27 tons of iron oxide have a volume of 5m^3 and can be stored in pretty much a hole in the ground.

2.7 tons of hydrogen have a volume of almost exactly 30000 m^3, requiring storing it under high pressure in specialized containers. Hydrogen is famous for being hard to store without losses.

For long-term storage storage and losses are a problem.

> But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%

Maybe I'm missing something, but why? As you mentioned it takes 29kj to restore 3 moles of H2 out of (3 moles of H20 + 1 mole of Fe2O3). Where does 50% comes from?