Hacker News Reader

Comment by deng | Hacker News Reader

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.

View on HN

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.

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.

Reply on news.ycombinator.com