Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reason why was it chosen.
This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...
Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.
Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.
However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.
There are basically no DC/DC converters that hit that efficiency at any load.
Not only did they mention (rephrasing) for unlimited money / resources AC / AC transformers could be far more efficient, but the part you are critiquing is simultaneously comparing COST for SAME efficiency parts (I assume typical) and resulting efficiency for similar COST.
And no, 99% isn't hard for a resonant switched capacitor converter. They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)
With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another.
I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).
- Switching AC is much more easy than DC.
- The grid we have today was not made for small small cellphone chargers. It was made for light and motors.
It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.