For safety alone DC is not going to replace AC. The author seems to be confusing local use, and distance transmission.
Why DC May Replace AC (2019)
51–60 of 139 posts
Re: Why DC May Replace AC (2019)
#52Earlier quoted context omitted.
> 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.
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...
Re: Why DC May Replace AC (2019)
#53If 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.
Re: Why DC May Replace AC (2019)
#54Earlier quoted context omitted.
> "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.
Re: Why DC May Replace AC (2019)
#55For 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.
Re: Why DC May Replace AC (2019)
#56The 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…
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.
Re: Why DC May Replace AC (2019)
#57The 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…
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 V…
In marketing speak, a variable frequency drive is often referred to as "inverter technology" in appliances.
Re: Why DC May Replace AC (2019)
#58Earlier quoted context omitted.
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 argu…
> 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.
Re: Why DC May Replace AC (2019)
#59There 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.
Re: Why DC May Replace AC (2019)
#60For safety alone DC is not going to replace AC. The author seems to be confusing local use, and distance transmission.
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.