Abstract:
An apparatus for converting Direct Current (DC) to Alternating Current (AC), comprising a plurality of inverters, each inverter having a separate DC input adapted to be coupled to at least one solar cell defining an area of a solar panel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a method and apparatus for converting a direct current (DC) to alternating current (AC). More particularly, the present invention relates to a method and apparatus for converting DC to AC utilizing a plurality of inverters. 
         [0003]    2. Description of the Related Art 
         [0004]    Historically, photovoltaic panels have been used mostly in remote applications where commercial power was not available. This was due to the high cost of an installation making photovoltaic panels an economical choice only when nothing else was available. However, photovoltaic panels are now well established that the fossil energy reserves, which are currently used to generate electricity, are being rapidly depleted. The worldwide growth of power demand is leading to a durable increase in energy cost. 
         [0005]    Photovoltaic generation systems require an inverter that converts Direct Current (DC) from solar cells to Alternating Current (AC) for use by household appliances, for example. It is important for any power generation system to produce and deliver electricity to electric appliances in the most efficient manner. A typical photovoltaic array comprises a plurality of sub-arrays, where each sub-array comprises a plurality of individual photovoltaic panels. A junction box combines the output of the various sub-arrays to form a DC signal that is supplied to an inverter. The inverter converts the DC to AC and supplies the AC to the power grid. The user uses power from the grid in a typical fashion; however, their cost of electricity from the grid is offset by the amount of electricity the photovoltaic generation system supplies to the grid. 
         [0006]    In such a system, the amount of power that is efficiently coupled to the grid is important for the cost recovery of the system. As such, the inverter must be as efficient as possible. In a typical large-scale installation, rows of panels are installed on a flat surface with the panels tilted at an angle depending on local conditions such as latitude. Typically, for a location in the Northern Hemisphere, the more tilt, the more efficient will be the collection in winter, while a smaller tilt will harvest more power in summer, when the sun is higher in the sky. Some installations have variable tilts, thus, panels are always perpendicular to the sun&#39;s rays in order to harvest the maximum amount of energy. To enable the sun to fully illuminate all the panels, the panels are widely spaced from one another to avoid one panel shadowing another when the sun is low on the horizon. Although spacing the panels in such a manner maximizes the panel illumination for a given area, such spacing causes the solar array to occupy an excessive amount of space to produce a required amount of power. Consequently, solar energy systems are impractical in densely populated areas or in areas of high real estate prices. 
         [0007]    More specifically, the current produced by the solar cell array is proportional to the incident solar energy. The cell should be biased at the Maximum Power Point (MPP) in order to extract the maximum power. This MPP changes based on temperature, incident light level and aging and cannot be predicted, but inferred. Due to manufacturing tolerances, all solar cells have slightly different characteristics that vary the MPP from cell to cell. As such, when the solar cells of an array are connected in series, the average MPP will be used for biasing the entire array, i.e., a “global” bias point. The “global” bias point is typically demonstrably less than the MPP of all the individual cells of the array. Additionally, if a row of cells is shaded, it cannot generate the current level that the other cells may generate in full sun. Any panel shading entails a very significant power production loss. In many cases, half a shaded panel produces no power at all. Thus, for an array that is partially shaded, the global MPP is substantially different than the MPP for either the shaded portion or full sun portion. Consequently, the individual cells are not biased optimally for peak power generation. In order to avoid shading of the panels on one another, as mentioned above, the panel rows are positioned a sufficient distance apart to minimize shadowing. 
         [0008]    Therefore, there is a need for a method and apparatus for efficiently converting DC to AC for a solar array. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment, the present invention discloses an apparatus for converting Direct Current (DC) to Alternating Current (AC), comprising a plurality of inverters, each inverter having a separate DC input adapted to be coupled to at least one solar cell defining an area of a solar panel 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  is a side view of an exemplary embodiment of the photovoltaic panel system; 
           [0012]      FIG. 2  is a front view of an exemplary embodiment of a photovoltaic panel of the photovoltaic panel system of  FIG. 1 ; 
           [0013]      FIG. 3  is a block diagram of an exemplary embodiment of a photovoltaic panel system in accordance with the present invention; 
           [0014]      FIG. 4  is a detailed block diagram of an exemplary embodiment of an inverter system of  FIG. 3  in accordance with the present invention; 
           [0015]      FIG. 5  is a block diagram of an exemplary embodiment of a dual stage inverter in accordance with the present invention; and 
           [0016]      FIG. 6  is a block diagram of an exemplary embodiment of a single stage inverter in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a side view of an exemplary embodiment of the photovoltaic panel system  100  that utilizes embodiments of the present invention. The photovoltaic panel system  100  includes a plurality of photovoltaic panels  102  (also referred to as solar cell arrays) and a sun  108   1  and  108   2 . In this embodiment, the photovoltaic panels  102  include three panels  102   1 ,  102   2  and  102   3  that are densely arranged, i.e., the panels shadow one another. Although three panels are shown, the photovoltaic panel system  100  may include any number of panels  102 . 
         [0018]    In this dense arrangement of panels, the sun  108  illuminates the photovoltaic panels  102   1 ,  102   2  and  102   3  such that, when the sun  108   1  has a high elevation above the horizon, each panel  102   1 ,  102   2  and  102   3  is fully illuminated. However, with a lower elevation of the sun  108   2  with respect to the panels  102 , each panel  102  comprises lit-areas  104   1 ,  104   2  and  104   3  and shaded-areas  106   1  and  106   2 . The lit-areas  104   1 ,  104   2  and  104   3  are areas that are illuminated with more light (direct sun) and thus optimally produce DC. Therefore, in the depicted arrangement, photovoltaic panel  102   3  is operating most efficiently of all the panels. The shaded-areas  106   1  and  106   2  are subjected to a shadow from a neighboring panel and are exposed to less light; thus, the shaded areas  106   1  and  106   2  may produce less DC than the areas exposed to direct sun. However, utilizing one embodiment of the invention as described below, a maximum amount of power can be extracted from the panel when fully illuminated or when shaded. 
         [0019]      FIG. 2  is a front view of an exemplary embodiment of a photovoltaic panel  102   1  of the photovoltaic panel system of  FIG. 1 . The panel  102   1  comprises rows  200  (rows  200   1  through  200   M , where M is an integer). The photovoltaic panel  102   1  has solar cells  110   1  in rows  200   1-4  that constitute the lit-area  104   1  and solar cells  110   2  in rows  200   S-M  that constitute the shaded-area  106   1 . Therefore, the cells in the lit-area  104   1  have a different MPP as compared to the MPP of the cells in the shaded-area  106   1 . 
         [0020]    To further maximize power from a given panel, the solar cells within a panel (or rows thereof) may be tested for efficiency. The cells (or rows thereof) having the highest efficiency are positioned in a location on the panel that receives full illumination for the greatest amount of time, while the cells (or rows thereof) having comparatively lower efficiency at full illumination are positioned lower on a panel when substantially more shade it produced. In thins manner, the panel is designed to maximize the amount of power generated by the cells that are most efficient. 
         [0021]      FIG. 3  is a block diagram of an exemplary embodiment of a photovoltaic panel system  100  in accordance with the present invention. This diagram only portrays one variation of the myriad of possible system configurations. The present invention can function in a variety of environments and systems. 
         [0022]    The power generation system  100  comprises a plurality of power generators  320   1 ,  320   2  . . .  320   N  (where N is an integer), a junction box  304 , an electric panel  306  and an electric meter  310 . Each power generator (for example generator  320   1 ) comprises a plurality of inverters  300   1  (see  FIG. 4 ) and a photovoltaic panel  102   1 . As such, inverters  300   1  through  300   n  are respectively coupled to panels  102   1  through  102   n . 
         [0023]    The system  100  supplies power to a power grid  312 , appliances  316 , or both. The photovoltaic panels  102   1 ,  102   2  . . .  102   n  are well known in the art and are used for generating DC power from solar energy. The plurality of photovoltaic panels  102   1 ,  102   2  . . .  102   n  (also referred to herein as solar panels or solar arrays) may be of any size or shape. Even though the system  100  shows four (4) photovoltaic panels  102   1 ,  102   2  . . .  102   n , the system  100  may include any number of the photovoltaic panels  102 . 
         [0024]    The inverter  300   1 ,  300   2  . . .  300   n  converts DC power generated by the plurality of photovoltaic panels  102   1 ,  102   2  . . .  102   n  into AC power. The inverters  300  of the present invention may produce current that is in-phase with the AC grid current and generate such current with low distortion. 
         [0025]    The inverters  300   1 ,  300   2  . . .  300   n  couple the output AC to an AC bus  314 . The AC bus  314  is terminated into a junction box  304 . Using such an AC bus  314  and discrete power generators  320   N , the system  100  is scalable and flexible to fit any user&#39;s needs. The structures of the inverters  300   1 ,  300   2  . . .  300   n  are discussed below (See  FIG. 5  and  FIG. 6 ). 
         [0026]    The junction box  304  generally connects together the outputs from all the inverters  300   1 ,  300   2  . . .  300   n  to form a single AC feed to the electric panel  106 . 
         [0027]    The electric panel  306  connects the power from the junction box  304  to the power grid  312  and, in some applications, to appliances  316  within a user&#39;s facility. For example, in a home, the electric panel  306  is a well-known AC distribution hub comprising various circuit breakers and/or fuses to distribute electricity to various circuits within the home. The electric panel  306  is coupled through the electric meter  310  to the power grid  312 . The meter  310  determines the amount of power supplied to the grid  312 , such that the owner of the system  100  can be compensated for supplying electricity to the grid  312 . 
         [0028]      FIG. 4  is a detailed block diagram of an exemplary embodiment of an power generator  320   N  in accordance with the present invention. This diagram only portrays one variation of the myriad of possible configurations for a power generator. 
         [0029]    The power generator  320  includes a photovoltaic panel  102  and a plurality of inverters  300 . The photovoltaic panel  102  includes solar cells  402 . The solar cells  402  are generally arranged in rows  404  and columns  406 , which may include any number of solar cells  402 . By way of example, the rows  404  include rows  404   1 ,  404   2 ,  404   3  and  404   4  and the columns  406  include columns  406   1 ,  406   2 ,  406   3  and  406   4 . For simplicity, a four by four cell array is depicted. Those skilled in the art will understand that any number of rows and columns may be used. 
         [0030]    The inverters  300  are adapted to couple the rows  404  and/or the solar columns  406 . Each inverter  300  comprises a plurality of inverters  400   1 ,  400   2 ,  400   3  and  400   4  (referred to herein as nano-inverters). Although four nano-inverters are shown, the system  100  may include any number of inverters  300  and/or nano-inverters  400 . 
         [0031]    In this embodiment, the nano-inverters  400   1 ,  400   2 ,  400   3  and  400   4  are coupled to the rows  404   1 ,  404   2 ,  404   3  and  404   4  of solar cells, respectively. Each of the nano-inverters  400   1 ,  400   2 ,  400   3  and  400   4  receive DC from a respective row  404   1 ,  404   2 ,  404   3  and/or  404   4 . The cells within a row are connected on series. The nano-inverters  400   1 ,  400   2 ,  400   3  and  400   4  convert the DC to AC from, for example, each of rows  404   1 ,  404   2 ,  404   3  and  404   4 , respectively. Thus, each of the row  404   1 ,  404   2 ,  404   3  and  404   4 , may be treated as a separate entity, i.e. independently analyzed, controlled and/or configured for optimizing the efficiency and DC production of the row  404 . For example, each inverter is controlled to bias the inverter&#39;s associated row of cells at the Maximum Power Point (MPP) for the row. In this manner, as the MPP varies for rows that are shadowed, the optimal bias may be applied to rows in full sun and rows in shade. Consequently, by operating each row at the row&#39;s MPP, the output of the entire panel is optimized. As such, in this embodiment, the panels may be installed much closer to each other, thereby, maximizing total power output for a given area of installation for the system. 
         [0032]      FIG. 5  is a block diagram of an exemplary embodiment of a dual stage inverter  500  that may be used as an inverter  300  within a power generator  320  in accordance with the present invention. 
         [0033]    The dual stage inverter  500  includes a first stage  502  and a second stage  504 . The first stage  502  comprises a plurality of boost circuits  508 , which include  508   1 ,  508   2  . . .  508   n . The second stage  504  comprises a DC/AC converter  510 . The nature and design of the boost circuits  508  and the DC/AC converter  510  are well known in the art, wherein the boost circuit performs DC to DC conversion and the DC/AC converter performs DC to AC conversion, such as, a pulse width modulator (PWM). A controller  512  provides an individual control signal to each boost circuit  508  to ensure that the rows of cells coupled thereto operates at the row&#39;s MPP. The boost circuits  508  are coupled by a DC bus  506 , which may be a high voltage bus. The bus  506  couples the combined DC outputs of the boost circuits,  508  to the DC/AC converter  510 . The controllers  512  controls DC/AC converter switching to convert DC to AC. 
         [0034]    In this embodiment, the boost circuits  508   1 ,  508   2  . . .  508   n  share a single DC/AC converter  510 . Specifically, the nano-inverter  400   1  includes boost circuit  508   1  and the DC/AC converter  510 , the nano-inverter  400   2  includes boost circuit  508   2  and the DC/AC converter  510 , and the nano-inverter  400   3  includes boost circuit  508   3  and the DC/AC converter  510 , and so on. In other embodiments, a shared DC/AC converter may not be used and each inverter may have a separate boost and DC/AC converter circuit. The boost circuit  508  receives DC from a DC source, such as, rows  404 , columns  406 , solar cell  402 , portions thereof and the like. The boost circuits  508  input DC into the DC/AC converter  510  via the bus  512 . The DC/AC converter converts the DC to AC. 
         [0035]      FIG. 6  is a block diagram of an exemplary embodiment of a single stage inverter  600  that may be used as an inverter  300  within a power generator  320  in accordance with the present invention. 
         [0036]    The single stage inverter  600  includes a plurality of single stage nano-inverters  602  and a bus  604 . The DC may be generated by a DC source, such as rows  404 , columns  406 , solar cell  402 , portions thereof and the like. The DC from each area, e.g., a row of cells, is coupled to an individual single stage nano-inverter  602 . The plurality of single stage nano-inverters  602  comprises single stage nano-inverter  602   1 ,  602   2 ,  602   3  and  602   4 . Each nano-inverter  602  converts DC to AC and couples the AC to an AC bus  604 , where the AC is combined to produce the output of the power generator. The bus  512  is shared by the plurality of single stage nano-inverters  602 . The structure of the nano-inverter is substantially the same as a conventional single stage inverter. A controller  606  provides signals to maintain the areas coupled to the inverter operating at the MPP for the area, e.g., row. 
         [0037]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.