A new take on a very old technology is about to beat batteries all hollow in terms of energy storage density and cost.      Its aimed at powering plugin cars.  EEstore, a leading startup in this area is mysterious, but news that does leak out indicates maybe...

" EESU is projected to offer up to 10x the energy density (volumetric and gravimetric) of lead-acid batteries at the same cost. In addition, the ESU is projected to store up to 1.5 to 2.5 times the energy of Li-Ion batteries at 12 to 25% of the cost."

http://thefraserdomain.typepad.com/energy/2007/01/eestor_in_techn.html

Ben Franklin used one of the first capacitors, the leyden jar, in his lightning experimentation.  The new ultracapacitors use a nanotech insulsating material between two metal sheets rolled up to boost this simple technology beyond the energy storage capability of batteriers.

My wild speculation is that the power transmission technology used right now, namely 500,000 volt direct current transmission, could be used for energy storage employing a nanotech ultracapacitor thousands of miles long that doubles as transmission line.  Another beautiful aspect is that unlike alternating current transmission lines, that lose too much power to ground when buried or running underwater, direct current transmission does not have this loss problem.

It can be buried out of sight and mind of NIMBYs.  Plus   (  this is a big plus), it does not emit electromagnmetic radiation like alternating current transmission, no stray voltage.  No problems with real or imagined health effects.

Using the principles of the basic physics of capacitors a rough estimate of storage available per mile of this proposed transmission/storage can be made.  The ultracapacitor in development by EEstore is purported to have aproximately 2   times the  electrical   storage   density of  lithium ion batteries.  

And the energy storage of a capacitor is directly proportional to voltage, the 500,000 volt ultracapacitor would store maybe 1000 times the power per area of metal plates rolled up into the device.  but of course the insulator would need to be thicker for 500,000 volts operating voltage than 500 volts.  The density would probably be 100 times the storage potential of the EEstore device, due to the much higher voltage.  depending upon the performance of nanotech materials.

I'll use 100 times for this guesstimate.  An EEstore electric car   ultracapacitor  could store around 70 kwh for the same size as the Altairnano li-ion plugin car battery.  About 7 kwh per cubic foot.  So I'm guessing around 500 kwh per foot of this transmission/storage line.  Around 2.5 million kwh per mile.  So in a thousand mile transmission loop that is 2.5 billion kwh.

The total power generation capacity of the US   is around 1 billion  kw.  That would mean that 50 of these 1000 mile loops, one per state on average, would store enough electricity to power the US for 125 hours with no generation input.  That would more than do the complete job of backing up renewables.

This is all wild speculation at this point.  Any utility engineers have any critique of this analysis?  Please be frank.