Q. When I run my simulation, the weather data reported in
the output file doesn't match the values in the weather file. Why
is that?
A. If you are reporting values hourly, the values should match. If
you are reporting values sub-hourly (i.e., detailed or timestep), then
the weather data are interpolated between the hourly values in the
weather file.
More explanation: 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.
This is also covered in the Tips & Tricks document.
At 04:30 PM 1/27/2012, Seth Moody wrote:
Would anyone have insight into
why my custom TMY file drybulb temperature doesn't match my model outdoor
drybulb temp?