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Is it at all practical to just run DC power lines through your house? Would that save some efficiency lost during transforming knowing that will end up as DC in

by tootie 5y ago

Is it at all practical to just run DC power lines through your house? Would that save some efficiency lost during transforming knowing that will end up as DC in a lot of household appliances?

rightbyte 5y ago
The contactors need to be much stronger to break a DC current compared to AC. The appliances would be quite expensive.
SavantIdiot 5y ago
One problem is the DC voltage requirement is different for every device. Look at the wall-warts in your house. I have in my lab: 24V, 19V, 17V, 12V, 5V, 3.3V. Yeesh. If you delivered 48VDC you'd still need a DC SMPS at each point, but an SMPS is way more efficient than a transformer.
- cadr 5y ago
  How are you comparing the efficiency of a SMPS and a transformer? They are different things. Would love to hear more.
  - Dylan16807 5y ago
    You look at the percentage, such as 95%? I'm not sure what you're asking. Add up the losses in each conversion, plus the losses from the house wiring.
  - MisterTea 5y ago
    Most AC power distribution transformers are 95% or better. As of 2016 the US EPA mandated even greater efficiency of 97% or greater: https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-431#subject-group-ECFRf791a7b3e315557 https://www.ecfr.gov/current/title-10/chapter-II/subchapter-...
- saalweachter 5y ago
  Every time I look into this the conclusion I end up with is "not enough things are produced using 24V/48V DC to be worthwhile". It seems like you could replace a certain fraction of room wiring with PoE from an energy budget point of view -- lights, lamps, chargers and other doodads don't need to draw more than a few watts each, so you could get by with just an outlet or two per room for large power draws everywhere but the kitchen, and PoE for all the rest. But the world just isn't built for it.
dijonman2 5y ago
There’s actually a paper on this: https://www.researchgate.net/figure/Single-DC-bus-distribution-for-a-household-The-bidirectional-power-converters-are_fig3_336136882 https://www.researchgate.net/figure/Single-DC-bus-distributi...
Syonyk 5y ago
No. If you run low voltage DC that matches what devices need (5V or 12V, typically), your wiring is staggeringly huge. Not only do you need the fat wire to carry the current safely, you don't have much room for voltage drop. Dropping 5V on 120V under load isn't a big deal, dropping 5V on 12V under load is literally unusable for most devices (12V stuff won't run on 7V). If you run something sane, like 48V or so, then you still need conversion electronics at each point of use, and you don't gain much over just running 120/240VAC, except that now your converters are really expensive because they're less common and "weird." 120VAC to 5V is super common, 48VDC to 5VDC is less so. Some switch mode power supplies will work on DC, some won't, and it's harder on the bridge rectifier diodes, so they don't tend to last very long in that sort of use. It's the sort of thing that sound good in isolation, but in practice leads to spending a lot of money to gain little or nothing in terms of efficiency. The exception is for a small system where you can run 12VDC to LEDs and stuff (think a camper), and avoid bringing the inverter online at all. Inverters have a significant idle draw more or less proportional to peak power delivered (my 2kVA inverter idles at 30W DC, a 6kVA one that can hold 18kVA for 20 seconds idles at 100W DC).
- tootie 5y ago
  I'm imagining a world where the converters are commodity but it seems they're not right now. I found this reddit thread of someone trying to do exactly this. Run 48VDC from solar panels to a usb-c PD device so it can power negotiation at each device. Seems like this is still really impractical. https://www.reddit.com/r/UsbCHardware/comments/phl211/48v_dc_to_usbc_pd_options/ https://www.reddit.com/r/UsbCHardware/comments/phl211/48v_dc...

by tootie 5y ago

Is it at all practical to just run DC power lines through your house? Would that save some efficiency lost during transforming knowing that will end up as DC in a lot of household appliances?

rightbyte 5y ago

The contactors need to be much stronger to break a DC current compared to AC. The appliances would be quite expensive.

SavantIdiot 5y ago

One problem is the DC voltage requirement is different for every device. Look at the wall-warts in your house. I have in my lab: 24V, 19V, 17V, 12V, 5V, 3.3V. Yeesh. If you delivered 48VDC you'd still need a DC SMPS at each point, but an SMPS is way more efficient than a transformer.

cadr 5y ago
How are you comparing the efficiency of a SMPS and a transformer? They are different things. Would love to hear more.
- Dylan16807 5y ago
  You look at the percentage, such as 95%? I'm not sure what you're asking. Add up the losses in each conversion, plus the losses from the house wiring.
- MisterTea 5y ago
  Most AC power distribution transformers are 95% or better. As of 2016 the US EPA mandated even greater efficiency of 97% or greater: https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-431#subject-group-ECFRf791a7b3e315557 https://www.ecfr.gov/current/title-10/chapter-II/subchapter-...
saalweachter 5y ago
Every time I look into this the conclusion I end up with is "not enough things are produced using 24V/48V DC to be worthwhile". It seems like you could replace a certain fraction of room wiring with PoE from an energy budget point of view -- lights, lamps, chargers and other doodads don't need to draw more than a few watts each, so you could get by with just an outlet or two per room for large power draws everywhere but the kitchen, and PoE for all the rest. But the world just isn't built for it.

dijonman2 5y ago

There’s actually a paper on this: https://www.researchgate.net/figure/Single-DC-bus-distribution-for-a-household-The-bidirectional-power-converters-are_fig3_336136882 https://www.researchgate.net/figure/Single-DC-bus-distributi...

Syonyk 5y ago

No. If you run low voltage DC that matches what devices need (5V or 12V, typically), your wiring is staggeringly huge. Not only do you need the fat wire to carry the current safely, you don't have much room for voltage drop. Dropping 5V on 120V under load isn't a big deal, dropping 5V on 12V under load is literally unusable for most devices (12V stuff won't run on 7V). If you run something sane, like 48V or so, then you still need conversion electronics at each point of use, and you don't gain much over just running 120/240VAC, except that now your converters are really expensive because they're less common and "weird." 120VAC to 5V is super common, 48VDC to 5VDC is less so. Some switch mode power supplies will work on DC, some won't, and it's harder on the bridge rectifier diodes, so they don't tend to last very long in that sort of use. It's the sort of thing that sound good in isolation, but in practice leads to spending a lot of money to gain little or nothing in terms of efficiency. The exception is for a small system where you can run 12VDC to LEDs and stuff (think a camper), and avoid bringing the inverter online at all. Inverters have a significant idle draw more or less proportional to peak power delivered (my 2kVA inverter idles at 30W DC, a 6kVA one that can hold 18kVA for 20 seconds idles at 100W DC).

tootie 5y ago
I'm imagining a world where the converters are commodity but it seems they're not right now. I found this reddit thread of someone trying to do exactly this. Run 48VDC from solar panels to a usb-c PD device so it can power negotiation at each device. Seems like this is still really impractical. https://www.reddit.com/r/UsbCHardware/comments/phl211/48v_dc_to_usbc_pd_options/ https://www.reddit.com/r/UsbCHardware/comments/phl211/48v_dc...