In the first case, the first statement is hypothetical (using the conditional mood), so considers a hypothetical situation where the machines would need to work, and weren't going to work without energy.
The other is a basic true statement (using the future tense) that simply states that the machines won't work without energy. I should note that this statement doesn't mean they can't work without energy, only that they won't. This is more to do with the idea of free will: consider a person who is able to work without biscuits but refuses to.
The second case is a lot more fun. Saying a machine can't (or cannot) work without energy sort of states that the machine couldn't work without energy even if it wanted to. In this case, because machines aren't sentient, it is identical to saying they won't work without energy.
However, the first sentence in the second group is either conditional or imperfect. In the case where it is conditional, it talks about a hypothetical situation like before. If, however, the tense is imperfect, it implies that the machines, in the past, weren't able to work without energy, but now supposedly can.
Note 1: Machines need energy to work. Fact of physics.
Note 2: I have no way of knowing what tense 'couldn't' is in. Normally you'd need to use the context to work it out.