.Buildings.BoundaryConditions.WeatherData.ReaderTMY3

Information

This component reads TMY3 weather data (Wilcox and Marion, 2008) or user specified weather data. The Modelica built-in variable time determines what row of the weather file is read. The value of time is the number of seconds that have passed since January 1st at midnight (00:00) in the local time zone. The local time zone value, longitude and latitute are also read from the weather data, such that the solar position computations are consistent with the weather data.

The weather data format is the Typical Meteorological Year (TMY3) as obtained from the EnergyPlus web site at http://energyplus.net/weather. These data, which are in the EnergyPlus format, need to be converted as described below.

Output to weaBus

The following variables serve as output and are accessible via weaBus:

Name Unit Description
HDifHor W/m2 Horizontal diffuse solar radiation.
HDifNor W/m2 Direct normal radiation.
HGloHor W/m2 Horizontal global radiation.
HHorIR W/m2 Horizontal infrared irradiation.
TBlaSky K Output temperature.
TDewPoi K Dew point temperature.
TDryBul K Dry bulb temperature at ground level.
TWetBul K Wet bulb temperature.
celHei m Ceiling height.
cloTim s One-based day number in seconds.
lat rad Latitude of the location.
lon rad Longitude of the location.
nOpa 1 Opaque sky cover [0, 1].
nTot 1 Total sky Cover [0, 1].
pAtm Pa Atmospheric pressure.
relHum 1 Relative humidity.
solAlt rad Altitude angle.
solDec rad Declination angle.
solHouAng rad Solar hour angle.
solTim s Solar time.
solZen rad Zenith angle.
winDir rad Wind direction.
winSpe m/s Wind speed.

Adding new weather data

To add new weather data, proceed as follows:

  1. Download the weather data file with the epw extension from http://energyplus.net/weather.
  2. Add the file to Buildings/Resources/weatherdata (or to any directory for which you have write permission).
  3. On a console window, type
      cd Buildings/Resources/weatherdata
      java -jar ../bin/ConvertWeatherData.jar inputFile.epw
    
    if inputFile contains space in the name:
      java -jar ../bin/ConvertWeatherData.jar "inputFile .epw"
    
    This will generate the weather data file inputFile.mos, which can be read by the model Buildings.BoundaryConditions.WeatherData.ReaderTMY3.

Location data that are read automatically from the weather data file

The following location data are automatically read from the weather file:

Wet bulb temperature

By default, the data bus contains the wet bulb temperature. This introduces a nonlinear equation. However, we have not observed an increase in computing time because of this equation. To disable the computation of the wet bulb temperature, set computeWetBulbTemperature=false.

Using constant or user-defined input signals for weather data

This model has the option of using a constant value, using the data from the weather file, or using data from an input connector for the following variables:

By default, all data are obtained from the weather data file, except for the atmospheric pressure, which is set to the parameter pAtm=101325 Pascals.

The parameter *Sou configures the source of the data. For the atmospheric pressure, temperatures, relative humidity, wind speed and wind direction, the enumeration Buildings.BoundaryConditions.Types.DataSource is used as follows:

Parameter *Sou Data used to compute weather data.
File Use data from file.
Parameter Use value specified by the parameter.
Input Use value from the input connector.

Because global, diffuse and direct radiation are related to each other, the parameter HSou is treated differently. It is set to a value of the enumeration Buildings.BoundaryConditions.Types.RadiationDataSource, and allows the following configurations:

Parameter HSou Data used to compute weather data.
File Use data from file.
Input_HGloHor_HDifHor Use global horizontal and diffuse horizontal radiation from input connector.
Input_HDirNor_HDifHor Use direct normal and diffuse horizontal radiation from input connector.
Input_HDirNor_HGloHor Use direct normal and global horizontal radiation from input connector.

Length of weather data and simulation period

If weather data span a year, which is the default for TMY3 data, or multiple years, then this model can be used for simulations that span multiple years. The simulation start time needs to be set to the clock time of the respective start time. For example, to start at January 2 at 10am, set start time to t=(24+10)*3600 seconds. For this computation, the used date and time (here January 2, 10 am) must be expressed in the same time zone as the one that is used to define the TMY3 file. This is usually the local (winter) time zone. The parameter `timZon` represents the TMY3 file time zone, expressed in seconds compared to UTC.

Moreover, weather data need not span a whole year, or it can span across New Year. In this case, the simulation cannot exceed the time of the weather data file. Otherwise, the simulation stops with an error.

As weather data have one entry at the start of the time interval, the end time of the weather data file is computed as the last time entry plus the average time increment of the file. For example, an hourly weather data file has 8760 entries, starting on January 1 at 0:00. The last entry in the file will be for December 31 at 23:00. As the time increment is 1 hour, the model assumes the weather file to end at December 31 at 23:00 plus 1 hour, e.g., at January 1 at 0:00.

Notes

  1. In HVAC systems, when the fan is off, changes in atmospheric pressure can cause small air flow rates in the duct system due to change in pressure and hence in the mass of air that is stored in air volumes (such as in fluid junctions or in the room model). This may increase computing time. Therefore, the default value for the atmospheric pressure is set to a constant. Furthermore, if the initial pressure of air volumes are different from the atmospheric pressure, then fast pressure transients can happen in the first few seconds of the simulation. This can cause numerical problems for the solver. To avoid this problem, set the atmospheric pressure to the same value as the medium default pressure, which is typically set to the parameter Medium.p_default. For medium models for moist air and dry air, the default is Medium.p_default=101325 Pascals.

  2. Different units apply depending on whether data are obtained from a file, or from a parameter or an input connector:

  3. Hourly and subhourly timestamp are handled in a different way in .epw files. From the EnergyPlus Auxiliary Programs Document (v9.3.0, p. 63): In hourly data the minute field can be 00 or 60. In this case as mentioned in the previous section, the weather data is reported at the hourly value and the minute field has to be ignored, writing 1, 60 or 1, 00 is equivalent. If the minute field is between 00 and 60, the file becomes subhourly, in this case the timestamp corresponds to the minute field in the considered hour. For example: 1, 30 is equivalent to 00:30 and 3, 45 is equivalent to 02:45.
    (Note the offset in the hour digit.)

  4. The ReaderTMY3 should only be used with TMY3 data. It contains a time shift for solar radiation data that is explained below. This time shift needs to be removed if the user may want to use the ReaderTMY3 for other weather data types.

Implementation

Start and end data for annual weather data files

The TMY3 weather data, as well as the EnergyPlus weather data, start at 1:00 AM on January 1, and provide hourly data until midnight on December 31. Thus, the first entry for temperatures, humidity, wind speed etc. are values at 1:00 AM and not at midnight. Furthermore, the TMY3 weather data files can have values at midnight of December 31 that may be significantly different from the values at 1:00 AM on January 1. Since annual simulations require weather data that start at 0:00 on January 1, data need to be provided for this hour. Due to the possibly large change in weatherdata between 1:00 AM on January 1 and midnight at December 31, the weather data files in the Buildings library do not use the data entry from midnight at December 31 as the value for t=0. Rather, the value from 1:00 AM on January 1 is duplicated and used for 0:00 on January 1. To maintain a data record with 8760 hours, the weather data record from midnight at December 31 is deleted. These changes in the weather data file are done in the Java program Buildings/Resources/bin/ConvertWeatherData.jar that converts EnergyPlus weather data file to Modelica weather data files, and which is described above. The length of the weather data is calculated as the end time stamp minus start time stamp plus average increment, where the average increment is equal to the end time stamp minus start time stamp divided by the number of rows minus 1. This only works correctly for weather files with equidistant time stamps.

Time shift for solar radiation data

To read weather data from the TMY3 weather data file, there are two data readers in this model. One data reader obtains all data except solar radiation, and the other data reader reads only the solar radiation data, shifted by 30 minutes. The reason for this time shift is as follows: The TMY3 weather data file contains for solar radiation the "...radiation received on a horizontal surface during the 60-minute period ending at the timestamp." Thus, as the figure below shows, a more accurate interpolation is obtained if time is shifted by 30 minutes prior to reading the weather data.

image

References

Revisions


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