[RE-wrenches] calculating low string voltage

David Brearley david.brearley at solarprofessional.com
Thu Dec 1 17:02:53 PST 2011


http://www.copper.org/applications/electrical/building/pdf/rooftop.pdf

³ASHRAE bases its ³warm-season temperature conditions² 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.²



On 12/1/11 5:57 PM, "Ray Walters" <ray at solarray.com> wrote:

>    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/inde
>> x.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
>>>  
>>>  
>>>  
>>>  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 1Ž2 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 don¹t 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 can¹t 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>
>>>>>> <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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David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
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