Environmental Engineering Reference
In-Depth Information
Figure 3.13
Photo of the Connections of the Evacuated Tubes to the
Solar Cycle
The entrance of atmospheric hydrogen into the vacuum cannot be avoided
even with nearly perfectly closed glass tubes because hydrogen atoms are
extremely small. This destroys the vacuum over a long period of operation.
Therefore, so-called getters, which can absorb hydrogen over a long period of
time, are installed inside the evacuated glass tube.
Evacuated tube collectors can obtain a significantly higher energy gain
especially in the cooler months of the year. Thus, a solar thermal system with
evacuated tube collectors needs a smaller collector area compared to standard
flat-plate collectors. The specific collector prices of evacuated tube collectors are
higher than that of flat-plate collectors. Furthermore it is not possible to integrate
tube collectors directly into a roof; they must always be installed on top of the
roof. This reduces the architectonical possibilities for collector integration.
Collector performance and collector efficiency
After explaining the assembly of collectors in the previous sections, this section
will describe the calculation of the collector performance and efficiency. This
is necessary for the further estimation of the output of the whole collector
system.
The collector converts solar irradiance
E
, which is transmitted through the
front glass cover with transmittance
onto the collector surface
A
C
, directly to
heat. The power output of the solar collector
Q
•
τ
out
is reduced by losses due to
reflection
Q
•
ref
, convection
Q
•
conv
and heat radiation
Q
•
rad
