Patent Publication Number: US-2011056533-A1

Title: Series solar system with current-matching function

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a solar system, and more particularly, to a solar system with current-matching function. 
     2. Description of the Prior Art 
     The solar panels are utilized for forming a solar system (power system) so as to convert the solar energy into electrical power. The solar panel can receive light beams and accordingly generates a photocurrent and a photovoltage. The solar system can be grid-connected for providing an output current and a load voltage. The solar system formed by the solar panels can be a series solar system (the solar panels are electrically connected in series), or a parallel solar system (the solar panels are electrically connected in parallel). Comparing with the parallel solar system, the series solar system can generate the higher load voltage and the smaller output current. Since the conduction loss can be reduced when the magnitude of the output current of the solar system is reduced, and, generally speaking, the voltage level of the load voltage required by the grid is quite high, the series solar system is more proper to be grid-connected than the parallel solar system. 
     Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram illustrating the relation between the photocurrent and the photovoltage generated by a solar panel. In  FIG. 1 , assume that the received light intensity of the solar panel is SUN H , and the current-voltage curve (photocurrent-photovoltage curve) of the solar panel is CV H . If the solar panel operates at the operating point O 1 , that is, when the photocurrent generated by the solar panel is I 1  and the photovoltage generated by the solar panel is V 1 , the solar panel generates the maximum output power. In other words, when the current-voltage curve of the solar panel is CV H , the optimum operating point of the solar panel is O 1 . When the received light intensity of the solar panel changes from SUN H  to SUN L , the current-voltage curve of the solar panel changes from CV H  to CV L . Meanwhile, if the solar panel operates at the operating point O 2 , that is, when the photocurrent generated by the solar panel is I 2  and the photovoltage generated by the solar panel is V 2 , the solar panel generates the maximum output power. In other words, when the current-voltage curve of the solar panel is CV L , the optimum operating point of the solar panel is O 2 . According to the above-mentioned, the optimum operating point of the solar panel varies with the received light intensity. In addition, when the current-voltage curve of the solar panel is CV L , the maximum magnitude of the photocurrent that the solar panel can generate is around I 2 . If the external circuit is to drain a current with a magnitude larger than I 2  (for example, I 1 ) from the solar panel, the solar panel may be damaged. Hence, in the prior art, a diode is connected to the solar panel in parallel for protecting the solar panel. 
     In the series solar system, assume that the current-voltage curve of each solar panel is the same as CV H  shown in  FIG. 1 . However, if one of the solar panels is covered by the falling leaves or the frost snow, the received light intensity of the covered solar panel decreases so that the current-voltage curve of the covered solar panel will change from CV H  to CV L . In this way, the maximum magnitude of the photocurrent that the covered solar panel can generate is around I 2 . Since in the series system, the magnitudes of the currents passing through the solar panels have to be the same, the photocurrents outputted by the other uncovered solar panels can not be larger than I 2 . In other words, the other uncovered solar panels can not operate at the optimum operating point O 1  (generating the photocurrent I 1  and the photovoltage V 1 ). Therefore, in the series system, when one of the solar panels is covered, all the other uncovered solar panel are affected and can not generate the maximum output power, reducing the energy conversion efficiency of the series solar system. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to provide a series solar system that can generate the maximum power. 
     The present invention provides a series solar system with current-matching function. The series solar system is utilized for providing an output current and a load voltage. The series solar system comprises a plurality of solar modules electrically connected to each other in series. Each solar module comprises a solar panel, a DC/DC converter, and a feedback circuit. The solar panel is utilized for receiving light beams and generating a photocurrent and a photovoltage according to a light intensity. The DC/DC converter is electrically connected to the solar panel. The DC/DC converter is utilized for converting the photovoltage into an output voltage and converting the photocurrent into the output current according to a power-feedback signal. The feedback circuit is electrically connected to the DC/DC converter. The feedback circuit is utilized for generating the power-feedback signal according to the output voltage and the output current. A sum of output voltages generated by the plurality of the solar modules is equal to the load voltage. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the relation between the photocurrent and the photovoltage generated by a solar panel. 
         FIG. 2  is a diagram illustrating a solar module of the present invention. 
         FIG. 3A  is a diagram illustrating the method that the controller adjusts the duty cycle of the power switch according to a first embodiment of the present invention. 
         FIG. 3B  is a diagram illustrating the method that the controller adjusts the duty cycle of the power switch according to a second embodiment of the present invention. 
         FIG. 4  is a diagram illustrating that the solar panel can operate at the optimum operating point when the received light beams of the solar panel changes. 
         FIG. 5  is a diagram illustrating a DC/DC converter according to another embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a series solar system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the series solar system of the present invention, the magnitudes of the currents passing through the solar panels of the series solar system do not have to be the same, by means of each solar panel connected to a DC/DC converter in parallel, and the photocurrent generated by each solar panel can be matching with the operating current corresponding to the optimum operating point. In this way, even one of the solar panels of the series solar system of the present invention is covered; each solar panel still can operate at the optimum operating point. Thus, each solar panel can generate the maximum output power, improving the energy conversion efficiency of the series solar system. 
     Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram illustrating a solar module SLM of the present invention. The solar module comprises a solar panel SP, a voltage-stabilizing capacitor C ST , a DC/DC converter  210 , and a feedback circuit FBC. The solar panel SP comprises solar cells SC 1 ˜SC x . The solar cells SC 1 ˜SC x  are electrically connected to each other in series. The solar panel SP is utilized for receiving light beams so as to generate a photocurrent I PH  and a photovoltage V PH . The voltage-stabilizing capacitor C ST  is electrically connected to the solar panel SP in parallel, and the voltage-stabilizing capacitor C ST  can stabilize the photovoltage V PH  generated by the solar panel SP. The feedback circuit FBC generates a power-feedback signal S PFB  according to an output voltage V OUT  and an output current I OUT  of the solar modules SLM. More particularly, the feedback circuit FBC detects the output voltage V OUT  and the output current I OUT  of the solar modules SLM, and accordingly calculates out the output power P of the solar modules SLM. For instance, the feedback circuit FBC can multiply the output current I OUT  and the output voltage V OUT  together for obtaining the output power P. In this way, the feedback circuit FBC can generate the power-feedback signal S PFB  representing the output power P. In this embodiment, the DC/DC converter  210  is a buck converter. The DC/DC converter  210  is utilized for converting the photovoltage V PH  into the output voltage V OUT , and converting the photocurrent I PH  into the output current I OUT  according to the power-feedback signal S PFB . The DC/DC converter  210  comprises an output capacitor C OUT , a diode D, an inductor L, a power switch Q PW1 , and a controller CL. The electrically connecting relations between the components of the DC/DC converter  210  are shown in  FIG. 2 , and hence will not be repeated again for brevity. The output capacitor C OUT  is utilized for generating the output voltage V OUT . The controller CL is utilized for controlling the power switch Q PW1  to be turned on or turned off. When the power switch Q PW1  is turned on, the output current I OUT  passes through the inductor L, the power switch Q PW1 , and the solar panel SP; meanwhile, the solar panel charges the inductor L. When the power switch Q PW1  is turned off, the output current I OUT  passes through the inductor L, and the diode D; meanwhile, the inductor L is in the discharging state for maintaining the magnitude of the output current I OUT . For the solar module SLM generating the maximum output power, the controller CL adjusts the duty cycle of the power switch Q PW1  according to the power-feedback signal S PFB , and the related operational principle is illustrated in detail as below. 
     Please refer to  FIG. 3A .  FIG. 3A  is a schematic diagram illustrating the method that the controller CL adjusts the duty cycle of the power switch Q PW1  according to the power-feedback signal S PFB , according to the first embodiment of the present invention. The periods of the solar module SLM operating can be divided into the first detecting periods T 11 ˜T 1K  and the second detecting periods T 21 ˜T 2K , wherein the period lengths of the first detecting periods T 11 ˜T 1K  and the second detecting periods T 21 ˜T 2K  are all equal to one cycle T. During the first detecting period T 11 , the controller CL controls the power switch Q PW1  operating with the first duty cycle DUTY 11 . That is, the DC/DC converter  210  operates with the first duty cycle DUTY 11  at the time. During the second detecting period T 21 , the controller CL controls the power switch Q PW1  operating with the second duty cycle DUTY 21 . That is, the DC/DC converter  210  operates with the second duty cycle DUTY 21  at the time. Assume that the second duty cycle DUTY 21  is smaller than the first duty cycle DUTY 21 . That is, the turned-on period of the power switch Q PW1  during the first detecting period T 11  is longer than the turned-on period of the power switch Q PW1  during the second detecting period T 21 . The controller CL receives the power-feedback signal S PFB21  corresponding to the second detecting period T 21  during the second detecting period T 21 . The controller CL compares the power-feedback signal S PFB21  with the power-feedback signal S PFB11 . When the power-feedback signal S PFB21  is larger than the power-feedback signal S PFB11 , it represents that the output power P 21  outputted by the solar module SLM during the second detecting period T 21  is larger than the output power P 11  outputted by the solar module SLM during the first detecting period T 11 . Since the second duty cycle DUTY 21  is smaller than the first duty cycle DUTY 11 , it represents that the DC/DC converter  210  has to decrease the duty cycle for the solar module SLM generating a larger output power at the time. Consequently, the controller CL decreases the first duty cycle from DUTY 11  to DUTY 12  during the succeeding first detecting period T 12  so the DC/DC converter  210  operates with the first duty cycle DUTY 12  smaller than the first duty cycle DUTY 11 , and the controller CL decreases the second duty cycle from DUTY 21  to DUTY 22  during the succeeding second detecting period T 22  so the DC/DC converter  210  operates with the second duty cycle DUTY 22  smaller than the second duty cycle DUTY 21 . If the received power-feedback signal S PFB22  of the controller CL during the second detecting period T 22  is smaller than the received power-feedback signal S PFB12  of the controller CL during the first detecting period T 12 , since the second duty cycle DUTY 21  is smaller than the corresponding first duty cycle DUTY 11 , it represents the DC/DC converter  210  has to increase the duty cycle for the solar module SLM generating a larger output power at the time. Therefore, the controller CL increases the first duty cycle from during the succeeding first detecting period T 13  so the DC/DC converter  210  operates with the first duty cycle DUTY 13  larger than the first duty cycle DUTY 12 , and the controller CL increases the second duty cycle during the succeeding second detecting period T 23  so the DC/DC converter  210  operates with the second duty cycle DUTY 23  larger than the second duty cycle DUTY 22 . Hence, the controller CL can repeatedly compare the received power-feedback signal during the first detecting period with the received power-feedback signal during the second detecting period by means of the above-mentioned method, and accordingly adjusts the duty cycle of the DC/DC converter  210  for the solar module SLM generating the maximum output power. 
     Please refer to  FIG. 3B .  FIG. 3B  is a schematic diagram illustrating the method that the controller CL adjusts the duty cycle of the power switch Q PW1  according to the power-feedback signal S PFB , according to the second embodiment of the present invention. The periods of the solar module SLM operating can be divided into the detecting periods T 31 ˜T 3K , wherein the period lengths of the detecting periods T 31 ˜T 3K  are all equal to one cycle T. In  FIG. 3B , the controller CL controls the power switch Q PW1  operating with the duty cycle DUTY 31  during the detecting period T 31 ; the controller CL controls the power switch Q PW1  operating with the duty cycle DUTY 32  during the detecting period T 32 , wherein the duty cycle DUTY 32  is smaller than the duty cycle DUTY 31 . If the received power-feedback signal S PFB32  of the controller CL corresponding to the detecting period T 32  is larger than the received power-feedback signal S PFB31  of the controller CL corresponding to the detecting period T 31 , it represents that the controller CL has to decrease the duty cycle of the power switch Q PW1  for the solar module SLM generating a large output power. As a result, the controller CL decreases the duty cycle of the power switch Q PW1  from DUTY 32  to DUTY 33  during the detecting period T 33 . When the received power-feedback signal S PFB33  of the controller CL during the detecting period T 33  is smaller than the received power-feedback signal S PFB32  of the controller CL during the detecting period T 32 , it represents that the controller CL has to increase the duty cycle of the power switch Q PW1  for the solar module SLM generating a larger output power. Thus, the controller CL increases the duty cycle DUTY 34  of the power switch Q PW1  during the detecting period T 34 . In this way, the controller CL can repeatedly compare the received power-feedback signal during a detecting period with the received power-feedback signal during a prior detecting period adjacent to the detecting period by means of the above-mentioned method, and accordingly adjusts the duty cycle of the DC/DC converter  210  for the solar module SLM generating the maximum output power. 
     Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram illustrating that the solar panel SP can operate at the optimum operating point when the received light beams of the solar panel SP changes. Assume that the output current I OUT  of the solar module SLM is limited to be I 3  by an external load. At the first, the received light intensity of the solar panel is SUN H , and the current-voltage curve of the solar panel SP is CV H . Meanwhile, the controller CL can adjust the duty cycle of the power switch Q PW1  by means of the methods illustrated in  FIG. 3A  and  FIG. 3B , so that the solar panel SP can operate at the optimum operating point O 1  (that is, the photocurrent generated by the solar panel SP is I 1 , and the photovoltage generated by the solar panel SP is V 1 ) of the current-voltage curve CV H . In  FIG. 4 , the curve CV SLMO1  represents the relation between the output current I OUT  and the output voltage V OUT  generated by the solar module SLM when the solar panel SP operates at the operating point O 1 , by means of the DC/DC converter  210 . Since the output current I OUT  of the solar module SLM is limited to be I 3 , the output voltage V OUT  generated by the solar module SLM is V 3  according to the curve CV SLMO1 . When the received light intensity of the solar panel SP changes from SUN H  to SUN L  (for example, the solar panel SP is covered), the current-voltage curve of the solar panel SP becomes CV L . The controller CL can adjust the duty cycle of the power switch Q PW1  by means of the methods illustrated in  FIG. 3A  and  FIG. 3B , so that the solar panel SP still can operate at the optimum operating point O 2  (that is, the photocurrent generated by the solar panel SP is I 2 , and the photovoltage generated by the solar panel SP is V 2 ) of the current-voltage curve CV L . In  FIG. 4 , the curve CV SLMO2  represents the relation between the output current I OUT  and the output voltage V OUT  generated by the solar module SLM when the solar panel SP operates at the operating point O 2 , by means of the DC/DC converter  210 . Since the output current I OUT  of the solar module SLM is I 3 , the output voltage V OUT  generated by the solar module SLM is V 4  according to the curve CV SLMO2 . Therefore, no matter the received light intensity of the solar panel SP is SUN H  or SUN L , the DC/DC converter  210  can adjust the duty cycle according the methods illustrated in  FIG. 3A  and  FIG. 3B  so that the output power of the solar panel SP can be maximized in the different condition of the received light intensity (for example, SUN H  or SUN L ). 
     Please refer to  FIG. 5 .  FIG. 5  is a schematic diagram illustrating a DC/DC converter  510  according to another embodiment of the present invention. The DC/DC converter  510  comprises an output capacitor C OUT , an inductor L, power switches Q PW1  and Q PW2 , and a controller CL. Comparing with the DC/DC converter  210 , the controller CL of the DC/DC converter  510  controls not only the power switch Q PW1 , but also the power switch Q PW2 . The power switches Q PW1  and Q PW2  are complementary to each other. That is, when the power switch Q PW1  is turned on, the power switch Q PW2  is turned off; when the power switch Q PW1  is turned off, the power switch Q PW2  is turned on. When the power switch Q PW1  is turned on and the power switch Q PW2  is turned off, the output current I OUT  passes through the inductor L, the power switch Q PW1 , and the solar panel SP. When the power switch Q PW1  is turned off and the power switch Q PW2  is turned on, the output current I OUT  passes through the inductor L and the power switch Q PW2 , meanwhile, the inductor L is in the discharging state for maintaining the magnitude of the output current I OUT . In addition, the DC/DC converter  510  further comprises a diode D (as shown in  FIG. 5 ). In this way, when the power switches Q PW1  and Q PW2  are in the dead-time state (that is, when the controller CL is to switch the power switches Q PW1  and Q PW2 , the power switches Q PW1  and Q PW2  may both be turned off for a short time), the output current I OUT  still can pass through the inductor L by the diode D, and the inductor L is in the discharging state for maintaining the magnitude of the output current I OUT  at the time. In the present embodiment, the controller CL of the DC/DC converter  510  still can control the solar panel SP operating at the optimum operating point by means of the methods illustrated in  FIG. 3A  and  FIG. 3B , so that the output power generated by the solar module SLM can be maximized. For example, by means of the method illustrated in  FIG. 3A , the controller CL controls the power switch Q PW1  operating with first duty cycles DUTY 11 ˜DUTY 1K  during the first detecting periods T 11 ˜T 1K  and operating with the second duty cycles DUTY 21 ˜DUTY 2K  during the second detecting periods T 21 ˜T 2K  according to the power-feedback signal S PFB . In this way, the controller CL can adjust the first duty cycle and the second duty cycle of the power switch Q PW1  by means of comparing the received power-feedback signals during the first detecting periods with the received power-feedback signals during the second detecting periods. In addition, the diode D is a Schottky diode, and the power switches Q PW1  and Q PW2  are both Metal Oxide Semiconductor (MOS) transistors. 
     Please refer to  FIG. 6 .  FIG. 6  is a schematic diagram illustrating a series solar system  600  of the present invention. The series solar system  600  is utilized for providing an output current I OUT  and a load voltage V L  to an external load LOAD. The series solar system comprises solar modules SLM 1 ˜SLM N , wherein the structures and operational principles of the solar modules SLM 1 ˜SLM N  are similar to the solar module SLM in  FIG. 2 . Since in the series solar system  600 , the output power generated by each solar module SLM 1 ˜SLM N  can be maximized by means of the methods illustrated in  FIG. 3A  and  FIG. 3B . Thus, the energy conversion efficiency of the series solar system  600  is improved. Besides, in the series solar system  600 , the received light intensities of the solar modules SLM 1 ˜SLM N  may be different. For instance, in the series solar system  600 , the solar panel SP 1  of the solar module SLM 1  is covered so the received light intensity of the solar panel SP 1  is SUN L , and the received light intensities of the other uncovered solar panels SP 2 ˜SP N  are equal to SUN H . In other words, the photocurrent correspond to the optimum operating point of the solar panel SP 1  of the solar module SLM 1  is different from the photocurrents corresponding to the optimum operating point of the other uncovered solar panel SP 2 ˜SP N . However, since in the series solar system  600 , the magnitudes of the currents passing through the solar panels SP 1 ˜SP N  do not have to be the same by means of each solar panel connected to a DC/DC converter in parallel, each solar panel SP 1 ˜SP N  still can operate at the optimum operating point. That is, the output power of each solar module SP 1 ˜SP N  is maximized by means of the DC/DC converters DCCR 1 ˜DCCR N  of the solar modules SLM 1 ˜SLM N  adjusting their duty cycles according to the illustration in  FIG. 4 . In addition, the magnitudes of the currents outputted by the solar modules SLM 1 ˜SLM N  are all equal to the output current I OUT  provided by the series solar system  600  at the same time. 
     In addition, in the above-mentioned solar module SLM, the DC/DC converter  210  (or  510 ) can be a boost converter or a boost-buck converter according to the requirement. For example, when the output current I OUT  of the series solar system  600  is mainly determined by the external load LOAD and the magnitude of the output current I OUT  determined by the external load LOAD is smaller than the current corresponding to the optimum operating point of the solar panel, each solar panel still can operate at the optimum operating point by means of realizing the DC/DC converter  210  (or  510 ) with a boost converter (or a boost-buck converter). Since the boost converter and the boost-buck converter are well known to those skilled in the art, the structures and the operational principles of them will not be illustrated for brevity. 
     In conclusion, the series solar system provided by the present invention has the current-matching function by means of the solar panel connected to the DC/DC converter in parallel. In this way, no matter the solar panel is covered or the magnitude of the current outputted by the solar module determined by the external load is smaller than the photocurrent corresponding to the optimum operating point of the solar panel, the DC/DC converter can adjust its duty cycle for the solar panel operating at the optimum operating point. Consequently, the output power of each solar module is maximized, increasing the energy conversion efficiency of the series solar system. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.