WO2013037952A2

WO2013037952A2 - Method for introducing protective gas into an absorber tube

Info

Publication number: WO2013037952A2
Application number: PCT/EP2012/068087
Authority: WO
Inventors: Marc MÖLLENHOFF; Oliver Sohr; Thomas Kuckelkorn; Guido KREMITZL
Original assignee: Schott Solar Ag; Trumpf Laser Gmbh & Co. Kg
Priority date: 2011-09-15
Filing date: 2012-09-14
Publication date: 2013-03-21
Also published as: DE102011082772B9; WO2013037952A3; DE102011082772B3

Abstract

The invention relates to a method for introducing protective gas that is located in a container which is arranged in the evacuated annular space of an absorber tube between its outer sleeve tube and inner metal tube. The container is opened by means of a laser drilling method, in which the laser beam is guided from outside through the sleeve tube onto the container, and the container is irradiated until an opening forms in the container and the protective gas is released.

Description

Process for introducing protective gas into an absorber tube

The invention relates to a method for introducing protective gas according to the preamble of claim 1.

Solar collectors can, for example, be equipped with a parabolic mirror, also called a collector mirror, and used in so-called parabolic trough power plants. In known parabolic trough power plants, a thermal oil is used as the heat transfer medium, which can be heated up to about 400 ° C. with the aid of sunrays focused on the absorber tube by the parabolic mirrors up to 400 ° C. The heated heat transfer medium is passed through the metal tube and fed to an evaporation process. with the help of which the heat energy is converted into electrical energy.

As a rule, the absorber tube consists of a metal tube which has a radiation-absorbing layer and a cladding tube which surrounds the metal tube. The cladding tube consists of a material which is transparent in the spectral range of the solar radiation, preferably of glass. The annular space formed between metal tube and cladding tube is usually evacuated and serves to minimize the heat losses on the outer surface of the metal tube and thus to increase the energy input. Such absorber tubes are known for example from DE 102 31 467 B4.

From DE 101 33 479 C1 it is known to apply a marking on the metal tube of an absorber tube by means of a laser beam. In this case, the laser beam penetrates the cladding tube and impinges on the metal tube, which has a coating on the outside. By the irradiation by means of Laser beam, the coating is only slightly removed. The pipe substrate and the lowest layer responsible for the emission protection remain completely intact. The thermal oil used as a heat transfer medium releases free hydrogen as it ages, which is dissolved in the thermal oil. The amount of dissolved hydrogen depends on the one hand on the thermal oil used and the operating conditions of Ölkreisiaufs, on the other hand also on the amount of water, which comes into contact with the thermal oil from. In particular, by leaks in heat exchangers, contact with water may occur more frequently. The released hydrogen passes as a result of permeation through the metal tube into the evacuated annular space, wherein the permeation rate also increases with increasing operating temperature of the metal tube. As a result of this, the pressure in the annular space also increases, which results in an increase in the heat conduction through the annular space, which in turn leads to heat losses and to a lower efficiency of the absorber tube or solar cultivator.

In order to at least reduce the pressure increase in the annulus and thus extend the life of the absorber tube, the hydrogen that has entered the annulus can be bound by getter materials. Absorber tubes, which are provided with getter materials in the annular space, are known, for example, from WO 2004/063640 A1. The absorption capacity of getter materials is limited. After reaching the maximum loading capacity, the pressure in the annulus increases until it is in equilibrium with the partial pressure of the free hydrogen which has passed from the thermal oil into the annulus. The hydrogen creates an increased heat conduction in the annulus with the aforementioned adverse consequences for the efficiency of the solar collector. From DE 10 2005 057 276 B3 an absorber pipe is known in which noble gas is introduced into the annular space when the capacity of the getter material is exhausted. The noble gas is located in a container sealed with solder, which will open at the appropriate time from the outside. This creates an H2 / noble gas mixture in the annular gap, the thermal conductivity of which is only slightly higher compared to the evacuated state. The placement of the container in the vacuum space of the absorber tube requires a non-contact opening from the outside. This can be done by heat input on the melting of a solder. The other possibility is to inductively or by heating an intermediate ring in the vicinity of the container is mounted to open the container. The disadvantage of this opening method is that the heat input can not be directed to a sufficient extent specifically on the solder cap of the container, but also heats all components in the vicinity of the container. In particular, when the cladding is made of glass, the joint of glass and metallic components {glass-metal compound) is at risk. The position of the container in the cladding tube basically has the disadvantage that the container is heated by the irradiation and the solder plug can open unintentionally. Other disadvantages can be seen in the embrittlement of the solder by hydrogen uptake. In addition, the complicated geometry of the container makes the manufacture of the closed-loop opening the entire container expensive.

DE 27 11 889 A1 discloses a method for excavating channels and holes in workpieces, wherein several hundred individual pulses are used per borehole and a collector is provided between the sample and a lens in order to deposit the material removed from the sample. to catch. The method is used eg in the fission product and contamination analysis.

DE 695 13 044 T2 discloses a method of forming a dispensing opening in a dispenser for an agent whereby the wall of the dispenser is burned by a laser drilling method to thereby release the agent.

It is therefore an object of the invention to provide a method for introducing protective gases into an absorber tube, which avoids the disadvantages of the prior art.

This object is achieved by a method having the features of claim 1. The method according to the invention is carried out by means of a laser drilling method. Laser drilling is a non-machining process in which lasers generate so much energy locally in the workpiece that the material melts and evaporates. The method has the advantage that the container can be opened without contact from the outside, without other components of the absorber tube are heated and thus damaged. The laser beam is aimed specifically at the container, which can be arranged at any point in the annulus. It is not necessary, for example, to protect the container from solar radiation, since the container is completely closed and has no prepared opening, which is closed with a heat-sensitive material, as is the case for example with a solder closure.

With appropriate laser power, the container can be opened in a very short time. The material of the container is evaporated or ejected in the laser bombardment against the incoming beam and settles in the annular space of the absorber tube. Here, the materia! u. U. also precipitate on the inside of the cladding tube. Due to the still ongoing laser bombardment, the precipitate and thus the cladding tube heats up. As a result of this heat, mechanical stresses can occur in the cladding tube which can damage the cladding tube. It is therefore preferable to collect the vaporized and ejected material of the container before it can precipitate on the inside of the cladding tube. Since the material is ejected against the incoming laser beam, it is advantageous if the ejected material is captured by an optical element, which can penetrate the incoming laser beam unhindered.

In the annular space, an optical element is preferably arranged adjacent to the container, which has the advantage that the material of the container, which is vaporized or ejected in the direction of the cladding tube during laser bombardment, is deposited on this optical element. It is thus prevented that this precipitate forms on the cladding tube.

The optical element may be arranged on the metal tube, on the cladding tube, on an element connecting the metal tube and the cladding tube, or on the container.

The optical element is preferably arranged in the region between the container and the cladding tube. Such an optical element may be a glass plate, in particular a planar glass plate. This glass pane absorbs the container material and thus protects the cladding tube. The laser drilling can be carried out for example with laser pulses. A short laser pulse with high power density brings the energy into the workpiece in a very short time during laser drilling. As a result, the material melts and evaporates. The larger the pulse energy, the more material melts and evaporates. During evaporation, the volume of material increases suddenly in the borehole and a high pressure is created. This vapor pressure drives the molten material out of the borehole. Preferably, laser pulses are used whose energy is 0.5 J to 80 J. With pulse energies in the range of 0.5 J to 4 J and without an optical element for collecting the ejected material, the container could be opened without causing damage to the cladding tube. With pulse energies in the range of 4 J to 80 J, the container could also be opened, but here an optical element was required to catch the ejected material to prevent damage to the Hüilrohr.

A special feature is the laser processing with ultrashort pulse lasers in the picosecond range. The material is evaporated by sublimation directly without material melting from the solid state. The container itself is heated only slightly.

Preferably, single-pulse drilling or percussion drilling is used. In the simplest case, a single laser pulse with comparatively high pulse energy and relatively long pulse duration generates the bore. In this way, many holes can be generated very quickly. Percussion drilling involves boring through several laser pulses with a shorter pulse duration and pulse energy. This drilling technique provides deeper and more precise holes than single-pulse drilling. Percussion drilling also enables smaller hole diameters Preferably, a diode pumped or flash lamp pumped pulse laser is used. Pulse lasers have the advantage that they have high peak intensities, which shortens the opening process.

Preferably, for opening a maximum of five pulses, in particular a maximum of 3 pulses and more preferably a pulse used. When using z. If, for example, two or more pulses have to be transmitted through the subsequent pulse, the material vaporized and ejected by the respective preceding pulse must be penetrated. The energy of the subsequent pulse arriving on the container can thereby be reduced. It is therefore preferable to use only a single pulse.

Preferably, laser pulses are used with a pulse duration in the range of 2 ms to 15 ms. Preferred pulse durations are 2 to 10 ms, in particular 3 to 6 ms. It has been found that even with Puisdauern of less than 2 ms, the container can be opened, especially if the diameter of the point of impact on the container is less than about 300 μρη. The smaller the diameter of the laser beam at the point of impact and the smaller the pulse duration, the less material of the container is vaporized and ejected. Thus, less material can be deposited on the cladding tube, which could cause damage there or prevent the penetration of the radiation of further laser pulses. It is therefore also preferable to use laser pulses with a pulse duration of 1 to <2 ms.

The power of the laser is preferably set to 2 kW to 12 kW, in particular to 6 kW to 0 kW. It has been shown that a laser power of just 1 kW is sufficient. In order to save energy, the laser power preferably set to 1 kW to <2 kW. Preferred wavelengths of the laser beam are at 400 to 1500 nm.

It has been shown that it is also possible to use wavelengths of> 1500 nm, preferably in the range of> 1500 nm to 1600 nm. The advantage of this wavelength range is that in this wavelength range, particularly little radiation energy of the laser is absorbed by the material of the cladding tube. This reduces the risk of damage to the cladding tube as a result of the laser bombardment.

Preferably, steel containers are used. According to EN 00 20, steel is one whose mass of iron is greater than that of any other element whose carbon content is generally <2% and which contains other elements. Steel is corrosion-resistant, gas-impermeable and mechanically stable and therefore particularly suitable as a protective gas container.

The melting point can be set in a wide range up to about 1500 ° C. It is therefore possible to optimally adjust the melting point of the container material including the wall thickness of the container and the laser parameters for opening the container.

The wall thickness of the container is preferably in the range of 0.5 to 1 mm, preferably 0.6 to 0.8 mm. It has been shown that containers with a wall thickness of 0.2 to <0.5 mm can be used.

Preferably, a laser beam is used whose diameter at the point of impact on the container 200 to 500 μηι, preferably 200 to 300 m.

The container is filled with a protective gas, eg a noble gas with low thermal conductivity. In particular, xenon or krypton are preferred. The method further provides that the cladding tube temperature is continuously measured. If this cladding tube temperature rises above a predetermined temperature value, which is an indication of increased heat losses in the absorber tube, it is necessary to open the container. The cladding tube temperature is measured, for example, with a thermocouple, a resistance thermometer or a pyrometer and a corresponding signal is displayed in a monitoring device. It is also possible from time to time to measure the cladding tube temperature on site. The control of the thermal conductivity of the annular space on the basis of the temperature of the cladding tube is known in principle from DE102009047548 A1.

If increased heat losses are detected, the container is preferably opened with a mobile laser. The judgment as to whether the container is open or not can again be judged from the tube tube temperature, which is still measured by the laser drilling method. When the tube temperature drops, it can be assumed that the container has been properly opened. Exemplary embodiments will be explained in more detail with reference to the drawings.

Show it:

1 shows a section through one end of an absorber tube, and

2 shows a cross section through an absorber tube according to another

Embodiment. In the figure 1, an end of an absorber tube 1 is shown schematically in section. The absorber tube 1 has a metal tube 10 through which heat exchanger liquid flows and, as has been described initially, has radiation-absorbing layers.

This metal tube 10 is arranged concentrically in a cladding tube 20, which consists of a transparent material for solar radiation, preferably made of glass. Between the metal tube 10 and the cladding tube 20, an annular space 5 is formed, which is evacuated.

At the free end face of the cladding tube 20, a transition element 22 is fixed, which has a radially inwardly facing collar 23. In the annular space 5 formed between the cladding tube 20 and metal tube 10, a strain compensation device 24 is arranged in the form of a bellows 25, which is fastened with its outer end 26 on the collar 23 of the transition element 22.

The bellows 25 extends below the transition element 22 in the annular space 5 and is fastened at the opposite end to a connecting element 27, which has an annular disk 28 for this purpose. At this annular disc 28, a container 30 is arranged with inert gas, which is designed according to the annular disc 28 bent and extends over a semicircle. A laser beam 50 which strikes the cladding tube 20 perpendicularly from above penetrates the cladding tube 20 and subsequently reaches the container 30.

During the drilling process, material of the container 30 is released and the container is opened. After laser drilling, the protective gas flows out of the container into the annular space 5. FIG. 2 shows a cross section through an absorber tube 1 with a metal tube 10 and a cladding tube 20, wherein an optical element 40 in the form of a planar glass plate 42 is arranged in the region between the container 30 and the cladding tube 20. During the drilling process, material of the container 30 is released, which deposits on the underside of the planar glass plate 42.

It is thus prevented that the container material is deposited on the cladding tube 20. The holding means for the tubular container 30 and for the optical element 20 are not shown. The corresponding holding means may be attached to the metal tube 10, to the cladding tube 20 or to an element connecting the metal tube and the cladding tube.

Embodiment 1

An absorber tube with the following specification is used:

Diameter of the metal tube: 70 mm, cladding made of coated borosilicate glass with a solar transmission of more than 96%, diameter of the cladding: 125 mm,

Wall thickness of the cladding tube: 3 mm. Such absorber tubes are distributed by the applicant under the name SCHOTT PTR 70 receiver.

In the annulus is a container made of steel with a wall thickness of 0.7 mm. For example, there is a lamp-pumped pulsed neodymium: YAG. Laser with a wavelength of 1064 nm is used. The laser power carries for example 8 kW. Alternatively, other solid-state lasers such as disk lasers, fiber lasers or diode lasers can be used provided they emit radiation of sufficient power in the appropriate wavelength range. The laser beam is directed perpendicular to the cladding tube, penetrates the cladding tube and optionally the optical element and is perpendicular to the wall of the container.

It is worked with a single laser pulse. The pulse duration is, for example, 5 ms.

After a laser pulse, the container is opened and the protective gas exits into the annulus. It has been shown that the container could be opened with pulse durations between 2 ms and 15 ms, without an unnecessarily large atehalabtrag precipitating on the cladding tube.

With laser powers between 1 kW and 12 kW, the container could be opened without excessive material removal. The cladding tube had not yet reached its critical load limit by this irradiation.

Embodiment 2: An absorber tube having the same characteristics as in Embodiment 1 is used.

Also in this example is located in the annulus a container made of steel, but with a wall thickness of 0.45 mm. For example, there is a lamp pumped pulsed neodymium: YAG. Laser with a wavelength of 1064 nm is used. The laser power is 1, 5 kW.

Again, the laser beam is directed perpendicular to the cladding tube, penetrates the cladding tube and optionally the optical element and is perpendicular to the wall of the container.

It is also worked with a single laser pulse. The pulse duration is only 1 ms, so that the total pulse energy is 1.5 J.

Also in this example, after a laser pulse, the container is opened and the inert gas exits into the annulus.

Embodiment 3

Absorber tube, laser type and container have the same properties as in Embodiment 2.

However, here the laser is set to a power of 1, 0 kW and a pulse duration of 1 ms, so that the total pulse energy is 1 J.

It also succeeds here with a single laser pulse to open the container, so that the protective gas exits into the annulus. Absorber tube

annulus

Metal tube cladding tube

Transition element

Federation

Expansion compensating means

bellow

outer end

connecting element

Ring disc container Optical element

Planar glass plate

laser beam

Claims

claims

Method for introducing inert gas, which is located in a container which is arranged in the evacuated annular space of an absorber tube between the outer casing tube and the inner metal tube, characterized in that the container is opened by means of a laser drilling method, wherein a laser beam of a laser from the outside through the cladding tube is directed onto the container and the container is irradiated until an opening forms in the container and the protective gas is released.

A method according to claim 1, characterized in that by the laser beam! evaporated and ejected material of the container is collected.

A method according to claim 2, characterized in that the material is collected by at least one adjacent to the container arranged optical element.

Method according to one of claims 1 to 3, characterized in that the laser drilling method is a Einzelpulsbohrverfahren or a Perkussionsbohrverfahren.

Method according to one of claims 1 to 4, characterized in that a diode-pumped or a flash lamp pumped pulse laser is used.

6. The method according to any one of claims 1 to 5, characterized in that laser pulses are used and that a maximum of five laser pulses are used.

7. The method according to any one of claims 1 to 6, characterized in that a laser pulse is used.

8. The method according to any one of claims 1 to 7, characterized in that the power of the laser during laser drilling is 2 kW to 12 kW.

9. The method according to any one of claims 1 to 7, characterized in that the power of the laser during laser drilling is 1 kW to <2 kW. 0. The method according to any one of claims 1 to 9, characterized in that the wavelength of the laser beam is 400 nm to 1500 nm.

11. The method according to any one of claims 1 to 10, characterized in that the wavelength of the laser beam is> 1500 nm to 1600 nm.

12. The method according to any one of claims 4 to 11, characterized in that laser pulses are used with a pulse duration of 2 ms to 15 ms.

13. The method according to any one of claims 4 to 12, characterized in that laser pulses are used with a pulse duration of 1 ms to <2 ms.

14. The method according to any one of claims 1 to 13, characterized in that a laser beam is used, whose diameter at the point of impact on the container is 200-500 pm.

15. The method according to any one of claims 1 to 14, characterized in that a laser beam is used, whose diameter at the point of impact on the container is 200 - 300 pm

16. The method according to any one of claims 1 to 15, characterized in that the temperature of the cladding tube is measured and that when a predetermined temperature is exceeded, the container is opened.

17. The method according to any one of claims 1 to 16, characterized in that a container made of steel is used.

18. The method according to any one of claims 1 to 17, characterized in that a container with a wall thickness of 0.5 to 1 mm is used.

19. The method according to any one of claims 1 to 17, characterized in that a container with a wall thickness of 0.2 to <0.5 mm is used.

20. The method according to any one of claims 1 to 18, characterized in that laser pulses are used whose energy is 0.5 J to 80 J.