[RE-wrenches] calculating low string voltage

Kirk Herander kirk at vtsolar.com
Wed Nov 30 08:04:54 PST 2011


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

NABCEPTM 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
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

NABCEPTM Certified installer Charter Member

NYSERDA-eligible Installer

VT RE Incentive Program Partner

 










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