[HN Gopher] Slow Electricity: The Return of DC Power? (2016)
___________________________________________________________________
Slow Electricity: The Return of DC Power? (2016)
Author : nf3
Score : 41 points
Date : 2023-12-31 06:25 UTC (1 days ago)
(HTM) web link (solar.lowtechmagazine.com)
(TXT) w3m dump (solar.lowtechmagazine.com)
| bad_alloc wrote:
| I wish there was some standard way to add DC wiring in parallel
| to AC. Seems like people who want to do this have to roll their
| own systems for now.
| amelius wrote:
| You could use USB wall sockets :)
| msandford wrote:
| Same here. The problem is that there's no consensus on what
| voltages. I might prefer 48v for lower losses but you might
| want 24v since it's "safer" and my neighbor might want 192v
| since he's already an electrician.
| Szpadel wrote:
| IMO just using the same voltage as in AC and multi voltage
| DC/DC adapter would be sweet spot
| PaulKeeble wrote:
| There are multiple things that we really need to make it
| viable. A standard for the plug that separates it from any AC
| standard, a standard voltage and current and then standard DC
| to DC convertors for adjusting the voltage. I guess also we
| need a completely different light bulb fitting socket too that
| only fits DC light bulbs. Lots of different commercial DC
| convertors of varying sizes to replace the AC convertors in all
| sorts of devices.
|
| I do wonder how much in practice would be gained given the DC
| to DC conversions are guaranteed for every device.
| eternityforest wrote:
| We do have a standard AC-DC converter, USB PPS, which can do
| most of these use cases just fine. DC-DC versions can be done
| too if one really wanted.
|
| But running low voltage DC power wiring is still probably not
| the best plan.
| adrianN wrote:
| Power over Ethernet perhaps?
| cesarb wrote:
| There's already a somewhat common standard for DC wiring in
| homes and offices, though it's probably not exactly what you
| had in mind: it's Power over Ethernet (PoE), which provides
| around 48V over standard twisted pair Ethernet wiring, while
| still allowing it to be used for data.
| ianburrell wrote:
| PoE is too low power to use for many things that would want
| DC power. 802.3bt tops out at 71W. You couldn't run a 100W
| USB-C adapter on it.
|
| Also, PoE is pretty high losses running power over Ethernet
| cable. The max power at source is 100W. PoE puts in higher
| voltage for account for the losses which means it isn't a
| straight 48V input to 48V output.
| JKCalhoun wrote:
| When I built out a van into an RV, I got to run a new
| electrical system for it -- one based around the 12V automotive
| battery.
|
| It turns out that there is a small industry out there that
| serves both RVs (and boats as well) that is 12V/DC based.
| Lights, pumps, refrigerators, fans, all running straight off a
| 12V power bank. "Cigarette lighter" outlets stand in for wall
| outlets for USB-style chargers, etc.
|
| The 12V/DC battery system in the van is of course charged from
| both solar on the roof of the van and from the van's alternator
| (when the van battery is topped off and the van is under power
| of course).
|
| I did add an inverter to supply 110V/AC for a pair of
| traditional electrical outlets I installed in the kitchen area
| of the van/RV. These are primarily used for plugging in wall-
| wart style chargers for the laptops.
|
| (My Kill-A-Watt suggests that my rice cooker and even the
| electrical kettle would, just barely, function on the current
| provided by the inverter but RV-life tends toward minimalism so
| those extra appliances I've left behind.)
|
| In any event, the whole experience did have me wondering if I
| could run a parallel 12V/DC electrical system in a new home and
| do away with a lot of the step-up/down of AC.
|
| Definitely could do with some kind of modern outlet (USBC?)
| rather than the cigarette lighter outlets, ha ha.
| Gibbon1 wrote:
| What would be nice is a residential power over Ethernet
| standard. With standard crimp connections.
|
| Some advantages power limited and low voltage means it's
| safer[1]. Ethernet means you can control lights and other
| devices. Smaller diameter wiring means it's cheaper. Not
| needing a licensed electrician to install it saves $$$.
|
| [1] Can imagine for a non North American not having to deal
| with 230VAC would be a big bonus.
| amelius wrote:
| > Dutch researchers managed to reduce total cable length in a
| house down from 40 metres to 12 metres. They did this by moving
| the kitchen and the living room (where most electricity is used)
| to the first floor, just below the roof (where the solar panels
| are), while moving the bedrooms to the ground floor. They also
| clustered most appliances in the central part of the building,
| right below the solar panels
|
| Now this is dedication ...
| kabouseng wrote:
| That's not great, since entertaining guests it is preferable to
| have these rooms on the ground floor, and the bedrooms which
| can be private on the upstairs floor.
| Epa095 wrote:
| While upstairs has a better view, more light coming in
| through the windows, and heat rises up (so the bedrooms can
| be kept comfortable cool). I guess it depends on the house
| and it's surroundings:-)
| brtkdotse wrote:
| > 40 metres to 12 metres
|
| So they sacrificed the freedom of design to save EUR100?
| huppeldepup wrote:
| Something I didn't find in the article is that in AC systems
| sparks are self-extinguishing because of the zero-crossing.
|
| There are a few domestic circuit breaker teardowns on youtube. I
| suggest watching one and then asking the question: how can we
| break a high current _DC_ surge?
| weare138 wrote:
| High voltage DC circuit breakers are a thing already.
| adrian_b wrote:
| Yes, but they are more complex, thus more expensive.
| bluGill wrote:
| They exist, but are much more complex. they do things like
| pair with an air compressor to blow the arc out.
| guenthert wrote:
| "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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| > 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 wrote:
| 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.
| cesarb wrote:
| > 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.
| deng wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Above 220/240 it's typically required a person be
| certified and wear certain fabric types to minimize
| hazards.
| Dylan16807 wrote:
| 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.
| andix wrote:
| 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).
| IndrekR wrote:
| 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
| zamadatix wrote:
| 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.
| amelius wrote:
| > I think we can reuse some of the strategies electric
| cars use.
|
| Sounds like it could be a patent minefield.
| willis936 wrote:
| 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 wrote:
| 100 amps through 16AWG? Man that's pushing it.
| Dylan16807 wrote:
| Especially for a 15 foot cable. I'd expect a cable >10x
| the thickness with <1/10th the resistance.
| tlb wrote:
| 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 wrote:
| 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
| amelius wrote:
| > 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.
| mananaysiempre wrote:
| 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 wrote:
| > 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 wrote:
| 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 wrote:
| 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 ?
| quickthrowman wrote:
| 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.
| tuatoru wrote:
| The biggest point loads are aluminum smelters. Probably next
| are electric arc furnaces for steelmaking, the so-called
| mini-mills.
| bob1029 wrote:
| 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.
| coryrc wrote:
| 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 wrote:
| > 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.
| amluto wrote:
| > 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.
| Physkal wrote:
| Would this make DC appliances cheaper or smaller since there's no
| need for rectification circuits?
| andix wrote:
| I don't think it would be significant.
| _fizz_buzz_ wrote:
| I think it could make a difference. A lot of devices have
| power factor correction (pfc) now. To draw a current without
| harmonics from the grid. You wouldn't need a pfc stage
| anymore.
| andix wrote:
| Some kind of voltage conversion would be needed anyway.
| Most electronic devices run at 5V, 3.3V or 1.8V. And that's
| limiting cable lengths to a few meters (probably less than
| 5m).
| _fizz_buzz_ wrote:
| Yeah of course, but the pfc stage boosts the voltage
| first up above the ac peak voltage and then a second
| stage steps it down to whatever voltage is needed. If one
| has a dc grid one doesn't need the pfc stage (but of
| course still the second stage).
| limaoscarjuliet wrote:
| All this requires the DC appliances to be the same voltage, as
| transformers on DC is a tough task.
| _fizz_buzz_ wrote:
| Most stuff is switched mode nowadays anyway, so this becoming
| less and less relevant. Not sure if there really are any
| transformer based appliances in my house anymore.
| Liftyee wrote:
| Transformers on DC are already a solved problem with various
| switched-mode converters that can boost or decrease voltages at
| will. In fact, many AC-DC supplies (phone charger, etc.) are
| already using such converters, first rectifying the incoming
| AC. Not needing a bulky transformer makes them smaller and
| lighter.
| George2515 wrote:
| That's a clever way to summarize it! The merge commit retains the
| history of changes in the feature branch, while squashing
| simplifies the history for the release branch, making it cleaner
| and easier to follow. It's all about balancing clarity and detail
| in version control.
| jakedata wrote:
| Just sitting here speculating, not even a web search yet...
|
| In the USA we have both ground and neutral wires at every outlet
| along with the hot (120VAC nominal) wire.
|
| Why couldn't we raise the ground wire to +12VDC (or whatever)
| with regard to the neutral? The ground would have a very
| sensitive detector so that if electrical conditions exceeded some
| preset threshold it would collapse to zero nearly instantly,
| allowing the circuit protection to function as expected.
|
| You could make a plug that only has a neutral and ground pin to
| tap into the low voltage side, or use all three pins to provide
| low power standby mode.
|
| Now I'll do a search to find out that it is impossible because of
| a, b or c reasons or that it has already been patented somewhere
| and abandoned. Yay Internet...
| tzs wrote:
| Ground and neutral are tired together at the electrical panel.
| jakedata wrote:
| That's why this could work - the panel is the logical place
| to insert the DC supply. If ground and neutral were bonded in
| multiple places then it would just be a DC short.
| sokoloff wrote:
| They are bonded there for safety reasons and that bond
| needs to be low impedance.
|
| The reason is that you don't want to allow a fault which
| puts a line voltage (120VAC) onto the chassis of a piece of
| equipment waiting for a human to come along and complete
| the circuit to ground. So, you use the protective earth to
| connect to the chassis, such that if a power line ever did
| contact it, it would be a dead short to ground, which would
| trip the OCPD (over-current protection device, typically a
| circuit breaker), clearing the fault.
|
| If you replace this safety mechanism with something that's
| high-enough impedance to maintain a 12VDC differential,
| you've eliminated this important safety mechanism in your
| AC distribution system.
| tzs wrote:
| I think this would also make another possible fault more
| dangerous: a break in neutral somewhere between an AC
| device and the panel.
|
| If there was also a DC device on the same side of the
| break as the AC device, then you'd have a possible return
| path for the AC current that goes through the DC device,
| the ground wire, and the DC power supply.
|
| In addition to that, would it also cause problems with
| GFCI?
|
| The 12 VDC return current would be on the neutral. I'd
| guess that a steady DC current on the neutral wouldn't
| cause problems, but whenever a DC device is switched on
| or off, or you use a DC device that has a fluctuating
| instantaneous power need, that should induce current in
| the GFCIs sensing coil that won't be canceled out by an
| opposite current induced from the load wire.
|
| I don't know if that mismatch would be long enough or
| large enough to trip the GFCI.
| sokoloff wrote:
| GFCIs will trip at ~5mA of current imbalance, so your
| point is an additional excellent one.
| RetroTechie wrote:
| Even if plug is polarized (plug goes in only in 1 orientation),
| you can't assume _any_ of the high voltage wires to be touch-
| safe. Wiring mistakes (swaps) can & do happen.
|
| In-house AC systems are designed to remain safe when that
| occurs. But it prevents using any of those wires in ways like
| what you suggested.
| _fizz_buzz_ wrote:
| The GND (or Earth/GND) is there for safety and equipotential
| bonding. It's not meant to carry current and running current
| through it will trigger your RCD, because that's how a ground
| fault is detected. And detecting that is important so when you
| trip with your hairdryer into the bathtub you won't kill
| yourself.
| ianburrell wrote:
| One problem is that many older houses don't have grounding.
| Half my outlets are ungrounded and I use GFCI to get three-
| prong plugs. Or the grounding is done through pipes and conduit
| which aren't suitable for running current.
|
| The standards for grounding make them unsuitable for running
| current. The ground wire in box are frequently bare. Metal
| boxes are grounded. The center screw is grounded. If you run
| current over the ground, those are live.
|
| You have invented a way to electrocute lots of people.
| RetroTechie wrote:
| Not mentioned so far: AC->DC adapters also provide galvanic
| isolation. In many power supplies, that electric isolation comes
| 'for free', as you don't want device(s) connected to high voltage
| wiring anyway. AC vs. DC as input isn't _that_ big a deal really.
|
| In situations where power source (solar!), storage and most
| consuming devices are low-voltage DC, just use a DC based setup
| if more practical.
|
| I'm on a boat & most everything electric here is low voltage DC.
| But I do have a 12V->230V AC converter when needed.
|
| In short: AC & low voltage DC can live happily side-by-side. The
| DC vs. AC debate is kind of a moot point these days.
| dang wrote:
| Related:
|
| _Slow Electricity: The Return of DC Power? (2016)_ -
| https://news.ycombinator.com/item?id=28216968 - Aug 2021 (230
| comments)
| Animats wrote:
| Yeah, we did this already.
|
| Few devices other than switching power supplies are designed to
| work over a wide range of DC input voltages. There's an
| electric pump which will run slowly on an low input voltage and
| faster at a higher voltage.[1] Those are driven from windmills
| and solar. It's not cheap. It has to be self-protecting against
| too high and too low voltages.
|
| [1] https://www.dankoffsolarpumps.com/product/solar-slowpump/
| Johnny555 wrote:
| If DC in houses became popular, it seems like they'd follow the
| same model as RV's -- low power devices like lighting, water
| pump, maybe refrigerator all run on 12VDC, high power devices
| like air conditioning, microwave, etc run on 120VAC (sometimes
| inverted from battery).
|
| That limits the high gauge wire to the short run from battery to
| inverter, otherwise you'd need very expensive and heavy large
| gauge cable running to those high power devices.
| amelius wrote:
| Why not step up the DC voltage instead of inverting to AC?
| Johnny555 wrote:
| It's harder to step DC than AC, you'd probably end up using a
| switching power supply, which gets you most of the way to an
| AC inverter.
| amelius wrote:
| Yes, but once you have DC your losses will be less.
| amluto wrote:
| I think that, if DC appliances as used in RVs became genuinely
| popular, they would move to reasonable voltages. 12VDC is
| absurd. 48VDC would be much, much better.
___________________________________________________________________
(page generated 2024-01-01 23:01 UTC)