Abstract:
In a method for forecasting shadowing of a photovoltaic (PV) system due to cloud formation or movement, part of the firmament is imaged with fisheye optics onto the input optics of a digital camera. Pixel groups associated with luminous intensity ranges are formed. The spatial arrangement of the groups is analyzed to forecast shadowing of a photovoltaic system. A line extending from the PV system to the sun is formed and is continuously tracked. A reference line located inside a region around the line is formed. Passage of clouds across a reference line is analyzed. The result of the analysis is used to increase the electric power from the PV system to a minimum value through supply of additional backup energy or to reduce the electric power consumption by disconnecting users to ensure that key users do not experience a drop in supplied power below a minimum value.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2009 024 212.0, filed Jun. 8, 2009, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and an apparatus for forecasting forthcoming shadowing of a photovoltaic system connected to a power grid due to cloud formation or cloud movement. 
     The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
     Supply contracts with associated energy providers frequently require that the solar power plants are designed to supply to the power grid an agreed-upon minimum power within a determined time interval. For example, a 2.4 MW solar power plant may be required to supply between 11 AM and 5 PM at least 60% of the rated power, meaning about 1.5 MW. Depending on the weather, this minimum power cannot always be generated by the solar system alone. Backup power plants supplying DC current, for example a battery or a fuel-cell, must be switched in. For a longer forecast decrease in energy, an additional power plant must be commissioned on the AC side. This is implemented either by purchasing the required energy from another energy supplier by adding an additional power plant on the AC current side, for example in form of a diesel generator or a gas power plant. The respective energy suppliers have different startup times until the energy can be supplied. This ranges from the almost immediate discharge of a battery to approximately 5 minutes for starting out an additional power plant. 
     The decision, which measures are potentially the most appropriate, depends to a large degree from the incident sunlight to be expected on the area of the photovoltaic system. The energy supplier is generally interested in being informed about an impending decrease in the available energy. This can be addressed, for example, by early disconnection of users from the grid, whereby their operation can then be resumed at a later time. Such approach is indicated, for example, in the production of hydrogen. 
     It would therefore be desirable and advantageous to provide a method and an apparatus for identifying a impending undersupply and to rank its expected duration and severity, so that a selection can be made, which additional energy supplier should be prepared and provisioned for supplying the energy, so that the desired minimum power can be called up at any time, or that alternatively the disconnection of users from the grid can be initiated. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for preventing an impending decrease in electric power supplied by a photovoltaic (PV) system connected to a power grid below a limit value due to forthcoming shadowing caused by cloud formation or cloud movement, or both, includes the steps of analyzing the cloud formation or the cloud movement in a predetermined region between the PV system and a part of the firmament visible from the PV system, and deciding, based on results of the analysis, a need for enacting a measure for maintaining a minimum of supplied power into the power grid, or disconnecting at least one of the users. 
     According to another aspect of the invention, a device for preventing an impending decrease in electric power supplied by a photovoltaic (PV) system connected to a power grid below a limit value due to forthcoming shadowing caused by cloud formation or cloud movement, or both, includes a digital camera having an image sensor composed of pixels, fisheye optics imaging a visible part of the firmament at least partially onto a predetermined region of the image sensor, a memory device storing images of the region recorded at predetermined time intervals, a storage device storing parameters representing a solar trajectory, and a computing device. The computing device is configured to receive as an input a position of the sun at the predetermined time intervals together with at least two stored images, to compare the at least two images with each other, and to analyze a change in luminous intensity of the pixels in the at least two images with respect to the position of the sun. 
     As already mentioned above, the aforementioned measure may be the supply of additional energy, in addition to the energy generated by the PV system, into the power grid from a supplier of additional energy, or the additional measure may be the disconnection of less relevant users with a correspondingly lower priority from the grid. 
     According to a first embodiment of the method, the predetermined region is recorded by a digital camera, wherein images of the region which are composed of a plurality of pixels are recorded during time intervals, wherein the computed position of the sun is taken into account in the images, wherein consecutive images are compared with one another, and wherein the change in the luminous intensity of the pixels is analyzed in relation to the position of the sun and the reference point. 
     With this approach, a large part of the firmament is advantageously observed and cloud changes and cloud movement can be tracked and analyzed almost to the horizon. Marking the position of the sun in the images does not necessarily imply a visible mark, but is meant to indicate instead It that information about the position of the sun in the sky at the time of the recording is included in the images. This may occur while the images are recorded or later during additional processing of the image information. 
     Not every individual pixel needs to be processed separately. Alternatively, several adjacent pixels may be combined depending on the resolution of the camera and viewed as a composite pixel point, a large pixel point or a pixel aggregation point. 
     The feature “that the change in the luminous intensity of the pixels in relation to the position of the sun and the reference point is analyzed” indicates that the change in the luminous intensity of each pixel is measured from one image to the next, and that it is analyzed to which extent shadowing of the solar radiation by clouds at the reference point can be inferred from this change. 
     The time intervals may be periodic, whereby consecutive images that directly compared with one another. For example, an image may be generated every 2 to 5 seconds which is then compared with the previous image. If there are no changes during a certain time interval, as is the case for example for sunny or overcast weather, then this time interval may be successively increased and then again decreased after identifying a change. 
     Patterns in the change may advantageously be identified by not only comparing consecutive images, but also groups of images. A rather unstable, linear interpolation would result if only immediately consecutive images were compared. By analyzing image sequences, stable linear trends can be identified which can then be extrapolated into the future. 
     For modeling a cloud as realistically as possible, a plurality of contiguous pixels, which have the same luminous intensity or a luminous intensity within a predetermined tolerance range, may be combined into a group, and the migration of this group across the image region to other pixels may be monitored. For example, dark pixels with a light intensity that is only 10% to 15% of the maximum intensity for the calculated position of the sun, are combined into a group. In the next image, some pixels may fall outside this group because they have become brighter and are then associated with a different group. Other, previously brighter pixels may become correspondingly darker and are the accepted into the respective group. Accordingly, a change in the luminous intensity distribution takes place from one image to the next in the monitored region. 
     Advantageously, several groups with identical or different luminous intensity may be formed, wherein a motion vector is associated with each group, the length of which corresponds to the average intensity or velocity of all contributing pixels in the respective group and the direction of which corresponds to the average displacement direction of the luminous intensities in the image. Conversely, a low illumination or luminous intensity, which suggests a cloud and may be significant for shadowing, forms a correspondingly longer vector. 
     From the two parameters i) spatial extent of the group and ii) migration direction of the group, conclusions can be drawn if and how the group traverses a line between the reference point and the sun mark. Additional parameters, for example the rotation, the increase, the decrease or the change in the intensity may also be considered. 
     The aforementioned first embodiment advantageously considers cloud movements regardless of their altitude or the prevailing wind direction. It is also measured when cloud movements occur in different direction in one and the same region of an image, for example when a small black cloud at low altitude moves in front of a brighter cloud at higher altitude. All cloud movements with a tendency to approach the reference point while shadowing the solar radiation can thereby be filtered out and analyzed more closely. Likewise, clouds can be filtered out which approach the reference point, but do not intersect its line to the sun. 
     The second embodiment is based on the concept that, if the wind direction is known, one can look from the reference point to a location adjacent to the sun to recognize cloud movements that take place towards the reference point and will cause shadowing at a later time. Starting with this basic approach, according to an alternative to the previous embodiment, a line from the PV system or its immediate surroundings to the actual position of the sun is determined, the predetermined region is formed around this line, and passage of the clouds through this region is measured. In this way, the region quasi migrates with the position of the sun, and the region between the PV system and the sun is always observed. To analyze cloud movement, a horizontal reference line is placed in the region, and the passage or crossing of the clouds relative to the reference line is evaluated. A second reference line placed parallel to the first reference line enables determination of the velocity of the cloud movement. 
     Because it is of no interest how the cloud movement or cloud formation continues to develop after passage across the PV system, the boundary of the region should be placed in relation to the line so that the boundary is located close to the line in the prevailing wind direction and far removed from the line in the direction opposite to the wind. The region is advantageously rectangular or elliptical, and the line is then located at the narrow edge of the rectangle or proximate to the focal point of the ellipse located in the wind direction. This will be explained hereinafter with reference to  FIG. 2 . 
     The size of the region changes permanently with the position of the sun, i.e., the angle between the line and the ground surface on which the photovoltaic modules are installed. When the sun is located vertically above the photovoltaic system, the region should have an extent of about 5 to 100 square kilometers, depending on the altitude of the cloud cover. A larger region is recommended for clouds passing at low altitudes than for clouds moving at high altitudes. 
     As described above, a measure based on the result of the analysis may be the supply of additional energy, in addition to the energy provided by the photovoltaic system, from a supplier of additional energy into the power grid. More particularly, a battery can be used to supply the additional energy, in particular when using electro-mechanical DC-AC converters with a DC motor in combination with a synchronous motor (where in the DC motor is powered by the photovoltaic system). If the power supplied by the solar system drops below the minimum power, then the deficit is supplied by the supplier of additional energy. 
     Because a battery can compensate the energy deficit only over a limited time of several minutes, it may make sense to prepare a backup power plant for supplying power into the grid, as soon as there is an indication that the drop below the minimum required power from the photovoltaic system is not just temporary, for example due to a single passing cloud. For this reason, connection of the battery should be synchronized with the startup of a backup power plant or with the connection of an already running backup power plant. The backup power plant is configured to supply DC current and may be a conventional diesel generator with a rectifier able to provide the additional power within about one minute, or a fuel cell. The elapsed time between the request from the control device to the actual correction of the DC motor is advantageously between 1 and 5 minutes, more particularly between 2 and 4 minutes. 
     One criterion leading to a request for additional power from a power plant may be, for example, that the electric power generated by the photovoltaic modules drops below a minimum value for a predetermined time interval. Another criterion may be that the product of the electric power generated by the photovoltaic modules, on one hand, and a time interval on the other hand (i.e., the integral over the power) drops below a predetermined minimum value. Additional criteria may take into account the nature of the observed power drop of the photovoltaic system, such as hard shadows, lighting changes, cirro-cumulus clouds, high fog and the like. 
     This forms the basis for the invention, in that these weather-related parameters are not evaluated based on the energy currently generated by the photovoltaic system, and to then decide after the fact, but to forecast the shadowing parameters in advance of the weather pattern, so as to make a decision about the most suitable approach for supplying additional energy (or disconnecting users), before the shadowing condition actually arrives. 
     Fisheye optics used in the apparatus which enables complete imaging of the visible firmament onto a camera is generally known in the art. With its large observation space, the fisheye optics can be arranged remote from an individual photovoltaic system. It may be, for example, located at the center of three solar fields (PV systems) and may accordingly consider in the calculation three reference points, one for each PV system. Early first forecasts are possible because the field of view essentially includes the horizon. A gathering thunderstorm may be identified very early. In addition, this embodiment does not depend on the wind direction, because the entire space above the PV systems can be observed. 
     The apparatus may include instead of the camera a light detector having a plurality of photocells arranged in a matrix and by an analysis device which analyzes the light incident on the photocells with respect to its distribution on the matrix and its intensity. 
     A lens which images the region onto the surface of the light detector is arranged above the light detector. Advantageously, a control device is provided which tracks the lens so that its center optical axis is oriented towards the sun. The light detector should be composed of a plurality of photocells, for example between 50 and 500 photocells, arranged in form of a matrix. If the region is elliptical, the lens should also be elliptical. An analysis device evaluates the distribution of the incident light on the plurality of photocells and its intensity with respect to the individual photocells or photocells combined into groups. The aforementioned horizontal reference line is formed by a number of photocells located on a straight line located preferably at the edge of the matrix. The position of the edge depends on the wind direction. For this purpose, the apparatus is configured to be rotatable, so it can be oriented according to the wind direction, so that the cloud migration is imaged on the matrix before reaching the photovoltaic system. 
     The analysis device will not be described further in this application. Such devices are generally known in the art, for example in the field of fire detection where, for example, the horizon is observed above a wooded area to detect spreading of white or dark fire plumes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic diagram of an electromechanical converter with two DC motors, a DC generator, a battery, a backup power plant, and an additional power plant; 
         FIG. 2   a - 2   c  is a schematic diagram of an apparatus for carrying out the method of the invention; 
         FIGS. 3   a - 3   b  shows a result of an analysis, obtained with a first embodiment of the invention; 
         FIG. 4   a - 4   b  is a schematic diagram depicting the operation of the method for two different positions of the sun and wind directions according to a second embodiment; 
         FIG. 5  is a schematic diagram of the apparatus for carrying out the method; and 
         FIG. 6  shows a matrix of photocells with a distribution of the incident light in the presence of a cloud. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a photovoltaic system A with a first subset of photovoltaic modules Q 1  (not individually shown in  FIG. 1 ) as a first energy source, which has an output first terminal pair  1  configured for supplying a DC voltage and a DC current. The output terminal pair  1  can be connected to a first externally-excited DC motor  5  by way of a first switch  3 , for driving the DC motor  5 . 
     Likewise, a second subset Q 2  of photovoltaic modules representing a second energy source, which has a second output terminal pair  7  supplying a DC voltage U and a DC current I are shown on the right side of  FIG. 1 . The output terminal pair  7  can be connected to a second externally-excited DC motor  11  by way of a second switch  9 , for driving the DC motor  11 . 
     A three-phase generator  13  is arranged between the two DC motors  5 ,  11 , with the shaft  15  of the three-phase generator  13  in alignment with the two motor shafts  17 ,  19  of the DC motors  5 ,  11 . The generator shaft  15  extends outwardly from the left side on the right side of the DC generator  13  and is provided on each of the respective free ends with a respective coupling  21  and  23 . The shafts  17 ,  19  of the DC motors  5 ,  11  can be separately connected to the three-phase generator  13 . Alternatively, a fixed connection without couplings  21 ,  23  can also be implemented. The three-phase generator  13  includes an electric terminal  25  with a switch for connection to a power grid (shown in  FIG. 1  outside of A). 
     Also provided is a control device  26  configured to control the switches  3  and  9  as well as to disconnect or connect the couplings  21 ,  23 . The control device  26  is also equipped with a signal generator which actuates a bridge switch  27  which can the output terminals  1  and  7  with one another. During startup of the first DC motor  5 , the bridge switch  27  is closed and the power from the second energy source Q 2  is available at the input of the first DC motor  5  in addition to the power from the first energy source Q 1 . When the power of the first DC motor  5  reaches a predetermined value, in particular its nominal power, the bridge switch  27  is opened. The electric energy generated by the energy source Q 2  is then used for a startup and subsequent operation of the second DC motor  11 . When the rotation speed of its shaft  19  is matched to that of the shaft  25 , the control device  26  transmits a signal for initiating the coupling process of the coupling  23 . After successful coupling, the shafts  17 ,  19  together with the shaft  15  form a connected rotating assembly. 
     A battery  31  can be connected to the second DC motor  11  via a battery switch  29 . The battery  31  is in turn connected with a charging device  33 , in particular a rectifier charging device, selectively by way of an additional unillustrated switch, wherein the rectifier charging device is powered from the power grid  28 . 
     If the terminal  25  is unable to supply a defined minimum power to the power grid  28 , then the control device  26  is configured to initially open the switch  9 , to regulate the voltage of the second DC motor  11  to match the battery open-circuit voltage through a change in the excitation of the second DC motor  11 , and to subsequently close the battery switch  29  for supplying additional energy into the system A in addition to the solar energy. The additional energy is supplied by decreasing the motor voltage on the second DC motor  11  after the battery switch  29  is closed, which causes power to flow from the battery  31  to the DC motor  11 . 
     Preferably, both the first DC motor  5  and the second DC motor  11  may be connected to the battery  31 . It is hereby immaterial if a change-over switch is provided or if the battery  31  is divided into two separate batteries, e.g.  31   a ,  31   b , with one of the two separate batteries  31   a ,  31   b  associated with the first DC motor  5  and the other battery  31   b ,  31   a  with the second DC motor  11 . This measure is important in the context of the following evaluation regarding heat-up of the DC current machines  5 ,  11 . 
     To prevent damage to the DC current machines  5 ,  11 , a temperature monitoring device is advantageously provided. To this end, the control device  26  receives from a sensor  32  a measurement value representative of the temperature of the first DC motor  5 . It will be assumed that the first DC motor  5  starts up and the second motor  11  runs on the battery  31 . The first motor  5  is at risk of overheating. A safety measure is implemented in that, when a predetermined temperature limit is exceeded, the battery  31  is switched over from the first DC motor  5  previously supplied by the photovoltaic system Q 1  and Q 2  to the other DC motor  11 , whereas the other DC motor  11  previously driven only from the battery  31  is now driven from the photovoltaic system Q 1 +Q 2 . The situation is therefore reversed. 
     Advantageously, both DC motors  5 ,  11  may be provided with a device for monitoring the temperature, because both the DC motor which receives the entire load of the photovoltaic system may overheat, as well as the other DC motor which is supplied with the high battery current at low voltage. The control device  26  must here provide equalization by optionally switching several times between the energy sources, namely solar energy on one hand, and battery  31  (backup source) on the other hand. 
     The individual DC motors  5 ,  11  may still be overloaded even if they only supply a portion of the total nominal power. This situation occurs when the minimum power of, for example, about 70% of the nominal power (from first and second energy source Q 1 , Q 2  combined) is not attained, but the PV system (Q 1  and Q 2  combined) still supplies 60% of its nominal power. In this situation, the energy supplied by the second energy source Q 2  is diverted to the first DC machine  5  by closing the bridge switch  27 , with the first DC machine  5  now receiving 60% of the total energy. This represents an overload of 20% relative to half of the total power for this DC motor  5 . 
     This will now be illustrated in the following numerical example: a solar system with a total of 2.4 MW is installed. This system is subdivided into parts of equal size, so that the power supplied by Q 1  is equal to the power supplied by Q 2 , namely 1.2 MW each. The actual incident solar radiation supplies a power of 1.4 MW; however, to meet the supply contract with the energy supplier, 70% of the nominal power, namely 1.68 MW, must be supplied. After the switch  9  is opened and the bridge switch  27  is closed, 1.68 MW is supplied to the first DC motor  5 . However, this represents 0.24 MW (=20% of the nominal power) more than the continuous power rating of the first DC motor  5 . The missing 0.24 MW is then supplied to the second DC motor  11  from the battery, which corresponds to a current of 2400 amperes at a battery voltage of 100 V. 
     Accordingly, both DC motors  5 ,  11  should be provided with a device for temperature monitoring to prevent overheating. If the reduced amount of power supplied by the photovoltaic system (Q 1 +Q 2 ) may last for an extended period of time of several minutes, then the embodiment illustrated in  FIG. 1  must be implemented in order to guarantee that a minimum quantity of energy is supplied to the power grid  28 . 
       FIG. 1  also shows a DC backup power plant  35  which can be connected to the second DC motor  11  by way of an additional switch  37 . The battery  31  can only be used to briefly compensate for an energy shortage in order to supply the contracted power from the PV system A. If the reduced power output continues, then the backup power plant  35  instead of the battery  31  may be connected to the second DC motor  11 . Depending on the selected backup power plant  35 , different startup times are required to set up a connection of the backup power plant  35 . The setup time for a diesel generator with a rectifier is about 1 minute until the final operating state is reached, whereas the setup time for a fuel cell may be several minutes. Accordingly, the control device  26  must be designed to start up the backup power plant  35  early, to avoid the risk that the battery  31  is drained before being connected. 
     According to one criterion, a connection may be initiated when the electric power generated by the photovoltaic modules drops below a minimum value for a predetermined time interval. This means, for example, that the connection process is initiated as soon as the photovoltaic system Q 1 , Q 2  remains below the minimum power level for about 1 minute and the battery  31  needed to be connected during this one minute. According to another criterion, a connection may be initiated when the product of the electric energy produced by the photovoltaic modules and a time interval is less than a predetermined minimum value. In other words, the backup power plant  35  may be connected when 20% of the battery capacity is used up over a time of 6 minutes, because energy from the battery was used to make up for the power deficiency in order to cover the contractually guaranteed power. 
     It is essentially unimportant if the criteria are designed to use as a basis for the calculation the time when the backup power plant is set up or the time when the actual connection is made. In a second case, the required setup time of the backup power plant must be included in the calculation. 
     The backup power plant  35  is therefore used, on one hand, to rapidly charge the battery  31  and, on the other hand, to provide the minimum power for a relatively short time of, for example, 5 to 20 minutes. 
     If it can be foreseen that the additional power from the battery  31  and the backup power plant  35  may be required for a longer time of, for example, between 20 minutes and several hours, then an additional power plant  39  is switched in on the AC side via a switch  38 . This additional power plant  39  may be located remote at a different location in the supply grid  28 . For example, the additional power plant  39  may be a gas-fired power plant which can be ready to supply energy after a setup time of about 4 minutes. The control device  26  can here also be used for start-up and/or connection of the additional power plant. 
       FIG. 2   a  shows fisheye optics  50  which collimates onto the optics  54  of a digital camera  56  solar radiation  52  incident from all directions. These devices are known in the art. The camera  56  has an electrical output  58  which is connected to the input  60  of a comparator  62 . The comparator  62  compares in particular consecutive images from the camera  56 , which may be accomplished, for example, by evaluating the incident luminous intensity pixel-by-pixel, as illustrated schematically in  FIG. 2   b . The temporal spacing of the individual consecutive images may preferably be identical. 
       FIG. 2   b  shows a small segment  64  of the pixel set from three digital images, which were sequentially produced at the times t n , t n+1  and t n+2  in form of a matrix. In the uppermost row of the matrix on the left, five adjacent pixels  66  with associated luminous intensity values 5-6-8-10-9 at the time t n  are illustrated. The row underneath has the luminous intensity values 6-7-7-8-9. A large luminous intensity value corresponds here to a large intensity. 
     In the subsequent image taken at the time t n+1  the cloud cover has moved somewhat with the result that in the new value of the leftmost pixel introduced in the evaluated segment of the matrix is equal to 6, while the other pixels  66  of the uppermost row have moved to the right without changes in their luminous values. The segment  64  of the pixel set shown at the time t n+2  shows an additional shift in the uppermost row, with the left pixel  66  now having the value 4, whereas all other pixels are shifted to the right unchanged. 
     The situation is different in the lower row on the pixels  66  that has intensity values. At the time t n+1 , the center pixel  66  previously assigned the value of 7 has become somewhat brighter and now has the value 8 (fourth pixel from the left). Likewise, the center pixel with the value 7 at the time t n+1  undergoes the same change one image later at the time t n+2 , having now the value 8. Depending on the employed camera  56 , this comparison is performed for thousands of pixels  66 . 
     The output signal of the comparison unit  62  is supplied to a computing or analysis unit  68  which evaluates the changes in the intensity distribution on the matrix  64  described with reference to  FIG. 2   a . The evaluation includes both a change in the value of the pixels  66  as well as an analysis of the shift of the light or dark regions. For example, a contiguous pixel group having brightness values between 6 and 10 can be formed. This pixel group includes, depending on the cloud cover, a number of pixels  66 . The analysis unit  68  then observes the displacement of this group as a whole, as illustrated in  FIG. 2   c . A somewhat larger matrix segment  64  is shown in the upper image at a time t p , wherein a group  70  has 12 contiguous pixels  66  each having an intensity value between 6 and 10. In a subsequent image at the time t p+1 , the group  70  has shifted somewhat to the right; it has at the same time become larger and now includes 17 pixels which fall into the predetermined brightness region or tolerance range between 6 and 10. Each group  70  can be assigned a vector or arrow  72  which characterizes the intensity distribution within the group  70 . The length L of the arrow  72  is a measure for the average brightness of the group  70 . The intensity distribution between the corresponding images can be determined by comparing the arrows  72  at different times, for example at t p  and t p+1 . This change can be most prominently illustrated by providing each group  70  with a vector  72 . In this way, a large number of groups  17  can be formed (also with overlapping sets of pixels) which, on one hand, represent the same intensity level and, on the other hand, cover regions with lower and higher intensity. 
     Returning to  FIG. 2   a , there is also shown a memory device  74  which stores the trajectory of the sun as seen from the fisheye optics  50 . When the fisheye optics  50  is located at the location of the photovoltaic system, it also forms a reference point. The trajectory of the sun may be stored, for example, by entering the coordinates describing the position of the sun at a predetermined time. It is sufficient if the coordinates of the associated position of the sun are stored for a duration of, e.g., 10 minutes, i.e., six coordinate pairs per hour. The trajectory of the sun need not be stored in table form, but maybe computed based on known formulae and stored for each image. 
     The input data, i.e., the camera images or, as illustrated in  FIG. 2   a , already their comparison with each other obtained with the comparison unit  68 , as well as the coordinates of the position of the sun, are processed in the analysis unit  68  and associated with the reference point(s), i.e., the location where the photovoltaic system is installed. It typically result is described with reference to  FIGS. 3   a  and  3   b.    
     The entire visible region of the fisheye optics  50  is shown in the upper  FIG. 3   a  as a camera recording, in a manner in which the camera recording is rendered when the entire visible firmament is to be observed. For sake of simplicity, the region is illustrated as a circle, wherein the locations of two photovoltaic systems PV 1  and PV 2  are shown as stationary reference points. Also included in the camera image is the visible position  76  of the sun. A plurality of groups  70  with their associated vectors  72  is located within the camera image. The image represented in  FIG. 3   a  is only an instantaneous image recorded at a fixed point in time t. This image is updated after each camera image has been recorded and analyzed. 
     The lower  FIG. 3   b  shows for the first photovoltaic system PV 1  an exemplary course  78  of the forecast intensity of the incident solar radiation for time of one minute. Within this time period, the sink  71  which belongs to the group  70  at the lower left, has migrated to the left. Typically, not only a single group  70  is observed. Instead, all vectors  72  are evaluated to determine which of the groups  70  approaches the reference point PV 1 . It is then determined for these groups when they will arrive the reference point PV 1  by taking account their migration speed and how long they will remain above the reference point PV 1 , before they move to a location where they no longer pose a risk of shadowing. In the illustrated example of  FIG. 3   a , these are the two groups  70  and  70 ″, where a comparison between t p  and t p+1  suggests that there is a risk that these groups move between the position of the sun  76  and the reference point PV 1 . As a result, according to  FIG. 3   b , a decrease in the intensity is to be expected in about three minutes, which will then have passed after about 4 minutes (corresponding to the width of the group  70 ′). The intensity in the sink  71  drops below the threshold value  80  which is 60% of the maximum luminous intensity expected for this position of the sun. If a drop below this threshold value  80  is indicated, then it would be advisable to inform the energy supplier or the operator of the system of an unacceptable undersupply, so that one of the aforementioned measures can be taken. It will be understood that these measures may also be implemented immediately with the help of the control device. 
     It will be understood that the described components, such as the memory unit  74 , the comparison unit  62  and the analysis unit  68 , may not be viewed as separate components but rather describe the function to be provided. Accordingly, these components can be realized in a single electronic component. The attributed function should also not be viewed selectively, meaning that comparison of the image data and their analysis may be combined as far as practicable. 
       FIGS. 4   a  and  4   b  show schematically a large photovoltaic system  103  with an output power of several megawatts which is installed on a ground  101  in North-South direction. The sun  105  which travels along the arc  107  in East-West direction is indicated symbolically above the ground  101 . 
     A line  111  is located between the sun  105  and a point  109  inside or proximate to the photovoltaic system  103 . This is a determined line  111  which changes with the change in the position of the sun along the arc  107 , see  FIG. 4   b . The line  111  can be determined, for example, by starting from the point  109  as the initial point and selecting the brightest point on the firmament as an endpoint, with the line  111  being formed as a straight line through these two points.  FIG. 4   b  shows the line  111  at a later time of the day than in  FIG. 4   a.    
     A region  113  is formed around the line  111  which marks the field within which the cloud movement occurs. The cloud movement is here symbolized by an arrow  115  which indicates the prevailing wind direction. The region  113  is here defined as the plane parallel to the ground surface  101 . For sake of clarity, the region  113  in  FIGS. 4   a  and  4   b  is tilted by 90° and shown as a corresponding ellipse. A reference line  117  is drawn inside the region  113 , extending essentially perpendicular to the wind direction  115 . Passage of clouds  121  across the reference line  117  is monitored with an evaluation unit (not shown) located on the ground. It is determined how long it takes for the cloud  121  to move through the reference line  117 . The actual or average width B of the cloud(s) is also determined. From these data, the time is determined before the cloud(s)  121  move(s) in between the PV system  103  and the sun  105 . The extent of shadowing can also be determined from the velocity and the width B of the cloud(s). The computed result are then used to assess the need for backup energy due to the expected shadowing, in order to determine the minimum power of the energy supplied by the photovoltaic system  103  to the grid  28 . For example, a backup battery is sufficient for individual clouds  121  traversing the reference line  117 . For a denser cloud cover, preparation for provisioning a fuel cell would be advisable. For a very dense cloud cover or high fog blocking the solar radiation, which may last for several hours, a backup power plant should be arranged. 
       FIG. 4   b  shows the same situation at a later position of the sun  105 . The line was adapted to the new position of the sun. The wind has also slightly shifted and now blows from a different direction  115 , so that the elliptical region  113  together with the reference line  117  were rotated accordingly. 
       FIG. 5  shows an optical system  123 , symbolically indicated by a simple convex lens, located near the point  109 . No fisheye optics is implemented. The optical system  123  collects the obliquely incident light rays from the region  113  and projects them as substantially parallel radiation onto a flat matrix  125  of photocells  127 , see  FIG. 6 . The optical system  123  together with the fixedly connected matrix  125  is housed in a housing  129  which is supported for rotation along a first double arrow  131  and for tilting along a second double arrow  133 . This can be implemented, for example, by a cardan joint or ball joint. The joint is controlled by an unillustrated control device so that the optical system  123  is always able, depending on the prevailing wind direction  115  and the prevailing position of the sun, to analyze the region  113  for the presence of clouds  121 , before these clouds  121  shadow the photovoltaic system. The optical system  123  is oriented so that its central optical axis  135  points to the center of the region  113 . The size of the matrix  125  is selected accordingly, so that the entire region  113  is imaged onto the matrix  125 . 
     This is shown more clearly in  FIG. 6 , which shows the matrix  125  in a top view. The matrix  125  consists of a plurality of photocells  127  arranged in a rectangle. The rectangle is dimensioned so that the imaged region  113 ′, which is indicated in  FIG. 6  by its outer peripheral line  137 , fits into the rectangle. The imaged region  113 ′ corresponds to the region  113  between the photovoltaic system  103  and the sun  105  processed by the lens system. In particular, the photocells  127  are individually connected to the evaluation unit  119 . The position of the photocells  127  inside the matrix is correlated in the evaluation unit  119  with the received signal strength of the associated photocell and also correlated with the imaged reference line  117 ′. The evaluation unit  119  is then able to recognize early on the decrease in luminous intensity expected during passage of clouds, and to initiate one of the aforementioned countermeasures. 
     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein.