You can use wood, coal, or CO2 from the air. The carbon from all the stuff we burned didn't go away, it's all still here. Most of it is in the ocean or in plants.

If you "extended" coal by adding hydrogen then you can quintuple the energy content (since each atom of carbon gets 4 of hydrogen, for longer chain hydrocarbons the ratio is less, but at a minimum it's tripled). So instead of the estimated 100 years of coal we now have, we'd have 500 years.

And on top of that most of the coal is used for electrical production (with the rest of steel making), if we stopped doing that and used nuclear power for electricity and saved the coal to make liquid fuel we would have at least 10,000 years of coal left.

> Conventional non-breeder uranium reactors will suffer from fuel shortages if used for significant amounts of power generation well within a century. Thorium breeders offer a possible alternative, though the technology is unproven.

So don't use non-breeder and switch to breeders. I once calculated that if we used breeding reactors, plus thorium, we have enough energy in those two elements alone for 10,000 to 100,000 years! (The range is because energy consumption will inevitably go up.)

So no, energy is NOT the problem. We have enough for longer than recorded history.

> On top of that, Fischer-Tropsch plant and capital are expensive and would take a long time and vast investments (trillions of dollars) to replace the 20 million barrels of oil consumed daily in the US.

Sure. But have you seen the unbelievable size of the equipment used today for petroleum production? If there was demand for these fuels, it would get built. No problem at all. No one is expecting this to happen overnight - I think it would take 20 to 50 years.

> You're side-stepping your original statement. Producing hydrocarbons at that scale from electricity is unproven.

Of course it's proven. And you don't make it from electricity, you make it directly from heat. The only reason it's not done right now is that it's not needed. But the technology is there.

> Producing hydrogen might work

?? Why would this work if producing hydrocarbons won't? You could make hydrogen the same way - by heating water. But hydrogen is extremely hard to transport or store. It leaks right through everything, and damages most metals if stored under pressure. You need special materials to store hydrogen, and those are expensive and not plentiful. Hydrogen is not the fuel of the future - methane is.

> My point was to demonstrate that where solar powered heavier-than-air craft are only just feasible

If I'm understanding you right, your argument is that we won't be able to fly anymore because we'll run out of energy (or fuel), and your proof is that solar powered airships are only just barely workable.

You mistake is a: thinking that we'll run out of energy, and we won't. And b: that we would use solar power for an air transport, and we won't do that either.