now, if, instead of recharging, you swap out batteries (that is, move mass that carries energy instead of moving energy aone), things get far more attractive. Except that people are currently a little leary of exchanging parts of their cars (can you imagine swapping tires every time you went to a filling station?). But that would allow a quick recharging.
The only solution that really makes sense for refueling by recharging is to do it while the vehicle is sitting idle when there is more time available, rather than being driven when there isn't. If you allow 20 hours for a recharge instead of 5 minutes, the power transfer rate drops to 20 kW which isn't so bad. Add in an order of magnitude higher efficiency vehicles and perhaps live with shorter distances between recharges, and you get down to the kilowatt range which is entirely doable (1.5kW can be supplied from a single, standard US household outlet).
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Re:A good electric Car. (Score:4, Insightful)
You realize that you have now committed the classic blunder (second only to getting involved in a land war in Asia). Millions of engineers are now scrambling to prove you wrong, at any cost!
Here is how I would do it: Battery in car is a one meter square, 2 cm thick. Charging station brings over their one meter square battery, places it on top of yours. Power is transfered at 50 volts x 100,000 amps - but that 100,000 amps is flowing through a "wire" half a square meter in area, which is the equivalent of 0.1 amps through a somewhat standard 1mm wire. In other words: the efficiency is basically 100% (it would be hard to estimate before doing it, but very high); the grid can see a long slow charge (as the Charging station can slow charge their transfer battery); the energy transfer is done at 5MW, so it takes only a few seconds to fill your car.
OK, I think you owe me lunch now!
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Re:A good electric Car. (Score:5, Insightful)
U = 1/2 * C * V^2
Where C is the capacitance, in Farads, and V the Voltage. For comparison's sake, a typical 1.5 Volt AA battery is rated for around 2000 milliamp-hours (why they use this ridiculous measurement, I don't know, but it's all I can find). So a tiny AA battery stores the potential energy
U_battery = 2000E-3 Amps * 1.5V * 3600 seconds/hours
Or, it stores 11,000 Joules. Now, searching for big capacitors on froogle [google.com], I came up with a link from Autotoys for a 1 Farad capacitor, on sale for a mere $42 (which is actually really cheap for one of those bad boys, but anyways..). It claims to have a "surge voltage" of 20V. So, assuming it's charged to 20V, the potential engergy in the capacitor is
U_cap =
So this $42, huge capacitor stores 200 Joules, in comparison with our AA battery that stores 11,000 Joules. In addition to the problems of price, miniscule total energy storage, storage space (making impractical for electrical car use.. you'd need a TON to power a car for an hour.. 100 HP = 75kW, for an hour, that's 270 MJ.. that's a lot of capacitors), in order to get the most out of capacitors you have to charge to a very high voltage (since U goes up with V^2), so you need a high voltage DC power supply, and finally, unlinke batteries, capacitors' voltage goes down exponentially with time, so you need clever (i.e. large, complicated) circuitry get out a constant voltage from a capacitor bank.
Basically, capacitors have their place (namely, smoothing voltages, or storing small amounts of power for quick discharge, i.e. camera flash), and batteries have theirs. The article is very light on specifics, but even if, say, the Cost / Farad goes down by an order of magnitude, and they manage to shrink the size as well.. I still don't see much changing. They also don't mention whether these things work at high-voltage. If they can't be charged up to 500+ Volts, they're not going to be able to store much energy. I'm not an expert on capacitor design, but if you look around for high-voltage capcitors (they go up to 10kV+), they pretty much all have tiny capacitances (e.g. 800pF, 10kV). I assume there must be some inherent difficulty in making them with both a large capacitance and high-voltage rating (or perhaps too dangerous.. who knows?). Don't get your hopes up just yet.
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Re:A good electric Car. (Score:5, Insightful)
I wonder what one of these big capacitors would do in a crash? At least they're not filled with so many chemicals as normal batteries, but what would happen?
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Re:A good electric Car. (Score:4, Insightful)
Imagine the size of a megawatt-hour capacitor!
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i remember discussing this back in physics class (Score:3, Interesting)
Re:i remember discussing this back in physics clas (Score:5, Insightful)
I'd gladly pay 4 times (or more) the price of regular batteries to have batteries that recharge in seconds and never need replacing. This will be great in cell phones and laptops, too.
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Re:i remember discussing this back in physics clas (Score:5, Interesting)
... And thus the comments about the mfg. process 'catching up'. I think we already don't use Li-Ion AA's and AAA's because they're cost-prohibitive, and the packaging is wasteful of space. I already wince at paying about US$2.50 per individual AAA for NiMH. But this technology promises features I think are worth paying for, just like having Li-Ion and Li-Polymer batteries in your cellphone, mp3 player, and PDA right now. Imagine when the battery for your cellphone or iPod is long-lived enough to be printed onto the circuit board and never replaced, and it can receive a charge in only a few seconds. If this is done properly, it'll eventually be the end of removable cells altogether.
This even opens up a lot of integration possibilities that just weren't there before, like peripherals that bring their own capacitor bank in to boost the system's capacity. Everything with a PCB can now cache its power, without all the bulk of a traditional battery. Imagine expansion cards that can carry the power needed for I/O (Wireless, Flash Memory, whatever) and charge with the system. You could even use the memory expansion slot as an auxiliary battery, like on some laptops how the optical drive can be replaced with another battery.
Take this with System-On-Package designs like were just recently discussed here, and we may get some really small electronics in our lifetime. You could even reduce capacity to save space -- I wouldn't mind charging my cellphone almost every night if it only took a few seconds.
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summary misses an important bit... (Score:3, Insightful)
Safety? Durability? (Score:5, Insightful)
First, safety. One of the amazingly cool things about capacitors is that they can deliver all their charge over the course of a few milliseconds. This makes them very useful for things like strobelights and subwoofers. But it can be very, very dangerous: What happens if you drop your in the toilet? Or you drop your iPod and it gets run over by a car? If they have batteries, a short circuit will cause the battery to get warm for a while, or it will release some slightly caustic goo and you have to wash your hands. But if they have capacitors, you get an explosion and a violent electrical arc.
Second, durability. You can beat the hell out of a chemical battery, expose it to shock and vibration to no end and it will continue to operate. These nanotubes, OTOH look awfully easy to break. Breakage could cause two things to happen: loss of capacitance, or worse, an internal short circuit, and see above.
It will be interesting to see how these two problems are addressed, or if these cool toys will be relegated to industrial and other controlled-environment applications.
Re:Safety? Durability? (Score:5, Interesting)
You have never created an internal short circuit on a conventional (rechargable) battery, did you? It is also able to deliver all the stored energy on an explosion that will take your hand away.
Now, batteries don't explode all the time, because they are well blinded. Capacitors are less dangerous (carry less energy), so they are not that well blinded, and explode often. There is nothing stopping the people from making blinded capacitos out of economics, and it could be even safer than battteries, because there is no ion trading going on.
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Re:Safety? Durability? (Score:5, Informative)
Sorry, that's incorrect.
Try shorting a car battery with a screwdriver and tell me there isn't a violent electrical arc. Also, NiCads (and I believe NiMH) have very low internal resistance - if shorted, they can literally explode as they overheat dramatically. You're confusing this with non-rechargeable batteries, which behave as you describe.
Also, capacitors deliver charge at a rate dependent on the impedance of the load they're driving. It would be very straightforward to put a small resistor in the package containing the capacitor, so that the current out of it is limited.
Regarding the short-circuiting, capacitors require overlapping surfaces that are electrically insulated from each other. That means if you're using nanotubes, you'll want both sides covered in nanotube "fuzz" and the two sides then pushed together so that the two intertwine. This means that one (or preferably both) sides need their nanotubes coated with some kind of insulating material for it to work, otherwise the nanotubes will simply short out, and then you won't have a capacitor any more. And that means you won't get short circuits from random broken nanotubes in the structure.
Fragility I don't know about, but since carbon nanotubes are the strongest substance currently known, I suspect it's not going to be a huge problem. Also consider that the whole thing could easily be encapsulated in some solid insulating block so that it's a single physical chunk (remember that carbon isn't a metal so there are no significant expansion/contraction issues with heat). Batteries are only as solid as they are because they've got a solid metal case encapsulating well-packed electrodes and electrolyte - try dropping a plastic-case car battery from a height and tell us how solid it is.
Given how desperate battery manufacturers are for any kind of edge, I imagine this will be rushed to market as fast as physically possible!
Grab.
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Capacity? (Score:3, Insightful)
out with a bang (Score:4, Funny)
But what kind of capacity will it have? (Score:3, Funny)
My experience with capacitors. (Score:4, Interesting)
Could someone explain pls? (Score:3, Insightful)
Seems like I miss something. It's not the area of the capacitor that matters (yes, I know the formula C=A/d for flat electrodes) but an "effective area". These capacitors are supposedly two flat or nearly flat substrate surfaces each coved with nanotube "fur". There is a gap between these two electrodes. The gap is much larger that the thickness of the nanotube. Consequently, the effective area of the capacitor is not much larger than the area of the flat substrate electode. What's the advantage of the "fur"? I would understand if [+] and [-] charged nanotubes were alternating inside the fur, but it's clearly not the case judging from the picture.
For instance, take a wire, cut it in half and separate two pieces by a small gap. That's a capacitor. Its capacitance is going to be somewhat larger than the A1/d where A1 is the area of the wire crossection, and a lot smaller than A2/d where A2 is the full surface area of the wire. The same applies to nanotubes.
So, obviously, they are doing it differently. How?
There's a limit (Score:3, Interesting)
It is hard to exceed a certain energy storage on a capacitor. As you move the plates together, the capacitance goes up and you can store more charge per volt. The breakdown voltage goes down as you move the plates together. So you can store a small charge at a high voltage or you can store a large charge at a low voltage. For a capacitor of a given volume, you can store only so much energy depending on the breakdown voltage of the dielectric material.
I don't doubt that you can double or triple the energy storage of capacitors compared with current technology. On the other hand, I am very skeptical about the possibility of getting enough capacitance to store enough energy to be a general purpose battery replacement.
I leave it to you as an exercise to calculate the capacitance of a 2 volt capacitor necessary to store one amp hour. ie. something similar to an AA battery cell.
Real-world example (Score:5, Interesting)
What about the energy-density ? (Score:5, Insightful)
The thing is, one kg of petrol holds around 45MJ of energy. One kg of NiMH batteries hold around 0.25MJ, a factor of almost 200 less. A lead-acid battery holds half that. A normal capacitor holds 0.002 MJ/kg.
So, even to compare with lead-acid batteries in energy-storage this thing needs to be 50 times better than normal capacitors.
Recharging in seconds is fine, assuming you can build a sensible car that goes oh say 100 miles at the least between recharges, that's perfectly acceptable for most people. Same for cellphones; faster recharging is very nice. But only if you can still go for 2-3 days without recharging, and talk on the phone for atleast an hour or two before its empty.
A car that could only go 20 miles between recharges would not be a hit, not even if the recharge was done in a minute.
Re:What about the energy-density ? (Score:5, Informative)
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Supercapacitors (Score:4, Informative)
However, there is now a lot of academic and business interest in them as they are ideal for a wide range of modern applications. Devices like UPS's and power smoothers still run on lead acid batteries, which are bulky, contain corrosives and are prone to unexpected failure (at least mine seems to be). There is also a big push from the electric vehicle crowd. Note though that they are unlikely to form the primary power source for an electric vehicle (they still have poor energy density compared to chemical technologies), but are extremely attractive for both initial power-up (i.e. heating a fuel cell to running temperature) and for sensible implementation of regenerative braking - charge the supercap when you brake, use the energy for short term bursts (driving up a hill, overtaking etc).
Everything old is new again (again!) (Score:4, Informative)
Power supply problems (Score:5, Informative)
Some math to back this up: My work laptop, a Dell Latitude D610, has a 53 WHr battery. My home laptop, an Apple 12" Powerbook, has a 46 WHr battery. These aren't huge laptops, mind, and battery capacity is only on the rise as consumers demand more.
Let's use the Dell example, 53 WHr. Change hours to seconds, that's 53 * 3600 = 190,800 Watt-seconds (more usually known as Joules). 191 kJ - that's a fair bit of electrical energy to store, either in battery or in capacitor form. Let's ignore losses that occur in the charger and energy storage device - assume everything is 100% efficient for a moment.
What if we wanted to charge up that 191 kJ capacitor in, say, 10 seconds. That would require a 191 kJ / 10 s = 19.1 kW power supply. Hmmmm, don't think we'll be seeing one of those in a laptop bag anytime soon.
Laptop batteries are a particularly high-energy example, but it illustrates the kind of power increases you'd need to accommodate if instead of charging in hours, you charged in seconds. If you had a battery that used to charge in, say, one hour (cellphone, PDA, whatever), and you instead wanted to charge it in (again, for example) 10 seconds, the charging power supply would need to put out 360x more power. Even to charge it in a minute would require a 60-fold increase in power. That'd be an amazing and fascinating power electronics problem to consider - how to make such charging devices as compact as today's.
Re:Power supply problems (Score:5, Informative)
The real gotcha is that the charge power is not anywhere close to constant like the first 80% of a charge to a conventional battery. Within the first 20% of the charge cycle you'll have pushed 2/3 of the total power that cap is going to draw if it's readily available. With that in mind they'll probably have a built in cut-off similar to those used in Li-Ion batteries that prevents the cap from discharging below a certain point. which would certainly limit the available power but lessen the demands during charging.
So basically if we want charging in seconds like the article suggests, we're working with overly large power requirements and/or diminished capacity. If we want minute scale chargnig we're looking at diminished capacity and reasonable power requirements.
There's also competition with newer Li-Ion and LiPoly configurations which, through the use of nano-tech as well, to give us 80% charges in 5-10 minutes. There are also quick-charge NiMH solutions already on the market which can pack about 40,000 joules into 4 cells in 8-15 minutes and are scalable to laptop level battery configurations.
I don't think this is going anywhere for a while, but it could end up with some use in industry eventually. And I certainly like the idea of large cheap caps even if they won't replace batteries any time soon.
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Re:Let me be among the first to say, (Score:5, Informative)
That said, I would not hold my breath waiting for this product to come out. The making of the nanotubes in the way that they have is not hard, but I would be suprised to learn that there is not some other performance or quality issue that needs to be struggled with.
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Re:Let me be among the first to say, (Score:3, Funny)
The image of nanotubes that they show are almost certainly nanotubes made by chemical vapor deposition (CVD). CVD is cheap, scalable, fairly easy, and found in every semiconductor fab you have ever gone to.
That said, I would not hold my breath
I would, given all that chemical vapor around. Speaking of which, this sounds like a great way of powering my Phantom console running Duke Nukem Forever.