The answer, in a word, is experience. The difference between the possible and the practical can only be discovered by trying things out. Therefore, even though the physics suggests that a thing will work, if it has not even been demonstrated in the lab you can consider that thing to be a long way off. If it has been demonstrated in prototypes only, then it is still distant. If versions have been deployed at scale, and most of the necessary refinements are of an evolutionary character, then perhaps it may become available fairly soon. Even then, if no one wants to use the thing, it will languish in the warehouse, no matter how much enthusiasm there is among the technologists who developed it.
It’s well worth considering what makes a potential technology easy or hard to develop, because a mistake can lead to unwise decisions. Take, for instance, the International Thermonuclear Experimental Reactor that’s now under construction in France at an estimated cost of US $22 billion. If governments around the world believe that this herculean effort will automatically lead to success and therefore to near-term commercial fusion reactors, and if they plan their national energy strategies around that assumption, their citizens may very well be disappointed.
Here I present a short list of technology projects that are now under way or at least under serious discussion. In each case I’ll point out features that tend to make a technology easy or hard to bring to market.