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System Advisor Model (SAM)

Time Step for SAM Model Simulations

pgilman
Topic Author

02 Dec 2014 18:23 #2757 by pgilman

Time Step for SAM Model Simulations was created by pgilman

According to our experience using SAM software we believe it is not possible to develop a performance model with a 15 minute time step because SAM calculations are hourly calculations.
SAM 2014.11.24 supports subhourly simulations and works with weather files with up to one-minute time steps. However, not all of the performance models have been tested at short time steps. Also, in order to run the models, you need a weather file with data for all of the required elements (solar radiation, wind speed, temperature etc.)
For more details, see "Time Convention and Sun Position" in the SAM 2014.11.24 Help system under "Weather Data."
For a discussion about subhourly simulations in SAM 2014.1.14 and older versions, see Time Step -- Accuracy of the Simulation .
Best regards,

Paul.

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Pablo Pueyo

03 Dec 2014 10:56 #2758 by Pablo Pueyo

Replied by Pablo Pueyo on topic Time Step for SAM Model Simulations

Thank you very much for your quick answer, Paul,

I've downloaded SAM 2014.11.24 and I still have several questions regarding this issue:
-According to the mentioned help, it is not possible to run Empirical CSP Trough model with 15-min time step, it is necessary to use Physical Model. Could you confirm?
-According to SAM help it is necessary to use a csv file format to run 15-min time step models. We have an excel file. How could I convert this file into a 15-min step csv format. I've tried with an excel macro that I used to use called 'Excel to tm2 converter and it doesn't work with so many rows

Thanks in advance

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pgilman
Topic Author

03 Dec 2014 16:45 #2759 by pgilman

Replied by pgilman on topic Time Step for SAM Model Simulations

Dear Pablo,

We have tested the physical trough model with subhourly time steps, but not the empirical trough model. The empirical trough model will run with subhourly weather data, but because the model was originally designed for hourly time steps, there may be some equations that are not suitable for smaller time steps. That is likely not the case for 15-minute time steps, but we have not verified that.

The new SAM CSV format is described in Help under Weather Data, Weather File Formats. You can use a spreadsheet program or text editor to create the file. You may want to refer to one of the files in the default solar resource library as an example (see the solar_resource folder in your SAM installation folder, c:\SAM\2014.11.24 by default).

Best regards,
Paul.

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Pablo Pueyo

04 Dec 2014 08:41 #2760 by Pablo Pueyo

Replied by Pablo Pueyo on topic Time Step for SAM Model Simulations

Thanks, Paul,

I have followed your advice and I have used an excel from the solar resource library as an example to build my Wheather File.
The problem that I have found doing this I guess that is regarding how to declare the hour of the day. I show you here below a row of my Wheather File as an example, because it's easier for me to explain the problem:

Year,Month,Day,Hour,GHI,DNI,DHI,Tdry,Twet,RH,Pres,Wspd,Wdir,Albedo
2015,1,1,1.25,0,0,0,17,0,50.6,1053,1.4,353,0 (example row for the hour 01:15 of First January, 2015)

As you could see in the previous row, I have declared the hour of this row as 1.25 because I would like to declare the hour 01:15 of First January. I've done the same for the rest of rows in the file. The problem is that, when I run the model with the 35040 rows declared as mentioned before and I check the results, I realise that only the first 8760 values have been considered by the software, so I understand I've made a mistake creating the wheather file.

Could you show me please the way to declare a 15 min time-step single row for this csv?

Thank you very much in advance.

Regards,

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pgilman
Topic Author

04 Dec 2014 17:35 #2761 by pgilman

Replied by pgilman on topic Time Step for SAM Model Simulations

Dear Pablo,

When you create a weather file with subhourly time steps, you should add a 'Minute' column, and use that to specify the minute you want SAM to use for sun position calculations.

Also, note that the first hour of the year is Hour zero, not Hour one.

Your example would look like this:

Year,Month,Day,Hour,Minute,GHI,DNI,DHI,Tdry,Twet,RH,Pres,Wspd,Wdir,Albedo
2015,1,1,0,0,0,0,0,17,0,50.6,1053,1.4,353,0
2015,1,1,0,15,0,0,0,...
2015,1,1,0,30,0,0,0,...

For weather files with subhourly data, SAM uses the time stamp to determine the minute to use for sun position calculations. In the example above, the minute column indicates the end of each 15-minute time step, so SAM would use the minute at the end of each time step determine the sun position for that time step. If you want SAM to use a different minute for the sun position, you can specify a different minute. For example Minute values of 0, 7.5 and 22.5 for the first three time steps would result in the sun position being calculated at the midpoint of each 15-minute time step.

For hourly data, you do not need to specify a minute. SAM uses the minute at the midpoint of the hour for hourly sun position calculations.

Best regards,
Paul.

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Pablo Pueyo

04 Dec 2014 18:42 #2762 by Pablo Pueyo

Replied by Pablo Pueyo on topic Time Step for SAM Model Simulations

Dear, Paul,

Thank you for your answer. I've built the wheather file as you mention. Here below you can see the 6 first rows of my file:
Year,Month,Day,Hour,Minute,GHI,DNI,DHI,Tdry,Twet,RH,Pres,Wspd,Wdir,Snow Depth
2015,1,1,0,0,0,0,0,17,0,50.6,1053,1.4,353,0
2015,1,1,0,15,0,0,0,17,0,50.8,1053,1.4,353,0
2015,1,1,0,30,0,0,0,17,0,50.9,1053,1.4,353,0
2015,1,1,0,45,0,0,0,17,0,51.1,1053,1.4,353,0
2015,1,1,1,0,0,0,0,17,0,51.3,1053,1.4,353,0

Then I put the wheather file csv in the SAM wheather libray and I try to run the model with this wheather file with 35040 rows. Then, the problem I had is still there and the result of the simulation is totally incoherent. I've tested the results with the same wheather file but with 8760 rows (each row the mean hourly value that I have previously calculated with excel) and the result with 15 min step is much lower, I mean 50,805,084 KWh of power output instead of 121,329,544 KWh which is the result with hourly step.

I beleive that the model only take into consideration the first 8760 rows of the csv file and I don't know why. I don't know which step I skip or what I am doing wrong.

Could you help me? It is something more that I have to do before running the model? Do you know where is the problem?

Thanks in advance,

Regards

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