Height of Wind Speed Data Used to Derive PVsyst Thermal Parameters

[kjs55]

I spoke to Tim Townsend who historically provided PVsyst with the recommended Uc and Uv values of 25 and 1.2 (which PVsyst attributes to PVUSA) derived during his time working at that same location in Davis, CA where PVUSA-related testing activities also took place. He confirmed to me that the height of the wind speed data used to derive those values via regression was 10 meters. (He said: “The pole was next to my office and plainly visible out the west windows of our building.”) I think this (the 10 meter height) is a critical piece of information to specify in the relevant section of the PVsyst Help Menu [1], so this is fundamentally my suggestion (to add this single bit of information to the help menu) and the reason for this PVsyst Forum post.

Note that since NSRDB PSM v3, that particular weather data set suddenly changed the wind speed height to 2 meters (PSM v2 and prior NSRDB data sets used the 10-meter height which is more standard aka traditional for TMY aka TGY files given that the particular ground surface coverage doesn't significantly impact the wind speed at that higher up height). Unfortunately, I can’t find any formal documentation of this change from 10 m to 2 m apart from this NSRDB webinar's Q&A [2] and a YouTube video (also in the Q&A of a NSRDB webinar) [3].

In such use cases (of 2-meter wind speed data), I don’t think the thermal parameters that were derived using 10-meter wind speed height would apply. I’m pointing this out because many PVsyst users are using {Uc=25, Uv=1.2} regardless of the weather data set in use (i.e., incl. NSRDB PSM v3), and my feeling is that's not correct. I suppose if the 2-m wind speed data was scaled to a 10-m height, that's one way to continue to apply those same thermal parameters historically provided to PVsyst by T. Townsend. I'm not aware of anyone who's doing this, though, and my understanding is this wind speed height translation is not happening internal to PVsyst (in the PVsyst algorithm) in such use cases of 2-m wind speed height data in the TMY aka TGY weather (meteorological) data set. Either way, the situation (state of affairs) needs to be clear to the PVsyst user which is why I'd like to see it stated in the PVsyst Help Menu that the values provided by T. Townsend are specific to a 10-m wind speed height (and then a description of any such wind speed height translations, e.g., to a standardized height of 10 meters if that's to occur within PVsyst OR the lack thereof of any such translations thus leaving it up to the user what to do with the wind speed height external to PVsyst, prior to importing the TMY aka TGY data, for the purpose of applying/pairing with a specific set of {Uc, Uv} within PVsyst).

[1] https://www.pvsyst.com/help/thermal_loss.htm

[2] https://webcache.googleusercontent.com/search?q=cache:dqpgedl3nbgJ:https://nsrdb.nrel.gov/nsrdb/NSRDB_Webinar_QA_-_Oct_6_2020.pdf

[3] https://www.youtube.com/watch?v=BzNoYACXCOY&t=4220s

[André Mermoud]

You are right, the wind velocity measured according to the WMO standards  (World Meteorological Organization) specify that the measurement should be done at 10 m height.  The Uv value of 1.2  W/m²K / m/s proposed by  PVUSA or Tim Townsend is referred to this measurements (i.e. values of all standard meteo data files).

This is well explained in the help  (topic "Array Thermal losses", paragraph "U-value"):