I happened across an interesting post on the promise of ultracapacitor startup EEstor at Ezra Klein's site. Guest poster John raises a few interesting questions and a commenter gets the last word in. The latter reminds us that 1/2 the energy stored in a capacitor eventually gets wasted. I rolled this assertion around in my head for a bit and essentially come to the same conclusion. If you consider that any power source has an internal resistance (R1=r) that must eventually get balanced by an external resistive sink (R2=R), the most efficient power transfer occurs when r=R.
P=I*V=I*(I*R)=I2RReduce r and you have to reduce R to get the current up. Increase r and you have to increase R to generate a significant voltage drop. And since equal amounts of power get consumed internally and externally when r=R, only 1/2 becomes usable for an application.
dP/dR = 0 when r=R
The same principle follows from audio amplifiers having an internal impedance designed to match that of your typical speaker (8 ohms). Half the power gets lost as heat emanating from your amplifier.
Another reminder that energy storage devices by themselves do not save us from dealing with the real issue at hand:
But these are all forms of energy storage, not sources of energy on their own. The primary form of energy for the United States would still, even if every car had one of these EEStor capacitors in it, still be coal and oil. (We could use a lot less, but still.) The objective still has to be reducing the amount of oil we use to avoid Peak Oil (whenever it happens, better to prepare early.) Quite aside from Peak Oil, climate change requires that we stop using fossil carbon altogether. Storage technologies, exciting as they are, are not by themselves the answer.
Update: The anonymous commenter has a good point, but consider that big-money military rail gun development and other directed energy weapons has predominantly funded the development of ultracapacitors. Here, and in other applications (like the Tesla Roadster racing) where you need huge amounts of instantaneous power, power extraction efficiency remains important. All those fast discharge losses add up. But I'm happy to see designs get above 80% efficiency (charge+discharge). I have to admit that I must have retained little from the switching power supply class I took in grad school circa 1986.