[RE-wrenches] calculating low string voltage

Thu Dec 1 13:57:51 PST 2011

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
>
>
> 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-bounces at lists.re-wrenches.org
>>>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of
>>>> *boB at 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).
>>>>
>>>> Kirk Herander
>>>>
>>>> VT Solar, LLC
>>>>
>>>> dba Vermont Solar Engineering
>>>>
>>>> NABCEP^TM Certified installer Charter Member
>>>>
>>>> NYSERDA-eligible Installer
>>>>
>>>> VT RE Incentive Program Partner
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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