Environmental Engineering Reference
In-Depth Information
Soldered
copper pipe
Extruded
copper pipe
Aluminium
rollbond
Pillow-shaped
absorber
Figure 3.9
Various Designs of Solar Absorber
The constants
c
1
and
c
2
are:
(3.11)
and
(3.12)
Ω
0
= 1 sr is needed for the correct balance of the units. The steradian (sr) is
the unit of the solid angle. The sun can ideally be considered as a black body
with a temperature of 5777 K.
The majority of the solar spectrum is in the wavelength range below 2 µm.
The absorber should have a very high absorptance in this range. The sun heats
up the absorber to about 350 K. The maximum of the corresponding
emittance spectrum is higher than 2 µm. Since the absorptance
α
is identical
to the emittance
according to Kirchhoff's law of emission of radiation, the
absorptance should be as low as possible in the range above 2 µm so that the
heated absorber emits only a little heat radiation to the environment.
Figure 3.11 shows the relative radiance referred to the maximum of the
corresponding spectra at 350 K and 5777 K as well as the absorptance of a
selective and non-selective absorber. Table 3.6 summarizes parameters of
selective and non-selective absorber materials. Many collectors that are sold
today use selective coatings such as TiNOX (TiNOX, 2004).
A
vacuum
between the front glass cover and the absorber can significantly
reduce convection heat losses due to air movements inside the collector. The
ε
Reflecting surfaces
reflect the solar radiation
Black surfaces
absorb the solar radiation
and emit back a high amount
of heat radiation
Selective coated surfaces
absorb the solar radiation
and emit back only a low
amount of heat radiation
Figure 3.10
Losses at Absorber Surfaces with Different Types of Coating
