In a couple of other threads much has been discussed about how certain energy sources become unreliable over time, and how others are too expensive to be commercially viable operations at their current level of technology. Since those discussion are getting well into the fine details of why this source or that source will or won't work, I decided to take a sideways step here for a moment and discuss another point about energy sources in a post apocalyptic world or environment.
For the purposes of discussion I intend to totally ignore aspects like corpses littering the landscape, major weather patterns or changes, and attacking enemies, but focus on aspects of the energy sources in the culture arising from the ruins.
1. The term commercially viable is no longer relevant, no one is going to be buying anything. But they will barter for what they see as something that works and they can use, or they'll attack to take it away from who has it.
2. Most technical gear will cease to function over time, especially if it wears out or needs maintenance that the current users don't know how to do or can't do.
3. Anything that can be done in a lab will be done in the working environment if it is physically possible to do so and someone has an interest in making it do it because they have a perceived need for it.
4. Technical gear and devices are of no use at all unless you have someone on hand who knows how to use it and whatever it needs to make it work properly for the purpose you want to use it for.
5. Anything that can't be immediately used or seen as usable in the near future will be cannibalized or re-purposed the moment someone can see a use for it or part of it.
When you take these points into account, a lot of the technical discussions of fine points of energy sources kind of work themselves out of relevancy in a very short time.
What is interesting is how many of the post apocalyptic stories that have a lot of today's civilization's artifact left lying around do not take all these matters into account. Some take a a few of them into account, but I've not yet found one that takes it all into account unless they have everything destroyed and nothing left at all.