Environmental Engineering Reference
In-Depth Information
Table 3.5
Absorption, Transmission and Reflection Factors for IR Glass
In
2
O
3
and ZnO
2
Compared with Ordinary Window Glass
Visible
Infrared
α
ε
τ
ρ
α
ε
τ
ρ
Material
=
=
Window glass
0.02
0.97
0.01
0.94
0
0.06
In
2
O
3
0.10
0.85
0.05
0.15
0
0.85
ZnO
2
0.20
0.79
0.01
0.16
0
0.84
Source:
Kleemann and Meliß, 1993
The material for the
rear heat insulation
must be temperature resistant and
highly insulating. Suitable materials are polyurethane foam or mineral fibre
slabs. The insulation and all other materials should not contain binding agents
that may evaporate at higher temperatures because these could condense on
the front glass pane and reduce the incoming solar radiation.
The collector can reach stagnation temperatures of about 200°C. All
materials used must therefore resist these temperatures. As a result, the
absorber is usually made of metal such as copper, steel or aluminium. Various
absorber designs are used as shown in Figure 3.9. Absorbers with soldered or
forced copper pipe are the most commonly used today. Aluminium is now little
used as absorber material, since it is not corrosion-resistant and its production
needs much more energy input than other materials.
It is well known that black materials absorb sunlight very well and warm
up to higher temperatures. However, metallic materials do not naturally have
black surfaces and must therefore be coated. Black lacquer is one option.
Temperature-resistant lacquer serves its purpose but there are more advanced
materials available for
absorber coating
. If a black surface heats up, it re-emits
part of the absorbed heat energy as heat radiation. This can be noticed with
electrical hotplates for cooking. The heat radiation can be felt on the skin
without touching the hotplate. A black lacquered absorber exhibits the same
effect. It transfers only a part of the absorbed heat to the water flowing
through; the rest of the heat is undesirably emitted as heat radiation to the
environment.
So-called
selective coatings
absorb the sunlight nearly as well as black
lacquered surfaces but re-emit a significantly smaller amount of heat radiation.
Materials used for these advanced coatings include black chrome, black nickel
or TiNOX. However, they need more complicated coating processes than black
lacquer and so incur higher costs.
Figure 3.10 shows the different material behaviours. It is necessary to look
at the spectrum of the heat radiation to describe the principle of selectively
coated absorbers. The
radiance L
e
depends on the absolute temperature
T
and
wavelength
λ
and is given according to Planck by:
(3.10)
