4 ms·

Everything in your house runs off and/or generates vastly different DC voltages. Yes, you could get rid of some AC-DC conversions in there, but your various sol

Everything in your house runs off and/or generates vastly different DC voltages. Yes, you could get rid of some AC-DC conversions in there, but your various solar panels, batteries, infeeds from the grid, electric car, etc are all going to function at dramatically different DC voltages, such that you’re going to need a ton of DC-DC converters on everything anyway. On top of that, high-voltage DC is quite a bit more dangerous than AC - it is much better at generating sustained arcs that are very good at catching things on fire.

hutzlibu 6y ago
Yeah, but isn't DC-DC volt conversion still much better, than DC - AC - AC- DC? Also, most devices have to internally change voltage for various elements anyway. So we do already have tons of DC-DC converters in everything. Another important advantage of DC only grid: it is smooth. DC from AC has lots of fluktations(because AC always goes up and down), which of course most device expect that and sensitive electronics like in hospital, does not run on the ordinary grid for that reason. But it would be nice if also common devices can have stable electric input. Who knows, maybe quite some tricky computer bugs or hardware failures have their root in this. Another aspect of DC: it does not constantly creates a changing magnetic field arround it, like any AC power line does.
- zaarn 6y ago
  DC-DC is fine at lower amperages, once you go into a few kW, then it gets more difficult. DC - AC - AC - DC has the advantage that you can use a transformer to shift voltage levels, which does this very efficiently, usually between 95% and 99% of the energy is passed along. A DC-DC converter on the other hand usually will top out at 90% in deal conditions, thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. You loose more if you want this to work bidirectional (ie, you can feed energy from both sides of the converter). There is also some other issues that DC brings along, if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting, DC cables can't be safely cut from power as easily as DC (it will ark and it will not stop arking until interrupted, you need special fuses, AC stops arking on the 0 crossing), switching heavy DC loads can cause cables to jump at much lower wattages, etc. Additionally, DC only grid would not be smooth. Every device that joins or leaves the grid will cause voltage fluctuations. It'll maybe be smooth within 10V or so. Within that it'll be moving fast. For bigger grids there is also the disadvantage that it's rather difficult to sync up a DC connection. Worst case you get a lot of current until the grid normalizes. On AC grids you only really need to synchronize the frequency, then you can connect and any currents should seize over the next zero crossing.
  - hutzlibu 6y ago
    "if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting" You mean every time gets switched on or off? (or changes in device energy demand) But with AC the magnetic field changes constantly all the time. That sounds not better. Or is it, that the constant change is pretty much constant, so the effective influence on other devices is lower?
    - zaarn 6y ago
      AC causes radio emissions (which is lost power), it doesn't affect the magnetic field unless it forms coils. And most coils on AC will be put into iron cores to contain them nice and close to where they belong. Even then an AC coil doesn't really behave magnetic very much since the field reverses polarity so often. A DC wire will constantly produce a static magnetic field. With only a few kilowatts, you can probably measurably shift a compass needle within your house, since all your wires will necessarily have to form a loop (positive and ground). Magnetic fields aren't created by shifting the DC voltage or current, they're created by current flowing through a wire, AC just has the advantage that 20ms later, the field goes the other direction so few things care. What will happen when DC load shifts is that the magnetic field changes and it'll dissipate or absorb power from the DC line to do so (Relays require you to put in a diode so that when they are switched off, they can dissipate the stored field energy safely without damaging electronics. This dissipation happens at much higher voltage levels than what induced it and is of opposite polarity to your input voltage.
  - dreamcompiler 6y ago
    > thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. IGBTs and SiC are plenty economical for boosting and bucking in the kilowatt range. For very light loads (like inside the home), switched supercapacitors would work fine and are quite efficient. The need for a big spool of copper is over.
    - zaarn 6y ago
      Last I checked, they are economical but not terribly efficient if you're using them in an environment with the power usage pattern of a household.
- dreamcompiler 6y ago
  > Yeah, but isn't DC-DC volt conversion still much better, than DC - AC - AC- DC? Yes. Dramatically better. Overall efficiency could move from 60-70% to 95%.
dreamcompiler 6y ago
In-house DC could easily be limited to 48V which is not dangerous. Efficient DC buck converters could be built for pennies inside each outlet and negotiate with the load to provide the right DC voltage.
- deleted 6y ago
  [deleted]
- URSpider94 6y ago
  If you want to cut the voltage by 60%, from 120V to 48V, then you’ll have to triple the thickness of all of the conductors in your walls. The costs would be immense, as would the challenge of stuffing such thick wires through the wall. That said, lots of people use 48V wiring for things like LED lighting and home audio. It makes perfect sense for those applications.
  - throw0101a 6y ago
    > If you want to cut the voltage by 60%, from 120V to 48V, then you’ll have to triple the thickness of all of the conductors in your walls. And that's not even getting into 240V appliances like dryers, ovens, cooktops, and (increasingly) EV chargers. The (e.g.) newly released VW ID.4 can charge at up to 11 kW, which at 240V, would use 45.8A, but (IIRC) you can only use up to 80% of a circuit, so you would need wiring that could handle ~60A. At -48V, we're talking >200A: those are some thick cables (or a busbar). So you need high-voltage (i.e. >48V) into the home anyway.
  - dreamcompiler 6y ago
    And most home applications are in that lower-power category. You'll still need 120VAC and 220VAC but only on particular, dedicated circuits. Like maybe one or two per room. Other outlets and lighting would work fine at 48VDC on 12 gauge Romex because 20A is more than sufficient for the majority of loads.

hutzlibu 6y ago

Yeah, but isn't DC-DC volt conversion still much better, than DC - AC - AC- DC? Also, most devices have to internally change voltage for various elements anyway. So we do already have tons of DC-DC converters in everything. Another important advantage of DC only grid: it is smooth. DC from AC has lots of fluktations(because AC always goes up and down), which of course most device expect that and sensitive electronics like in hospital, does not run on the ordinary grid for that reason. But it would be nice if also common devices can have stable electric input. Who knows, maybe quite some tricky computer bugs or hardware failures have their root in this. Another aspect of DC: it does not constantly creates a changing magnetic field arround it, like any AC power line does.

zaarn 6y ago
DC-DC is fine at lower amperages, once you go into a few kW, then it gets more difficult. DC - AC - AC - DC has the advantage that you can use a transformer to shift voltage levels, which does this very efficiently, usually between 95% and 99% of the energy is passed along. A DC-DC converter on the other hand usually will top out at 90% in deal conditions, thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. You loose more if you want this to work bidirectional (ie, you can feed energy from both sides of the converter). There is also some other issues that DC brings along, if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting, DC cables can't be safely cut from power as easily as DC (it will ark and it will not stop arking until interrupted, you need special fuses, AC stops arking on the 0 crossing), switching heavy DC loads can cause cables to jump at much lower wattages, etc. Additionally, DC only grid would not be smooth. Every device that joins or leaves the grid will cause voltage fluctuations. It'll maybe be smooth within 10V or so. Within that it'll be moving fast. For bigger grids there is also the disadvantage that it's rather difficult to sync up a DC connection. Worst case you get a lot of current until the grid normalizes. On AC grids you only really need to synchronize the frequency, then you can connect and any currents should seize over the next zero crossing.
- hutzlibu 6y ago
  "if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting" You mean every time gets switched on or off? (or changes in device energy demand) But with AC the magnetic field changes constantly all the time. That sounds not better. Or is it, that the constant change is pretty much constant, so the effective influence on other devices is lower?
  - zaarn 6y ago
    AC causes radio emissions (which is lost power), it doesn't affect the magnetic field unless it forms coils. And most coils on AC will be put into iron cores to contain them nice and close to where they belong. Even then an AC coil doesn't really behave magnetic very much since the field reverses polarity so often. A DC wire will constantly produce a static magnetic field. With only a few kilowatts, you can probably measurably shift a compass needle within your house, since all your wires will necessarily have to form a loop (positive and ground). Magnetic fields aren't created by shifting the DC voltage or current, they're created by current flowing through a wire, AC just has the advantage that 20ms later, the field goes the other direction so few things care. What will happen when DC load shifts is that the magnetic field changes and it'll dissipate or absorb power from the DC line to do so (Relays require you to put in a diode so that when they are switched off, they can dissipate the stored field energy safely without damaging electronics. This dissipation happens at much higher voltage levels than what induced it and is of opposite polarity to your input voltage.
- dreamcompiler 6y ago
  > thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. IGBTs and SiC are plenty economical for boosting and bucking in the kilowatt range. For very light loads (like inside the home), switched supercapacitors would work fine and are quite efficient. The need for a big spool of copper is over.
  - zaarn 6y ago
    Last I checked, they are economical but not terribly efficient if you're using them in an environment with the power usage pattern of a household.
dreamcompiler 6y ago
> Yeah, but isn't DC-DC volt conversion still much better, than DC - AC - AC- DC? Yes. Dramatically better. Overall efficiency could move from 60-70% to 95%.

dreamcompiler 6y ago

In-house DC could easily be limited to 48V which is not dangerous. Efficient DC buck converters could be built for pennies inside each outlet and negotiate with the load to provide the right DC voltage.

deleted 6y ago
[deleted]
URSpider94 6y ago
If you want to cut the voltage by 60%, from 120V to 48V, then you’ll have to triple the thickness of all of the conductors in your walls. The costs would be immense, as would the challenge of stuffing such thick wires through the wall. That said, lots of people use 48V wiring for things like LED lighting and home audio. It makes perfect sense for those applications.
- throw0101a 6y ago
  > If you want to cut the voltage by 60%, from 120V to 48V, then you’ll have to triple the thickness of all of the conductors in your walls. And that's not even getting into 240V appliances like dryers, ovens, cooktops, and (increasingly) EV chargers. The (e.g.) newly released VW ID.4 can charge at up to 11 kW, which at 240V, would use 45.8A, but (IIRC) you can only use up to 80% of a circuit, so you would need wiring that could handle ~60A. At -48V, we're talking >200A: those are some thick cables (or a busbar). So you need high-voltage (i.e. >48V) into the home anyway.
- dreamcompiler 6y ago
  And most home applications are in that lower-power category. You'll still need 120VAC and 220VAC but only on particular, dedicated circuits. Like maybe one or two per room. Other outlets and lighting would work fine at 48VDC on 12 gauge Romex because 20A is more than sufficient for the majority of loads.