Fresnel linear focusing systems relationship between "General optical derate" and "Receiver optical derate"

Dear Paul,

my doubt refers to the way SAM processes the two input parameters:

"General optical derate" at the upper right area of the sheet "Collector and Receiver"

and the

"Receiver optical derate" at the buttom of the same sheet.

The "Receiver optical derate" according to my trials is influenced only in case the "Evacuated Tube Model" is selected to determine the "Receiver Geometry and Heat Loss".

In my understanding however, the Optical efficiency to determine how much irradiation is absorbed by the solar field, is determined by multiplying:

1. Tracking error derate
2. Geometry effects derate
3. Solar-weighted mirror reflectivity
4. Dirt on mirror derate
5. General optical derate
AND THE RECEIVER SPECIFIC OPTICAL PROPERTY, i.e.
6. Receiver optical derate

Nevertheless, in case I choose the "Polynomial heat loss model" to account for the "Receiver Geometry and Heat Loss"
the Receiver optical derate is always equal 1 (not impacted as in case I choose the "Evacuated tube model").

Looking in the USERGUIDE, the optical efficiency is defined as being:

Optical Efficiency ="Total Thermal Energy Absorbed by Receiver ÷ ( Direct Normal Irradiance × Actual Aperture Area )"

I.e., this includes the absorptivity of the receiver. In fact, depending on which model I select, the output difference is considerable (more than 4% net output less for the "Evacuated Tube Model" choice).

On the other hand, in case I set all collector derate parameters to 1, and insert the "General optical derate" to the maximum optical efficiency of the collector (in case of normal solar incidence), this is the maximum value that can be found in the output hourly results, no matter which model is selected for the Receiver.

How can this be explained and is it possible that in case of choosing the "Evacuated Tube Model", the absorptivity of the Receiver is considered twice, shrinking the net output of the overall system in comparison to the "Polynomial heat loss model"?

The analyses is even worse when considering that, in case I insert the overall optical efficiency in the field "General optical derate", and set the parameters in "Evacuated Tube Model" such as to achieve the value "1" in the field "Receiver optical derate", even though I get a much lower performance compared to in case "Polynomial heat loss model" is selected.

Thank you very much for any help on this issue.

Kind regards

Dietmar