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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#101

Post by Upitor »

sandworm101 wrote: post #49
Reason077 wrote: post #35

Earlier 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.

They are still not syncing directly to the AC grid. They are not going 50 rounds per second

Re: Why DC May Replace AC (2019)

#102

Post by coding123 »

zw123456 wrote: post #67
dhdc wrote: post #66

Earlier quoted context omitted.

Since 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.

Right, 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.

There are a few people that use 24 - 48V DC batteries/solar charge controllers in RVs and then step down for the motors, lights, etc.. for the rest of the rig. Pretty simple conversion for an RV.

Re: Why DC May Replace AC (2019)

#103

Post by coding123 »

dreamcompiler wrote: post #85
zw123456 wrote: post #59

There 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…

I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads. When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit…

That's going to be some thick ass wires for the oven, water heater and a few other high wattage items.

Re: Why DC May Replace AC (2019)

#104

Post by YZF »

Upitor wrote: post #101
sandworm101 wrote: post #49

Earlier quoted context omitted.

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.

They are still not syncing directly to the AC grid. They are not going 50 rounds per second

They wouldn't necessarily need to do 50 rounds per second to generate 50 Hz. There could be any number of poles on the generator. At least in theory... Not saying you're wrong about not being synced but just that they wouldn't need to do 50 rps...

Re: Why DC May Replace AC (2019)

#105

Post by cat_plus_plus »

Consider 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.

Re: Why DC May Replace AC (2019)

#106

Post by unnouinceput »

sabareesh wrote: post #7

Pretty 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.

Cool, let me know how much the copper will cost you when you'll need to wire your entire household with at least 2 tons (and I am on low estimate here) of them, because you'll need them thick to drive the increased Ampere juice. Remember P (power) == U (voltage) x I (intensity). As you'll lower your voltage, your intensity will rise to meet the same power consumption. Meaning thick copper wires, at least 2cm diameter, not your current 3mm diameter you currently have.

Re: Why DC May Replace AC (2019)

#107

Post by unnouinceput »

cfraenkel wrote: post #63
thehappypm wrote: post #55

Earlier quoted context omitted.

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.

It 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-a...

That'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.

Re: Why DC May Replace AC (2019)

#108

Post by unnouinceput »

thehappypm wrote: post #95
BenjiWiebe wrote: post #75

Earlier quoted context omitted.

Don't forget that AC can flow through capacitors.

What do you mean by that exactly?

Get a capacitor that supports 700V minimum as highest applicable voltage, and a wire. Go to your socket outlet, put the capacitor in one hole, the wire in the other hole and see if you get shocked or not. Do the same (if you're still alive) with a DC source. Comeback here and let me know how the experiment went and also tell me why you skipped physics class in high-school when this was taught.

Re: Why DC May Replace AC (2019)

#109

Post by unnouinceput »

dhdc wrote: post #60
Overtonwindow wrote: post #51

For 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.

170V is peak to median. Peak to peak is double that (340V).

More precisely the math formulae is: "AC voltage" x sqrt(2) - for median. And you double that for peak-to-peak

Re: Why DC May Replace AC (2019)

#110

Post by julosflb »

Gravityloss wrote: post #81
upofadown wrote: post #54

Earlier quoted context omitted.

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.

Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower. 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...

Blade pitching first use is to reduce the aerodynamics power above the rater power point of the turbine and avoid overload.
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