Receiver system for a fresnel solar plant

Abstract

A receiver system for a Fresnel solar plant is provided. The system includes an absorber tube defining a longitudinal direction and a mirror array that runs parallel to the longitudinal direction. The mirror array has a mirror-symmetrical curve profile having at least one top apex for concentrating light beams onto the absorber tube. The mirror array has ventilation holes in the region of the apex.

Claims

1. A receiver system for a Fresnel solar plant, comprising: an absorber tube defining a longitudinal direction; a mirror array parallel to the longitudinal direction, the mirror array having a mirror-symmetrical curve profile with at least one top-lying apical point for concentration of light rays on the absorber tube; and ventilation openings disposed in the mirror array in a region of the apical point.

2. The receiver system according to claim 1, further comprising a receiver tube comprising the absorber tube and a sleeve tube disposed around the absorber tube at least in sections.

3. The receiver system according to claim 2, wherein the mirror array has separate first and second mirror elements in the longitudinal direction with a gap lying in between.

4. The receiver system according to claim 3, wherein the mirror array has a mirror segment that at least partially optically closes the gap.

5. The receiver system according to claim 4, further comprising a first air gap between the first mirror element and the mirror segment and a second air gap between the second mirror element and the mirror segment, the first and second air gaps forming the ventilation openings.

6. The receiver system according to claim 3, wherein the mirror array has first and second profile elements associated with the first and second mirror elements, respectively, the first and second profile elements housing the associated first and second mirror element on a side facing away from the absorber tube.

7. The receiver system according to claim 6, further comprising ventilation openings between the first and second mirror elements and the first and second profile elements.

8. The receiver system according to claim 6, wherein the first and the second profile elements are L-shaped.

9. The receiver system according to claim 6, wherein the first and second mirror elements are fixed, in each case, by a compensation device, at least on one side, on the first and second profile elements, respectively, the compensation device permitting different expansions of the first and second mirror elements and the first and second profile elements, respectively, in the longitudinal direction.

10. The receiver system according to claim 4, wherein the mirror segment is a third mirror element.

11. The receiver system according to claim 10, wherein the mirror array has a third profile element associated with the third mirror element, the third profile element housing the third mirror element on a side facing away from the absorber tube.

12. The receiver system according to claim 11, wherein the third profile element is U-shaped.

13. The receiver system according to claim 11, wherein the third mirror element is fixed to the third profile element by a compensation device, at least on one side, the compensation device permitting different expansions of the third mirror element and the third profile element in the longitudinal direction.

14. The receiver system according to claim 4, wherein the mirror segment is a reflective surface of the sleeve tube.

15. The receiver system according to claim 6, further comprising a support frame for the absorber tube and the mirror array, a first suspension holding the absorber tube, a second suspension holding at least parts of the mirror array, the support frame and first and second suspensions being mounted independently from one another, wherein the first suspension has first compensation device and the second suspension has second compensation device, and the first and second compensation devices permitting different expansions of the absorber tube and the mirror array in the longitudinal direction.

16. The receiver system according to claim 15, wherein the support frame has a longitudinal support member on which the first suspension is mounted and which runs parallel to the absorber tube and the mirror array.

17. The receiver system according to claim 16, further comprising a receiver tube comprising the absorber tube and a sleeve tube disposed around the absorber tube at least in sections, wherein the mirror array has separate first and second mirror elements in the longitudinal direction with a gap lying in between, and wherein the first suspension is guided from the longitudinal support member through the gap between the first and the second mirror elements to the absorber tube.

18. The receiver system according to claim 15, wherein the second suspension has, as a second compensation device, a first joint that connects the support frame on one side to the mirror array or to at least parts of the mirror array on the other side.

19. The receiver system according to claim 18, wherein the first joint is a solid joint.

20. The receiver system according to claim 16, wherein the first suspension has, as first compensation device, a roller bearing or slider bearing arrangement disposed to travel along the longitudinal support member.

21. The receiver system according to claim 15, wherein the first and second profile elements are fastened to the second suspension.

22. The receiver system according to claim 15, wherein the second suspension has a second joint that connects the support frame on one side to the mirror array or at least to parts of the mirror array on the other side and defines one degree of freedom crosswise to the longitudinal direction.

23. The receiver system according to claim 22, wherein the second joint is a rotating joint for swinging out the mirror array or at least parts of the mirror array.

24. The receiver system according to claim 1, wherein the mirror-symmetrical curve profile has two apical points lying on top, the ventilation openings then being disposed in each case in a region of the two apical points.

25. The receiver system according to claim 2, wherein the sleeve tube is disposed eccentrically around the absorber tube, at least on a central longitudinal segment.

26. The receiver system according to claim 2, wherein the sleeve tube is tapered at least on a central longitudinal segment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Additional features and advantages of the invention will be explained on the basis of embodiment examples, which are shown in the figures described below. Herein:

(2) FIG. 1 shows a schematic representation of the initial situation that was the basis for the invention;

(3) FIG. 2 shows a three-dimensional representation of the support frame of the receiver system according to the invention, with first and second suspensions;

(4) FIG. 3 shows an end view onto an example of embodiment of the receiver system according to the invention;

(5) FIG. 4 shows an isolated representation of the support frame of the receiver system according to the invention in the end view;

(6) FIG. 5 shows a lateral view onto the detail of the support frame for illustration of the second suspension for holding the mirror array;

(7) FIG. 6 shows a lateral view onto a segment of the receiver system for illustration of the mirror array;

(8) FIG. 7 shows a lateral view onto the detail of the receiver tube together with the first suspension of the receiver system;

(9) FIG. 8 shows a perspective representation of the detail of the first suspension;

(10) FIG. 9 shows a schematic cross section through the receiver system in a first embodiment for illustration of a divided mirror array;

(11) FIG. 10 shows a schematic cross section through a second embodiment of the receiver system for illustration of an alternative division of the mirror array;

(12) FIG. 11 shows a schematic cross section through the receiver system similar to the embodiment according to FIG. 9 for illustration of an aeration of the mirror elements;

(13) FIG. 12A shows a lateral view of a first embodiment of the receiver tube with eccentric arrangement of the sleeve tube; and

(14) FIG. 12B shows a lateral view onto a second embodiment of the receiver tube with tapered sleeve tube.

DETAILED DESCRIPTION

(15) The initial situation in order to explain the problem that is the basis of the invention is sketched in FIG. 1, based on a schematic representation of a Fresnel solar plant 100. This plant has a field of parallelly aligned primary concentrator mirrors 102, which are mounted close to the ground and are aligned on a receiver system 104 in such a way that the solar beams striking the primary concentrator mirrors 102 are reflected from them essentially upward and reach the detection region of the receiver system 104 as much as possible without loss. The receiver system 104 essentially has three functional components, namely a receiver tube 106, which in turn is composed of an absorber tube 108 and a sleeve tube 110 disposed around the absorber tube 108, at least in segments, in the longitudinal direction; a mirror array 112, which is parallel to the longitudinal direction, as a secondary concentrator for the concentration of the light rays reflected from the primary concentrator onto the absorber tube 108; and a support frame 114, which is shown here greatly simplified, with a support post 116 and a crosswise support 118.

(16) Proceeding from here, the invention is concerned with the question of how a heat-caused deformation of the optical components, in particular the mirror array, can be reduced or equilibrated as efficiently and cost-effectively as possible. According to the invention, the aeration of the mirror array explained on the basis of FIG. 11 is used for this purpose, and secondly, there is a fastening that provides sufficient degrees of freedom for the compensation of different longitudinal expansions of the individual components. Up until now, both the receiver tube as well as the mirror array have been accommodated in a common housing, which is disposed like a roof over the mirror array, protecting it from weather effects. Inside the housing, common holders engage the receiver tube and the mirror array from above and fix them in place. These holders also, in fact, permit a compensation movement in the longitudinal direction, but only a common movement for both the receiver tube and the mirror array. In order to do justice to the very different longitudinal expansions, the mirrors of the mirror array are fixed only on one side and are guided only on the other side. However, this has not led to reliable movements of the components and regularly leads to an optical maladjustment.

(17) First, it will be explained in more detail on the basis of FIGS. 2 and 3, how the fastening of the mirror array or of parts thereof and the fastening of the absorber or receiver tube can be designed advantageously, whereby in FIG. 2 essential parts of the receiver system are omitted for purposes of clarity, these parts being explained with reference to the cross section in FIG. 3.

(18) The receiver system 204 has a receiver tube 206 of the above-described type, a mirror array 212, and also a post 216 and a crosswise support 218 as parts of the support frame 214.

(19) The support frame 214 further has a frame element 220, which connects the crosswise support 218 to a longitudinal support member 222 running above the receiver tube 206 and the mirror array 212. In addition, the support frame 214 has a first suspension 323 for holding the receiver tube 206 and a second suspension 324 for holding the mirror array or at least parts of the mirror array. The two suspensions 323 and 324 are disposed on the frame element 220, independent of one another.

(20) More precisely, the first suspension 323 has first compensation means in the form of a trolley 326, which is disposed so as to be able to travel along the longitudinal support member 222 in the longitudinal direction. In this way, the longitudinal support member forms a rail guide for the first suspension that could compensate for an almost unlimited longitudinal expansion and thus takes into account the large expansion of the greatly heated absorber tube.

(21) The second suspension 324 has second compensation means in the form of tabs 328, which are joined in one piece at their fixed end to the frame element 220 of the support frame 214, and are connected on the other side at their free end to parts of the mirror array 212. The tabs 328 each form a monolithic and flat solid unit in the longitudinal direction, i.e., crosswise to the plane of the illustration, thus defining one degree of freedom essentially in the longitudinal direction on their free end. In this way, the tabs 328 form a first joint in the form of a solid joint, which could compensate for only a limited longitudinal expansion in the longitudinal direction, but which is sufficient for the relatively small expansion of the mirror array. Moreover, the deflection of the joint can be adjusted in principle in a simple way to the required expansion length by suitably selecting the length of the tabs 328 and the distances from one another of the following two suspensions in the longitudinal direction or the length of the mirror elements disposed between them.

(22) Further details, in particular of the second suspension 324, will be explained in more detail based on FIG. 4. In addition to the tab 328 forming a first joint, the second suspension 324 has a second joint 430, which is formed as a rotating joint for swinging out the mirror array or parts of the mirror array crosswise to the longitudinal direction. The mirror array is shown in the operating position by means of solid lines and in the lateral and upwardly swung-out position for maintenance or revision by means of dotted lines. In the revision position, the mirror array releases the receiver tube disposed in the center (see FIG. 3) for an intervention on all sides, and, in particular, protects the sensitive mirror elements in this way from damage due to the intervention.

(23) In the embodiment shown here, the mirror array has separate first and second mirror elements 432 and 434 having a gap 436 running in between them in the longitudinal direction. At the same time, the gap 436 forms the ventilation opening in the region of the apical point of the mirror array and an opening in the mirror array through which is guided the first suspension 323 from the longitudinal support member 222 to the receiver tube 206; see FIG. 3. In addition, the mirror array has a first and a second profile element 438, 440, each associated with the first or the second mirror element 432, 434, the profile element housing the associated mirror element on its side turned away from the receiver tube and thus protects it from weather effects and dust. The first and the second profile elements 438, 440 are both configured L-shaped.

(24) Further details, particularly of the support frame, can be recognized in FIG. 5. The latter shows the support frame in the region of the previously described frame element 220 above the support post 216. In the lateral view, it is shown that two segments of the support frame 214 are adjacent to one another in the region of the support post. Thus, a frame element 220 is screwed onto each side of the crosswise support 218, and each of these elements supports one of the adjacent segments of the support frame 214 on one side. This can be recognized in that a separate element of the longitudinal support member 222 is screwed to each of the two frame elements 220, whereby a joining 542 is formed between the two elements of the longitudinal support member 222. This joining serves as an expansion joint, since the elements of the longitudinal support member also expand during heating. However, unlike the case for known constructions, the longitudinal support member is not found inside a housing together with the optical elements greatly heated during operation, but rather is found above the housing of the mirror elements 430, 434, which lies underneath, so that a much smaller expansion reserve is sufficient.

(25) In addition, it can be recognized that two sequential segments also meet up at this place with L-shaped profile elements 438 and 440 that are each suspended on one side on one of the two frame elements 220 by means of tabs 328 belonging thereto. A joining 544 is formed as an expansion gap in the longitudinal direction, both between the first and second profile elements (only one of which can be recognized in the illustration) and the associated first and second mirror elements (not recognizable).

(26) A complete segment of the support frame between two adjacent support posts is shown as 646 in FIG. 6. A joining 542 can also be recognized here between two adjacent longitudinal support members 222 mounted on the same support post as well as a joining 544 between two adjacent profile elements 438 and 440. In addition, it can also be seen that several first and second mirror elements 432, 434 which are spaced at a distance in the longitudinal direction are disposed on the profile element 438 and analogous to the mirror-image profile element 440. Three mirror elements per profile element are illustrated, but the number can vary as desired. The joinings 648 which are formed in between each time within the same associated profile element serve once again as expansion joints and take into consideration the circumstance that temperature differences and expansion differences can also form, in particular during operation, between the mirror elements 432, 434, on the one hand, and the associated profile element on the other hand. The first and second mirror elements 432, 434 are fixed to the associated first and second profile elements, thus preferably on one side in each case by means of compensation means, whereas they are connected rigidly to the profile element belonging thereto on the other side in each case in the longitudinal direction. The compensation means may comprise slots, for example.

(27) FIG. 7 shows a schematically simplified representation of detail aspects of the first suspension in lateral view. The receiver tube 206, which is composed of the absorber tube 208 and the sleeve tube 210 concentrically surrounding it on a longitudinal segment, can be seen. The sleeve tube 210 is connected at both ends to the absorber tube 208 by means of known compensation elements in the form of expansion bellows 754.

(28) It is clear here that the receiver tube 206 also has a segmentation that is predetermined by the length of the sleeve tube 210, whereas the absorber tube 208 continues without break (almost endlessly). Of course, this must be the case, since the fluid transporting the heat can flow unhindered through the absorber tube 208 from one end to the other. A gap that serves on one side as expansion compensation for the sleeve tube and in which, on the other side, a short section of the absorber tube lies free on which the first suspension 323 engages on its free end with a clip 750 for fastening the absorber tube 208 is left each time between two adjacent sleeve tubes 210. The longitudinal support member 222, on which the first suspension 323 having a trolley 326 as the first compensation means is disposed so that it can travel in the longitudinal direction, is in turn again shown above the receiver tube 206.

(29) In this construction, the sleeve tube 210 moves relative to the longitudinal support member 222 with a longitudinal expansion of the absorber tube 208. A differential length change of the absorber tube 208 relative to the sleeve tube 210, based on different heating and different materials, is made possible by the expansion bellows 754 alone.

(30) In the comparison of FIGS. 6 and 7, it is also clear that the suspension and segment length of the receiver tube is independent from the suspension of the mirror array and segment length of the longitudinal support member, profile elements or mirror elements.

(31) In addition, a third profile element 756 for a third mirror element (which cannot be recognized here) is shown in FIG. 7 directly above the receiver tube 206, which is fastened to the first suspension 323 on both sides. The third profile element 756 is connected on one side by means of compensation means, here in the form of a slot 758, and on the other side, it is connected rigidly to the associated first suspension 323, thus permitting a different lengthwise expansion of the absorber tube 208 on one side and of the profile element 756 on the other side.

(32) Further details, in particular of the first suspension 323, are shown in FIG. 8, in which this is shown in section and in perspective. The first suspension 323, as stated, has on its upper end a trolley 326, which is suspended by means of two pairs of rollers 860, only one of which can be seen in the half section, and which can travel in the longitudinal direction on the longitudinal support member 222. On its lower end, the trolley 326 is connected by means of a bracket 862 to the clips 750, which fix in place the absorber tube 208.

(33) In this illustration, the third profile element 756, which houses the third mirror element 871 on its side facing away from the receiver tube 206, can be well recognized. The third profile element 756 is configured essentially U-shaped for this purpose, whereby it has at the ends of its two legs, lateral projections 866, which form a covering with the L-shaped first and second profile elements 438 and 440. The mirror array lying thereunder, which is composed of the first, second and third mirror elements 432, 434, 871, is sufficiently protected in this way from environmental effects and dust.

(34) FIG. 9 shows, in a schematically very simplified form, the optical components of the receiver system according to the invention according to a first embodiment. The receiver tube 906 having an absorber tube 908 and a sleeve tube 910, which surrounds the absorber tube 908 concentrically, can be seen therein. Also counted among the optical components is the mirror array 912 above the receiver tube 906, which is separated in the longitudinal direction into the two first and second mirror elements 932 and 934 with a gap 936 lying in between, directly over the receiver tube 906, which is closed optically at least partially by a mirror segment 968. Closed optically at least partially in this case means that, considered in cross section, an air gap 970 remains between the first mirror element 932 and the mirror segment 968 as well as between the second mirror element 934 and the mirror segment 968. Both air gaps 970 provide sufficient ventilation openings in the region of the two apical points of the mirror-symmetrical profile of the mirror array.

(35) The gap 936 between the first and the second mirror elements 932 and 934 further serves as the opening through which the first suspension is guided from the longitudinal support member (which is not shown here) above the optical components to the receiver tube. In this way, it is possible to form the first suspension for holding the receiver tube 906 and the mirror segment 968 independently from the second suspension for holding parts of the mirror array, i.e., the first and the second mirror elements 932, 934, so that they permit different expansions of the absorber tube 908 on one side and the first and second mirror elements 932, 934 on the other side.

(36) In this embodiment, the mirror segment 968 is formed as a third mirror element 971, which is positioned without direct contact with the sleeve tube 910 and above it.

(37) FIG. 10 shows a second variant of embodiment of the optical components of the receiver system according to the invention. This variant has a receiver tube 1006 composed of an absorber tube 1008 and a sleeve tube 1010 as well as a mirror array 1012 composed of a first mirror element 1032, a second mirror element 1034 and a mirror segment 1068, which at least partially optically closes the gap 1036 between the first and the second mirror segments. An essential difference from the embodiment example according to FIG. 9 consists in the fact that the mirror segment 1068 is formed as a reflective surface 1072 of the sleeve tube 1010. This variant of embodiment has the advantage of a simpler construction, since the third mirror element together with its fastening can be omitted, because here the mirror segment forms an integral component of the receiver tube and thus is also always automatically guided together with the receiver tube.

(38) In addition, in this embodiment, air gaps 1070 can also be recognized between the first mirror element 1032 and the mirror segment 1068 as well as between the second mirror element 1034 and the mirror segment 1068, providing the ventilation openings in the region of the one apical point of the mirror array, whereby the air gaps 1070 are found only slightly underneath the apical point. The vertical distance to the apical point, however, preferably amounts to no more than 10% of the total height of the curve profile.

(39) Finally, a first profile element 1038, which is associated with the first mirror element 1032, and a second profile element 1040, which is associated with the second mirror element 1034, are shown in FIG. 10. The two profile elements 1038 and 1040 are configured L-shaped and house the first and second mirror elements on their side facing away from the receiver tube 1006.

(40) A third embodiment of the optical components of the receiver system according to the invention is shown schematically in FIG. 11. With respect to the mirror array 1112, this essentially corresponds to the embodiment of FIG. 9. As it was therein, the mirror array is composed of a first, a second, and a third mirror element 1132, 1134, and 1171.

(41) A difference from the example of embodiment in FIG. 9 consists of a modified shape of the receiver tube 1106, in which the absorber tube 1108 is disposed eccentrically in the sleeve tube 1110. Stated more precisely, the absorber tube 1108 is shifted up, so that the gap between the lateral surface of the absorber tube 1108 and the lateral surface of the sleeve tube 1110 is reduced on the upper side facing the third mirror element 1171. In this way, radiation losses are minimized; see also FIG. 12A.

(42) In addition, a first profile element 1138, which is associated with the first mirror element 1132; a second profile element 1140, which is associated with the second mirror element 1134; and a third profile element 1156, which is associated with the third mirror element 1171, are illustrated in FIG. 11. The two first and second profile elements 1138, 1140 are L-shaped and house the associated first and second mirror elements. The third profile element 1156 is U-shaped and houses the third mirror element 1171 on its side facing away from the receiver tube 1106. At the same time, the three profile elements overlap in such a way that the mirror elements and the receiver tube are protected on the upper side against environmental effects, such as precipitation, sunlight, wind or dust, for example. A further protection can be achieved in a way known in and of itself by suitable beading of the edges as well as the lateral projection, which is not shown here (compare FIGS. 3 and 8).

(43) Further, it is illustrated in FIG. 11 that the entire mirror array has a plurality of air gaps. On the one hand, these are the air gaps 1170 between the first and the second mirror elements 1132, 1134 on one side and the mirror segment formed in the shape of the third mirror element 1171, which have already been discussed above. Another air gap is formed between the first profile element 1138 and the third profile element 1156 as well as, symmetrically thereto, between the second profile element 1140 and the third profile element 1156.

(44) It can also be recognized that the mirror array in cross section has a mirror-symmetrical curve profile with two apical points lying on top, whereby the air gaps 1170 are each found in the region of these apical points. In the region of the apical points also includes a region underneath the apical point, or here the apical points, of the curve profile of up to 10% of the total height of the curve profile. In any case, the air gaps 1170 disposed in the region of the apical points contribute to the fact that rising hot air does not build up in the mirror array but can be drawn off upward through the air gaps, so that the temperature fluctuations to which the individual mirror elements are subjected can be reduced.

(45) So that the rising hot air can completely exit the housing formed from the three profile elements 1138, 1140, and 1156, the gaps 1174 are provided on both sides between the profile elements. An exhaust air flow is characterized by means of arrows.

(46) Additional gaps are found between the first and second mirror elements 1132 and 1134 and the associated profile elements 1138 and 1140. Here, the gaps 1176 are found on the lower end and the gaps 1178 are found on the upper end, permitting an air circulation also on the outer side of the mirror elements 1132 and 1134, so that an optimal cooling of these mirror elements is assured.

(47) FIGS. 12A and 12B show two different embodiments of modified receiver tubes 1206 and 1206. Each of these has a structurally identical absorber tube 1208, but different sleeve tubes 1210 and 1210. The sleeve tube 1212 is offset in a central segment of the receiver tube, so that the absorber tube1208 runs eccentrically in the sleeve tube 1212 in this segment. Different from this, the sleeve tube 1210 according to FIG. 12B is only tapered in this segment, the absorber tube 1208 and the sleeve tube 1210 thus forming a concentric arrangement. Both variants have as a consequence the fact that the gaps 1280 and 1280 between the absorber tube and the sleeve tube taper in the longitudinal direction of the central segment of the receiver tube on its upper side, so that a smaller radiation loss occurs at this site, as was already explained on the basis of FIG. 11. Both measures, an offset sleeve tube and a tapered sleeve tube can also be basically combined.

(48) Moreover, the connection between the absorber tube 1208 and the sleeve tube 1210 or 1210, which is configured identically in both cases, can be recognized. It provides for a sealing of the volume enclosed by the sleeve tube 1210 or 1210, which is usually evacuated. In addition to sealing, the connection 1282 simultaneously has the function of an expansion compensation. Thus it has an expansion bellows in a way known in and of itself.

(49) It will still be noted by way of explanation that the FIGS. 12A and 12B show an end segment of the receiver tube, which continues toward the left and has a mirror-reversed closure on the opposite-lying side.

LIST OF REFERENCE CHARACTERS

(50) 100 Fresnel solar plant 102 Primary concentrator mirror 104 Receiver system 106 Receiver tube 108 Absorber tube 110 Sleeve tube 112 Mirror array 114 Support frame 116 Support post 118 Crosswise support 204 Receiver system 206 Receiver tube 208 Absorber tube 210 Sleeve tube 212 Mirror array 214 Support frame 216 Support plate 218 Crosswise support 220 Frame element 222 Longitudinal support member 323 First suspension 324 Second suspension 326 Trolley 328 Tab 430 Second joint, rotating joint 432 First mirror element 434 Second mirror element 436 Gap, opening 438 First profile element 440 Second profile element 542 Gap 544 Gap 646 Segment of the support frame 648 Gap 750 Clip 752 Segment of the first suspension 754 Expansion bellows 756 Third profile element 758 Compensation means 860 Pair of rollers 862 Bracket 866 Lateral projection 906 Receiver tube 908 Absorber tube 910 Sleeve tube 912 Mirror array 932 First mirror element 934 Second mirror element 936 Gap 968 Mirror segment 970 Air gap 971 Third mirror element 1006 Receiver tube 1008 Absorber tube 1010 Sleeve tube 1012 Mirror array 1032 First mirror element 1034 Second mirror element 1036 Gap 1038 First profile element 1040 Second profile element 1068 Mirror segment 1070 Air gap 1072 Reflective surface 1106 Receiver tube 1108 Absorber tube 1110 Sleeve tube 1112 Mirror array 1132 First mirror element 1134 Second mirror element 1138 First profile element 1140 Second profile element 1156 Third profile element 1170 Air gap 1171 Third mirror element 1174 Air gap 1176 Air gap 1178 Air gap 1180 Gap between absorber tube and sleeve tube 1206 Receiver tube 1206 Receiver tube 1208 Absorber tube 1208 Absorber tube 1210 Sleeve tube 1210 Sleeve tube 1280 Gap between absorber tube and sleeve tube 1280 Gap between absorber tube and sleeve tube 1282 Connection