Environmental Engineering Reference
In-Depth Information
As only direct radiation can be concentrated, the use of concentrating collectors
from a technical point of view only makes sense in areas with a high level of di-
rect radiation. In Central and Northern Europe they are practically not used at all.
6,000
Typical w
orking tem
perature
Maximum theoretical temperature
5,000
4,000
3,000
2,000
1,000
0
1
10
100
1,000
10,000
100,000
Concentration ratio
Spherical paraboloidal
mirror
Parabolic
heliostats
Parabolic
cylinder collectors
Flat-plate collector
Fig. 4.5
Theoretical maximum absorber temperature and actual temperatures of concen-
trating collectors (e.g. /4-1/, /4-2/, /4-3/)
Data and characteristic curves.
Optical and thermal losses are the decisive fac-
tors for the collector efficiency (Chapter 4.1.4). Optical losses are determined by
the product of the cover transmission coefficient and the collector absorption co-
efficient. This loss is only dependent on the material and - approximately - radia-
tion and temperature-independent. Thermal losses are described together with
other non-constant losses by a constant heat transition coefficient (Equa-
tion (4.11), Chapter 4.1.4). As a first approximation, this loss is linearly depend-
ent on the difference between the absorber and the ambient temperature and in-
versely proportional to radiation (Equation (4.14), Chapter 4.1.5).
The resulting efficiency curve for a single flat-plate collector is shown in
Fig. 4.6. In the case of large temperature differences, assuming the linear depend-
ency on the temperature, an increasing deviation from the real efficiency curve is
observed. The reason for this is the non-linear increase in heat radiation within
this temperature difference range. Therefore the collector Equation (4.12) or the
efficiency Equation (4.17) is applied in many cases - the approximation of heat
radiation is performed by using a square term in that case.
Additionally, Fig. 4.6 shows the course of the characteristic curve for the same
collector at different levels of radiation. It becomes obvious that the approxima-
tion line for the efficiency is getting flatter with an increase in radiation and thus a
change in the temperature difference between absorber and the environment has
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