Why DC May Replace AC (2019)
electricalindustry.ca
electricalindustry.ca
"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.
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.
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).
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).
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...
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.
Then at the ground station where it's connected to the grid, convert to AC, whatever kilovolts are needed.
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).
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.
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.
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.
>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.
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.
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.
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...
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.
Citation needed? Other then HVDC links or micro-generation I can't see a practical use for DC unless you are entirely off-grid.
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.
Distribution implies a network that keeps getting downstepped until it reaches consumer nodes.
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.
Lead pipes are an entirely different matter, and remediation is usually (wrongly) deferred due to cost.
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.
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.
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.
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.
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.
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.
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?
see for instance: https://www.vicorpower.com/documents/whitepapers/wp-boosting...
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?
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.
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.
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.
Couldn't have said it better, I had a good laugh.
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.
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.
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.
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.
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).
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.
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 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.
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.
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=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.
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...
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.
https://en.wikipedia.org/wiki/High-voltage_direct_current#Di...
Not to mention that circuit breakers are harder in DC.
[Citation needed]
(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".
High power DC-DC conversion equipment is still more expensive than AC and the technology simply didn’t exist during the current wars.
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
But right now, resistive heat losses make DC a silly solution. That’s why we rectify only when the energy reaches “the edge.”
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.
I believe you meant DC fast charging, unless you were referring to level 2 charging.
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.
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.
They do need controllers that use AC, so I don't think that existing devices would work on DC.
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.
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.
More precisely the math formulae is: "AC voltage" x sqrt(2) - for median. And you double that for peak-to-peak