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If it's so fast then why hasn't it already happened? Why is only 4% of production from solar, instead of e.g. 40%?Basically because economic growth is exponential, and exponential growth starts slow. If a certain alga doubles its population every day and takes 30 days to cover a certain lake, how long does it take to cover half the lake?
29 days. On what day does it cover 4% of the lake?
Day 25.
Until about 8 years ago, solar was too expensive in most countries to compete with fossil fuels without subsidies, and then there was a period of four years when the Chinese cartel announced in https://www.reuters.com/article/us-davos-meeting-solar-gcl-i... fixed prices to keep them artificially high, as you can see on https://www.solarserver.de/photovoltaik-preis-pv-modul-preis.... Since that cartel evidently collapsed in late 02022, the cost of "mainstream" solar panels in the Preisindex has gone from €0.34/Wp to €0.11/Wp, and the cost of "low cost" panels has dropped from €0.22/Wp to €0.065/Wp. This month, for the first time, the Preisindex is no longer rounding prices to the nearest €0.01/Wp.
A crucial question, then, is what the exponential growth rate is. https://en.wikipedia.org/wiki/Growth_of_photovoltaics says that from 02016 to 02022 the growth rate was 26%. At that rate, growing from 4% to 100% would take 14 years; if 4% were now, that would mean producing more energy from solar than from all sources today in 02038.
That doesn't mean people will stop burning coal, of course. It just means it'll be a marginal activity.
> How far do we get on accelerating growth before it flattens out because the low-hanging fruit is picked and the remaining demand is a bad match for solar's generation profile?
That's another crucial question, and one that is very hard to predict, but most likely at a few orders of magnitude larger than current world marketed energy consumption.
Solar energy is cheap enough to be applied to many applications for which previous energy sources were too expensive. €0.065/Wp at a capacity factor of 20% is €0.33/W; financed forever at an interest rate of 5%/year that would be €0.016/W year. A watt year is 8.77 kilowatt hours, so that works out to be €0.0018 per kilowatt hour, or €0.51/GJ. This is 200× lower than what people in California are paying today. This is a rough estimate. You can figure in the panels' nominal 20-year lifetime to get a more precise figure, though the "low cost" category generally comes with no such warranty. Also, utility companies can usually get a better interest rate than 5%. But it's in the ballpark.
For many purposes, this works out to basically unlimited energy as long as you only need it when the sun is shining. Atmospheric carbon capture is a crucial one.
> What if [sodium-ion battery technology] doesn't [become a thing],
People will make do with lithium-ion and other forms of energy storage such as sensible heat energy storage, phase-change material energy storage, and TCES.