[EnergyPlus_Support] Re: EPW values different than E+ output of weather data

["china4ever2003" <lisa.ng@xxxxxxxx>]

Thank you!

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Linda Lawrie <linda@...> wrote:
>
> Standard reply from the helpdesk:
> 
> 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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