Environmental Engineering Reference
In-Depth Information
Table 3.4 Heat Transition Coefficient k and Total Energy Transition
Coefficient ( g -value) of Various Conventional Materials and Transparent
Insulation Materials (TIMs)
Conventional
TIM between low-iron glass with
glass covers
air gap
Material
k in
g -value
Material
k in
g -value
W/(m 2
K)
W/(m 2
K)
1 layer of glass
5.9
0.86
Aero gel
0.85
0.4
(4 mm)
granulate
(20 mm)
2 layers of
3.0
0.77
Polycarbonate
0.7
0.66
insulating glass
honeycomb
(20 mm)
structure (100 mm)
3 layers of glass
1.0-1.2
0.53-0.62
Polycarbonate
0.7
0.64
with IR coating
capillary structure
(36 mm)
(100 mm)
Note: IR = infrared reflecting
construct. If the system is used in regions with a danger of frost, heat losses
cool both the collector and storage; ultimately, they can freeze and be
damaged. A double-circuit system with frost protection is not possible to
realize as an integral collector storage system. A way needs to be found for
achieving significant reduction of the collector heat losses. Better insulation on
the back is not a problem - the problem is heat losses through the front cover.
Sunlight must pass through the front with low absorption and reflection losses.
The cover must therefore be transparent, and yet this leads to large heat losses
through the cover. A vacuum can reduce the heat losses, but not as much as is
necessary to design an integral collector storage system.
New so-called transparent insulation materials (TIM) brought a solution
to these problems (Lien et al, 1997; Manz et al, 1997). These materials have a
slightly lower transmittance compared to low-iron solar safety glass. However,
the heat transition coefficient is significantly lower so that the heat losses are
reduced to levels acceptable for ICS systems. Table 3.4 compares various
conventional and TIM covers.
Figure 3.5 shows a sketch of an ICS system. The hot water tank is made of
stainless steel. The back is perfectly insulated. Reflectors on the inner side of
the back reflect the light to the storage, which is also the absorber of this
system. The transparent insulation material is under the glass front cover. A
system covering two square metres has a storage volume of about 160 litres.
ICS systems do not need an external heat storage tank, which is necessary
for other collector systems. The total system is simpler: some components
found in other collector systems are not necessary and this reduces the cost. If
the water temperature in the storage tank is too low, an auxiliary, thermostat-
controlled water heater can boost the temperature up to the desired level.
 
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