Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why DC May Replace AC (2019)

electricalindustry.ca

Post reply on HN
Print view View original

41–50 of 139 posts

Re: Why DC May Replace AC (2019)

#41

Post by jacquesm »

borodi wrote: post #36
jacquesm wrote: post #30

Earlier quoted context omitted.

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 arrangeme…

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).

Re: Why DC May Replace AC (2019)

#42

Post by dpierce9 »

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.

Re: Why DC May Replace AC (2019)

#43

Post by Animats »

Youden wrote: post #16

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> B…

The author seems very confused about whether they're talking about the grid or devices.

Which is surprising, given his background, a degree in electrical engineering and jobs with power companies.[1]

This may be an argument for using more DC-DC converters and fewer transformers. The classic problem with shipping DC around is that voltage conversion is expensive. DC-DC converters have improved a lot. This article may be a dumbed-down version of that argument.

A nice thing about large transformers is that those big hunks of copper and iron have a lifespan of 30 to 75 years. Replace those with a DC-DC converter, and it will probably have semiconductor lifespan problems. Plus someone will add on a data connection, firmware updates, a web server, and an antivirus program.

[1] https://grid.pitt.edu/people/gregory-f-reed

Re: Why DC May Replace AC (2019)

#44

Post by klondike_ »

WorldMaker wrote: post #14
castratikron wrote: post #11

Earlier quoted context omitted.

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 w…

"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

Re: Why DC May Replace AC (2019)

#46

Post by upofadown »

Youden wrote: post #16

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> B…

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.

Re: Why DC May Replace AC (2019)

#47

Post by Arainach »

danachow wrote: post #33
seanalltogether wrote: post #17

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 i…

> 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 le…

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.

Re: Why DC May Replace AC (2019)

#48

Post by [deleted] »

bsder wrote: post #25
jacquesm wrote: post #24

Earlier quoted context omitted.

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.

[deleted]

Re: Why DC May Replace AC (2019)

#49

Post by sandworm101 »

Reason077 wrote: post #35
Youden wrote: post #16

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> B…

> "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.

Re: Why DC May Replace AC (2019)

#50

Post by xattt »

Mizza wrote: post #27
InTheArena wrote: post #18

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?

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.
Post reply on HN