This is expected. The difference comes from interpolating hourly weather
data
for subhourly timesteps in EnergyPlus. In an hourly weather file, the
temperatures and other state-point readings are the value at the time the
reading was taken. For example, in the USA_IL_Chicago-OHare_TMY2.epw
file, the
outdoor dry bulb value for July 2, hour 1, is 19.4C. This is the
temperature
at 1:00 am.
If you set Timestep = 1, then EnergyPlus will report 19.4C for 07/02
01:00 and will use that value for the entire one hour timestep.
If Timestep = 4, then 19.4C is used only for the time step which ends
at 01:00. The other timesteps use linearly interpolated values between
the
hourly weather file values. When you report at the "hourly"
frequency in
EnergyPlus, you see the average temperature over the hour. If you report
at
the "timestep" frequency, you will see the values from the
weather data file
appear at the last timestep of each hour.
In summary:
The standard EnergyPlus weather files (epw) contain hourly data.
For state variables such as temperature and humidity, the values in the
epw file are the instantaneous measurements at 1:00 am, 2:00 am, and so
on. Because EnergyPlus uses zone time steps of less than one hour,
these values are linearly interpolated for each time step. Weather
data values reported from EnergyPlus at the Timestep frequency will match
the weather file one timestep per hour on the hour. Values reported
at the Hourly frequency represent the average value over the hour, so
they will not match the instantaneous value in the weather file.
The solar radiation values in the weather file are integrated total
values for the hour, so the interpolation must be done differently.
The total radiation in [Wh/m2] is used as an average rate for the hour in
[W/m2], and this average rate is assumed to be the value at the midpoint
of the hour. Again, linear interpolation is used to determine the
value for each timestep. The reported solar values in the output
will match the value in the weather file on the half hour.
At 07:52 AM 4/7/2011, china4ever2003 wrote:
I've opened the EPW file and the
E+ output:variables CSV file to compare. I've noticed that the
temperatures, wind speed, etc. (outdoor data) are different. Can someone
tell me why? THANKS!
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