Capacitors to Replace Batteries? 499
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.'"
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
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.
Capacity? (Score:3, Insightful)
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.
Re:i remember discussing this back in physics clas (Score:1, Insightful)
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?
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?
Re:A good electric Car. (Score:5, Insightful)
Re:Fascinating (Score:2, Insightful)
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:A good electric Car. (Score:3, Insightful)
What seems more likely is a swap-a-cap, drive in, old cap is pulled out, freshly charged one is popped in. The empty ones are sent to a big recharging center, probably attached to a nuclear power station, one station could charge a lot of caps.
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).
Re:What about the energy-density ? (Score:3, Insightful)
Schindall says, "Small devices such as hearing aids that could be more quickly recharged where the batteries wouldn't wear out; up to larger devices such as automobiles where you could regeneratively re-use the energy of motion and therefore improve the energy efficiency and fuel economy."
He doesn't say it will replace the main battery of a hybrid car. The bulk of gas mileage gain of such car comes from the regenerative braking. Gas engine running at constant optimum RPM and load is another, smaller source of gain.
Regenerative braking requires an energy storage device with characteristics that precisely match those of ultracapacitors: moderate energy storage density and ability to take a huge spike of recharge current in seconds or faster.
Toyota Prius still has the regular brakes for this reason - the battery can not absorb all the energy released during hard braking.
Re:A good electric Car. (Score:4, Insightful)
Imagine the size of a megawatt-hour capacitor!
Re:What about the energy-density ? (Score:3, Insightful)
If you go to discover.com and track down their version of this story you'll find the blurb below. It still doesn't say comparable energy density, but at least it says comparable amounts of energy.
More worrying to me is the dreaded "five years away".
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.
And the Sony car (Score:3, Insightful)
Seriously, compatibility has always struck me as the weak point of the battery-swap idea. You would have to get all the car manufacturers in the world to agree to a standard size, shape, connection, and electric properties. This would prevent Ford (for instance) from saying "The new Escape has a battery that lasts four times longer than the competition" and would discourage battery improvements, because when you dropped your "improved" battery at the station, who is to say if you'd get the same "improved" type in exchange?
Indeed, there's a similar problem for recharging battery-powered cars, as you'd have to have standard charging paddles. But at least you could upgrade your batteries (or the charging equipment) and keep the old charging system.
The big benefit that gas currently has (aside from high energy density) is it's a physical substance that's easily used by "common" physical interfaces. A BMW, Ford, and Renault may all have different length/shape/diameter filling tubes, but as long as it's "close enough" you can get the end of the pump nozzle into the hole.
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!
Can you provide actual numbers ? (Score:1, Insightful)
Ratios between capaciter and equivelent battery with the same energy stored
- Volume ratio
- Weight ratio
- Recharge time ratio
- Estimated number of recharges during its lifetime
The lack of basic facts in the article should be corrected.
Re:Riverworld anyone? (Score:3, Insightful)
The popular myth is way more dramatic though. Thus why the Mythbusters probably chose to duplicate IT, rather than the actual experiment.