TMY averaging of weather variables within SAM

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williampaulbell
Topic Author

23 Aug 2014 04:21 #2510 by williampaulbell

TMY averaging of weather variables within SAM was created by williampaulbell

Hello Paul

Could you clarify the following query about SAM's use of TMY files?

The values for weather variables in TMY files are the average of the previous hour.

However, I notice that SAM takes the values from the TMY file and produces the average of the current hour and previous hour in the hourly data output tables.

Is this in effect doubling up the averaging over the previous hour?

For instance

From SAM
Dry bulb temperature (C) hourly
0
24.5
24.1
23.7

From TMY
Dry-bulb (C)
24.7
24.3
23.9
23.5

I have attached the TMY file.

Regards

William Bell

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Paul Gilman

26 Aug 2014 00:52 #2511 by Paul Gilman

Replied by Paul Gilman on topic TMY averaging of weather variables within SAM

Dear William,
SAM assumes that the temperature values in the weather file are instantaneous values at the end of each hour, not average values as you describe above. That convention is described briefly in SAM's Help system ( SAM Help -- Weather Data Overview , bottom of page), and also in the documentation that accompanies the standard NREL TMY2 and TMY3 data sets.
(That's different from the solar radiation values, which are totals over the previous hour. SAM assumes that the solar position at the midpoint of the hour applies to a given hour for the sun angle calculations.)
Best regards,

Paul.
 
 

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