That's still 5-10 times higher than lithium based batteries (0.3-1MJ/kg) and those are being used in cars and almost good enough for some airplane use cases. Heat engine efficiency isn't as good as electric motors, but it isn't bellow 20%. Downside is that recharging by converting back to iron probably can't be done within device. While pouring in metal powder in for fast refilling might be doable getting it out is probably messier.
Yes, but even best combustion engines have at very best 50% efficiency using direct transformation of thermal energy through gas expansion into mechanical energy. This involves numerous technologies, which better mix fuel and air, inject fuel directly, adjust timing of injection and exhaust following the RPMs, etc. Cars don't generate steam or use any other indirect method of transformation of thermal energy into mechanical energy, due to losses and impracticality (otherwise we would see other types of combustion engines). Burning of powder iron would not work with normal ICEs since powders are not liquids, they are abrasive and burn residuals are not gaseous (rust dust, which is even more abrasive). At very least this is non trivial engineering challenge. So 5-10 times lower specific energy of lithium batteries wins because of simplicity (and lower weight) of whole power-train (inverters, engines) and their high overall efficiency (90-95%). But indeed, on big ships (e.g. tankers), which can have a steam turbines, iron might be a feasible fuel (especially if using onboard solar power while cruising it is possible to recover part of it as a fuel again).
I this is is largely on point - this should work for larger ships that have steam turbines. Also, said turbines have better efficiency than an ICE engine.
They don't, actually. Steam turbines are less, not more efficient than marine diesel engines. In modern marine propulsion, turbines are used pretty much only on gas and coal carriers.
You would need a steam engine to convert heat into work, which is clearly out of the question for planes, and might make cars and trucks impractical. Maybe rail and boats though ?
