Environmental Engineering Reference
In-Depth Information
1.0
1.0
Non-selective absorber
Selective
absorber
0.8
0.8
Black body
350 K
Black body
5777 K
0.6
0.6
0.4
0.4
0.2
0.2
0
0
0.1
1
10
100
Wavelength in
λ
µ
m
Figure 3.11
Spectra of Black Bodies at 5777 K and 350 K and the
Absorptance of Selective and Non-selective Absorbers
evacuated flat-plate collector uses this principle. Since the ambient air pressure
might press the front cover against the absorber, small supports between the
back of the collector and the front cover must keep the front cover in position.
However, it is difficult to maintain the vacuum over a long period of time.
Ambient air nearly always finds a path between the glass and collector housing
to get into the collector airspace. Therefore, an evacuated flat-collector must
be evacuated from time to time with a vacuum pump, which is connected to a
valve at the collector. The evacuated tube collector, which is described in the
next section, can avoid these disadvantages.
Evacuated tube collectors
The high vacuum inside the closed
glass tube
of the evacuated tube collector is
easier to maintain over a long period of time than that in an evacuated flat-
plate collector. Glass tubes can resist the ambient air pressure due to their
shape so that no supports are necessary between the back and front sides.
A metal absorber sheet with a
heat pipe
in the middle is embedded inside a
closed glass tube with a diameter of a few centimetres. A temperature sensitive
working medium such as methanol is used inside the heat pipe. The sun heats
up and vaporizes this heat pipe fluid. The vapour rises to the condenser and
heat exchanger at the end of the heat pipe. There, the vapour condenses and
transfers the heat to the heat carrier of the solar cycle. The condensed heat
pipe fluid flows back to the bottom of the heat pipe where the sun starts
heating it again. To work properly, the tubes must have a minimum angle of
