Hacker News Reader

Comment by typedef_struct | Hacker News Reader

TIL one gallon of gas is 120 lightning bolts, and the relative cost of energy between electricity and fuel is near equilibrium.

View on HN

To illustrate that even more vividly: if a typical lightning bolt is a few km long, the energy released is equivalent to detonating a stream of gasoline roughly 0.1mm in diameter (not much thicker than a human hair).

When you phrase it like that, it does feel like the quoted energy figure is too low - after all, lighting that tiny a stream of gasoline isn't going to illuminate the sky anywhere as bright as a lightning bolt.

Perhaps the 'nickel of electricity' is what's remaining in electrical energy after all the rest has been used up as light, heat and sound across the sky?

Not in visible light, but I can easily believe it would be as bright if we could both see infrared and also if we made all that gasoline burn as fast as lightning propagates.

> lighting that tiny a stream of gasoline isn't going to illuminate the sky anywhere as bright as a lightning bolt.

Gasoline will burn for much longer so the energy will be released slower, so the peak will be much lower. And we perceive the peak light not the total amount of energy (see 1000 lumen stroboscope going 1 ms on - 999 ms off vs 1 lumen light turned on constantly).

Also gasoline will release more radiation in infrared part of the spectrum.

You are comparing two different time scales and two different spectra.

The lightning finishes in microseconds. The flame front speed of well-mixed gasoline/air mixture is about 16.5 m/s (see https://en.wikipedia.org/wiki/Flame_speed ) so 6 microseconds to cross that 0.1 mm.

Seems comparable, right? But that's the entire length of the lighting bolt in microseconds, not just one patch. Plus, the 0.1mm calculation assumed only gasoline, not a gasoline/air mixture with a 12:1 compression. Any guidance from a real-life comparison would be affected by the diffusion speed of oxygen. If it takes significantly longer to burn the same energy then the intensity (energy/time) will be significantly lower.

In addition, the spectra are different. Have you ever seen a fuel-based camping lantern? They use a mantle to make the light significantly brighter. (See https://www.youtube.com/watch?v=F3rncxf4Or8 for details). This mean the visible light from burning fuel isn't a good guide for the amount of visible light which can be generated from the same amount of energy.

But the energy release from the gasoline would be much slower, right?

Reminds me of the units used for vehicle fuel efficiency: litres per 100 km.

But liters is a unit of volume (length x length x length) and kilometers is a unit of distance (length). Hence, this efficiency metric is equivalent to L^3/L or just L^2. That is, the unit of vehicle fuel efficiency is a measure of area!

The area of what, you ask?

If you made the contents of your tank into a long thin stream of fuel as your vehicle moved along, then its cross section is the instantaneous fuel usage. You can imagine your car driving along, "sucking up" this long thin streamer of fuel as it moves.[1] The thicker this line of fuel, to more it needs for the same distance.

My car gets about 7L/100km, which works out[2] to just 0.07 mm^2, which is surprisingly thin!

[1] I can't take credit for this concept, I got it from XKCD's What If section: https://what-if.xkcd.com/11/

[2] https://www.wolframalpha.com/input?i=7+L+%2F+100+km+in+mm%5E...

You have remember that most of the energy in a storm is more to do with its exchange of air masses. It's more like a boiler than a dynamo. So it doesn't surprise me that lightning in practice isn't energy dense. But figuring out how to capture the energy generated seems like a fun experiment just to see how far we can go with material science than anything else.

Reply on news.ycombinator.com