Negative AC net monthly output for High-X CPV model

I am using SAM version 2012.5.11. I am attempting to model a High-X concentrating PV (HCPV) array. For some months, I am seeing a negative AC net monthly output. Other months have a net AC output that is near zero. Do you have any ideas on what could cause this?

There are two things that could cause the system to have negative or near-zero output: Tracking parasitic power, and nighttime consumption by the inverter.

For tracking power, please see the "Single tracker power" input on the Array page. In SAM 2012.5.11, this input is a fixed value in Watts, and if you change the module size without changing the tracker power, you can end up with a tracker that consumes more power than the module generates, resulting in negative power output. (SAM assumes that the system is connected to the grid, so negative power output represents electricity consumed by the system.) In SAM 2012.11.30, we changed that input so that it is a fraction of the DC nameplate capacity instead of a fixed capacity -- hopefully that change will help avoid this issue.

Inverter nighttime losses may result in negative output values for hours between sundown and sunrise, but should not result in negative output over an entire month.

Best regards,
Paul.

Hello Peter,

The original question was about the high-concentration photovoltaic model (HCPV). It sounds like you are having a similar issue with one of the concentrating solar power (CSP) systems. Can you tell me which model you are using (physical trough, power tower, etc.)?

Best regards,
Paul.

Hi Peter,
Thank you for the clarification. I set up two cases as you described, but could not recreate the situation with negative monthly output. Could you [url=mailto:sam.support@nrel.tov?subject=Trough%20examples%20with%20negative%20monthly%20output&body=See%20attached%20.zsam%20file t=_self]email me[/url] a copy of a SAM file (.zsam) that shows an example?
Best regards,

Paul.