Why DC May Replace AC (2019)
electricalindustry.caThe 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.
> "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).
Wind turbines, the big ones, regulate themselves to turn in sync with the grid. The adjust thier blades as needed to keep pace/power. That's why huge fields of them all turn in perfect lockstep.
From some random reading on the net, it is the smaller ones that can be directly connected. Above 2MW things are not as easy and it is better and more efficient to convert to DC.
Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower.
The blade angle is adjusted to get optimum power extraction. Rotor RPM is completely independent of produced power frequency.
References: https://www.vestas.com/en/products/offshore/V236-15MW and https://library.e.abb.com/public/bf09cdf11d234241845c79ac343...
You're talking past each other.
The electronics perform two key functions:
* Choosing pitch angles for efficiency and turbine safety. You can, for smaller turbines, just synchronize the turbine to the grid, but this is becoming uncommon practice.
* Converting the produced AC power to DC, and then choosing the proper frequency output and voltage to feed power to the grid, and inverting the DC to make this power. This should usually be trying to "speed up" the frequency of the grid a little if it's not already way too fast and regulate the voltage appropriately.
The second link you have, on page 3, shows (active) rectification (d) of the wind turbine AC power to make positive and negative DC buses, and then inversion of that DC (f) to make 3 phase AC output power.
Yeah, I don't know if for example in big wind offshore wind parks, since it's connected with a DC cable to mainland anyway, you would convert all the turbine generator outputs just to DC.
Then at the ground station where it's connected to the grid, convert to AC, whatever kilovolts are needed.
> you would convert all the turbine generator outputs just to DC.
You could. But you probably still need a DC/DC conversion step or boosting in order to let the power flow from each of the turbines fairly. (The synchronous conversion to DC from AC provides opportunity to slightly change the voltages you get out, but not terrifically so).
Blade pitching first use is to reduce the aerodynamics power above the rater power point of the turbine and avoid overload.
Yup, that's why the word "safety" is in there. I probably could have put that better. (It also includes e.g. feathering rather than just trying to stay close to rated power, speed, and torque limits)
> "That's why huge fields of them all turn in perfect lockstep."
Not true for any modern wind farm. While they might appear to be turning together, that's just because the wind conditions are relatively consistent across the whole area. Not because they are actually synchronised! Most modern turbines will target a certain rated/optimal/maximum rpm once a certain wind speed has been reached, but are free to rotate more slowly (while still generating power) in light wind conditions.
They are still not syncing directly to the AC grid. They are not going 50 rounds per second
They wouldn't necessarily need to do 50 rounds per second to generate 50 Hz. There could be any number of poles on the generator. At least in theory... Not saying you're wrong about not being synced but just that they wouldn't need to do 50 rps...
Gearboxes. Windmills of the same design will turn at the same rate in lockstep with each other. A gearbox converts that rate into what is necessary for the generator. The generator turns in sync with the grid.
Likewise for solar -- the DC voltage and current a panel puts out is dependent on both insolation and load -- only in the most simple applications can you just hook in directly to a panel, without involving a DC-DC converter. Even grid-tie installations often use DC-DC converters, called "power optimizers" [1], to optimally tie multiple panels to a single bus.
I have personally worked on a tens-of-kW-scale wind turbine which was directly connected to the grid and was fixed speed and blade angle. It's quite common at lower power ratings because the cost/complexity of pitch control isn't justifiable.
It wasn't much more complicated than a three phase electric motor bolted to high-angle-of-attack blades, connected to the grid via contactors controlled by a microcontroller and a grid-intertie monitor.
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.
> "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 is more efficient for transmission too.
The reason the grid uses AC is that high voltage is always[1] more efficient for transmission than low voltage--regardless of whether you're talking about AC or DC--and in 1900 the only way to step voltages up or down was to use transformers, and transformers require AC. That's unfortunate because for a given voltage, it's more efficient to transmit it as DC than AC.
But today we have power electronics which can step DC voltages up or down, which means we are free to convert the grid to DC.
[1] The single exception is superconducting cables which can transmit low voltage power just as efficiently as high voltage power. But they are not yet cheap enough to be practical except over short distances.
I fully agree with what you said about AC/DC, just want to point out that DC benefits superconductors as well since superconductors have critical current densities where they lose their super conductivity.
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.
Indeed. There seems to be a lot of confusion about the types of AC. "AC" can mean "50 or 60 Hz sinusoid synchronized to the grid" or it can mean "a waveform that is not a constant voltage." Most motors require some form of the latter. Older motors required the former, but there are very few reasons to build motors that way today. The ones that remain in service can easily be driven by electronic inverters from a DC supply.
In marketing speak, a variable frequency drive is often referred to as "inverter technology" in appliances.
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.
If you wanted to efficiently make an efficient deeply variable speed modern motor run on AC, you might very well turn the AC into DC then back into AC in a brushless motor controller.
Indeed, changing the frequency of an AC wave programmatically is incredibly difficult (but possible through a CVT, I guess), you're better off turning it into DC then back into AC through some some form of function generator.
So indeed, the widespread use of BLDC motors is a point in favour of DC electric circuits in the home.
Same goes for variable velocity generators, you will generally have an AC-DC-AC conversion in a variable speed generator. Either that, or a gearbox, those are your two options.
Youden, your comment is a great example of why I love HN so much. Thanks for sharing this, and clearing a few important things about the article.
The case for DC is named microgrids: keeping the frequency on a microgrid is hard, inverters react too slowly for most loads (that does not ramp up/does down power slowly) while DC devices can be far quicker. The case for AC is large grid and safety.
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
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).
You could also have a continuously variable gearbox, which is still hugely problematic, but would waste less energy.
The author gets a few things wrong or mixed up (mostly covered in other comments).
None of the other comments also talk about galvanic isolation, you need transformers (therefore AC) for that.
But the article is trying to sell you on DC, like there are lot of marketing around hydrogen and such. I would say the right approach would use the right technology where it is the best option. HVDC to replace HV AC transmission, sure if it makes sense for your use case. DC-ify all homes and appliances just because? Definitely not. DC-ify parts of home lighting? Why not. Tho lighting is usually at 24VDC or 48VDC and then you are going to need either very thick cables (insane waste) or DCDC converters and where is the advantage there?
Besides transmission line level of DC, rest of the article is pretty much noise, generation is where we have big problems at the moment.
Also AC frequency is currently used as signaling mechanism to regulate grid power-balance and stability, spinning reserve and all that jazz. With DC you need some other signaling mechanism, first to mind is Voltage but with all the smart semiconductor devices my guess is that the signal gets lost as these devices tend to compensate. So you'd need a dedicated signaling mechanism like software...
You don't need AC for isolation, you need transformers. Isolated DC to DC is a big area of power supplies. You've also got cabling size backwards: you need thinner conductors for higher V due to lower I.
Transformers only work with AC. Isolated DC-DC has also transformers where there is really high frequency AC. DC->AC||AC->DC. It's all hidden and due to high frequency quite small. But still AC.
Re cabling: currently we have 230V at home, going to 24VDC would mean 100x more losses. So I would say I got it correctly when I said going to 24V or 48V needs thicker cables due to increased I. As for your sentence, you don't NEED thinner conductors when voltage goes up but you would be quite wasteful if you didn't.
Yes, but it's not AC in the sense that most people think of AC, where it's oscillating around 0V. Cable comment was related to you mentioning 12 and 24, it didn't seem compared against 120V in your comment.
> 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.
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.
But DC is used for distribution, just at extremely high voltages.
HVDC is used in distribution for specific transfer between point A and point B, HVDC is not used _for_ distribution.
Distribution implies a network that keeps getting downstepped until it reaches consumer nodes.
San Francisco has a “secret dc grid serving 900 customers” - the problems described are now solvable.
I am aware of at least one American managed company here in China selling DC-all-the-things.
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 not high-value areas.
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.
It's a tangent, but worth pointing out that like asbestos, lead paint remediation is often more dangerous than doing nothing. Paint over it, and it isn't hurting anyone. Sand it off, and now you have super fine lead particles floating in the air and settling all over the place just waiting to be disturbed and kicked up again.
Lead pipes are an entirely different matter, and remediation is usually (wrongly) deferred due to cost.
Lead remediation isn’t just sanding and covering with paint is not always a good idea. First windows and doors are painted friction surfaces which create lead dust. Second, painting over it assumes the bond between the substrate and the lead paint will hold in perpetuity. In practice it doesn’t but yes often painting is a good form of remediation. This does not work outside though. Third, sanding is not great for all of the reasons you mentioned but not the only way to remediate (e.g., steaming, chemical stripping, physical encapsulation, or removal are all options). Fourth, it was really just an example of how the built environment is incredibly sticky to things which are downright dangerous, known to be dangerous, and yet continue to persist because of the associated expense. Your example is also good.
K&T is not uninsulated.
Can confirm. My previous house had some still in service for some general hallway lighting and other light loads. It was not jacketed (like modern non-metallic cable is), but it was definitely insulated.
I misspoke. The K&T I had in my house had gaps in the loom because it failed. You also can’t insulate around it.
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.
There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and battery system.
That is not a new idea obviously, that exact approach has been in use in the telecom industry for decades. There is a AC feed to a system of rectifiers that converts the AC from the grid to -48VDC and continuously charge a string of batteries. There is a Generator that will go on automatically if the power from the grid fails and takes over.
The -48VDC is distributed throughout the central office to the equipment bays. There are a number of benefits to doing it this way, the batteries help maintain a constant voltage level and provide back up until the generator can fire up or if the generator should fail to start it buys you time to get it going.
For a home, I could see having a DC distribution system using USB 48V standard or maybe a 12VDC system with a wall battery and perhaps Solar system. Assuming that you could power all your devices off DC, it would eliminate the need for an Inverter. Most devices in homes today can be powered with 12VDC versions, with some exceptions.
It's an interesting idea.
Since we are doing DC-DC, I'd say we go with the highest DCV we can get away with. 12V is just simply too low and the associated I^2R losses will be unacceptable.
Right, agree. But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. But strictly from a technical point of view you are spot on.
> But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating.
Which is exactly why we need to get away from those 12V old garbages asap. 12V is so bloody wasteful, in terms of both wasted power and wasted conductor material. The only reason why its still around is people got used to it and don't wanna change.
what is the optimal voltage based on empirical data ?
> highest we can get away with
Assuming this means highest voltage that's still mostly safe and does not require a whole bunch of insulation, we might be looking at something from 80 to 100 VDC? No idea where I read it from but apparently it takes about that much DC voltage for people to "feel something" when touching conductors with dry skin.
The EU safety regs have a cutoff of 50VAC and 75VDC. (Those are only ~5% different in peak voltage.)
Below those levels, you have only general product safety regs to comply with. At/above those and up through all “reasonably household” voltages, you (probably) have to comply with the EU low voltage directive. “Probably” because the LVD itself isn’t law but member states have generally implemented it in their laws.
You break skin at about 50VDC. We could keep ~120V for distribution. There are problems tuning control loops of buck converters dropping more than about 30V (though you can just have multiple buck converters in a row).
Correcting the power factor from 120VAC gives you a boost circuit that gives you 360-400VDC. Some motor control and battery technology standardizes around this voltage. Cars are a big one, but also PFC direct to inverter motor control, which is becoming popular in white goods.
> You break skin at about 50VDC.
What does breaking skin mean in this context? My understanding was that humans largely act like a resistor with some parasitic capacitance and inductance. Wouldn't more voltage equal more current in a mostly linear relation?
You'll feel it shock you. It overcomes the surface resistance of not particularly moist skin and makes your nerves tingle or your muscles twitch.
A lot of industry like datacenters and vehicles are moving to 48V. 48V is right on the limit of what is considered low-voltage and therefor doesn't have special safety requirements. And because it's a multiple of 12V, it can work well with old equipment by putting exiting supplies or batteries in series, and using very efficient constant-ratio down-converters.
see for instance: https://www.vicorpower.com/documents/whitepapers/wp-boosting...
There are a few people that use 24 - 48V DC batteries/solar charge controllers in RVs and then step down for the motors, lights, etc.. for the rest of the rig. Pretty simple conversion for an RV.
Correct me if I'm wrong but isn't the whole point of using AC that it's easy to convert voltages super-efficiently with transformers?
Having DC power at home seems like the worst of both worlds. Low voltage DC at home causes large losses in the wiring, and high voltage DC causes large losses in the step-down regulator (leaving you with a buck-type regulator operating at the low extreme of its duty cycle). After all, DC-DC conversion relies broadly on turning the DC into something vaguely sinusoidal and using an inductor - so its basically DC-AC-DC anyways.
Am I missing something?
Switchmode power supplies already tend to rectify and filter the high voltage AC before stepping it down. At high frequency, the transformer can be a lot smaller.
Not sinusoidal: Boost and buck converters use PWM square waves. Because the power transistors are almost always fully on or fully off the I2R losses in the transistors are quite low.
I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads.
When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit would be expensive.
That's going to be some thick ass wires for the oven, water heater and a few other high wattage items.
The big loads would have to remain at 120 or 240vac.
> 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.
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.
At your typical power distribution frequencies (50 Hz, 60 Hz) the skin effect is negligible.
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.
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.
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.
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).
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.
Why won’t it replace it, out of interest?
Same reason why it didn’t win out 100 years ago. It isn’t as efficient.
I don't think it is efficiency, we had no way to step up and step down DC as we can do AC
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.
> DC-AC-DC "sandwiches"
Couldn't have said it better, I had a good laugh.
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.
There is an interesting effect with AC distribution that leads to the statement "the amount of power produced must balance that consumed" that you sometimes hear.
On first thought this is a ridiculous statement: voltage is a measure of potential energy after all. If load is reduced, the voltage just hangs there and less power will be consumed.
But with AC distribution what you have is essentially a large rotating machine. The more power you put into it, the faster it spins. When you connect a generator to the grid, you phase match the AC waveforms, connect it, then start pushing the grid faster to inject power.
So the statement is true. If the load is too low, the frequency starts to go up. But it's also not true.. if each generator independently self limits the frequency, we would be back to the potential energy situation. But power plants want to push power into the grid- this is how they earn money.
So some plants self regulate and some do not, see:
https://www.e-education.psu.edu/ebf483/node/705
Ones that self regulate get to charge a premium for their unused capacity.
Even with DC only, you would be in the same situation. A power plant wants to make money, so it will want to push power into the grid, which for DC means pushing the voltage up.
> There is an interesting effect with AC distribution that leads to the statement "the amount of power produced must balance that consumed" that you sometimes hear.
You have the same problem with DC. It nothing to do with 'AC' per say. It's a issue of large scale power plants.
> On first thought this is a ridiculous statement: voltage is a measure of potential energy after all. If load is reduced, the voltage just hangs there and less power will be consumed.
Voltage is a measurement of the difference of two electrical potentials. "Potential Energy" is something else entirely.
Voltage is not a measurement of energy.
Voltage works in a similar way that pressure does. Imagine you have two pressure cylinders and one of them is 200 PSI and the other is 220 PSI. If you were using a voltage-style measurement between the two cylinders you'd say that the there is 20 PSI different or 'potential' between them.
That gives you zero information as far as the actual energy potential. A 16 ounce canister at 100 PSI is going to have a lot less energy potential then a 500 gallon tank at 100 PSI, for example.
This is why you can go and get a static electrical shock that involves thousands of volts and your skin doesn't burst into flames.
> But with AC distribution what you have is essentially a large rotating machine.
No with AC, or DC, what you is LITERALLY a large rotating machine. A rotating machine larger then most houses running of of hydro electric, coal, or nuclear power take time to have their energy output adjusted.
They don't work like car motors were you press a button to go "zoom" and another to go "woah".
The generators that can quickly adjust are small ones. Generally natural gas turbines. They are a lot like jet engines. In fact many of them used to be the same type of engines used in jet planes.
And they are much more expensive to operate and less efficient overall, but they are the ones people are moving to because they can keep up with the extremely poor quality electrical output (read: highly unpredictable) you get from solar and wind.
> Even with DC only, you would be in the same situation. A power plant wants to make money, so it will want to push power into the grid, which for DC means pushing the voltage up.
You somehow seem to have "laws of physics' confused with "making a profit".
Of course you are not wrong with the power plants operators wanting to get paid to work for a living and there are plenty of shady things they do that you should be irritated about, but you are barking up the wrong tree here.
For example: the massive scam that is government-subsidized grid-tied residential solar. How that the plant operators have colluded with the regulators to ensure that they have remote control over your inverter's output. Which means that with the hundreds of thousands of solar panel installations that people are proud of and think they can make money from 'selling back to the power plant' are actually operating at a only a tiny fraction potential output.
Which means that home owners that do pay tens of thousands of dollars for these setups are getting burned WHILE accomplishing nothing to help the environment.
And when the grid goes down so will those grid-tied installations. For "safety" reasons, despite the fact that ICE-based generators have had reliable failsafes for generations that automatically prevent any electrical feedback into downed power lines.
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The fact of the matter why DC is better then AC for power transmission has to do with inductance.
Every time the wire is subject to electrical current it generates a magnetic field. The higher the current the more powerful the magnetic field. By winding a cable around a iron core you can concentrate that field and make a powerful electro-magnet. The effect is still present in the miles of electrical cable used for power distribution. It's just spread out over a massive area.
With AC that magnetic field needs to be torn down to zero, reversed, and then torn down to zero again 60 times a second. Sure much of that energy is returned to the wire on each field collapse, but it is still something you have to deal with and fight with. It shows up as significant inductance.
With DC you don't have to do that.
The AC/DC debate has raged for over a hundred years because each side has some merits over the other, but overall the differences are relatively minor compared to the cost of ripping up an existing AC or DC system and replacing it, or even switching to one system or the other going forward. PG&E isn't going to scrap all of the transmission towers they have on order just because DC might save them a few dozen watts.
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?
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.
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.
It might not scale for washing machines, but at least for home electronics.
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.
> 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.
https://www.youtube.com/watch?v=S7C5sSde9e4&t=3m18s
https://www.youtube.com/watch?v=S7C5sSde9e4&t=7m34s
The thermal properties of AC for comparable amounts of power in a conductor are significantly better.
Copper is expensive, and having dealt with just the difference between connecting 12 gauge and 14 gauge to outlets and pulling it through conduits I'm certainly not enthusiastic about needing larger conductors.
He was running ten amps over thin wires; of course that doesn't work. The problem there was that he was transmitting significant power at low voltage and high current. That doesn't say anything about the superiority of AC or DC. AC has historically been the more practical choice because you can trade AC voltage for current easily with a transformer, but there are ways to do that with DC now too. Maybe there's an argument to be made that transformers are cheaper or more reliable than boost converters; I don't know one way or the other.
If wikipedia is to be believed, DC is better in terms of conductor material costs and transmission losses than AC, at least for long distance high-power high-voltage transmission lines.
> A long-distance, point-to-point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme. HVDC conversion equipment at the terminal stations is costly, but the total DC transmission-line costs over long distances are lower than for an AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect.
> Depending on voltage level and construction details, HVDC transmission losses are quoted at 3.5% per 1,000 km, about 50% less than AC (6.7%) lines at the same voltage.
https://en.wikipedia.org/wiki/High-voltage_direct_current#Ad...
> If wikipedia is to be believed, DC is better in terms of conductor material costs and transmission losses than AC, at least for long distance high-power high-voltage transmission lines.
This is correct. DC is a much better choice for long distance transmissions. You don't need to worry about reactive power, skin effect, reactive elements of the line, whether the generators and the grid are synced or not etc. The problem with DC voltage conversion are mostly solved as well.
> The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance. The cost of the converters may not be offset by reductions in line construction cost and lower line loss.
https://en.wikipedia.org/wiki/High-voltage_direct_current#Di...
Not to mention that circuit breakers are harder in DC.
> Not to mention that circuit breakers are harder in DC.
[Citation needed]
I can believe this is true. It's easier to make reliable AC switches because whatever arcing you get is self-extinguishing because the voltage passes zero 120 times a second. With DC, if you get an arc it'll just keep going as long as conditions allow. That means if you use a designed-for-AC switch in a DC application with the same voltage, it's likely to destroy itself sooner or later.
(Apparently this is one of the reasons why cars stick with 12V for accessories, because if they used higher voltages the electrical switches would be more expensive and less reliable.)
I don't know how this is normally overcome. In a lot of cases, the solution might just be "use a fuse instead".
Yes, opening a DC circuit under load can be very surprising. The arcs can just stay on forever, and pose a big fire hazard.
A big capacitor?
He’s demonstrating the use of a transformer to step up the voltage. The thermal properties are because of the lower current - not the line frequency. The transformer requires AC to work, but that is not the only way to convert power, and once the voltage is stepped up, it can be rectified back to DC and will actually be more efficient than AC - hence the reason that HVDC is a thing.
High power DC-DC conversion equipment is still more expensive than AC and the technology simply didn’t exist during the current wars.
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.
The physical design of the switch ought to be easy enough to make physically interrupt the arc.
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
Consider also having to rebuild things from scratch. Say a solar flare knocked out all your electronics or you are at war with your only supplier of rare earth metals. How quickly can you replace missing generators and appliances with devices that may be less efficient or clean and may only supply a small fraction of current power levels, but will at least let you turn lights and fridges back on? If mechanical turbines can generate A/C without advanced electronics and mechanical motors can run on that, it's not a bad precaution to keep these around.
I can't find the date this article was written. Is it 20 years old or 2 days old?
This may be a dumb idea, but, would it be more efficient and useful to use DC battery storage in series up to 120V rather than up/down-converting between 120 and 12?
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.”
Ohms law is the same for AC and DC at any given moment in time. That is, in an instant where an AC voltage in a wire at 120V, it will have identical resistive loss as DC voltage at 120V.
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.
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.
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.
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.
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.
All charging is done with DC internally. Level 1 and 2 charging uses a rectifier inside the car to convert AC to DC. Fast charging simply bypasses the rectifier.
The reason Level 2 charging is current-limited by your car -- even if you had a very high-current AC source available -- is that to take advantage of a high-current AC source your car would have to carry around a bigger, heavier rectifier. Which would decrease your EV's efficiency just by virtue of being big and heavy.
Yep, that's all true. I meant that L1 and L2 use an external AC power source.
"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
Not necessarily. Brushless DC motors are not required to be 3-phase or even to use sinusoidal waveforms. Most small BDC hobby motors (such as those on drones) are not even remotely sinusoidal.
> AC fast charging
I believe you meant DC fast charging, unless you were referring to level 2 charging.
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.
No. AC motors are more reliable simply by having fewer parts that can fail.
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.
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.
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.
Cool, let me know how much the copper will cost you when you'll need to wire your entire household with at least 2 tons (and I am on low estimate here) of them, because you'll need them thick to drive the increased Ampere juice. Remember P (power) == U (voltage) x I (intensity). As you'll lower your voltage, your intensity will rise to meet the same power consumption. Meaning thick copper wires, at least 2cm diameter, not your current 3mm diameter you currently have.
For safety alone DC is not going to replace AC. The author seems to be confusing local use, and distance transmission.
In what way is AC safer than DC? Ohm’s law says V=IR, that means your resistive body is gonna get shocked regardless of whether the voltage is constant or changing.
It isn't - both can kill you. DC results in a continuous muscle contraction, where AC makes for many contractions; DC 'let go' current is higher than AC; lethal AC current is also lower than DC. But we're talking mA here - house current will kill either way if it goes through the heart. source: https://www.brighthubengineering.com/power-plants/89792-ac-a...
That's because the 220V AC is the integral (as in that math you learn in high-school) part of it. That 220V AC outlet actually has 630 V peak-to-peak voltage difference. That's why AC voltage is riskier than its DC counterpart, but that's because the numbers for AC are somehow misleading. Anyone who ever build an AC-to-DC converter knows that after you run your AC source through a rectifier bridge your get 315V at its exit. Then you use capacitors and maybe induction coils to "smooth" it, and that smoothing is what gets you your 220V DC source now.
Don't forget that AC can flow through capacitors.
What do you mean by that exactly?
Get a capacitor that supports 700V minimum as highest applicable voltage, and a wire. Go to your socket outlet, put the capacitor in one hole, the wire in the other hole and see if you get shocked or not. Do the same (if you're still alive) with a DC source. Comeback here and let me know how the experiment went and also tell me why you skipped physics class in high-school when this was taught.
I know all about RLC circuits, I just fail to see how this makes it any safer than DC. Conversely, DC can flow through inductors, AC can’t.
AC can and will definitely flow through inductors, especially the 220V/60Hz one you have at outlet. I challenge you to connect an inductor, even one of several Henry's as value, to your outlet and to your hand and comeback here and tell me if it shocked you or not.
To transfer an equivalent amount of power to a given load, AC has a peak to peak voltage of about 170V compared to DC's constant 120V.
170V is peak to median. Peak to peak is double that (340V).
More precisely the math formulae is: "AC voltage" x sqrt(2) - for median. And you double that for peak-to-peak
An AC computer is more efficient that a DC one, because the machine operates on cycles, or repeating interval-segments.
I suppose that might be true if you're using a 60hz clock speed.
After skimming this post history I'm not sure if it's AI, a joke, a troll or something else all together. AC computers and sin + cos = consciousness are just the beginning.