Farhang, and others interested in processing their own weather data,
Please note there's a difference in convention between the US and Europe (don't know the situation in other places) in how climate variables (esp. solar radiation) are recorded. In the US, the data are over the previous time step (e.g., the previous hour), whereas in Europe, the data are around the time step (e.g. -30 minutes to +30 minutes). This difference has an appreciable effect on the solar radiation data as well as the calculation of the sun position. Since the EnergyPlus Weather web site contains a number of weather files produced in Europe and elsewhere, those files may have solar radiation that's 30 minutes early unless the files have been adjusted in being converted to EPW. A spot check I did recently on one such file picked at random to the IWEC2 file for the same location did show a small shift in the solar radiation data.
In terms of calculating the sun position for an EPW, it should be the average of the sun positions at the current and the previous time stamp. However, this gets a little tricky at sunrise and sunset hours, where the sun position should be the average only for the time when the sun is above the horizon.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 7/30/2013 2:34 AM, Farhang wrote:
Hi,
While making an actual-year weather data file, I got confused about time stamps and time zone fields.
To put measured data in an EPW file:
1) I assume that the time stamps are in UTC,
2) I set the timeZone field in the EPW file based on my local winter time (+1 in my case).
Now, Let's think about simulating building performance in Jan 1st 13:00 local time in Vienna (+1):
I assume that, EnergyPlus calculates the sun position and puts it in south (longitude: 180), but uses the radiation and other weather data of 12:00, because the timezone in EPW file is 1.
Are these assumptions correct? What other approach would you recommend to put measured data in an EPW file? How would one then interpret the time stamps in EnergyPlus output files?
Regards,
M.Sc. Farhang Tahmasebi
PhD Candidate / Project Assistant
Department of Building Physics and Building Ecology
Vienna University of Technology
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