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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