2 strings 90 degree apart in azimuth

[Bob]

Hi all,
I have a proposal from a PV installer to put 2 parallel strings of 8
panels on two different parts of my 5 and 12 (22 degree) roof. The two
sections of roof face 240 (WSW) and 150 (SSE) degrees (true) and I live
at 37 degrees north latitude. It seems to me that the optimum
installation would have both strings facing the 150 degree azimuth but
there isn't enough room on that section of roof.

I had an idea to tilt the panels on the WSW roof up (prop the north edge
up) by about 20 degrees. This would swing the azimuth around by about 43
degrees to end up at 200 (SSW). It would also pitch the elevation down,
of course, to about 60 degrees from horizontal.

I can do the rotational math (see http://bmary.com/SolarAngle.html ) but
I don't have access to anything that will integrate the insolance over
the seasons. So, can anybody tell me how much I would lose by mounting
flat on the two roofs and how much I might gain back by tilting the WSW
panels?

TIA,
Bob

 

[Martin Riddle]

If I understand your question correct, your asking what is the gain or loss at 200deg.

You might find an answer on the Sandia web site, take a look at http://photovoltaics.sandia.gov/docs/PDF/IEF5CE~1.PDF.
The last part has tests of AOI, angle of incidence. The test show that up to 50deg the output is rather flat.
Our local university, SUNY Farmingdale, has a 30Kw array facing SE and one 20Kw facing south. From speaking with people in
the department there was little lost with the SE configuration. I dont remember the specific numbers but there wasnt a big
difference. That makes sense since upto 50deg there is little change in collector output.

This might be of intrest. http://yachtsoftware.org/SolarPanelCalculator.aspx

Cheers

 

[[email protected]]

Take a look at the PVWATTS calculator at the NREL web site:
http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/

You can plug in your location, the azimuth and angle of your proposed and
possible panels, and see the change in predicted energy production.

PVWatts was going to be my first suggestion. The values used are actual
observed insolation over a period of years, with "typical" years used in
the calculations.

The other thought is that if you have Time of Use Net Metering, the optimum
azimuth for dollar generation is not the same as the optimim angle for
energy generation. I can't locate the URL, and it only showed the results
for Northern California, but the optimum for the E-7 TOU schedule for PG&E
was 217 azimuth, 31 degree elevation.

240 is too far north even for that, but a few degrees might not make enough
difference to bother with the complexity and appearance of any special
setup.