That is the trade off why the most realistic chance for viable Fusion, ITER, is around $65billion and still only a research prototype.
Just like fission, they will suffer from the lack of scale economics and the significant build time and thus ROI will make them prohibitive.
Renewables and BESS will continue to become exponentially cheaper.
Why? I think they're more likely to become more expensive as they stop being subsidised so much and they become more complex as a result of trying to improve efficiency and because a vast overcapacity is required due to the inefficiencies and the lack of reliability and control. For example, it's common for wind in the UK to generate less than 10% of the nameplate capacity and it averages ~25% and that's despite the fact that the UK is about as good as it gets for wind.
But I think a more important point is that very few areas could function solely on renewables. Norway can manage it because it has a lot of geothermal and it has exceptionally good conditions for hydroelectric and it has an extremely low population density. Few areas have that combination of factors.
Hydroelectric is the key factor...and hydroelectric also has a lack of scale economics and a significant build time and high maintenance costs if a reasonable level of safety is wanted. It's also more dangerous than fission, let alone fusion. Once again, Norway is an extreme outlier because its unusual geography and very low population density means that far fewer people would be killed by a dam breach.
Energy storage could theoretically be combined with a vast overcapacity to make 100% renewables viable in more places, but only if it existed and it doesn't. Energy storage adequate for that task is no closer than fusion, might be more expensive and will definitely be more dangerous. The least bad way of doing it so far is pumped hydro, which is very wasteful and carries the same degree of risk as any form of hydro. Also, it's nowhere near adequate for the task and never will be. There's a superb pumped hydro setup in the UK. Dinorwig. A brilliant feat of engineering. Excellent design, superb implementation, minimal environmental impact, as low risk as hydro can be. Extremely expensive. Holds about 8 minutes worth of energy in theory, although in practice less than that. Nowhere near adequate for storage for a renewables grid, not even if we had hundreds of them (which is impossible - there aren't that many places with suitable conditions). It's excellent for the purpose for which it was built - jump-starting the grid in the event of a complete shutdown - and of some use as energy storage. But the scale isn't there and never could be.
Another theoretically possible solution would be a highly transnational renewables grid with HVDC connections and free flow of electricity and overcapacity everywhere. So excess generated from wind during ideal conditions in the UK could fill in a shortfall in solar at night in Morocco, excess generated from solar in Libya during the day could fill in for a shortfall in wind in Poland, that sort of thing. That could be done with existing technology. But the cost would be immense and it's impossible anyway for political reasons and it would be extremely vulnerable to attack.
I think that we need and will continue to need a reliable, controllable baseline generation. The cost of that will be much less than the collapse of modern civilisation, which is what will happen without reliable energy supply. We need it and we can't afford to gamble it on a single type of system that relies on potential future technology that doesn't exist yet. I consider that degree of gambling quite horrifying.
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