Choppy Field Collector IAM from physical parabolic trough simulation

After performing a physical parabolic trough LCOE calculation, when I look at the variation of the Field Collector Incidence Angle Modifier over a single day, I get some very unexpected results - a very choppy curve:

I found the equations for IAM in the Technical Manual For the SAM Physical Trough Model (p. 20-25) and using those equations, I get tracking angles as expected, but the IAM does not match the SAM result.  The IAM I calculate is, as I would expect, a very smoothly varying curve over the day.  The plot below compares the calculations from equations vs the SAM simulation results.

If it helps, my IAM coefficients are a1=1, a2 = -0.008, a3=-0.1117
 

Hi Matthew,

I cannot replicate those results in SAM. Please attach a .sam file and the weather file that demonstrate these results if you would like me to help troubleshoot.

Also, to include an image in a post on this forum, please attach the image and then insert it. Copying and pasting images into the editor does not work.

Best regards,
Paul.

Thanks for replying, Paul.

Further investigation showed that the choppiness originates in the incidence angle itself and I still can't figure out why.  I've attached the .sam file along with an image of the incidence angle and zenith angles (latter is smooth). 

 

Matthew

Sorry, I forgot to attache weather file - here it is.

Also, I'm using SAM 2022.11.21 on MacOS if that matters.

Thanks,

Matthew

Hi Matthew, 

The choppiness of the Field Collector Incidence Angle Modifier and Solar Incidence Angle parameters are caused by the system switching between operating modes during the day. The model only calculates the incidence angle when the field is “on”. 

In this case, on June 21-22, the field starts "off" and recirculates HTF while it heats up. The HTF eventually hits a startup temperature and turns “on”, passing the fluid to the rest of the system. However, the trough field does not supply enough heat to keep the htf in the field above the startup temperature, causing the field to turn "off" and recirculate again. This process repeats as the field starts up and shuts down. The output parameters “Field total mass flow delivered” and “Field total mass flow recirculated” demonstrate this behavior. 

Decreasing the minimum single loop flow rate (i.e., 2 kg/s) makes it easier for the HTF temperature to rise and allows it to stay above the startup temperature throughout the day. Therefore, the model calculates the incidence angle and shows expected behavior. 

We recommend changing the system design to ensure that the HTF in the field is supplied enough heat to remain above the startup temperature, especially in design conditions.

Best regards,
Paul.

Thanks for your answer Paul.  The one thing I don't understand is that other quantities - like power absorbed at the receiver - are calculated from IAM.  So if IAM isn't calculated because there's a flow issue, then model results say there's no thermal power incident on the receiver at those times.  But unless the reflectors are detuned, there will be optical power on the receivers, no?  And if you are saying that there's detuning involved, why doesn't that show up in the fraction of focused SCAs or something similar?

Matthew