Solar Radiation Graphs

[J.P.M.]

I hear you. Wished I had known you lived in SD back in 2016. I was there late March 2016. We stayed in a area you might know of or about where it is. We stayed at a old friend of mine house from the Navy that works for Northrop Grumman in El Cajon. It is on a hill and was an old Stage Couch Station. Just off the intersection of Washington and Anza. Fantastic view of the city. He owns most of the property on top of the hill. The the old Stage Couch Station is still there and historically protected.

a hill ~~ 3/8 of a mile ESE of the intersection ? Got some structures on top of it ? Lots of single lane roads ?

Know where it is but don't get there much. Looks like kind of an eclectic neighborhood.

[Sunking]

Around San Diego, being a weatherman means being able to say "cloudy early, then sun".

I hear you. Wished I had known you lived in SD back in 2016. I was there late March 2016. We stayed in a area you might know of or about where it is. We stayed at a old friend of mine house from the Navy that works for Northrop Grumman in El Cajon. It is on a hill and was an old Stage Couch Station. Just off the intersection of Washington and Anza. Fantastic view of the city. He owns most of the property on top of the hill. The the old Stage Couch Station is still there and historically protected.

[J.P.M.]

My bad and thanks for the correction. Solar South = Solar Noon down here. Where I live we point the panels straight up and level it. Pour a glass ow water on the panel, tap the south side of the panel with the bracket loosened up a bit, until the water just drains off, and tighten it down Perfect. No DST down here or anywhere south of TX has DST that I know of. In the USA daylight hours vary significantly from Summer to winter. Down here daylight hours do not vary much from Wet to Dry season. Err I should say Wet to Less Wet season. Almost exactly 1 hour difference. Shortest day of the year is 11 hours and 36 minutes, longest day is 12 hours and 39 minutes.

Being a Weatherman is an easy job here. Forecast for every day is low in the mid 50's, highs in the mid 80's with a chance of rain. It is spring everyday Panama City is even easier to forecast Hot and humid with a chance of rain. About the only thing that changes is humidity. Wet season is sticky, and dry season is less sticky and humid.

Remember when 25 cents would by a cup of coffee and a conversation at any dinner? That still exist down here and we have the best coffee in the world.

RUA ! (Read, Understood, Acknowledged). No correction, just clarification. I learn more terms every day. Never knew coke meant any flavor of soda until I spent some time in AL w/in-laws. DST doesn't usually enter into the geometric calcs/solar position algorithms until the end, if at all.

Around San Diego, being a weatherman means being able to say "cloudy early, then sun".

[Sunking]

One common term in use for that time is "solar noon".

My bad and thanks for the correction. Solar South = Solar Noon down here. Where I live we point the panels straight up and level it. Pour a glass ow water on the panel, tap the south side of the panel with the bracket loosened up a bit, until the water just drains off, and tighten it down Perfect. No DST down here or anywhere south of TX has DST that I know of. In the USA daylight hours vary significantly from Summer to winter. Down here daylight hours do not vary much from Wet to Dry season. Err I should say Wet to Less Wet season. Almost exactly 1 hour difference. Shortest day of the year is 11 hours and 36 minutes, longest day is 12 hours and 39 minutes.

Being a Weatherman is an easy job here. Forecast for every day is low in the mid 50's, highs in the mid 80's with a chance of rain. It is spring everyday Panama City is even easier to forecast Hot and humid with a chance of rain. About the only thing that changes is humidity. Wet season is sticky, and dry season is less sticky and humid.

Remember when 25 cents would by a cup of coffee and a conversation at any dinner? That still exist down here and we have the best coffee in the world.

[J.P.M.]

Just a note here. Magnetic South does not equal Solar South. You want Solar South. A one-hour time zone is 15 degrees.

I'm not sure how the term solar south is defined, but if it means the time when the sun crosses the meridian of the site's local longitude, that is, when the azimuth angle of the sun at a location is 180 degrees, that time will vary for each day of the year and can be conceptualized or graphed as something called the analemma of the sun. It's that funny looking figure 8 you used to see on some globes. Also check out "analemma + Stanford Solar Center" on Google for some neat pictures and some text.

One common term in use for that time is "solar noon".

Relative to civil clock time (and leaving D.S.T. adjustments out of it for this discussion) which is partially based on based on each day being exactly 24 hours in length, and a non elliptical orbit, the variation in the time when solar noon occurs is mostly caused by the slightly elliptical nature of the earth's orbit, and to a lesser degree by the wobbling of the earth on its axis (like a top) called precession, and some gravitational effects from the moon and the sun.

The magnitude of the annual variation of solar noon is about 30 minutes or so, from earliest to latest, with the variation being asymmetrical about civil clock mid day (noon) time and the degree of asymmetry f(longitude). The actual daily change is of the order of a few seconds/day.

Example: yesterday (12/16/2017) at my location, solar noon was at 11:44:17 P.S.T. and some fraction of a second. Today it was at 11:44:45 P.S.T.and some fraction of a second.

The earliest solar noon at my location occurs at 11:31:54 P.S.T. on 11/01 to 11/03.
The latest solar noon at my location occurs at 12:02:37 P.S.T.on 01/10/ to 02/02.

Those dates also wobble around by less than about 24 hours or so over a few years +/- some. .

Consult the nautical almanac for details.

All that and 5 bucks or more will get you a Starbucks.

[Sunking]

Just a note here. Magnetic South does not equal Solar South. You want Solar South. A one-hour time zone is 15 degrees.

[J.P.M.]

I knew that you knew the difference so much that you assumed everyone else did .
I learned in land navigation class in high school but we see it all the time with customers though usually they are just guessing anyway occasionally they use their smart phones which tend to default to megnatic north.

Point taken. But in the same assumptive sense, I also assume people know their right from their left. I guess I'll plead guilty to selective assumption ranting. My rant mode about things falling into a state of idocracy gets the best of me more often than is politically correct.

Lots of ways to find true north. Many have no need of a compass - just a sunny day.

[ButchDeal]

True, but a sad comment on the times that it even needs to be brought up. I was shooting azimuths with a compass and correcting for magnetic declination when I was 10 or less, and so were my peers.

I knew that you knew the difference so much that you assumed everyone else did .
I learned in land navigation class in high school but we see it all the time with customers though usually they are just guessing anyway occasionally they use their smart phones which tend to default to megnatic north.

[J.P.M.]

Note that there is a difference between magnetic and true. True is the azimuth you need to use not magnetic. Most smart phones have a setting to use true instead of magnetic for the compas app.

True, but a sad comment on the times that it even needs to be brought up. I was shooting azimuths with a compass and correcting for magnetic declination when I was 10 or less, and so were my peers.

[ButchDeal]

As for surface azimuth angles, 180 is dead (true) south. 90 is east, 270 is west. Zero is due north.
.

Note that there is a difference between magnetic and true. True is the azimuth you need to use not magnetic. Most smart phones have a setting to use true instead of magnetic for the compas app.

[J.P.M.]

I have played a little with PVWatts in the past to try and learn it, but have not tried to compare with actual field readings.

I do have a question about tilt angle that I get readily confused by, and I think it has to do with how it may be defined or used differently by different sources on the internet. Is tilt angle measured from the horizontal from the top of the platform? Horizontal from the base of the platform? Or from the vertical?
Every time that i think I understand, then I get confused again, depending on the article that I read.

A common situation. and confusion.

Zero tilt is horizontal, or more accurately, a plane that is normal or vertical to the direction of gravity.
90 deg. "tilt" is a vertical surface like a common wall of a house.
If on an untilted surface (level, flat ground), an array or flat surface tilted at, say, 10 degrees will be "mostly" horizontal". A surface sloped at 80 degrees will be "mostly" vertical.

As for surface azimuth angles, 180 is dead (true) south. 90 is east, 270 is west. Zero is due north.

Tilt and azimuth are both necessary to specify array orientation.

[citabria]

I have played a little with PVWatts in the past to try and learn it, but have not tried to compare with actual field readings.

I do have a question about tilt angle that I get readily confused by, and I think it has to do with how it may be defined or used differently by different sources on the internet. Is tilt angle measured from the horizontal from the top of the platform? Horizontal from the base of the platform? Or from the vertical?
Every time that i think I understand, then I get confused again, depending on the article that I read.

[J.P.M.]

OP:

If you're interested, 2 things:

1.) Google "NREL + Bird" and download the Richard E. Bird Clear Sky Broadband Solar Radiation Model. I've found it easy to use and FWIW, seems to give a pretty good fit with my measurements. It also allows some input if you know/guess atmos. constituents like precipitable H2O, ozone, and other stuff.

2.) Download and run PVWatts for Silver City, NM. I'd use Deming - close enough. Use a horizontal array orientation (zero tilt angle) and get/choose the hourly output option. If you then look for clear days, which are those that are symmetric about noon and have high values of P.O.A. (Plane of Array) irradiance, which will be the same as the G.H.I. (Global Horiziontal Irradiance), you'll get another approximation of clear sky insolation on an hourly and daily basis.

If you compare the two models you'll find they are a pretty good indicator of potential clear sky GHI and will probably agree with one another to a pretty good degree.

I lived in NM for several years. About the best solar climate on the planet.

[J.P.M.]

Sensij: Thanx.

J.P.M.

[sensij]

Here is an image snip of the pdf.

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