Absorber tube and method for the reversible loading and unloading of a getter material

Abstract

An absorber tube, especially for solar collectors in solar thermal power plants with at least one collector mirror, is provided. The absorber tube includes a metal tube for supplying and heating a heat transfer medium, a sheath tube surrounding the metal tube to form an annular space that can be evacuated, a wall extending through the sheath tube and the metal tube to seal the annular space, and a getter material binding free hydrogen in the annular space. The absorber tube has a temperature variation device that changes the temperature of the getter material and the wall.

Claims

1. An absorber tube for solar collectors in solar thermal power plants, comprising: a collector mirror that reflects solar irradiation and directs the reflected solar irradiation onto the absorber tube; a metal tube that conducts and heats a heat transfer medium; a sheath tube surrounding the metal tube, the sheath tube forming an annular space that can be evacuated; a wall running between the sheath tube and the metal tube to seal the annular space; a getter material configured to bind free hydrogen that is present in the annular space; a membrane in the wall that is permeable to the free hydrogen, and purges free hydrogen from the annular space; and a temperature changing device that changes a temperature of the getter material, the wall, and the membrane.

2. The absorber tube according to claim 1, wherein the wall comprises a transition element and an outer ring, the membrane being disposed in or on the transition element and/or the outer ring.

3. The absorber tube according to claim 1, further comprising an expansion bellows that equilibrates expansion differences between the sheath tube and the metal tube.

4. The absorber tube according to claim 1, wherein the membrane comprises a material selected from the group consisting of iron, palladium, and niobium.

5. The absorber tube according to claim 3, wherein the expansion bellows and/or the wall comprise(s) a doped region that increases permeability for the free hydrogen.

6. The absorber tube according to claim 2, wherein the getter material is in a container that is fastened to the outer ring and/or to the transition element.

7. The absorber tube according to claim 6, wherein the container comprises a fabric sock.

8. The absorber tube according to claim 2, wherein the outer ring has an opening that points towards the getter material and that is sealed by the membrane.

9. The absorber tube according to claim 2, wherein the outer ring and/or the transition element has a section that at least partially encloses the getter material.

10. The absorber tube according to claim 9, wherein the membrane is disposed in the section.

11. The absorber tube according to claim 9, wherein the temperature changing device is adjacent to the section.

12. The absorber tube according to claim 3, wherein the temperature changing device runs at least partially in folds of the expansion bellows.

13. The absorber tube according to claim 1, wherein the temperature changing device comprises a heating device.

14. The absorber tube according to claim 13, wherein the heating device comprises a heating filament.

15. A method for changing the temperature of a membrane of an absorber tube, the absorber tube comprising: a collector mirror that reflects solar irradiation and directs the reflected solar irradiation onto the absorber tube; a metal tube that conducts and heats a heat transfer medium; a sheath tube surrounding the metal tube, the sheath tube forming an annular space that can be evacuated; a wall running between the sheath tube and the metal tube to seal the annular space; a getter material configured to bind free hydrogen that is present in the annular space; a membrane in the wall that is permeable to the free hydrogen, and purges free hydrogen from the annular space; and a temperature changing device that changes a temperature of the getter material, the wall, and the membrane, the method comprising the steps of: reflecting solar irradiation and directing the reflected solar irradiation onto the absorber tube with a collector mirror; changing the temperature of a getter material by a temperature changing device; and changing the temperature of the membrane by the getter material.

16. A method for the reversible loading and unloading of a getter material with free hydrogen in an absorber tube, comprising the following steps: reflecting solar irradiation and directing the reflected solar irradiation onto the absorber tube with a collector mirror; loading the getter material at a first temperature (T.sub.1); changing the temperature of the getter material to a second temperature (T.sub.2); unloading the getter material at the second temperature (T.sub.2); and changing the temperature of getter material to the first temperature (T.sub.1).

17. A device for discharging free hydrogen from an annular space of an absorber tube, comprising: a collector mirror that reflects solar irradiation and directs the reflected solar irradiation onto the absorber tube; a metal tube that conducts and heats a heat transfer medium; a sheath tube surrounding the metal tube, the sheath tube forming an annular space that can be evacuated; a wall running between the sheath tube and the metal tube to seal the annular space; a getter material configured to bind free hydrogen that is present in the annular space; a membrane in the wall that is permeable to the free hydrogen, and purges free hydrogen from the annular space; and a temperature changing device that changes a temperature of the getter material, the wall, and the membrane; a temperature measuring unit that determines a temperature value of the sheath tube; and a comparison unit that compares the temperature value of the sheath tube to a selectable critical temperature value.

18. A method for discharging free hydrogen from an annular space of an absorber tube, comprising the following steps: reflecting solar irradiation and directing the reflected solar irradiation onto the absorber tube with a collector mirror; determining a temperature value of a sheath tube of the absorber tube by a temperature measuring unit; comparing the temperature value with a selectable critical temperature value by a comparison unit; and changing the temperature of a getter material, a wall, and/or a membrane of the absorber tube with a temperature changing device so that hydrogen that is bound to the getter material is released and is discharged from the annular space.

Description

(1) The invention will now be described in detail based on the preferred examples of embodiment with reference to the figures. Here:

(2) FIG. 1 shows a schematic representation of a solar collector,

(3) FIG. 2 shows a first example of embodiment of an absorber tube according to the invention in a half-sectional representation,

(4) FIG. 3 shows a second example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(5) FIG. 4 shows a third example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(6) FIG. 5 shows a fourth example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(7) FIG. 6 shows a fifth example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(8) FIG. 7 shows a sixth example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(9) FIG. 8 shows a seventh example of embodiment of the absorber tube according to the invention in a half-sectional representation,

(10) FIG. 9 shows an eighth example of embodiment of the absorber tube according to the invention in a half-sectional representation, and

(11) FIG. 10 shows a graphic representation of the method for the reversible loading and unloading of a getter material with free hydrogen.

(12) A solar collector 10 of the known type is shown in FIG. 1. Solar collector 10 comprises a collector mirror 12, which reflects solar irradiation 14 and directs the reflected solar irradiation 16 onto an absorber tube 18. Collector mirror 12 is configured in trough shape, so that it brings about a focusing of the reflected solar radiation along a focal line through which runs a longitudinal axis 20 of absorber tube 18. Absorber tube 18 has a metal tube 22 and a sheath tube 24. Metal tube 22 is coated with a radiation-absorbing layer and a heat transfer medium flows through it. Sheath tube 24 surrounds metal tube 22, so that an annular space 26 is formed between metal pipe 22 and sheath tube 24. Sheath tube 24 is typically composed of glass. Based on the trough-shaped configuration of collector mirror 12, absorber tube 18 can be divided into one half 28 facing collector mirror 12 and one half 30 turned away from it.

(13) The flow direction of the heat transfer medium is indicated by the arrow P. By flowing through metal tube 22, the heat transfer medium will be heated by reflected solar radiation 16. The temperature that can be reached amounts to approximately 400 C. The heated heat transfer medium is introduced into a process that is not shown in more detail here, in which electrical energy is obtained. The half 30 of absorber tube 18 that is turned away from collector mirror 12 is cooled by mixed convection, thus by natural convection and by forced convection due to wind, for example, which leads to heat losses and thus adversely affects the heating process of the heat transfer medium. Thus, one attempts to reduce the heat conduction from metal tube 22 outwardly as much as possible, which is carried out by means of the annular space 26 formed with sheath tube 24. The latter is evacuated, whereby the heat conduction through annular space 26 is reduced and thus the heat losses are limited.

(14) FIG. 2 shows a first example of embodiment of absorber tube 18 according to the invention in a half-sectional view. Absorber tube 18 comprises a wall 32, which is composed of a transition element 34 and an outer ring 36 in the example of embodiment shown, the transition element 34 being connected to sheath tube 24. This wall 32 seals annular space 26 in a gas-tight manner in the axial direction of the longitudinal axis 20 of the absorber tube.

(15) A container 40 filled with a getter material 38 is fastened to outer ring 36, e.g., by tacking, soldering or gluing. A fastening to transition element 34 could also be provided. Outside annular space 26, adjacent to container 40, a temperature changing device 42 is introduced, which is disposed so that it can change the temperature of wall 32, in the example shown, the temperature of outer ring 36. For this purpose, temperature changing device 42 comprises a heating device 48 and a cooling device 49. Since container 40 is fastened to outer ring 36, the change in the temperature of outer ring 36, in particular, also effects a change in the temperature of getter material 38 due to heat conduction.

(16) Further, absorber tube 18 comprises a connection element 44, which is connected to metal tube 22, and an expansion bellows 46, which equilibrates differences in the expansion of sheath tube 24 and metal tube 22 during the operation of absorber tube 18. In this embodiment, outer ring 36 is applied to connection element 44, but can be displaced axially on it.

(17) A second example of embodiment of absorber tube 18 according to the invention is shown in FIG. 3. The temperature changing device 42 is designed as heating device 48, which comprises a heating filament 50, which is installed in turn in folds 52 of expansion bellows 46. Here, outer ring 36 is not applied to connection element 44, so that expansion bellows 46 is accessible on the side turned away from annular space 26. Container 40, in which is found getter material 38, is designed as a fabric sock 54, which is installed in annular space 26 on expansion bellows 46. A membrane 56, through which free hydrogen can diffuse upon a change in the temperature of membrane 56 is disposed in transition element 34. The heating filament 50 first heats getter material 38 by expansion bellows 46, and the getter material in turn heats membrane 56, so that membrane 56 is indirectly heated.

(18) A third example of embodiment of absorber tube 18 is shown in FIG. 4. Here, getter material 38 is found in turn in container 40, which has a number of holes 58 that are dimensioned so that free hydrogen can easily pass through, but getter material 38 remains in container 40. Both transition element 34 and the outer ring have membrane 56. Temperature changing device 42 in this case is designed as a heat pipe or as a photovoltaic module 59. These serve for heat transport over longer distances. Thus, they make possible producing heat at any place and guiding it to where it is required. On the other hand, they can also discharge heat from places that must be cooled. In the present case, heat is produced at one end 57 of heat pipe 59 and released in the direct vicinity of membrane 56 of outer ring 36. The latter is first heated thereby, so that getter material 38 is heated indirectly here. This also applies analogously to a reduction in temperature. Photovoltaic module 59 in addition makes possible the very favorable and environmentally-friendly production of heat.

(19) In the fourth example of embodiment, which is shown in FIG. 5, outer ring 36 has a projection 60 which extends into getter material 38 and a recess 62, which is found in the direct vicinity of projection 60. Heating filament 50 of heating device 48 runs in this recess 62, so that the temperature of getter material 38 can be increased very effectively. Further, outer ring 36 has a doped region 64, which is also found in the direct vicinity of projection 60 and includes projection 60.

(20) A fifth example of embodiment of absorber tube 18 according to the invention is shown in FIG. 6. Here, outer ring 36 has an opening 66, which projects into getter material 38 and is closed by membrane 56. In the example shown, membrane 56 is designed as a cap 68, which seals opening 66. Temperature changing device 42 is disposed in the direct vicinity of opening 66, so that it can change the temperature of membrane 56 and getter material 38.

(21) In the sixth example of embodiment of absorber tube 18, which is shown in FIG. 7, temperature changing device 42 comprises an electrical coil 70 and a metal disk 72. Metal disk 72 and electrical coil 70 are thus aligned so that metal disk 72 can be heated inductively with electrical coil 70. In this case, metal disk 72 is disposed in getter material 38, so that getter material 38 can be directly heated. In this example of embodiment, membrane 56 is disposed in expansion bellows 46 and is heated indirectly via getter material 38.

(22) A device 74 for discharging free hydrogen from an annular space 26 of an absorber tube 18 is shown schematically in FIG. 8. It comprises absorber tube 18 according to the sixth example of embodiment, which is shown in FIG. 2, all other examples of embodiment also being able to be used. Temperature changing device 42 is connected to a comparison unit 76, which in turn is connected to a temperature measuring unit 78. The connection in this case is provided via a cable 80, a wireless connection also being conceivable. A computer or microcomputer can serve as comparison unit 76; a temperature measuring unit 78 can be designed as a thermal imaging camera or a temperature sensor.

(23) Temperature measuring unit 78 determines the value of the temperature of sheath tube 24 and further guides the determined value to comparison unit 76, which compares this with a critical temperature value that can be selected and introduced into comparison unit 76. If the determined temperature value of sheath tube 24 exceeds the critical temperature value, this is a sign of an accumulation of free hydrogen in annular space 26 of absorber tube 18 and that the absorption capacity of getter material 38 for free hydrogen is exhausted. In this case, comparison unit 76 can cause temperature changing device 42 to reduce the temperature of getter material 38 in order to increase its absorption capacity. Alternatively, comparison unit 76 can effect an increase in the temperature of getter material 38 and wall 32, whereby the bound hydrogen is released from getter material 38 and is discharged from annular space 26 through membrane 56.

(24) In the example of embodiment shown in FIG. 9, outer ring 36 has a section 82 that at least partially encloses getter material 38. Membrane 56 is disposed in region 82 in the example shown. Temperature changing unit 42, which is designed, for example, as heating device 48, is also disposed in this region 82.

(25) The method for the reversible loading and unloading of a getter material 38 with free hydrogen is shown graphically in FIG. 10. Here, the dependence of the pressure p in annular space 26 is plotted for an isothermally running increase in the concentration of free hydrogen in getter material 38 for a first temperature T.sub.1 and a second temperature T.sub.2 of getter material 38. The first temperature T.sub.1 is thus lower than the second temperature T.sub.2. In this case, a line L marks the maximum absorption capacity of getter material 38 for free hydrogen. It can be seen that the absorption capacity of getter material 38 in the case of the lower temperature T.sub.1 is higher than for the higher temperature T.sub.2 (points of intersection of the isotherms with line L).

(26) The hydrogen entering into the annular space is bound to getter material 38 until the maximum absorption capacity of getter material 38 is reached. This loading occurs at the temperature T.sub.1 in the example shown. Upon reaching the maximum absorption capacity for the first temperature T.sub.1 or, as shown, just prior to it, by activating temperature changing device 42, the temperature of getter material 38 increases from the first temperature T.sub.1 to the second temperature T.sub.2. The increase in the temperature prior to reaching the maximum absorption capacity prevents particle formation of the getter material. At the second temperature T.sub.2, the getter material 38 has a smaller absorption capacity, so that the bound free hydrogen is released until the maximum absorption capacity of getter material 38 is reached for the second temperature T.sub.2. At this point (point of intersection of the isotherm T.sub.2 with L, the temperature of getter material 38 is reduced to the first temperature T.sub.1 at which the absorption capacity is higher, so that getter material 38 can absorb additional free hydrogen.

(27) The changes in the temperature of getter material 38 accompany changes in the temperature of wall 32 and/or membrane 56 and/or expansion bellows 46. Since wall 32 is usually at least partially produced from metal, in particular from iron-containing materials, it has a temperature-dependent permeability that increases with increasing temperature. The same also applies to expansion bellows 46, as long as it is produced from metal materials as well as for membrane 56 and the doped region 64 of wall 32 or of expansion bellows 46.

(28) The invention has been described in detail on the basis of several preferred embodiment examples. Modifications or variations resulting in an obvious way for a person skilled in the art from the description do not deviate from the concept that is the basis for the invention and are contained within the protective scope, which is defined by the following claims.

LIST OF REFERENCE CHARACTERS

(29) 10 Solar collector

(30) 12 Collector mirror

(31) 14 Solar irradiation

(32) 16 Reflected solar irradiation

(33) 18 Absorber tube

(34) 20 Longitudinal axis

(35) 22 Metal tube

(36) 24 Sheath tube

(37) 26 Annular space

(38) 28 Half of the absorber tube facing the collector mirror

(39) 30 Half of the absorber tube turned away from the collector mirror

(40) 32 Wall

(41) 34 Transition element

(42) 36 Outer ring

(43) 38 Getter material

(44) 40 Container

(45) 42 Temperature changing device

(46) 44 Connection element

(47) 46 Expansion bellows

(48) 48 Heating device

(49) 49 Cooling device

(50) 50 Heating filament

(51) 52 Folds

(52) 54 Fabric sock

(53) 56 Membrane

(54) 57 End

(55) 58 Holes

(56) 59 Heat pipe, photovoltaic module

(57) 60 Projection

(58) 62 Recess

(59) 64 Doped region

(60) 66 Opening of the outer ring

(61) 68 Cap

(62) 70 Electrical coil

(63) 72 Metal disk

(64) 74 Device

(65) 76 Comparison unit

(66) 78 Temperature measuring unit

(67) 80 Cable

(68) 82 Section

(69) L Line

(70) T.sub.1 First temperature

(71) T.sub.2 Second temperature