[RE-wrenches] calculating low string voltage

Ray Walters ray at solarray.com
Thu Dec 1 12:48:31 PST 2011


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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