Sources of data - EPW files
The Energy Plus site
Weather Data for Simulation provide, for free, the data for more than 3 thousand sites. They can be downloaded in a zip file for each site contain:
- A .txt file, specifying a disclaimer about the origin of data and their copyright and a warranty about their us.
- A .stat text file, CSV formatted, presenting some statistics about the following data.
- A .ddy file, containing few basic Design Day of the year data or the typical extreme days
- An .epw text file, CSV (comma separated values) formatted, hourly data series to record EnergyPlus-ready data, for building energy simulation. The format is widely used but not the only one. This format has become a standard and many other softwares use and read it without problems.
Our main interest refer to the you open the EPW file with a text editor or in a spreadsheet (sometime it is better to change its extension to
.txt, just take care to set the proper numerical format, i.e. the the Decimal separator and Thousands separator - see also the
excel support page about) you will find a strange file.
First, you will see that it has a long “header” (not necessarily filled). In the header we find some interesting data \cite{file}:
- In the first row of the header we can recognize few fundamental information: it begins with the name of the city and ends with four numbers: (1) the Latitude (North when positive, South when negative), (2) the Longitude (East of Greenwich when positive, West of Greenwich when negative), (3) the Time Zone (Greenwich Mean Time - GMT) and (4) the Elevation above the sea level.
- the second row of the header is about Design Conditions Header/Data (CSV)
- the third row reports the typical Extreme Periods
- the fourth row reports the Ground Temperatures, typically for 3 depths, the assumed soil conductivity, density and specific heat) and the monthly “undisturbed” ground temperatures.
- the fifth row is related to Holiday/Daylight Saving Time Data: default weather processing contains no holidays or daylight saving period.
- other rows are available for comments and other technical information.
After the header rows, eventually, Data Records come: usually hourly data. Their structure is the following (35, comma separated, columns):
- the date occupy the first three columns: year, month and day of the month
- the time the following two columns: hour of the day and minute
- the Datasource (col.6) shows a long code like "C9C9C9C9*0?9?9?9?9?9?9?9A7A7A7A7A7A7*0E8*0*0"
- the DryBulb Temperature (col.7) [°C]
- the Dew-point Temperature (col.8) [°C]
- the Relative Humidity (col.9, missing value 999) [%];
- the Atmospheric Pressure (col.10) [Pa]: a standard Barometric Pressure (altitude based);
- the Extraterrestrial Horizontal Radiation (col.11, often missing value = 9999, since there is no use in EnergyPlus calculation) [Wh/m2];
- the Extraterrestrial Direct or normal Radiation (col.12 often missing value = 9999, since there is no use in EnergyPlus calculation) [Wh/m2];
- the Horizontal infrared radiation intensity from the sky vault (col.13) [Wh/m2];
- the Global Horizontal Radiation (col.14) , the amount of direct and diffuse solar energy received on a horizontal surface for the specified number of minutes preceding the indicated time); this value is not currently used in EnergyPlus calculations (missing value for this field is 9999) [Wh/m2];
- The Direct Normal Radiation (col.15), the amount of solar radiation in received directly from the solar disk on a surface perpendicular to the sun’s rays [Wh/m2];
- the Diffuse Horizontal Radiation (col.16) or the amount of solar energy received from the sky and not from the solar disk on a horizontal surface [Wh/m2];
- the Global Horizontal Illuminance [lux] (col.17) is the total amount of direct and diffuse illuminance in hundreds of lux on a horizontal surface; not currently used in EnergyPlus calculations, missing value is 999999
- the Direct Normal Illuminance [lux] (col.18);
- the Diffuse Horizontal Illuminance [lux] (col.19);
- the Zenith Luminance [Cd/m2] (col.20);
- the Wind Direction (col.21) in degrees: North = 0.0, East = 90.0, South = 180.0, West = 270.0. If calm, direction equals zero. Missing value is 999,
- the Wind Speed (col.22) in m/sec; Values rangING from 0 to 40. Missing value 999;
- the Total Sky Cover (col.23) is amount of sky dome covered by clouds at the hour indicated (in tenths of coverage: i.e. if it is =1 it means that it is 1/10 covered, while if it is = 10 it means "total coverage"); missing value is 99;
- the Opaque Sky Cover (in tenths of coverage) it is used to calculate Horizontal Infrared Radiation Intensity; missing value is 99 (col.24);
- the Visibility [km] (col.25);
- the Ceiling Height [m] (col.26);
- the Present Weather Observation is the Indicator of observation at that hour: it may be 0 (= Weather observation made) or 9 (= Weather observation not made, or missing) and its primary use is for rain/wet surfaces (col.27);
- the Present Weather Codes follows the TMY2 conventions: a text field of 9 single digits each of which represents an event (i.e. precipitation) (col.28);
- the Precipitable Water [mm] (col.29) is not currently used in EnergyPlus calculations because of the unreliability of the reporting of this value (missing value is 999);
- the Aerosol Optical Depth [in thousandths], it is not currently used in EnergyPlus calculations (col.30); missing value is .999;
- the Snow Depth (col.31), in cm, is used to tell when snow is on the ground and the ground reflectance may change; missing value is 999;
- the Days Since Last Snowfall (col.32) is not currently used in EnergyPlus calculations; its missing value is 99;
- the Albedo (col.33) is the ratio of reflected solar irradiance to global horizontal irradiance. It is not currently used in EnergyPlus.
- the Liquid Precipitation Depth (col.34) is the amount of liquid precipitation (mm) at the indicated time; if not missing, it is used and overrides the “precipitation” flag as rainfall; if the precipitation flag shows rain and this field is missing or zero, it is set to 1.5 (mm).
- the Liquid Precipitation Quantity (col.35) is the period of accumulation (hr) for the liquid precipitation depth field. It is not currently used in EnergyPlus.
Morphic transformations of the reference year
Attachments
Glossary
- DDY: it is the extension for Climatic Data files and it means Design Day Year)
- STAT: it is the extension for Climatic Data files and it means "statistics" and it is referred to EnergyPlus weather data
- EPW: means EnergyPlus weather file
- TRY: Test Reference Year
- CSV: comma separated values" format
- TMY (TMY2): Typical Meteorological Year
- WYEC2: Weather Year for Energy Calculations
- FTMY: future Typical Meteorological Year