Solar Radiation Graphs

[citabria]

I just wanted to show the variance in solar radiation and insolation, for June and December, 2017 for my approximate location. Data was collected from one weather station at weather underground. I snipped pieces of the html page using a previously unknown windows program called (snipping tool).

[sensij]

Yeah, the snipping tool was one of my favorite Windows 7 improvements.

Can you explain your thinking with this statement:

Figure 3: December 10, 2017 - cloudy day. The clouds are interfering to the point that calibration scale error may be a factor in the printed data. It is truncated at about 600 watts/m2 , but from the previous day we see the peak radiation at about 600 watts/m2 .

[citabria]

I'll try to explain my comment on Figure 3 by comparing it to Figure 2. On Figure 2, we see a nice un-interrupted graph that peaks at just over 610 watts/m^2. Now looking at Figure 3, we see the abrupt interference of the cloud cover and the curve is just under the 600 watts/m^2, and the curve while squiggly, sort of remains there from about 11 am through 2 pm. That is basically all that I noticed and commented on. I do not know anything about the quality of solar radiation detector or weather station to know what is happening as the cloud cover comes and goes.

[sensij]

Ok. I don't think there is any particular reason in the data to doubt the quality of the station. On partly cloudy days, you can get reflections off the sides of clouds that increase the local irradiance above what clear sky levels would be for the same conditions (at the expense of areas that are getting blocked by the clouds). That is what can cause the spikes up to 800 W/m2.

[J.P.M.]

For some reason I can't load the pictures.

[sensij]

Here is an image snip of the pdf.

snip.JPG

[J.P.M.]

Sensij: Thanx.

J.P.M.

[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.

[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.]

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.

[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.]

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]

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.]

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.

[Sunking]

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