Local min. incidence angle.

**randomuser** · 04-26-2016, 02:24 PM

Thanks, good info to know JPM. Temps are cool this week, which is nice but dang, partly cloudy the next few days with forcast of rain on Thursday. When is the other time of year we should look forward to in San Diego?

**J.P.M.** · 04-26-2016, 02:49 PM

Originally posted by randomuser

Thanks, good info to know JPM. Temps are cool this week, which is nice but dang, partly cloudy the next few days with forcast of rain on Thursday. When is the other time of year we should look forward to in San Diego?

You're welcome. Take it for what it's worth. To reiterate: that's a time, not an entire day. If it's very clear, system output in Watts MAY be close to system max. mostly because the incidence angle at that time will be quite small with a correspondingly large cosine (close to 1.0), and the summer heat has not developed to any large degree., meaning the array temp. might be low(er) relative to the higher plane of array irradiance.

I Haven't worked the second date/time for my array, but it'll be sometime about equidistant datewise on the other side of the summer solstice, or around the middle of August or so fort a few days.

**azdave** · 04-26-2016, 05:41 PM

East of you about 350 miles in Phoenix here...

Yesterday was one of my best collection days but it was because it was extremely windy here in Phoenix and cloud-free. My panels were running nice and cool all day. By the end of May, the increasing daylight does not outweigh the lower efficiency I get due to higher panel temperatures. In a few days, my banked energy will be zero'd (account credit at wholesale). It's the worst time of year to zero the account but that is the fiscal year end for my POCO and it can't be changed. I fill the bank all I can in May because we still have some cool days and I don't typically turn on the air conditioner until June. Last year I made it through the summer without any out-of-pocket costs using banked energy and account credits to reach September.

**sensij** · 04-26-2016, 11:36 PM

Thanks for starting this thread. About a month ago, I had wrote to a friend:

No, the conversion from horizontal irradiance to plane of array irradiance is a bit complicated and requires some modeling on the fraction that come directly from the sun and that which is from diffuse sources (atmosphere and ground reflection). NREL has a spreadsheet to do the calculations:

Bird Clear Sky Model

http://rredc.nrel.gov/solar/models/clearsky/

According to it, I should have measured around 904 W/m2 GHI at noon today, pretty darn close. It says the GHI I measure should peak at around 1025 W/m2 around June 9. The zenith angle then is 10 deg, really, the best day for my system is probably closer to April 30, when the zenith angle at solar noon matches the 18.5 deg tilt of my south facing panels. If it is clear then, the GHI I measure should be around 1000 W/m2.

My GHI estimates appear to be high... at 971 over the weekend (compared to 950 at Boulder Knolls), it doesn't look likely to get to 1000, so I've probably got some assumptions wrong in the Bird spreadsheet.. I've had some significant events that required my attention, but I love that the data are collecting every day for review and eventual processing to take the next step forward in my understanding.

**J.P.M.** · 04-27-2016, 11:18 AM

Originally posted by J.P.M.

For a lot of reasons, this is one of 2 times/year when I look for and note the time of minimum, that is, ~ 0 degree incidence angle on my array.

For those in San Diego: If you have an array, and it faces south at ~~ an 18 deg. tilt (4/12 pitch), the incidence angle on your array will be close to zero for the next few (4 or 5 days maybe) at about 12:45 P.M., Pacific Daylight Time. Move that time back by about 4 min. for every degree of array azimuth to the east of south and move it forward by about 4 min. for every degree of array azimuth west of south.

If it's clear and not too warm with a breeze, this is the time of year in this area where arrays with the orientation described will probably have the best chance of maximum power output at the time described. That information may be useful to some.

Note to Sensij FYI and FWIW : If I have your data correct, the incidence angle on your array will be ~~ 0.13 degrees on 04/27/2016, and ~~ 0.19 degrees on 04/28/16 at 12:45 P.D.T. each day. One small point: Those estimated angles include very approx. adjustments for atmos. parallax. This time of year my array temp. runs about 24 deg. C. warmer than the roof ambient air temp. under full sun w/ave, wind velocity ~ 2m/sec. and ~ 10" clear space under the array.

In checking my above post from yesterday, I discovered an error: The time adjustment for minimum incidence angle as f(array azimuth) is NOT 4 minutes per deg. of array azimuth change away from true south as I wrote in that post.

At this approx. latitude and time of year, the minimum incidence angle time adjustment for arrays with off south orientation of up to ~ 15 deg. or so with relatively shallow tilt angles of ~ 20 deg. or so should be approximately 1.25 minutes (that is, ~ 75 seconds) per degree of array azimuth change, earlier than 12:45 P.D.T. for eastward array orientation, later for westward orientation.

For example, my array azimuth is 195.75 deg. My min. incidence angle time is approx. 13:06 P.D.T. today (04/27/2016). Solar noon on 04/27/2016 at my location is ~ 12:46:15 which would be minimum incidence angle time if my array faced true south, or 1.27 min./deg. of array azimuth change away from true south.

My error was jumping to using the hour angle adjustment of 4 min./degree as the azimuthal angle change instead of thinking.

I apologize for any wasted time and confusion my error may have caused. Not an excuse, but when working for a living, someone, or several others were always checking my work and vice versa. Dumb stuff errors like this were less frequent.

Local min. incidence angle.

Local min. incidence angle.

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