<html><body><span style="font-family:Verdana; color:#000000; font-size:10pt;"><div>I don't know if this is what you are looking for or not, however Dr. Perez in Albany NY was working on developing a weather based performance guarantee for NY. It may help with your research.</div>
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<div><A href="http://www.cleanpower.com/Content/Documents/research/capacityvaluation/WeatherAdjustedPerformanceGuarantees.pdf">http://www.cleanpower.com/Content/Documents/research/capacityvaluation/WeatherAdjustedPerformanceGuarantees.pdf</A></div>
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<div>Jamie</div>
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<DIV id=wmQuoteWrapper>-------- Original Message --------<BR>Subject: [RE-wrenches] Cell temperature (back of module temp) effects<BR>on yearly kWh predicted generation<BR>From: Jim MacDonald <<a href="mailto:jmac@solaresystems.com">jmac@solaresystems.com</a>><BR>Date: Wed, January 12, 2011 3:03 pm<BR>To: RE-wrenches <<a href="mailto:re-wrenches@lists.re-wrenches.org">re-wrenches@lists.re-wrenches.org</a>><BR><BR>
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<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Hello wrenches<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><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></div>
<div class=MsoNormal style="font-size:12pt;"><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\u2019m 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></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Each Celsius degree over 25 drops a standard C-Si module\u2019s power output by about a half a percent.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><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></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Im wondering how much that effects kWh over the course of a year.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">If we have 2 meteorological data sets to judge an arrays performance:<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">#1: only insolation data<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">#2: Insolation data and cell temp data<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">How much will the expected yearly PV AC kWh # diverge from those 2 scenarios?<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">For scenario #1:<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Say we got 1500 kWh/ m2 per year for 2010, recorded by your pyranometer.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">For a 1 kWdc C-Si system, we would expect 1500 kWh (without derate.)<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Derate the system by .85, and we\u2019re at 1275 kWh. <o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">For scenario #2:<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Now, factor in cell temp (of course this varies based on mounting system, geography etc.)<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Consider a system flush mounted on a standing seam metal roof in NY.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><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></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><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></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Therefore 5.6% less than 1275 kWh/yr.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">I\u2019m hoping this logic makes sense.<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">How bout out in the desert? 10%+ losses?<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Thanks for your thoughts on this<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><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></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d"><o:p> </o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Jim MacDonald<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Solar Energy Systems, LLC<o:p></o:p></SPAN></div>
<div class=MsoNormal style="font-size:12pt;"><SPAN style="COLOR: #1f497d">Brooklyn, NY <o:p></o:p></SPAN></div></DIV>
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