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
NABCEP^TM Certified installer Charter Member
NYSERDA-eligible Installer
VT RE Incentive Program Partner
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