Principles; Parameters For Collectors; Area Designations; Collector Efficiency - Viessmann VITOSOL 100-F SV Technical Manual

Principles

(cont.)

1.2 Parameters for collectors

Area designations

Flat-plate collector
B
A
– 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 efficiency 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 reflection
and absorption at the glass pane and through absorber reflection. The
ratio between the insolation striking the collector and the radiation that
is converted into heat on the absorber is used to calculate the optical
efficiency η .
0
When the collector heats up, it transfers some of that heat to the ambi-
ence through thermal conduction of the collector material, thermal
radiation and convection. These losses are calculated by means of the
heat loss factors k and k and the temperature differential ΔT (given
1 2
in K) between the absorber and the surroundings:
k 1 . ΔT k 2 . ΔT²
- -
ŋ = ŋ 0
E g E g
VITOSOL
C
Vacuum tube collector
C
B
Efficiency curves
The optical efficiency η and the heat loss factors k
0
with temperature differential ΔT and the irradiance E
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 temperature, 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 with respect to the
absorber surfaces of the collectors.
A
and k together
1 2
are sufficient to
g
VIESMANN
1
7