Environmental Engineering Reference
In-Depth Information
coefficient for the outside pane is defined as
α
1
and for the internal pane as
α
2
. The
secondary heat emission degree also depends on the layer thicknesses
s
1
,
s
2
and heat
conductivities
λ
1
,
λ
2
of the two panes and on the thermal resistance
R
air
of the standing
air layer between the panes:
⎛
⎞
α
1
+
α
2
α
2
⎝
⎠
+
s
1
λ
1
+
s
2
λ
2
h
o
R
air
+
=
q
i
(18.3.7)
1
h
i
+
1
h
o
+
s
1
λ
1
+
s
2
λ
2
R
air
+
Q
trans
results
The solar radiation let through (transmitted by) the glazing into the room
directly from the product of the g-value and the solar irradiance:
Q
trans
=
gG
(18.3.8)
The calorific loss through the window is deducted from the transmitted power, which is
characterized by the heat transfer coefficient of the glazing
U
g
or of the entire window
including the frame
U
w
. Double-glazing coated panes filled with heavy noble gases
achieve a minimum
U
g
value of 1.0 W m
−
2
K
−
1
and triple glazing at best reaches a
U
g
value of 0.4 W m
−
2
K
−
1
. Even at a glazing with a
U
g
value of 1.3 W m
−
2
K
−
1
, a wooden
or plastic frame increases the window's
U
w
value slightly. For passive house concepts,
specially insulated expanded polystyrene frameworks must be used to avoid worsening
the low glazing values of triple glazing by the frame proportion.
The passive solar gain
Q
u
usable in a room results from the balance of losses and
gains. The losses are calculated from the
U
w
value of the window of surface
A
w
and
from the temperature difference between the room air
T
i
and the outside air
T
o
:
Q
u
A
w
=
−
−
U
w
(
T
i
T
o
)
gG
(18.3.9)
From the available energy balance, an effective U-value
U
eff
can be defined, which
is often used for monthly or annual balance calculations with mean temperature
differences and irradiances.
Q
u
A
w
(
T
i
−
G
T
i
−
U
eff
=
T
o
)
=
U
w
−
g
(18.3.10)
T
o
Balanced over a heating season, for example about 400 kWh m
−
2
a
−
1
of solar irradi-
ance is available on a south-facing façade in Germany. The so-called heating degree
day number is obtained by multiplying the mean temperature difference between the
inside and outside, about 17
◦
C, by the number of days in the heating season. There are
about 3500 Kelvin-days per year on average in Germany. Thus, the maximum usable
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