[RE-wrenches] calculating low string voltage

Thu Dec 1 05:30:47 PST 2011

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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