Module Voltage Question for Off-Grid DesignersFor larger off grid systems - pretty much anything over 600 watts - we have been recommending only MPPT type controllers, such as the Outback & Xantrex. Since they can match almost any panel size to any battery size, it has reduced our dependence on the often hard to get 12 volt panels.
.................................................................................................. Northern Arizona Wind & Sun - Electricity From The Sun Solar Discussion Forum: http://www.wind-sun.com/ForumVB/ .................................................................................................. ----- Original Message ----- From: Allan Sindelar To: 'RE-wrenches' Sent: Wednesday, December 10, 2008 1:16 PM Subject: [RE-wrenches] Module Voltage Question for Off-Grid Designers Wrenches, Here's an issue I haven't seen directly addressed, and I would like to know what the rest of you are doing. It's getting harder to get 24V nominal modules. Because the bulk of industry growth is high-voltage grid-tied applications, modules no longer have to be a standard voltage. Just pick the number of modules to best fit the chosen inverter's string voltage range. But for off-grid, the odd cell counts and resulting "nontraditional" DC input voltages challenge the traditional rules of off-grid design. I may be fighting a losing battle here, as the industry changes so rapidly and fundamentally. I have been trying to stay with 72-cell modules for offgrid, as this most readily combines with existing systems with 36-cell modules (12V) and 72-cell modules (24V). For example, I will use the new Canadian Solar 170-200W modules. They are 60-cell modules, with a Vmppt of around 28 volts - too low to charge 24V batteries with a standard charge controller. We can use them in any new systems with an MX60 or similar voltage-converting MPPT controller. Two or three in series would charge a 24V battery; 3 in series would charge 48V. Four in series would violate Code, as low-temp voltage would easily exceed 150V. Unless 60-cell modules remain a standard in the future, any future array additions would have to be on their own charge controller, in order to match a different I-V curve and MPP voltages into the same battery bank. Is this prohibitive? No, it just runs counter to the longstanding standards of off-grid design that allow modules to be added in the future: these modules will not add well to existing systems, and will not easily allow additional dissimilar modules to be added later. I had this same objection to using Day4 modules, although they were better at 16Vnom. Three made up a standard 48V array, so strings could be combined with two-module strings of similar 24Vnom modules. Who else is trying to stay with 24V modules? Anyone still using 12V modules in off-grid (residential-scale, not little apps) designs? Who has a crystal ball and knows what modules will be like in 20 years, or even two years? Wat are the rest of you doing? Thanks, as usual. Allan Allan Sindelar allan_(at)_positiveenergysolar.com NABCEP certified solar PV installer Positive Energy, Inc. 3225A Richards Lane Santa Fe NM 87507 505 424-1112 ------------------------------------------------------------------------------ _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
_______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
