Lukas,
Thanks for the information. I must admit I never use WeatherConverter to create a weather file from raw data (I have other software to do that), but I find what you describe troubling, because it means a user in the US with raw data but no solar, which is 95% of the time, better make sure to input a TimeHourOffset=-0.5 or else the user will get an EPW file with the solar in European convention, i.e., from -0:30 - +0.30. Furthermore, I see no advantage at all for WeatherConverter to calculate the solar radiation in European convention since EnergyPlus always assumes the data is in North American convention, i.e., from -1:00 - 0.00. Repeating what I had written before, the only time that the WeatherConverter should use a TimeHourOffset=0 is if the GHI is already in European convention and WeatherConverter is being used to calculate the DNI (Direct Normal).
I've thought of doing what you've done in rebinning the data, but came to the conclusion that simply moving the longitude is much simpler and also avoids both the problem of smoothing that you've mentioned, plus another effect that's more important, which is the solar intensity at sunrise and sunset hours. These are very nonlinear with time, particularly the DNI, and if you just do linear interpolation you may see very wierd effects, like DNI or even GHI being more than Extraterrestrial, or being zero when the sun is above the horizon for a fraction of the hour. At the sunrise and sunset hours, you cannot just use the average solar position for that hour, you must calculate the fraction of hour when there is sun and then calculate the sun position only for that fractional hour. This is because while the average solar position may be below the horizon, there can still be up to 30 minutes when the sun is above the horizon.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 205A Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data http://www.whiteboxtechnologies.com (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
Joe,
First off, the default for custom files is quite surly set to TimeHourOffset=0 (I tested it on data using both time stamp conventions). So you need to specify the offset to -0.5 an hour if you have solar data as -1:00-0:00 and need to let the WC tool calculate either DNI or DHI.
I've GHI and DNI on the full hour (instantaneous values, in practice same as -0:30-0:30). I "shift" the data half an hour backwards by linear interpolation and then calculate DHI with the TimeHourOffset=-0.5, thus getting the solar data as -1:00-0:00. This is not ideal as the interpolation does some smoothening on the solar data. Subtracting 7.5 from the longitude in the epw file doesn't seem as a robust solution, an alert user could change it back to the "right" longitude.
IDA-ICE treats all parameters including solar data as instantaneous and have 8761 or 8785 rows in an weather file. So when using e.g. your IWEC2 files users should shift the longitude by 7.5 degrees (I don't remember in which direction, but should be the opposite of what Energyplus need to do with files using the -0:30-0:30 convention). This issue is supposed to be dealt with in the upcoming 4.7 version of IDA-ICE.
Best regards,Lukas
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Posted by: Joe Huang <yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx>
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