Minimum DNI Value for electric power generation

I have simulated using parabolic trough solar field and found that electric generation is possible if DNI level is less than 170 w/m2. Is it possible? What is the minimum Direct normal irradiation (W/m2) hourly consider for electric power generation from parabolic trough plant? What is the methodology followed by SAM to discard DNI during start up and shut down of the plant for the power generation?
Please suggest me.
thanks
vineet

Dear Vineet,
The answer to your question depends on your assumptions. If your system includes storage or fossil backup (or both), then yes, the system could generate electricity at low incident solar irradiance levels.
For details about the model, including how it handles start-up and shut-down, see the physical trough reference manual, which you can download from the Performance Model Documentation page .
Best regards,

Paul.

Dear Paul,
I have taken zero value for the both (Storage and fossil backup)assumption and still power is generated if DNI level is less than 170 W/m2. Please suggest me.

thanks and regards,
Vineet

Dear Vineet,

Without seeing you SAM file (.zsam) it is difficult for me to tell why you are seeing those results. Would you mind attaching your SAM file to your original message? (To do so, click Edit and then attach the file.)

Best regards,
Paul.

Dear Paul,
Files are attached as requested.

Thanks and regards,
Vineet

Dear Vineet,
Thank you for attaching the file. In order to completely replicate your analysis, I also need a copy of your weather file. (Note that you can embed weather data in a SAM file by clicking "Copy to project" on the Location and Resource page.)
However, I think I can explain what you observed without your weather file. I ran a simulation in your file using the TMY2 file for Phoenix, Arizona to generate the following scatter plot of hourly power output (kWh) vs direct normal irradiance (W/m2). As you can see, there are a few hours between about 110 W/m2 and 430 W/m2 where the system generates power, but most of the power is generated at higher DNI levels.
/sites/default/files/content/images/scatter-power-vs-dni.bmp
The following hourly graph shows once instance where the DNI (red) is 120 W/m2, and the power output (green) is about 11 MWh/h. You can see that it is an hour that follows several hours of sunshine, and that the field HTF temperature drop (blue) lags the drop in solar resource. In other words, the HTF temperature fluctuates at a slower rate than the fluctuation in DNI because the fluid stores heat. That could be one explanation for why there are some hours that the system generates electricity during periods of low DNI.
/sites/default/files/content/images/hourly-dni-temp-output.bmp
Best regards,

Paul.