Agreed on the HVDC lines. They end up reducing the cost of electricity too, because they let us use all of the really cheap wind on the plains, and sun from the southwest.
Don’t forget long distance transmission in that mix: HVDC can be done with losses of only 3.5%/1000km, which makes it a cost/geopolitics issue for your night to be someone else’s day, your winter someone else’s summer.
A large enough HVDC grid would almost totally eliminate the need for backup power. There's always wind or sun somewhere and HVDC transmission losses are <5% per 1,000mi. It's possible to build a grid where pumped and battery storage alone can smoothen out any fluctuations.
HVDC does have advantages in certain scenarios (very long transmission lines, for example) but parent is still correct--the majority of the grid makes way more sense with AC.
And DC avoids problems caused by induced currents due to geomagnetic storms (you get a voltage shift, but that's no big deal, compared to what happens to transformers.)
Last time I looked at this, antipodal HVDC was several (2? 3? Can't remember) orders of magnitude cheaper than batteries at the scale of continental electrical requirements; the limit is needing so much metal you become a dominant player in the world metal mining industry just for this project.
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