Power Production vs. Time of Day for different sub-arrays

I have a simulation which has two arrays that are facing 180 degrees apart.  One faces generally Westward (Array 1), the other Eastward (Array 2).  I am using microinverters on each panel.

I would expect that the East facing array would generate power mostly in the morning and mid-afternoon, while the Westward facing one would generate power mostly in the mid-afternoon to early evening.

However, when I look at the "DC Gross Power" hourly data, I found a very curious thing.  Both arrays start and stop producing power at exactly the same time each day.  For example, at 6:00 AM on Aug 19, Array 1 and Array 2 both start producing energy.  Array 1 produces 0.12 kW, and Array 2 produces 0.45 kW.  This is not entirely surprising, as the sun rises in my location on that day at around 6:40 AM.

However, both array's output drops completely to zero in the 7:00 pm hour.  This is despite the fact that the sun doesn't set until nearly 8:20 PM. 

I would expect that either both arrays produce some energy throughout all the sunlight hours (sunrise to sunset) or that at least Array 1 shuts down after Array 2.

Is there something in the simulation that is causing this?  Or do I just have the wrong idea about how this should work?

There is not significant shading in the westward direction, so that's not the reason.

thank you.

Hi Ron,

Perhaps it was cloudy during that time, so the entire array's output dropped. You could check that by plotting "Irradiance DNI from weather file" to see if it drops during that time.

Best regards,
Paul.

Ron, I have a similar situation. Two arrays, one due east, one due west with the same number of modules, same tilt, same row spacing, etc. etc.

Both turn on at the same time and the east facing subarray ALWAYS shows higher power than the west facing subarray, with rare exceptions at the very end of the day. This happens throughout the year, so is not a clouding or some other astronomical effect. The subarrays also turn on and off at the same time. So something is not working right in how the irradiance is modeled on the panels. I saw on another forum, an article that described how the horizontal irradiance is used and how it does not apply well to E-W arrays.

I'd like someone with more knowledge of how the model works give us some insight into whether we should even be trying to use SAM for E-W arrays.

Thanks,

Les

Hi Les,

Attached is a .sam files for a 5 kW PV system configured with two subarrays. Both have a tilt angle of 20 degrees with Subarray 1 facing West and Subarray 2 facing East. The table below shows results for a system with a fixed array (no tracking) for January 1:

• In the morning, the East-facing subarray is exposed to more sunlight and generates more power than the East-facing array.
• At Noon, both subarrays generate about the same amount of power.
• In the evening, the West-facing array generates more power than the East-facing one.

You can see results for the entire year and for a one-axis tracking array in the .sam file.

If you have a file that shows different results, please attach it and I would be happy to review it.

Best regards,
Paul.

HI Paul,

I'm still working on this. Your file shows the same result as mine. The entire array shuts down at the same time, even though the west facing panels are producing at very high levels. Does the model just assume that both subarrays must operate in order for the full array to be in production? Seems rather odd that we can't take advantage of the late time of day benefits of E-W. Any ideas?

Thanks,
Les

Hi Paul,

The attached screenshot shows the output from a 25 degree E-W array modeled in Topsham Maine. East and west subarrays are identical. You can see that the array turns on at 4:00 a.m. at about 100kW output for each subarray. That is VERY early in the morning for Maine!! All goes as expected until 6:00 p.m. when the array shuts off. West is still over 1200 kW and east is producing over 400kW. Makes no sense at all. It looks time shifted to early in the day. The result is that the array appears to not be generating during some of the most valuable time of day, when pricing is peak. Any ideas at all on this? Both the time shift and the apparent premature shutdown in the model are of concern.

Thanks again,

Les Fritzemeier