Civil Engineering Reference
In-Depth Information
100
Semiconducting
Metallic
80
60
40
20
0
0.05
0.1
0.15
Thickness (
0.2
0.25
0.3
μ
m)
100
Semiconducting
Metallic
80
60
40
20
0
0.05
0.1
0.15
Thickness (
0.2
0.25
0.3
μ
m)
11.7
Computed data on luminous (upper panel) and solar (lower
panel) transmittance as a function of the thickness of VO
2
fi lms in
semiconducting (
τ
<
τ
c
) and metallic (
τ
>
τ
c
) states. From Li
et al.
(2010).
The fi ndings in Fig. 11.7 permit an identifi cation of two properties of
thermochromic VO
2
fi lms that severely limit their applicability to energy-
effi cient fenestration:
•
the modulation of solar energy, given by
Δ
T
sol
≡
T
sol
(
τ
<
τ
c
) -
T
sol
(
τ
>
τ
c
),
is no larger than
∼
10%, and
•
T
lum
is no higher than
∼
40% for fi lms with a signifi cant magnitude of
Δ
T
sol
, which is too low for most applications to windows in real buildings
(Wigginton 1996).
A third limitation, obviously, is that the change of the solar transmittance
should take place in the vicinity of a comfort temperature of
∼
25°C, whereas
•
τ
c
≈
68°C for bulk VO
2
(Morin, 1959).
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