Patent Publication Number: US-2011056483-A1

Title: Self-learning solar collector orientation control system

Description:
The invention relates to a method for orientation control of a solar collector and/or of a plurality of solar collectors arranged in a row, in particular parabolic channel collectors, having a heat collecting element which is in each case arranged on the focal line, wherein the temperature and/or the collected amount of heat in the heat carrier medium flowing through the heat collecting element is measured in the area of each solar collector such that it can be associated therewith, and the determined temperature values and/or heat amount values are supplied to a control unit which controls the orientation of the respective solar collector and orients the respective solar collector within the scope of a defined orientation parameter, in particular an orientation range and/or orientation travel, with respect to the sun. The invention also relates to a method such as this, in which a plurality of solar collectors which are arranged in a row interact. 
     Solar-thermal power stations use the energy captured from the sun by means of an absorber and absorbed by a heat carrier medium which is flowing in the absorber. The heat emitted from the sun is therefore used as a primary energy source. In this case, the concept of solar-thermal power stations with focusing of the direct radiation consists of using reflectors to focus the direct radiation from the sun onto a solar absorber. These power stations use concentrating reflector surfaces in order to focus the incident sunlight onto the absorber. The reflectors and the absorber are oriented toward the sun. The collector array of a solar farm power station in this case normally consists of a large number of parabolic channel collectors which are connected in parallel and/or in series. 
     Parabolic channel collectors consist of curved mirrors which focus the sunlight onto an absorber tube which runs on the focal line. Depending on the type, the length of such collectors is between 20 and 200 m. The concentrated solar radiation is converted to heat in the absorber tubes, and is emitted to a circulating heat carrier medium. The parabolic channels are generally oriented toward the sun in only one axis, for cost reasons. They are therefore arranged in the North-South direction and are oriented toward the sun from East to West over the course of the day. This orientation can be controlled by time, or else can be carried out controlled by means of a solar sensor. The individual collectors, which have a length of up to 200 m, consist of a plurality of segments which are arranged in series for flow purposes. Each collector in each case has an associated drive for its orientation, a temperature measurement, a device for determining the time of day, and an orientation detector. The collectors are oriented toward the sun over the course of the day. This is done either by means of a light-sensitive sensor, which produces information about the position of the collector relative to the position of the sun, or via an algorithm, which calculates the position of the sun and a position sensor which supplies the collector position. The orientation of a solar collector is optimum when the heat amount absorbed in its extent by the heat carrier medium is a maximum. 
     However, there are problems in optimally designing the orientation and the alignment of the solar collectors. Because of the physical size and the mechanical loads associated with it for the adjustment of the collectors, it is difficult to align them optimally with respect to one another. The optimum alignment cannot be detected geometrically since, in general, the mirror elements of a solar element are not all precisely aligned with the focal line, and the various segments of a collector are not aligned optimally with respect to one another. Furthermore, the alignment of the sensors, both of the solar sensor and of the (angle) position sensor, can shift relatively with respect to the respectively associated solar collector. In addition, their initial and original alignment is not optimum in every case. Finally, a considerable amount of weight has to be moved in the relatively long, individual segments of a collector when they are oriented on one axis about the longitudinal axis, as a result of which torsion and therefore twisting of a segment about its longitudinal axis occur during the orientation movement, thus changing the relative position of the individual mirrors with respect to the focal line and the absorber arranged therein. 
     These problems and disadvantages lead to the individual solar connectors which are connected in a row in a strand or circuit element not in each case being optimally aligned toward the sun and being oriented toward the sun over the course of the day. The evaluation of the respectively detected sensor signals and the calculation of the orientation position do not reflect the optimum position of the respective overall solar collector. In practice, the geometric determination of the optimum collector position is possible only approximately. 
     In contrast, the invention is based on the object of providing a solution which allows improved orientation of a solar collector. 
     In the case of a method of the type mentioned initially, this object is achieved according to the invention in that the method comprises the following steps:
     a) determination of the heat amount collected in the respective solar collector;   b) variation of an orientation parameter of the respective solar collector by an increment in the direction of the solar movement or a decrement in the opposite direction to the solar movement;   c) comparison of the heat amount collected in step a) with the heat amount collected in the respective solar collector after carrying out step b) and   d) if the heat amount detected in step c) is greater than the heat amount determined in step a), storage of the orientation parameter, which has been modified by the increment or the decrement, as the new nominal value for the orientation control of the respective solar collector in the control unit.   

     In the case of a method of the type mentioned initially, and in which furthermore a plurality of solar collectors which are arranged in a row interact, this object is likewise achieved in that the method comprises the following steps:
     a1) determination of a sliding mean value of a first temperature difference, which is associated with a first solar collector, of the heat carrier medium between a first and a second temperature measurement point;   a2) determination of a sliding mean value of a second temperature difference, which is associated with another solar collector, of the heat carrier medium between two temperature measurement points, at least one of which is different from the first and/or second temperature measurement point;   b1) variation of the orientation parameter, in particular of the orientation range and/or of the orientation travel, of the first solar collector by an increment in the direction of the solar movement or by a decrement in the opposite direction to the solar movement;   c1) comparison of the leading mean values, which then result, of the first and second temperature difference and   d1) if the sliding mean value of the first temperature difference is increased relative to the sliding mean value of the second temperature difference, storage of the orientation parameter, in particular the orientation range and/or orientation travel, which has been modified by the increment or the decrement, as the new nominal value for the orientation control of the first solar collector in the control unit.   

     Refinements and developments of the invention are specified in the dependent claims. 
     The method according to the invention makes it possible to carry out a relative power measurement of the respective solar collector or of the series-connected solar collectors in a collector array or a circuit element during operation and to determine collector-specific correction values, that is to say correction values associated with each individual collector of possibly a plurality of solar collectors, for orientation control, and in this way to orient each solar collector individually, as optimally as possible. The orientation parameters are optimized by variation of the orientation parameters, in particular of the orientation range and/or of the orientation travel, of a/each collector in operating conditions which are otherwise constant, and by comparison of the changes which then result in the heat amount collected in the respective collector, in particular by comparison of the changes in a sliding mean value of a temperature difference or a plurality of temperature differences relative to one another. In this case, in the situation in which the comparison value for the change in the heat amount rises, the orientation parameter is stored as the new nominal value for the orientation control in the control unit. If the comparison of the changes results in a comparison value which is lower than the initial value, then the orientation parameter is adjusted by one increment or one decrement in the opposite direction with respect to the initial position, that is to say it is adjusted by two increments or two decrements in the opposite direction with respect to the already changed position. If the comparison which is now carried out with the original initial value likewise indicates no change in the heat amount or in the sliding mean value of a temperature difference, then the orientation parameter or the orientation parameters is or are reset to the original initial values. A series and sequence of control steps are defined in this way, which lead to the respective solar collector or, in the case of a plurality of series-connected solar collectors, each individual solar collector, each being aligned optimally toward the sun, and being oriented with the profile of the sun through the day. In the case of a plurality of series-connected solar collectors, this optimization method is determined by a comparison between a solar collector whose orientation parameter remains unchanged and a solar collector whose orientation parameter is changed. When a plurality of solar collectors are combined in series in a row to form a circuit element, the optimization method is subsequently repeated for each individual solar collector in the circuit element. The respectively optimized orientation parameters are adopted by the collector control system, that is to say they are stored here and are processed by means of a microprocessor and/or fuzzy logic. The orientation control system starts a respective optimization cycle either on a time-control basis or controlled by the solar sensor. As soon as the defined time period has passed or the solar sensor indicates defined further movement of the sun on its daily path, the orientation control system varies the respective orientation parameter, that is to say in particular the orientation range and/or the orientation travel of a respective connected solar collector. In particular, the orientation travel and the mid-position of the solar collector are varied, and the control unit uses the change which then occurs in the heat flow, or uses the heat amount, to determine the current orientation strategy in the sense of the procedure described above, which is then optimum for the respective solar collector and the respective current radiation condition. This results in a self-learning orientation system. The respective solar collector is also optimally oriented with respect to the sun throughout the day by carrying out this control cycle repeatedly a number of times in the course of the day. 
    
    
     
       The invention will be explained in more detail in the following text with reference to one exemplary embodiment and a drawing, in which: 
         FIG. 1  shows a schematic illustration of a solar collector, 
         FIG. 2  shows a schematic illustration of the orientation of a solar collector following the course of the sun from East to West, 
         FIG. 3  shows a schematic illustration of a circuit element, which consists of four solar collectors, with four temperature measurement points, and 
         FIG. 4  shows a circuit element, which consists of four solar collectors, with five temperature measurement points. 
     
    
    
     The solar collector, which is annotated  1  overall in  FIGS. 1 and 2 , consists of two segments  2 ,  3  which themselves each comprise six reflector elements  4 , each with twenty-eight mirrors. Overall, the collector  1  therefore has twelve reflector elements  4 . These are each rigidly mounted on a frame  5  which is resistant to twisting. Each segment  2 ,  3  is mounted on a support. In this case, the outer supports form end supports  6   a ,  6   b , and the center support forms a drive support  7 . The drive support  7  has a drive unit which comprises a hydraulic unit and orients the entire collector  1  on one axis about its longitudinal axis  8 , pivoting such that it follows the sun  9  over the course of the day from East to West, as is illustrated schematically in  FIG. 2 . An absorber  10  is formed and arranged as a heat collecting element on the focal line of all the reflector elements  4 , which are aligned as far as possible in the same way as one another. A heat carrier medium flows through the absorber or the heat collecting element  10 , and the absorber or the heat collecting element  10  is oriented toward the sun in the direction of the arrow  11  over the course of the day such that the solar radiation occurs as far as possible at all times directly corresponding to the solar radiation incidence  12  illustrated schematically in  FIG. 2 , and therefore as great a heat amount as possible is transmitted to the heat carrier medium flowing in the absorber/heat collecting element  10 , at all times during the course of the day, by direct radiation from the sun. 
     The orientation is carried out with the aid of an orientation control system. This orientation control system receives input signals from solar sensors, angle position sensors and temperature sensors, which are associated with each individual solar collector  1 . The respective solar sensor and the respective angle position sensor are associated with the drive unit, and are generally arranged on the drive support  7 . A temperature measurement point for the respective solar collector  1  is likewise arranged there. As can be seen from the exemplary embodiment shown in  FIG. 4 , it is, however, also possible to provide temperature measurement sensors in front of and behind each solar collector  1 , if these are connected or arranged in series in a row in the form of a circuit element. 
     By way of example, a conventional solar sensor consists of two photovoltaic cells which are arranged alongside one another and are arranged on the respective solar collector  1  such that the absorber  10  throws a shadow onto the two photovoltaic cells when subject to solar radiation. In this case, the solar sensor is now ideally aligned and is oriented with the course of the sun—together with the segments  2 ,  3  and the reflector elements  4 —such that the shadow that is thrown is distributed uniformly over the two photovoltaic cells, that is to say a minimum difference voltage occurs between the two individual solar/photovoltaic cells in the solar collector. As soon as the sun now moves onward and the difference voltage changes and reaches a specific, defined value, an orientation cycle is carried out by the control unit associated with the respective solar collector  1 . The control unit detects and processes the signals received from the angle position sensor, by means of which the angular position of the segments  2 ,  3  and therefore of the respective solar collector  1  is detected with respect to the vertical longitudinal axis of the supports  6   a ,  6   b  and  7 . In this case, the position in which the concave mirror surface is aligned to face completely East is the 0° position, the opposite complete alignment to the West is the 180° position, and the alignment of the reflector elements  4  which is approximately horizontal and is located between them is the 90° position. 
       FIGS. 3 and 4  each show a circuit element  13 , which consists of four collectors K 1 , K 2 , K 3  and K 4 , of a solar-thermal power station. By way of example, as a heat carrier medium, thermal oil flows through the absorber/the heat collecting element  10  of the circuit element  13  from a cold distributor  14  to a hot gatherer  15 . In a thermal solar power station or solar-thermal power station, a plurality of these circuit elements  13  are connected in parallel, and are each connected to the cold distributor  14  and to the hot gatherer  15 . The solar collectors of each of these circuit elements  13  each always have the same mass flow rate. 
     In the exemplary embodiments shown in  FIGS. 3 and 4 , one circuit element  13  is in each case illustrated, consisting of four solar collectors K 1 , K 2 , K 3  and K 4 . The major difference is that, in the case of the exemplary embodiment shown in  FIG. 3 , the temperature sensors T 1 , T 2 , T 3  and T 4  are provided, which are each arranged at the center of each collector K 1 , K 2 , K 3  and K 4  between the respective two segments  2  and  3 , and detect the temperature of the heat carrier medium there, while, in the case of the exemplary embodiment shown in  FIG. 4 , the temperature measurement points T′ 1 , T′ 2 , T′ 3 , T′ 4  and T′ 5  are in contrast each located in front of and/or behind a solar collector K 1 , K 2 , K 3  and K 4 . 
     Since the sun continues to move continuously during the course of the day, the signal emitted from the solar sensor to the control unit varies continuously if the solar collector does not move. This signal, and therefore the relative position of the respective solar collector with respect to the sun, can be modeled by a number in degrees, a signal voltage or else by a time period. The orientation parameter that is used to orient the respective solar collector is then based on these respectively chosen values, or else on a plurality of these chosen values. In this case, an orientation process is not carried out whenever the signal changes, but only when the signal moves into or out of an “orientation window”. For the purposes of the self-learning optimization process described in the following text, or the self-learning orientation control system, this “orientation window” is in each case adjusted or shifted by an increment or a decrement, wherein, in the exemplary embodiment, a single step or an increment/decrement means a shift through 0.5 degrees. 
     The optimization sequence for the orientation control cycle is carried out only when the solar array, that is to say the circuit element  13 , is in a time interval of quasi-steady state. This is the case when the solar radiation and the mass flow in the respective collector K 1 , K 3 , K 3  or K 4  to be oriented are sufficiently constant, and this is checked by a weather station for the solar power station and the superordinate solar array control system. In a situation such as this, all the solar sensors which are associated with the respective solar collectors K 1 , K 2 , K 3  and K 4  indicate a difference voltage which is in the “orientation window”. The temperatures and temperature differences T 2 −T 1 , T 3 −T 2  and T 4 −T 3  measured by the temperature sensors are recorded continuously as a sliding mean value. 
     When the orientation optimization cycle is initiated, the mid-position of the “orientation window” of the first solar collector K 1  in this case is now shifted through one step of 0.5 degrees, for example in the westerly direction. The first collector K 1  in this case is thus shifted through one increment of 0.5 degrees in the westerly direction from its previous alignment, that is to say it is aligned shifted through one more such 0.5° step in the westerly direction. 
     When the sliding mean value of the temperature difference T 2 −T 1 , which is now once again measured, rises relative to one and/or both likewise measured sliding mean values of the temperature differences T 3 −T 2  and T 4 −T 3 , then this means that a greater heat amount is being collected by the heat carrier medium in the collector K 1 , that is to say the solar radiation is being better utilized. This is interpreted as an optimum position by the control unit, and the “orientation window” that is being shifted through 0.5 degrees to the west is stored as the new nominal value for the solar collector K 1 . 
     If, in contrast, this measure finds that the sliding mean value of the temperature difference of T 2 −T 1  relative to the sliding mean values of the temperature differences T 3 −T 2  and/or T 4 −T 3  has fallen, the orientation window is shifted in the opposite direction, that is to say through two decrements, that is to say through two 0.5 degree steps, and in consequence through a total of one degree to the east. This means that the orientation window is now in a position which has been shifted through 0.5 degrees to the east in comparison to the original initial position. The sliding mean values of the temperature differences T 2 −T 1 , T 3 −T 2  and T 4 −T 3  are once again compared. If it is now found with this setting that the sliding mean value of the temperature difference of T 2 −T 1  has risen in comparison to the sliding mean values of the temperature differences T 3 −T 2  and/or T 4 −T 3 , this position of the “orientation window” which has been shifted to the east is now stored in the orientation control device as the new nominal value for the solar collector K 1 . 
     If it is also found with this second measure that there is once again no increase in the sliding mean value of the temperature difference T 2 −T 1  relative to the sliding mean values of the temperature differences T 3 −T 2  and/or T 4 −T 3 , then the position of the “orientation window” is then not reoriented, that is to say the nominal value originally stored in the orientation control device is maintained for the orientation control system. 
     The orientation range of the solar collector K 1  was adjusted or optimized in the procedure described above. The entire “orientation window” was adjusted. However, in this case, the orientation travel or the width of the “orientation window”, that is to say the angle through which the sun has moved further, remained unchanged until the solar collector was readjusted, corresponding to a change in the “window width”. This angle also once again corresponds to a time interval or a signal variable of the solar sensor. 
     An optimization process corresponding to the procedure described above is also carried out for this purpose. In order to optimize the orientation travel, the orientation window for the orientation travel is once again increased, for example by 0.75 degrees, in a quasi-steady state for the solar collector K 1  as the first solar collector. If no relative increase in the sliding mean value of the temperature difference T 2 −T 1  is found during this procedure, in comparison to the sliding mean values of the temperature differences of T 3 −T 2  and/or T 4 −T 3  in the sliding mean value now being determined of the temperature difference T 2 −T 1 , the travel about the mid-position is decreased by 0.75 degrees as a decrement. If no relative increase in the sliding mean value of the temperature difference T 2 −T 1  associated with the first solar collector K 1  can be found even then, the old nominal value is retained in the control unit, for control of the orientation travel. However, if an increase in the temperature difference associated with the solar collector K 1  is found in one of the two steps, the orientation travel value associated with this temperature difference is stored as the new nominal value for the solar collector K 1  in the orientation control unit. 
     Once the optimization method or methods described above has or have been carried out for the solar collector K 1 , these orientation control optimization cycles are then carried out for the other solar collectors K 2 , K 3  and K 4 , during which process these are then, for the purposes of the subjects of the invention, dealt with and associated with the first solar collector, per se. 
     The following procedure, in particular, is preferably adopted for the optimization of the circuit element  13  according to the exemplary embodiment shown in  FIG. 3 : 
     With the solar collector K 1  as the first solar collector, the relative change in the sliding mean value of the temperature difference of T 2 −T 1  is considered with the comparison value of the temperature difference T 4 −T 3 . 
     With the solar collector K 2  as the first solar collector, the change in the sliding mean value of the temperature difference of T 2 -T 1  is likewise considered in comparison with the temperature difference T 4 -T 3 . 
     With the solar collector K 3  as the first solar collector, the change in the sliding mean value of the temperature difference T 4 −T 3  is considered in comparison with the temperature difference T 2 −T 1 , and with the solar collector K 4  as the first solar collector, the change in the sliding mean value of the temperature difference T 4 −T 3  is considered in comparison with the temperature difference T 2 −T 1 . The sensitivity and accuracy of the method can be improved by alternatively positioning the temperature sensors or temperature measurement sensors between in each case two collectors, which then results in there being one more temperature sensor for each circuit element  13 . This is illustrated in the exemplary embodiment shown in  FIG. 4 . In this case, the circuit element  13  has the temperature measurement sensors and temperature measurement positions T′ 1 , T′ 2 , T′ 3 , T′ 4  and T′ 5 . In this case, with the solar collector K 1  as the first solar collector, the change in the sliding mean value of the temperature difference T′ 2 −T′ 1 , in the case of the solar collector K 2  as the first solar collector, the change in the sliding mean value of the temperature difference T′ 3 −T′ 2 , in the case of the solar collector K 3  as the first solar collector, the change in the sliding mean value of the temperature difference T′ 4 −T′ 3 , and in the case of the solar collector K 4  as the first solar collector, the change in the sliding mean value of the temperature difference T′ 5 −T′ 4 , are considered relative to at least one of the respective other temperature differences, and are evaluated in the orientation control optimization cycle. 
     The invention results in the optimum mean value of the orientation of the respective collector K 1  or of all the collectors K 1  to K 4  with respect to the sun being determined in each case, even when the respective total of twelve segments  2 , 3  of the four solar collectors K 1  to K 4  have been shifted or twisted with respect to one another or in their own right. The self-learning orientation control method is used to compensate for imprecisely adjusted settings of the various sensors, in particular of the solar sensors relative to the respective collector or relative to all the collectors. The optimum mean value of the orientation of the collectors or of the respective collector with respect to the sun is also used for control purposes when the segments are twisted relative to one another or in their own right during the readjustment or the orientation process. Overall, the method according to the invention improves the efficiency of a solar collector, and an improvement of 5-10% was determined using the method according to the exemplary embodiment. 
     In order to carry out the self-learning orientation or orientation control, the control unit is equipped with a microprocessor and/or a fuzzy logic unit in which, on the one hand, the optimization rules for the method are stored and which, on the other hand, is connected such that the sensor signals received are processed appropriately. 
     Alternatively, the incremental or decremental adjustment steps of the respective orientation parameter are also continued and repeated in one direction when an increased heat amount or temperature difference is found, until the heat amount or the temperature difference falls again, that is to say until the maximum has been passed over. The last value of the respective orientation parameter before the decrease again is then stored as the new nominal value.