issue in field pressure drop and field optical end loss.

Hi Paul,
While reviewing the simulation results of a CSP parabolic trough system using the Physical Model in SAM, I observed an aspect that I am finding difficult to interpret and would appreciate your guidance on.

At certain time stamps in the results, the reported resource pressure appears to be lower than the corresponding field pressure drop. Conceptually, my understanding is that for the heat transfer fluid to be successfully pumped through the solar field, the available resource (or pump outlet) pressure should be higher than the total pressure drop across the field. Otherwise, the fluid should not be able to flow through the system.
However, despite observing instances where the resource pressure is lower than the field pressure drop, the simulation runs without any errors or warnings. This has led to some confusion regarding whether this is actually an error on SAM's side or I am conceptually wrong.

While analyzing the results of a CSP parabolic trough workfile, I came across the term “Field Optical End Loss” in the data table section after the simulation. I am finding it difficult to clearly understand the physical meaning of this term and how it is evaluated within SAM, and why its value is around 0.9, although it is a loss.

regards,
Abhishek Meena 

Hi Abhishek,

The "Resource Pressure (mbar)" output variable is the atmospheric pressure value from the weather file, not a pressure drop in the system.

The "end loss" label is misleading: That variable is a factor rather than a reduction, so a value of 0.9 would be equivalent to a 10% reduction. The equations for that value are here in the SSC source code:

The end loss:

github.com/NREL/ssc/blob/0a05b2dfa33c325ff59f00e97a3c84122ec0a542/tcs/csp_solver_trough_collector_receiver.cpp#L1854

The average end loss is the value that SAM reports in the results:

github.com/NREL/ssc/blob/0a05b2dfa33c325ff59f00e97a3c84122ec0a542/tcs/csp_solver_trough_collector_receiver.cpp#L1888

Best regards,
Paul.
 

Thanks Paul,
I just want to confirm one more thing, there is a term System pressure what it means. And also after simulations in the output there is multiple options like hourly data, monthly data single values and like that so what is the meaning of 92 values what it means.

Hi Abhishek,

I don't see a variable named "system pressure" in the inputs or outputs for the physical trough model. Can you be more specific about the term you are asking about?

The Data Tables tab on the Results page displays output variable values in tabular form. The table organizes variables into groups of variables of the same length: The "Single Values" group shows variables that have one value, "Hourly Data" shows variables with 8760 values (the number of hours in one year), "Monthly Data" shows variables with 12 values, etc.

For the physical trough model, the "Data: 92 values" group shows variables with 92 values, which happen to be variables associated with the field piping model. These variables have a number of values that depends on the number of loops in the solar field (see Actual number of loops at the top of the Solar Field page), where the number of values is the number of loops divided by two.

For more information about the field piping model, see:

• Section 2.5 of Wagner, M. J.; Gilman, P. (2011). Technical Manual for the SAM Physical Trough Model. 124 pp.; NREL Report No. TP-5500-51825. ( PDF 3.7 MB )
• B. Kelly and D. Kearney. Parabolic trough solar system piping model: Final report. Technical Report NREL/SR-550-40165, National Renewable Energy Laboratory, Golden, Colorado, USA, 2006.

Best regards,
Paul.