Wrenches, Bill Hoffer's comments below are correct relative to "hot spots". Mac's comments are also correct relative to overall PV module temperature under open-circuit versus "loaded" conditions.
To Bill's point, if one or more cells are fractured, or buss solder bonds or other connections are defective, the increased resistance at those points will result in significantly increased spot temperatures when current is flowing. The higher the current, the hotter the spot temperature. When no current is flowing, the energy that would otherwise be converted to electricity has to go somewhere .. and in that case it becomes heat. Conversely, buss conductors and connections operate hotter on a localized basis when a PV module is connected to a load. Overall, however, the mean temperature of the entire module is cooler when conducting than when open circuit. Yes, this goes against what one might call "intuitive", but it's fact. This was reported in a study conducted by Matt Mueller (et. al.) of NREL. Multiple degradation mechanisms exist within PV modules. Generally, the most common failure mode is associated with thermal cycling. Given the temperature differential between open-circuit and fully loaded module operation, it could be argued there's an effect on overall module life either way, but the effects of either are difficult to quantify. Temperature cycling still remains "king" in that realm. Additional information is available on the NREL and Sandia websites. Regards to all, Dan Lepinski -------------------------------------------- On Wed, 9/10/14, Mac Lewis <maclew...@gmail.com> wrote: Subject: Re: [RE-wrenches] Cooler Modules To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Date: Wednesday, September 10, 2014, 11:23 PM Mark, This question depends on what where you draw your system boundary. If you are analyzing a module in its entirety, the module that is converting irradiance energy into electrical current will be cooler overall (less thermal energy) because less of that irradiance energy is turning into thermal energy. However, if you look at individual cells, the module that is conducting current can transfer some of that electrical energy into thermal energy in bad cells, which will make localized heating. Effectively, the modules producing useful work outside of your boundary won't be be converting all of that radiation energy into thermal energy inside of your boundary. Hence, open circuit module warmer than conducting module. This assumes that the only energy entering the system is from radiation, and that energy can leave the system as electrical energy. I like the question On Wed, Sep 10, 2014 at 10:10 PM, William Miller <will...@millersolar.com> wrote: Colleagues: Thanks for the scintillating and relevant discussion. This brings up a question on my radar: Will unloaded modules fail sooner than loaded modules? Thanks again. Great Group! William Lic 773985 millersolar.com 805-438-5600 From: RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Hoffer Sent: Wednesday, September 10, 2014 9:04 PM To: RE-wrenches Subject: Re: [RE-wrenches] Cooler Modules Mark Loaded modules will have hotter cells. I know this from searching for bad cells with a thermal camera. At open circuit I could not see a difference, but short circuit or loaded I could see 20 - 30 C increase in mismatched cells . Bill _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
