Knowledgebase : Weather

Question: Where can I get value for Site:GroundTemperature:Deep?  Can I get it from the weather file or anywhere else?

Answer:  One source is the statistics summary of the epw file (.stat file included in epw zip files). If you do not have a .stat file, you can create it for an epw file using the EnergyPlus Weather Statistics and Conversion utility (see Chapter 2 Weather Converter Program in AuxiliaryPrograms.pdf)



Site:GroundTemperature:Deep
Site:GroundTemperature:Deep are used by the Pond Ground Heat Exchanger and Vertical Ground Heat Exchanger objects (i.e. objects: GroundHeatExchanger:Pond and GroundHeatExchanger:Vertical). Only one deep ground temperature object can be included.
Note that the ground temperatures included in full year weather files may be suitable of being used for the values in these fields – namely, the 4 m depth temperatures that are calculated for “undisturbed” soil of “typical” conditions. However, you may wish to use some other change effect – based on the weather conditions or special knowledge of the building location.

The easiest way is:
1) run the 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


The weather data by request from the EnergyPlus team was a volunteer (unpaid) activity which used Meteonorm -- a program that created weather files by interpolation based on climate data statistics. Because a number of groups now provide weather data for virually any location in the world at a reasonable price, the EnergyPlus team will no longer provide weather files from Meteonorm. The Yahoo Groups site is at capacity.

Two places you can obtain weather files in EPW format are WeatherAnalytics and Meteonorm:

Weather Analytics (www.wxaglobal.com) - Site specific weather files in EnergyPlus format based on the latest 30 years of hourly data are now available from the private sector company Weather Analytics for any official weather station or over 600,000 35-km grid tiles across the globe. These files are built by integrating hourly weather station observations and the new NOAA reanalysis data sets. Both Typical Meteorological Year (TMY) files and individual, Actual Meteorological Year (AMY) files are available as well as files constructed from the previous 12 months.


Meteonorm (www.meteonorm.com) - Files for specific locations can be purchased in EnergyPlus format from Meteonorm company. Meteonorm extrapolates hourly data from statistical data for a location. Where statistical data aren't available, Meteonorm interpolates from other nearby sites. Generally a statistical approach is a last resort--weather files generated from statistics will not demonstrate the normal hour-to-hour and day-to-day variability seen in measured data.


Information on selecting weather data appropriate for energy simulation 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
The EnergyPlus team can provide weather files appropriate for energy simulation created using Meteonorm. These are posted on the EnergyPlus YahooGroup under Files > Meteonorm_Weather_Files. Locations are listed alphabetically by continent (WMO region) and country. More than 300 locations are currently available--look there first before asking for a new weather file.
Link to the file: http://tech.groups.yahoo.com/group/EnergyPlus_Support/files/Meteonorm_Weather_Files/Meteonorm_Weather_File_Inventory.xlsx

Information on selecting weather data appropriate for energy simulation 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

Requesting Meteonorm Weather Data for Your Location

Send the required information described below

In your message, you will need to send:

Name of Location (including country and state/province, if appropriate)
Latitude (N+/S-)(decimal format required)
Longitude (E+/W-)(decimal format required)
Time Zone (+/- GMT)
Elevation (meters)

For convenience, you can convert lat/long in degrees minutes seconds to the decimal format by visiting this web link: http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html

NOTE: It can take a few days for the weather files to be created. Creation of the weather files is done on an unpaid, volunteer basis.

And you should be aware of Meteonorm limitations:

Meteonorm extrapolates hourly data from statistical data for a location. Where statistical data aren't available, Meteonorm interpolates from other nearby sites. Generally a statistical approach is a last resort--weather files generated from statistics will not demonstrate the normal hour-to-hour and day-to-day variability seen in measured data.

Each .ZIP includes a .STAT (EnergyPlus weather data statistics), .EPW (EnergyPlus weather file), and .INFO (Information about the source data and limitations from Meteonorm).

IN ALL CASES, YOU SHOULD REVIEW THE .STAT FILE FOR THE LOCATION BEFORE USING ANY OF THESE FILES TO ENSURE THAT IT REPRESENTS THE CLIMATE OF THE LOCATION AS YOU UNDERSTAND IT. IN MANY CASES, A NEARBY LOCATION WITH MEASURED DATA MAY BE MORE APPROPRIATE THAN ONE DERIVED FROM STATISTICS.

YOU USE THESE FILES ENTIRELY AT YOUR OWN RISK.

As always, if you know of sources of weather data that we might be able to share with the EnergyPlus community, please contact us.

EnergyPlus Development Team
Q: I have a question about the following metrics in weather file. Can they be directly measured by some inexpensive devices?

Extraterrestrial Horizontal Radiation {Wh/m2}
Extraterrestrial Direct Normal Radiation {Wh/m2}
Horizontal Infrared Radiation Intensity from Sky {Wh/m2}
Global Horizontal Radiation {Wh/m2}
Direct Normal Radiation {Wh/m2}
Diffuse Horizontal Radiation {Wh/m2}
Global Horizontal Illuminance {lux}
Direct Normal Illuminance {lux}
Diffuse Horizontal Illuminance {lux}*

A: You can't measure extraterrestrial unless you're in outer space, but then it's assumed to be constant anyway. For the various radiation and illuminance values, they can measured by various instrumentation ranging from the very cheap to the very expensive. Properly, radiation needs to be measured with a pyranometer (Eppley), which is pricy, but I'm also seen people use simpler apparatus (Lycors) that are really photometers. Direct beam is generally not measured, but derived by subtracting the diffuse from the global. Diffuse is measured by adding a shadow band over a pyranometer to block out the direct beam. Pyranometers measure heat, photometers measure light. Don't know what people in our field do to measure illuminance. I suppose it must be simple (lots of photometers around), I just seen any. All the illuminance on the weather files are derived from the radiation and sky conditions.

Do not forget that the quantities you list are in fact the inputs to the models that are used to derive the variables you really need in practice: irradiance and illuminance on the facets of the building (windows especially). These facets are usually NOT horizontal. Measuring all the components for all tilts and azimuths can be a costly proposition, and that's why it is rarely done (hence the need for models), but that's what should be done in serious experiments to remove the (large) uncertainties in modeled radiation.

Illuminance is measured with photometers (from, e.g., Licor), which resemble silicon-based pyranometers. Both are less costly than thermopile radiometers, which are normally the best in terms of accuracy. Measurements obtained with silicon-based pyranometers need various corrections to account for their limited spectral range. No correction is needed for photometers, though. So you have this issue of accuracy vs cost to consider.

Direct irradiance is measured with a pyrheliometer, which tracks the sun and is therefore costly, but also the most accurate of all radiometers. Obtaining direct irradiance by subtracting diffuse from global is convenient, but not accurate, as shown in recent publications.

The sources for all the EnergyPlus weather data are available here:
http://apps1.eere.energy.gov/buildings/energyplus/weatherdata_sources.cfm
Here is a caveat about Meteonorm files:
Over the last few years, a number of users have needed weather data to use
with EnergyPlus where we don't have measured data (on the EnergyPlus web
site: http://www.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm).
When possible, we have created weather data using the Meteonorm
software. Meteonorm extrapolates hourly data from statistical data for a
location. Where statistical data aren't available, Meteonorm interpolates
from other nearby sites. Generally a statistical approach is a last resort--
weather files generated from statistics will not demonstrate the normal
hour-to-hour and day-to-day variability seen in measured data.

Each .ZIP includes a .STAT (EnergyPlus weather data statistics), .EPW
(EnergyPlus weather file), and .INFO (Information about the source data and
limitations from Meteonorm).

IN ALL CASES, YOU SHOULD REVIEW THE .STAT FILE FOR THE LOCATION BEFORE USING
ANY OF THESE FILES TO ENSURE THAT IT REPRESENTS THE CLIMATE OF THE LOCATION AS
YOU UNDERSTAND IT. IN MANY CASES, A NEARBY LOCATION WITH MEASURED DATA MAY BE
MORE APPROPRIATE THAN ONE DERIVED FROM STATISTICS.

YOU USE THESE FILES ENTIRELY AT YOUR OWN RISK.

As always, if you know of sources of weather data that we might be able to
share with the EnergyPlus community, please contact us.
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.