...or that most of the time, PV systems do not operate at the array and
inverter maximum power points...or that test instruments at the best labs are
only accurate to 3% to 5% (K. Emrey, NREL) which brings to question some of the
tight module specifications we now see...or that a one-time snapshot is not as
accurate as a moving picture. Nevertheless, an experienced diagnostician can
tell if everything (but not necessarily any specific thing) is ok or something
is amiss.
Joel Davidson
----- Original Message -----
From: b...@midnitesolar.com
To: RE-wrenches
Sent: Tuesday, November 29, 2011 10:02 PM
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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