In his "Energy Transitions", Vaclav Smil evaluates one of Jacobson's and Delucchi's (Note, in an earlier post, which I think was upthread although I can't find it, I mis-remembered it as Isaacson) previous studies, which claimed that it would be possible to go 100% fossil-fuel free by 2050 (this was 2009). Smil pointed out not only most of the systems of the sizes used in the study either didn't exist yet or were just entering service, and that even if they did exist, the numbers called for in that study would require scaled-up deployments amounting to 1000s of %, all of which would supposedly take place within 21 and 41 years, and cost was said to be no problem! Here's part of the abstract of that study, which gives the numbers required:
. . . 3,800,000 5 MW wind turbines, 49,000 300 MW concentrated solar plants, 40,000 300 MW solar
PV power plants, 1.7 billion 3 kW rooftop PV systems, 5350 100 MW geothermal power plants, 270
new 1300 MW hydroelectric power plants, 720,000 0.75 MW wave devices, and 490,000 1 MW tidal
turbines can power a 2030 WWS world that uses electricity and electrolytic hydrogen for all purposes.
Such a WWS infrastructure reduces world power demand by 30% and requires only 0.41% and 0.59%
more of the world’s land for footprint and spacing, respectively. We suggest producing all new energy
with WWS by 2030 and replacing the pre-existing energy by 2050. Barriers to the plan are primarily social
and political, not technological or economic. The energy cost in a WWS world should be similar to
From memory, Smil pointed out that there were no 5 MW turbines at the time (the largest one now is 9.5 MW offshore, but the structural issues get harder and harder to solve as you try and go bigger), no 300MW CSP plants (IIRR Ivanpah was the first @ 392 MW, the Mojave parabolic trough plant is rated at 354 MW, and those remain the largest in the world as of 2017), no wave generators or tidal turbines of the size given (ignoring siting limitations, the power density of these sources, and the near total lack of any commercially deployed units at the time and mostly still true since) and so on through the list.
The land use claims are equally airy-fairy. So, while we will probably be able to do it eventually and can and IMO should speed the process up considerably, studies like that one fall in the realm of fantasy. Hopefully the more recent one here has been toned down to better reflect real-world constraints.