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How Tesla Batteries Will Force Home Wiring To Go Low Voltage
CIStud writes with a story at CEPro suggesting that solar power and home batteries like Tesla's PowerWall "will force the reinvention of home wiring from primarily AC high voltage to DC home-run low voltage to reduce power conversion loss," writing "To avoid the 20% to 40% power loss when converting from DC to AC, home wiring will have to convert to home-run low-voltage, and eventually eliminate the need for high-voltage 110V electrical wiring." As a former full-time Airstream dweller, I can attest to the importance of DC appliances when dealing with batteries.
You'd think the fight between Edison and Tesla would have ended long after their deaths. Clearly not. It is a good thing their graves aren't near each other, if they were, there would surely be lighting bolts going back and forth.
Kind of ironic. Nikola Tesla fought to champion AC power, and the company named after him will bring Edison's dream of DC-sourced homes to reality.
...albeit this has already happened on a smaller scale before. All you need to do is ask anyone who owns or has owned an RV or Camping trailer.
I dealt with it myself when I had an RV: a bank of huge batteries, an inverter, and a generator. In Tesla's instance, you replace "generator" with "local power grid", but otherwise it's the same routine: Your lights and similar are low-voltage (just like most RVs), but you use an inverter for any general consumer item (TV, computer/laptop, hair dryer, whatever).
I think the only diff would be in the appliances... most RV appliances (e.g. the refrigerator, furnace blower, AC units) are made to run off of 12v DC, but most RV appliances are pretty small when compared to their house-made counterparts.
Maybe ask folks who do the hardcore solar/wind thing?
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It's not like there is one single standard DC voltage that everything runs off of. Switching between different DC voltages incurs a loss just like switching between the current AC standard and a given DC voltage incurs a loss.
If one were deploying everything from scratch, one could pick a standard. Right now, everyone is going to want to run the stuff they have, and the AC to DC converters on that stuff, even when they are exposed (i.e. wall-warts) instead of embedded in the device, are converting to a variety of different DC values.
Forgive me if I have this wrong, but if we start wiring houses for low voltage DC, won't this mean huge fat copper cables to deal with the current implications of a washing machine or oven pulling tens, even hundreds of amps because of Ohms law?
If we were starting out then maybe, but there are just so many things that can be plugged into an AC socket. It's pretty amazing that you can take anything from the last 50 years or more that has the right plug on it, shove it into a wall socket, and off it goes. The current system is a very good standard, and it will be hard to change things. Further, one of the original reasons Tesla (Nikola) won out is that the induction motor is an extremely good motor design (safe, reliable, quiet). Lots of things still have AC induction motors (heatpumps, your fridge) and these require, well AC. If you don't have that then you need a motor driver for them (or brushes I suppose) which is just a three-phase inverter anyway.
Also 20-40% power loss is crazy. More like 5-10% with modern semi-conductors and getting better/cheaper all the time.
I'm not buying it. Voltage x Amperage = Wattage. So long as Wattage stays the same (think 1,800W hair dryers here), your Amperage must proportionately increase if the Voltage drops... This can only be accomplished by using LARGER wires to deliver the Amps... This is why wires on your car battery or golf cart are so large... Imaging the COST of wiring a home with large (lower Voltage) conductors like that... Ask yourself why Europe uses a ~230V/240V electricity in homes and how much cost savings there must be by delivering all the wattage at half the conductor size compared to the North American 120V household standard... Smarter people than us have all thought this stuff through many decades ago... Tesla is trying to push battery tech and if it were affordable and better than a $500 gas generator, we'd already have it installed. Cool technology, way too expensive and I'm not rewiring my house.
Well, as soon as someone invents the AC battery we can switch back...
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This is a very poorly researched article. They talk about getting 12V from a solar panel. No modern home-scale solar system runs at 12V. The power loss due to resistance is much too high until you use wires that are much too large.
The real solution would be to standardize on some type of home HVDC distribution in the 150-300VDC range. This would help keep the DC/DC conversion in roughly the 2:1 voltage ration range, which helps efficiency. It would also help keep the wire gauge reasonable. I'm not sure how the article's author envisions running things like a modern HE washing machine with build in heater from, say, 12V. It would take about 100-150 amps and require about 2/0 gauge wire to keep the losses manageable.
Just because we're all dim bulbs doesn't mean we're low voltage. There is a lot of resistance around here.
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NO. That will not happen. Power equals voltage times current. To deliver the same power load at a lower voltage would require higher current, and household wiring is already designed to carry as much current as it safely can. Lowering voltage would thus require new, much bulkier wiring, which can't easily be retrofitted in older structures. Conduits would be able to carry far less of it, so those two would have to be overhauled. Last but not least, wireless charging and better batteries will eliminate much of the need for the lower-power wiring in the first place. There are very few things that I can confidently predict about the future, but one of those things is that mains (110-220v) voltage is not going to change drastically anytime soon. I'd be willing to bet every single powered appliance in my home on it.
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Low voltage is not going to happen, if only because the costs for copper wire would be astronomical. If you take your standard 1500w electrical outlet, at 120v it only needs #14 gauge wire to run 54 feet @1800watts because it's only 15 amps. If you take that down to 24V, you need #2 gauge wire to run the same distance, and you are only getting 1200watts, at 50 amps! #14 wire is about $0.17 per foot, where as #2 wire is (from what I could find) about $7.50 per foot.
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This is largely what I was thinking.
As it currently stands, commercial buildings often have 277V lighting circuits (this is in the US) because it involves installing less copper in the ceilings.
From this, one can intuit that lowering the voltage will significantly increase the amount of copper, but let's take an example and make it more solid.
Let's say, for the sake of example, that we were considering 48V DC as an alternative to 120V AC (I personally would not want to consider anything lower than 48V in a home environment). If you need to deliver 1200W from point A to point B, it will require 10A at 120V, and 25A at 48V.
That 10A could be safely delivered on a 14 ga. wire in most domestic contexts, but will probably be delivered on 12 ga. For 25A, however, you're going to need 10 ga.*
A 250' roll of wire is ~$43 for 14 ga, $95 for 12 ga., and $138 for 10 ga. See the problem?
For the next challenge, you will also need to use different, more expensive switches and circuit breakers, or drop back to using fuses. This is because an AC arc self-quenches in half a cycle or less, and won't re-establish until the contacts are brought close enough together. The DC arc, on the other hand, is continuous, and requires additional effort to quench. Just for the record, there is an arc every time that a circuit breaker or switch is opened under load. This is the reason why you will often see switches and breakers labelled "AC Only".
Now, this is not to say that these problems won't be overcome or that a different variant might come about. Who knows? Maybe they'll gravitate towards 120V AC or some such, in which case it will be 1915** all over again.
(*For the non-Americans and uninitiated, US wire gauge is backwards: larger numbers are smaller wires. 14, 12 and 10 gauge are ~2.1, 3.3 and 5.3 mm^2, respectively)
(**There is nothing special about 1915, but I live in a house that was built in 1915 and was electified from day one. It would have had DC delivered to it in those early days, courtesy of Mr. Edison's various efforts in my current home town of Schenectady.)
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I'm not sure that home batteries will drive a switch to low voltage DC. There's a ton of inertia to overcome. The cost of retrofitting the wiring to handle the higher amperage of using lower voltage alone will be thousands of dollars for every single house, apartment, and office. A simple 20A 120V circuit changed over to 12V will draw 200A. You're going to need to upgrade to 4 or even 2 gauge wire at a minimum to handle that kind of current. And that's a lot of money.
The switch from AC to DC inside the home might be feasible but there's no way you can convert the entire grid. You'd have to rebuild the whole grid from scratch to convert from AC to DC. The transformers to step the voltages up or down simply don't work unless they're pushing AC so how do you handle industrial level supply being stepped down to household voltages at the neighborhood transformer? And who's going to pay for the switch? And what about the industrial users who don't need to run low voltage DC? How do you satisfy their demand?
Then you have to deal with how a substantial number of appliances are built. Many are designed for AC current and won't work with DC, regardless of the voltages. Sure, you can swap out the power supply in your desktop PC to take a DC feed without a lot of trouble. And if electronics retailers had a standard DC wall voltage to work with, you'd see most consumer electronics move to those standards. But how do you deal with a cable modem that needs 12V and a home router that takes 9V? Who wants to go out and replace all of their equipment that is running just fine right now? Who has the money to do that?
And here's the kicker. What real benefit do we gain from a switch over to low voltage DC in the house? Sure, some of the consumer electronics we use won't need that big wall vampire to supply them with power. And sure, we don't really need to run our lights from 120V when 12V can still drive enough light from LEDs without any trouble. But what about the appliances in the house that really draw the bulk of the power in the house? The 240V electric stove or the heat and AC systems? What about your refrigerator and your washer/dryer? Hell, can you imagine the amperage draw trying to recharge your electric car with 12V? And are you going to just skip using those appliances when you're running on battery power?
So if you're going to have to keep your 120V AC based house wiring for your major appliances, do you really want to spend all the money installing a low voltage subsystem for a few consumer electronic devices to supplement the wiring you already have? I know I wouldn't want to.
Like everything else that is poised to "fundamentally change the way we do things", the dreamers never consider the practical reality of actually making the change. In reality, I think we're going to have to deal with the inefficiency of converting from DC battery power to 120V AC for the home. There's just too many things to overcome for little to no benefit.
The USA is running on 220-250V AC for residential (exact voltage varies per locale). It's single-phase with a center-tap neutral, sometimes called "split phase"; Typically, a neighborhood will be on one phase of three-phase distribution system. Split phase allows one get two half-phases of about 120V (typical U.S. receptacle, a.k.a. "power outlet"), but you still have 240V available for large appliances: electric stoves/ranges, furnaces, installed heaters (baseboard or in-wall), clothes dryers, and/or sometimes a welding receptacle in the garage.
Split phase is occasionally incorrectly referred to as "two phase", which actually only exists with one old electrical distribution system near Niagra.
Seems premature to me. An awful lot of things have to work out just right for whole-home battery systems to make much sense.
Even then low-voltage DC plants don't make much sense. Your microwave oven consumes 1100+ watts. Know what amperage that is at 5 volts DC? You'd barely be able to wrap your hand around the power cord.
Even at 48 volts DC, the power plant in a telephone company central office is really something to behold.
Also, AC/DC conversion isn't as dire as stated. Sloppy cheap converters do indeed operate at around 75% effeciency with the remaining 25% lost as heat. But look at the "80+" computer power supply standards. The "80+ platinum" standard requires 95% efficiency. Those power cost twice as much but "pure science" does not prevent their operation. They work as promised.
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This is strange. "20 to 40% power loss" seems to be an awfully poor inverter; existing inverters are 4-8 % loss.
Rather than rewire every house in America, wouldn't it make more sense to just design better inverters?
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DC has very rapid power loss over any kind of distance.
No it doesn't. Losses are related to current, not AC vs. DC. A higher current in the same sized conductor equates to higher loss. You can get around this by raising the voltage (traditionally easier with AC), thus transferring the same amount of energy with less current, or you can increase the size of the conductor. DC can actually transfer more energy than AC on a similar sized conductor because it doesn't have to deal with skin effect.
I could link all of these terms to applicable articles for you but I'm feeling lazy and this is all common knowledge stuff anyway.
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> If you're using somewhere near the inverter's peak output, then you can get as much as 90% efficiency
These days inverters are much better than that. To use a random product that is currently shipping, an SMA Sunny Boy 5000 runs at 95.5-97% efficiency. Bigger inverters are even better with some commercial scale monsters at 98% efficiency.
The original article is pure nonsense. There are already three port inverters on the market. Those ports are: your 120V AC, your solar array, your battery bank. If the energy is going from the solar array to the battery there is simply no intermediate conversion to AC. With a three port inverter, there is only ever a single conversion from DC to AC. And, as I previously mentioned, will only get hit with a 3-4.5% loss. There is simply no way the world is going to change how electricity is delivered to avoid that.
Since the Tesla Power Wall is pretty much for sure going to be a high volume product, there are inverter manufacturers falling all over themselves to design and build three port inverters specifically optimized for the Tesla product.
We need to use HIGH voltage DC at about the same voltage as your house is now, forget about going "low voltage" DC. MOST things in your home will run JUST FINE on DC with a few notable exceptions. AC induction motors will NOT work, nor will anything that involves an old fashioned transformer, but most modern electronics with switching power supplies work great on anywhere between about 90V to 200V DC without modification. Most switching power supplies just convert the AC into DC right up front and won't know the difference. So, all you do is provide inverters for the things you cannot easily change (like for your appliances) and just feed DC to the rest of the stuff that doesn't care. What you DON'T do is go to low voltage DC and suggesting this is just crazy talk. Why?
1. Most stuff just works on high voltage DC as discussed above. Most switching power supplies simply don't know or care about AC or DC and due to their efficiency switching power supplies are used in almost everything electronic.
2. It's easier (and more efficient) to use high voltage DC for charging the batteries. All you need is a rectifier to convert that 220 into about 250V DC and charge the batteries, which is about as simple and efficient as it comes.
3. It's easer (and more efficient) to make an inverter that uses high voltage DC as input. It's pretty easy to just flip the current one way then the other to get AC sufficient to run most induction motors and transformer powered devices.
4. It's more efficient to use higher voltage in terms of wire size because IxR losses are less for the same power transfer. Chances are the same wires you have now will be fine, but if you go to low voltage (say 13.8V like in your car) you are going to need bigger conductors to avoid the voltage drops over long high current runs. Use higher voltage and lower current, and stick with the wires you have.
5. Current battery technology for EV's and hybrids uses about 200V DC to start with, so there are less modifications to the technology when adapting to a home use. If we stick with a common battery pack voltage it will increase the economies of scale in their production and allow the use of old automobile packs that have reduced capacity as power storage in homes where the size and weight of the battery is less important. If you go low voltage, you either have to convert the 200V down to 12 or 48 (and incur the conversion loss) or modify the battery pack to operate at the lower voltage.
I know that traditional DC systems run at multiples of 12 Volts because they are usually built on Lead-Acid batteries and that much equipment is commercially available that uses 12 and 48 volts based on this. But going to 12 or 48 volts is not the right answer. It's really just the traditional solution based on past thinking and limitations. Running 200V DC is a more viable and long term solution that will work fine with a lot of existing AC equipment, plus is compatible with a ready source of batteries which are commercially available (and if purchased used, pretty cheap).
So, NO, we DON'T want to start using low voltage DC... We want to use HIGH voltage DC.
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This is strange. "20 to 40% power loss" seems to be an awfully poor inverter; existing inverters are 4-8 % loss.
Rather than rewire every house in America, wouldn't it make more sense to just design better inverters?
Or just run at 120V DC, as renewable energy systems did (and occasionally still do) before so many appliances were AC-only that it made sense to use an inverter.
Dropping voltage means you have to replace the copper wiring with MUCH HEAVIER wiring - by a square law - to carry a given amount of power with the same loss - and thus wiring heating inside the walls, where it can set the house of fire.
Switching to 120V just means using DC-capable appliances and replacing the breakers (DC is harder to interrupt) and must-be-GFCI outlets (normal GFCI devices use a transformer to sense unbalanced load).
The 48V standard was about having a voltage that was low enough that touching it was typically survivable, so working on or near it is (relatively) safe. The boundary between the hard part and the easy, "low-voltage", part of the electrical code is 50V (BECAUSE of phone companies B-) ). Medium power (>1KW) home Renewable Energy systems tend to be at 48V so much of the wiring falls under the easier part of the code, and because of the availability of
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I do this for a living as an facilities electrical engineer who works closely with electricians. The phase between lines on the primary side of a single-phase stepdown transformer is irrelevant to the secondary side. Indeed, sometimes the distribution lines are Y configuration rather than delta, so the inputs to the single-phase transformer is sometimes line-neutral instead of line-line. In most systems worldwide the single-phase transformer has two poles on the secondary side, one of which is grounded locally and is connected to the neutral conductor, the other pole is connected to the "hot" conductor or "line voltage". There is typically about 240V between hot on neutral. A main electrical panel for residential will have 2 bus bars in this case.
In the U.S., the transformer is typically has a three-pole secondary with a center-tap connected to the center of the secondary coil. The center tap is connected to local ground as well as the neutral conductor, and the other two poles at opposite ends are each hot conductors. Since there is only one coil on the transformer secondary this results in two hots that while measured against neutral are 120V, but each 180 degrees out of phase with the other for a result of 240V between lines. A main electrical panel will have 3 bus bars in this case. You can confirm this with a voltmeter. (If they were 120-degrees out of phase, you would measure a SQRT(3) ratio of V_lineline/V_lineneutral.
Occasionally in a commercial or industrial facility, you may find a 2-pole electrical panel that is a sub-circuit to a three-phase Y-configured panel (120/208V Typical configuration). These tend to be remodel conversions from when the building mains were swapped from single-phase to three-phase. In this one case, you will get the 120-degree difference between lines. When this is the case you have to be extra careful when connecting loads to the subpanel, because the difference in line-line voltage is less than what you would expect at first glance, and some equipment may fail to operate, or operate in a degraded state, because of that.
Someone is pushing some other agenda here.
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I can see AC to the doorstep a big efficient whole house power supply that has 12vdc and 48vdc rails that are distributed thorough the house and battery backed, and few 220v "appliance circuits" off the AC.
48V and 12V lines are far too low to be sage and/or sensible. Remember that the power used is equal to the voltage times the current and that the heating of the wire carrying the current goes as the square of that current. Typical house wiring is good for ~30A of current and supplies several plugs in a room typically. With a 12V circuit you limit the power of all the devices connected to this circuit to 360W vs. the 6.6kW you get now (or 3.3kW if you live in North America). Even with a 48V circuit you only get 1.44 kW.
The result is that either you need to rewire the entire house with massively thick, and therefore expensive, cables to carry the far higher currents or you need to use a higher voltage for transmission. Even the factor of two reduction between Europe and Canada/US is noticeable for some devices: electric heaters are far punier than their European counterparts, kettles take far longer to boil, and Electric lawnmowers are practically useless etc. If you drop the voltage by another factor of 2-10 below even Canada/US then almost all devices will be impacted.