Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 123

α + ρ + τ = 1

(4.1)

4.1.2 Optical features of absorbers

Absorbers have to absorb radiation and partially convert it into heat. The absorber
is, amongst other things, characterised by being opaque for radiation ( τ = 0) as
expressed in Equation (4.2); the sum of absorption α and reflection ρ at the ab-
sorber area is one.

α + ρ = 1

(4.2)

An ideal absorber does not reflect any short-wave radiation ( ρ = 0) and thus -
in line with Equation (4.2) - completely absorbs solar radiation within this wave
range ( α = 1). For long-wave radiation above a certain boundary wavelength, the
situation is exactly the opposite. Given an ideal absorber, it reflects all of the ra-
diation and does not absorb any at all. Accordingly, the emission in this wave
range is zero (Kirchhoff's law). Fig. 4.1 shows the wavelength dependency of the
absorption and reflection coefficient in the case of an ideal absorber.
Ideal scenarios cannot be completely recreated in real life. So-called selective
surfaces (or selective coatings) are close to the optimal absorber features
(Fig. 4.1). Within the spectrum of solar irradiance, the reflection coefficient ρ real is
close to zero, in the infrared spectrum (> 3 µm) close to one. The absorption coef-
ficient α real demonstrates exactly the opposite.
Table 4.1 shows the absorption coefficient for various different materials and
the transmission and reflection coefficients for the solar irradiance and the infra-
red range of the solar radiation spectrum. Compared to the non-selective absorber,
selective absorber surfaces show high degrees of α s / ε I .

Table 4.1 Optical features of absorbers (according to /4-1/)

Solar irradiance Infrared-radiation α S / ε I

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

Selective
Absorber

Black nickel
Black chromium
Aluminium grid
Titanium-oxide- nitride

0.88
0.87
0.70
0.95

0
0
0
0

0.12
0.13
0.30
0.05

0.07
0.09
0.07
0.05

0
0
0
0

0.93
0.91
0.93
0.95

12.57
9.67
10.00
19.00

Non-selective absorber 0.97 0 0.03 0.97 0 0.03 1.00

α S is the absorption coefficient in the spectrum of solar irradiance, ε I is

the emission coefficient in the infrared radiation spectrum. Such surfaces are thus
also called α / ε -surfaces. For the selective absorbers described in Table 4.1, the