I’ve always been told that transmission of electricity is hard or inefficient. What is HVDC and why is it better?

With AC transmission, there are a couple of small loss terms that are not present in DC transmission. To the extent that the world surrounding the lines is conductive, there is a parasitic transformer between the transmission lines and the world. The parasitic transformer is particularly bad in salt water, which is why you see so many HVDC links crossing bodies of water. Also, the capacitive reactive current in the lines adds up over many kilometers.

HVAC distribution wins hands-down for short to medium distances, or when there are many places where you need to step up/down the voltage. It will always be cheaper to step HVAC up/down in voltage at any single point relative to DC. But HVDC transmission wins over sufficiently long distances or when crossing major bodies of water.

So I don't think we'll ever see a complete overhaul of the US distribution grid using HVDC. But we might see long-distance HVDC distribution to do things like ship solar power from the desert southwest to the west coast, or ship wind power from the Atlantic to the east coast.

High voltage DC. High voltage means it’s more efficient (power dissipation depends on current, not voltage, so for the same power, higher voltage means much less dissipation). DC (direct current instead of alternating) makes it easier to connect disparate grids as you don’t have to worry about phasing.

Also, I recall reading that if you run undersea cables with AC, the sea water sucks out energy from them.

Can’t tell if trolling or not.

The cables have water barriers so the conductors + insulation remain quite dry.

I believe the parent meant that the alternating current creates an electric field which is then absorbed by the water, leading to energy loss. This happens to some extent to all AC transmission, but I don't know enough to say if seawater+AC is a particularly bad combination. It would make sense though, because it's more conductive.

EDIT: brandmeyer's comment explains it well: https://news.ycombinator.com/item?id=16903431

Yes, that's precisely the thing I was referring to. Wasn't sure about the correct terminology (not an EE) so I didn't try to make the comment sciency-souding :).

I work installing these cables and never heard this discussed, maybe at the voltages / transmission subsea cables work at (220kV, 1000s of amps) makes this effect negligible?

I honestly don't know. I'm not an EE, I only reported on what I've heard reported on the topic (and that passed my "basic physics sniff test").

Are they AC? My impression was that practically all vaguely modern undersea cables are high voltage DC.

It depends, when talking about interconnecting cables between countries, yes the majority are DC, both to minimise transmission losses and also to tie together grids working at different frequencies....

In terms of km installed I think it will be HVAC just from the sheer number of offshore windfarms built over the last 10yrs in Europe. These things can be huge - 60km in length, 220kV ~1500A....

Salt water is a conductor. Two conductors separated by insulation make a capacitor. This extra capacitance makes AC less efficient.

For a fun rabbit hole: https://en.wikipedia.org/wiki/Transposition_(telecommunicati...

Nat Geo article that explains it.

https://news.nationalgeographic.com/news/energy/2012/12/1212...

Thanks to both of you for the details.

The future of energy is mostly black hole farming.

Nice, how you sneaked nuclear in there.

I think at extreme latitudes, wind and hydro might not be available everywhere and it may be easier to do nuclear. In general I hope nuclear isn’t a big part of the solution because of cost alone: nuclear is quite expensive and I don’t really see the pathway to make it a whole lot cheaper.