ONE-AXIS TRACKING WITH SOLAR PV ARRAYS: SURFACE TILT VERSUS SUN ALTITUDE

My starting assumptions are:

1. Location in  Northern Hemisphere, flat ground.
2. Solar pv panels face due South at all times. For the moment I'm assuming an isolated  solar PV array with no shading effects present (no row-to-row shading effects, or any other shading).
3. One axis tilting of the array follows only the altitude of the Sun above the local horizon, during the local day time.

I notice that, for some reason, the SAM algorithm assumes that the tilt angle (relative to the horizontal) for the solar pv panels never exceeds 45 degrees - while the Sun altitude is much less than 45 degrees for much of the day time, particularly in the winter months, and is always 0 degrees at Sun-up and Sun-down. 

What is the reason for the SAM algorithm assuming that the tilt angle is never more than 45 degrees (corresponding to a Sun altitude of about 45 degrees, when doing one axis tilt simulations)?

I can understand making a preliminary initial design assumption like this when you are simulating a system consisting of a large number of solar panels arranged in two or more rows of equal length (aligned in at East/West  direction) and with some row-to-row shading present on account of site area limitations. In practical large size systems, some row-to-row shading will always be present - even if this is small enough to result in an output loss of (say) only 1% of the "zero shading" output. 

On the other hand, when you are simply making basic performance comparisons between a system with one-axis tracking versus one having the tilt angle fixed (but possibly allowing for manual adjustment of the tilt angle at intervals during the year), it helps if you can do simulations for the one-axis tracking case with the 45 degree tilt angle limitation absent (e.g. allow the tilt angle to be 85 degrees when the Sun is at 5 degrees above the horizon. etc. ) - in order to get valid comparisons with the simulation results for fixed tilt angles.

Is there any way to bypass the 45 degree tilt angle limitation when doing SAM simulations involving one axis tracking?

Robert T. Chisholm

Hi Robert,

You can change the value of Tracker rotation limit on the System Design page. It is set to 45 degrees by default, but you can set it to a maximum value of 85 degrees to minimize the effect of the limit.

Best regards,
Paul.

OK, Paul, great - and many thanks!

Regards, Robert

Hi Paul,

What is the Tracker rotation limit considered in PVWatts as there is no option for this in PVwatts?

Hi Abbas,

PVWatts v8 assumes a rotation limit of 45 degrees for the one-axis tracking option.

Best regards,
Paul.