PV module temp [RE-wrenches]

Joel Davidson joeldavidson at earthlink.net
Tue Sep 25 16:55:36 PDT 2001


The formula for converting C. to F. is ((C + 40)1.8) - 40 = F.

The formula for converting F. to C. is ((F + 40)0.555) - 40 = C.

Cell temperature is approx. 20 degrees C. (68 F.) above ambient air temperature.
If ambient air temperature is 80 F., I expect cell temperature to be approx. 148
F and find it to be so. An open rack PV system in Pasadena receiving 700 to 900
W/m2 at ambient 33 C. consistently measures 55 C. cell temperature since 1996.

In Tokyo, a roof mounted single crystal array with black tedlar and black frame
raised and boxed in over an existing roof with no air circulation measured 83 C.
cell temperature in September. (The IEEE report on this test is somewhere in my
office, but I can't put my hands on it. I think the testing was done in 1993 or
1994.) For many years, I told engineers in Japan not to use black tedlar or box
in the array. They said that the customer wants a uniform all black appearance
and does not want to provide bird nesting places.

If the peak voltage of a solar module at 25 C. is 17 volts, at 83 C. module
voltage is 12.62 volts.
83 C - 25 C = 58 C difference in temperature x 0.0021 generic voltage
coefficient for single crystal cells x 36 cells = 4.38 volts drop.

Bill Brooks wrote:

> Joel,
>
> Careful with your units. The boys on the other side of the globe might get a
> little testy. You are referring to 20C hotter than ambient temp (probably
> more than that for roof mounted arrays) which is only 36F hotter than
> ambient (not 68F--68F=20C when we are talking actual temperatures, not
> temperature difference). Degrees C is 1.8 times degrees F when looking at
> differences.
>
> On a 40C day in full sun (1000 W/m2), I measured 70C (158F) on an open rack
> array. That's 30C which is worst case rather than 20C which might be an
> annual average.
>
> Bill.
>
> > -----Original Message-----
> > From: Joel Davidson [mailto:joeldavidson at earthlink.net]
> > Sent: Tuesday, September 25, 2001 2:53 PM
> > To: RE-wrenches at topica.com
> > Subject: Re: PV module temp [RE-wrenches]
> >
> >
> > Single crystal and polycrystalline cell temperature is approx. 20
> > degrees C. (68 F.)
> > above ambient air temperature. The sensor should measure the cell
> > temperature, not
> > the atmosphere at the back of the panel.
> >
> > Smitty wrote:
> >
> > > Trav, the insulation is probably giving you a high reading. The
> > sensor should be
> > > exposed to the same atmosphere that the back of the panel is. Smitty.
> > >
> > > "Travis Creswell, Ozark Solar" wrote:
> > >
> > > > Hey Gang,
> > > >
> > > > I have hooked up a Heliotrope Quadra Temp four channel temp
> > monitor that
> > > > uses 10k sensors.  One of the channels is monitoring temp on
> > the back of a
> > > > module in my roof mounted PV array.  I'm seeing temps of
> > nearly 140 F on
> > > > sunny days that the ambient temp doesn't get past 80 F.  That
> > seems pretty
> > > > high.  The sensor is fine as it reads ambient temp accurately
> > after sunset
> > > > plus I checked it before I installed it.  I siliconed the
> > sensor along with
> > > > a small piece of Armaflex insulation on the back
> > approximately in the center
> > > > of the module.
> > > >
> > > > I insulated it so the temp would be more accurate but now I
> > am wondering if
> > > > I'm not holding heat in with the insulation causing the temp
> > to be too high?
> > > >  But if I don't insulate it won't it read too cool?
> > > >
> > > > Suggestions?
> > > >
> > > > Thanks in advance.
> > > >
> > > > Travis Creswell
> > > > Ozark Solar
> > > >
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