[RE-wrenches] calculating low string voltage

Thu Dec 1 13:50:57 PST 2011

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-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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