How Pressure and Temperature is considered in SRW/SAM CSV files for Wind Project

Hello everyone,

I downloaded data from NREL Wind Resource Database ( wrdb.nrel.gov/ ) selecting all wind speed, wind direction, temperature and pressure fields. Including "surface air pressure (Pa)".
Specially because I noted this database has 3 height levels for pressure: 200m, 100m, and "surface" (what I guess could be considered 2m instead 0m, It would be great if somebody could confirm it).

After conducting some simulations with this file and also with the version of this converted to the old SRW format, some questions came to my mind:
(1) It seems that SAM ignores pressure data from the column "surface air pressure (Pa)" since it is not named like "air pressure at 2m (Pa)" Could someone confirm that?
(2) Also, case (1) is confirmed, and the target wind turbine hub height is for instance 50m, then SAM will assume that air pressure at hub height is the same of the nearest height with valid pressure data, in this case 100m. Otherwise, case we rename the column to "air pressure at 2m (Pa)", it will do a linear interpolation between 100m and 2m pressure levels to calculate it at hub height of 50m. Could we conclude this?
(3) Can we also conclude that for temperature either SAM will assume the nearest temperature or do a linear interpolation?
(4) When comparing some simulations with SRW file and  SAM CSV Format for Wind  , I could not find differences. Could anyone please confirm that internally the results are really the same? Maybe I am missing something here.
(5) Is the elevation information either in SRW file and SAM CSV Format for Wind being considered internally for any calculation? I guess it is not being considered, but I would appreciate if anyone could confirm it.
(6) How SAM calculates the air density for the wind projects? Does it consider only dry air

Maybe in future releases SAM could consider the elevation together the international standard atmosphere equations for pressure and temperature to  to fill gaps in the data and also to be considered in the interpolation as well.
Also, as per SAM help, the air density is calculated considering the ideal gas equations for dry air instead humidity air. It would be great if SAM team could improve this calculation in the future by considering the humidity air equations to improve accuracy similar what is done by other wind industry softwares. As a matter of effect, this parameter is normally measured at met masts during a wind resource assessment campaign and also NREL WRDB contains relative humidity data at 2m that we are not using in SAM. 

Thanks in advance for your attention.

Kind Regards,
Erick B.

Hi Erick,

The NREL WRDB provides access to data from the NREL WIND Toolkit API developer.nrel.gov/docs/wind/wind-toolkit/ .

Different endpoints of the API deliver files in different formats and containing types of data, so they cannot all be readily used with SAM. See this SAM forum discussion for a list of endpoints with data that works in SAM: sam.nrel.gov/forum/forum-general/4171-how-to-download-edit-a-nrel-wind-toolkit.html#11985

NREL is working to fix this so that all endpoints deliver files in the same format and more consistent data.

To answer your questions:

1. SAM does not read the pressure column if the heading includes "pres" and "surface" because the wind power model does not use surface pressure data.

2. Yes, SAM interpolates pressure data from the closest available measurement height. You can see what data elements are interpolated in the source code here: github.com/NREL/ssc/blob/31d4a54a01f2531fbb412aee93e5ac39a973a3ce/shared/lib_windfile.cpp#L200

3. Yes. SAM also interpolates temperature data from the nearest measurement height.

4. The SRW file format is an older format. SAM CSV for Wind is a newer format and is the preferred format. The wind power model can read either format, and should give the same results if the files contain the same data.

5. No. The elevation about sea level is not used by the wind power model.

6. The air density is used for wake calculations and is for dry air: github.com/NREL/ssc/blob/31d4a54a01f2531fbb412aee93e5ac39a973a3ce/shared/lib_windwatts.cpp#L158

Thank you for your suggestions to improve the wind power model.

Best regards,
Paul.