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-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of
*b...@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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