Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 124

ratios α s / ε I are between 9 and 19. Titanium oxide with 19 for example shows a
particularly high α s / ε I -ratio.

1

α ideal

ρ real

0,8

0,6

0,4

0,2

α real

ρ ideal

0

0,1 0,2 0,3 0,5 1 2 3 5 10

Waveleght in µm

Visible
light

Ultraviolet Infrared

Fig. 4.1 Absorption ( α ) and reflection coefficient ( ρ ) of an ideal ( ideal ) and a standard real
absorber ( real )

4.1.3 Optical features of covers

In order to reduce the convective thermal losses of the absorber to the environ-
ment (Chapter 4.1.4), in many cases absorbers used in solar thermal systems have
a transparent cover. Ideal covers have a transmission coefficient of one in the
range of solar radiation, whereas reflection and absorption coefficient equal zero
in this spectrum.

Table 4.2 Optical features of covers (according to /4-1/)

Solar spectrum Infrared radiation

α S ( ε S ) τ S ρ S α I ( ε I ) τ I ρ I

Sheet glass
Infrared reflecting glass (In 2 O 3 )
Infrared reflecting glass (ZnO 2 )

0.02
0.10
0.20

0.97
0.85
0.79

0.01
0.05
0.01

0.94
0.15
0.16

0
0
0

0.06
0.85
0.84

In real life such conditions cannot be achieved. Table 4.2 shows the attributes
of the different cover materials. According to that table, glass fulfils the required
optical features within the luminous spectrum very well. Infrared light emitted by
the collector , however, cannot pass through, but is mainly absorbed. If the degree
of absorption is high, the temperature of the glass cover rises and the radiation
losses to the environment are correspondingly high according to Kirchhoff's law.
These losses can be reduced by vacuum-coating of layers that reflect infrared
light.