Graphs of SAM outputs show strange correlations

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Javier Ojeda
Graphs of SAM outputs show strange correlations

I am simulating a 100 MW CSP Plant with 10 hours storage, and I am trying to assess and interpret the results by SAM.
To that end, I export the hourly data to Excel and plot pairs of values to see their correlation.

For instance, if I plot the Thermal Power into the Storage, it should be almost proportional to the HTF mass flow rate into storage, the proportionality constant being HTF_Specific_Heat x HTF_Delta_T_across_HEX.

Then I loop through various pairs of variables, which I understand should be linear correlated, and have found several results that astonish me. The Plots can be found in the attachment with some screenshots of the input data to SAM, as well as some of my remarks and annotations.

The one that baffles me the most is when I plot the System Power Generated (MWe) versus the Cycle Thermal Power Input (MWt).
Considering that the Rankine Cycle has an efficiency of around 40%, to produce 100 MWe, we should have around 250 MWt in the Cycle.

Well, I get exactly that, but not ONLY THAT! I get a clear linear correlation at half the power. That is, for 250 MWt, the Generated power amounts only 50 MWe approx. This means, that the system is considering a Rankine efficiency of just half (20%).

I would appreciate some light on these questions.

Javier Ojeda

Sorry, I just realized that the uploaded file does not include the graphs. They were removed when uploading.

Paul Gilman

Dear Javier,

You might want to look at the time series data in SAM, and plot the "PC efficiency: gross" variable along with the various other ones. I think the low power cycle (PC) efficiency values you observe may be occurring at startup. Let me know if that helps.

Best regards,
Paul.

Javier Ojeda

Dear Paul,

thanks for the advice. The fact is that I should have had a look first, but the linear correlation at half the rated power is so well defined that I just jumped in to ask.

I have checked the time series, and YES, the majority of the points occur the first generating hour of day. Then, it seems to me that SAM calculates with a time step less than one hour (I thought that the time step would be one hour, since the weather file is hourly defined).

However, the gross efficiency is calculated at 40% approx. So I am still baffled. I upload a new .pgn file with the graphs.

I picked a random day, where this phenomenon is occurring, and you can see from the data values that the Gross Efficiency is calculated as 40% approx.

Kind regards and thanks for your help.

Javier

Javier Ojeda

New file uploaded is "All year gross efficiency..."

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