Still confused on panel angle

rgk1

I know this has been asked before, but I did not really see a direct answer. I am at about 34.75 Lat. and I am about to do a final mount of my panels and decided to go with the middle of the road angle that is my Lat. 35 degrees. Can someone tell me how to arrive at that angle. I percieve 0 degrees to be straight up/skyward and 90 degrees parallel to the ground aiming toward the horizon. Do I start from the face of the panel aiming straight up and tilt "down" toward the horizon till I get to 35 degrees (leaving 55 more degrees to arrive at the 90 parallel to the ground mark, or do I go "up" 35 degrees from the horizon (leaving 55 more degrees till the panel would be strait up at 0 degrees? I guess simply asked, do I tilt up 35 degrees from the horizon, or down 35 degress from staight up? Probably a silly question, but 20 degrees difference coupled with 15 off off peak angle twice a year and it becomes substantial for half the year. On a side note, my magnetic declination is 2 deg. W. Is it safe to say magnetic south would be ok for me? Looking at the online tools, it appears to make very little difference over the course of the year.

BB.

Re: Still confused on panel angle

Raise up 35 degrees from ground. Zero degrees is at the equator and the panel is pointing straight up to sun.

2 degrees is a don't care for pointing south.

-Bill

niel

Re: Still confused on panel angle

i'm not sure bb's explanation would be fully understood as at ground means it is laying flat on the ground and that is 0 degrees. hope this helps you arrive at the starting latitude angle and i say starting because it can be adjusted seasonally, for max yearly power, or optimized some for winter.

solarix

Re: Still confused on panel angle

This web site has the definitive calculations/charts for your PV angle.

http://www.macslab.com/optsolar.html

For a fixed tilt mount in the U.S. the optimum angle for overall year-round production, apparently is a little less than your latitude angle -
Latitude x 0.76 + 3.1deg to be exact. 29.5deg at your latitude.

You probably would'nt be able to notice the difference in power but
"hey - lets get it right!" (Jim Lovell of Apollo 13)

rgk1

Re: Still confused on panel angle

Thanks for the responses. It makes sense to me know. I wasn't thinking of the equator as the 0 degree reference point.

RCinFLA

Re: Still confused on panel angle

Zero degrees is flat to ground.

Assuming panels are pointing directly south, average is latitude minus a few degrees. The minus a few degrees gives you a little more morning and afternoon power at slight drop in solar noon maximum.

Having reasonable access to panels, it helps if you can adjust tilt two to four times a year. There is a 23.5 axis tilt on the earth so N. hemisphere summer is latitude - 23.5 degs, where winter is latitude + 23.5 degs.

The charts here, http://rredc.nrel.gov/solar/pubs/redbook/ , shows average insolation hours for various tilts versus months of year. It also has one and two axis trackers. It is alway interesting to see what your fixed mount is compared to optimum two axis tracker where the panels are facing directly toward sun all day.

ggunn

Re: Still confused on panel angle

RCinFLA wrote: »

Zero degrees is flat to ground.

Assuming panels are pointing directly south, average is latitude minus a few degrees. The minus a few degrees gives you a little more morning and afternoon power at slight drop in solar noon maximum.

Having reasonable access to panels, it helps if you can adjust tilt two to four times a year. There is a 23.5 axis tilt on the earth so N. hemisphere summer is latitude - 23.5 degs, where winter is latitude + 23.5 degs.

Just to clarify, the maximum elevation of the sun at solar noon on the winter solstice in the northern hemisphere is 90 degrees minus the latitude minus 23.5 degrees. For the summer solstice it is 90 degrees minus the latitude plus 23.5 degrees. Note that if you are closer to the equator than 23.5 degrees that number will be greater than 90 degrees for the summer solstice; that means that in that location at solar noon on the summer solstice the sun is actually in the northern half of the sky.

Tilt is sort of the reciprocal of elevation, as it is measured from straight up. Tilt + elevation = 90 degrees.

Dave Angelini

Re: Still confused on panel angle

The easiest way to understand this is to watch a dual axis for a year. The Wattsun has a nice program for rainy weather weather and goes flat to capture the scattered light in the clouds. It is doing this now as we got a couple inchs of rain last night.

rgk1

Re: Still confused on panel angle

The basis for my original question was half wrong. I didnt know if I should point the panels from straight overhead toward the horizon the appropriate degrees or (incorrectly) start the degree calculation from the horizon and tilt upward the from there. As I understand it now, assuming a fixed angle year round, I would want to point directly toward where the sun is during the equinox. Since I am 35 degrees north of the equator, then I am really just correcting by tilting the panels toward the south those 35 degrees. At the peak of summer, the sun would be 23.5 deg higher or about 12-13 deg from being directly overhead for me. Does this sound right?

niel

Re: Still confused on panel angle

i think he's got it.:D

ggunn

Re: Still confused on panel angle

rgk1 wrote: »

The basis for my original question was half wrong. I didnt know if I should point the panels from straight overhead toward the horizon the appropriate degrees or (incorrectly) start the degree calculation from the horizon and tilt upward the from there. As I understand it now, assuming a fixed angle year round, I would want to point directly toward where the sun is during the equinox. Since I am 35 degrees north of the equator, then I am really just correcting by tilting the panels toward the south those 35 degrees. At the peak of summer, the sun would be 23.5 deg higher or about 12-13 deg from being directly overhead for me. Does this sound right?

Tilt them 35 degrees toward true south from laying flat on their backs, correct. Then the modules will point directly at the sun on the equinoxes (equinoxen?) at solar noon. This may or may not maximize your output, though. If it is chronically cloudy where you are in the mornings or afternoons it will affect the best azimuth orientation, or in the summer or winter it will affect the optimum tilt. Run a few simulations through PVWatts for your location to see, though read the caveats carefully. NREL quotes a somewhat pessimistic +/- 10 to 12% confidence envelope around their results, so if you are seeing a 1% variation (for example), that really is no difference, statistically speaking.

rgk1

Re: Still confused on panel angle

I have played with PVWatts quite a bit and unless I move way East or West, or go beyond a 20 or 50 degree tilt, I can't see more than about 5% variance over the course of the year. I will order my 2nd KD135 panel on Monday and that will give me 270w on the panels and probably be too much at times for my CC. I figure the fixed angle won't make much difference in my case. This is mostly an interesting hobby, project, learning experience for me. It's keeps a few convieniences going overnight is case of an outage and I don't have to run electrical from the house to the shed for interior lights, outside dusk to dawn light, charge various power tools and the kids toys. Right now the system will easily put back anything I take from it on a daily basis. I am sure I will find other things to run off of it. I have put in some components that are unnecessary now, but hey, when I first started reading the forum everyone said plan for expansion as the loads tend to grow.