Thursday, November 25, 2010

I have finally got the psa algorithm to match the NREL code. Not really sure why I got the errors in the previous post but I suspect it was to do with the inputs I gave the NREL calculator, and not with the simpler psa code. Anyway now I'm getting results that match within ~0.5deg. However I am still having issues with my rig drifting out of alignment over time.

I did a little experiment today and set up a sundial with a thin vertical stick in the centre. I marked the position of the shadow with the current calculated azimuth value and waited for the calculated angle to move 30deg (2hours from 1:30 to 3:30).

The rig tracked nicely around 30deg but had over-rotated and was casting a shadow. When I came to measure the new shadow position on the sundial it was only about 20deg from the old mark... So the results that I'm getting from both NREL and the psa algorithms aren't matching what's happening in the real world.

I have a new piece of code from gabriel over at, which works from tabulated data. I'll give that a go and hope for the best.

Finalised Circuit in Place

I've just finished transferring my prototype bread board circuit to a more permanent soldered perf-board setup. Been having issues with bad connections here and there so this should finally sort that out.

Also added a circuit diagram page done quickly in publisher.

Monday, November 15, 2010

Sun Position Calc Error Checking

I've run through a little accuracy test of the sun position code I'm using right now, as provided by Mowcius. To check the accuracy I compared results calculated by Mowcius' code and the NREL online solar position calculator solpos (uncertainty of +/- 0.01 degrees).

Results were calculated every 10 minutes for the 11th of November using location data - Longitude = 172, Latitude = -39.

Here is the plot showing the error during the day:

Turns out Mowcius' code is not extremely accurate. But the errors are somewhat consistent and the results for each calculation could be improved, for this day at least, by adding constants (Elevation -2deg and Azimuth +4deg say). Hopefully I can track down a more accurate position calculator, but right now at least I know how much error to expect from the calculation.