Fuel cells, like batteries, perform direct electrochemical conversion of chemical energy to electrical energy. No pesky steam.

The problem is chemical bonds are super low energy density.

The only way we know to achieve something resembling that level of energy density is fusion or fission, both of which typically make steam, but that's not the only way to use them.

Satellites use radioisotope thermoelectric generators for power, which have no moving parts and are about 30-40% efficient

In order to make something high powered you can strap onto your chest, you would need some pretty miraculous innovation in:

• radiation shielding, because duh
• efficiency AND heat shielding. I'd wager, without doing the math, even at 99% efficiency, the equipment in Iron Man's suit would start to boil him alive if he didn't have a magic heat management system. Less efficient would of course = more heat to dissipate

If you can't manage those, then you need a whole new form of energy conversion. Some sort of matter to energy fuel cell ala Project Hail Mary Solarphage, or a means of efficiently bending space time to cheat modern physics.

Also it's fundamentally a heat engine. Even a fusion reactor is a heat engine bounded by Carnot efficiency, which is the maximum theoretical efficiency of a system determined by the absolute temperature of your hot and cold sides.

Which means his arc reactor should be hot enough to cook him unless he's got some crazy external cooling. Our bodies produce around 100w of heat, and some of the stuff ironman does with his suit would require a few million watts of energy to pull off.

I have heard of very few sources of energy that do not rely on turbines. Solar is the big one but one of the ones that might be most interesting in future is 'plug flow.' Much like with solar and other non-chemical systems, the challenge seems to be scale, because there is just an insane amount of energy density in hydrocarbons, so much that we can run our society on them while wasting huge amounts of it. But if we could find ways to scale these kinds of effects, it would pull energy from natural cycles, like a dam, but with fewer moving parts. It's still using solar energy with conversions but maybe it'd use less REM.