Solar System Design Guide
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Parameters for Collectors
Area designations
– Gross area
A
Describes the external dimensions (length x width) of
a collector. It is important when planning the installation
and when calculating the required installation area, as
well as when applying for subsidies.
– Absorber area
B
The area of the collector that is selectively coated and
is set into the collector. This is where the sun’s energy
is captured and turned/converted into heat.
– Aperture area
C
This is the opening or the area in the collector which
solar energy can pass through such that it reaches the
absorber
sheet.
Flat-plate collector aperture area:
Area of collector cover through which solar rays can
enter.
Flat-plate collector
Principles
The efficiency of a collector (see chapter “Specification”
for the relevant collector) specifies the proportion of
insolation hitting the absorber area that can be converted
into usable 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
0
.
When the collector heats up, it transfers some of that
heat to the ambient area through thermal conduction of
the collector material, thermal radiation and convection.
These losses are calculated by means of the heat loss
factors k
1
and k
2
and the temperature differential
∆
T (given in Kelvin) between the absorber and the
surroundings:
Efficiency curves
(based on collector absorber areas)
The optical efficiency
0
and the heat loss factors
k
1
and k
2
together with temperature differential
∆
T
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
∆
T and the thermal losses are zero. The
higher the collector temperature, the higher the thermal
losses and the lower the efficiency.
Collector Efficiency
