<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=ISO-8859-1" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.19154"></HEAD>
<BODY bgColor=#ffffff text=#000000>
<DIV><FONT size=2 face=Arial>...or that most of the time, PV systems do not
operate at the array and inverter maximum power points...or that test
instruments at the best labs are only accurate to 3% to 5% (K. Emrey, NREL)
which brings to question some of the tight module specifications we now
see...or that a one-time snapshot is not as accurate as a moving picture.
Nevertheless, an experienced diagnostician can tell if everything (but not
necessarily any specific thing) is ok or something is amiss.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Joel Davidson</FONT></DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=boB@midnitesolar.com
href="mailto:boB@midnitesolar.com">boB@midnitesolar.com</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=re-wrenches@lists.re-wrenches.org
href="mailto:re-wrenches@lists.re-wrenches.org">RE-wrenches</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Tuesday, November 29, 2011 10:02
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [RE-wrenches] calculating
low string voltage</DIV>
<DIV><BR></DIV><BR><BR>If you really want to throw a monkey wrench into the
mix, bring up partial shading...<BR><BR>Or, maybe that's just not allowed in
that debate ?? Just a
thought.<BR><BR>boB<BR><BR><BR><BR><BR><BR><BR><BR>On 11/29/2011 8:39 PM, Kent
Osterberg wrote:
<BLOCKQUOTE cite=mid:4ED5B371.3070705@coveoregon.com
type="cite">Kirk,<BR><BR>You've got some good feedback on this from Ray and
Bill. I'll try to add a little more. Most module datasheets show a normal
operating cell temperature, NOCT, value that's typically 47.5°C. That's 20°C
ambient temperature, 800 W/sq m, and calm wind and nothing blocking the
airflow on the back of the module. That 27.5°C temperature rise should be
pretty close to the temperature rise that occurs for a pole-top mount. It's
common to see people use 25°C for modules on a pole and 30°C or 35°C on a
roof and there are data that support these "typical" values. With 1000 W/sq
irradiance, the temperature rise can obviously be more too. Between the
intensity of the sun, the direction of the sun, the color of the roof, the
spacing off the roof, and the wind speed there is a lot that is different
from one system to the next or even one day to the next.<BR><BR>Most PV
module spec sheets don't give you a temperature coefficient for Vmp. I've
seen people use the the Voc coefficient, usually expressed as a percentage,
for both Voc and Vmp. Big mistake. Data from NREL indicates as Bill said,
the temperature coefficient for Vmp is higher than that the temperature
coefficient for Voc. That's particularly true when the coefficient is
expressed as a percentage per °C. Since there are very few manufacturer's
that give both temperature coefficients, I'll use a value from an old
Evergreen module for an example. The Evergreen ES-195 datasheet shows Voc =
30.5 volts with a coefficient of -0.34%/°C and Vmp = 27.1 volts with a
coefficient of -0.47%/°C. Since one shouldn't add volts and percents, I'll
put the temperature coefficients in volts/°C: Voc = 30.5 V - 0.10 V/°C and
Vmp = 27.1 V - 0.13V/°C. So Vmp is moving faster than Voc, but not a lot
faster. That's generically true for c-Si or poly-Si.<BR><PRE class=moz-signature cols="72">Kent Osterberg
Blue Mountain Solar, Inc.
<A class=moz-txt-link-abbreviated href="http://www.bluemountainsolar.com" moz-do-not-send="true">www.bluemountainsolar.com</A>
t: 541-568-4882</PRE><BR>On 11/29/2011 11:48 AM, Kirk Herander wrote:
<BLOCKQUOTE cite=mid:00b201ccaecf$cf13b3e0$6d3b1ba0$@vtsolar.com
type="cite">
<META name=Generator
content="Microsoft Word 14 (filtered
medium)">
<STYLE>@font-face {
font-family: Calibri;
}
@page WordSection1 {size: 8.5in 11.0in; margin: 1.0in 1.0in 1.0in 1.0in; }
P.MsoNormal {
MARGIN: 0in 0in 0pt; FONT-FAMILY: "Calibri","sans-serif"; FONT-SIZE: 11pt
}
LI.MsoNormal {
MARGIN: 0in 0in 0pt; FONT-FAMILY: "Calibri","sans-serif"; FONT-SIZE: 11pt
}
DIV.MsoNormal {
MARGIN: 0in 0in 0pt; FONT-FAMILY: "Calibri","sans-serif"; FONT-SIZE: 11pt
}
A:link {
COLOR: blue; TEXT-DECORATION: underline; mso-style-priority: 99
}
SPAN.MsoHyperlink {
COLOR: blue; TEXT-DECORATION: underline; mso-style-priority: 99
}
A:visited {
COLOR: purple; TEXT-DECORATION: underline; mso-style-priority: 99
}
SPAN.MsoHyperlinkFollowed {
COLOR: purple; TEXT-DECORATION: underline; mso-style-priority: 99
}
SPAN.EmailStyle17 {
FONT-FAMILY: "Calibri","sans-serif"; COLOR: windowtext; mso-style-type: personal-compose
}
.MsoChpDefault {
FONT-FAMILY: "Calibri","sans-serif"; mso-style-type: export-only
}
DIV.WordSection1 {
page: WordSection1
}
</STYLE>
<!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<DIV class=WordSection1>
<P class=MsoNormal>I am in debate with a PE over calculation of low
voltage of a series string on a hot day. He insists that an arbitrary high
cell temp is factored in, not just ambient temperature. Could someone
please give an accepted formula for this calculation? Thanks. I cannot
find a clear reference to low voltage calculation on a hot day (but every
reference material is clear on how to calculate high voltage on a cold
day).<O:P></O:P></P>
<P class=MsoNormal><O:P></O:P></P>
<P class=MsoNormal>Kirk Herander<O:P></O:P></P>
<P class=MsoNormal>VT Solar, LLC<O:P></O:P></P>
<P class=MsoNormal>dba Vermont Solar Engineering<O:P></O:P></P>
<P class=MsoNormal>NABCEP<SUP>TM </SUP>Certified installer Charter
Member<O:P></O:P></P>
<P class=MsoNormal>NYSERDA-eligible Installer<O:P></O:P></P>
<P class=MsoNormal>VT RE Incentive Program Partner<O:P></O:P></P>
<P class=MsoNormal><O:P></O:P></P></DIV><BR>
<FIELDSET class=mimeAttachmentHeader></FIELDSET> </BLOCKQUOTE><BR>
<FIELDSET class=mimeAttachmentHeader></FIELDSET> <BR><PRE wrap="">_______________________________________________
List sponsored by Home Power magazine
List Address: <A class=moz-txt-link-abbreviated href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</A>
Options & settings:
<A class=moz-txt-link-freetext href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</A>
List-Archive: <A class=moz-txt-link-freetext href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</A>
List rules & etiquette:
<A class=moz-txt-link-abbreviated href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</A>
Check out participant bios:
<A class=moz-txt-link-abbreviated href="http://www.members.re-wrenches.org">www.members.re-wrenches.org</A>
</PRE></BLOCKQUOTE><BR>
<P>
<HR>
<P></P>_______________________________________________<BR>List sponsored by
Home Power magazine<BR><BR>List Address:
RE-wrenches@lists.re-wrenches.org<BR><BR>Options &
settings:<BR>http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org<BR><BR>List-Archive:
http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org<BR><BR>List
rules & etiquette:<BR>www.re-wrenches.org/etiquette.htm<BR><BR>Check out
participant
bios:<BR>www.members.re-wrenches.org<BR><BR></BLOCKQUOTE></BODY></HTML>