Re: [EnergyPlus_Support] Re: Changes to epw file

[Linda Lawrie <linda@xxxxxxxxxxxxxx>]

Search on "interpolation" in the document and you
will find it.

(You can have multiple interval files, see the Auxiliary Programs
document).  If you find that your output does not match the EPW
values:

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 06:58 AM 3/17/2012, sachin sharma wrote:

Hey Linda,

Thanks a lot that was of great help. I followed the steps but there is one issue which I came across while going through the steps. The weather file has hourly report, however, in energyplus if we use the output variable of OA DBT we can get temperature of OA with timesteps of 15mins interval as well. So, how does energyplus calculate the OA DBT for that interval, as it is not mentioned in the weather file. Or does it work on some sought of algorithm.

regards,
SACHIN SHARMA



--- On Fri, 3/16/12, L Lawrie <linda@xxxxxxxxxxxxxx> wrote:

From: L Lawrie <linda@xxxxxxxxxxxxxx>

Subject: [EnergyPlus_Support] Re: Changes to epw file

To: EnergyPlus_Support@xxxxxxxxxxxxxxx

Date: Friday, March 16, 2012, 4:07 AM

 

My suggested set of steps is similar to the other user:

How do I modify an EPW file?

The easiest way is:

1) run the EnergyPlus weather converter on the original file

2) convert the file to "EnergyPlus CSV"

3) open the EnergyPlus CSV file in your favorite spreadsheet program.

4) make the mods, saving it as CSV from the spreadsheet program.

5) run the weather converter again, this time selecting the CSV and make sure the File type is "EnergyPlus CSV"

6) convert it to EPW.

if your regional settings aren't like US, you could run into a couple of problems, such as the spreadsheet program changing the DataPeriods line from 1/1, 12/31 to 01-Jan -- you'd need to hand edit that before step 5

Be aware that specific year data is not usually deemed appropriate for Energy Analysis.

See this document for more details:

Information on selecting weather data is described in this paper:

Drury B. Crawley. 1998. "Which Weather Data Should You Use for Energy Simulations of Commercial Buildings?" in ASHRAE Transactions, pp. 498-515, Vol. 104, Pt. 2. Atlanta: ASHRAE. (PDF 197 KB)

PDF: http://energyplus.gov/pdfs/bibliography/whichweatherdatashouldyouuseforenergysimulations.pdf

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