Abstract:
An AC photovoltaic module includes a DC photovoltaic module for converting solar energy to DC electrical power, and an inverter for converting DC electrical power to AC electrical power, the inverter being adapted for connection to a frame portion of the module and being sized and configured, and provided with arrangements of electrical components thereof, to dispense heat from the inverter, whereby to prolong operational life and reliability of the inverter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application No. 61/128,219, filed May 20, 2008 in the names of Miles Clayton Russell, Gregory Allen Kern, Ruel Davenport Little and Zachary Adam King. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to solar electric power systems, and more particularly to an AC photovoltaic module including an inverter sized and configured so as to collect less heat, and to dissipate heat, and to operate under relatively cool conditions. 
     2. Description of the Prior Art 
     It is known to provide an AC photovoltaic module including a DC photovoltaic module for converting solar energy to DC electrical energy, and an inverter for converting the DC electrical energy to AC electrical energy, and for feeding the AC electrical energy to an AC grid. See, for example, International Patent Application No. PCT/US2009/038547. 
     The inverter typically is mounted at or near the center of the DC photovoltaic module. It is known that the center of the DC module suffers the greatest elevation of heat during a sunny day, and after the sun has lowered and disappeared, the greatest change in temperature. 
     The extremes of temperatures of the inverter lead to a relatively short life span of the inverter. Inasmuch as each AC module is provided with an inverter, keeping all inverters active in an array of numerous modules can be problematic. 
     An object of the invention is therefore to provide an AC photovoltaic module having an inverter of a beneficial size and configuration, and mounted on the DC photovoltaic module at a relatively cool peripheral portion of the DC module. 
     A further object of the invention is to provide an arrangement of components in the inverter such as to concentrate heat in the inverter on one side thereof, and to provide heat sink means for dissipating the heat from that side to the surrounding environment. 
     A still further object of the invention is to provide film capacitors in the inverter circuitry, which operate more reliably and more effectively, with less internal resistance, and therefore generally less heat, than the same assembly circuitry with commonly used electrolytic capacitors. 
     SUMMARY OF THE INVENTION 
     With the above and other objects in view, a feature of the present invention is the provision of an AC photovoltaic module including a DC photovoltaic module for converting solar energy to DC electrical power, and an inverter for converting DC electrical power to AC electrical power, the inverter being adapted for connection to a peripheral frame portion of the DC photovoltaic module and being sized and configured to dispense heat therefrom, to prolong operational life and reliability of the inverter. 
     In accordance with a further feature of the invention, there is provided an AC photovoltaic module comprising a DC photovoltaic module for producing DC electrical power, and an inverter for converting the DC electrical power to AC electrical power, wherein the inverter is mounted on the DC photovoltaic module, and wherein the inverter comprises a narrow elongated body mounted proximate an outer edge of the DC photovoltaic module, such that an elongated side of the inverter is fixed in abutting relationship to the outer edge of the DC photovoltaic module. 
     In accordance with a further feature of the invention, there is provided an AC photovoltaic module comprising a DC photovoltaic module for producing DC electrical power, and an inverter for converting the DC electrical power to AC electrical power, the inverter having film capacitor means therein for storing and releasing electrical energy, and the inverter being mounted on the DC photovoltaic module proximate an edge of the DC photovoltaic module. 
     In accordance with a still further feature of the invention, there is provided an inverter assembly for converting a DC electric power input to an AC electrical power output, the inverter assembly comprising means for receiving the DC electrical power from a DC power source, one or more film capacitors for filtering input current switching ripple of a DC/DC converter, the DC/DC converter being adapted to convert input voltage from the DC power source to voltage suitable for a DC/AC inverter, a second set of one or more film capacitors for filtering switching ripple output of the DC/DC converter and input switching ripple of the DC/AC inverter, the DC/AC converter being adapted to convert DC power to AC current and feed the AC current into an AC power grid. 
     In accordance with a still further feature of the invention, there is provided an inverter assembly comprising means for receiving DC electrical power from a DC power source, a DC/AC inverter adapted to convert the DC electrical power to AC electrical power, and a set of one or more film capacitors for (a) filtering switching ripple of input current to the DC/AC inverter, and for (b) filtering energy difference between the inverter DC input power and the inverter AC output power, and the DC/AC inverter being adapted to feed AC current into an AC power grid. 
     The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of the invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings in which are shown illustrative embodiments of the invention, from which its novel features and advantages will be apparent, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a plan view of a framed photovoltaic module, illustrating typical gradients of heat exhibited by a module subjected to sunlight; 
         FIG. 2  is a back dimensional view of an AC photovoltaic module, including an inverter mounted on a frame portion of the module; 
         FIG. 3  is a perspective view of a frame portion of a photovoltaic module with an inverter fixed thereto; 
         FIG. 4  is a length-wise cross-sectional view of the inverter of  FIG. 3 ; 
         FIG. 5  is a width-wise cross-sectional view of the inverter of  FIG. 4 ; 
         FIG. 6  is an end view of an inverter mounted on a frame member, and a heat sink in the form of a plate fixed to the inverter and extending therefrom, and in phantom shows an optional heat sink feature; 
         FIG. 7  is a diagrammatic view of an inverter adapted to receive DC electrical current from a DC source and to provide AC electrical current to an AC grid; 
         FIG. 8  is a diagrammatic view of an alternative inverter adapted to receive DC electrical current from a DC source and to provide AC electrical current to an AC grid; 
         FIG. 9  is a graphical illustration of DC input power, constant with time, and AC output power pulses; and 
         FIG. 10  is a graphical illustration showing ripple voltage that typically appears on an energy storage capacitor over time. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , it will be seen that a DC photovoltaic module  20  illuminated by sunlight, and in the absence of air flow, will exhibit gradients of heat, with a hottest area H appearing in a generally central area of the module. Extending outwardly from the central area are progressively cooler areas, H 2 , H 3 , H 4 , and a coolest area C at an outermost edge of the module which is typically bounded by a frame  22 . 
     At sunless times, and particularly at night time, the module  20  can cool to ambient, and below ambient, temperatures with the central area of the module experiencing the greatest temperature drop. Such change in temperature, particularly over an extended time, is conducive to condensation occurring in the module, which can lead to freeze-thaw mechanized stress in the unit, corrosion, and/or loss of insulation resistance. Accordingly, the most beneficial location for an inverter is proximate the frame  22 , that is, in an area well removed from the central area of the module  20 . 
     As shown in  FIG. 2 , an AC photovoltaic module  24  includes a DC photovoltaic module  20  and an inverter  30 , the DC module converting solar energy to DC electrical power, and the inverter converting DC electrical power to AC electrical power. 
     Referring to  FIGS. 2 and 3 , it will be seen that in accordance with the invention an inverter  30  is provided which is mechanically attachable to the frame member  22 . 
     The inverter  30  is of a narrow elongated configuration, such that throughout the length of the inverter, the inverter is disposed in the coolest area C of the DC module  20 . Further, the inverter  30  is mounted on a back portion of the frame  22  so as to be out of direct sunlight. Still further, the inverter is spaced from the back of the solar cells of the module  20 . 
     The frame  22  is typically of metal, usually aluminum, which provides a heat sink for the inverter  30  mounted thereon. Thus, heat from the inverter is transferred throughout the frame  22  which, in turn, is located in the coolest region of the module  20 . 
     The narrow, elongated configuration of the inverter  30 , typically about 12 inches in length and 2×2 inches in cross-section, permits an extended inverter-to-frame contact surface so that heat is readily transferred to the frame and much less likely to build up in the inverter. 
     Referring to  FIG. 4 , it will be seen that the inverter  30  comprises a housing  32  in which is disposed a series of electrical components  34  known in the art. The electrical components  34  are mounted on an elongated circuit board  36  extending throughout most of the length of the housing  32 . 
     Within the inverter housing  32 , the electrical components  34  are disposed in a manner facilitating the removal of heat from the inverter. With that in view, the cooler and taller components  40 , such as inductors and capacitors are mounted on a first side  44  of the circuit board  36 , while heat-producing and shorter components  42 , such as diodes and transistors, are mounted on a second side  46  of the circuit board  36 . 
     Thus, as seen in  FIGS. 4 and 5 , the heat-producing components  40  are necessarily disposed proximate a wall  48  of the housing  32 , the wall  48  thereby serving as a heat sink for dissipation of heat generated internally of the inverter  30 . 
     To further expedite the removal of heat from the inverter  30 , there may be provided in addition to, or instead of the housing wall  48 , an inverter heat sink  50  ( FIG. 5 ) for conveying heat from inside the inverter housing  32  to the surrounding environment. The heat sink  50  may be provided with a plate portion  52  ( FIGS. 3 and 6 ) extending outwardly from the inverter housing  32 , and/or may be provided with a portion  54  ( FIG. 6 ) extending from the heat sink  50  into the housing  32  proximate the “hot” components  42 . 
     The heat sink  50  preferably is of aluminum and of a thickness of about 0.125 inch. 
     To still further expedite the removal of heat from the inverter  30 , the wall  56  of the inverter housing  32 , which faces away from the module frame  22 , may be provided with a highly emissive coating or treatment ( FIG. 6 ) to increase radiative heat transfer from the inverter. 
     To still further expedite the removal of heat from the inverter  30 , the cavity of the inverter may be filled with pottant, which provides heat transfer away from the hot components. In addition, the pottant provides thermal mass which limits temperature excursions of the hot components and the rate of change of the thermal components under varying generation of the DC source. 
     There is thus provided an inverter which, by nature of its size and configuration and location, and the arrangement of its internal components, provides a longer life cycle than was heretofore customary. 
     Another avenue by which to extend the life of the inverter is to avoid the use of historically troublesome electronic components. The most vulnerable component at present is the state-of-the-art electrolytic capacitor, which is inexpensive and adapted to store a relatively large amount of energy, but is known to deteriorate in a typical AC module environment of high temperature, temperature cycling, and voltage spikes. 
     In  FIG. 7 , there is shown an inverter  60  in electrical communication with a DC source, which may be a DC photovoltaic module, wind, hydro, battery, fuel cell, or the like. Between the DC source and the DC/DC converter  62  is a capacitor  64 , used to filter the input current switching ripple of the DC/DC converter  62 . The DC/DC converter  62  converts the input voltage from the source to a voltage level suitable for a DC/AC inverter  66 . Between the DC/DC converter  62  and the DC/AC inverter  66  is a further capacitor  68 , for filtering the switching ripple output of the DC/DC converter  62  and the input switching ripple of the DC/AC inverter  66 . 
     The capacitor  68  may also be used as a main energy storage element to make up for any difference in input DC power and output AC power as a function of time; refer to  FIGS. 9 and 10 . 
     The DC/AC inverter  66  converts a DC voltage to an output AC current which is injected into an AC power grid  70 . In the AC power grid  70 , voltage is typically regulated by a utility or a local generation facility. 
     As noted above, the DC source may be a photovoltaic module  20 . The capacitor  64  is a film capacitor for filtering the input current ripple of the DC/DC converter  62 . The DC/DC converter  62  is a full bridge converter. The capacitor  68  stores the difference in energy input from the DC input to the AC output. This difference in energy causes a ripple voltage to appear on capacitor  68 . The ripple voltage on capacitor  68  is sinusoidal under ideal operating conditions, at twice the frequency of the AC grid  70 . 
     The DC/AC inverter  66  is a full bridge inverter operated in discontinuous conduction mode. The DC/AC inverter  66  is operated such that the output current is a controlled low (5%) total harmonic distortion current wave-form. The AC grid  70  is a 120 volt 60 Hertz AC circuit. 
     It will be understood that many changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as explained in the appended claims.