I feel that there should be at least one dungeon that has no energy cost to play. This dungeon should of course have very limited rewards, offering very little money, possibly no heat, only the most basic materials, and low quantities of minerals.

It could also include many energy-locked doors which contain somewhat better rewards.

The point would be to give players something to do while they're out of energy, as well as serve as a good place to test out new weapons and strategies.