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Energy Plus - How to create and use epw and ddy files?

asked 2015-01-13 03:33:09 -0500

OlgaF's avatar

updated 2017-05-29 17:41:58 -0500

Hi, I would like to ask about .epw and .ddy weather files and their use in Energy Plus.

I have specific hourly data that I would like to use, e.g. temperature, relative humidity, wind speed and wind direction for a typical day for each month of the year (12 typical days).

Do I have to create both files (.epw, .ddy) for an annual simulation of a building and its HVAC systems and if yes, how to do it?

Many thanks in advance

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This looks like a duplicate of this question. Have a look there and if it doesn't completely answer your problem then you can edit this question to let us know what's missing.

Jamie Bull's avatar Jamie Bull  ( 2015-01-13 09:39:36 -0500 )edit

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answered 2015-01-15 12:34:55 -0500

Joe Huang's avatar

Just to be clear, *.epw and *.ddy are simply file formats used by EnergyPlus. You could conceivably put any weather data into those formats and EnergyPlus would run (barring reality checks in the program for values outside of physical possibility...). Whether you will get meaningful results, though, is a different question.

If you just have typical day information (hourly values or averages?), how would you expand that to the other days of the month? You can just repeat them, but the weather file in my opinion would be too unrealistic to provide meaningful results. You also did not mention solar radiation, which is a main driver for cooling loads.

No, my recommendation is that you don't try to turn such sparse data into an *.epw, but that you go find a real weather file for your location of interest and use that. That would also be a lot less work, since creating an *.epw is not something that can be explained on a bulletin board.

The *.ddy files supplied with the *.epw files on the EnergyPlus Weather web site are taken from the ASHRAE Handbook of Fundamentals for the same WMO station (if available). If you know the lat and lon for your location, you can find the closest HOF station in the database (see PreProcess/WeatherConverter/ASHRAE2013YearlyDesignConditions.xls) and use that. Having said that, I feel there's an over-reliance on the ASHRAE design tables. There ARE different design procedures in different countries, and real engineers actually tweak the values to meet their needs.

Yes, you do need multi-year records to calculate design conditions, but these are not so hard to obtain anymore. Perhaps a bigger barrier now is that people lack the software (or don't have enough interest) to extract the design conditions, which really isn't that hard.

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answered 2015-01-14 07:36:49 -0500

Archmage's avatar

As for the .ddy file, it is not something EnergyPlus really uses directly like it does the .epw. The .ddy file is just a handy collection of the climate design data, used for sizing, that are in E+ input format (see the object SizingPeriod:DesignDay). If you make your own SizingPeriod:DesignDay objects and put them in the .idf, then you don't also need to create a .ddy file. If you are not using autosizing, then you don't need any design day objects.

However, if the climate design data already exist for the location, then they don't necessarily need to be changed since they are a different sort of thing than hourly weather. To do better than handbook data you would need many many years of observations to capture extremes.

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Asked: 2015-01-13 03:33:09 -0500

Seen: 5,335 times

Last updated: Jan 15 '15