Details:ShortWave

Short-wave Radiation Absorptivity

The heat transfer coefficients already contain a component which describes the long-wave radiation exchange with other surrounding surfaces. The temperature of these surfaces is assumed to be approximately equal to the air temperature.

The solar radiation, however, which shows a pronounced diurnal variation, can by no means be related to the air temperature. Since its effect on the surface temperatures cannot be neglected, it must be treated separately.

Whereas the radiation exchange between the building component and the surroundings (T ≈ 300 K) essentially takes place in the infrared, the spectrum of the solar radiation (T ≈ 6000 K) also contains a considerable amount of short-wave radiation. For this reason, the solar radiation is also simply called 'short-wave radiation'.

The short-wave radiation absorptivity indicates the fraction of the total (i.e., long- and short-wave) solar radiation incident on the component surface which is absorbed:

q = a_s · I
 

q [W/m²]: heat flow due to solar radiation
 
a_s [-]: 'short-wave' radiation absorptivity
 
Iv[W/m²]: solar radiation vertical to component surface

WUFI calculates the radiation load vertical to the surface from the data in the climate file, multiplies it - if positive - by the s.r.a. and applies the result as a heat source at the surface of the component. (If the radiation load computed from the climate data is negative, it is multiplied by the long-wave radiation emissivity.) [##update##]

Examples:

	plaster, white (aged)		0.4
	roof tile, red		0.6
	klinker brick, dark red		0.7
	roof tile, brown		0.8
	bituminous roof covering		0.9
	wood (spruce):
	untreated		0.4
	weathered (silver-gray)		0.7
	painted brown		0.8
	Schilf sandstone:
	dry		0.7
	wet		0.85
	red Main sandstone		0.75
	sandstone with patina		0.9

WUFI offers a list of predefined surfaces with their respective short-wave absorptivities for selection, a user-defined value may be entered as well.

The absorptivity coefficients are entered in the dialog "Surface Transfer Coefficients".