Thanks for all the good response. Speaking of monkey wrenches, these are Sanyo HIT Double panels, mounted on a rack which is 5 ½ feet above the roof surface. They will absorb reflected light on the backside, so I assume the cells will operate at a higher temp. But they are elevated enough to simulate a pole-mount, thus lowering operating cell temp. So what operating temp to use? If I use 25 C as the operating temp, the low voltage calc comes out to 301 vdc (inverter min is 300 using SMA US8000). If I use 15 C the calc is 308 vdc. This is for a 6 panel string. Unfortunately I really dont want to use 7 panel strings due to the layout. If I used a US7000 it would work since min vdc in is 250. However the inverters are part of a SunnyTower and I cant swap a 7000 for an 8000 without voiding the Tower UL listing.
Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEPTM Certified installer Charter Member NYSERDA-eligible Installer VT RE Incentive Program Partner From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of b...@midnitesolar.com Sent: Wednesday, November 30, 2011 1:03 AM To: RE-wrenches Subject: Re: [RE-wrenches] calculating low string voltage If you really want to throw a monkey wrench into the mix, bring up partial shading... Or, maybe that's just not allowed in that debate ?? Just a thought. boB On 11/29/2011 8:39 PM, Kent Osterberg wrote: Kirk, 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. 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. Kent Osterberg Blue Mountain Solar, Inc. www.bluemountainsolar.com t: 541-568-4882 On 11/29/2011 11:48 AM, Kirk Herander wrote: 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). Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEPTM Certified installer Charter Member NYSERDA-eligible Installer VT RE Incentive Program Partner _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
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