Negative Energy?

Hi all,

This is my first time running SAM in about 3 years, so bear with me. I am doing a detailed PV simulation, but am getting negative AC production in the summer months. Does anyone know what some of the typical causes for that might be? I do believe it is something in the weather file, because the results look normal if I use a weather file from the solar resource library. I downloaded a TMY2 from NSRDB, saved it to disk, then checked the box to "use a specific weather file on disk". Is there any additional formatting I should have to do in order to use a TMY2?

-Jen

Additionally, if I try to download a TMY weather file from within SAM using latitude and longitude, I get the error message attached.

Dear Jen,
There is a problem with downloading TMY files from the NSRDB in SAM 2016.3.14, which is why you are seeing that error message. That will be fixed in a patch that we hope to release later today or tomorrow.
In the meantime, you can download TMY files by hand directly from the NSRDB website , or use SAM to download files for specific years instead of TMY, or, as you did, use a weather file from SAM's solar resource library.
Best regards,

Paul.

Paul,

Thanks for the quick response. I'm still getting negative AC output during the summer if I use a TMY2 downloaded directly from the NSRDB website. Any thoughts on why that might be?

-Jen

Hi Jen,
Are you getting those values at night? If so, that might be due to the inverter's nighttime consumption. If that doesn't explain it, please [url=mailto:sam.support@nrel.gov?subject=Negative%20output%20for%20PV%20model t=_self]email me[/url] your SAM file and I'll see what I can find.
Best regards,

Paul.