Yet another discussion on space solar power on reddit pointed out the difficulties (both real and imagined) of transferring energy harvested in space back to Earth. Since this particular discussion centered around transferring energy from the Moon, I wondered if there might be some other way to transmit the energy from a body such as the moon.
What if we drop some sort of energy storage device from the moon onto Earth. Since Luna* is high up in Terra's gravity well and has a low escape velocity, doing so shouldn't be too difficult. So the question becomes, "What sort of energy storage device?"
*What are the preferable names between Luna and Moon and Terra and Earth?
Three possibilities came to mind: superconducting electromagnetic storage, fly wheel storage and thermal storage. All three benefit from something space has in abundance -- vacuum. Vacuum combined with low background temperature makes achieving the low temperatures necessary for superconducting storage a matter of the proper shading, at least during the trans-Terra flight. Flywheels don't lose energy due to drag and thermal storage benefits from the ease at which a vacuum thermos can be created.
Presumably the cost of transporting material to the moon would be fairly high, so I decided to limit my consideration to devices that could be constructed on the moon. I started with the flywheel. Aluminum is very common on the moon, so I came up with the maximum flywheel storage for a very strong Aluminum-Lithium alloy, Weldalite 048-T8. I came up with (and this is just the mass of the flywheel, not any of the parts needed to get it safely to Earth) an energy density of 273.1 kJ/kg. That's about 1/100th the energy density of coal, though extraction is more efficient, so call it about 1/60th the usable energy density of coal.
Waving aside all of the considerable other technical challenges, is the low energy density a show stopper for aluminum flywheel based interplanetary energy transfer?
Wikipedia gives world energy use as about 474 exajoules. That would require 1735 trillion kg of aluminum per year. While that doesn't push things in the impossible realm (Luna has a lot of aluminum), I'd say it's definitely infeasible. Particularly when you add the inefficiency of due to extracting the aluminum from regolith, at least 50 MJ/kg.
Well, maybe one of the other options is more workable. I hope to actually detail my (really simple) analysis of this, um, out of the box, idea and look at thermal storage in an upcoming post.