I've been hearing about and hoping for fusion power since probably the 1980s. The more I think about it the more I think it's probably a pipe dream.
The idea appears so simple an elusive: combine Hydrogen into Helium (and there are variations that use Helium, particularly He-3 as a fuel), which releases energy. Hydrogen is essentially limitless (He, particularly He-3, less so). Stars do it all the time.
The two big problems are of course:
1. Containment. How do you contain something that's heated up to millions of degrees? Magnetic containment has been the obvious candidate since hydrogen heated sufficiently loses it's electrons and becomes a H+ ion but this so far seems far easier said than done; and
2. Neutrons. Fusion using deuterium, tritium, He-4 and He-3 releases varying amounts of neutrons. These can't be magnetically contained (obviously) and they're destructive. We can achieve fusion but neutrons end up pretty quickly destroying the containment vessel.
Stars solve the first problem quite easily: with gravity. This isn't something that's useful to us. A star exists in a state of a balance of two opposing forces: gravity from the sheer their sheer mass trying to crush them vs the outward pressure of the fusion processes they're undergoing.
The differences in potential size are massive. A star like our own sun will eventually become a red giant extending about to the orbit of the earth. That's 150 _million_ km. Likewise a star larger than ours can end up compressed to something 10 miles or less across. This just illustrates the differentials in forces involved.
At this point I'm honestly not convinced that this is an economically solvable problem. It's not sufficient to output more energy than you input either.
Let's say a fusion reactor costs $1B to produce, has maintenance costs of $20m/year and has a life of 50 years. That's (at least) $2B you need to recover in capital costs from your "free" energy.
I honestly think burning a fuel of some sort will be here for a long, long time. I don't want to discount the rising importance of wind and solar. I do believe that with the ever decreasing costs these will become even more important, even critical, in the future. They simply won't completely replace some kind of combusted fuel.
Now what that fuel economy looks like is an unknown. There is a course a limit in what we can dig up out of the ground and to continue using it or something akin to it we need to solve the carbon problem. So what we really need is:
1. Some method of sequestering atmospheric carbon at scale; and
2. Some method of storing excess renewable energy.
There are multiple solutions to (2), the most common of which today seems to be using batteries. Current battery technology has come a long way but it has a lot of disadvantages, not the least of which is its current reliance on something that itself is limited (lithium). Plus obtaining all the materials for batteries seems to be highly environmentally destructive.
It's unclear to me that without some major innovation batteries will only ever be a niche technology for the likes of a few Teslas and airplanes. Now I'm not saying that innovation won't happen. I'm simply saying current battery tech isn't there yet.
Another potential solution for (2) is to use energy to construct a fuel that can be transported and combusted eg [1].
Synthetic hydrocarbons have a lot of advantages. Convenience, relatively simple tech and high energy density (important for weight). Large scale carbon sequestration remains a huge sticking point however. Plus actual fossil fuels remain cheaper than synthetics but this will eventually change either because fossil fuels become much rarer or renewable energy costs get sufficiently low.
[1]: https://www.quora.com/How-do-I-get-gasoline-from-wind-power