The present invention relates to an Improved Absorber pipe for solar heating
application. More particularly, the invention relates to an absorber pipe especially
for a parabolic trough collector in a solar heat collecting apparatus.
BACKGROUND OF INVENTION
Parabolic collectors are described, for example, in "Journal of Solar Energy
Engineering", Vol. 124, p. 109 f, May 2002.
Typical of these absorber pipes are US Patent Numbers US 2004/0050381 A1, US
7013887 B2. These inventions relates to an absorber pipe, especially for a
parabolic trough collector in a solar heat collecting apparatus, which comprises a
central metal pipe, a glass sleeve tube surrounding the central metal pipe to
form an annular space between it and the central metal pipe, and a glass-metal
transitional element arranged on a free end of the glass sleeve tube, wherein the
central metal pipe and the glass-metal transitional element are connected with
each other by means of folding bellows so that they can move relative to each
other in a longitudinal direction.

The glass-metal connection is characterized by a direct bonding of materials
without additional use of adhesives (metal with glass or glass solder-low-melting
glass). The boundary surface contact is achieved by pre-oxidation of the metal.
The glass thus forms a permanent chemical bond with the metal oxide.
The absorber pipe usually comprises an inner radiation-absorbing coated steel
pipe and a surrounding glass tubular sleeve. The individual absorber pipes are
about 4m long and are assembled to form a solar field loop with a length of up
to 800m. The solar radiation is concentrated on the pipe by a mirror arranged
behind the pipe. A glass-metal transitional member connects the glass tube and
metal pipe with each other in a glass-tight manner. The intervening space
between the metal pipe and the glass tube is evacuated to minimize the heat
loss and to raise the energy yield. Expansion compensation is required between
the metal pipe and tubular glass sleeve, because of the different thermal
expansion coefficients of metal and glass and because of the great difference in
heating operation in which the metal pipe reaches about 400°C and the glass
tube only 100°C. This expansion compensation is generally provided by a metal
folding bellows, as described in "Journal of Solar Energy Engineering", May 2002,
Vol 124, p. 109 and following, page 115.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose 'an improved absorber pipe
for solar heating application', especially for a parabolic trough collector, which is
capable of enhancing the service life than the absorber pipe of the prior art.
Another object of the invention is to propose an improved absorber pipe for solar
heating application especially for a parabolic trough collector, which is capable of
reducing the heat losses of the absorber pipe by providing ultra high vacuum
double walled glass tube.
Further object of the invention is to propose an improved absorber pipe for solar

heating application especially for a parabolic trough collector, which has a
simpler design of absorber tube by using double walled glass tube with high
vacuum and avoiding glass to metal seal and folding bellows.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1 - Shows an absorber tube according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invention discloses an improved absorber pipe, especially for
parabolic trough collector, which has a greater service life and much simpler
design by avoiding complicated glass-metal seal folding bellows. These absorber
pipes are used especially for parabolic trough collector in a solar heat collecting
apparatus as explained above.
Figure 1 shows a solar heat collecting apparatus, which comprises a central
metal pipe with selective coating (1), double-walled glass tube (2) having an
evacuated annulus (3) surrounding the central metal pipe to form an annular
space between it and the central metal pipe, and 0 rings (4) on a free ends of
the double-walled glass tube to hold the metal pipe and isolate the gases in the
space between metal pipe and double walled glass tube. The selective coating
used is an alloy of Co-70.6%, Cr-21%, AI-8% and Y-0.4%. Any other coatings
such as Ni-Cr-AI-Y and Co-Ni-Cr-AI-Y can also be used.

WE CLAIM
1. An improved absorber pipe, for solar heating application especially for a
parabolic collector in a solar heat collecting apparatus, said absorber pipe
comprising:
a central metal pipe (1) with selective coating;
a double-walled glass tube (2) having an evacuated annulus (3)
surrounding the central metal pipe to form an annular space between it and
the central metal pipe and 0 rings (4) on a free ends of the double-walled
glass tube.
2. The improved absorber pipe as claimed in claim 1, where in selective coating
used is an alloy of Co-70.6%, Cr-21%, AI-8% and Y-0.4%.