WO1997000408A2 - Trough-type solar collector - Google Patents

Abstract

The invention concerns a solar collector (1) comprising: a trough-shaped primary mirror (2) which is approximately parabolic in the plane vertical to its length and deflects the incident radiation onto a cylindrical absorber (3) which extends in the longitudinal direction of the primary mirror (2); and a secondary reflector (5) which reflects onto the absorber radiation already reflected by the primary mirror (2). The mid-point of the absorber (3) is shifted on the optical axis A of the primary mirror (2) from the focus of the primary mirror (2) towards the primary mirror (2), and the secondary reflector (5) consists of an involute section (51) and (optionally) a second reflector section (52) joining on thereto.

Description

 Channel-type solar collector

description

The invention relates to solar collectors according to the preambles of patent claims 1, 2, 3, 14 and 15 and a lighting fixture according to claim 13.

Solar collectors of the generic type generally serve to direct the radiation from the sun via a parabolic deflecting mirror to an absorber which uses the energy received either thermally by heating a medium flowing through the absorber or photovoltaically by directly converting it into electrical energy. The collectors are channel-shaped, since it is therefore technically simple to uniaxially track the collectors with respect to the moving sun. In addition, the deflecting mirror and the absorber are also relatively easy to build. Usually, the parabolic deflecting mirror and the absorber are matched to one another in such a way that the light cone coming from the sun hits the absorber in the focus of the deflecting mirror at every point of the deflecting mirror. If the absorber has a larger diameter, there are unnecessary radiation losses, if it is too small, some of the radiation is lost unused.

As already mentioned, uniaxially tracking, concentrating solar collectors can comparatively easily follow the sun in comparison to biaxially tracking collectors. However, the concentration that can be achieved is smaller; consequently, when using the uniaxial concentration, it is particularly important to almost approach the achievable concentration factor. Arrangements with a (parabolic) primary mirror and a cylindrical absorber, however, are, as explained below, in the concentration clearly below the theoretically possible upper limit. By increasing the concentration, a smaller absorber can be used. Immediate advantages of this are the reduction in thermal losses, the reduction in the costs for the absorber or photovoltaic surface or heat transfer medium, and the reduction in inertia by the heat transfer medium.

From US-A-5 154 163 a solar collector according to the preamble of claim 1 or 2 is known which, in addition to the above-mentioned deflection mirror, has a set of further deflection mirrors, so-called secondary reflectors, which have the task of the incident light continue to bundle and point to a reduced diameter absorber. The solar collector described in the above-mentioned publication uses a secondary reflector which comprises one or more parabolic elements (CPC). Involute mirror sections are arranged after each of these parabolic elements in order to redirect all incident radiation onto the absorber. With this concept, high concentration factors can only be achieved with complicated geometry; several differently glimmered mirrors must be used; and the average number of reflections is high. The latter generally leads to high losses due to imperfect reflection. For example, the version with 6 CPS's, which achieves the highest concentration factors, requires 12 additional parabolic mirrors, another 8 involute mirrors and fiber optic mirrors.

In "D.R. Mills and J.E. Giutronich. Asymmetry non-imaging cylindrical solar concentrators. Solar Energy, 20: pages 45-55, 1978", only low concentrations (C <10) are achieved. In the arrangement published in "R. Winston and WT Welford. Design of nonimaging concentrators as second stages in tandem with image-forming first-stage concentrators. Applied Optics, 19: 347-351, 1980", a flat absorber is used considered; the concentration achieved with a CEC (Compound Elliptic Concentrator) as the secondary concentrator is given as only around 1/3 of the thermodynamically possible value.

In "E.M. Kritchmann. Asymmetrie second-stages concentrators. Applied Optics, 21: Pages 870-873, 1980", asymmetric configurations are discussed.

The secondary concentrator described in "Robert P. Friedman, JM Gordon, and H. Ries. New high-flux two-stage optical design for parabolic solar concentrators. Solar Energy, 51: pages 317-325, 1993" is designed for flat absorbers; For larger opening angles Θ _{Λ of} the primary reflector, the concentration is limited by design, for example less than 8 for Θ _Λ = 55 °. Further concepts with a concentration that is clearly below the attainable level can be found, for example, in US Pat. No. 4,505,260 and WO-A-90/10182.

The aim of the described invention is to create a solar collector with the highest possible concentration factor using the simplest possible means; the lowest possible number of reflections should be achieved.

According to the invention, this object is achieved by the features specified in claims 1, 2, 3, 14 and 15.

In addition, it is an object to provide an illuminating device for a cylindrical illuminant which enables the most uniform possible illumination of a given surface. This object is achieved by the features of claim 13. The invention and its configurations and advantages are explained below with reference to the attached figures. It shows:

1: a schematic representation of a conventional trough-shaped solar collector; 2: a schematic representation of a solar collector according to the invention; 3: a solar collector according to the invention in a proportional representation; Fig. 4 - 11 different absorber secondary collector arrangements according to a first

Embodiment of the invention; 12 shows a second embodiment of the absorber secondary collector according to the invention.

Arrangement invention in cross section; 13 shows a third embodiment of the invention in cross section; and Fig. Heine fourth embodiment of the invention in cross section.

The concentration of solar radiation on a cylindrical absorber can be increased by a factor C2 by a secondary concentrator. The maximum possible concentration factor for a secondary reflector can be calculated from basic conservation parameters. He is

C ^ 2, max = -. ^ _Λ - 'sιnθ _ß where Θ _{R is} half the opening angle of the parabolic trough, ie the angle between the axis of symmetry and the connecting line between the focal point (focus) and the outermost end of the parabola.

1 of the accompanying drawings shows a cross section through a trough-like solar collector 1 with a parabolic trough-shaped primary reflector 2, which illuminates a cylindrical absorber 3. The size of the absorber 3 is such that the absorber 3 is seen from the end 2a of the reflector 2 at the same half opening angle α as the sun 4. In the figure, the angle α, and thus the absorber 3, is compared to the primary reflector 2 shown greatly enlarged. In addition to the central ray of the sun cone 4a, an edge ray 4b is also shown in FIG. 1, which ideally is reflected from the sun's edge onto the edge of the absorber 3. The marginal ray 4b which strikes the absorber 3 from the reflector end 2a is referred to below as the right marginal ray.

As can be seen from the above equation, a concentration factor greater than or equal to π is possible in any case. However, for theoretical reasons, this maximum concentration factor given above can only be achieved with a Secondary reflector that extends down to the parabolic trough. In such an extreme case, the secondary concentrator would of course completely shade the parabolic mirror. However, if a lower than the maximum concentration factor is selected, this can also be achieved with small and compact secondary reflectors.

FIG. 2 shows a solar collector 1 according to the invention with an absorber 3, which is again shown greatly enlarged with respect to the primary mirror 2, wherein the right half of the secondary reflector 5 according to the invention can also be seen to scale relative to the absorber 3. The left half of the secondary reflector is shown in dashed lines. FIG. 3 shows a completely to scale cross section through the solar collector 1, the size relationships between the primary reflector 2 and the two-leaf secondary reflector 5 being recognizable.

A first embodiment of the present invention is dimensioned as follows:

The design of a secondary reflector 5 is started by choosing an actual concentration C2 which is slightly less than the maximum value given in the above equation.

The radius of the absorber tube 3 when using a secondary reflector 5 is therefore RAI / C2. The design of the secondary reflector 5 is started by first determining the position of the now reduced absorber tube 3.

At each point of the primary reflector 2, the sun rays fall from a cone 4a with an opening angle equal to the opening angle α of the sun 4. They are reflected so that the central beam crosses the focal point F of the primary reflector 2. The two marginal rays 4b, 4c of the sun 4 are reflected in a cone around the focal point F. These two marginal rays 4b, 4c now represent the family of marginal rays on which the secondary mirror 5 is designed or tailored. The position of the reduced absorber tube 3 is now determined such that the reduced absorber 3 is tangent to the lower marginal rays 4b of the sun 4 from the end points 2a of the parabolic reflector 2, as can be seen in FIG. 2.

Here and in the following, as well as in the patent claims, the description of the geometry of the solar collector 1 and the secondary reflector refers in particular to the two-dimensional space lying in the plane of the drawing. Of course, however, all parts are three-dimensional elements which extend further in the direction perpendicular to the plane of the drawing. In the following, one of the two marginal rays 4b of the sun 4 will now be considered. It is also only given how one side of the secondary reflector 5 is defined. In Fig. 2 only the right wing of the secondary reflector is shown for reasons of clarity. Of course, on the left above the absorber 3 in FIG. 2 is the left wing of the secondary reflector 5, which is symmetrical with respect to the axis A in the illustration plane. The other side results from reflection about the optical axis A and the properties which are necessary for a family of Border rays, for example the right edge rays, are also valid for the other for symmetry reasons. In the following we therefore consider the marginal rays 4b which are reflected by the primary mirror 2 from the left edge of the sun 4, so that they pass the focal point F on the right. The position of the absorber tube 3 is determined so that the edge beam from the left end 2a of the parabolic primary reflector 2 is just tangent to the lower end of the absorber 3.

2 shows the parabolic primary reflector 2, the absorber 3 and the secondary mirror 5. Furthermore, the size of a necessary absorber 3a for the primary mirror 2 shown is shown in dashed lines if no secondary reflector 5 would be present. The considerably smaller size of the absorber 3 according to the invention can be seen.

The secondary reflector in the embodiment shown consists of a first 51 and a second 52 reflector section, the first reflector section 51 as the involute to the absorber 3 from the upper vertex of the absorber 3 to an intersection U with the tangential edge beam of a point P2, the properties of which are further below are explained. The second reflector section is shaped in such a way that all right marginal rays 4d, 4e from points beyond, ie in FIG. 2 to the right of P3 ₅ , are reflected tangentially to the absorber 3 by the secondary reflector 5 (dashed lines 4d, 4e). For clarification, the sun angle α has been chosen to be very large in FIGS. 1 and 2. 3 shows primary reflector 2 and secondary reflector 5 together in one image for α = 0.4 °, ie somewhat larger than the actual sun angle of 0.27 °.

Depending on the parameters selected so far, there are two case distinctions for the precise design of the secondary reflector and in particular the transition point U between the first and the second reflector sections 51, 52:

Case distinction 1: It is possible that for points just below the point P1 on the left edge 2a (ie in the drawing to the right thereof) of the primary reflector 2, the corresponding edge rays intersect the absorber 3 (do not hit it tangentially). If this is the case, then there is a second point P2 on the primary reflector 2, located more to the apex thereof, which has the property that the edge ray 4f emanating therefrom is also tangent to the absorber 3. Otherwise, P2 = Pl. The secondary reflector 5 begins with an invented reflector section 51, which extends from the upper vertex of the absorber 5 until it intersects the edge ray 4f from this point P2 mentioned above. U is the intersection of the involute 51 with the edge ray from P2 and thus the transition point between the first reflector section 51 and the second reflector section 52.

Case differentiation 2: It is now possible for points on the primary reflector 2 to the right of P2 to pass the associated marginal rays on the left at point U, that is, to hit the first reflector section 51. If this is the case, there is a further point P3 located to the right of P2, with the property that the associated marginal ray 4g also passes through U. Otherwise P3 = P2-

Non-imaging reflectors can be designed (tailored) in such a way that edge rays of a certain amount are reflected on edge rays of another amount.

The second (tailor-made) reflector section 52 of the secondary reflector 5 extends from the point U and is shaped in such a way that the right-hand edge beam from points of the primary reflector 2 reflects on the left or lower edge of the absorber tube 3 to the right of P3 become. This is shown in Fig. 2. The end of the secondary reflector 5 is reached when the associated point on the primary mirror 2 has reached the outermost right edge 2b.

The construction of the left half of the secondary reflector 5 ensures that all left marginal rays of the sun 4 also hit the absorber 3. The left wing of the secondary reflector 5 is obtained simply by reflection on the optical axis A. It analogously ensures that all right marginal rays of the sun 4 also strike the absorber 3. Since the entire secondary reflector 5 thus complies with the marginal ray principle, it is guaranteed that all sun rays (between the left and right margins) are concentrated on the absorber 3.

The shape of the secondary mirror 5 that results from this is in FIGS. 3 to 11 for different values of ΘR and C2 / C2. _ma χ shown. In this representation, the vertex of the parabolic mirror is always at the point (0, - 100). The shape depends on the values of ΘR and C2. The sun angle α was always chosen to be 0.4 °. In many cases, this shape almost corresponds to a straight line on the outside. 4 (Θ _Λ = 30 °, C ₂ / C _{2 max} = 0.7, C ₂ = 4.4) shows a situation in which the case distinctions P ₂ = P _x and P ₃ = P _{2 are} selected. In Fig. 5 (Θ _Ä = 45 °, C ₂ / q _raax = 0.5, C ₂ = 2.22) and

Fig. 6 (Θ _Λ = 45 °, C ₂ / C _{2 max} = 0.7, C ₂ = 3.11), P ₂ = P _V but P ₃ ≠ P _{2 should} be selected. For the images with larger θ _ß , Fig. 7 (Θ _Ä = 60 °, C ₂ / q _max = 0.5, C ₂ = 1.81),

Fig. 8 (Θ _A = 60 °, C ₂ / C _{2> raax} = 0.7, C ₂ = 2.54),

Fig. 9 (θ „= 90 °, C ₂ / C _2jraax = 0.5, C ₂ = 1.57),

Fig. 10 (Θ _Ä = 90 °, C ₂ / C _{2 raax} = 0.7, C ₂ = 2.2), and

Fig. 11 (Θ _Ä = 120 °, C ₂ / C _{2 raax} = 0.5, C ₂ = 1.81), P ₂ ≠ P _λ and P ₃ ≠ P ₂ apply.

FIG. 12 shows a further embodiment of the invention, in which a secondary reflector 5 is in turn attached to the absorber 3b on its side facing away from the primary reflector, not shown, and furthermore a fin 53 aligned with the optical axis on the opposite side of the absorber 3b is appropriate. In this embodiment, the involute shape of the first reflector section 51 is shaped in accordance with the entirety of the absorber 3b + fin 53.

A further embodiment of the invention shown in FIGS. 13 and 14 provides that the surface of the absorber tube 3 c and the losses associated therewith can be reduced by giving the absorber tube 3 c a different cross-section deviating from the circular one , which nevertheless receives all rays from the primary reflector 2. In a wide range of opening angles of the parabolic trough around the value θ _β = 90 °, the optimal cross-sectional shape of the absorber tube is given as in FIG. 13; there it is shown for Θ _Ä = 80 °. It extends between the upper and lower intersection (S \, S2) of the marginal rays 61, 62 and 71, 72 from the two ends of the primary reflector, begins with two straight lines 31, 32 and 33, 34 each of these Intersections S \, S2 are tangent to Kaustik 35. Of course, any other non-convex shape, but the convex hull of which corresponds to the shape given in Fig. 13, is equivalent.

A further embodiment of the invention is shown in FIG. 14, which proves to be particularly practical. It consists of: a circular tube 80, to which two fins 81, 82 connect in the direction of the optical axis A, which extend to the intersection S \, S2 of the marginal rays from the left and right edges of the primary reflector (not shown in the drawing). The absorber tube 80 is smaller by a factor of 2 than the smallest circular absorber tube 3a (from FIG. 3), which receives all rays directly. Together with the fins 81, 82, the effective area of this absorber is reduced by approximately 30% compared to a circular absorber tube 3 a according to the current state of the art.

The displacement of the absorber is an essential criterion for the training according to claims 1 to 3 of the solar collector. The design according to claim 1 has such a size of the absorber that the secondary reflector consists only of one interior volute exists. The embodiment of claim 2 corresponds to the form shown in FIGS. 3 and 4. The embodiment of claim 3 is shown in Fig. 12.

In many cases, this shape corresponds almost outwardly to a straight line, as stated in claim 7. The straight mirrors specified in this claim are particularly simple to manufacture. With the embodiment according to claim 10, possible heat losses of the absorber to the mirror can be reduced by a small gap between the involute part and the reduced absorber, which in turn is associated with correspondingly low energy losses. Claim 1 leads to a very simple and compact mirror construction.

According to claim 14, it is also possible to increase the concentration compared to the cylindrical absorber 3a without a secondary concentrator by means of a modified absorber shape 3c. Claim 15 realizes this with a design suitable for heat recovery with a tube and two heat-conducting absorber fins, as shown in FIG. 14.

According to claim 4, this absorber combination can be attached concentrically to the focal point of the primary reflector 2, ie to the original absorber tube 3 a, for the simplified conversion of existing systems without secondary reflector. Such an arrangement is shown in Fig. 12, for θ _β = 90 °.

A transparent covering 6 can be used to reduce heat losses not caused by radiation and to protect against degradation, as explained in claim 11. 8 is designed in such a size that the transition point U between the first 51 and the second 52 reflector section of the secondary reflector 5 coincides with the casing 6. In other words, the first reflector section 51 is fitted inside and the second reflector section 52 outside the casing 6. The casing 6 is preferably evaluated or filled with a suitable gas. With a suitable size of the secondary reflector 5, it can also be arranged entirely within the envelope 6.

According to claim 12, the concentrated radiation can be used for heat recovery, e.g. Oil or water as a heat transfer medium, for electricity or for the production of photochemical reaction products. The version is particularly suitable for simple assignment with photovoltaic elements.

As explained in claim 13, the reversal of the principle according to the invention or of the beam path, in which the absorber becomes the radiation source, can lead to a uniform illumination of a given area. In this case, the radiation source is preferably a fluorescent tube.

Claims

Patent claims Solar collector with a trough ^- shaped primary mirror (2), approximately parabolic in the plane perpendicular to the direction of extension, which deflects the incident radiation onto a cylindrical absorber (3) running in the longitudinal direction of the primary mirror (2), and with one of two symmetrical to each other ¬ secondary reflector (5) consisting of right and left reflector wings, which reflects the radiation reflected by the primary mirror (2) onto the absorber (3), characterized in that the center of the absorber (3) is in a viewing plane perpendicular to the direction of extension a). on the optical axis (A) of the primary mirror (2) and between the focus (F) of the primary mirror (2) and the primary mirror (2) is arranged in such a way that b) an inner marginal ray (4b) lying in the viewing plane ) from the edge (2a) of the primary mirror (2) hits the edge of the absorber (3) facing the primary mirror (2) tangentially, c) each wing of the secondary reflector (5) is an involute to the absorber (3), d) the starting point of the right involute is the intersection of the optical axis (A) with the absorber wall facing away from the primary mirror (2), and the end point (U) of the secondary reflector (5) is given as the intersection of the right edge ray (4f) lying furthest to the right of one of the Primary reflector (2) reflected radiation cone, which does not intersect the absorber (3), with the involute. Solar collector with a trough-shaped primary mirror (2), approximately parabolic in the plane perpendicular to the direction of extension, which deflects the incident radiation onto a cylindrical absorber (3) running in the longitudinal direction of the primary mirror (2), and with one of two mutually symmetrical Right and left reflector wings existing secondary reflector (5), which reflects radiation reflected by the primary mirror (2) onto the absorber (3), characterized in that in a viewing plane perpendicular to the direction of extension CORRECTED SHEET (RULE 91) ISA/EP a) the center of the absorber (3) is arranged on the optical axis (A) of the primary mirror (2) and between the focus (F) of the primary mirror (2) and the primary mirror (2) in such a way that b) an inner edge ray (4b) lying in the viewing plane strikes tangentially from the edge (2a) of the primary mirror (2) onto the edge of the absorber (3) facing the primary mirror (2), c) the secondary reflector (5) consists of at least a first reflector section ( 51) and an adjoining second reflector section (52), whereby d) the first reflector section (51) is an involute section to the absorber (3), which extends from the intersection of the optical axis (A) with that of the primary mirror (2) facing away from the absorber wall. 3. Solar collector with a trough-shaped primary mirror (2), approximately parabolic in the plane perpendicular to the direction of extension, which deflects the incident radiation onto a cylindrical absorber (3) running in the longitudinal direction of the primary mirror (2), and with one of two symmetrical to each other ¬ secondary reflector (5) consisting of right and left reflector wings, which reflects radiation reflected by the primary mirror (2) onto the absorber (3), characterized in that the center of the absorber (3) is in a viewing plane perpendicular to the direction of extension a). ) is arranged on the optical axis (A) of the primary mirror (2), b) a fin (53) is attached to the side of the absorber (3) facing the primary mirror (2), c) an inner one lying in the viewing plane Edge beam (4b) from the edge of the primary mirror (2) hits the free end point of the fin (53), d) the secondary reflector (5) consists of at least a first reflector section (51) and an adjoining second reflector section (52). , where e) the first reflector section (51) is an involute section for the combination of absorber (3) and fin (53), which starts from the intersection of the optical axis (A) with the absorber wall facing away from the primary mirror (2). 4. Solar collector according to claim 3, characterized in that the center of the absorber (3) lies in the focus (F) of the primary mirror (2). CORRECTED SHEET (RULE 91) ISA/EP 5. Solar collector according to claim 2 or 3, characterized in that the second reflector section (52) is designed as a flat mirror which is arranged in such a way that essentially all marginal rays striking it from the primary reflector (2) tangentially onto the absorber (3) be redirected. 6. Solar collector according to claim 2 or 3, characterized in that the second reflector section (52) has such a contour that all marginal rays striking it are deflected tangentially from the primary reflector (2) onto the absorber (3). 7. Solar collector according to claim 6, characterized in that the second reflector section (52) has such a contour over up to 10% of its length that all marginal rays from the primary reflector (2) striking it tangentially onto the absorber (3) are deflected, and in the remaining course is designed as a flat mirror, which is arranged in such a way that essentially all marginal rays striking it are deflected tangentially from the primary reflector (2) onto the absorber (3). 8. Solar collector according to one of claims 2 to 7, characterized in that the connection point (U) between the first and second reflector sections (51, 52) of the right secondary reflector wing is given as the intersection of the right edge ray (4f) lying furthest to the right Radiation cone reflected by the primary reflector (2), which does not intersect the absorber (3), with the first reflector section (51). 9. Solar collector according to one of claims 2 to 8, characterized in that the second reflector section (52) has a length which corresponds to at most twice the diameter of the absorber (3). 10. Solar collector according to one of the preceding claims, characterized in that a gap is provided between the secondary reflector (5) and absorber (3). 11. Solar collector according to one of the preceding claims, characterized in that the absorber (3) and at least part of the secondary reflector (5) are internal half of a transparent casing (6) are attached, and a remainder of the secondary reflector (5), if present, is continued outside the casing. 12. Solar collector according to one of the preceding claims, characterized gekennzeich¬ net that the absorber (3) is a thermal absorber for heat generation and / or a photovoltaic absorber and / or a photochemical reactor. 13. Lighting arrangement, which is constructed essentially the same as the solar collector according to one of the preceding claims, wherein a lighting element is used instead of the absorber (3), and the radiation from this lighting element through the secondary reflector (5) and then through the primary reflector ( 2) leads to uniform illumination of the area that was reserved for the entry of solar radiation in applications as a solar collector. 14. Solar collector with a trough-shaped mirror, approximately parabolic in the plane perpendicular to the direction of extension, which deflects the incident radiation onto a cylindrical absorber (3 c) which runs in the longitudinal direction of the mirror and is arranged in the focus of the mirror, characterized in that the absorber (3 c) has a cross-sectional shape that originates from the intersection points (S^ S2) of the edge rays (61, 62; 71, 72) from the outermost points of the mirror, and from these two intersection points (Si, S2) from tangents (31 , 32, 33, 34) are drawn to the caustics of the rays coming from the mirror, and the cross-sectional contour of the absorber (3 c) between these tangents corresponds to the caustics. 15. Solar collector with a trough-shaped mirror which is approximately parabolic in the plane perpendicular to the direction of extension and which deflects the incident radiation onto a cylindrical absorber (3d) which runs in the longitudinal direction of the mirror and is arranged in the focus of the mirror, characterized in that the absorber ( 3d) has a cross-sectional shape which a) consists of a cylindrical tube (80), the contour of which is a circle inscribed by the caustics (83, 84) of the rays coming from the mirror, b) on which the optical axis (A) of the Mirror's aligned flat fins (81, 82) are formed, which extend from the outermost points of the mirror to the intersection points (S\, S2) of the edge rays (61, 62; 71, 72). CORRECTED SHEET (RULE 91) ISA/EP