Environmental Engineering Reference
In-Depth Information
Table 3.7
Optical Efficiencies
η
0
and Loss Coefficients
a
1
and
a
2
of
Real Collectors with the Collector Absorber Area
A
C
as Reference
η
0
Name
Type
a
1
in
W/(m
2
K)
a
2
in
W/(m
2
K
2
)
A
C
in
m
2
Paradigma Solar 500
Flat-plate
0.805
3.79
0.009
4.7
Solahart M
Flat-plate
0.746
4.16
0.0084
1.815
Solahart OYSTER Ko
Flat-plate
0.803
2.49
0.0230
1.703
Sonnenkraft SK 500
Flat-plate
0.800
3.02
0.0013
2.215
Wagner Euro C18
Flat-plate
0.789
3.69
0.007
2.305
Microtherm Sydney
SK-6
Evacuated tube
0.735
0.65
0.0021
0.984
Thermolux 2000-6R
Evacuated tube
0.801
1.13
0.008
1.05
Ritter CPC 12 OEM
Evacuated tube
0.617
1.04
0.0013
2.01
Sunda SEIDO 5-16
Evacuated tube
0.736
1.78
0.0130
2.592
Source:
SPF Institut für Solartechnik, 2002
high errors. The coefficient
a
must be estimated separately from measurements.
The collector reference surface
A
C
must always be given with the collector
parameters. It is possible to determine the collector efficiency parameters
referring to the absorber area, aperture area or total collector area. The
absorber area is used as a reference for the following calculations.
The
collector efficiency
η
C
can be calculated using the power output of the
solar collector
Q
•
as well as the solar irradiance
E
, which reaches the
out
collector surfaces
A
C
.
With
, the
collector efficiency
becomes:
(3.18)
Figure 3.14 shows the typical collector efficiencies of a flat-plate collector. The
thermal losses increase with rising temperature differences between the collector
and ambient air. At low solar irradiances the efficiency decreases faster. For
instance, at a solar irradiance of only 200 W/m
2
the output of the collector of
this example becomes zero at a temperature difference of about 40°C.
The
stagnation temperature
of the collector can also be found from the
figure. The stagnation temperature is the temperature at which the collector
power output and the collector efficiency are equal to zero (
η
C
= 0). At an
irradiance of 400 W/m
2
, the stagnation temperature of the collector is about
75°C above the ambient temperature. The stagnation temperature can rise
above 200°C at irradiances of 1000 W/m
2
. Therefore, collector materials must
be chosen that can resist these relatively high temperatures over a long period
of time.
The calculation of the collector efficiency given above is only valid if there
is no wind; convective thermal losses will increase with the wind speed.
