[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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





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!


__._,_.___


Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are currently allowed but be mindful that not everyone has a high speed connection.  Limit attachments to small files.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.




Your email settings: Individual Email|Traditional
Change settings via the Web (Yahoo! ID required)
Change settings via email: Switch delivery to Daily Digest | Switch to Fully Featured
Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe

__,_._,___





=======
Email scanned by PC Tools - No viruses or spyware found.
(Email Guard: 7.0.0.21, Virus/Spyware Database: 6.17260)
http://www.pctools.com
=======