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Comment by guenthert | Hacker News Reader

"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.

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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...

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.

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.

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.

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...

Above 220/240 it’s typically required a person be certified and wear certain fabric types to minimize hazards.

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.

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.

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).

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

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.

> 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.

> I think we can reuse some of the strategies electric cars use.

Sounds like it could be a patent minefield.

Since we’re talking about power distribution in homes, the National Electric Code and local zoning apply. That means low voltage for the purpose of the article will be limited to 0-49v. All solar panels for homes that I’m aware of are under 49v per panel so fair enough for the article.

The panels on my house are 44V each but are connected with 11 in series. That means you can expect to see 484V in an open circuit situation with full sunlight.

Most grid-tie systems use multiple solar panels wired in series to give the high voltage DC needed to simplify the conversion to Mains AC.

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.

100 amps through 16AWG? Man that's pushing it.

Especially for a 15 foot cable. I'd expect a cable >10x the thickness with <1/10th the resistance.

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.

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?

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

>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.

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.

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?

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.

> 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.

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.)

> 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.

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.

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 ?

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.

Motors consume around half of the world’s electricity, and nearly all of the power consumed by motors connected to the grid is consumed by three-phase A/C induction motors. Motors aren’t just in factory equipment, HVAC systems have all sorts of motor driven equipment.

VFDs have made A/C motors far more efficient than one driven by an across-the-line starter.

The biggest point loads are aluminum smelters. Probably next are electric arc furnaces for steelmaking, the so-called mini-mills.

Semiconductor factories are not far down the list either. Texas will probably see quite a few come online within the next decade.

When I was working at SAS in ATX circa 2013, the 2 manufacturing lines were responsible for something like 10% of the city's average power consumption. I can't imagine the bleeding edge will have improved much - EUV light sources aren't exactly Energy Star compliant.

> a frequency of 50 or 60Hz yields what are, by modern standards, comically huge step-down transformers

Should switch to 400Hz like aircraft do. Not just comically huge transformers, but also motor/generator magnets.

> A special generator was designed to create an output of 400 Hz. This allowed a motor which was the size of a watermelon to be replaced by one the size of a one-pound coffee can which could do the same work.

> The saving of weight allowed increased cargo capacity and decreased fuel consumption. Power at 400 Hz for aviation was a success and became the standard of modern AC-powered aircraft.

> Airports all around the world standardized on the same power system. This included the physical plug and cable as well as the 400 Hz power so that aircraft from anywhere in the world could land and be serviced wherever they landed. The aviation power system of 400 Hz became one of the first worldwide-adopted standards.

https://fcxinc.com/why-the-aviation-industry-operates-on-400...

> Should switch to 400Hz like aircraft do.

Higher frequency lets you get away with smaller transformers, but decreases the skin depth on transmission lines which increases losses.

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.

> 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.

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.

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.

AC can mean "sinusoidal" or it can more generally mean "any non-constant voltage [or current]." It was clear from the context that the parent meant the latter.

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.

> 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.

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.

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)

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.

> 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.

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.

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)

> 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.

> 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.

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.

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.

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.

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.

Square wave would be very noisy in terms of EMI

You would need to forget about any Radio Communications, eg Medium Wave Broadcast.

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