By Stuart Morgan & David Wren
This release of MEPworx includes the ability to make use of hourly weather data in common use by Energy Plus, Open Studio and many other Energy Simulation Programs. There is a wealth of data available on the web in this file format.
We are shipping this release of MEPworx alongside a new Viewer/Weather chooser program.
The weather Chooser utility will allow you to geolocate your nearest weather data for your project and download this in ZIP format.
The Chooser will extract the required files (*.EPW & *.DDY) into the appropriate folder, which is in the standard MEPworx installation folder
MEPworx 2024\DB\Weather\EPW.
Currently the location Chooser will allow you to download Energy Plus Weather data (EPW files) from either National Renewal Energy Laboratory (NREL)
Or from OneBuilding.org.
(There will be a separate follow-up article with more information about the EPW Chooser facility).
However you may obtain EPW/DDY files from any source, as this format for weather data is very widely used. More information regarding the Energy Plus Weather file format can be found here.
This change of data-source for weather, allows MEPworx to have access to rich hourly weather data for tens of thousands of locations around the world. This is instead of the limited brief summary data previously used by MEPworx that had to be manually entered into the software.
Currently MEPworx makes use of various ASHRAE design day data in the *.DDY (Design Day files). A Design Day file typically contains Winter (Heating Design days) as well as Summer (Cooling Design days). For definitions of the various Design Day criteria please see the ASHRAE standard definitions
A typical Design Day extract is shown below:
======================= Annual Design Conditions Sizing Period:DesignDay ======================
The following annual design conditions are reproduced from the climate data shown above.
Heating - 99%, 99.6%; Cooling - .4%, 1%, 2% for several humidity, etc conditions !
Bournemouth.AP_ENG_GBR Extreme Annual Wind Speeds, 1%=11.0m/s, 2.5%=9.7m/s, 5%=8.7m/s
Bournemouth.AP_ENG_GBR Extreme Annual Temperatures, Max Dry bulb=29.0C Min Dry bulb=1.6C
Bournemouth.AP_ENG_GBR ASHRAE Climate Zone=4A
Bournemouth.AP_ENG_GBR Annual Heating Design Conditions Wind Speed=1.5m/s Wind Dir=350
Coldest Month=JAN
Bournemouth.AP_ENG_GBR Annual Heating 99.6%, Max DB=-4.3C
SizingPeriod:DesignDay,
Bournemouth.AP Ann Htg 99.6% Condns DB, !- Name
1, - Month
21, - Day of Month
Winter Design Day,- Day Type
-4.3, - Maximum Dry-Bulb Temperature {C}
0.0, - Daily Dry-Bulb Temperature Range {C}
Default Multipliers, - Dry-Bulb Temperature Range Modifier Type
, - Dry-Bulb Temperature Range Modifier Day Schedule Name
Wet bulb, - Humidity Condition Type
-4.3, - Wet bulb at Maximum Dry-Bulb {C}
, - Humidity Indicating Day Schedule Name
, - Humidity Ratio at Maximum Dry-Bulb {kgWater/kgDryAir}
, - Enthalpy at Maximum Dry-Bulb {J/kg}
, - Daily Wet-Bulb Temperature Range {deltaC}
101186., - Barometric Pressure {Pa}
1.5, !- Wind Speed {m/s} design conditions vs. traditional 6.71 m/s (15 mph)
350, - Wind Direction {Degrees; N=0, S=180}
No, - Rain {Yes/No}
No, - Snow on ground {Yes/No}
No, - Daylight Savings Time Indicator
ASHRAE Clear Sky, - Solar Model Indicator
, - Beam Solar Day Schedule Name
, - Diffuse Solar Day Schedule Name
, - ASHRAE Clear Sky Optical Depth for Beam Irradiance (taub)
, - ASHRAE Clear Sky Optical Depth for Diffuse Irradiance (taud)
0.00 - Clearness {0.0 to 1.1}
As a User you can add additional design days, or edit existing design days, using a simple text editor such as Notepad etc.
Within the Building program you can set the weather location from the Edit-General Information dialog, on the Further Details tab.
When a weather location has been selected for use in Building, that data is also available in the Energy program’s Edit- General Plant dialog.
After selecting an EPW file, in Building’s Edit-General Information dialog, you can use the View button (or the previously described Weather button in Energy) to review the data being used by MEPworx for the selected weather location.
The ‘EPW/DDY Weather Data’ dialog is broken into three sections.
Section i (Design Day Data)
You can choose one of many Heating Design Day conditions. Typically, the 99.6 percentile heating day is worst case scenario but other design days may be considered (such as the windiest heating design day). Once a selection of heating design day is made, then the statistics used for that day will be shown.
Note, the first dataset on the list may not necessarily reflect the highest or lowest temperature so it is worth perusing months conducive to the calculation i.e. July, Aug, Sept selections for Cooling and Dec, Jan, Feb for the lowest winter temperatures.
e.g. The external DB temperature assumed, the date, the wind speed etc.
A Selection should also be made for the Cooling Design Day you wish to use in the calculations for heat-gains. Again after making a selection (typically the 0.4% DB>MWB is the worst case design day in terms of external DB temperature), the design days used will affect the calculation for both heat-losses and heat-gains.
The pull-down lists can be used to select conditions for recorded Winter Design Days.
Also, to select conditions for recorded Summer Design Days.
As monthly data is available throughout the year the user, could, if they wish select December to look at a heat gain calculation where, if shading is used for summer months, glazing gains can be examined which may actually be higher as the sun is lower in the sky. This affords the user more flexibility compared with the existing CIBSE/MET office data previously used.
Section ii - Location Data
The Project’s location (Latitude/longitude, height above sea level (M), time zone etc) will all be automatically set up from the data in the EPW file.
Section iii (Hourly/Monthly Weather Statistics)
The spread sheet will show climatic hourly/monthly summaries for which ever item is currently
selected in the drop down list.
The Direct and Diffuse Solar Radiation figures for the selected weather location will be directly utilised for the solar gain calculation. This will produce much more accurate predictions of solar gain at any given location than in previous versions of MEPworx, which used a very simplified solar model based on project Latitude. Observed/measured hourly values taken from a weather station will produce more reliable estimates of solar gain W/m2.
Dry/Wet Bulb temperatures and ranges are also used in the calculation for summer-time temp to setup sinusoidal day/night temp ranges.
Also included in Building’s Face Data dialog, Ground Temperature from the EPW Weather data can be used to evaluate losses through exposed floors that are in direct contact with the ground.
You can select the Exposed Floor to be at room temperature less ground temperature, by checking the box for Auto.
Then when calculating the room losses the dt used will be the room design temperature minus the ground temperature for the selected heating design day.
When the Auto box is checked, when calculating the room losses, the temperature used will be the room temperature minus the ground temperature for the selected heating design temperature.
Please also refer to MEPworx in-program’s Help (F1) for more context appropriate information.
There will be further use made of the data available in the weather files in future releases of MEPworx.