Superconducting magnets become cheap and widely available which allows for maglev trains at massive scale. Costs for the LHC and similar experiments would drop dramatically. MRIs would only require air conditioning, if that; Modern cell phones are sufficient to compute tomography. Magnetic confinement fusion also becomes cheaper and easier. Electric cars could use superconducting motor magnets allowing for even greater power to weight ratios and efficiency.
Just a few things off the top of my non-mechanical-engineer head.
Undersea cables are a pie in the sky; current high-load cables in urban an industrial areas could be made much smaller, simpler, and lossless.
I wonder if transformers, currently huge and expensive, could be made better with this, too; at least the ohmic losses could be removed, and thus a lot of need for cooling, and the fire hazards.
Not really, when the sun is up over the Pacific ocean, there's not that much sun over land. Maybe a global grid happens anyway, but cabling losses aren't the only source of cost, so I'd put my money on more localized improvements.
Better interconnection between and within local grids (maybe a viable Tres Amigas interconnection, but even just better connections between sections of the major grids would help with grid management. Improvements in motors, MRIs, magnetic bearings, transformers, etc.
[edit: typo]
https://en.wikipedia.org/wiki/Superconducting_magnetic_energ...
I'm not an expert, and everything that follows comes from a quick reading of this Wikipedia article.
It seems like (counter-intuitively) refrigeration isn't a significant cost compared to all the other stuff that's necessary. So at first glance it seems like high-temperature superconductors might not make a big difference.
However, that Wikipedia article does say this:
> The critical temperature of a superconductor also has a strong correlation with the critical current. A substance with a high critical temperature will also have a high critical current. This higher critical current will raise the energy storage exponentially. This will massively increase the use of a SMES system.
Right now, superconducting energy storage has a lot of advantages, but it doesn't have very good energy density (by mass). Not even a tenth of what lithium-ion batteries have. I assume you couldn't power a car with it. But it has some compelling advantages in other areas. It has unlimited charge/discharge cycles. It has zero self-discharge. It has unlimited (in theory) power density, so you could charge or discharge it arbitrarily fast.
Depending on what the energy density ends up being, it might suddenly become way more useful. It would have to be a gigantic leap in energy density, though.
Also, not needing refrigeration could potentially open up smaller scale applications. Maybe you could have a residential superconductor storage system for your solar panels. (Although I don't know about its safety, so maybe not.)
All this assumes the cost to build it is reasonable compared to other alternatives, that the discovery is real, etc.