7 ms·

Yes, you cannot reasonably run these high-power devices on low-voltage DC (12-48V), this is also said in the article. But the reason is not that these devices c

by deng 3y ago

Yes, you cannot reasonably run these high-power devices on low-voltage DC (12-48V), this is also said in the article. But the reason is not that these devices cannot run on DC in principle, the problem is cable loss. For running devices which need >1kW on low-voltage DC, we are talking at least 20-80A, depending on the voltage. So you need thick cables, which are expensive and hard to deal with (amount of space needed, securing connections, avoiding bending, everything becomes problematic with these cables). Any little mistake done with cabling is immediately a fire hazard. So in the end, the article says: say good bye to your washing machine, dish washer, electric kettle, electric stove, etc. I mean, the site is called "low tech magazine" for a reason...

andix 3y ago
I don't see any future in 12V systems, even in cars (with rather short cable lengths) there are discussions to switch to 48V, like Tesla is doing it in the Cybertruck. I was thinking more like 200-400V DC systems, that could power most devices, but comes with a lot of challenges.
- deng 3y ago
  Yes, but this is not what this article is about, which is to avoid the loss of efficiency by converting low-voltage DC solar power to AC through an inverter, only to convert it back to low-voltage DC again for most devices. If we are talking high-voltage DC, you'd just replace the old DC-AC conversion with low/high-voltage DC-DC conversion, which I don't think is much more efficient. Also, I wouldn't want to live in a house which runs on high-voltage DC, purely because I don't want to instantly die if I accidentally touch a life wire.
  - posterboy 3y ago
    I don't see how that would be any different from unprotected "high" voltage lines, or preferably protected ones for that matter. In house installations around 500V count as Low Voltage in power engineering, by the way.
    - deng 3y ago
      That might very well be, but I have touched 240V AC accidentally, and I'm fine. Don't think that would still be the case for 400V DC, whether it counts as "low-voltage" or not. EDIT: After reading a bit, it seems that the "what is more deadly" discussion regarding AC/DC is much more complicated than I thought, so the above might very well be false...
      - mc32 3y ago
        Above 220/240 it’s typically required a person be certified and wear certain fabric types to minimize hazards.
      - Dylan16807 3y ago
        240V AC peaks at 340V and most systems would rate 400V DC as being pretty similar. Significant risk of shock, not much risk of arcing.
    - xenadu02 3y ago
      Any arc source is vastly more destructive with DC. That makes switching and circuit protection more difficult/expensive. AC helps you because the voltage crosses zero twice every cycle so normal voltages don't tend to be sufficient to sustain much of an arc in air. Every time you flip a switch you cause an arc, but the same DC voltage eats up the contacts more quickly. Even silly things like unplugging a running appliance does way more damage to the receptacle and the plug under a DC load.
  - andix 3y ago
    I think we need to talk about the definition of "low voltage". In my understanding it's everything below 1000V. Solar power systems often provide a voltage up to 1000V, by serially chaining modules. Running them in parallel with ~50V would require quite massive cables. Edit: About the danger of DC wires: There are technical solutions to mitigate risks. They may be too expensive though. I think we can reuse some of the strategies electric cars use. It's the only end-user application I know that uses DC powered cables with up to 900V (Lucid Air).
    - deleted 3y ago
      [deleted]
    - IndrekR 3y ago
      I (as someone who designs electric and electronic equipment every day) agree. Low Voltage is a very specific term and typically means <1kV AC and <1.5kV DC in the industry: https://en.wikipedia.org/wiki/Low_voltage https://en.wikipedia.org/wiki/Low_voltage
      - paleface 3y ago
        > Low Voltage is a very specific term […] It’s not a very specific term, at all - and is dependent upon context - per the first sentence, of the wikipedia article, you linked, stating: > In electrical engineering, low voltage is a relative term, the definition varying by context. The various standards, mentioned in that article, are in relation to “installations”, and power distribution. The standard in the UK (BS 7671), defines “high voltage”, as >600VAC difference between conductor(s), and earth. A voltage greater than mains voltage, in any item of domestic electrical equipment, is absolutely “high voltage”. Hence why valve amp power supply terminals, are considered high tension (HT, i.e., high voltage), despite not exceeding the threshold, claimed in wikipedia.
      - zamadatix 3y ago
        This is a great piece of data, thanks for posting it. Without this comment I would never have realized low voltage referring to ~50 volts or less was primarily a US thing.
  - willis936 3y ago
    Switching losses are less than ohmic losses. A few chips cost less than a lot of copper. The crossover point depends on the cable lengths, cable size, load, and voltage in question. Let's do a quick example. 16 AWG carrying 12V. 16 AWG is 4 mOhm/ft. Let's say you have a 1200 W load (100 A, 0.12 ohm). Ohmic losses per foot of cable are 3%. So if you have a 15 foot cable it will use as much power as the load. Edit: I think I responded to the wrong comment.
    - amelius 3y ago
      100 amps through 16AWG? Man that's pushing it.
      - Dylan16807 3y ago
        Especially for a 15 foot cable. I'd expect a cable >10x the thickness with <1/10th the resistance.
      - nine_k 3y ago
        Absolutely. But with the voltage 10x as high, the current goes down 10x, too, and the power loss goes down, too, while the power stays the same. That's how things run on AC normally.
- tlb 3y ago
  200-400V DC distribution systems are dangerous, both for electrocution and fire. Though maybe modern GFCI and arc detection breakers could make them adequately safe. Could someone who knows the state of the art comment?
- jbotdev 3y ago
  They’ve actually had 48V systems in production cars for several years now. Tesla is late to the game on that one. https://en.m.wikipedia.org/wiki/48-volt_electrical_system https://en.m.wikipedia.org/wiki/48-volt_electrical_system
- serf 3y ago
  >I don't see any future in 12V systems, even in cars why not? it works, it has industry wide support and a huge range of products, and it's safe without excess need for shielding and isolation. it's closer to the actual operating range of most equipment, so it produces , in most cases, less intrinsic need for shifting voltages around, and the wiring that would get substantially smaller is already near size limits for dealing with vibration and harshness. aside from trying to get closer to the operating voltages of whatever arbitrary energy pack we're specifically talking about , what's the point? if anything it would just wreck any hopes of cross-industry compatibility for a long time for very little manufacturing efficiency gains or energy savings. tl;dr: the vast majority of car 12v is lights and logic, and they're closer to 12v than 200v in the vast majority of cases.
  - Podgajski 3y ago
    I agree. Probably 90% of the power I consume is from a 100 watt solar panel and a 12v solar battery as I currently live out of a minivan.
    - callalex 3y ago
      I guess that really depends on how you define “consume”. For example do you do your laundry by hand or do you use a machine that lives outside your van?
  - theluketaylor 3y ago
    You're massively underestimating just how much wiring is in a modern car. The car industry has been trying to move to 48V for decades to save money on wiring and Tesla just succeeded with Cybertruck. High consumption devices like electric power steering, electric A/C, and active anti-roll bars all require huge amperage to run at 12V and are only becoming more common on cars. Car makers project big savings in moving to 48V and it's going to become a lot more common.
amelius 3y ago
> the problem is cable loss One solution may be to make a cable with a step-up converter at the beginning and a step-down converter at the end.

by deng 3y ago

andix 3y ago

I don't see any future in 12V systems, even in cars (with rather short cable lengths) there are discussions to switch to 48V, like Tesla is doing it in the Cybertruck. I was thinking more like 200-400V DC systems, that could power most devices, but comes with a lot of challenges.

deng 3y ago
Yes, but this is not what this article is about, which is to avoid the loss of efficiency by converting low-voltage DC solar power to AC through an inverter, only to convert it back to low-voltage DC again for most devices. If we are talking high-voltage DC, you'd just replace the old DC-AC conversion with low/high-voltage DC-DC conversion, which I don't think is much more efficient. Also, I wouldn't want to live in a house which runs on high-voltage DC, purely because I don't want to instantly die if I accidentally touch a life wire.
- posterboy 3y ago
  I don't see how that would be any different from unprotected "high" voltage lines, or preferably protected ones for that matter. In house installations around 500V count as Low Voltage in power engineering, by the way.
  - deng 3y ago
    That might very well be, but I have touched 240V AC accidentally, and I'm fine. Don't think that would still be the case for 400V DC, whether it counts as "low-voltage" or not. EDIT: After reading a bit, it seems that the "what is more deadly" discussion regarding AC/DC is much more complicated than I thought, so the above might very well be false...
    - mc32 3y ago
      Above 220/240 it’s typically required a person be certified and wear certain fabric types to minimize hazards.
    - Dylan16807 3y ago
      240V AC peaks at 340V and most systems would rate 400V DC as being pretty similar. Significant risk of shock, not much risk of arcing.
  - xenadu02 3y ago
    Any arc source is vastly more destructive with DC. That makes switching and circuit protection more difficult/expensive. AC helps you because the voltage crosses zero twice every cycle so normal voltages don't tend to be sufficient to sustain much of an arc in air. Every time you flip a switch you cause an arc, but the same DC voltage eats up the contacts more quickly. Even silly things like unplugging a running appliance does way more damage to the receptacle and the plug under a DC load.
- andix 3y ago
  I think we need to talk about the definition of "low voltage". In my understanding it's everything below 1000V. Solar power systems often provide a voltage up to 1000V, by serially chaining modules. Running them in parallel with ~50V would require quite massive cables. Edit: About the danger of DC wires: There are technical solutions to mitigate risks. They may be too expensive though. I think we can reuse some of the strategies electric cars use. It's the only end-user application I know that uses DC powered cables with up to 900V (Lucid Air).
  - deleted 3y ago
    [deleted]
  - IndrekR 3y ago
    I (as someone who designs electric and electronic equipment every day) agree. Low Voltage is a very specific term and typically means <1kV AC and <1.5kV DC in the industry: https://en.wikipedia.org/wiki/Low_voltage https://en.wikipedia.org/wiki/Low_voltage
    - paleface 3y ago
      > Low Voltage is a very specific term […] It’s not a very specific term, at all - and is dependent upon context - per the first sentence, of the wikipedia article, you linked, stating: > In electrical engineering, low voltage is a relative term, the definition varying by context. The various standards, mentioned in that article, are in relation to “installations”, and power distribution. The standard in the UK (BS 7671), defines “high voltage”, as >600VAC difference between conductor(s), and earth. A voltage greater than mains voltage, in any item of domestic electrical equipment, is absolutely “high voltage”. Hence why valve amp power supply terminals, are considered high tension (HT, i.e., high voltage), despite not exceeding the threshold, claimed in wikipedia.
    - zamadatix 3y ago
      This is a great piece of data, thanks for posting it. Without this comment I would never have realized low voltage referring to ~50 volts or less was primarily a US thing.
- willis936 3y ago
  Switching losses are less than ohmic losses. A few chips cost less than a lot of copper. The crossover point depends on the cable lengths, cable size, load, and voltage in question. Let's do a quick example. 16 AWG carrying 12V. 16 AWG is 4 mOhm/ft. Let's say you have a 1200 W load (100 A, 0.12 ohm). Ohmic losses per foot of cable are 3%. So if you have a 15 foot cable it will use as much power as the load. Edit: I think I responded to the wrong comment.
  - amelius 3y ago
    100 amps through 16AWG? Man that's pushing it.
    - Dylan16807 3y ago
      Especially for a 15 foot cable. I'd expect a cable >10x the thickness with <1/10th the resistance.
    - nine_k 3y ago
      Absolutely. But with the voltage 10x as high, the current goes down 10x, too, and the power loss goes down, too, while the power stays the same. That's how things run on AC normally.
tlb 3y ago
200-400V DC distribution systems are dangerous, both for electrocution and fire. Though maybe modern GFCI and arc detection breakers could make them adequately safe. Could someone who knows the state of the art comment?
jbotdev 3y ago
They’ve actually had 48V systems in production cars for several years now. Tesla is late to the game on that one. https://en.m.wikipedia.org/wiki/48-volt_electrical_system https://en.m.wikipedia.org/wiki/48-volt_electrical_system
serf 3y ago
>I don't see any future in 12V systems, even in cars why not? it works, it has industry wide support and a huge range of products, and it's safe without excess need for shielding and isolation. it's closer to the actual operating range of most equipment, so it produces , in most cases, less intrinsic need for shifting voltages around, and the wiring that would get substantially smaller is already near size limits for dealing with vibration and harshness. aside from trying to get closer to the operating voltages of whatever arbitrary energy pack we're specifically talking about , what's the point? if anything it would just wreck any hopes of cross-industry compatibility for a long time for very little manufacturing efficiency gains or energy savings. tl;dr: the vast majority of car 12v is lights and logic, and they're closer to 12v than 200v in the vast majority of cases.
- Podgajski 3y ago
  I agree. Probably 90% of the power I consume is from a 100 watt solar panel and a 12v solar battery as I currently live out of a minivan.
  - callalex 3y ago
    I guess that really depends on how you define “consume”. For example do you do your laundry by hand or do you use a machine that lives outside your van?
- theluketaylor 3y ago
  You're massively underestimating just how much wiring is in a modern car. The car industry has been trying to move to 48V for decades to save money on wiring and Tesla just succeeded with Cybertruck. High consumption devices like electric power steering, electric A/C, and active anti-roll bars all require huge amperage to run at 12V and are only becoming more common on cars. Car makers project big savings in moving to 48V and it's going to become a lot more common.

amelius 3y ago

> the problem is cable loss One solution may be to make a cable with a step-up converter at the beginning and a step-down converter at the end.