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The Register reports
In a paper published at [Proceedings of the National Academy of Sciences] (abstract), the researchers claim capacitance of more than 1,100 Farads per cubic centimetre--or around 1,145 Farads per gram, which is about as much as they reckon you could get out of the manganese dioxide (MnO2) in the cap.
Using a combination of graphene and MnO2, the researchers say the energy density they can achieve can be as high as 42 Watt-hours per litre, which is getting close to that of a lead acid battery.
It's not much yet: the demonstrator pictured below from the UCLA California NanoSystems Institute is one-fifth the thickness of paper, however it can hold charge long enough to power the demo LED overnight.
That, the university claims, beats a thin-film lithium battery on a pound-for-pound (or rather gram-for-gram) basis.
Manganese dioxide is cheap and plentiful, and is good at storing charge--which is why it's popular in dry-cell batteries and alkaline batteries.
The combination of the MnO2 and laser-etched graphene--the secret sauce in all of this--can be produced without dry rooms or extreme temperatures.
(Score: 3, Interesting) by richtopia on Monday April 06 2015, @11:11PM
I like them, and I hope they work out, however they are fundamentally flawed. A capacitor has a linear discharge curve (from the physics of two plates losing a potential between them). This means that at 100% SOC you have full voltage, but as you drain the capacitor the voltage drops.
Lithium batteries have very flat discharge curves. This is frustrating for calculating the energy left in them, but it helps keep the performance constant of the vehicle over the discharge of the battery.
Gasoline would compare to a flat discharge curve. No matter how much is in the tank the car performs the same. With a capacitor, the car would be peppy at first but performance would drop as the voltage drops. I don't think people would be on-board with that concept.
(Score: 2, Interesting) by _1156277 on Tuesday April 07 2015, @12:15AM
I'd imagine (I'm not an EE) that the current could be maintained through a switching power supply.
(Score: 0) by Anonymous Coward on Tuesday April 07 2015, @04:18AM
A capacitor has a linear discharge curve
I'm sure you meant to say something entirely different and had a keyboard malfunction.
The discharge curve of a capacitor is really heinous. [learningaboutelectronics.com]
For those still interested: Charge curve. [learningaboutelectronics.com]
As _1156277 indicated, you'll be wanting a buck-boost switching regulator in this application.
-- gewg_
(Score: 1, Informative) by Anonymous Coward on Tuesday April 07 2015, @09:44AM
There's nothing heinous about an exponential curve.
(Score: 1, Insightful) by Anonymous Coward on Tuesday April 07 2015, @08:09PM
The systems that will be consuming this energy will be looking for a constant voltage.
The discharge of a rechargeable battery (the power source this will be replacing) is pancake-flat until it falls off a cliff at the very end.
Trying to replacing that with [E * (1 - T/tau)] is a really lousy fit.
The buck-boost regulator mentioned can deal with input voltages that are higher than or lower than the desired output voltage.
Even then, that regulator's compliance range is limited.
Compared to what the system is expecting, "heinous" is an apt description for this square-peg/round-hole situation.
-- gewg_
(Score: 2) by Zinho on Tuesday April 07 2015, @02:32PM
Yes, he did mean something different. If you make a graph of stored charge vs output voltage you'll get a perfectly straight line from the origin out to the rated max voltage of the capacitor. The slope of this line is so perfectly predictable that it's considered a fundamental characteristic of the part, "Capacitance". As the GP stated, this makes it very easy to read the current stored charge, at the cost of having a rapidly reducing voltage as that charge is used up.
In contrast, chemical batteries tend to produce a nearly-constant output voltage over a large portion of their discharge time. A new 9V battery may produce 9.5V, and will degrade non-linearly down to 8.5V when discharged. That makes it able to produce useful amounts of current for its entire lifespan, but harder to guess current charge state just based on its momentary voltage.
Honestly, I think we all three agree on this point, we're just saying the same thing different ways. If someone wanted to use a capacitor as a battery replacement it would only produce the correct voltage over a narrow range of its discharge time; your graph showed that beautifully. As such, it wouldn't be able to be connected directly in to the circuit the way a battery is. Options for dealing with this would include: increasing the voltage on the capacitor and use a regulator to reduce voltage provided to the circuit, or perhaps using some control circuitry to step up the voltage provided by the capacitor to the circuit. The first option would flatten the discharge curve significantly, but increases the max voltage you've got in your system; not sure how people would respond to having a few kV under the seat of their car, it brings new meaning to "no user-serviceable parts inside". The other option would make the discharge curve steeper, so if you think the curve you linked now is heinous, the new one would be worse.
Anyhow, as long as total stored energy is equivalent and there are advantages like weight, lifespan, or cost I'm sure someone will build it.
"Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin
(Score: 2) by richtopia on Tuesday April 07 2015, @06:40PM
Thank you for the clarification, I admit my original post was lacking.
I was recalling the major takeaway from a presentation from Dennis Corrigan, so my memory of the finer points may be flawed. From the abstract it looks like this paper covers the topic if anyone is interested (although it is paywalled; I imagine most universities could access ECS papers).
(Score: 2) by maxwell demon on Tuesday April 07 2015, @07:37PM
Well, people were fine for quite some time having quite a few kV in boxes in their living rooms.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by bookreader on Tuesday April 07 2015, @10:30AM
.. A capacitor has a linear discharge curve (from the physics of two plates losing a potential between them). This means that at 100% SOC you have full voltage, but as you drain the capacitor the voltage drops. .. With a capacitor, the car would be peppy at first but performance would drop as the voltage drops. I don't think people would be on-board with that concept.
This might be true if the capacitor is directly connected to the motor, and at the same time the motor is of a type that changes its torque and speed characteristics when the voltage changes.
Isn't there some inverter in electric cars, which sits between the power source and the electric motor and takes care that the motor does what the driver wants it to do?