[HN Gopher] Why DC May Replace AC (2019)
___________________________________________________________________
Why DC May Replace AC (2019)
Author : aqeelat
Score : 29 points
Date : 2022-04-04 18:17 UTC (4 hours ago)
(HTM) web link (www.electricalindustry.ca)
(TXT) w3m dump (www.electricalindustry.ca)
| b33j0r wrote:
| If we discover room-temperature superconductivity and it is
| industrial-scalable, I could see DC taking over AC transmission
| lines.
|
| But right now, resistive heat losses make DC a silly solution.
| That's why we rectify only when the energy reaches "the edge."
| DoingIsLearning wrote:
| > In China and Europe, new cities and villages are being
| envisioned that will be entirely DC powered.
|
| Citation needed? Other then HVDC links or micro-generation I
| can't see a practical use for DC unless you are entirely off-
| grid.
| naikrovek wrote:
| lots of things are envisioned every day. everyone has ideas;
| calling it "envisioned" doesn't make those things any more
| feasible or realistic.
|
| article author is either sole owner of a huge copper deposit or
| isn't articulating themselves very well. DC makes no sense for
| distribution at all.
| dpierce9 wrote:
| The major thing that is unaddressed is the inertia of the extant
| built environment.
|
| We know that lead paint is bad for people, especially kids, but
| we haven't remediated it in much of the pre-1976 housing stock.
| Why? It is expensive and the places where it is worst are by
| definition not worth it.
|
| Similarly, there are many homes in America with low voltage knob
| and tube which is an uninsulated wire. Would you retrofit homes
| with a second DC circuit or use the existing AC infrastructure
| and be constrained by the choice of 12/14GA wire? Would new homes
| have two systems? Would you have a second set of DC distribution
| wires or a home inverter (with its own inefficiencies and failure
| modes)?
|
| The supposed efficiency of DC for residential applications will
| be overwhelmed by the efficiency of doing nothing.
| Youden wrote:
| The author seems very confused about whether they're talking
| about the grid or devices.
|
| "DC power is significantly more energy efficient than AC power."
| -> the examples go on to specify end points for electrical energy
| but we already use DC there, AC is mainly used in transmission,
| so the claimed advantages of DC are irrelevant.
|
| "DC motors and appliances have higher efficiency and power to
| size characteristics." -> Brushed DC motors aren't efficient,
| just cheap. Brushless DC motors actually require a separate
| circuit to turn DC into something resembling a sinusoidal current
| (i.e. AC).
|
| "DC is inherently compatible with renewable sources of energy
| such as solar and wind." -> solar generates DC but wind generates
| AC.
|
| "requiring storage (batteries)" -> chemical batteries require DC
| but other forms of storage like dams require AC to drive motors
| or turbines.
|
| "Most energy storage technologies are DC-based" -> at a local
| level (mobile phones, cordless power tools), sure. At a grid
| level, we're often talking about hydro.
|
| "Electronic equipment operates on DC power." -> equipment that
| deals with computation. Electric fans, washing machines and many
| industrial consumers of electricity use AC. Plus, the existing
| grids and existing generation infrastructure are built on AC.
| upofadown wrote:
| DC is generically more efficient for transmission than AC. That
| is why it is used for very long lines. It used to be that you
| needed to stay AC to use transformers to step up the voltage
| but those days are long past.
|
| >Brushless DC motors actually require a separate circuit to
| turn DC into something resembling a sinusoidal current (i.e.
| AC).
|
| Typical split phase AC induction motors used in residential
| applications are not very efficient and have various other
| deficiencies. There is a tendency to do a AC>DC>AC thing to a 3
| phase these days for smaller electric motors and get variable
| speed as a bonus.
|
| >...wind generates AC.
|
| But not at any particular frequency. So typical wind turbines
| have a AC>DC>AC converter to allow them to sync up with the
| grid.
| danhor wrote:
| AFAIK wind turbines, as opposed to conventional turbines in
| power plants, don't directly connect to the 60Hz Grid but go
| through DC and an inverter. This is done so they can
| efficiently work at different speeds at not just a few
| mechanically selectable ones
| shadowgovt wrote:
| They have to, because the variable wind means that they need
| to be spinnable at a continuous range of speeds.
|
| Components connected to the AC grid need to synchronize with
| the grid's frequency. Since we can't force the wind to blow
| at a particular rate, we'd either need a lot of fancy
| mechanics on the turbines themselves to drop their speed
| (which would waste energy) or we decouple their spin rate
| from the grid frequency with the AC -> DC -> AC converter
| (which _also_ wastes energy, but probably less and with much
| less cost than complicated spin-rate-stabilizing machinery).
| Animats wrote:
| _The author seems very confused about whether they 're talking
| about the grid or devices._
|
| Which is surprising, given his background, a degree in
| electrical engineering and jobs with power companies.[1]
|
| This may be an argument for using more DC-DC converters and
| fewer transformers. The classic problem with shipping DC around
| is that voltage conversion is expensive. DC-DC converters have
| improved a lot. This article may be a dumbed-down version of
| that argument.
|
| A nice thing about large transformers is that those big hunks
| of copper and iron have a lifespan of 30 to 75 years. Replace
| those with a DC-DC converter, and it will probably have
| semiconductor lifespan problems. Plus someone will add on a
| data connection, firmware updates, a web server, and an
| antivirus program.
|
| [1] https://grid.pitt.edu/people/gregory-f-reed
| Reason077 wrote:
| > _" solar generates DC but wind generates AC."_
|
| Yes, but a wind turbine is allowed to spin at variable speeds -
| its rotation is not synchronized to the grid frequency in the
| same way that hydroelectric and thermal turbines are.
|
| In order to get a wind turbine's power output to match the grid
| frequency, it goes through an AC -> DC -> AC conversion in a
| component known as a double-fed induction generator (DFIG).
| jbay808 wrote:
| In terms of motors, yes -- virtually all motors need to provide
| AC to the coils to run, so DC motors need to use an inverter.
|
| But increasingly these days, even AC motors are being run from
| variable-frequency drives, in order to squeeze out a bit more
| efficiency, because the savings from better matching the load
| more than makes up for the losses in the drive. Many
| jurisdictions are starting to incentivize or require VFDs for
| HVAC applications. And typically the first thing the VFD does
| is rectify the AC input to DC.
| seanalltogether wrote:
| AC is still way better for power transmission, and I don't mean
| giant power lines spanning from one city to another, I mean from
| the curb to your home, or within the walls of your home.
| Electroboom has a video on this topic
| https://www.youtube.com/watch?v=S7C5sSde9e4 and its been repeated
| elsewhere, but transmitting dc power with any meaningful voltage
| is dangerous, like burn your whole house down dangerous, and if
| its not high voltage, you're just losing too much power to
| resistance.
| elihu wrote:
| AC won out historically because it's easy to change the voltage
| of AC using a transformer. If we reach a point where modern DC-
| DC converters are cheaper or better than a traditional
| transformer, then I don't see why we wouldn't just use DC
| everywhere. (I don't know if we're actually there yet.) With
| DC, you can transmit more power over the same wire you'd use
| for AC (no skin effect), electric shock from moderate DC
| voltages isn't as bad as AC, and you mostly get rid of 60-hz RF
| noise.
|
| One argument for AC though is that it's easier to make AC
| switches, since those have a self-extinguishing arc. Maybe even
| household light switches can be replaced by solid-state
| devices?
|
| Aside from the difficulty of making reliable switches, I'm not
| aware of anything about DC that makes it inherently more
| dangerous than an equivalent AC voltage.
| danachow wrote:
| > AC is still way better for power transmission, and I don't
| mean giant power lines spanning from one city to another, I
| mean from the curb to your home, or within the walls of your
| home.
|
| First you say AC is way better for transmission, which is false
| - you should look into what HVDC is.. and second what you're
| talking about "curb to home" is power distribution, not
| transmission - so your post is confused on a few levels - it's
| hard to understand what you're trying to even claim.
|
| > its been repeated elsewhere, but transmitting dc power with
| any meaningful voltage is dangerous, like burn your whole house
| down dangerous
|
| Would you like to specifically reference in your linked video
| where that claim is made? Because I didn't see that, and I am
| puzzled what you're referring to. Quite the opposite he
| demonstrates at household voltage, DC is safer than AC from a
| shock standpoint (see 2:08). Why do you think high voltage DC
| is inherently less safe than AC?
|
| This video seems to demonstrate the basic historical concept
| that AC is superior for transmission due to the typical ease in
| converting to high voltage low current and back - the key point
| is that it is high voltage for lower current and lower loss and
| this has traditionally been easier achieved with AC. It doesn't
| really get into modern power conversion which has changed
| things somewhat.
| dudeofea wrote:
| I'm not sure if that's the moral of that video. Just watched
| (had seen before), and his conclusion is that you shouldn't
| transmit power with low voltage because it means you need lots
| of current.
|
| HVDC has some tradeoffs over HVAC, but you should be able to
| transmit power just fine with either.
|
| HVDC doesn't suffer from the skin effect that AC does:
| https://www.allaboutcircuits.com/textbook/alternating-curren...
|
| HVDC is however harder to make/break contact with compared to
| HVAC as AC crosses zero volts many times:
| https://electronics.stackexchange.com/a/325608
|
| _EDIT_ : Sorry, didn't mean to pile on this comment with
| everyone else
| InTheArena wrote:
| I keep wondering if there might be some value in a derivative
| high-efficiently USB-C PD standard (since distance, and other
| factors come in) for whole house. IE, could you add DC power via
| USB connections to a bunch of different devices with a high-
| efficiency power supply for all of the different connections,
| rather then having low-efficiency power supplies in lots of other
| devices?
| danhor wrote:
| Most modern (GaN) USB-C chargers are already highly efficient.
| Since USB-C PD can only be used as a point-to-point connection
| (since a voltage level is negotiated), a sane architecture
| would likely use a high voltage (>100V to minimize resisitve
| losses) DC line with local step-down. At that point, the dc-dc
| step down shouldn't be integrated into the cabeling but into an
| external unit or into tne device, since the idle power, size
| and cost of a 500W PC power supply and a 5W headset charger is
| very different (even using DC).
|
| The losses from the AC conversion aren't very high and the most
| energy intensive consumers (resisitive loads, ACs, Fridges)
| don't benefit much from switching to DC.
|
| With high voltage DC safety becomes another concern, with
| arcing being a huge issue.
| Mizza wrote:
| I've always wanted something like this, and I imagine the
| global efficiency benefit would be monumental in the long term,
| despite the enormous cost of enforcing a change. Though I also
| have trouble wrapping my head around a USB washing machine.
| sabareesh wrote:
| Pretty much all device in a household can function using only DC
| and now we can also generate DC directly DC at home, thanks to
| Solar. So we can cut the losses switching to AC and DC.
| nickff wrote:
| Counter-example: Motors consume >= 25% of all electrical power,
| and the majority of household and industrial motors are AC.
| Think air conditioners, fans, compressors, etc.
| castratikron wrote:
| Question is whether those devices would be better off using
| brushless DC or some other DC motor? Tesla uses a switched
| reluctance motor (basically stepper motor) instead of
| induction motor on their low end vehicles for example.
| WorldMaker wrote:
| Just about every electric vehicle at this point uses DC
| induction motors. Many support AC fast charging with
| various sorts of battery hacks, but Lithium Ion batteries
| are kind of inherently DC when it comes to applying power
| to the motors. At this point DC motors generally seem to
| out-class their AC counterparts _other than_ the efficiency
| of using the same current as walled outlets in homes.
|
| The article puts it this way in a bullet point toward the
| top:
|
| > DC motors and appliances have higher efficiency and power
| to size characteristics.
| klondike_ wrote:
| "Brushless DC" motors are actually 3 phase AC synchronous
| motors with an integrated DC-> AC converter. In
| industrial settings with 3 phase grid power they are very
| efficient
| xxpor wrote:
| > AC fast charging
|
| I believe you meant DC fast charging, unless you were
| referring to level 2 charging.
| WorldMaker wrote:
| Yes, I was referring to Level 2+ charging. Some EVs can
| charge surprisingly efficiently that way through some
| interesting engineering hacks, but yes overall the
| industry has moved on to DC fast charging standards with
| "AC fast charging" a fallback.
| elihu wrote:
| Actually, induction motors seem to be losing popularity
| in EVs, being replaced by permanent magnet motors which
| are more efficient (which also makes them easier to
| cool). And they're usually regarded as AC motors because
| they're fed 3-phase AC power from a motor controller
| (also called an inverter). The entire motor controller /
| motor system runs on DC power, so sometimes it's referred
| to as a brushless DC motor.
|
| Fast charging is done with DC. Level 1 and level 2
| charging uses 110 or 220 volt AC and is quite slow by
| modern standards.
| kevin_thibedeau wrote:
| No. AC motors are more reliable simply by having fewer
| parts that can fail.
| watermelon0 wrote:
| I thought newer devices (washing machines, ACs, etc.)
| mostly used brushless DC / ECM motors, since they are
| more efficient and quieter?
|
| They do need controllers that use AC, so I don't think
| that existing devices would work on DC.
| candiodari wrote:
| Yes but they work on feedback (if the washing machine has
| 5kg load, use frequency X, voltage Y, if machine has 10kg
| load ... they don't measure the load they measure how the
| motor reacts to their first guess voltage (this is called
| startup) and then adjust). Even comes with mechanical
| advantages: instead of using brakes, you just use the
| same motor and reverse the feedback.
|
| Now if you want the ability to adjust frequency and
| voltage, at large power levels, you're talking about
| changing the parameters of an inverter. So what it's
| going to do with AC input voltage is AC -> DC -> AC* (*
| with different frequency and voltage, synchronized to the
| rotation angle changes of the drum of your washing
| machine). This comes with a second advantage: it's easier
| (and cheaper) to be tolerant to frequency and voltage
| changes in the wall plug, maybe even tolerant enough to
| have one device that works in US and EU (and ...)
|
| You're doing this because the power plant is not going to
| change frequency or voltage based on how fast your
| washing machine is turning, but doing that makes the
| washing machine much more efficient.
| jbay808 wrote:
| The trend is towards DC motors that are
| _electromechanically_ AC motors, but supplied with AC
| current generated by an inverter from a DC power source.
| The inverter can control the motor more precisely than a
| fixed-frequency grid voltage can, and the DC source can
| be supplied by a battery.
|
| In terms of reliability, the inverter is still an extra
| part that can fail, but on the other hand, it's also much
| less likely to blow a fuse when your motor shaft stalls
| on startup.
| fatnoah wrote:
| The author appears to be Gregory Reed, Professor at the
| University of Pittsburgh who focuses on grid things. He's also
| the Chief Science Advisor for: https://www.emergealliance.org/
|
| Definitely seems like someone who would have appropriate
| knowledge to make statements like those in the article, though
| maybe with a vested interest in things.
| chrisp_how wrote:
| An AC computer is more efficient that a DC one, because the
| machine operates on cycles, or repeating interval-segments.
| kimpeek wrote:
| > Direct Current (DC) electric power is an emerging disruptive
| technological area that has the potential to stimulate economic
| growth, inspire innovation, increase research and development
| opportunities, create jobs, and simultaneously advance
| environmental sustainability.
|
| Was this published in the early 1900s? There is no date and DC is
| definitely not emerging nor disruptive.
|
| DC won't replace AC for those who rely on remote power
| production.
| orf wrote:
| Why won't it replace it, out of interest?
| wbsss4412 wrote:
| Same reason why it didn't win out 100 years ago. It isn't as
| efficient.
| sabareesh wrote:
| I don't think it is efficiency, we had no way to step up
| and step down DC as we can do AC
| WorldMaker wrote:
| Yup, transistors especially have given us major
| breakthroughs in the ability to step up/step down DC.
|
| Also, there have been huge breakthroughs in High-Voltage
| DC: https://en.wikipedia.org/wiki/High-
| voltage_direct_current
|
| At certain huge (grid) scales they have found that AC and
| DC swap "efficiencies" again and we're increasingly
| starting to see current flows as DC-AC-DC "sandwiches"
| with DC used by the majority of consumer electronics and
| DC used for extremely high scale grid transport, and AC
| still useful in the mid-range transport.
| vegetablepotpie wrote:
| Yes, this is true. To step up/down AC voltages, you only
| need a transformer, a pair of coiled wire. This is very
| simple tech.
|
| To step up/down DC. There are ways to do it with solid
| state electronics. One of the ways I've seen is to
| transform the dc to ac internally, change the voltage,
| and convert and output DC.
| jacquesm wrote:
| For grid transmission over longer hauls it will definitely be
| the standard, for shorter runs and local distribution we will
| likely be using AC for a long time to come, possibly forever.
| bsder wrote:
| This is, in fact, precisely backward.
|
| DC is _great_ for transmitting power. You crank the voltage,
| use all of the copper wire (no pesky skin effect), and sync to
| the grid at the DC-AC conversion point.
|
| The limiting factor to DC was conversion losses. The Pacific DC
| Intertie needed to use gigantic, toxic mercury vapor tube
| diodes for the conversion for a very long time.
|
| Now that we use high voltage semiconductors, that's no longer a
| problem. We easily convert between DC voltages as well as AC
| with quite remarkable efficiency.
| jacquesm wrote:
| At your typical power distribution frequencies (50 Hz, 60 Hz)
| the skin effect is negligible.
| bsder wrote:
| You are simply wrong.
|
| Skin effect at 60Hz is about 8mm. Power transmission
| (especially the long distance ones) conductors are normally
| quite a lot larger than that. Even the wires coming into
| your house are probably pretty close to that so there will
| be some effect even if it's not huge.
| jacquesm wrote:
| 8 mm (in copper, which is rarely used for powerlines, if
| at all, it is super expensive and heavy) is huge for a
| single conductor, and your typical overhead powerline is
| concentric shells of tens of conductors. Negligible: has
| no practical effect on the construction. It's in the 4th
| significant decimal or so for a typical powerline
| segment, dwarfed by plain resistive losses.
|
| You want those multiple conductor arrangements anyway to
| reduce the corona discharge.
|
| If you go up to multiple KHz then it will become a
| problem.
| borodi wrote:
| Reactive power losses due to the cable's inductance are
| probably the largest factor in the efficiency boost of
| using DC. Also you can use a somewhat higher voltage
| since you don't have to account for the AC peak, it does
| mean however that breaking a DC arc is harder since the
| is no 0 crossing.
| jacquesm wrote:
| Depends greatly on the material used and the cable
| construction. Typical: 60 strand aluminum (better skin
| effect properties than copper by the way) around a steel
| carrier. And yes, those are the largest factors in the
| boost to DC, but that's mostly because the resistance
| losses are there regardless so there isn't much else that
| you could improve on.
|
| If you transmit a lot of power over very long distances
| then the higher the voltage the lower the current and DC
| gets rid of the skin losses so there's the case for HVDC
| transmission lines (which are _extremely_ impressive
| feats of engineering, as are the substations).
|
| Finally found a good picture of a cross section of a HV
| AC transmission cable:
|
| https://en.wikipedia.org/wiki/Aluminium-conductor_steel-
| rein...
|
| Based on that ruler that makes the AL wires about 3 mm
| each, and the skin depth at 50 Hz would be about 11.5 mm
| or so, so _well_ within the range where the skin losses
| are extremely small (they are still there though, and
| when you 're transferring Gigawatts every little bit
| helps).
___________________________________________________________________
(page generated 2022-04-04 23:01 UTC)