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Why DC May Replace AC (2019)
- jl2718 4y agoThis 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?
- system2 4y agoI can't find the date this article was written. Is it 20 years old or 2 days old?
- DoingIsLearning 4y ago> 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 4y agolots 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.
- Skunkleton 4y agoBut DC is used for distribution, just at extremely high voltages.
- DoingIsLearning 4y agoHVDC 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.
- twobitshifter 4y agoSan Francisco has a “secret dc grid serving 900 customers” - the problems described are now solvable. https://news.ycombinator.com/item?id=26431569 https://news.ycombinator.com/item?id=26431569
- deleted 4y ago[deleted]
- contingencies 4y agoI am aware of at least one American managed company here in China selling DC-all-the-things.
- kimpeek 4y ago> 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 4y agoWhy won’t it replace it, out of interest?
- wbsss4412 4y agoSame reason why it didn’t win out 100 years ago. It isn’t as efficient.
- sabareesh 4y agoI don't think it is efficiency, we had no way to step up and step down DC as we can do AC
- WorldMaker 4y agoYup, 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 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.
- dhdc 4y ago> DC-AC-DC "sandwiches" Couldn't have said it better, I had a good laugh.
- sabareesh 4y agoPretty 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 4y agoCounter-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 4y agoQuestion 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 4y agoJust 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.
- xxpor 4y ago> AC fast charging I believe you meant DC fast charging, unless you were referring to level 2 charging.
- WorldMaker 4y agoYes, 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.
- fatnoah 4y agoThe 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/ 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.
- Youden 4y agoThe 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.
- jbay808 4y agoIn 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.
- fy20 4y agoIn marketing speak, a variable frequency drive is often referred to as "inverter technology" in appliances.
- dreamcompiler 4y agoIndeed. 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.
- seanalltogether 4y agoAC 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 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 4y agoAC 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.
- ncmncm 4y agoThe physical design of the switch ought to be easy enough to make physically interrupt the arc.
- danachow 4y ago> 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.
- InTheArena 4y agoI 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?
- Mizza 4y agoI'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.
- xattt 4y agoIt might not scale for washing machines, but at least for home electronics.
- danhor 4y agoMost 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.
- b33j0r 4y agoIf 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.”
- thehappypm 4y agoOhms 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.
- dpierce9 4y agoThe 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.
- zdragnar 4y agoIt'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.
- dpierce9 4y agoLead 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.
- userbinator 4y agoK&T is not uninsulated.
- chrisp_how 4y agoAn AC computer is more efficient that a DC one, because the machine operates on cycles, or repeating interval-segments.
- cfraenkel 4y agoI suppose that might be true if you're using a 60hz clock speed.
- CyberDildonics 4y agoAfter 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.
- Overtonwindow 4y agoFor safety alone DC is not going to replace AC. The author seems to be confusing local use, and distance transmission.
- thehappypm 4y agoIn 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.
- cfraenkel 4y agoIt 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-and-dc-shock-comparison/ https://www.brighthubengineering.com/power-plants/89792-ac-a...
- unnouinceput 4y agoThat'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.
- BenjiWiebe 4y agoDon't forget that AC can flow through capacitors.
- thehappypm 4y agoWhat do you mean by that exactly?
- zw123456 4y agoThere 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.
- dhdc 4y agoSince 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.
- zw123456 4y agoRight, 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.
- dhdc 4y ago> 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.
- stevespang 4y ago
- phendrenad2 4y agoThe 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.
- jhallenworld 4y agoThere 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 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.
- hushpuppy 4y ago> 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. -------- 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.
- cat_plus_plus 4y agoConsider 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.
- jnsaff2 4y agoThe 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...
- R0b0t1 4y agoYou 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.
- jnsaff2 4y agoTransformers 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.
- R0b0t1 4y agoYes, 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.