GB2053453A

GB2053453A - Solar collector and heat pipe particularly suitable for use therein

Info

Publication number: GB2053453A
Application number: GB8021014A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-06-29
Filing date: 1980-06-26
Publication date: 1981-02-04
Also published as: DE3024129A1; SE8004720L; GB2053453B; FR2460456B1; FR2460456A1; IT8023068A0; AU5959080A; IT1132137B; NL7905057A; AU538910B2; JPS5610656A; CA1173318A

Abstract

A solar collector for collecting solar radiation comprising a plate-like absorber (1) connected to an evaporator section (3) of a heat pipe (2), said heat pipe (2) containing isobutane as a working medium. The condenser section (4) of the heat pipe (2) is connected to a further heat exchange system (5). <IMAGE>

Description

SPECIFICATION Solar collector and heat pipe particularly suitable for use therein The invention relates to a solar collector, comprising an absorber for collecting solar radiation which is thermally conductively connected to an evaporation section of a heat transport system which is constructed as a heat pipe and which, in use, can exchange heat with a further heat transport system by way of a condenser section, the heat pipe containing a working medium having a critical temperature which is lower than the permissible maximum temperature in the further heat transport system.

A solar collector of the kind set forth is known from United States Patent Specification 3,390,672.

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 the absorber, the working medium starts to evaporate. The vapour flows to the condenser section where it gives off heat to a medium in the further heat transport system. The condensate subsequently returns to the evaporator section.

The transport of heat from the evaporator to the condenser continues for as long as a phase transition of the working medium is possible. For example, if the temperature of the heat pipe rises beyond the critical temperature of the working medium, the ability to transfer heat is almost eliminated. Thus, the maximum temperature at which the transfer of heat ceases can be determined by selection of the working medium.

This is of major importance, because the further heat transport system whereto the heat is applied usually may not exceed a given temperature.

Said United States Patent Specification 3,390,672 mentions working media for a number of temperature ranges, such as ethyl alcohol ethyl ether, Freon 1 Freon 113. Working media for heat pipe applications are also known from other literature, for example, ammonia, propane and a number of other Freons. It is to be noted that Freon is a registered trademark. A drawback of all these known working media consists in that they start to decompose after some time, thus producing residual gases which no longer participate in the evaporation/condensation process and which collect at the highest part of the condenser. As a result, a part of the condenser, and after a prolonged period of time possibly even the entire condenser becomes inactive, and hence also the heat pipe.

One object of the present invention is to provide a solar collector of the described kind in which the working medium used in the heat pipe is a medium which does not decompose in the course of time and which has a heat transfer capacity which varies rather steeply with the temperature, so that at the normal operating temperature there is a high heat transfer capacity which subsequently very rapidly decreases to the value zero at the critical temperature. Further heat transport then takes place only under the influence of heat conduction and convection.

According to the present invention there is provided a solar collector comprising a plate-like absorber for collecting solar radiation which is thermally conductively connected to an evaporator section of a heat transport system which is constructed as a heat pipe and which, in use, can exchange heat with a further heat transport system by way of a condenser section, the heat pipe containing a working medium having a critical temperature which is lower than the permissible maximum temperature in the further heat transport system is characterized in that the working medium is isobutane.

Isobutane offers 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 1 350C which is an acceptable maximum temperature for the further heat transport system.

The heat transfer capacity of isobutane varies rather steeply with the temperature. This means that while the heat transfer capacity is zero at the critical temperature, it already has a substantial value at the desired operating temperature of approximately 900 C. A high heat transfer capacity in operating conditions is important, because the surface of the condenser may be smaller as the heat transfer capacity is higher.

The invention furthermore relates to a heat pipe which is particularly suitable for use in a solar collector of the aforesaid kind and which is characterized in that the heat pipe contains isobutane as the working medium.

The invention will be described in detail hereinafter with reference to the accompanying drawing which diagrammatically shows a solar collector by way of example.

The solar collector shown comprises a plateshaped absorber 1 which is thermally conductively connected to the evaporator section 2 of a tubular heat pipe 3. The upper side of the absorber 1 is provided with a solar heat absorbing layer.

The heat pipe 2 furthermore comprises a condenser section 4 which exchanges heat with a liquid in a further heat transport system 5 which is only partly shown. The absorber plate 1 and the evaporator section 3 of the heat pipe 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 9 of a suitable metal, for example, an alloy of chromium/iron which is soldered or welded to the heat pipe 2 in a vacuum tight manner.

Thermal stresses between the heat pipe and the glass envelope are substantially avoided by this construction. 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 1 2 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 heat reflective layer of, for example, zinc-doped indium oxide.

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 1. The vapour flows to the condenser where it condenses while giving off its evaporation heat which is transferred to the liquid in the system 5 via the condenser wall.

The operating temperature of the solar collector is preferably restricted to approximately 900 where the isobutane still has a favourable heat transfer capacity.

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 3000 C, in given circumstances, and it has been found that even at this temperature isobutane does not decompose.

The heat transfer capacity by the evaporation/condensation cycle then rapidly decreases, until it becomes zero at approximately 1 350C which is the critical temperature of isobutane. Further heat transport to the system 5 is effected only by conduction and convection, so that the temperature of this system will not become too high.

In comparison with all other known working media in systems operating with comparatively low maximum permissible 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 operation of the heat pipe.

In addition to this high stability, isobutane also has a high heat transfer capacity at the normal operating temperature, which means that the surface of the condenser section 4 may be comparatively small.

Claims

1. A solar collector, comprising a plate-like absorber for collecting solar radiation which is thermally conductively connected to an evaporator section of a heat transport system which is constructed as a heat pipe and which, in use can exchange heat with a further heat transport system by way of a condenser section, the heat pipe containing a working medium having a critical temperature which is lower than the permissible maximum temperature in the further heat transport system, characterized in that the working medium is isobutane.

2. A heat pipe which is particularly suitable for use in the solar collector claimed in Claim 1, characterized in that it contains isobutane as the working medium.

3. A heat pipe comprising a plate-like absorber which is thermally conductively connected to an evaporator section of a heat transport system and which in use can exchange heat with a further heat transport system by way of a condenser system, the heat pipe containing a working medium having a critical temperature which is lower than the permissible maximum temperature in a further heat transport system characterized in that the working medium is isobutane.

4. A heat pipe collector as hereinbefore described with reference to the figure of the accompanying drawing.

5. A solar collector as hereinbefore described with reference to the figure of the accompanying drawing.