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<TITLE>Re: [RE-wrenches] calculating low string voltage</TITLE>
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<FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'><a href="http://www.copper.org/applications/electrical/building/pdf/rooftop.pdf">http://www.copper.org/applications/electrical/building/pdf/rooftop.pdf</a><BR>
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“ASHRAE bases its “warm-season temperature conditions” for each city on annual percentiles of 0.4%, 2 %, 5% and 10%. As an example, the June 2.0% dry-bulb design temperature for Atlanta is 91.7°F. Therefore, based on a 30-day month (i.e., 720 hours), the actual temperatures can be expected to exceed 91.7°F for total of 14 hours a month. The corresponding 5.0% design temperature (89.3°F) can be expected to be exceeded for 36 hours a month; while the 0.4% design temperature (94.7°F) can be expected to be exceeded for 3 hours a month.”<BR>
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On 12/1/11 5:57 PM, "Ray Walters" <<a href="ray@solarray.com">ray@solarray.com</a>> wrote:<BR>
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</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'> Fantastic Info everyone, I think I've got a full grasp on this topic now, from using the correct coefficient to the correct weather data, to the adders for installation type, and a nice link to the applicable data without spending $180 for the ASHRAE book. <BR>
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I'm guessing the 2% number is 2% of the time year round 24/7, not 2% of the time the sun is shining, so losses could be higher than 2% of production? Sorry to keep asking questions, but I know my class will probably ask the same thing.<BR>
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Ray<BR>
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On 12/1/2011 2:57 PM, Gary Willett wrote: <BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'> </SPAN><FONT SIZE="4"><SPAN STYLE='font-size:14pt'>RAY:<BR>
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This "Solar Reference Guide Map" web site is also linked to on the SolarABCs site and provides the ASHRAE 0.4% & 2% high temperatures, as well as the PV module max temp based on distance above the roof (0.5, 3.5, and 12").<BR>
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<a href="http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html">http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html</a><BR>
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Regards,<BR>
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Gary Willett, PE<BR>
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On 12/1/2011 14:56, Gary Willett wrote: <BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'> </SPAN><FONT SIZE="4"><SPAN STYLE='font-size:14pt'>Ray: <BR>
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Look at Appendix F in the Expedited Permit Process for PV Systems - A Standardized Process for the Review of Small-Scale PV Systems Prepared by Bill Brooks, P.E. Brooks Engineering<BR>
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This document provides the ASHRE temperatures for a lot of locations around the USA.<BR>
<BR>
<a href="http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf">http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf</a><BR>
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Regards,<BR>
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Gary Willett, PE<BR>
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On 12/1/2011 14:48, Ray Walters wrote: <BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>That's more detailed info, but I still am wondering what ambient temp to apply (which AHSRAE #) with the temp adder. Daryl suggested using a 2% high temp. Also, where are you all getting your weather data? I found the NEC suggested ASHRAE manual to be close to $200. That's a bit much just to get a couple of numbers for our design. <BR>
It seems this calculation of low voltage is far from being entirely codified, and I'm trying to standardize it somewhat for my PV class. Non of the design manuals I have currently are even close to this level of complexity, yet this is obviously an important calculation. Should I suggest that my students just use the inverter manufacturers' online tools, and forget about trying to run this voltage calc themselves? <BR>
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Thanks, <BR>
<BR>
Ray <BR>
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On 12/1/2011 6:30 AM, Dave Click wrote: <BR>
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</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>At an SMA training last year they recommended the following temperature adders: <BR>
<BR>
Open field: +22°C <BR>
Rooftop, lots of ventilation: +28°C <BR>
Rooftop, some ventilation: +29°C <BR>
Rooftop, little ventilation: +32°C <BR>
Rooftop BIPV, no ventilation: +43°C <BR>
Façade, some ventilation: +35°C <BR>
Façade, little ventilation: +39°C <BR>
Façade BIPV, no ventilation: +55°C <BR>
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For your Sanyo project, I'd agree that the pole mount would be a good fit and +25C would seem to be OK. As you know, the 301VDC VMP on an inverter with a 300V+ tracking window wouldn't work very well during the summers for very long. My own rooftop system has a design VMP of around 294 at 70C module temperature and I've seen it a bit lower than that on a sunny day-- I think it's because in irradiances below 1000Wm/2, the modules can still get plenty hot but the lower irradiance doesn't bring the VMP up to 100%. So all told, maybe plan for an array minimum of: <BR>
<BR>
Inverter Tracking Minimum <BR>
/0.85 (degradation and voltage tolerance) <BR>
/0.95 (effect I just described) <BR>
/0.825 (60C operating temp) <BR>
= minimum string VMP at STC <BR>
<BR>
(similar to what Bill said) <BR>
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On 2011/11/30 11:04, Kirk Herander wrote: <BR>
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</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Thanks for all the good response. Speaking of monkey wrenches, these are <BR>
Sanyo HIT Double panels, mounted on a rack which is 5 ½ feet above the <BR>
roof surface. They will absorb reflected light on the backside, so I <BR>
assume the cells will operate at a higher temp. But they are elevated <BR>
enough to simulate a pole-mount, thus lowering operating cell temp. So <BR>
what operating temp to use? If I use 25 C as the operating temp, the low <BR>
voltage calc comes out to 301 vdc (inverter min is 300 – using SMA <BR>
US8000). If I use 15 C the calc is 308 vdc. This is for a 6 panel <BR>
string. Unfortunately I really don’t want to use 7 panel strings due to <BR>
the layout. If I used a US7000 it would work since min vdc in is 250. <BR>
However the inverters are part of a SunnyTower and I can’t swap a 7000 <BR>
for an 8000 without voiding the Tower UL listing. <BR>
<BR>
Kirk Herander <BR>
<BR>
VT Solar, LLC <BR>
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dba Vermont Solar Engineering <BR>
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*From:<a href="*re-wrenches-bounces@lists.re-wrenches.org">*re-wrenches-bounces@lists.re-wrenches.org</a> <BR>
[<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] *On Behalf Of <BR>
<a href="*boB@midnitesolar.com">*boB@midnitesolar.com</a> <BR>
*Sent:* Wednesday, November 30, 2011 1:03 AM <BR>
*To:* RE-wrenches <BR>
*Subject:* Re: [RE-wrenches] calculating low string voltage <BR>
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If you really want to throw a monkey wrench into the mix, bring up <BR>
partial shading... <BR>
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Or, maybe that's just not allowed in that debate ?? Just a thought. <BR>
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boB <BR>
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On 11/29/2011 8:39 PM, Kent Osterberg wrote: <BR>
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Kirk, <BR>
<BR>
You've got some good feedback on this from Ray and Bill. I'll try to add <BR>
a little more. Most module datasheets show a normal operating cell <BR>
temperature, NOCT, value that's typically 47.5°C. That's 20°C ambient <BR>
temperature, 800 W/sq m, and calm wind and nothing blocking the airflow <BR>
on the back of the module. That 27.5°C temperature rise should be pretty <BR>
close to the temperature rise that occurs for a pole-top mount. It's <BR>
common to see people use 25°C for modules on a pole and 30°C or 35°C on <BR>
a roof and there are data that support these "typical" values. With 1000 <BR>
W/sq irradiance, the temperature rise can obviously be more too. Between <BR>
the intensity of the sun, the direction of the sun, the color of the <BR>
roof, the spacing off the roof, and the wind speed there is a lot that <BR>
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 <BR>
Vmp. I've seen people use the the Voc coefficient, usually expressed as <BR>
a percentage, for both Voc and Vmp. Big mistake. Data from NREL <BR>
indicates as Bill said, the temperature coefficient for Vmp is higher <BR>
than that the temperature coefficient for Voc. That's particularly true <BR>
when the coefficient is expressed as a percentage per °C. Since there <BR>
are very few manufacturer's that give both temperature coefficients, <BR>
I'll use a value from an old Evergreen module for an example. The <BR>
Evergreen ES-195 datasheet shows Voc = 30.5 volts with a coefficient of <BR>
-0.34%/°C and Vmp = 27.1 volts with a coefficient of -0.47%/°C. Since <BR>
one shouldn't add volts and percents, I'll put the temperature <BR>
coefficients in volts/°C: Voc = 30.5 V - 0.10 V/°C and Vmp = 27.1 V - <BR>
0.13V/°C. So Vmp is moving faster than Voc, but not a lot faster. That's <BR>
generically true for c-Si or poly-Si. <BR>
<BR>
Kent Osterberg <BR>
<BR>
Blue Mountain Solar, Inc. <BR>
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www.bluemountainsolar.com <<a href="http://www.bluemountainsolar.com">http://www.bluemountainsolar.com</a>> <<a href="http://www.bluemountainsolar.com">http://www.bluemountainsolar.com</a>> <BR>
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t: 541-568-4882 <BR>
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On 11/29/2011 11:48 AM, Kirk Herander wrote: <BR>
<BR>
I am in debate with a PE over calculation of low voltage of a series <BR>
string on a hot day. He insists that an arbitrary high cell temp is <BR>
factored in, not just ambient temperature. Could someone please give an <BR>
accepted formula for this calculation? Thanks. I cannot find a clear <BR>
reference to low voltage calculation on a hot day (but every reference <BR>
material is clear on how to calculate high voltage on a cold day). <BR>
<BR>
Kirk Herander <BR>
<BR>
VT Solar, LLC <BR>
<BR>
dba Vermont Solar Engineering <BR>
<BR>
NABCEP^TM Certified installer Charter Member <BR>
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