Solar energy use in buildings - Solar Energy Sciences and Engineering Applications - page 592

Table 18.2.1 U values required to reach passive standard for different European climates

U values Rome Helsinki Stockholm

/Wm − 2 K − 1 Passive building Passive building Passive building

Wall 0.13 0.08 0.08

Window 1.4 0.7 0.7

Roof 0.13 0.08 0.08

Ground 0.23 0.08 0.1

Mean 0.33 0.16 0.17

Figure 18.2.2 Wavelength-dependent transmittance of a single glazing.

18.2.1 Passive solar gains by glazing
Glazing is characterized by high transmittances for short-wave solar irradiance up to
wave lengths of 2.5

m and an impermeability to long-wave radiant heat emitted from
building components with a maximum intensity at wavelengths of around 10

µ

m, as
demonstrated by Figure 18.2.2. This transmission characteristic results in a greenhouse
effect and the conversion of solar irradiance by absorption in structural elements into
heat, the long-wave radiation proportion of which is not transmitted through the
glazing.

µ

18.3 TOTAL ENERGYTRANSMITTANCE OF GLAZING

Aside from the direct transmission of short-wave solar radiation with a transmittance
τ , part of the irradiance is absorbed in the panes, as illustrated by Figure 18.3.1.
Heating the panes causes a heat flow towards the room, which contributes to the total
energy transmission factor (g-value). The absorption coefficient of single glazing can
be up to 50% for special sun-protection glazing. The secondary heat emission degree
q i is defined as the relation of the heat flow on the room side Q i per square metre of
window area A w to the impacting solar radiation G. The value of q i is calculated by