Re: [RE-wrenches] calculating low string voltage
At an SMA training last year they recommended the following temperature adders: Open field: +22°C Rooftop, lots of ventilation: +28°C Rooftop, some ventilation: +29°C Rooftop, little ventilation: +32°C Rooftop BIPV, no ventilation: +43°C Façade, some ventilation: +35°C Façade, little ventilation: +39°C Façade BIPV, no ventilation: +55°C 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: Inverter Tracking Minimum /0.85 (degradation and voltage tolerance) /0.95 (effect I just described) /0.825 (60C operating temp) = minimum string VMP at STC (similar to what Bill said) On 2011/11/30 11:04, Kirk Herander wrote: 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 don’t 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 can’t swap a 7000 for an 8000 without voiding the Tower UL listing. Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEP^TM 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 http://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
Re: [RE-wrenches] calculating low string voltage
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. 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? Thanks, Ray On 12/1/2011 6:30 AM, Dave Click wrote: At an SMA training last year they recommended the following temperature adders: Open field: +22°C Rooftop, lots of ventilation: +28°C Rooftop, some ventilation: +29°C Rooftop, little ventilation: +32°C Rooftop BIPV, no ventilation: +43°C Façade, some ventilation: +35°C Façade, little ventilation: +39°C Façade BIPV, no ventilation: +55°C 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: Inverter Tracking Minimum /0.85 (degradation and voltage tolerance) /0.95 (effect I just described) /0.825 (60C operating temp) = minimum string VMP at STC (similar to what Bill said) On 2011/11/30 11:04, Kirk Herander wrote: 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 don’t 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 can’t swap a 7000 for an 8000 without voiding the Tower UL listing. Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEP^TM 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
Re: [RE-wrenches] calculating low string voltage
Ray: 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 This document provides the ASHRE temperatures for a lot of locations around the USA. http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf Regards, Gary Willett, PE On 12/1/2011 14:48, Ray Walters wrote: 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. 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? Thanks, Ray On 12/1/2011 6:30 AM, Dave Click wrote: At an SMA training last year they recommended the following temperature adders: Open field: +22°C Rooftop, lots of ventilation: +28°C Rooftop, some ventilation: +29°C Rooftop, little ventilation: +32°C Rooftop BIPV, no ventilation: +43°C Façade, some ventilation: +35°C Façade, little ventilation: +39°C Façade BIPV, no ventilation: +55°C 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: Inverter Tracking Minimum /0.85 (degradation and voltage tolerance) /0.95 (effect I just described) /0.825 (60C operating temp) = minimum string VMP at STC (similar to what Bill said) On 2011/11/30 11:04, Kirk Herander wrote: 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 don’t 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 can’t swap a 7000 for an 8000 without voiding the Tower UL listing. Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEP^TM 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
Re: [RE-wrenches] calculating low string voltage
ASHRAE temp data as applicable to PV can be found (with some digging) on the Solar ABCs website. http://solarabcs.org/about/publications/reports/expedited-permit/map/map_int ro.html -Glenn -Original Message- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Ray Walters Sent: Thursday, December 01, 2011 3:49 PM To: RE-wrenches Subject: Re: [RE-wrenches] calculating low string voltage 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. 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? Thanks, Ray On 12/1/2011 6:30 AM, Dave Click wrote: At an SMA training last year they recommended the following temperature adders: Open field: +22°C Rooftop, lots of ventilation: +28°C Rooftop, some ventilation: +29°C Rooftop, little ventilation: +32°C Rooftop BIPV, no ventilation: +43°C Façade, some ventilation: +35°C Façade, little ventilation: +39°C Façade BIPV, no ventilation: +55°C 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: Inverter Tracking Minimum /0.85 (degradation and voltage tolerance) /0.95 (effect I just described) /0.825 (60C operating temp) = minimum string VMP at STC (similar to what Bill said) On 2011/11/30 11:04, Kirk Herander wrote: 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 NABCEP^TM 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
Re: [RE-wrenches] calculating low string voltage
Ray, I'd use the 2% high temp. The 0.4% high temp would only be reached a few hours out of the year. However, the 0.4% high is only a few degC warmer than the 2% high, so that would change my fudge factor below by a max of 1.8-2.0%. On 2011/12/1 15:48, Ray Walters wrote: 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. 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? Thanks, Ray On 12/1/2011 6:30 AM, Dave Click wrote: At an SMA training last year they recommended the following temperature adders: Open field: +22°C Rooftop, lots of ventilation: +28°C Rooftop, some ventilation: +29°C Rooftop, little ventilation: +32°C Rooftop BIPV, no ventilation: +43°C Façade, some ventilation: +35°C Façade, little ventilation: +39°C Façade BIPV, no ventilation: +55°C 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: Inverter Tracking Minimum /0.85 (degradation and voltage tolerance) /0.95 (effect I just described) /0.825 (60C operating temp) = minimum string VMP at STC (similar to what Bill said) On 2011/11/30 11:04, Kirk Herander wrote: 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 don’t 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 can’t swap a 7000 for an 8000 without voiding the Tower UL listing. Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEP^TM 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
Re: [RE-wrenches] calculating low string voltage
Ray, There is an explanation on the www.solarabcs.org/permitting webpage for the interactive solar map. The 2% ASHRAE data represents 14 hours in a month-meaning temperature will be at or above that value for 14 hours in a typical month. This is likely to correspond to at least one 3-hour timeframe which is considered continuous in the NEC. The Copper Development Association has chosen this number for ambient temperature calculations in the NEC. The NEC Handbook (not the NEC) uses this number in their example problems. The 0.4% ASHRAE date represents 3 hours in a month. It is much more conservative number. I put it in the data to provide a context for the 2% number and it allows engineers to consider a more conservative number. Of course, the mean extreme minimum drybulb temperature is also on that website for 690.7 max system voltage calculations. Enjoy-its free thanks to SolarABCs. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Ray Walters Sent: Thursday, December 01, 2011 3:58 PM To: RE-wrenches Subject: Re: [RE-wrenches] calculating low string voltage 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. 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. Ray On 12/1/2011 2:57 PM, Gary Willett wrote: RAY: 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). http://www.solarabcs.org/about/publications/reports/expedited-permit/map/ind ex.html Regards, Gary Willett, PE On 12/1/2011 14:56, Gary Willett wrote: Ray: 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 This document provides the ASHRE temperatures for a lot of locations around the USA. http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Ex permitprocess.pdf Regards, Gary Willett, PE ___ 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
Re: [RE-wrenches] calculating low string voltage
swap a 7000 for an 8000 without voiding the Tower UL listing. Kirk Herander VT Solar, LLC dba Vermont Solar Engineering NABCEP^TM 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 http://www.bluemountainsolar.com http://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 NABCEP^TM 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 mailto: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-wrench es.org List rules etiquette: www.re-wrenches.org/etiquette.htm http://www.re-wrenches.org/etiquette.htm http://www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org http://www.members.re-wrenches.org http://www.members.re-wrenches.org ___ 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 http://www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org http://www.members.re-wrenches.org ___ 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
Re: [RE-wrenches] calculating low string voltage
Again Thanks Bill and David; Continued excellent information. I just jumped on Solectria'a string sizing online tool, and what are they using for the Vmpp coefficent? just a tiny bit higher than the Voc coefficient. SMAs new design tool just lists the Voc coeff. I think some manus might need to be following this thread as well, and tweak their sizing tools appropriately. Ray On 12/1/2011 5:54 PM, Bill Brooks wrote: Ray, There is an explanation on the www.solarabcs.org/permitting http://www.solarabcs.org/permitting webpage for the interactive solar map. The 2% ASHRAE data represents 14 hours in a month---meaning temperature will be at or above that value for 14 hours in a typical month. This is likely to correspond to at least one 3-hour timeframe which is considered continuous in the NEC. The Copper Development Association has chosen this number for ambient temperature calculations in the NEC. The NEC Handbook (not the NEC) uses this number in their example problems. The 0.4% ASHRAE date represents 3 hours in a month. It is much more conservative number. I put it in the data to provide a context for the 2% number and it allows engineers to consider a more conservative number. Of course, the mean extreme minimum drybulb temperature is also on that website for 690.7 max system voltage calculations. Enjoy---its free thanks to SolarABCs. Bill. *From:*re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Ray Walters *Sent:* Thursday, December 01, 2011 3:58 PM *To:* RE-wrenches *Subject:* Re: [RE-wrenches] calculating low string voltage 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. 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. Ray On 12/1/2011 2:57 PM, Gary Willett wrote: RAY: 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). http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html Regards, Gary Willett, PE On 12/1/2011 14:56, Gary Willett wrote: Ray: 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 This document provides the ASHRE temperatures for a lot of locations around the USA. http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf Regards, Gary Willett, PE ___ 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
Here¹s a link to an Excel version of our c-Si PV module specifications table: https://solarprofessional.com/webexclusive/ It includes coefficients for Voc, Pmp and Isc. For Vmp calculations, use the coefficient of Pmp, which is primarily voltage driven. (Temperature coefficients of current are VERY small, as seem in the Isc values.) David Brearley, Senior Technical Editor SolarPro magazine NABCEP Certified PV Installer david.brear...@solarprofessional.com Direct: 541.261.6545 On 11/30/11 12:18 AM, Ray Walters r...@solarray.com wrote: I pulled the voltage temp coefficient off of an article from Solar Pro Magazine. It is indeed listed for Voc. I have never seen a separate coefficient for Vmpp. Maybe we should use the Power mpp coefficient and then factor the slight rise in current by temp (which would make Vmpp even worse). My Solar world example shows a Pmpp coeff. of -0.47, while the current coeff. (short circuit though) is positive 0.04 so correcting just for voltage, that would be at or slightly worse than Bill's suggested -0.5 coefficient for Vmpp. Which would pull the Vmpp from 35 to 28 in my example, not 30.4 v. The degradation and mismatch Bill points out make sense, but is it really another 15% combined? Would it be fair to say different modules and manufacturers would degrade at different rates? Finally, the ambient temp I picked was the opposite of the NEC suggested ASHRAE low temp. Is that really the best number for calculating this? Or is that too extreme a temperature, that wouldn't occur most of the time? Now I have to correct that coefficient for my next class..but that's why I asked for everybody to look it over. Thanks Everyone, Ray On 11/29/2011 8:08 PM, Doug Wells wrote: Ray, I agree with what you have calculated, but the temp. coef. listed is for a Voc. For voltage sag, the temp. value would be a Vmp value and I don't know how much of a difference that makes. I have seen some manufacturers offer a temp. coefficient for Vmp as well as Voc. Not sure what the difference would be, but if it gets down to a few volts with the PE, it is would be worth checking. Doug Wells The Solar Specialists Morrisville, VT 05661 (p) 802-223-7014 (c) 802-498-5856 www.thesolarspecialists.com http://www.thesolarspecialists.com On Nov 29, 2011, at 5:55 PM, Ray Walters wrote: I'm prepping for a class on this very subject tonight, so I'll just wrap up everybody's good points: V min = Vmpp x (1- temp diff x Temp coefficient) So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean Maximum, then add Scott's adder (ie roof mount off 6 = +30C) so let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp then, 65C - 25C(STC) = a differential of 40 deg C from standard test conditions The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, Solar world for example is -0.33 %/degC With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2% So just assuming Vmpp = 35v, the adjustment would be 35v x (100%-13.2%), or 86.8% of 35v, which gives a Volt min of 30.4 v Somebody please correct my math if I got that wrong. Also probably more interesting is the actual Power coefficient, which shows the total power losses from higher temperatures, not just voltage. The calculation would be similar. Ray On 11/29/2011 2:07 PM, Darryl Thayer wrote: I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT From: Kirk Herander k...@vtsolar.com mailto:k...@vtsolar.com To: RE-wrenches re-wrenches@lists.re-wrenches.org mailto:re-wrenches@lists.re-wrenches.org Sent: Tuesday, November 29, 2011 1:48 PM Subject: [RE-wrenches] calculating low string voltage 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
Re: [RE-wrenches] calculating low string voltage
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 ___ 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
[RE-wrenches] calculating low string voltage
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
Re: [RE-wrenches] calculating low string voltage
Usually you increase the temperature above ambient by an amount which depends on the mounting method. Close to the roof is +35 degrees C, more than 6 inches above the roof is +30 degrees C, and pole or ground mount is +25 degrees C. Scott On Nov 29, 2011, at 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
As far as I know, the temperature coefficient holds true whether you are increasing or decreasing the temperature around the STC temperature of 25C. Of course, there is going to be a point at which it goes non-linear, but that is not stated. On Tue, Nov 29, 2011 at 3:58 PM, Scott McCalmont re-wrenc...@scottmccalmont.com wrote: Usually you increase the temperature above ambient by an amount which depends on the mounting method. Close to the roof is +35 degrees C, more than 6 inches above the roof is +30 degrees C, and pole or ground mount is +25 degrees C. Scott On Nov 29, 2011, at 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 ___ 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 -- Chris Mason President, Comet Systems Ltd mas...@cometsystems.co www.cometsystems.co Cell: 264.235.5670 Int: +1305.767.2094 Skype: netconcepts ___ 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
Re: [RE-wrenches] calculating low string voltage
I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT From: Kirk Herander k...@vtsolar.com To: RE-wrenches re-wrenches@lists.re-wrenches.org Sent: Tuesday, November 29, 2011 1:48 PM Subject: [RE-wrenches] calculating low string voltage 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___ 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
Re: [RE-wrenches] calculating low string voltage
I'm prepping for a class on this very subject tonight, so I'll just wrap up everybody's good points: V min = Vmpp x (1- temp diff x Temp coefficient) So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean Maximum, then add Scott's adder (ie roof mount off 6 = +30C) so let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp then, 65C - 25C(STC) = a differential of 40 deg C from standard test conditions The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, Solar world for example is -0.33 %/degC With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2% So just assuming Vmpp = 35v, the adjustment would be 35v x (100%-13.2%), or 86.8% of 35v, which gives a Volt min of 30.4 v Somebody please correct my math if I got that wrong. Also probably more interesting is the actual Power coefficient, which shows the total power losses from higher temperatures, not just voltage. The calculation would be similar. Ray On 11/29/2011 2:07 PM, Darryl Thayer wrote: I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT *From:* Kirk Herander k...@vtsolar.com *To:* RE-wrenches re-wrenches@lists.re-wrenches.org *Sent:* Tuesday, November 29, 2011 1:48 PM *Subject:* [RE-wrenches] calculating low string voltage 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 NABCEP^TM 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 mailto: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 ___ 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
This reply from Scott actually answered the original question. To that end, I would like to include my vote for the accuracy of his information. Measurements conducted here at Exeltech affirm the ~30C temperature delta directly behind any cell near the center of the PV for an array with sufficient area to allow convective cooling. We have also found a few other variables impact cell temperature *slightly*, but not enough to worry about. The figures given by Scott below are as accurate a value as can be utilized. Dan Sr. Engineer Exeltech --- On Tue, 11/29/11, Scott McCalmont re-wrenc...@scottmccalmont.com wrote: From: Scott McCalmont re-wrenc...@scottmccalmont.com Subject: Re: [RE-wrenches] calculating low string voltage To: RE-wrenches re-wrenches@lists.re-wrenches.org Date: Tuesday, November 29, 2011, 1:58 PM Usually you increase the temperature above ambient by an amount which depends on the mounting method. Close to the roof is +35 degrees C, more than 6 inches above the roof is +30 degrees C, and pole or ground mount is +25 degrees C. Scott On Nov 29, 2011, at 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 HeranderVT Solar, LLCdba Vermont Solar EngineeringNABCEPTM Certified installer Charter MemberNYSERDA-eligible InstallerVT 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 -Inline Attachment Follows- ___ 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
Folks, One factor often overlooked when calculating minimum system voltage is degradation of the array and rating variations of the modules. If you assume 0.5% loss in voltage per year, you need to factor a 10% higher voltage at year one. Manufacturers tolerance on voltage and current can be 5-10% (rare in real life). It is healthy to add another 5% for voltage tolerance. Voltage tolerance can be much larger than power tolerance that may only be +/- 3%. The voltage tolerance numbers used by Ray are for Voc not Vmp. Vmp numbers are closer to 0.45% to 0.5%. This why the power loss due to temperature is about 0.5%/C-all of it is in voltage, not current. Therefore, for a 40C rise in temperature, there will be a 20% loss in voltage. Factoring another 15% loss for degradation and module tolerance, the total loss in voltage would be: Vmin = 35V x 0.8 x 0.9 x 0.95 = 23.94V = 24 Volts, not 30 Volts. It is always lower than you think unless you are Eeyore. This is one reason why the most common string sizing mistake is not enough modules in series. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Ray Walters Sent: Tuesday, November 29, 2011 2:55 PM To: RE-wrenches Subject: Re: [RE-wrenches] calculating low string voltage I'm prepping for a class on this very subject tonight, so I'll just wrap up everybody's good points: V min = Vmpp x (1- temp diff x Temp coefficient) So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean Maximum, then add Scott's adder (ie roof mount off 6 = +30C) so let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp then, 65C - 25C(STC) = a differential of 40 deg C from standard test conditions The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, Solar world for example is -0.33 %/degC With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2% So just assuming Vmpp = 35v, the adjustment would be 35v x (100%-13.2%), or 86.8% of 35v, which gives a Volt min of 30.4 v Somebody please correct my math if I got that wrong. Also probably more interesting is the actual Power coefficient, which shows the total power losses from higher temperatures, not just voltage. The calculation would be similar. Ray On 11/29/2011 2:07 PM, Darryl Thayer wrote: I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT From: Kirk Herander mailto:k...@vtsolar.com k...@vtsolar.com To: RE-wrenches mailto:re-wrenches@lists.re-wrenches.org re-wrenches@lists.re-wrenches.org Sent: Tuesday, November 29, 2011 1:48 PM Subject: [RE-wrenches] calculating low string voltage 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 ___ 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
Ray, I agree with what you have calculated, but the temp. coef. listed is for a Voc. For voltage sag, the temp. value would be a Vmp value and I don't know how much of a difference that makes. I have seen some manufacturers offer a temp. coefficient for Vmp as well as Voc. Not sure what the difference would be, but if it gets down to a few volts with the PE, it is would be worth checking. Doug Wells The Solar Specialists Morrisville, VT 05661 (p) 802-223-7014 (c) 802-498-5856 www.thesolarspecialists.com On Nov 29, 2011, at 5:55 PM, Ray Walters wrote: I'm prepping for a class on this very subject tonight, so I'll just wrap up everybody's good points: V min = Vmpp x (1- temp diff x Temp coefficient) So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean Maximum, then add Scott's adder (ie roof mount off 6 = +30C) so let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp then, 65C - 25C(STC) = a differential of 40 deg C from standard test conditions The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, Solar world for example is -0.33 %/degC With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2% So just assuming Vmpp = 35v, the adjustment would be 35v x (100%-13.2%), or 86.8% of 35v, which gives a Volt min of 30.4 v Somebody please correct my math if I got that wrong. Also probably more interesting is the actual Power coefficient, which shows the total power losses from higher temperatures, not just voltage. The calculation would be similar. Ray On 11/29/2011 2:07 PM, Darryl Thayer wrote: I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT From: Kirk Herander k...@vtsolar.com To: RE-wrenches re-wrenches@lists.re-wrenches.org Sent: Tuesday, November 29, 2011 1:48 PM Subject: [RE-wrenches] calculating low string voltage 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 ___ 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 ___ 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
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 NABCEP^TM 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
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 NABCEP^TM 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 ___ 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
Re: [RE-wrenches] calculating low string voltage
I pulled the voltage temp coefficient off of an article from Solar Pro Magazine. It is indeed listed for Voc. I have never seen a separate coefficient for Vmpp. Maybe we should use the Power mpp coefficient and then factor the slight rise in current by temp (which would make Vmpp even worse). My Solar world example shows a Pmpp coeff. of -0.47, while the current coeff. (short circuit though) is positive 0.04 so correcting just for voltage, that would be at or slightly worse than Bill's suggested -0.5 coefficient for Vmpp. Which would pull the Vmpp from 35 to 28 in my example, not 30.4 v. The degradation and mismatch Bill points out make sense, but is it really another 15% combined? Would it be fair to say different modules and manufacturers would degrade at different rates? Finally, the ambient temp I picked was the opposite of the NEC suggested ASHRAE low temp. Is that really the best number for calculating this? Or is that too extreme a temperature, that wouldn't occur most of the time? Now I have to correct that coefficient for my next class..but that's why I asked for everybody to look it over. Thanks Everyone, Ray On 11/29/2011 8:08 PM, Doug Wells wrote: Ray, I agree with what you have calculated, but the temp. coef. listed is for a Voc. For voltage sag, the temp. value would be a Vmp value and I don't know how much of a difference that makes. I have seen some manufacturers offer a temp. coefficient for Vmp as well as Voc. Not sure what the difference would be, but if it gets down to a few volts with the PE, it is would be worth checking. Doug Wells The Solar Specialists Morrisville, VT 05661 (p) 802-223-7014 (c) 802-498-5856 www.thesolarspecialists.com http://www.thesolarspecialists.com On Nov 29, 2011, at 5:55 PM, Ray Walters wrote: I'm prepping for a class on this very subject tonight, so I'll just wrap up everybody's good points: V min = Vmpp x (1- temp diff x Temp coefficient) So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean Maximum, then add Scott's adder (ie roof mount off 6 = +30C) so let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp then, 65C - 25C(STC) = a differential of 40 deg C from standard test conditions The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, Solar world for example is -0.33 %/degC With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2% So just assuming Vmpp = 35v, the adjustment would be 35v x (100%-13.2%), or 86.8% of 35v, which gives a Volt min of 30.4 v Somebody please correct my math if I got that wrong. Also probably more interesting is the actual Power coefficient, which shows the total power losses from higher temperatures, not just voltage. The calculation would be similar. Ray On 11/29/2011 2:07 PM, Darryl Thayer wrote: I am on road so I have no texts with me, but the same formula applies about 0.4% per degree (less voltage) or use the module label voltage coefficent. However you must use the MPPT voltage and the temperature = (temperature ambient (2% high) plus roof temperature warming) The roof temperature is hard to find good values because it depends upon spacing between roof and module. I add 15 deg C for most situations. Best I can do from memory. DT *From:* Kirk Herander k...@vtsolar.com *To:* RE-wrenches re-wrenches@lists.re-wrenches.org *Sent:* Tuesday, November 29, 2011 1:48 PM *Subject:* [RE-wrenches] calculating low string voltage 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 NABCEP^TM 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 mailto: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 ___ 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
