CSP - reported wet bulb temperature

Hello:

After running the CSP model, I looked at the hourly wet bulb temperatures. The temperature reported as wet bulb matches exactly with the dew point temperature in raw TMY3 data from NSRDB. However in the 'Resource and Location' tab in SAM, when I view the hourly data, I see that the wet bulb and dew point temperatures are slightly different (which is what I would expect). Was the wet bulb temperature calculated?

Thanks,

Ana Dyreson

Dear Ana,
I wasn't sure which CSP model you are using, so I checked results in the physical trough and molten salt power tower model with a couple of different NSRDB TMY3 files from SAM's solar resource library. In my tests, the temperature data that SAM shows for the weather file and in the results seem consistent.
For example, here is data from SAM for the physical trough model with the "USA AZ Tucson International Ap (TMY3)" weather file:

Are you seeing something different?
Best regards,

Paul.

Hi Paul:

I was comparing the original TMY3 data to the data I see in SAM when I run the physical trough model. I got the raw TMY3 data from here: rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/

Here is a snapshot beginning on the first hour of the year. The problem is that the dewpoint (from original TMY3) and the wet bulb (as reported in SAM) are identical. I would expect them to be different, if only slightly.


TMY3 Dry bulb [C] TMY3 dew point [C] SAM dry bulb [C] SAM wet bulb [C]
7.2 3.3 7.2 3.3
7.2 3.3 7.2 3.3
6.7 3.3 6.7 3.3
6.7 3.3 6.7 3.3
5 2.8 5 2.8
4.4 2.2 4.4 2.2
3.3 1.7 3.3 1.7
3.9 2.2 3.9 2.2
6.7 2.8 6.7 2.8
9.4 3.9 9.4 3.9

I am not sure that it matters much but I want to make sure I know what data I'm using from SAM.

Thanks,

Ana

Dear Ana,

It turns out that the TMY3 data in SAM's solar resource library has the dew point temperature mislabeled as wet bulb temperature. We will fix that for the next version of SAM, which we hope to release in January 2016.

Sorry for all the confusion, and thank you for helping us find that problem with the data.

This only affects the CSP model's heat loss calculations, and as you note, only introduces a small error.

In the meantime, you can correct the problem by using a text editor or spreadsheet program to change the header in any of the TMY3 files you want to use with one of the CSP models: Just change the "Twet" column heading in the file to "Tdew." The files are in the solar_resource folder of your SAM installation.

Note that this is only a problem with the TMY3 files in SAM's library -- it is not a problem with the original TMY3 files from the National Solar Radiation Database (NSRDB). The files in the SAM library contain data that we copied from the NSRDB files, and we made a mistake in label for that column heading.

Best regards,
Paul.