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:Not sure how this works (Score:5, Informative)
Isn't this pretty old news? (Score:2, Informative)
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.
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.
Re:Fascinating (Score:3, Informative)
Re:summary misses an important bit... (Score:2, Informative)
The current needed to charge them so fast is tremendous, the cells would explode.
For example, for a AA cell of 2000 mAh, you would need 720 Amperes to charge in 10 seconds, or 1.44 KA to charge in 5 seconds.
Re:Not sure how this works (Score:4, Informative)
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.
Re:Not sure how this works (Score:3, Informative)
One solution to avoid a switching supply, would be to create a simple circuit that ties capacitors series/parallel as they discharge, to keep a more or less constant voltage.
BTW, everyone is focused on the power storage applications, but let's not forget the implications in electronics.
Electrolytic capacitors are some of the largest electronic components, so large capacities in small volumes would help miniaturization quite allot.
Re:Not sure how this works (Score:3, Informative)
Q = CV (where Q = charge, C = capacitance and V = voltage).
There's absolutely no problem regulating the voltage as it comes off of the capacitor, the biggest problem is getting the maximum Q high enough to supply more than a few minutes of constant voltage. It's not hard to get a high Q by increasing the total voltage across the capacitor, but that's extremely dangerous. If you accidentally discharge it, you can do some serious damage to yourself and anything the capacitor is hooked to. Presumably, this technology is being used to increase the Capacitance of the capacitor, which is roughly proportional to the surface area of each of the plates. The end result would be a MUCH higher Q at a much lower V, allowing for hours of sustained use and near instantanious recharge.
This could make electric motors for cars more fesible, as well as replace batteries in electric appliances like laptops.
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).
Re:Not sure how this works (Score:2, Informative)
Some comments below this one indicate that CVD (chemical vapour deposition) is used to grow the nanotubes. A link in the article to some information about a British effort along the same lines indicates that they grow the carbon nanotubes to build the basic hair-like structure. After this, they deposit a layer of conductive material (the first plate) onto the nanotubes. After this, a layer of dielectric (insulating material) is deposited. Finally, another layer of conductive material (the second plate) is deposited.
This is just what I have picked up.
Please note, I am not an electrical engineer or a physicist.
I just code 3D modelling software (http://gsculpt.sf.net/ [sf.net]) for fun.
Everything old is new again (again!) (Score:4, Informative)
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.
Re:A good electric Car. (Score:2, 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.
Article is not news... (Score:2, Informative)
Re:The article is really annoying (Score:2, Informative)
A battery is simply a collection of related things intended for use together.
What you think is a battery may be a collection of electric cells, but is more likely to be a single electric cell.
You can also have a battery of capacitors. In fact, the term battery was first used in electricity to describe a collection of Leyden jars, otherwise known as capacitors.
Re:What about the energy-density ? (Score:5, Informative)
Re:i remember discussing this back in physics clas (Score:3, Informative)
On the plus side, its discharge curve is more abrupt, so it tends to be better for powering electronics. Further, it provides many more charge cycles, has no memory effect, and has great shelf life (won't discharge as quickly as NiMH if not used).
Re:charge density. (Score:3, Informative)
60Wh/kg is a measure of energy density, which is to say, joules per kilogram in a charged state - they are just using units of watt-hour, which can be more convenient for energy storage measurements.
To put it into a normalised form, we have
100,000 J/s*kg (joules per second per kilogram) for the power density,
and
216,000 J/kg (joules per kilogram) for the energy density.
So, one kilo of capacitor could dump about 216,000 joules of energy into something in slightly over two seconds.
I believe that also runs the other way around, with a two-second charge.
But IANAEE.
Re:Oh great (Score:5, Informative)
Re:A good electric Car. (Score:3, Informative)
They contain equal abounts of positive and negitive charge. When the charges meet, they neutrilise.
When a large capacitior is shorted, it will likely cause damage to whatever shorted it. In the case of a car, this is likely to be a pieice of chassis, bodywork or the car's electronics. The energy released will most likely melt these, so the only real danger is that it could quite easily ignite any conventional fuel around, such as in a hybrid or a collision with a conventional vehicle.
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.
Re:A good electric Car. (Score:2, Informative)
Hopefully the same thing 10 or 15 gallons of gas does in current cars: Nothing. The easiest thing to do would be to short the terminals with enough resistance to avoid excessive heat but drain the capacitor within a few minutes or seconds, depending on the quality of the capacitor. Surrounding the whole thing in an insulating blanket would prevent physical damage that would short it internally and also prevent it from shorting to ground or the frame of the car.
Re:Let me be among the first to say, (Score:3, Informative)
Since the capacitor's charge is stored at the contact between the conducting and insulating parts, the benefit of this nanotube idea is that having a 'forest' of nanotubes poking out of the electrode will greatly boost the contact area, in the same way a heatsink's fins increase its own ability to dissipate heat.