I thought the original question was about wiring an off grid house for low voltage DC versus throughout and using all DC appliances and equipment instead of 120/240 VAC like all other houses....
-----Original Message----- > I think Lee was referring to how you wire the panels together, > not the house. You could wire your panels in parallel and, > as long as your inverter is near the panels, not incur any more line > losses > than a series system. Peri ------ Original Message ------ From: "Robert Bruninga via EV" <[email protected]> To: "Electric Vehicle Discussion List" <[email protected]> Sent: 08-Jun-16 5:47:59 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:[email protected]] On Behalf Of Lee Hart via >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) > > _______________________________________________ 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)
