Author Topic: Polar Axis Sun Tracker Project  (Read 7853 times)

Gabriel

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Polar Axis Sun Tracker Project
« on: December 20, 2009, 01:15:59 PM »
I couple of weeks ago, I received an e-mail from Starsailor asking what my thoughts were on solar trackers. As it turns out, I had recently gotten my hands on some solar cells and was planning on putting them together and seeing if I could track the sun myself to help increase their output.

I won't go into the details of the various emails that we sent back and forth, but I will show some of the experiments I did in Sketchup to test out the polar axis sun tracker that I had wanted to build.

If you don't know what a polar axis sun tracker is, go to this Wikipedia link and scroll down. http://en.wikipedia.org/wiki/Solar_tracker


I'll start by showing a picture of a simple sun tracker that I drew in Sketchup. Notice how there is a rectangular block that sticks out of the front of it.


With Sketchup, it is possible to accurately simulate the shadows cast by the sun during any time of the year or any location on Earth. This means that it is pretty easy to tell if the sun tracker is pointing at the sun or not by looking at the shadows cast by the rectangular block.

To open the shadow window, go to the menu bar, click window, and go to shadows.
To set your latitude and longitude, go to the menu bar, click window, and go to model info.

Notice that there isn't any shadow in this picture. This means that the sun tracker is pointing directly at the sun.



In this picture though, you can see a shadow, which means that the sun tracker isn't pointing at the sun.



I first simulated the sun tracker's behavior during the winter solstice, the shortest day of the year. In order to do so, I needed to make the angle of the sun tracker the same as the sun's declination at this time of the year. Here is a table of the sun's declination values. http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html

Here is the sun tracker lying flat.



The latitude where I live is about 39 degrees, so I first rotate the sun tracker forward 39 degrees to compensate.



The sun's declination during the northern hemisphere's winter solstice is about -23.5 degrees, so I will next rotate the sun tracker forward 23.5 degrees more.



The basic idea for this sun tracker is for it to rotate 360 degrees every day along the polar axis, or 15 degrees per hour. This means that it will end up in the same spot at noon every day. This picture shows the axis that it turns on from above.


In this picture, you can see that it is facing directly at the sun at noon on the winter solstice, which is good.


I moved the sun forward 3 hours and rotated the sun tracker 45 degrees. You can see how a polar axis sun tracker is doing a pretty good job of tracking the sun on this day. There is a little bit of a shadow up at the top, but, for the most part, it is pointing right at the sun.



I next tried the experiment out during the summer solstice by first changing the declination angle to +23.5 degrees.

At noon, there wasn't any significant shadow, which meant that the sun tracker was pointing directly at the sun.

I then moved the sun forward 6 hours and turned the sun tracker 90 degrees. Here you can see that there is a fairly long shadow.



The fact that the shadow was so much longer confused me for awhile. I had thought that the polar axis sun tracker more accurate than that.

I decided to try another approach. I next drew lines from the sun to the sun tracker using the Sketchup Sun Position plugin.  http://cerebralmeltdown.com/projects/sunplugin/default.htm


This made it a little bit more obvious why the sun tracker wasn't pointing at the sun. It might be kind of hard to tell from the picture, but the sun's azimuth has increased so much that it is basically behind the sun tracker.


Here is another angle. The line from the sun to the sun tracker is highlighted in blue. The line also looks like it agrees perfectly with the shadow.



It looks like a polar axis sun tracker is only so accurate. I still think that it will work well with solar panels, so I'm still planning on using, especially because it is so simple. It doesn't look like it would work very well for everything though. A solar concentrating parabolic dish, for example, needs to be more accurate than that for it work correctly.

Once I actually get the chance to build this thing, I will have to put up some pictures. It could be awhile though before I have the time for it, so, for now, I'll just have to play around with this cad model to keep myself occupied.  ;D
« Last Edit: December 20, 2009, 01:20:19 PM by Gabriel »


Starsailor

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Re: Polar Axis Sun Tracker Project
« Reply #1 on: December 21, 2009, 03:08:28 AM »
very nice post Gabriel, thank you very much!
unfortunately during the summer solstice the Azimuth changes don't follow a regular rotation and that's more evident as you move further from noon. so that's probably why at 6 PM you had all that shadow.

Anyway that still looks like a relatively small error if you plan to use the tracker with a solar panel.
I have to make my tracker work as an heliostat so I had to make the microcontroller calculate the Julian Day, then the Declination and finally the Azimuth. so far using a PIC16F887 I am using about 90% of the available ROM and I still have to finish the program that increments the solar Azimuth every 3-4 minutes (with its precise value), I tried to save all the different values on a 4k EEPROM external memory that I interfaced with the PIC I2C module but I am having some trouble during the reading routine. I feel like I'm pretty close to the end of the program but I'll probably have still a lot of work to do before this looks like a sellable product.

I'll post here the pictures of my project as well. thanks again Gabriel!


  
« Last Edit: December 21, 2009, 03:10:40 AM by Starsailor »


trevorb

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Re: Polar Axis Sun Tracker Project
« Reply #2 on: August 06, 2013, 07:22:06 PM »
I have looked at polar trackers, I think in the fifth picture down, you may need to set the pin 23.5 degree angle clockwise instead of tilting the entire axis.  in this way, I believe you will get the proper and correct result with no shadow.

Gabriel

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Re: Polar Axis Sun Tracker Project
« Reply #3 on: August 08, 2013, 10:20:47 AM »
Yep you're right! If you look at the attached image, you'll see what trevorb said in pictures. Basically the pole that the surface attaches to (which rotates to follow the sun) is at the same angle as the machine's latitude. On the left, the surface is turned back 23.5 degrees for the summer solstice. In the middle it is in the equinox position, 0 degrees. And on the left it is in the winter solstice position, or -23.5 degrees.

In these positions, the surface will indeed point directly at the sun as you rotate it.

I guess what I was originally thinking of was a single axis tracker more than a polar tracker. I don't know, it was 4 years ago, and I've learned a lot since then. :)

It does seem as though most of these types of trackers aren't designed with a tiltable surface such as this one. I wonder why that is? I guess it's just to help keep costs down.

trevorb

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Re: Polar Axis Sun Tracker Project
« Reply #4 on: November 11, 2013, 07:04:08 PM »
Ha, did not realize your original work was from 2009.