Abstract:
A flexible space structure such as a solar array is composed of multiple solar cell modules (SCMs) each supporting an arrangement of solar cells on a frontside layer and incorporating a backside layer with a surface opposite from the frontside layer having a conductive coating. A selected portion of the SCMs have structural ground extension harnesses intermediate the frontside layer and backside layer. Conductive tapes secure vertically adjacent SCMs by attachment to the conductive coating and electrical jumpers interconnect the structural ground extension harnesses across gapped hinge lines of laterally adjacent SCMs.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under contract number FA9453-10C-0206 awarded by the United States Air Force. The government has certain rights in this invention. 
    
    
     REFERENCE TO RELATED APPLICATIONS 
     This application is copending with application Ser. No. 12/818,255 entitled SOLAR CELL module filed on Jun. 18, 2010 by inventors Andrew R. Streett, Ray A. Stribling and Darren S. Cokin and application Ser. No. 12/907,273 entitled CARBON NANOTUBE COATED STRUCTURE AND ASSOCIATED METHOD OF FABRICATION by inventors Andrew R. Streett filed on Oct. 19, 2010, both of which have a common assignee with the present application and both of which are incorporated herein by reference as though fully set forth. 
     BACKGROUND INFORMATION 
     1. Field 
     Embodiments of the disclosure relate generally to the field of space structures and more particularly to embodiments for a flexible solar array having panel elements interconnected with layered conductive tapes and layered electrical jumpers providing electrostatic current dissipation across the array. 
     2. Background 
     Large solar cell arrays are typically provided for generation of electrical power for satellites and other space craft. Heritage, rigid solar array substrates consist of an aluminum honeycomb core covered by composite facesheets. These substrates may be thick, heavy, and not easily scalable. Furthermore, these large rectangular panels are not ideal for packing into the cylindrical shroud of a launch vehicle. Approaches for solar arrays have been proposed for reductions in weight and stowage volume. Bonded photovoltaic solar cells on a flexible substrate incorporated with flexible printed circuitry, provide a reduction of the substrate mass and wiring. Thin solar panels have an extremely efficient volumetric packing factor allowing higher total power generation with respect to an equivalent rigid solar array. This allows mass budget to be transferred to the payload or dramatic cost reductions and/or downsizing to a smaller and cheaper class of launch vehicle can be accomplished. 
     Heritage rigid solar panels, as well as other large space structures in general, dissipate electrostatic charge through, a grounding path of graphite facesheets, conductive overlays, aluminum core, and copper wire. In contrast, a flexible solar array or other space structure whose substrate would otherwise be fully insulating may be subject to damage from charge build up and electrostatic discharge. 
     It is therefore desirable to provide an electro-mechanical configuration and fabrication process which does not impede the advantages of the mechanical structure of advanced bonded solar arrays and similar space structures while allowing dissipation of electrostatic charges that build up on the structure due to incident radiation (i.e. charged particle) in the space environment. 
     SUMMARY 
     Embodiments described herein provide a solar array composed of multiple solar cell modules (SCMs). Each SCM supports an arrangement of solar cells on a frontside layer and incorporates a backside layer with a surface opposite the frontside layer having a conductive coating. A selected portion of the SCMs have structural ground extension harnesses intermediate the frontside layer and backside layer. Conductive tapes secure vertically adjacent SCMs by attachment to the conductive coating and electrical jumpers interconnect the structural ground extension harnesses across gapped hinge lines of laterally adjacent SCMs. 
     The elements of the disclosed embodiments provide an electrostatic charge dissipation system for a solar array by using a conductive coating on solar cell modules (SCMs) supporting an array of solar cells on a frontside layer and incorporating a backside layer with a surface opposite from the frontside layer carrying the conductive coating. Structural ground extension harnesses are provided intermediate the frontside layer and backside layer in a selected portion the SCMs. Conductive tapes secure vertically adjacent SCMs by attachment to the conductive coating. Electrical jumpers interconnect the structural ground extension harnesses of laterally adjacent SCMs across gapped hinge lines. 
     Using the embodiments disclosed for a method of dissipation of electrostatic charge on a solar array, charge from a conductive coating on a plurality of Solar Cell Modules (SCMs) is conducted into an electrical jumper. The charge buildup in the conductive coating layer is routed through conductive adhesive strips of the electrical jumper and through a conductive polyimide layer into a highly conductive layer. Current is dissipated from the conductive layer through joints welded or soldered through windows in isolating layers between the conductive layer and a structural ground harness. Current is then dissipated through the structural ground harness to a spacecraft ground. 
     In certain embodiments, the SCMs are vertically attached by conductive tapes. Buildup of electrostatic charge due to incident radiation in a space environment is conducted through a conductive carbon nanotube structure of the conductive coating on the SCMs in which the buildup dissipates through the conductive tape into a vertically adjacent SCM. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. l is a schematic flow diagram of system elements and assembly for an example solar array in which embodiments disclosed may be employed; 
         FIG. 2  is an exploded view of a Solar Cell Module (SCM); 
         FIG. 3  is a detailed view of a partial detail view of a subgroup of interconnected SCMs; 
         FIG. 4  is a section view of an SCM; 
         FIG. 5  is a front view of a layered conductive tape for vertical connection of SCMs; 
         FIG. 6  is a side section view of the layered conductive tape of  FIG. 5 ; 
         FIG. 7  is a front view of an electrical jumper for interconnection of structural ground extension harnesses between SCMs; 
         FIG. 8  is a side section view of the electrical jumper of  FIG. 7 ; 
         FIG. 9  is a schematic representation of electrostatic charge dissipation by an example embodiment; and 
         FIG. 10  is a flow chart of electrostatic dissipation as accomplished by the embodiments disclosed. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments described herein provide a flexible solar array with connection elements that utilize layered and segregated conductive and non-conductive tapes and adhesives as well as conducting substrates to provide an enhanced mechanical structure while allowing dissipation of electrostatic charges that build up on the structure due to incident radiation in the space environment. In addition, a carbon nanotube coating provides both protection from atomic oxygen and, as incorporated in the overall enhanced electro-mechanical structure, a continuous electrostatic dissipative path to spacecraft ground, between areas not covered by the conductive tape or adhesive. The coating is also highly emissive, thereby improving thermal management and efficient operation of the solar array. 
     As shown in  FIG. 1 , a solar array employing the embodiments described in detail herein typically incorporates solar cells  10  which are electrically connected in series (approximately 20 solar cells in an exemplary embodiment) defined as a solar cell module (SCM)  12 . SCMs are then combined in circuit panels  14 , which for the embodiment shown contain five SCMs connected laterally. Circuit panels are then combined into subgroups  16 ; vertical combinations of seven circuit panels for the embodiment shown and subgroups are then again laterally combined in groups  18  with five subgroups. Groups are then combined in a blanket  20  with two blankets forming a solar array  22  for a spacecraft  24 . 
       FIG. 2  shows an exploded view of an SCM  12  which includes a frontside layer  30  carrying the individual solar cells  10  and a backside layer  32 . Backside layer  32  includes connection windows  34  for end of circuit requirements and extension harness connector windows  36  which provide for interconnection of extension harnesses  38   a,    38   b  and  38   c  between SCMs in a circuit. As shown in  FIG. 3 , flexible jumpers  40 , special versions of which to be described in greater detail subsequently, are employed to splice together the extension harnesses. The arrangement of SCMs and flexible jumpers allows the solar cell panel formed by circuits  14  to fold along gapped hinge lines  42  between laterally connected SCMs  12  shown in  FIG. 3 . The general configuration and arrangement of the SCMs may be as disclosed in co-pending application Ser. No. 12/818,255 entitled SOLAR CELL module.  FIG. 4  shows an exemplary cross sectional arrangement of the frontside and backside layers. Frontside layer  30  includes the solar cells  10  with an adhesive layer  44  securing the cells to a top polyimide insulating layer  46  such as 1, 2, 4 or 5 mil thickness yellow KAPTON®, a polyimide film, provided by DuPont Chemical Co. The backside layer  32  includes a bottom polyimide insulating layer  50  which is coated on a surface  52  opposite from the frontside layer with a carbon nanotube polymer conductive coating  54  providing high thermal emissivity, protection from atomic oxygen degradation and Electrostatic Discharge (ESD) conductivity as will be described in greater detail subsequently. The conductive coating for an example embodiment may be as disclosed in co-pending application Ser. No. 12/907,273 entitled CARBON NANOTUBE COATED STRUCTURE AND ASSOCIATED METHOD OF FABRICATION. The extension harnesses  38   a,    38   b  and  38   c  are contained between the top and bottom polyimide insulating layers  46 ,  50  on the frontside layer  30  and backside layer  32  respectively. 
     Returning to  FIG. 3 , vertical interconnection of SCMs  12  in circuit panels  16  is accomplished using layered conductive tapes  60 . As shown in detail in  FIGS. 5 and 6 , the layered conductive tapes  60  incorporate a carbon loaded polyimide layer  62  such as Black Kapton®, a black polyimide film coated with a silicone adhesive, manufactured by DuPont, of approximately 0.002″ thickness in an example embodiment, backed by an atomic oxygen (AO) protection barrier  64  as disclosed in application Ser. No. 12/907,273. Electrically conductive adhesive strips  66  are attached on a side of the polyimide layer  62  opposite the AO protection harrier to secure the layered conductive tapes to the backside layers  32  of vertically adjacent SCMs  12  (as seen in  FIG. 3 ). For an exemplary embodiment, a  9713  PSA adhesive is employed manufactured by Minnesota Mining and Manufacturing Co. Additional fastening capability for the layered conductive tapes is provided by structural adhesive strips  68  outboard of the conductive adhesive stripes  66 . For an exemplary embodiment, a 3M  100  series adhesive from Minnesota Mining and Manufacturing is used. A gap  70  separates the conductive adhesive strips and similarly, the adhesive strips terminate at the lateral edges of the layered conductive tapes to provide end gaps  72  so that no adhesive is exposed between the edges of SCMs  12  as arranged in the circuit panels  16  where it might result in SCMs  12  or circuit panels  16  sticking together when stowed (folded adjacent one another). In an exemplary embodiment the SCMs are approximately 7″ in width and 14″ in length. The layered conductive tapes are approximately 1.5″ in width and 7″ in length. Conductive adhesive stripes are approximately 0.25″ wide and structural adhesive stripes are approximately 0.25″ wide with gaps  70  and  72  approximately 0.25″ in width. 
     As previously described, interconnection of extension harnesses across adjacent SCMs is accomplished using flexible jumpers  40 . Interconnection of at least one of the extension harnesses provides a structural ground for the SCMs in a panel  20 . Shown in  FIGS. 7 and 8  (thicknesses exaggerated for clarity), an electrical jumper  80  is employed for interconnection of the structural ground extension harnesses  38   b  in adjacent SCMs  12 . Electrical jumper  80  incorporates copper or other conductive layer  82  which is encased in a carbon loaded polyimide layer  84 . For the embodiment shown, the conductive layer  82  has conduction diffusion pads  86  and a conductive neck  88  extending across the gap of the hinge line  42  (best seen in  FIG. 7 ). The conductive neck is sufficiently flexible to allow folding of the SCMs  12  along hinge line  42  as previously described. A service loop  90  in the conductive neck may be employed to allow for manufacturing tolerances and thermal contraction/expansion in the SCMs  12 . A “valley” fold configuration and “mountain” configuration for inward or outward folding of the SCMs may be employed in certain embodiments. The carbon loaded polyimide layer  84  of the electrical jumper  80  is attached to the conductive coating  54  on the individual SCMs  12  using conductive adhesive strips  90  which may be curved around the periphery of the jumper. An outer conductive coating layer  92  is provided on the outer surface of the electrical jumper  80  for emissive and conductive continuity (the outer conductive coating is not shown in  FIG. 7  to reduce the number of hidden layers). An insulating polyimide strip  94  is affixed to the carbon loaded polyimide layer  84  between the conductive adhesive strips  90  for isolation of the conductive elements of the electrical jumper over the gapped hinge line  42 . Additionally, the carbon loaded polyimide layer  84  may include notches  96  consistent with the gap at the hingeline  42  (best seen in  FIG. 7 ) allowing greater flexibility in the electrical juniper at the neck  88  of the conductive metal  80 . 
     Electrical interconnection of the structural ground extension harnesses  38   b  in the opposing SCMs  12  is accomplished in the electrical jumper  80  by providing windows  98  extending through the conductive coating layer  92  and carbon loaded polyimide layer  84  on the outboard side of the highly conductive layer  82  and extending through the insulating polyimide strip  94  and outer insulating polyimide layer  50  of the SCMs  12  to expose copper on the structural ground extension harnesses to allow for welding (or soldering) as represented by arrows  99 . Alternative joining techniques to achieve a conductive path may be used such as electric current and pressure to form a silver-to-silver thermal compression bond similar to that employed in interconnects for solar cells or ultrasonic bonding. 
     The layered elements of the electrical jumper  80  allow current dissipation of electrostatic build up in the SCMs by providing multiple current flow paths. As shown in  FIG. 8 , charge buildup in the conductive coating  54  on the SCMs is routed as shown by arrows  100  and  102  through the conductive adhesive strips  90  of the electrical jumper  80  and through the conductive polyimide layer  84  into the highly conductive layer  82 . From the highly conductive layer  82  the current may be dissipated as shown by the continuation of arrows  100  and  102  from the highly conductive layer  82  through the welded or soldered joints and windows  98  to the structural ground harness and to the spacecraft ground. Current flow from an upstream structural ground harnesses  38   b  in the SCMs also flows through the electrical jumper as shown by arrow  103  routed from the upstream structural ground harness through the upstream welded or soldered joint, through the highly conductive layer  82  in the electrical jumper, back through the downstream welded or soldered joint to the downstream structural ground harness. 
     As shown in  FIG. 9 , the layered conductive tapes  60  interconnecting the SCMs vertically provide conductive paths for electrostatic charge dissipation. Buildup of electrostatic charge due to incident radiation in a space environment, represented as a localized charge  110 , is conducted, as represented by arrow  112 , through the conductive carbon nanotube structure of the conductive coating  54  of the SCM(s) in which the buildup occurs, through the conductive tape  60  into the vertically adjacent SCM where the charge is conducted, arrow  114 , into the electrical jumper  80  for dissipation into the structural ground harness  38   b  as described previously with respect to  FIG. 8  and represented by arrow  116 . The embodiment disclosed provides an electrostatic dissipation system for the entire solar array. 
     The embodiments disclosed provide dissipation of electrostatic charge as shown in  FIG. 10  by conducting buildup of electrostatic charge due to incident radiation in a space environment through the conductive carbon nanotube structure of the conductive coating  54  of the SCM(s) in which the buildup occurs, step  1002 , through the conductive tape  60  into the vertically adjacent SCM, step  1004 . The charge is conducted from the conductive coating into the electrical jumper  80 , step  1006 , for dissipation into the structural ground harness  38   b  as described previously with respect to  FIG. 8 . Charge buildup in the conductive coating  54  on the SCMs is routed through the conductive adhesive strips  90  of the electrical jumper  80 , step  1008 , and through the conductive polyimide layer  84 , step  1010  into the highly conductive layer  82 , step  1012 . From the highly conductive layer  82  the current may be dissipated as shown by the continuation of arrows  100  and  102  from the highly conductive layer  82  through the welded or soldered joints between the highly conductive layer and structural ground harness  38   b  through the windows  98 , step  1014  and through the structural ground harness  38   b  to the spacecraft ground, step  1016 . 
     Having now described various embodiments of the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.