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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