Hi Robert and All, You way overstate your case in
several ways. First one can just use the same RMS voltage DC as AC. It's not
like we EV people are not use to it especially, it's arcing problem are greatly
exaggerated with good DC practice. Next most things even on
12vdc run on under 2 amps, most under .5amp. My fan is just 1 amp, LED lights
.5 amp, computer 2amp, phone 1 amp charging, fridge 5 amps when running.
I cook with just 5-20 amps, e blanket 2 amps at 12vdc. Other than
kitchen, bath, other high power unit like an A/C that one can bunch together
you really don't need more than present 120vac wire size. And even some home
model switches work at 120vdc IIRC, SqD?. Or use a low cost
inverter for these . I use a 3k wt peak, 2kw cont $129 inverter runs my a/c
window unit. My recent disassembly of a Miata for my next EV
suspension also gave me an a/c unit I can drive very efficiently with a PM
motor from 24vdc could make my a/c only be 200wt at about 3500btu.
And many power supplies now work off 300vdc+ from rectified ac so
running 350vdc could end up using less copper if you want to get picky. And
homes run on used EV packs as pulled from wrecks.. A well
designed home will have centralized power, utilities, kitchen, bath making big
copper runs very short and the rest little different. The other savings and
reliability of battery DC with modern DC power supplies makes DC at least
competitive if not better. Kind of like EVs fighting the
established ICE machine, DC does have a future as homes, building go to clean
and battery power. And EVs will change to be part of these
systems, especially 200 mile EVs with their excess capacity with 20-50kwhr to
play with and still have 100 mile range. Many homes, businesses will be powered
by these charged by solar. Jerry Dycus
From: Robert Bruninga via EV <ev@lists.evdl.org>
To: Electric Vehicle Discussion List <ev@lists.evdl.org>
Sent: Wednesday, June 8, 2016 8:47 AM
Subject: Re: [EVDL] Off-grid solar house and electric car charging
> The only time high voltage helps is when you need to have long wire
runs...
The operative word is "long" And when you wire a house for every room
and for every appliance and for every outlet (whether used fully or not)
then every wire is "long".
The academic argument below is like saying there is nothing wrong with
falling out of an airplane. Its only when you hit the ground that you
have a problem...
Bob
-----Original Message-----
From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Lee Hart via EV
Sent: Tuesday, June 07, 2016 11:24 PM
To: Larry Gales; Electric Vehicle Discussion List
Subject: Re: [EVDL] Off-grid solar house and electric car charging
Larry Gales via EV wrote:
> Thanks, I was somewhat aware of the increased use of copper, but not
> to the extent that you specify, so it looks like AC is the way to go,
> even for off-grid solar.
Lower voltage means higher current and bigger wires; but it's not as bad
as you think.
First, consider a motor or transformer. You would think that winding it
for a lower voltage / higher current would require more copper... but it
doesn't. Motors and transformers are exactly the same size, have the same
efficiency, same power rating, and use the same amount of copper no matter
what voltage they are built for.
Here's why: If you halve the voltage, you double the current (to get the
same power). But half the voltage requires half the turns. So the wire is
twice as think, but half as long. The total amount of copper thus stays
the same. This only breaks down if the voltage is so low that you need
less than 1 turn, or if the voltage is so high that excessive amounts of
space are taken up by insulation instead of copper.
Now consider a pair of identical 12v batteries. You can wire them in
series (24v), or parallel (12v). For the same power, you'll have the same
current in each battery (since their voltages are all the same).
So, the same wire size to every battery. For the sake of argument, let's
assume you connect a 12" piece of wire to every battery post, and it has
1 milliohm of resistance.
If they're in series, you have a total of 4 feet of wire total, all in
series, and so 4 milliohms of resistance. if the load is 24v at 100 amps,
then this 4 milliohms is burning up I^2R = 100^2 x 0.004 = 40 watts as
heat.
If they're in parallel, the free ends of the + wires connect together, and
the free ends of the - wires connect together. Now you have two parallel
strings, each with 2 feet of wire in it; so each string has half the
resistance or 2 milliohms. But there are two of these strings in parallel,
so the total resistance is 1 milliohm. The same load power is 12v at 200a.
I^2R losses are 200^2 x 0.001 = 40 watts.
Exactly the same size and length of wire, and exactly the same losses!
The same thing happens with PV panels, power semiconductors, and just
about any power devices. Arranging them for low voltage/high current
results in the same losses as arranging them fro high voltage/low current.
The only time high voltage helps is when you need to have long wire runs.
If your PV panels are far from your inverter, then high voltage for the
wires between them will the reduce the amount of copper needed and/or
lower your losses. However, if you're using small low-voltage individual
inverters mounted right on each panel to one big central inverter located
far away, then the small inverters can "win" and use less copper overall.
You have to carefully consider the specifics of the situation, and not
make snap judgements about low voltages being automatically worse.
--
"IC chip performance doubles every 18 months." -- Moore's law "The speed
of software halves every 18 months." -- Gates' law
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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