Abstract:
A tracking type photovoltaic power generator includes a clock for clocking a time and a sun direction sensor for outputting a signal indicative of a sun direction, a controller which calculates a first time when the sun the sun arrives at a culmination azimuth on the basis of the time clocked by the clock, detects a culmination azimuth on the basis of a difference between a second time when the culmination azimuth is detected and the first time. Accordingly, the generator automatically corrects the error of the time clocked by the clock.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tracking type photovoltaic power generator and an error correction method of its built-in clock. 
     2. Description of the Related Art 
     Conventionally, a tracking type photovoltaic power generator has been well known. This tracking type photovoltaic power generator, as conceptually shown in FIG. 1, has a power generation module  12  supported so as to be rotatably in a direction of an azimuth angle “A” and a direction of an elevation angle “Z”. Also, this tracking type photovoltaic power generator has a sun direction sensor rotated together with the power generation module  12 , a clock and a controller although they are not shown. A signal obtained from the sun direction sensor for indicating a sun direction is sent to the controller. 
     This conventional tracking type photovoltaic power generator uses the following two methods to detect the sun direction. In the first method, the sun direction is detected by the calculation based on a position (represented by a longitude and a latitude) at which the tracking type photovoltaic power generator is installed, and a date and a time. In this case, the date and the time are obtained from the clock. In the second method, the sun direction is detected based on a signal from the sun direction sensor. 
     This conventional tracking type photovoltaic power generator is operated as follows. That is, on an arrival of an operation start time (for example, a time of a sunrise), the power generation module  12  is firstly turned to the sun direction. That is, the controller detects the sun direction by using the first method, and rotates the power generation module  12  to the detected direction. 
     In the normal operation after that, if the sun is behind a cloud, the first method is used to detect the sun direction at a predetermined time interval. If the sun is not behind the sun, the second method is used to detect the sun direction at the predetermined time interval. Based on those detected results, the power generation module  12  is rotated at the predetermined time interval so that a light reception surface of the power generation module  12  is faced to the sun. Thus, it is controlled such that the light reception surface of the power generation module  12  is always opposite perpendicularly to the sun, namely, the power generation module  12  tracks the sun. 
     By the way, a quartz oscillation clock similar to a home clock is used as the above-mentioned clock. An error (an oscillation stability) of this clock is 11 ppm (corresponding to one second/day) if an ambient temperature is 10 to 30° C. Also, if the ambient temperature is −40 to 85° C., the error is 150 ppm (13 seconds/day). If it is converted into an annual error, the error becomes 80 minutes/year. 
     The occurrence of such an error causes the direction of the light reception surface of the power generation module  12  to be deviated from the actual sun direction. For example, the delay of the clock such as two minutes and fifty seconds may cause the following situation. That is, as shown in FIG. 2, although a culmination time (also referred to as “meridian passage time”) determined by a calculation is forty minutes thirty seconds past eleven, a time when the light reception surface of the power generation module  12  is turned to a true south in the case of the tracking operation using the sun direction sensor is forty-three minutes twenty seconds past eleven. The excessively large deviation causes the sun to depart from a visual angle of the sun direction sensor, when the control based on the sun direction detected by the first method is switched to the control based on the sun direction detected by the second method. Thus, there may be the case that the tracking is impossible. 
     This error of the clock can be manually corrected by a person at the position of the installation of the tracking type photovoltaic power generator. However, if the installation position of the tracking type photovoltaic power generator is located at a remote place, various difficulties are brought about. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a tracking type photovoltaic power generator that can automatically correct an error of a clock, and an error correction method of its built-in clock. 
     In order to attain the above-mentioned object, a tracking type photovoltaic power generator according to a first aspect of the present invention comprises a clock for clocking a time, a sun direction sensor and a controller. The sun direction sensor outputs a signal indicative of a sun direction. The controller corrects an error of the time clocked by the clock based on the signal from the sun direction sensor. 
     In the tracking type photovoltaic power generator according to the first aspect of the present invention, the controller may be composed of a calculating section, a detecting section and a correcting section. The calculating section calculates a first time when the sun arrives at a predetermined azimuth based on the time clocked by the clock. The detecting section detects the predetermined azimuth based on the signal from the sun direction sensor. The correcting section corrects the error of the time clocked by the clock based on a difference between a second time when the detecting section detects the predetermined azimuth and the first time calculated by the calculating section. 
     In this case, the detecting section can be configured so as to detect the predetermined azimuth based on the signal from the sun direction sensor within a predetermined time period before or after the first time calculated by the calculating section. Also, the correcting section can be configured so as to correct the error of the time clocked by the clock in a time band except the predetermined time period. Moreover, the correcting section can be configured so as to correct the error of the time 6. 
     An error correction method of its built-in clock in the tracking type photovoltaic power generator according to a second aspect of the present invention includes the steps of: detecting an error of a time clocked by the built-in clock based on a signal from a sun direction sensor; and correcting the error of the time clocked by the built-in clock based on the detected error. 
     In the error correction method of its built-in clock in the tracking type photovoltaic power generator according to the second aspect of the present invention, the step of detecting the error can be composed so as to include the steps of: calculating a first time when the sun arrives at a predetermined azimuth based on the time clocked by the built-in clock; detecting the predetermined azimuth based on the signal from the sun direction sensor; and detecting the error of the time clocked by the built-in clock based on a difference between a second time when the predetermined azimuth is detected and the calculated first time. 
     In this case, the correcting step can be configured so as to correct the error of the time clocked by the built-in clock when the difference becomes equal to or greater than a predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a configuration of a conventional tracking type photovoltaic power generator; 
     FIG. 2 is a view describing a deviation between a sun direction detected by a calculation and a sun direction detected by a sun direction sensor, which is brought about because of an error of a time clocked by a clock; 
     FIG. 3 is a view describing a sun direction at a certain location on an earth surface; 
     FIG. 4 is a view showing a configuration of a tracking type photovoltaic power generator according to an embodiment of the present invention; and 
     FIG. 5 is a flowchart showing a process for correcting an error in a tracking type photovoltaic power generator shown in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, an embodiment of the present invention will be described below with reference to the attached drawings. It should be noted that, in the following description, as “predetermined azimuth” in the present invention, a culmination azimuth of the sun, that is, the azimuth of the meridian line of the position at which the tracking type photovoltaic power generator is installed is used. However, the predetermined azimuth in the present invention is not limited to the culmination azimuth. Any azimuth may be used. 
     Typically, a sun direction at a certain position on an earth surface can be defined from a zenith angle “Z” (an angle between a sun direction and a vertical line on the position) and an azimuth angle “A” at which a culmination is zero, as shown in FIG.  3 . The following equations (1) to (4) are established: 
     
       
         cos Z=cos D′×cos L′+sin D′×sin L′×cos H  (1) 
       
     
     
       
         tan A=(sin D′×sin H)/(sin D′×cos L′×cos H-cos D′×sin L′)  (2) 
       
     
     
       
         cos D′=sin 23.5°×sin((360°×n days)/365.25 days)  (3) 
       
     
     
       
         H=(360°/24 hours)×t hours  (4) 
       
     
     Here, “L” is a complementary angle of a latitude “L” of an observation position, “n” is the number of elapse days from the spring equinox, and “t” is a time from the culmination time. The negative “t” indicates the morning, and the positive “t” indicates the afternoon. 
     Here, with reference to the equations (2) and (4), at t=0 (the culmination time), the azimuth angle “A” is always zero (A=0). Thus, in this embodiment, a time when an azimuth angle of a power generation module indicates the culmination is regarded as the culmination time, and a clock is corrected. 
     Now, it is specifically described below. FIG. 4 is a view showing a configuration of the tracking type photovoltaic power generator according to the embodiment of the present invention. This tracking type photovoltaic power generator is designed such that a power generation module  12  is mounted in a frame  11  rotatably supported on a pole  10 . In this tracking type photovoltaic power generator, an electric power generated by the power generation module  12  is sent through a wire to an inverter  13 . The inverter  13  converts an inputted direct current power into an alternating current power. The alternating current power outputted by this inverter  13  is used for consumption. 
     Also, an azimuth actuator  14  for rotating the entire frame  11  in an azimuth angle direction is mounted at a lower end portion of the pole  10 . A motor  15  drives this azimuth actuator  14 . Also, a linear actuator  16  for rotating the entire frame  11  in an elevation angle direction is mounted at an upper end portion of the pole  10 . A motor  17  drives this linear actuator  16 . Each of the motors  15 ,  17  is constituted by, for example, a stepping motor in which a rotation direction and a rotation amount can be easily controlled. 
     A sun direction sensor  18  for detecting a sun direction is mounted at a predetermined portion of the frame  11  (an upper portion of FIG.  4 ). This sun direction sensor  18  outputs a signal for indicating whether or not a vertical line to a light reception surface of the power generation module  12  is deviated from the sun direction. The signal from the sun direction sensor  18  is sent to a controller  20 . 
     The controller  20  contains a CPU (Central Processing Unit), a program memory composed of read only memories (ROMs), and a work memory composed of random access memories (RAMs). The program memory stores a control program to operate the controller  20 , fixed data and the like. The work memory is used as a temporary memory when the CPU carries out various processes. 
     A culmination time register, an error time register, an elevation angle register, an azimuth angle register and the like are defined in the work memory. The culmination time register stores a culmination time calculated based on a time indicated by a clock  21 . The error time register stores a difference between the culmination time and a time indicated by the clock  21  when the power generation module  12  is turned to the culmination direction. The elevation angle register stores a current elevation angle of the power generation module  12 . Also, the azimuth angle register stores a current azimuth angle of the power generation module  12 . The elevation angle register and the azimuth angle register are updated each time the direction of the power generation module  12  is changed. 
     The clock  21  for clocking a time of a position at which the tracking type photovoltaic power generator is installed is connected to the controller  20 . The controller  20  generates tracking control signals for actuating the azimuth actuator  14  and the linear actuator  16 , on the basis of time data from the clock  21  and the signal from the sun direction sensor  18 , and sends them to the motors  15 ,  17 , respectively. 
     The motors  15 ,  17  are rotated in the directions and by the amounts that are indicated by the tracking control signals. Those rotations drive the azimuth actuator  14  and the linear actuator  16 , respectively. Accordingly, the power generation module  12  mounted in the frame  11  is rotated in the azimuth angle direction and the elevation angle, respectively. Thus, it is possible to carry out the control so that the light reception surface of the power generation module  12  is always faced to the sun, namely, the tracking control of the sun. 
     A process for correcting an error of the time clocked by the clock  21  (hereafter, referred to as “error correction process”) among the processes executed by the controller  20  will be described below with reference to a flowchart shown in FIG.  5 . It should be noted that it is assumed that the data indicative of longitude and latitude of a position at which the tracking type photovoltaic power generator is installed is stored in the work memory of the controller  20  when installing the tracking type photovoltaic power generator. 
     In this error correction process, a culmination time is firstly calculated (Step S 10 ). This calculation is carried out on the basis of a time indicated by the clock  21 . Thus, when an error is included in the time indicated by the clock  21 , the error is also included in the calculated culmination time. The thus-calculated culmination time is stored in the culmination time register. 
     Next, a tracking start process is carried out (Step S 11 ). In this tracking start process, a sun direction at a time indicated by the clock  21  is calculated. Specifically, an elevation angle and an azimuth angle indicative of the sun direction are calculated. They are stored in the elevation angle register and the azimuth angle register, respectively. 
     After that, the tracking control signals are generated on the basis of the contents of the elevation angle register and the azimuth angle register, and sent to the motors  15 ,  17 . Accordingly, the azimuth actuator  14  and the linear actuator  16  are driven. Thus, the power generation module  12  is rotated so as to face to the sun direction detected by the calculation. 
     Then, it is investigated whether or not a current time indicated by the clock  21  is present in a range of ten minutes before or after the culmination time stored in the culmination time register (Step S 12 ). If it is judged to be present in the range of the 10 minutes before or after the culmination time, it is then investigated whether or not the tracking is possible (Step S 13 ). This investigation is carried out by measuring whether or not an illumination of a certain value or more can be obtained for a certain period, by using an illumination meter (not shown). In this case, if the illumination of the certain value or more can not be obtained, for example, because of cloudy sky, or if the illumination of the certain value or more is only intermittently obtained since the sunlight is shielded by a mass of moving clouds, it is judged that the tracking is impossible. 
     It should be noted that, whether or not the tracking is possible can be judged by using another method. For example, it can be also judged by investigating whether or not an electric power of a certain value or more can be obtained from the power generation module  12 , continuously for a certain period. 
     At the step S 13 , when it is so judged that the tracking is impossible, the sequence returns back to the step S 12 . On the other hand, when it is judged that the tracking is possible, the tracking process is carried out by using the sun direction sensor  18  (Step S 14 ). That is, the contents of the elevation angle register and the azimuth angle register are updated on the basis of the signal from the sun direction sensor  18 . After that, the tracking control signals are generated on the basis of the contents of the elevation angle register and the azimuth angle register, and sent to the motors  15 ,  17 . Thus, the azimuth actuator  14  and the linear actuator  16  are driven. Hence, the power generation module  12  is rotated so as to face to the sun direction detected by the sun direction sensor  18 . 
     Next, it is investigated whether or not the power generation module  12  is turned to a true south (Step S 15 ). This is carried out by investigating whether or not the content of the azimuth angle register updated at the step S 14  is data indicative of an azimuth angle of the true south (for example, zero). 
     At the step S 15 , if it is so judged that the power generation module  12  is not turned to the true south, the sequence returns back to the step S 12 . On the other hand, if it is judged that the power generation module  12  is turned to the true south, a time at that point is read from the clock  21  (Step S 16 ). The time stored in the culmination time register is subtracted from the read time and stored as an error time in the error time register (Step S 17 ). After that, the sequence returns back to the step S 12 . The operations similar to those of the above-mentioned description are repeatedly executed. 
     In the process of this repetition execution, if is judged at the step S 12  that the current time indicated by the clock  21  is not present in the range of the 10 minutes before or after the culmination time, it is then investigated whether or not the error is 24 seconds or more (Step S 18 ). In this Step S 18 , if it is judged to be 24 seconds or more, an error correction of the clock  21  is carried out (Step S 19 ). That is, a time stored in the error time register is added to the time read from the clock  21 , and its result is reset for the clock  21 . If the error is less than 24 seconds at the step S 18 , the clock correction process is skipped. The error correction process is completed by the above-mentioned operations. 
     According to the above-mentioned embodiment of the present invention, the error of the time clocked by the built-in clock  21  is corrected at the predetermined time interval. Thus, the tracking type photovoltaic power generator is never at the state that the tracking is impossible. Therefore, the operation among long-range is possible. Also, it is possible to reduce a cost necessary for the correction of the error of the time clocked by the clock  21 . 
     It should be noted that the above-mentioned embodiment is designed such that the error time is calculated if the current time indicated by the clock is present in the range of the 10 minutes before or after the culmination time. However, it is not limited to the 10 minutes before or after the culmination time. Any time may be used if it is in a range of an error time that can be varied by an accuracy of the clock  21  and an execution period of the error correction process. 
     Also, this is designed such that the clock correction is carried out only when the error is the 24 seconds or more. However, it is not limited to the 24 seconds. It can be defined as any predetermined time. In this case, the reason why the clock is corrected after waiting for the fact that the error exceeds a certain time is that a slight error may be brought about even if the sun direction sensor  18  is used to track the sun since the sun direction sensor  18  also has an error. 
     Moreover, the above-mentioned embodiment is designed such that the clock correction is carried out when the current time indicated by the clock  21  becomes outside the range of the 10 minutes before or after the culmination time. However, it can be designed such that the clock correction is carried out at another timing, for example at timing when a daily tracking operation is ended. 
     As detailed above, according to the present invention, it is possible to provide the tracking type photovoltaic power generator that can automatically correct the error of the time clocked by the clock, and the error correction method of its built-in clock.