Hi, I took part in discussions on various forums on viability of large-scale solar power as a primary energy source (on Earth), and on in-situ resource utilization (ISRU) for space missions. One idea discussed for Mars base was to produce CO and LOX from Mars CO2 atmosphere, obviating the need to find, scoop and process soil/ice/etc. I thought "hmm, interesting that Mars has this possibility to make fuel simply from air, while we on Earth do not". Then I realized that this is actually not true, there are compounds which can be synthesized from N2 and O2 which then can be used as fuel. Availability of some water vapor open up more possibilities. We don't use these fuels today on large scale because they are not very convenient, we have better ones derived from oil and gas. However, this may change. Large-scale solar power is finally happening. PV is no longer a "fancy green toy power source, not really competitive". It is competitive, and improving still. And as its market share grows, it will encounter the problem of needing to store energy during daytime for night-time load. So, to the question. Let's say we have huge PV installations whose peak power is significantly more than demand, making peak daytime electricity basically free. Let's say these installations are in Sahara and they are powering Europe. Can this excess power be used to produce fuels from the air, and use them at night to produce power? What those fuels can be? Is it economically viable? One possible fuel is N2O, nitrous oxide. It is non-toxic, it is not too difficult to store (and we already practice storage of many ton tanks of it), it is a monopropellant with a completely harmless exhaust. Questions: do current production processes for N2O use only air/N2/O2, or they use other routes? Another possible "fuel" is simply compressed air. I know that compressed air energy storage is practiced, but it has a problem: energy is lost when heated compressed air loses heat while in storage, and then on decompression it becomes very cold, nearly to liquefaction temps - which severely impacts amount of extracted energy. Because of this, commercialized compressed air energy storage schemes add a bit of natural gas burning to the generating machinery. This makes it "not exactly energy storage": now you need other fuel. Well, we have that N2O turbine running, don't we? And its exhaust is *warm* N2/O2 mixture. Can we use this heat to fix "decompressed air is cold" problem and use both schemes? Next. Compressing air allows to collect ambient water vapor. It can be used to produce other compounds: ammonia NH3, hydrazine N2H4, hydrogen peroxide H2O2. Can those be used as a fuel? H2O2 and N2H4 each can be used as monopropellant, and variously combined with N2O - as fuel/oxidizer pair. Since water is less abundant in air than N2/O2, and variable, I would imagine some sort of "N2O decomposition augmented by N2H4 burning" may end up being the best scheme. Other nitrogen oxides can be synthesized too. N2H4 + N2O4 is actually a quite potent fuel/oxidizer pair. There are toxicity and corrosion problems, however. Can you guys comment on economics and synthesis-from-only-O2-N2 difficulties for these compounds?