> 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 _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA) _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
