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Energetic properties of a glaz facade
The quality of a transparent facade is - besides the undoubted, most important appearance - also reflected by it´s energetic properties. These are the basis of the energetic evaluation of the facade - and also of it´s energetic optimization.
Subsequent the dominant energetic properties of transparent facades are listed; opaque (i.e. tight for radiation) components of the facad are mot considered here.
Solar heat gain and daylight entry
In respect of energy transmission through a glazed facade it must be differentiated between the solar heat gain and daylight entry. The relevant properties for energy transmission - the solar heat gain coefficient (shgc) or solar factor (sf) and the daylight transmission - are explained in detail later. For a start the physical background of these properties of a transparent facade are discussed.
Solar radiation incident on the earth´s surface covers a wavelength between approx. 300...2500 nm and shows a distribution of intensity, which can be seen in the figure above. The spectral distribution of the solar radiation gets some grooves while passing the earth´s atmosphere, which are caused by certain gaz-molecules in the atmosphere (e.g. vapour, carbon-dioxide, etc.).
For the energetic properties of a facade it is very important, that the visible light - the daylight from the sun - is a certain part (approx. 380...780 nm) of the complete spectrum of solar radiation (approx. 300...2500 nm, called "short-wave radiation" also). This is the background why all properties concerning daylight (e.g. daylight transmission, colour of a surface = daylight reflection, etc.) are relating to the wavelength of visible light, which is approx. 380...780 nm. On the other hand all properties concerning solar heat gain (e.g. solar heat gain coefficient/shading factor, radiation transmission, etc.) are relating to the complete wavelength of solar radiation, which covers approx. 300...2500 nm.
The ratio of daylight transmission and solar heat gain coeficient is called selectivity s.
This relationship on different ranges of wave length plays an important role for the optimization of transparent facades. These different spectral distributions are also the reason, why a glazing can show different values for daylight transmision and solar radiation transmission.
Summer resp. cooling
Solar heat gain coefficient (shgc)
Shgc glazing
Design of a insulating glazing
Posibilities for optimization
Spectral distribution
Calculation method
Software
- Softwarepaket WINDOW/OPTICS/THERM des Lawrence Berkeley National Laboratory, USA, mit der International Glazing Database (IGDB)
- Software Advanced Window Information System (WIS) des Window Energy Data Network (WinDat), coordinated by TNO Building and Construction Research (TNO Bouw), Department: Sustainable Energy and Buildings (DEG), The Netherlands
Shgc of a facade with a shading system - shading factor
fundamental classes of shading systems
Definition of the shading factor
Colour of shading components
Sun incident angle and slope angle of venetian blinds
Natural ventilation of shading system
Detailled calculation methods
daylight situation
Winter resp. heating
Heattransferkoefficient (U-Value)
Heattransferkoefficient of a window
Heattransferkoefficient of a curtain wall facade
Aims and purpose of the facade's energetic properties
Topics concerning facade construction
Approaches for facade optimization
... to be completed soon !
