"Although its specific energy is a relatively poor 1.4 kWh/kg, meaning that for a given amount of energy, iron powder will take up a little bit less space than

"Although its specific energy is a relatively poor 1.4 kWh/kg, meaning that for a given amount of energy, iron powder will take up a little bit less space than gasoline but it’ll be almost ten times heavier." Sounds like a good candidate for grid storage if it can be reversed somewhat efficiently, the weight isn't as big a deal in grid, especially if it is compact. The high weight could probably be used to store gravitational potential energy as well in hilly areas. However, if we had a crapton of iron, it would probably be better to seed plankton.