Create *.WAC climate files
The “boundary conditions” required for the calculation describe the thermal and hygric influences that the environment exerts on the component. They are usually expressed in meteorological quantities such as air temperature, humidity, etc. WUFI typically reads these values from weather files containing hourly weather data measured at a specific geographic location or simulated for a location. There are numerous different weather file formats, some of which use different physical quantities to describe the same boundary conditions (e.g. relative humidity or water vapor partial pressure or wet bulb temperature, etc. to describe the moisture content of the air).
In addition, heat radiation (such as radiated by the sun and surrounding surfaces) and rain (especially driving rain) are directional quantities. Their influence on the observed surface therefore depends on the inclination and orientation of the surface. Sometimes measurements of these directional quantities are available, obtained directly on the area under study, which can then be used directly by WUFI. As a rule, however, there are no direct measurements, and the radiation or rain input to the surface must be derived from other measurements, usually from measurements on a horizontal receiving surface.
WUFI’s *.WAC format for weather files allows flexibility in the selection and compilation of different physical or meteorological data and in the handling of directional quantities. WUFI itself carries out the physical or geometric conversions if necessary.
In order to calculate the solar radiation incident on any surface from the values measured on a horizontal receiving surface, WUFI needs the diffuse as well as the direct or global radiation incident on the horizontal surface, the geographical coordinates and the time zone.
To calculate the driving rain falling on any surface, WUFI needs the normal rain (i.e. the rain falling on a horizontal surface), the wind speed and the wind direction.
To simplify the creation of a *.WAC file, the Excel worksheet CreateClimateFile.xls is supplied with WUFI (in the Tools folder). The user only needs to copy the hourly data into the columns of the worksheet and select the appropriate column description. By pressing a button, the creation of a *.WAC file with the entered data is started.
The following demo films show some possible combinations of input data for a *.WAC file. It is assumed that the user has already obtained the required data and copied it into the worksheet using copy and paste. The columns can be in any order.
Creation of an indoor climate file
For an indoor climate file, only indoor air temperature and humidity are required. In this example, we are even satisfied with using constant climate values. It is therefore sufficient to fill the columns of the worksheet by copying the same value into all rows using Copy & Paste or using Excel’s AutoFill function.
Creation of a weather file with directional data
Creation of an outdoor climate file (1)
An outdoor climate file is created for a west-facing vertical surface using solar radiation and driving rain data measured directly on the surface in question.
In this case, standard radiation and precipitation data (i.e. measured on a horizontal surface) are not needed and WUFI does not need to make any directional conversions. WUFI’s calculation results are not affected by any inadequacies of the conversion models, but the weather file can only be used for surfaces of the given slope and orientation.
Creation of an outdoor climate file (2)
An outdoor climate file that can be used for any oriented and inclined surfaces is created. It contains the global and diffuse solar radiation measured on a horizontal surface, from which WUFI calculates the radiation on any surface. In this example, rain is not an issue and no rain data is needed. If hourly air pressure data is available, altitude does not need to be specified.
Creation of an outdoor climate file (3)
An outdoor climate file that can be used for any oriented and inclined surfaces is created. WUFI calculates the solar radiation and driving rain input on an arbitrarily inclined and oriented surface from the standard data in the file.
Creation of an outdoor climate file suitable for evaluating the ‘explicit radiation balance’
In building physics, the long-wave radiation exchange between the facade and the surroundings is usually not explicitly considered, but is only taken into account by adding a suitable addition to the heat transfer coefficient.
For a more precise calculation of the surface temperatures, the ‘Explicit radiation balance’ calculation mode can be activated in WUFI, in which all short- and long-wave radiation components that occur are explicitly taken into account.
Of course, the user must then also provide sufficient data about all of these radiation components. If necessary, WUFI can convert short-wave or long-wave radiation measured on a horizontal surface to the area under study, and it can estimate long-wave radiation data from other weather data.
Creation of an outdoor climate file (4)
In this example, the user has global and diffuse radiation, measured on a horizontal surface (WUFI automatically converts to the inclination and orientation of the surface to be examined), as well as long-wave radiation, measured directly on the surface to be examined (so that WUFI does not need to convert the long-wave radiation data).Creation of an outdoor climate file (5)
Here the user does not have any long-wave radiation data measured directly on the surface in question. WUFI must therefore convert the long-wave radiation falling on the surface to be examined from standard data (i.e. measured on a horizontal surface). For this conversion, WUFI requires separately the atmospheric and terrestrial long-wave counter-radiation on a horizontal surface.
Creation of an outdoor climate file (6)
In this example, WUFI is supposed to work in ‘Explicit Radiation Balance’ mode, but no data on atmospheric and terrestrial long-wave counterradiation is available. In this case, WUFI can attempt to estimate long-wave radiation from air temperature and humidity as well as the degree of cloudiness (colud index).
The atmospheric counterradiation is estimated from the air temperature and humidity as well as the cloud index. Terrestrial counterradiation is estimated assuming that the entire terrestrial environment is at air temperature and has the terrestrial emissivity entered by the user in WUFI’s Explicit Radiation Balance dialog. Depending on the inclination of the surface under study, the respective components of the atmospheric and terrestrial counterradiation are added together and result in the total long-wave counterradiation incident on the surface.