The "Critical Success Factor" for energy independence lies in creating a device with
high energy density, both by unit mass and volume, that can be quickly re-charged.
As discussed in the energy generation section, this device does not need to be as
dense as gasoline, only about 1/4 as dense. Quick re-charging is necessary for mass
conversion to electric vehicles because it is not always practical to take a long
time to charge a vehicle battery. Many urban dwellers live in places where they
cannot charge their vehicles overnight. Travelers on long journeys want to quickly
re-supply their vehicle with energy rather than wait a long time for a re-charge.
Electrical energy may be stored by converting it to chemical
energy in a battery, or as charge in a capacitor. Capacitors are more efficient
but typically have much lower capacity and storage lifetimes compared to batteries.
Capacitors can charge very quickly compared to batteries, which gives them the
quick charge characteristic we need for automobiles. Where batteries take hours
to re-charge a large capacitor could charge in minutes. So if a capacitor could
be designed with adequate capacity for an automobile it could be
recharged as quickly as a tank of gas in a car can be refilled.
Nanotechnology now gives us an unprecedented opportunity. With the advent of
carbon nanotube supercapacitors we possibly could create a capacitor with the
capability of replacing a tank of gasoline. The basic idea for why these
capacitors have greatly increased capacity over other capacitors can be
understood by looking at the basic equation for capacitance of a parallel-plate
capacitor:
Where ε is the permittivity of the material between the plates
(just think of it as a constant that depends on the material being used), and A is the area
of the plates, and d is the distance between the plates. Now picture two parallel plates
held a small distance apart separated by a small layer of plastic film. Ok that is your
basic parallel plate capacitor. Now take these flat plates and cover them with carbon
nanotubes only several nanometers in diameter. These nanotubes are very closely spaced on
the plates. Sort of like short hair on a scalp but the nanotubes are much finer
(about a million times) than hair and spaced much closer together than hair. The result
is the surface area of the plates has increased thousands of times and thus the
capacitance of the device will increase proportionately.
"Supercapacitors" like these are being worked on in several places. EEstor
in Texas has a patent in the area and has received several million dollars in venture
capital funding. Riccardo Signorelli, Joel Schindall, and John Kassakian at MIT
are working this see: Carbon Nanotube Enhanced Double Layer Capacitor and also Carbon Nanotube Enhanced Ultra Capacitor.