Environmental Engineering Reference
In-Depth Information
house (see Equation (3.3)). However, transmission losses must not be mistaken
for the transmission coefficientτ
e
of transparent structural elements.
m
(
(
)
)
&
=
∑
=
Q
U
⋅
A
⋅
θ −
θ
(3.3)
T
n
n
i
,
n
e
n
1
3.2.2
System components
Passive solar systems may consist of transparent covers (such as windows, trans-
parent thermal insulation), absorbers, heat stores and/or shading devices. Follow-
ing is a detailed description of all system components.
Transparent covers.
As an example, Fig. 3.2 illustrate the energy flow through
double-glazing. Only one part of the incident solar radiation is transferred into the
interior, whereas the remaining part is reflected from the outer pane surface. The
radiation share directly transferred into the interior through both panes is indi-
cated, in proportion to the radiation incidence on the outer pane surface, by the
transmission coefficient τ
e.
Another portion of the incident solar radiation is ab-
sorbed by the glass panes and heats up the gap between the two panes, and thus
leads to further heat transmission into the interior by long wave radiation and
convection. The
g
-value or energy transmittance factor indicates the ratio of total
heat transferred into the interior and the incident radiation.
100 % global
radiation
50 % solar
radiation
transmission
26 % reflection
12 % secondary
outward heat
emission by
long wave
radiation and
convection
12 % secondary
inward
heat flow
by long wave
radiation and
convection
Fig. 3.2
Total energy transmittance factor of an average thermal insulation double-glazing
(see /3-1/)
Transparent covers (such as windows) serve to transmit a maximum share of
solar radiation to the interior and ensure at the same time utmost insulation from
the outside. Typically, these two properties are expressed by the
g
-value (energy
transmittance factor) and the
U
-value (thermal transmittance coefficient).
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