Patent Publication Number: US-2013239948-A1

Title: Linearly concentrating solar collector and method for reflector tracking in such a solar collector

Description:
The present invention relates to a linearly concentrating solar collector comprising a receiver tube mounted in an elevated manner for the absorption of thermal energy and a plurality of reflectors, arranged on both sides of the receiver tube and pivoted around their longitudinal axis, for the reflection of incident sunlight onto the receiver tube, and a method for reflector tracking in such a linearly concentrating solar collector. 
     Such a device and method for reflector tracking have already been described in European patent application EP 1 754 942 A1. The object of this application is a Fresnel solar collector arrangement that is operated as a solar thermal power plant. In principle such an arrangement comprises initially a receiver mounted in an elevated manner that is embodied in the form of a receiver tube surrounded by a receiver cover. The receiver tube contains a heat-conducting medium that can remove thermal energy hitting the receiver tube, which energy can then be processed for use, for instance converted into electrical energy. 
     The necessary thermal energy comes from the solar irradiation that falls on the reflectors arranged around the receiver tube and is reflected and preferably also focused by them onto the receiver tube. Great precision of the reflection and focusing is very important in this connection, so that the individual reflectors actually reflect the reflected light back onto the receiver tube and the least possible loss occurs. 
     During a sunny day, the reflector arrangement must therefore track the current position of the sun in order to ensure that the reflected light continues to hit the receiver tube despite the changing position of the sun. To that end, the reflectors are pivoted in the direction of their longitudinal expanse so that the reflection can occur at a suitable angle, in each instance, in order to hit the receiver tube. 
     Usually an arrangement of receiver and reflectors is chosen such that both the individual reflectors and the receiver are erected in parallel to one another. The reflectors are arranged in parallel rows on both sides of the receiver, whereby the above state of the art provides for coupling of the individual reflectors relative to one another, because although each individual reflector must take up its own position, over time the change of angle is the same for all reflectors. The reflectors are therefore connected to one another by a common connecting rod that, if displaced, brings about a change of angle by way of a lever mechanism. The degree by which the angle is changed is determined by a control system based on sun position algorithms. The calculation of the position of the sun is compared with a measurement using clinometers or similar devices on the reflectors in order to establish a closed control loop. The problem with this is, firstly, that these clinometers, or angle transmitters in general, do not have sufficient accuracy. This can lead to the calculation already performed coinciding with the actual setting actually measured, but the receiver tube being missed because of the inaccuracy. 
     Such solar thermal power plants are also usually erected in regions in which high solar irradiation is to be expected. Usually such regions make relatively high demands on the material, which is therefore subject to both elongation and compression, and this can in turn lead to inaccuracies in tracking, in each instance. Likewise, bending can occur due both to wind pressure and to different rates of thermal expansion, so that all in all, optimal tracking is not ensured in the best possible way by following calculated sun positions. 
     In light of this, the present invention is based on improving the tracking of the reflectors in a linearly concentrating solar collector, in particular also increasing the thermal energy yield obtained. 
     This is achieved by a linearly concentrating solar collector according to the characteristics of the main claim. It also succeeds by the application of a method according to the independent claim  12 . Further practical embodiments of the linearly concentrating solar collector and of the method for reflector tracking can be seen in the respective dependent claims. 
     To that end, a linearly concentrating solar collector according to the invention provides that the receiver tube mounted in an elevated manner, through which a heat-conducting medium flows for the absorption of thermal energy, has at least one sensor arrangement, which is fitted with sensors for detecting the intensities of radiation, on both longitudinal sides. The sensors are arranged in such a way that from the aspect of a reflector, the receiver tube is respectively arranged between both sensors. To that extent, the at least two sensor arrangements may each have a first sensor, which is oriented in the direction of reflectors on a first side of the receiver tube, while additionally or alternatively, second sensors may also be present, which are oriented toward the reflectors on a second side of the receiver tube. The orientation of the first and/or second sensors is thus always effected solely onto a group of reflectors coupled to one another in a transverse direction. 
     It is only essential, in this connection, that one and the same reflector is in a position from which, depending on its setting, it can direct sunlight onto sensors either of both sensor arrangements or of no sensor arrangement. At the same time, it cannot reach any second sensor with the light reflected by it. It is then verified whether a predetermined relationship of the intensity of radiation, for instance the same intensity of radiation, can be determined in both sensors oriented in the same direction. In this example, it must be assumed that both sensors on average receive light equally strongly, whereby the maximum intensity of radiation would then have to occur in the middle between the two sensors. If the receiver tube is not located centrally between the first and/or second sensors as viewed from the reflectors, a certain relationship of the respectively measured intensities of radiation that corresponds to the desired location of the maximum is aimed at. If the intended orientation is present, the receiver tube is situated at this location, so that an optimal yield is achieved if this configuration exists. However, if one of the two sensors oriented in the same direction were to receive less or more than the expected relationship specifies, tracking is required to the effect that the reflectors are adjusted until the intensity of radiation has fallen at the sensor with the excessively high intensity of radiation and has risen at the sensor with the excessively low intensity of radiation, so that the relationship of the measured intensity of radiation is achieved again. 
     If only first sensors or only second sensors are present, only the detectors of one side of the receiver tube are considered for tracking of the reflectors, so that in this case, tracking of the reflectors of the other side also must be made dependent on the result of this measurement. In this case, the mechanical coupling, for instance, of reflectors of both sides with one another is recommended. 
     If, however, both first sensors and second sensors are present, the reflectors on the two sides of the receiver tube can be coupled with one another, each side by itself, at least in rows, so that all reflectors of one side are coupled with one another and all reflectors of the other side are coupled with one another. This further enhances the accuracy of tracking. 
     Specifically, in the installation of the sensors in the area of the receiver tube, at least one first sensor and at least one second sensor can be arranged in a common housing, whereby the two sensors point in two opposite directions out of the housing. The sensors can be accommodated not only in front of the housing but also in the housing, or can even penetrate the housing wall. The two sensors thus reach through openings into opposite surfaces of the housing, whereby these opposite surfaces preferably form an acute angle. This is due to the position of the reflectors relative to the sensors and permits a large angle of absorption for the rays of light reflected toward the receiver tube by the reflectors. The opposite dispositions of the sensors in the sensor arrangement also ensure that only the light reflected by the reflectors on one side of the receiver tube or by the reflectors on the other side of the receiver tube can be received, so that there is no diffusion to cause inaccuracies in the measurement. To that end, the sensors are essentially oriented transverse to the receiver tube. 
     Additionally, shields to repel dispersed radiation may be arranged around the receptor openings of the sensors, which radiation can, for instance, hit the sensors directly from the sun. Again, this improves the accuracy of measurement relative to the radiation received only by the reflectors. 
     The receiver tube is usually surrounded by a receiver cover that, on the one hand, guarantees thermal insulation of the receiver tube and, on the other hand, in the area above the receiver tube, has a secondary mirror, which reflects radiation dispersed past the receiver tube back onto the receiver tube. In the area of this receiver cover, there is a bottom edge to which the sensor arrangements on both sides of the receiver tube can be secured. This is a favorable location given that greater incoming radiation than above the bottom edge of the cover is not possible in any case, due to the receiver cover. The disposition of one sensor arrangement on each side of the receiver is sufficient for the measurement itself, but additional sensor arrangements disposed in pairs on the receiver cover may improve the measurement by enabling the results to be averaged. 
     Tracking of the reflectors is preferably effected mechanically in that a connecting rod connects several of the reflectors in such a way that the setting angle of the reflectors is adjusted by a displacement of the connecting rod transversely to the receiver tube. The adjustment is effected to the same extent for all reflectors, whereby a different absolute oblique position allows each individual reflector to be tracked exactly and separately. The displacement of the connecting rod is effected against a fixed bearing and by means of a servomotor and, in the case that only first or only second sensors are present, brings about tracking of detectors on both sides, and, if first and second sensors are present, tracking of reflectors on one side only, in each instance, whereby the reflectors on the respective other side are coupled by their own connecting rods. 
     Specifically, to this end the reflectors can each have at least one swiveling lever whose fixed end is non-rotatably connected to the reflector and whose free end can be coupled to the connecting rod. 
     The fixed bearing against which the connecting rod is displaced can be a bearing receiver mast or be connected to the latter. An adjusting element is mounted between the fixed bearing and the connecting rod, which element can be telescoped, for instance by means of the servomotor, and thereby can bring about displacement of the connecting rod. However, there are also other possibilities for the use of adjusting elements, such as a linear motor or the like. 
     To enable verification of the rough correct orientation of the reflectors, these additionally have clinometers so that the angle of inclination of the reflectors can thereby be determined again and can be compared with the corresponding specifications. A rough preliminary orientation conforming to the state of the art described initially can additionally be performed. 
     The sensors are advantageously photovoltaic cells that convert the incident radiation directly into an electric current. Such sensors give a current signal of approx. 0 to 30 mA, which is fed into an AD converter, converted by the latter and relayed to a data processing system by way of a bus, for example a field bus. However, other sensors may also be used, such as temperature sensors, which measure the generation of heat. Also conceivable is the use of a waveguide arrangement that relays the light received to a suitable detector. 
     The invention described above will be explained in more detail below, using an exemplary embodiment. 
    
    
     
       The drawing shows: 
         FIG. 1  a linearly concentrating solar collector in a perspective view obliquely from above, 
         FIG. 2  the solar collector according to  FIG. 1  in a lateral top view, 
         FIG. 3  the receiver of the collector according to  FIG. 1  in cross-section, and 
         FIG. 4  the receiver according to  FIG. 3  with a variant of the sensor arrangements in cross-section. 
     
    
    
       FIG. 1  shows a concentrating collector of a solar thermal power plant essentially comprising a receiver  20  mounted in an elevated manner and reflectors  30  oriented toward it. The reflectors  30  are mounted on a support frame  40  and can be pivoted on this frame in such a way that each reflector  30  can direct the incident sunlight that impacts it directly to the receiver  20  mounted in an elevated manner. The receiver  20  is supported on receiver masts  41 , which can be braced against the carrier frame  40  by means of bracing cables  42 . 
       FIG. 2  shows the collector described above in a lateral view, which shows the ability of the reflectors  30  to pan. The reflectors  30  each have a swiveling lever  31  on their downward-facing side, by way of which lever they are connected to the carrier frame  40  in articulated manner. The free end of the swiveling lever  31  is connected, on the underside of the carrier frame  40 , to a connecting rod  32 , which thereby mechanically couples all reflectors  30  located on a side  43 ,  44  of the receiver  20 . In the event that the reflectors  30  are tracked by means of a displacement of the connecting rod  32 , all reflectors  30  are thus rotated to the same extent, whereby their different absolute positions ensure that every single reflector  30  can direct the incident sunlight directly onto the receiver  20 , from its individual position. 
     The connecting rod  32  is displaced by means of a servomotor  35 , which operates a telescoping adjusting element  33 . This element is connected, on the one hand, to the connecting rod  32 , and, on the other hand, to a fixed bearing  34 , whereby the fixed bearing  34  is located on a receiver mast  41 . The arrangement shown is repeated multiple times in the longitudinal direction of the collector of the solar thermal power plant, as shown in  FIG. 1 . 
     Because of the mechanical coupling, the reflectors  30  of the first side  43  of the receiver  20  are coupled to one another, in each instance, and the reflectors  30  of the second side  44  of the receiver are likewise coupled to one another. 
       FIG. 3  shows the receiver  20  in a cross-sectional view, whereby the receiver  20  essentially comprises a receiver tube  21  in which the medium to be heated is guided, and a receiver cover  22 , which surrounds the receiver tube  21 . Downward, the receiver cover  22  is sealed off by a glass plate  24 , so that for one thing, less heat is lost around the receiver tube  21 , and for another, contamination of the receiver tube  21  and the secondary reflector  23  arranged on the inside of the receiver cover  22  is likewise avoided. The secondary reflector in question reflects the sunlight that is directed past the receiver tube  21  back onto the receiver tube  21 , and thereby enhances the effectiveness of the tube once again. The layers provided between the secondary reflector  23  and the outer shell of the receiver cover  22  accommodate not only switching elements, where required, but also insulating material, in order to improve the generation of heat around the receiver tube  21 . 
     Sensor arrangements  10  are disposed on the bottom edges of the receiver cover  22 , on both sides, whereby each of these sensor arrangements  10  has a first sensor  11  and a second sensor  12 , in each instance. The first sensor  11  of the two sensor arrangements points in the direction of the reflectors  30  of the first side  43  of the linearly concentrating solar collector, and is protected, by means of a shield  13 , against not only direct solar irradiation but also the incidence of reflected light of the reflectors  30  of the second side  44 . The light received by the first sensors  11  is checked with regard to its intensity of radiation, and a difference between the intensities of radiation of the two first sensors  11  is formed. If the difference between the two first sensors  11  is equal to zero, it is assumed that a maximum of intensity of radiation lies exactly between the two sensors, in other words directly on the receiver tube  21 . In this case, the reflectors of the linearly concentrating solar collector are tracking exactly and no intervention is required. 
     If, however, the difference between the intensities of radiation of the two first sensors  11  is not equal to zero, tracking of the reflectors  30  on the first side  43  of the receiver  20  is performed, to the effect that the intensity of radiation is decreased at the first sensor  11  with the greater intensity of radiation, and increased at the first sensor  11  with the lesser intensity of radiation. Tracking is performed until the values are balanced again and the difference approaches zero again. In this connection, tracking is effected by the servomotor  35 , which brings about a change of angle at the reflectors  30  by way of displacement of the connecting rod  32 . This is achieved using a traditional proportional regulator, which has the difference of the intensities of radiation of the first sensors  11  as its input signal, and the actuating signal at the servomotor  35  as its output signal. 
     Corresponding tracking is brought about by the measurements performed by the second sensors  12 . In the configuration presented here, these again only have an effect on the reflectors  30  of the second side  44 . 
       FIG. 4  shows an alternative embodiment of the sensor arrangements  10 , whereby these are fully integrated into a housing, which has openings for the sensors  11 ,  12  on the surfaces  14 , which are angled away from one another. Here too, a shield  13  is provided, which is intended to prevent direct solar irradiation on the sensors. 
     Thus, a linearly concentrating solar collector with improved tracking of the reflectors is described above, which tracking brings about an orientation of the maximum radiation directly onto the receiver tube by means of suitable placement of sensors in the area of the receiver tube, and promotes exact tracking in the event of a deviation. 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  Sensor arrangement 
           11  First sensor 
           12  Second sensor 
           13  Shield 
           14  Surface 
           20  Receiver 
           21  Receiver tube 
           22  Receiver cover 
           23  Secondary reflector 
           24  Glass plate 
           30  Reflector 
           31  Swiveling lever 
           32  Connecting rod 
           33  Adjusting element 
           34  Fixed bearing 
           35  Servomotor 
           40  Support frame 
           41  Receiver mast 
           42  Bracing cable 
           43  First side 
           44  Second side