The invention relates to a Fresnel solar collector arrangement.
This is understood to mean a line-focusing system in which multiple mirror strips disposed parallel to a receiver are made to track the position of the sun, and the solar radiation is guided onto a fixed absorber tube in which a heat storage medium flows. In addition, a secondary reflector assigned to the absorber tube guides the radiation onto the focal line essentially formed by the absorber tube. The absorber tube and the secondary reflector form the receiver disposed in elevated manner above the mirror strips. Such a Fresnel solar collector is currently in operation in Australia, for example, in a field trial. The heat that is produced can be utilized as process heat, or it can be converted into an electric current, for example by means of a Stirling motor.
The advantage of Fresnel solar collectors as compared with conventional parabolic trough collectors lies in their significantly simplified structure. Parabolic trough collectors consist of a reflector that has the shape of a parabolic cylinder. Here also, the light is focused onto a line, the focal line. The absorber tube of the parabolic trough collector, which absorbs the concentrated radiation and passes it on to the medium flowing through, is situated in this line. In this connection, the medium is typically heated to values of approximately 400° C. In order to improve the efficiency, the absorber can be surrounded by a glass tube. A vacuum prevails in the interstice between absorber tube and glass tube, for insulation. The “solar steam” produced in this way can also be utilized directly for process heat applications, or for conventional steam power plants and cogeneration power plants.
Alternatively, flat collectors and CPC collectors are known as further types of collectors.
In this connection, the efficiency of the Fresnel solar collector essentially depends on how well the reflected solar radiation is focused onto the absorber tube. For this purpose, it is practical to make the primary mirrors assigned to the absorber tube track the sun. Only in this way can acceptable efficiencies be achieved for the system. This usually takes place by means of an electric motor assigned to each primary mirror. In turn, the electric motors are usually provided with a timing device, so that tracking is more a question of controlling than of regulating.
A significant problem with Fresnel solar collectors is that such arrangements achieve their best efficiencies in regions with the highest incoming solar radiation, for example in desert regions, where extreme temperature variations from degrees below zero to degrees above zero of far in excess of 40° C. are at least not unusual. The materials and supporting structures used are exposed to considerable stresses, in this connection, whereby thermal deformations of the material are practically unavoidable and therefore can lead to angular deviations within the entire system, which can be manifested in the double-digit percentage range in the efficiency of the entire system. Even a small angle deviation in the supporting structure of the mirror arrangement can lead to having a large part of the radiation reflected by the primary mirrors not focused onto the absorber tube but rather reflected past the absorber tube. Furthermore, individual control, i.e. individual regulation and coordination of the various electric motors for tracking, i.e. controlling the panning movement of the mirrors is accompanied by considerable regulation and control effort, making the system somewhat susceptible to malfunctioning.
Proceeding from this prior art, the invention is based on the task of configuring the system more robustly, overall, and of improving its efficiency as much as possible.
This task is accomplished by means of a Fresnel solar collector arrangement according to the main claim. Advantageous embodiments can be seen in dependent claims 2 through 11.
Because, according to the main claim, the mirror supporting framework is mounted in a fixed position in the region of the receiver supporting framework and/or in connection with the receiver supporting framework, and furthermore is mounted in sliding manner, i.e. free from constraint, it is assured, in the case of unavoidable thermal expansion of the supporting framework as a result of the effects of heat, that the mirror supporting framework balances out these corresponding changes.
This succeeds even better if the receiver supporting framework and the mirror supporting framework are essentially made from the same material and are essentially mounted in fixed manner at the same location. If thermal expansions or contractions of the material occur, one can at least approximately assume that the alternating expansions of the supporting frameworks take place to the same extent. For example, the receiver mast is then expanded, as the result of the effect of heat, in approximately the same way as the mounting rails of the primary mirrors disposed as the mirror framework. Because the receiver framework and the mirror supporting framework are at least essentially disposed orthogonal relative to one another and are made from the same material, and therefore have the same expansion coefficient, it is assured that the angle relationships do not change relative to one another or, at most, change only slightly. However, this is possible only if both the receiver framework and the mirror supporting framework are mounted in constraint-free manner, i.e. only one of at least two required supports is fixed in place. This surprisingly simple solution eliminates complicated re-adjustments for material expansions or contractions, or making a largely hopeless attempt to use materials that are more or less temperature-independent. The use of such materials is usually ruled out for cost reasons alone.
In a concrete embodiment, the receiver of the Fresnel solar collector arrangement can be mounted as an absorber tube on a row of receiver masts, whereby the mirror supporting framework can also be mounted in a fixed location at the same point, if necessary using the same concrete pedestal. In this connection, receiver mast and mirror supporting framework are advantageously made from steel 37, in each instance, and therefore exhibit largely the same expansion coefficient.
In an advantageous further development, some of the primary mirrors mounted on the mirror supporting framework are combined to form a primary mirror group, which in turn are mechanically coupled by means of a common mechanical setting element, for tracking purposes, and thus are made to track the sun. Because of the use of a common setting element, complicated coordination, complicated control and regulation of the electric motors used is eliminated, at least within the primary mirror group in question. Instead, the entire primary mirror group can be adjusted by means of a common setting element, whereby the relative angle relationship between the primary mirrors is maintained at all times. Again, this is based on the actually trivial recognition of the law governing radiation, that the relative angle adjustments of the primary mirrors required in the course of tracking the sun, where these mirrors are disposed one behind the other, in an imaginary orthogonal line relative to the absorber tube, which is disposed at a distance and elevated, are the same relative to one another. Incidentally, this would also apply to the primary mirrors disposed on an imaginary line parallel to the absorber tube.
In this connection, the embodiment of the collector arrangement explained above, with mechanical coupling for the common panning movement of the primary mirrors by means of a common setting element, is also advantageous independent of the constraint-free mounting of the mirror supporting framework.
This common panning movement is achieved as a result of connecting the primary mirrors of a primary mirror group by means of a tracking shaft. Due to the movement of the connecting rod when aligning the primary mirrors, a rotation of the tracking shaft is brought about, which is uniformly transferred to the entire primary mirror group by means of the connection.
It is advantageous if the tracking shaft is mounted, at regular intervals, in roller bearing blocks that surround the shaft but that only support it using roller elements. These roller elements permit axial rotation of the tracking shaft, and are formed in barrel-like shape, in other words are essentially cylindrical, whereby their mantle surfaces bulge out. This shape makes it possible to dispose the tracking shaft not only along planar surfaces but also, if required, to guide it along its path over different heights. The shaft can be positioned at a slant on the roller elements, so that simultaneous slanted positioning of the roller bearing block can be eliminated.
In a concrete embodiment, mechanical coupling of the primary mirrors combined to form a group can be implemented by means of a common connecting rod, by way of which the primary mirrors mounted on the mirror supporting framework so as to pivot are pivoted relative to the absorber tube, as a function of the position of the sun, i.e. the time of day or, to say it better, they are made to track the sun.
In an advantageous embodiment, the connecting rod is driven by an electric motor, using a linear motor, whereby the connecting rod, which is disposed orthogonally relative to the longitudinal expanse of the absorber tube, is moved inward or outward, as a function of the sun's position, by means of the linear motor.
In an advantageous embodiment, water vapor or thermal oil flows inside the absorber tube and is heated to a temperature of up to approximately 400° C. by the reflected radiation. The thermal medium heated in this way can then be passed to further use, in known manner, or can be used to produce electricity.
In order to further improve the efficiency of the arrangement despite the improved angular accuracy of the arrangement, a secondary reflector is additionally assigned to the absorber tube, which reflector surrounds the absorber tube essentially like a shield, and thus captures and deflects possible scattered radiation from the primary mirrors, in such a way that this scattered radiation is also focused onto the absorber tube.
Thus the secondary reflector is also disposed so that the absorber tube lies essentially in the focal line of the secondary reflector.
In a further embodiment of the electric motor drive, the linear motor is also disposed essentially centrally, i.e. approximately in the region of the imaginary line formed by the receiver masts disposed in a row. In this connection, when one and the same linear motor is used for turning, one or more primary mirror groups driven by one or more connecting rods, on the left of the absorber tube, and one or more primary mirror groups driven by one or more connecting rods, on the right of the absorber tube, can be driven in such a way that a time-controlled panning movement of the primary mirrors, i.e. a panning movement that tracks the sun, takes place relative to the absorber tube.
The necessarily opposite movement of the primary mirrors on the right of the absorber tube in comparison to the primary mirrors on the left of the absorber tube is implemented by means of a deflection mechanism for the linear movement of the connecting rod, assigned to only one of the two sides.
In an advantageous embodiment, the linear motors can be connected to a common control and/or regulation unit, since the relative movements to be carried out by the connecting rods are exactly identical over the entire length of the absorber tube, and thus common regulation is possible for the entire system.
The invention will be explained in greater detail below, using an exemplary embodiment only shown schematically in the drawings.
The drawing shows:
According to the illustration in
Relative to the setup base, the mirror supporting framework 4 itself is, in turn, mounted with foot elements 7 connected only by means of slide bearings to the supporting rails 5, which extend in fixed manner, orthogonal relative to the longitudinal expanse of the receiver 1. Thus, in concrete terms, the receiver mast 2 and the supporting rails 5, which are disposed one behind the other in the longitudinal expanse of the receiver 1, are fixed in place only in the fixed bearing 3, and otherwise are mounted in constraint-free manner, so as to slide. Since both the receiver mast 2 and the supporting rails 5 are made from steel 37 and therefore possess essentially identical expansion coefficients, any thermal expansion of the two supporting frameworks is also essentially the same. The longitudinal expansion of the receiver mast 2 is thus essentially compensated in that any angle error in the arrangement, with the possible consequence that the absorber tube moves out of the focal line of the mirror arrangement, is compensated by a similar expansion of the supporting rail 5.
The Fresnel solar collector arrangement according to
According to
In this connection, according to the schematic diagram in
Thus a Fresnel solar collector arrangement is described above, which is temperature-compensated, to a great extent, in that materials having the same thermal expansion coefficients are used for the supporting rails 5 of the mirror supporting framework 4 and the receiver masts 2 and that furthermore, the receiver mast and the mirror supporting framework 4 are mounted in constraint-free manner. Beyond that, tracking of the primary mirrors 6, 6′ to follow the position of the sun is significantly simplified by means of mechanical coupling of the primary mirrors 6, 6′.
Number | Date | Country | Kind |
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10 2005 039 404.3 | Aug 2005 | DE | national |
06002605.1 | Feb 2006 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/DE2006/001441 | 8/18/2006 | WO | 00 | 5/22/2008 |