Field Incident Power to Receiver Incident Power - missing factor

I'm trying to understand the relationship between incident power outputs for my physical parabolic trough model and solar resource file (attached), particularly in relationship to the technical manual equations 2.44 (energy incident on solar field) and 2.36 (energy absorbed by absorber tube i).

My first question is about terminology.   In the technical manual. Equation 2.44 says it gives "energy incident on solar field" but since it includes a term to account for the non-perfect reflection from the parabolas, isn't it really the energy reflected by the reflectors?

Second, in SAM, there are two field thermal power outputs:  "field thermal power incident" and "field thermal power incident after cosine".  It's my understanding that the IAM accounts for foreshortening/cosine effects - is that correct?  If so, then the field thermal power incident after cosine would be the same as the power in 2.44, right?

Finally, it's also my understanding that the energy absorbed by the absorber tube should be reduced from the "energy incident on the solar field" by the optical derate factors listed on the receiver page:  absorber absorptance, envelope transmittance, bellows shadowing, and dirt on receiver.

However, when I look at the ratio of Receiver thermal power incident to Field thermal power incident, the value is different than the product of these factors.  In my model, the product is 0.875, but the ratio of powers, on June 21st is mostly constant at 0.7668.  See attached screenshot of the two curves.

So I'm missing an additional factor of 0.863.  Can you let me know where that comes from?

Thanks,

Matthew

 

Hi Matthew,

The technical manual does seem imply that "field thermal power incident" includes the mirror derates. However, after looking at the code, we note that the SAM output "field thermal power incident" is the incident power on the mirrors before any optical losses. 

Regarding IAM vs cosine effects, the IAM accounts for all solar positioning losses that aren't explicitly modeled, such as the cosine effect and row losses. That is why field thermal power incident and field thermal power incident after cosine have different values. 

Finally, the ratio of receiver thermal power incident to field thermal power incident is dependent on multiple optical derates. For the collectors these are: incidence angle modifiers, tracking error, general optical error, geometry effects, mirror reflectance, and dirt on mirror. Row shadowing and end losses also affect the collector performance. 

For this case, the receiver envelope is not considered because the glass is broken. Therefore, the remaining optical derates for the receiver are bellows shadowing and dirt on receiver. Currently, the incident power on the receiver does not include the power reflected away from the receiver via it's absorptance term, but we have created an issue to include this power in future versions: github.com/NREL/SAM/issues/1441 .

Best regards,
Paul.