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  • Confusing Test Question

    Hello All - I am new to the forum and this is my first post. If I'm in the wrong category I apologize up front. Anyway, I downloaded a practice test for the nabcep exam and this one question has me stumped. Either the answer key is wrong or I'm unable to grasp why the answer is correct. So any help understanding this is much appreciated. Below is the question (from pvstudent.com) and the answer key shows C - 6 months as correct. I answered A - 0 months. I answered 0 months because the most north latitude the earth will tilt directly at the sun is 23.5deg during the summer solstice. If the panels are above that at 38 degrees and completing upright and facing away from the sun, how do they get any light hitting the panels?

    If you had modules facing directly north and oriented vertically (90 degrees tilt) would
    there be a time of the year when the sun rays would hit the panels at 38 degrees latitude?
    If so, how many months of the year would the sun hit the north facing modules?
    a. 0 months
    b. 3 months
    c. 6 months
    d. 9 months

  • #2
    Your thinking is correct at noon, but the sun will come up and set quite a ways farther north at 38 deg latitude. I'm at 35 deg and a vertical panel would see a bit of sun at sunrise. Still its a pretty idiotic question and its disappointing to see NABCEP coming up with stuff that is of no practical value. The test is probably looking for answer a) although I'd say answer b) is correct according to the way the question is stated.
    BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed

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    • #3
      I think the answer is 6 months. No matter what the latitude, the spring Solstice is the point of equal daylight with daylight increasing over 12hours until the summer solstice and tapering off to the fall solstice. The farther north one goes the more the extreme to the point where the sun never sets on the north pole during peak summer . With a vertical fixed array the angle of incidence to the panel is poor so the actual generation is low. I have several tracking arrays near me and they start their day during summer facing roughly Northeast tilted full vertical and end their day facing NW full vertical at the end of the day. They track to a programed location based on day and latitude so even if the horizon is obscured they still go to where the sun is supposed to be. My guess with low sun angle the irradiance is poor during this early morning and late evening generation but it is generation. As importantly, it looks cool as the economics of a tracking solar array with a market full of cheap panels is extremely poor except for the gee whiz "my array is better than yours factor"
      Last edited by peakbagger; 07-19-2021, 02:26 PM.

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      • #4
        Ahh..OK. So if I'm understanding this, because the earth is a sphere and tilted, solar noon is actually our most southern apparent position of the day relative to the sun. Sunrise and sunset are the most east and west but also the most NORTH of the day relative to the sun. Is this correct? I also agree the question is a bit much especially without a sun path chart. Unless the tester is referring to a formula for this?

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        • #5
          Sorry I was rewording it prior to your post.

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          • #6
            Thank you to everyone for your help. I look forward to being part of the forum!

            Comment


            • #7
              Charts and sun path diagrams are not necessary - just some critical thinking skills.

              For the latitude and panel orientations given in the problem, the question becomes: At what times (or days) of the year is the solar azimuth angle less than 90 degrees (and so greater than 270 degrees).

              That condition will exist for any day when the sunrise azimuth angle is less than 90 degrees (or when the sunset azimuth angle is greater than 270 degrees). Since those conditions occur at the equinoxes and equinoxes are 6 months apart, the answer to the question is "C".

              It's also a sloppy question. For one thing, what if my panels were at 38 degrees south latitude ?

              Also, the conditions given in the question as worded will exist at any latitude for half a year making the 38 degree latitude condition either a trip wire or confirming the ignorance on the part of whoever came up with the question.

              Another example of how our educational system has gone in the toilet. Moron formula pluggers and keyboard jockeys leading the ignorant.

              Welcome to the forum of few(er) illusions.
              Last edited by J.P.M.; 07-23-2021, 01:12 AM.

              Comment


              • #8
                Originally posted by J.P.M. View Post
                Charts and sun path diagrams are not necessary - just some critical thinking skills.

                That condition will exist for any day when the sunrise azimuth angle is less than 90 degrees (or when the sunset azimuth angle is greater than 270 degrees). Since those conditions occur at the equinoxes and equinoxes are 6 months apart, the answer to the question is "C".
                I’m going to go for less days months, with B.

                Last year during my morning walks, I was expecting the sun to rise at 90 and set at 270, but it didn’t. The day was 12 hours long, but the sun was still a few degrees off (10 degrees?) of the 90 and 270 azimuths. Had a bit to do with the 32 degree latitude I’m at. I tried googling some formulas to see if I could make a spread sheet for myself.

                My interest in this was all the solar panels on the Northern side of the roofs installed in my neighborhood. North facing panels seem like a waste, but perhaps it catches enough early morning and late afternoon energy in my neighborhood to keep ACs powered earlier.

                The more I thought about the sun being off the azimuth at the equinox, the more it made sense to me that the because of the earths rotation and “Straight up” not being straight at the sub compared to your latitude and the earth turning, the sun would not rise and set at 90 and 270 at the equator and the extreme Northern latitudes.

                There’s probably a formula for this, but I’m not sure what it is,

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                • #9
                  For a thorough dive into solar tilt angles go to solarpaneltilt.com. And no, its not as simple as tilt the panels at whatever your latitude angle is...
                  BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed

                  Comment


                  • #10
                    Originally posted by chrisski View Post

                    I’m going to go for less days months, with B.

                    Last year during my morning walks, I was expecting the sun to rise at 90 and set at 270, but it didn’t. The day was 12 hours long, but the sun was still a few degrees off (10 degrees?) of the 90 and 270 azimuths. Had a bit to do with the 32 degree latitude I’m at. I tried googling some formulas to see if I could make a spread sheet for myself.

                    My interest in this was all the solar panels on the Northern side of the roofs installed in my neighborhood. North facing panels seem like a waste, but perhaps it catches enough early morning and late afternoon energy in my neighborhood to keep ACs powered earlier.

                    The more I thought about the sun being off the azimuth at the equinox, the more it made sense to me that the because of the earths rotation and “Straight up” not being straight at the sub compared to your latitude and the earth turning, the sun would not rise and set at 90 and 270 at the equator and the extreme Northern latitudes.

                    There’s probably a formula for this, but I’m not sure what it is,
                    Do as you wish but "B" is the wrong answer for reasons I explained. In the northern hemisphere, a surface oriented as in the problem will see some irradiance from the day of the vernal equinox until the autumnal equinox, or half the year. The daily total irradiance will be smallest at the equinoxes and greatest at the summer solstice (but even at the solstice it won't be much because of the panel orientation.

                    As for a spreadsheet, NOAA has done that. See: "NOAA_Solar_Calculations_Year.xls".

                    Or, to do your own, consult any decent solar textbook. Duffie and Beckman is the bible in this area. The required calculations are not complicated but rather involved.

                    Even if net metering is available, or even if it's not, your notion of waste is correct. For most any likely application it's a waste to face panels away from the equator.

                    Your notion of catching early and late day irradiance by facing panels away from the equator is also a waste of time, material and money, mostly because of two factors: The short times of panel irradiance and the low incidence angles obtained in such panel orientations.

                    I do not understand your statement: "...the sun being off the azimuth at the equinox ...". Can you explain ?

                    Comment


                    • #11
                      Originally posted by J.P.M. View Post

                      Do as you wish but "B" is the wrong answer for reasons I explained. In the northern hemisphere, a surface oriented as in the problem will see some irradiance from the day of the vernal equinox until the autumnal equinox, or half the year. The daily total irradiance will be smallest at the equinoxes and greatest at the summer solstice (but even at the solstice it won't be much because of the panel orientation.

                      As for a spreadsheet, NOAA has done that. See: "NOAA_Solar_Calculations_Year.xls".

                      Or, to do your own, consult any decent solar textbook. Duffie and Beckman is the bible in this area. The required calculations are not complicated but rather involved.

                      Even if net metering is available, or even if it's not, your notion of waste is correct. For most any likely application it's a waste to face panels away from the equator.

                      Your notion of catching early and late day irradiance by facing panels away from the equator is also a waste of time, material and money, mostly because of two factors: The short times of panel irradiance and the low incidence angles obtained in such panel orientations.

                      I do not understand your statement: "...the sun being off the azimuth at the equinox ...". Can you explain ?

                      My mystery I'm trying to solve is a little different than the OPs question, and is why did the sun not rise at a true heading of 090 on the day of the equinox.

                      On my morning walks, the main roads all run true North/South or True East/West. A huge benefit to bein in the flat, desert Southwest. I've confirmed this off the USGS maps also.

                      Because of this, I was excited to walk on the equinox, thinking that the sun would rise directly down the road as I'm walking to the East. It did not it was about 10 degrees off. I don't know why. I think it's got to do with the fact of time lapse videos of a sunrise or sunset drawing a line that looks like the red ones:

                      Sunrise.jpg
                      I know that mountains can change the exact angle based off the height of the mountains, but my nearest mountains obscuring the sunrise are about 40 miles away and have a 700' difference in elevation.

                      When I was watching the sunrise last year, I expected the sun to rise and set at true 090, and set at true 270 and it didn't. I also expected the sun to be directly overhead at solar noon. I now understand because of my latitude why it was not directly overhead. Of course on the equinox the son won't be overhead at the same angle from the equator to the poles.

                      I can think of no real reason to put North Panels in the Northern Hemisphere also, but there are enough in my neoghborhood, I'm hoping the installer did not just put them on to meet a customer requirement for "I want XXXX kw of panels on my roof.

                      Comment


                      • #12
                        Originally posted by chrisski View Post


                        My mystery I'm trying to solve is a little different than the OPs question, and is why did the sun not rise at a true heading of 090 on the day of the equinox.

                        On my morning walks, the main roads all run true North/South or True East/West. A huge benefit to bein in the flat, desert Southwest. I've confirmed this off the USGS maps also.

                        Because of this, I was excited to walk on the equinox, thinking that the sun would rise directly down the road as I'm walking to the East. It did not it was about 10 degrees off. I don't know why. I think it's got to do with the fact of time lapse videos of a sunrise or sunset drawing a line that looks like the red ones:

                        Sunrise.jpg
                        I know that mountains can change the exact angle based off the height of the mountains, but my nearest mountains obscuring the sunrise are about 40 miles away and have a 700' difference in elevation.

                        When I was watching the sunrise last year, I expected the sun to rise and set at true 090, and set at true 270 and it didn't. I also expected the sun to be directly overhead at solar noon. I now understand because of my latitude why it was not directly overhead. Of course on the equinox the son won't be overhead at the same angle from the equator to the poles.

                        I can think of no real reason to put North Panels in the Northern Hemisphere also, but there are enough in my neoghborhood, I'm hoping the installer did not just put them on to meet a customer requirement for "I want XXXX kw of panels on my roof.
                        Well you know what the Hopi Indians think if the sun is not in the right place at sunrise?

                        Comment


                        • #13
                          Originally posted by chrisski View Post


                          My mystery I'm trying to solve is a little different than the OPs question, and is why did the sun not rise at a true heading of 090 on the day of the equinox.

                          On my morning walks, the main roads all run true North/South or True East/West. A huge benefit to bein in the flat, desert Southwest. I've confirmed this off the USGS maps also.

                          Because of this, I was excited to walk on the equinox, thinking that the sun would rise directly down the road as I'm walking to the East. It did not it was about 10 degrees off. I don't know why. I think it's got to do with the fact of time lapse videos of a sunrise or sunset drawing a line that looks like the red ones:

                          Sunrise.jpg
                          I know that mountains can change the exact angle based off the height of the mountains, but my nearest mountains obscuring the sunrise are about 40 miles away and have a 700' difference in elevation.

                          When I was watching the sunrise last year, I expected the sun to rise and set at true 090, and set at true 270 and it didn't. I also expected the sun to be directly overhead at solar noon. I now understand because of my latitude why it was not directly overhead. Of course on the equinox the son won't be overhead at the same angle from the equator to the poles.

                          I can think of no real reason to put North Panels in the Northern Hemisphere also, but there are enough in my neoghborhood, I'm hoping the installer did not just put them on to meet a customer requirement for "I want XXXX kw of panels on my roof.
                          What day (date) were you observing the sunrise and what is the latitude and longitude of your location when you made those observations ? Also, how did you measure the solar angles, and how far off from 90 degrees was the solar azimuth ?

                          A couple of comments: Because everything in the sky is constantly moving, and because the earth is neither smooth nor completely spherical, and because of celestial and solar geometric realities and a bunch of other facts of life including precession, atmospheric refraction and some other tuff, and unless you believe in the alternate facts way of seeing reality, anecdotal observations can be poor indicators of reality.

                          I'll continue to put my faith in the U.S. naval observatory ways of doing celestial mechanics as well as the large body of open literature information found in the journals as well as a lot of time spent studying and measuring, and writing software and troubleshooting the code of others when writing about these sorts of things. Believe me or not as you see fit when I write that the (apparent) azimuth angle of the sun at its apparent location at sunrise on the day of a solstice is pretty close to, but not exactly 90 degrees anywhere on earth.

                          As for facing panels away from the equator, to the degree that happens is an indication of how folks are ignorant of the waste that such orientation incurs and have already swallowed the media's pap and B.S. about solar and alternate energy being the magic bullet to saving the planet, making them all softened up and primed for the "you could just throw a bunch of panels at your roof and 'POOF', you problems and electric bills are a thing of the past." that the solar peddlers feed their marks.

                          Comment


                          • #14
                            Originally posted by J.P.M. View Post

                            What day (date) were you observing the sunrise and what is the latitude and longitude of your location when you made those observations ? Also, how did you measure the solar angles, and how far off from 90 degrees was the solar azimuth ?

                            A couple of comments: Because everything in the sky is constantly moving, and because the earth is neither smooth nor completely spherical, and because of celestial and solar geometric realities and a bunch of other facts of life including precession, atmospheric refraction and some other tuff, and unless you believe in the alternate facts way of seeing reality, anecdotal observations can be poor indicators of reality.

                            I'll continue to put my faith in the U.S. naval observatory ways of doing celestial mechanics as well as the large body of open literature information found in the journals as well as a lot of time spent studying and measuring, and writing software and troubleshooting the code of others when writing about these sorts of things. Believe me or not as you see fit when I write that the (apparent) azimuth angle of the sun at its apparent location at sunrise on the day of a solstice is pretty close to, but not exactly 90 degrees anywhere on earth.

                            As for facing panels away from the equator, to the degree that happens is an indication of how folks are ignorant of the waste that such orientation incurs and have already swallowed the media's pap and B.S. about solar and alternate energy being the magic bullet to saving the planet, making them all softened up and primed for the "you could just throw a bunch of panels at your roof and 'POOF', you problems and electric bills are a thing of the past." that the solar peddlers feed their marks.
                            This will have to fall into the anecdotal evidence. I would like to better measure this next year. The day was the equinox that I measured. Lat and Long is pretty close to: 33.4942° N, 111.9261° W

                            I did use to do some surveying with theodolites 25 years ago when they first came out with digital compasses and laser range finders built in, so I do have some basic understanding. I'm positive the road I was walking down is oriented within 2 degrees of true North and South. As to how far off the sun was, that was a much less accurate guess of it actually rising well to one side of the road. Maybe it was a bit less than 10. A picture would have helped.
                            Last edited by chrisski; 07-23-2021, 05:49 PM.

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