Note: Descriptions are shown in the official language in which they were submitted.
~733~.~
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"Solar coll0ctor and heat pipe particularly suitable for
; ~ use therein".
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The invention relates to a solar collector, com-
~- prising an absorber for collecting solar radiation which is
-~; thermally conductively connected to an evaporat:Lon section
, of a heat transport system which is constructed as a heat
i` - 5 pipe and which can be made to exchange heat with a further
heat transport system by way of a condensor section, the
heat pipe containing a working medium having a critical
temperature which is lower than the permissible maximum
temperature in the further h0at transport system.
~ 0 A solar collector of the kind set forth is known
;~ from United States Patent Specification 3,3907672.
;~ - In solar collectors of this kind, the condensed
`. working medium collects in the evaporator section of the
~ heat pipe in the cold condition. When heat is applied from
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the absorber, the working medium starts to evaporate. The
vapour flows to the condensor section where it gives off
heat to the medium in the -further heat transport system.
; The condensate subsequently returns to the evaporator
section.
The transport of heat from the evaporator to the
condensor continues for as long as a phase transition of
the working medium is possible. For example, if the tempe-
rature of the heat pipe rises beyond the critical tempera-
ture of the working medium, the ability to transPer heat
is almost eliminated. Thus, the maximum temperature at which
the transfer of heat ceases can be determined by selection
of the working rnedium. This is of major importance, because
the further heat transport system whereto the heat is ap-
plied usually may not exceed a given temperature.
3~ Said United States Patent Specification 3,390,672
mentions working media for a number of temperature ranges,
- such as ethyl alcohol, ethyl ether, Freon 11, ~reon 113.
Working media for heat pipe applications are also known
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; ! from other literature, for example, ammonia, propane and a
number of other Freons. It is to be noted that Freon is a
registered tradernark. A drawback of all these known working
media consists in that they start to decompose af-ter some
~; 5 time, thus producing residual gases which no longer parti-
~` cipate in the evaporation/condensation process and which
collect at the highest part of the condensor. As a result,
a part of the condensor, and aPter a prolonged period of
~ time possibly even the entire condensor becomes inactive,
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and hence also the heat pip9.
The invention has for its object to provide a
~ solar collector of the described kind in which the working
<i medium used in -the heat pipe is a medium which does not
; decompose in the course of tirne and which has a heat -trans-
fer capacity which varies rather steeply with the tempera-
ture, so that a-t the normal operating temperature there is
~` a high heat transfer capacity which subsequently very
- rapidly decreases to the value zero at the critical tempe-
~ rature. Further heat transport then talces place only under
;~ 20 the inPluence of heat conduction and convection.
The solar collector in accordance with the in-
vention is characterized in that the working medium is
isobutane.
Isobutane ofPers the major advantage that even at
higher temperatures it is a very stable compound which does
not decompose.
Furthermore, isobutane has a critical temperature
; of 135 C which is an acceptable maximum temperature Por the
further heat transport system.
; 30 The heat transfer capacity oP isobutane varies
- rather steeply with the temperature. This means that while
the heat transfer capacity is zero at the critical tempera-
ture, it already has a substantial value at the desired
operating tempera-ture of approximately 9OC. A high heat
transPer capacity in operating conditions is important,
; because the surface oP the condensor may be smaller as the
-;` heat transPer capacity is higher.
~` The invention Purthermore relates to a heat pipe
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~.~733~
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` which is particularly suitable for use in a solar collector
and which is characteri~ed in that it contains isobutane
as the working medium.
The invention will be described in detail herein-
after with reference to the accompanying drawing which dia~
~,i grammatically shows a solar collector by way o~ example.
The solar collector shown comprises a plate-shaped
` absorber 1 which is thermally conductively connected to the
evaporator section 2 of a tubular heat pipe 3. The upper
~; lO side of the absorber 1 is provided with a solar heat absorb-
ing layer.
The heat pipe 2 ~urthermore comprises a condensor
section 4 which exchanges heat with a liquid in a further
heat transpor-t system 5 which is only partly shown. The ab-
lS sorber plate 1 and the evaporator section 3 of the heatpipe are accommodated in a glass envelope 6 having a round
cross-section. The lower side 7 of this envelope is sealed.
The upper side of the envelope is sealed by a part 8, also
made of glass, which is sealed -to a thin-walled sleeve ~
of a suitable metal, for example, chromium iron which is
soldered or welded to the hea-t pipe 2 in a vacuumtight man-
ner.
Thermal stresses between the heat pipe and the
glass envelope are substantially avoided by this construct-
; 25 ion. Subatmospheric pressure prevails in the glass envelope.
At the ends of the plate-like absorber 1 there
are provided supporting plates 11 and 12. The plate 12 at
the same time serves for centring the heat pipe.
Because the absorber 1 and the evaporator section
3 are arranged in a vacuum envelope, the heat losses remain
small. A further improvement in this respect can be obtained
by providing part of or the entire inner side of -the glass
- envelope with a selective hea-t reflective layer of, ~or
example, zinc-doped indium oxide.
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The working medium present in the heat pipe 2
is isobutane which evaporates due to the heat given off
to the evaporator section 3 by the absorber plate l. The
vapour flows to the condensor where it condenses while
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giving off its evaporation heat which is transferred to the
liquid :in the system 5 via the condensor wall.
The operating temperature of the solar collector
~` is preferably restricted to approximately 9O where the
isobutane still has a favourable heat transfer capacity.
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;Should the temperature of the collector rise for
some reason, for example, because the system 5 withdraws
too little heat, the temperatures may become very high,
~even beyond 3OO C, in given circumstances, and i-t has been
`i 10 found that even at this temperature isobutane does not de-
compose. ~he heat transfer capacity by the evaporation/
condensation cycle then rapidly decreases~ until it becomes
zero at approximately 135 ~ which is the crltical tempe-
rature of isobutane. ~urther heat transport to the system 5
is effecte~ only by conduction and convection, so that the
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tempera-ture of this system will not become -too high.
In comparison wi-th all other known working media
in systems operating with comparatively low maximum permis-
sible temperatures, isobutane offers the advantage that it
is a thermally very stable compound, which means that no
gaseous parts are separated which could disturb the opera-
tion of the heat pipe.
In addition to this high stability, isobutane
also has a high heat transfer capacity at the normal ope-
rating temperature, which means that the surface of thecondensor section 4 may be comparatively small.
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