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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Hello wrenches<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>We have a customer that wants us to promise kWh generation on a system, by relying on just an irradiance sensor.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>I think that a cell temperature sensor should be used as well to get a more accurate picture of the actual weather conditions that the array will experience. I’m trying to figure out what percent that hot cell temperature will effect an arrays output over the course of a year. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Each Celsius degree over 25 drops a standard C-Si module’s power output by about a half a percent.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Really hot solar noons in the NE (37 celsius ambient, 57 celsius cell temp) mean losing 16% right off the bat.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Im wondering how much that effects kWh over the course of a year.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>If we have 2 meteorological data sets to judge an arrays performance:<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>#1: only insolation data<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>#2: Insolation data and cell temp data<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>How much will the expected yearly PV AC kWh # diverge from those 2 scenarios?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For scenario #1:<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Say we got 1500 kWh/ m2 per year for 2010, recorded by your pyranometer.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For a 1 kWdc C-Si system, we would expect 1500 kWh (without derate.)<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Derate the system by .85, and we’re at 1275 kWh. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For scenario #2:<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Now, factor in cell temp (of course this varies based on mounting system, geography etc.)<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Consider a system flush mounted on a standing seam metal roof in NY.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>How much further could that 1275 kWh/yr be dragged down by factoring in cell temp? 5%? 10%?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>PV Watts shows 1203 kWh for a 1kWdc system @ .85 derate, (1500 kWh/m2/yr) (4.11 kWh/m2/day) <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Therefore 5.6% less than 1275 kWh/yr.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>I’m hoping this logic makes sense.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>How bout out in the desert? 10%+ losses?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Thanks for your thoughts on this<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>PS- I know PVSyst will tell me this, havent figured out how to ask it properly yet.. ;)<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Jim MacDonald<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Solar Energy Systems, LLC<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Brooklyn, NY <o:p></o:p></span></p></div></body></html>