- #36
pbuk
Science Advisor
Homework Helper
Gold Member
- 4,657
- 2,926
No, it is created by electrolysis. I am assuming that this work will have to be done, in addition to breaking the chemical bonds (presumably half as much energy again will need to be used in forming the oxygen too, but I'm thinking maybe you could use a different electrolyte to avoid this: it is a detail less relevant at the moment).mfb said:Creating a vacuum costs 100kJ, but that's not what you want to do. The hydrogen does not appear out of nowhere.
Obviously, but I'm not bringing it back to the ground, I'm burning it at altitude and reclaiming the chemical energy to help power the next cycle of electrolysis.mfb said:And bringing it back to the ground (to re-use it) will cost those 500kJ again.
Isn't that pretty much how an air source heat pump works to extract thermal energy from the atmosphere? Commercial pumps operate at about 250% "efficiency" I believe.mfb said:There is a nice related thought experiment:
Imagine a closed box with gas at a pressure p in a perfect vacuum, thermally coupled to some very large object (so the gas is always at the same temperature).
As the temperature and the amount of gas is constant, p*V is constant. If you slowly expand it from volume V1 to V2, you get an energy of ##E=pV_1 \ln\left( \frac{V_2}{V_1}\right)##. This has no upper limit - you can get as much energy as you want!