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You can distill the water using solar energy, even without converting it to electricity beforehand, which insanely increases the efficiency. Also, it just requires some initial materials, and then the process basically runs itself. It absolutely could be done at a large scale, especially in places where there is much sun and much saltwater (equatorial coastlines), but the process is veeery slow and there is not enough demand for freshwater (yet) to warrant the construction of gigantic solar stills. As a bonus, global warming increases the efficiency of the process AND the process cools the Earth down somewhat (separating the water and dissolved stuff is a thermodynamically entropy-decreasing process).
Which is also called rain.
The thing is more environmental protection than technical feasibility. What you described is well true; but do people really want to terraform large parts of the planet? Wouldn’t that lead to more harm than good? Maybe we should just live with the ecology/natural circumstances that we’re given.
You hit the nail in the head, with this one. The only way to do it at scale with solar power alone is already being done by the planet, through evaporation.
Desalination is energy-expensive. If you could somehow collect all the rain that falls over oceans, we’d have more than enough fresh water. That’s its own hard problem, though.