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Slow Electricity: The Return of DC Power? (2016)

lowtechmagazine.com

31–40 of 233 posts

Re: Slow Electricity: The Return of DC Power? (2016)

#31
post #26

There is a line that’s incorrect in the article: > AC won, mainly because of its higher efficiency when transported over long distances. AC is not actually more efficient at long distance transmission, it’s less efficient due to the skin effect. AC is just easier to convert.

Yeah I seem to recall that many extremely high voltage long distance lines are actually DC

Indeed. This is because you get phase differences (due to finite speed of signal in wire, even though it's near c) over those distances. Converting to DC and then back to AC at the destination avoids any concern about phase mismatch.

Re: Slow Electricity: The Return of DC Power? (2016)

#32

There is a line that’s incorrect in the article: > AC won, mainly because of its higher efficiency when transported over long distances. AC is not actually more efficient at long distance transmission, it’s less efficient due to the skin effect. AC is just easier to convert.

It's not "incorrect". They literally explain the reason why they say it is so in the very next sentence. Only taken out of context does it appear incorrect.

Re: Slow Electricity: The Return of DC Power? (2016)

#34

What I'd love is a standardized DC outlet wired into homes that can power anything from a phone to a desktop computer.

Connect a hefty DC power supply to a power strip and just use that? Get a power strip of some obscure country for your DC so that you don't accidentally mix up 120VAC appliances with it.

Re: Slow Electricity: The Return of DC Power? (2016)

#35
"Early DC power stations had a dynamo for every light bulb. Source unknown."

I have no idea where they got that picture either. Edison's Pearl St. station did not work that way, and that was the first power station. The generator shown is Edison's famous "long-waisted Mary Ann". But Edison never built a one generator per bulb power station. He was able to calculate ROI [1]. This is probably some conceptual drawing of what would have been necessary had the "subdivision of the electric light" (a hot issue around 1889)[2] had not been accomplished.

"In other words, a DC electrical system could make a solar PV system more energy efficient." Not clear that it matters much in terms of raw conversion efficiency. Solar inverters are now up around 96-98% efficiency. If you're going to have batteries, you need something that handles battery charging, discharging, and some constant output, and while you can get all that stuff for 12VDC or 24VDC, it's not any more efficient than outputting 120VAC.

The real argument they're making is to save power by using small boat or RV sized appliances. They're available, efficient, undersized, and about 3x as expensive as line-powered units.[3]

[1] https://www.hup.harvard.edu/catalog.php?isbn=9780674423640

[2] https://www.nature.com/articles/040152a0.pdf

[3] https://www.thecabindepot.com/collections/appliances

Re: Slow Electricity: The Return of DC Power? (2016)

#36

What I'd love is a standardized DC outlet wired into homes that can power anything from a phone to a desktop computer.

USB-C?

USB-C sucks, I break a USB-C cable a week. They're physically too weak. I've very rarely broken 120VAC plugs or cables.

Also you can't exactly have a USB-C PD power strip of 10+ plugs without it costing a fortune because of all the negotiation needed per-port. Now they're onto some GaN bullshit just to make the PD adapters smaller.

The standards are a shitshow, there are incredible number of non-compliant USB-C cables.

802.3at PoE is slightly better, you can get 24+ ports for a couple hundred bucks, most cables advertised to meet PoE specs do in fact meet them.

But really the best would be straight up 48V on 2 rails and a bunch of plugs. No negotiation, no bullshit, no GaN, just 2 wires. Equipment, cabling, power strips would be dirt cheap. Individual devices can have very compact, efficient buck converters to get the voltage they actually need.

Re: Slow Electricity: The Return of DC Power? (2016)

#37
I read the whole article with sympathetic interest, until the conclusion:

> One way to solve the problem of high power devices is simply not to use them -- this is the approach that's followed in sailboats, motorhomes and caravans

> Obviously, this strategy implies a change in our way of life. It would mean that electricity is used only for lighting, electronics and refrigeration, while non-electric alternatives are chosen for all other appliances. Not coincidentally, this is quite similar to how DC grids were operated in the late nineteenth century, when the only electric load was for lighting -- first arc lamps and later incandescent bulbs.

> Thus, no dishwasher, but doing the dishes by hand. No washing machine, but doing the laundry in a laundromat or with a manually operated machine. No tumble dryer, but a clothes line. No convenient and time-saving kitchen appliances like electric kettles, microwaves and coffee machines, but a traditional cooking stove operated by (bio)gas, a solar cooker, or a rocket stove. No vacuum cleaner, but a broom and a carpet-beater. No freezer, but fresh ingredients. No electric warm water boiler, but a solar boiler and a small wash at the sink if the sun doesn't shine. No electric car, but a bicycle.

Re: Slow Electricity: The Return of DC Power? (2016)

#38

What I'd love is a standardized DC outlet wired into homes that can power anything from a phone to a desktop computer.

USB Power Delivery 3.1 specifies power over USB-C up to 5A @ 48V. 240W doesn't give you a very powerful desktop: probably an SSD, a 65W CPU and a low power GPU. But that's still a decent desktop.

I don't necessarily think it's wise to do 5 amps over USB-C, but it is a standardized DC outlet. Personally, USB-A is still a more useful power outlet, but nobody is insane enough to shove 240W over it.

Re: Slow Electricity: The Return of DC Power? (2016)

#40

Earlier quoted context omitted.

> An LED doesn't run on 24V DC It can. Cree's JR5050 24‐V LEDs run on 24V. They have 30V and 36V versions as well.

That is because that package internally steps down that voltage. Any LED that produces light in the visible range will have a bandgap in the 1 to 2 Volt range. Feeding it a higher voltage than that would lead to disastrous current run away. The package that contains the LED and the current limiter and whatever other electronics are in there might of course accept 24V DV, or 110V AC or 230V AC or 17.4V at 4kHz. But th…

LEDs are usually current-controlled, not voltage controlled.

You might have several LEDs in series and they might very well have a voltage drop across all of them of 24V or even much higher when maintaining an operational amount of current.

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