Green Car Congress reports on a joint announcement by Nanotek Instruments and Angstron Materials of a supercapacitor which has energy storage over 80 Wh/kg. This is in the performance region of nickel metal hydride materials and getting close to lithium-ion, in a material which can handle 1000 A/kg. The claimed energy capacity of 85.6 Wh/kg, assuming a working voltage range of 0 to 4 volts, gives a capacitance of about 38 farads per gram; charging and discharging at 1 A/g, this could go from empty (45% voltage) to full in 80 seconds... or the reverse.
Charging or discharging, this is pretty impressive. 1 A/g @ 2 V is 2 kW/kg, or 200 kW from a 100 kg capacitor. That's 268 horsepower at the low end of the discharge curve (half voltage to full). You could get some pretty serious punch out of this, or cram lots of regenerative braking power into it. The Buick LaCrosse BAS system could do with a 10 kg capacitor instead of a battery 3 times as heavy.
But that's just the beginning. There's another energy-storage mode in this material which can boost the yield in two ways! But to explain this, I need to go back to basic electronics and capacitors.
The classic capacitor stores energy by storing electrons on one plate, which attracts positive charges to (or repels electrons from) the other and holds energy in the electric field between them. Dielectrics can increase the charge stored and also introduce non-linearities such as hysteresis losses. But by and large, the charge and energy follow relatively simple equations relating the capacitance C, charge Q, voltage V and energy E:
But all this assumes that capacitance is a constant. If a charge is put on a capacitor and the capacitance is varied, voltage and energy vary with the capacitance:
What does this mean, given that the capacitance of this material rises with increasing temperature? It means that the energy stored can be increased by cooling it, releasing both electrical and heat energy at the same time.
The specific heat of the material is not given, and I won't estimate it. But if the capacitor is cooled from 80°C to "room temperature" while holding the voltage constant at maximum followed by a normal discharge, the energy available on a cycle rises from 136 Wh/kg to a whopping 186 Wh/kg. This is in addition to the heat released, which is (at least initially) hot enough to provide fast windshield defrosting and a nice, warm cabin within seconds of activation. Instead of being disadvantaged in the cold, the batacitor-electric car could wind up having better creature comforts than anything currently on the market.
If 1/4 to 1/2 of the storage system is active material, such a temperature-managed capacitor would eke out 45-90 Wh/kg. A Chevy Volt-sized pack storing 10 kWh would be about 111-222 kg (245-490 lb). This brackets the weight of the actual Volt battery (375 lb). It would also be able to take a recharge in less than 2 minutes.
With "range anxiety" eliminated and cycle lifetime and charging time removed from the list of people's concerns, the only barrier keeping PHEVs from a full takeover of the auto industry is the ability to manufacture the capacitors themselves. Graphite is a very cheap material, and hardly scarce worldwide.
In a world of even 45 Wh/kg graphene capacitors it makes no sense for any vehicle not to be partially electric. Whole sectors could switch from liquid fuels to grid power. It's the death knell for petroleum. It's the end of the world as we know it, and I feel fine!
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