10 ms·

"Because many modern devices operate internally on direct current (DC)" But notably the high power consumers like electrical stove, air conditioning, washing ma

"Because many modern devices operate internally on direct current (DC)" But notably the high power consumers like electrical stove, air conditioning, washing machines etc. do not. And it's not that with DC throughout there are no conversions. Solar cells' output varies greatly, hence it needs to be DC/DC converted to a fix voltage to be of any use. The same, albeit to a much lesser extend, applies to batteries. Further, there is no one single voltage, AC or DC, which is optimal for all uses. Even for devices which receive DC voltage, e.g. laptops, there are internal DC/DC converters. Removing the AC/DC conversion seems hardly worth the trouble. High efficiency AC/DC converters can be constructed. It's rather a question on how much one is willing to spend on such.

kzrdude 3y ago
With more and more DC generation and consumption on the network, I think the AC mains is becoming very noisy, i.e. there are spikes, which we are not so used to handling.
- Liftyee 3y ago
  That is true. There is a whole field of power factor correction and filtering that tries to reduce the noise injected into the mains (hello, EMC regulations) but some devices do that better than others.
andix 3y ago
Most of the high power consumers could run on DC power with only slight modifications (in design, not aftermarket). Heating elements can run on DC more or less unmodified. Also electric motors can run on DC efficiently (see electric cars). Back in the days 3-phase power was needed to run strong electric motors (very common in Europe, but i think it exists also in the US for factories and businesses). But there are many easy solutions nowadays to just run them on DC.
- Liftyee 3y ago
  > Heating elements can run on DC more or less unmodified. ..but to get the same power (heat) output you need more current in a low voltage DC system. And power loss in the supply cables scales with the square of current, so you'll need thick (expensive) wiring in the wall or you'll risk the walls becoming an equally effective heating element. Electric motors that run on DC are either brushed or have extra driver/conversion circuitry to pulse their coils in the right order. Both of these are not perfectly efficient.
- eternityforest 3y ago
  High voltage DC is hard to switch with mechanical switches, and you have to convert it back to AC to run motors(Unless you're willing to accept brushed motors), so it wouldn't be trivial.
  - Scoundreller 3y ago
    A lotta motors are moving to variable drive to have a continuous duty cycle and do that by internally converting whatever is coming in into AC. They should happily accept DC since they're just rectifying the AC anyway. My fridge is "inverter drive" and just runs continuously at a low duty cycle all day. Most heat pumps are like this too to dial up and down instead of strictly being on or off. (Unsure what voltage the motors are running at though, so maybe AC-in is preferred to step it down before rectifying and chopping)
    - eternityforest 3y ago
      That's pretty neat! Nobody I know has many appliances from the last 20yrs so I'm a bit out of the loop on those... I guess all we'd really need is reliable switches and breakers that could be made affordably, and connectors that didn't create a huge arc every time someone unplugged a running space heater? Still a pretty big challenge, the lack of arcing seems like a really big advantage.
- cesarb 3y ago
  > Also electric motors can run on DC efficiently (see electric cars). It's quite likely that the motors on electric cars (and other efficient electric motors) are actually using an inverter to turn the DC into multi-phase AC and power an AC motor; this allows varying both the voltage and frequency of the AC fed into the motor.
  - phkahler 3y ago
    Bingo. The frequency is proportional to motor/vehicle speed. The inverters are at least 98 percent efficient at high power, and a good EV motor is in the 90s at high power.
- Scoundreller 3y ago
  Increasingly, heat pump devices, like fridges, air conditioners, heat pumps, water heaters, are inverter drive, so they just rectify all the AC into DC and start chopping it to get to their desired frequency. (Also dumb that electric codes require dedicated circuits for many of these things, despite them not having the surge like they used to)
mananaysiempre 3y ago
In-sync AC grids are an engineering marvel; high-voltage AC lines are comparatively lossy due to the skin effect among other things; a frequency of 50 or 60Hz yields what are, by modern standards, comically huge step-down transformers; etc. It's worth considering whether the relative ease of running AC motors makes all the difficulties of AC supply worth it. (It's worth remembering that the "modern", i.e. 1930s, three-phase power grid is built primarily to power motors on factory floors. I don't know if these are the highest load these days, but wouldn't be surprised if not.)
- VBprogrammer 3y ago
  > It's worth remembering that the "modern", i.e. 1930s, three-phase power grid is built primarily to power motors on factory floors. Small nitpick, I don't believe 3 phase power was picked primarily because it was convenient for powering 3 phase motors - it was picked because it's convenient for generating and transmission. There is no need for a neutral conductor in a balanced AC transmission system for example and it can very efficiently be converted to high voltage and stepped down again. It also results in modest power pulses though the generator compared to single phase etc.
  - tyingq 3y ago
    Feels like it's both to me. 3 phase, 3 wires is optimal for the amount of conductor material needed to distribute it. But it's also optimal for motors for a few reasons. Easy to reverse direction of the motor, self starting without a capacitor, and higher overall power and efficiency.
- algo_trader 3y ago
  If you are building a custom mini grid from scratch, how do you get from batteries/panels to distribution-level 11kv DC without going through AC ?
  - 20after4 3y ago
    All of the DC/DC boost converters that I am aware of use inductors to transform low voltage to high voltage. It's technically pulse-width-modulated DC instead of AC but there isn't a lot of difference between the operation of a boost converter and the operation of a high frequency inverter.
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.
coryrc 3y ago
So all efficient (switching) voltage conversion involves AC. Your stock plug-in DC power supply is actually a AC->DC->AC->DC device yet it's trivial to get above 97% and miniscule quiescent (minimum) load. High-frequency AC isn't a problem for efficiency. What's NOT easy is efficiently making 60 Hz AC at low quiescent power. Most inverters always burn 2-5% of maximum power with no load, while 1% would be considered far excessive if the output was DC (maybe 0.1% would be generally okay?). Part of this is lack of regulation; consumer devices have low-quiescent power supplies originally because of government mandates and, now, economies of scale. The government mandate didn't apply to inverters so there isn't an initial push to shift to low-quiescent topologies and prevent the first movers from being undercut. So just for that reason, lower-voltage DC distribution can overall be more efficient if you are off-grid, even if some other portions of the system have increased losses. 120Vdc distribution would be superior from an efficiency standpoint in every way, but you're going to spend a lot on switches and protection equipment.
- calamari4065 3y ago
  > Your stock plug-in DC power supply is actually a AC->DC->AC->DC device That is actually not at all how switching converters work. Pulsed DC is not AC. Those are two very different things.
  - coryrc 3y ago
    I'm not even sure how to reply to how wrong you are. You could only argue that with flyback topology (I'd still say you are wrong) but try a standard full-bridge and tell me the current isn't alternating. Then check out some ZCS variants and tell me which of those current waveforms don't look like Alternating Current.
  - nine_k 3y ago
    Pulsed DC is not DC,:) Since all such power supplies prominently feature transformers, they can't operate on DC alone. But the frequency they use is high (a few kHz), the transformers are small, unlike the huge beasts in traditional 50/60 Hz power supplies.
  - 3y ago
amluto 3y ago
> But notably the high power consumers like electrical stove, air conditioning, washing machines etc. do not. An electric resistance stove (radiant or exposed coil) would work just fine on DC, although, if it’s controlled using a TRIAC, that would need to change. An induction stove uses rather high frequency AC, and that could be generated just fine from a DC supply. Modern air conditioners use variable frequency drives, and those generally work by first converting the AC supply to DC. I imagine that many modern washing machines also use some sort of variable frequency drive or DC-powered motor.
marcosdumay 3y ago
Still, I wonder if the 50/60Hz pure sine wave has any quality for in-house transmission. Maybe outputting a square wave from that solar generator would be optimal.
- j16sdiz 3y ago
  Square wave would be very noisy in terms of EMI
- Johnythree 3y ago
  You would need to forget about any Radio Communications, eg Medium Wave Broadcast.
hinkley 3y ago
It is easier to turn AC at the wrong voltage into DC than it is to turn DC at the wrong voltage into the right voltage. Particularly if you care about efficiency of the device.
Johnythree 3y ago
> Solar cells' output varies greatly, hence it needs to be DC/DC converted to a fix voltage to be of any use. This is simply wrong. It is true that modern MPPT solar regulators use PWM to extract the maximum power, but for years, a simple on/off regulator was used to prevent overcharging. The reality is that a solar panel is essentially a "Constant Current" supply, so the panel puts out the same current over a wide range of battery voltages. It is the battery which puts out a Constant Voltage at whatever current the load requires.

kzrdude 3y ago

With more and more DC generation and consumption on the network, I think the AC mains is becoming very noisy, i.e. there are spikes, which we are not so used to handling.

Liftyee 3y ago
That is true. There is a whole field of power factor correction and filtering that tries to reduce the noise injected into the mains (hello, EMC regulations) but some devices do that better than others.

andix 3y ago

Most of the high power consumers could run on DC power with only slight modifications (in design, not aftermarket). Heating elements can run on DC more or less unmodified. Also electric motors can run on DC efficiently (see electric cars). Back in the days 3-phase power was needed to run strong electric motors (very common in Europe, but i think it exists also in the US for factories and businesses). But there are many easy solutions nowadays to just run them on DC.

Liftyee 3y ago
> Heating elements can run on DC more or less unmodified. ..but to get the same power (heat) output you need more current in a low voltage DC system. And power loss in the supply cables scales with the square of current, so you'll need thick (expensive) wiring in the wall or you'll risk the walls becoming an equally effective heating element. Electric motors that run on DC are either brushed or have extra driver/conversion circuitry to pulse their coils in the right order. Both of these are not perfectly efficient.
eternityforest 3y ago
High voltage DC is hard to switch with mechanical switches, and you have to convert it back to AC to run motors(Unless you're willing to accept brushed motors), so it wouldn't be trivial.
- Scoundreller 3y ago
  A lotta motors are moving to variable drive to have a continuous duty cycle and do that by internally converting whatever is coming in into AC. They should happily accept DC since they're just rectifying the AC anyway. My fridge is "inverter drive" and just runs continuously at a low duty cycle all day. Most heat pumps are like this too to dial up and down instead of strictly being on or off. (Unsure what voltage the motors are running at though, so maybe AC-in is preferred to step it down before rectifying and chopping)
  - eternityforest 3y ago
    That's pretty neat! Nobody I know has many appliances from the last 20yrs so I'm a bit out of the loop on those... I guess all we'd really need is reliable switches and breakers that could be made affordably, and connectors that didn't create a huge arc every time someone unplugged a running space heater? Still a pretty big challenge, the lack of arcing seems like a really big advantage.
cesarb 3y ago
> Also electric motors can run on DC efficiently (see electric cars). It's quite likely that the motors on electric cars (and other efficient electric motors) are actually using an inverter to turn the DC into multi-phase AC and power an AC motor; this allows varying both the voltage and frequency of the AC fed into the motor.
- phkahler 3y ago
  Bingo. The frequency is proportional to motor/vehicle speed. The inverters are at least 98 percent efficient at high power, and a good EV motor is in the 90s at high power.
Scoundreller 3y ago
Increasingly, heat pump devices, like fridges, air conditioners, heat pumps, water heaters, are inverter drive, so they just rectify all the AC into DC and start chopping it to get to their desired frequency. (Also dumb that electric codes require dedicated circuits for many of these things, despite them not having the surge like they used to)

mananaysiempre 3y ago

In-sync AC grids are an engineering marvel; high-voltage AC lines are comparatively lossy due to the skin effect among other things; a frequency of 50 or 60Hz yields what are, by modern standards, comically huge step-down transformers; etc. It's worth considering whether the relative ease of running AC motors makes all the difficulties of AC supply worth it. (It's worth remembering that the "modern", i.e. 1930s, three-phase power grid is built primarily to power motors on factory floors. I don't know if these are the highest load these days, but wouldn't be surprised if not.)

VBprogrammer 3y ago
> It's worth remembering that the "modern", i.e. 1930s, three-phase power grid is built primarily to power motors on factory floors. Small nitpick, I don't believe 3 phase power was picked primarily because it was convenient for powering 3 phase motors - it was picked because it's convenient for generating and transmission. There is no need for a neutral conductor in a balanced AC transmission system for example and it can very efficiently be converted to high voltage and stepped down again. It also results in modest power pulses though the generator compared to single phase etc.
- tyingq 3y ago
  Feels like it's both to me. 3 phase, 3 wires is optimal for the amount of conductor material needed to distribute it. But it's also optimal for motors for a few reasons. Easy to reverse direction of the motor, self starting without a capacitor, and higher overall power and efficiency.
algo_trader 3y ago
If you are building a custom mini grid from scratch, how do you get from batteries/panels to distribution-level 11kv DC without going through AC ?
- 20after4 3y ago
  All of the DC/DC boost converters that I am aware of use inductors to transform low voltage to high voltage. It's technically pulse-width-modulated DC instead of AC but there isn't a lot of difference between the operation of a boost converter and the operation of a high frequency inverter.

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.

coryrc 3y ago

So all efficient (switching) voltage conversion involves AC. Your stock plug-in DC power supply is actually a AC->DC->AC->DC device yet it's trivial to get above 97% and miniscule quiescent (minimum) load. High-frequency AC isn't a problem for efficiency. What's NOT easy is efficiently making 60 Hz AC at low quiescent power. Most inverters always burn 2-5% of maximum power with no load, while 1% would be considered far excessive if the output was DC (maybe 0.1% would be generally okay?). Part of this is lack of regulation; consumer devices have low-quiescent power supplies originally because of government mandates and, now, economies of scale. The government mandate didn't apply to inverters so there isn't an initial push to shift to low-quiescent topologies and prevent the first movers from being undercut. So just for that reason, lower-voltage DC distribution can overall be more efficient if you are off-grid, even if some other portions of the system have increased losses. 120Vdc distribution would be superior from an efficiency standpoint in every way, but you're going to spend a lot on switches and protection equipment.

calamari4065 3y ago
> Your stock plug-in DC power supply is actually a AC->DC->AC->DC device That is actually not at all how switching converters work. Pulsed DC is not AC. Those are two very different things.
- coryrc 3y ago
  I'm not even sure how to reply to how wrong you are. You could only argue that with flyback topology (I'd still say you are wrong) but try a standard full-bridge and tell me the current isn't alternating. Then check out some ZCS variants and tell me which of those current waveforms don't look like Alternating Current.
- nine_k 3y ago
  Pulsed DC is not DC,:) Since all such power supplies prominently feature transformers, they can't operate on DC alone. But the frequency they use is high (a few kHz), the transformers are small, unlike the huge beasts in traditional 50/60 Hz power supplies.
- 3y ago

amluto 3y ago

> But notably the high power consumers like electrical stove, air conditioning, washing machines etc. do not. An electric resistance stove (radiant or exposed coil) would work just fine on DC, although, if it’s controlled using a TRIAC, that would need to change. An induction stove uses rather high frequency AC, and that could be generated just fine from a DC supply. Modern air conditioners use variable frequency drives, and those generally work by first converting the AC supply to DC. I imagine that many modern washing machines also use some sort of variable frequency drive or DC-powered motor.

marcosdumay 3y ago

Still, I wonder if the 50/60Hz pure sine wave has any quality for in-house transmission. Maybe outputting a square wave from that solar generator would be optimal.

j16sdiz 3y ago
Square wave would be very noisy in terms of EMI
Johnythree 3y ago
You would need to forget about any Radio Communications, eg Medium Wave Broadcast.

hinkley 3y ago

It is easier to turn AC at the wrong voltage into DC than it is to turn DC at the wrong voltage into the right voltage. Particularly if you care about efficiency of the device.

Johnythree 3y ago

> Solar cells' output varies greatly, hence it needs to be DC/DC converted to a fix voltage to be of any use. This is simply wrong. It is true that modern MPPT solar regulators use PWM to extract the maximum power, but for years, a simple on/off regulator was used to prevent overcharging. The reality is that a solar panel is essentially a "Constant Current" supply, so the panel puts out the same current over a wide range of battery voltages. It is the battery which puts out a Constant Voltage at whatever current the load requires.