1.3 Parameters for collectors
Area designations
Flat-plate collector
Vacuum tube collector
C
A
B
A
C
B
–
Gross area
A
Describes the external dimensions (length x width) of a collector. It is decisive when planning the installation and when calculating the roof
area required, as well as for most subsidy programs when applying for subsidies.
–
Absorber area
B
Selectively coated metal area, which is set into the collector.
–
Aperture area
C
The aperture area is the technically relevant specification for designing a solar thermal system and for the use of sizing programs.
Flat-plate collector:
Area of collector cover through which solar rays can enter.
Vacuum tube collector:
Sum of longitudinal sections of the single tubes. Since the tubes are smaller at the top and bottom with no absorber area, the aperture
area of these devices is slightly larger than the absorber area.
Collector efficiency
The efficiency of a collector (see chapter "Specification" for the rele-
vant collector) specifies the proportion of insolation hitting the
absorber area that can be converted into useable heat. The effi-
ciency depends, among other things, on the operating conditions of
the collector. The calculation method is the same for all collector
types.
Some of the insolation striking the collectors is "lost" through reflec-
tion and absorption at the glass pane and through absorber reflec-
tion. The ratio between the insolation striking the collector and the
radiation that is converted into heat on the absorber is used to calcu-
late the
optical efficiency η
0
.
When the collector heats up, it transfers some of that heat to the
ambience through thermal conduction of the collector material, ther-
mal radiation and convection. These losses are calculated by means
of the heat loss factors k
1
and k
2
and the temperature differential ΔT
(given in K) between the absorber and the surroundings:
ŋ = ŋ0
k1 . ΔT - k2 . ΔT²
Eg
Eg
-
Efficiency curves
The optical efficiency η
0
and the heat loss factors k
1
and k
2
together
with temperature differential ΔT and the irradiance E
g
are sufficient
to determine the efficiency curve. Maximum efficiency is achieved
when the differential between the absorber and ambient temperature
ΔT and the thermal losses is zero. The higher the collector tempera-
ture, the higher the heat losses and the lower the efficiency.
The typical operating ranges of the collectors can be read off the effi-
ciency curves. This gives the adjustment options of the collectors.
Typical operating ranges (see following diagram):
1
Solar thermal system for DHW at low coverage
2
Solar thermal system for DHW at higher coverage
3
Solar thermal systems for DHW and solar central heating
backup
4
Solar thermal systems for process heat/solar-powered air condi-
tioning
The following diagrams show the efficiency curves relative to the
absorber surfaces of the collectors.
Principles
(cont.)
6
VIESMANN
VITOSOL
1
5822 440 GB
