[RE-wrenches] calculating low string voltage

Thu Dec 1 15:57:32 PST 2011

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
>>
>>
>> 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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