Abstract:
The inventions described herein generally relate to photovoltaic or solar module design and fabrication and, more particularly, to modules utilizing thin film solar cells. In one aspect is described a solar module and method of making the same that has a shield material that is both an electrical insulator and a moisture barrier provided at a location corresponding to at least one hole that is used to route a wiring member, such that the shield material seals the at least one hole against moisture entering into the internal space and electrically insulates the wires of the wiring member from the at least one metallic layer of the back protective sheet.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/143,744 filed Jan. 9, 2009 entitled “Reliable Thin Film Photovoltaic Modules Structures”, the entirety of which is incorporated by reference. 
     
    
     FIELD OF THE INVENTIONS 
       [0002]    The aspects and advantages of the present inventions generally relate to photovoltaic or solar module design and fabrication and, more particularly, to modules utilizing thin film solar cells. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. 
         [0004]    Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se) 2  or CuIn 1-x Ga x  (S y Se 1-y ) k , where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. It should be noted that although the chemical formula for CIGS(S) is often written as Cu(In,Ga)(S,Se) 2 , a more accurate formula for the compound is Cu(In,Ga)(S,Se) k , where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. 
         [0005]    The structure of a Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te) 2  thin film solar cell is shown in  FIG. 1 . A photovoltaic cell  10  is fabricated on a substrate  11 , such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber film  12 , which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over a conductive layer  13  or contact layer, which is previously deposited on the substrate  11  and which acts as the electrical contact to the device. The substrate  11  and the conductive layer  13  form a base  20  on which the absorber film  12  is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of  FIG. 1 . If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film  12 . Radiation  15  enters the device through the transparent layer  14 . Metallic grids (not shown) may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. The preferred electrical type of the absorber film  12  is p-type, and the preferred electrical type of the transparent layer  14  is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of  FIG. 1  is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te) 2  absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. 
         [0006]    There are two different approaches for manufacturing PV modules. In one approach that is applicable to thin film CdTe, amorphous Si and CIGS technologies, the solar cells are deposited or formed on an insulating substrate such as glass that also serves as a back protective sheet or a front protective sheet, depending upon whether the device is “substrate-type” or “superstrate-type”, respectively. In this case the solar cells are electrically interconnected as they are deposited on the substrate. In other words, the solar cells are monolithically integrated on the single-piece substrate as they are formed. These modules are monolithically integrated structures. For CdTe thin film technology the superstrate is glass which also is the front protective sheet for the monolithically integrated module. In CIGS technology the substrate is glass or polyimide and serves as the back protective sheet for the monolithically integrated module. In monolithically integrated module structures, after the formation of solar cells which are already integrated and electrically interconnected in series on the substrate or superstrate, an encapsulant is placed over the integrated module structure and a protective sheet is attached to the encapsulant. An edge seal may also be formed along the edge of the module to prevent water vapor or liquid transmission through the edge into the monolithically integrated module structure. 
         [0007]    In standard single or polycrystalline Si module technologies, and for CIGS and amorphous Si cells that are fabricated on conductive substrates such as aluminum or stainless steel foils, the solar cells are not deposited or formed on the protective sheet. They are separately manufactured and then the manufactured solar cells are electrically interconnected by stringing them or shingling them to form solar cell strings. In the stringing or shingling process, the (+) terminal of one cell is typically electrically connected to the (−) terminal of the adjacent device. For the Group IBIIIAVIA compound solar cell shown in  FIG. 1 , if the substrate  11  is conductive such as a metallic foil, then the substrate, which is the bottom contact of the cell, constitutes the (+) terminal of the device. The metallic grid (not shown) deposited on the transparent layer  14  is the top contact of the device and constitutes the (−) terminal of the cell. In shingling, individual cells are placed in a staggered manner so that a bottom surface of one cell, i.e. the (+) terminal, makes direct physical and electrical contact to a top surface, i.e. the (−) terminal, of an adjacent cell. Therefore, there is no gap between two shingled cells. In contrast, for solar cells that are strung together, solar cells are placed side by side with a small gap (typically 1-2 mm) between them and using conductive wires or ribbons that connect the (+) terminal of one cell to the (−) terminal of an adjacent cell. Solar cell strings obtained by stringing (or shingling) individual solar cells are interconnected through “busing” or “bussing” to form circuits. Circuits may then be packaged in a protective shell or package to seal the module. Each module typically includes a plurality of strings of solar cells which are electrically connected to one another. The two leads (the positive and negative leads) of the interconnected circuit are typically taken out through openings in the back protective sheet of the module structure and these leads are connected to terminals placed in a junction box on the back of the module. 
         [0008]      FIG. 2A  shows a top (illuminated side) view of a prior art module  200 .  FIG. 2B  is a bottom view of the same module  200 , while  FIG. 2C  is a cross sectional view taken across the line “ 2 C- 2 C”. The module  200  has two solar cell strings  201 A and  201 B, each string containing six solar cells  210  strung together using conductive ribbons  211 . The solar cell strings  201 A,  201 B, are electrically connected by buss conductors  202 A,  202 B and  202 C. As shown in  FIG. 2B , the back surface  203  of the module  200  (which is the same as the exposed surface of the back protective sheet  208 ) has a junction-box  204 , within which two electrical terminals,  205 B and  205 C are located. The two terminals,  205 B and  205 C, are electrically connected to the buss conductors,  202 B and  202 C, respectively, and they constitute the two terminals of the module. For module structures employing CIGS type solar cells fabricated on metallic foil substrates, the terminal  205 B is a (+) terminal and the terminal  205 C is a (−) terminal of the module. As can be seen from  FIGS. 2A and 2C , the module structure comprises a frame  206  (which is optional), a transparent front protective sheet  207 , a back protective sheet  208  and an encapsulant layer  209  which is between the front protective sheet  207  and the back protective sheet  208  and surrounds the solar cell strings as well as the electrical connections and wirings in the module structure. A variety of materials may be used as encapsulant for packaging solar cell modules. These materials include ethylene vinyl acetate copolymer (EVA), thermoplastic polyurethanes (TPU), and silicones. An edge seal  209  may also be employed around the perimeter of the module to act as a moisture barrier. As can be seen from  FIG. 2C , the bus conductors  202 B and  202 C are electrically connected to terminals  205 B and  205 C through two holes opened in the back protective sheet  208 . It is also possible to have a single hole rather than two separate holes opened in the back protective sheet  208  for this purpose. The junction-box  204  is typically attached on the exposed surface  203  of the back protective sheet  208  using moisture barrier adhesives, and the holes in the back protective sheet  208  are sealed against water seepage using potting materials such as silicone, epoxy, and urethane containing materials. It should be noted that the placement of the junction-box  204  in PV modules may vary. For example, it is common to place the junction box on the back surface of the module right below the solar cells. This way, the size of the module is reduced and the frame is brought very close to the edges of the first solar cells near the edge. 
         [0009]    The nature of the protective shell (which includes the front protective sheet  207 , the back protective sheet  208 , and optionally the edge seal  209  around the module perimeter) determines the amount of water or water vapor that can enter a module. Thin film solar cells are more moisture sensitive than the crystalline Si devices. Therefore, materials with moisture barrier characteristics need to be used in the module structure. The front protective sheet  207  is typically glass which is water impermeable. For flexible module structures employing thin film solar cells such as CIGS solar cells, thin, flexible and transparent polymeric sheets with moisture barrier coatings are used. For standard crystalline Si modules and monolithically integrated CdTe, CIGS and amorphous Si based modules the back protective sheet  208  may be a sheet of glass. For standard Si modules a polymeric sheet comprising a UV resistive material such as TEDLAR® (a product of DuPont) is also commonly used as the back protective sheet. 
         [0010]    In rigid and flexible module structures employing thin film solar cells, it is important to minimize moisture permeability of the module structure while assuring that the structure passes the electrical safety tests necessary for safe operation in the field. From the foregoing, there is a need to develop solar module structures with minimum moisture permeability. 
       SUMMARY 
       [0011]    The inventions described herein generally relate to photovoltaic or solar module design and fabrication and, more particularly, to modules utilizing thin film solar cells. 
         [0012]    In one aspect is described a solar module and method of making the same that has a shield material that is both an electrical insulator and a moisture barrier provided at a location corresponding to at least one hole that is used to route a wiring member, such that the shield material seals the at least one hole against moisture entering into the internal space and electrically insulates the wires of the wiring member from the at least one metallic layer of the back protective sheet. 
         [0013]    In a particular aspect is described a solar module connectable to an external terminal, comprising: a protective shell including an internal space defined by a transparent front protective layer, a back protective layer and a moisture barrier seal extending between and sealing edges of the transparent front protective layer and the back protective layer, wherein the back protective sheet is a multilayer composite including at least one metallic layer and at least one insulator layer bonded to the at least one metallic layer, and wherein the back protective sheet includes at least one hole extending through the back protective sheet, between an inner surface and an outer surface thereof; at least one solar cell disposed within the internal space so that a light receiving side of the at least one solar cell faces the front protective layer and a back side of the at least one solar cell faces the back protective sheet; a wiring member for electrically connecting the at least one solar cell to the external terminal, wherein the wiring member is routed through the at least one hole formed through the back protective sheet, the wiring member including a first wire and a second wire; an encapsulant material that fills a remainder of the internal space and surrounds the at least one solar cell; and a shield material that is both an electrical insulator and a moisture barrier provided at a location corresponding to the at least one hole, such that the shield material seals the at least one hole against moisture entering into the internal space and electrically insulates each of the first wire and the second wire from the at least one metallic layer of the back protective sheet, wherein a water vapor transmission rate of the shield material is lower than the water vapor transmission rate of the encapsulant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic view a thin film solar cell; 
           [0015]      FIG. 2A  is a schematic top view of a prior art photovoltaic module; 
           [0016]      FIG. 2B  is a schematic bottom view of the module of  FIG. 2A ; 
           [0017]      FIG. 2C  is a schematic cross sectional view of the module of  FIG. 2A , taken along the line  2 C- 2 C; 
           [0018]      FIG. 3  is a schematic cross sectional view of a back protective sheet and shielded leads according to an embodiment; 
           [0019]      FIG. 4  is a schematic cross sectional view of a module including a junction box electrically connected to the module using the shielded leads; and 
           [0020]      FIGS. 5A and 5B  are schematic views of the structure of various shielded lead embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The preferred embodiments described herein provide module structures and methods of manufacturing rigid or flexible photovoltaic modules employing thin film solar cells fabricated on flexible substrates, preferably on flexible metallic foil substrates. The solar cells may be Group IBIIIAVIA compound solar cells fabricated on thin stainless steel or aluminum alloy foils. The modules include a moisture resistant protective shell within which the interconnected solar cells or cell strings are packaged and protected. The moisture resistant protective shell comprises a top or front protective sheet through which the light enters the module, a back protective sheet, a support material or encapsulant covering at least one of a front side and a back side of each cell or cell string. The support material may preferably be used to fully encapsulate each solar cell and each string, top and bottom. The protective shell additionally comprises a moisture sealant that is placed between the front protective sheet and the back protective sheet along the circumference of the module and forms a barrier to moisture passage from outside into the protective shell from the edge area along the circumference of the module. The back protective sheet may be non-transparent and may comprise a composite structure, i.e., multiple layers stacked and bonded, including one or more metallic layers such as an aluminum layer to improve moisture resistance of the back protective sheet. The metallic layer may be interposed between polymeric layers such as TEDLAR® layers or TEDLAR and PET layers, or other polymeric material layers such as PVDF (poly vinyledene fluoride) or UV stabilized PET. The front protective sheet is typically a glass, but may also be a transparent flexible polymer film such as TEFZEL®, or another polymeric film such as Fluorinated ethylene propylene (FEP) or poly methyl methacrylate (PMMA). TEDLAR and TEFZEL are brand names of fluoropolymer materials from DuPont. TEDLAR is polyvinyl fluoride (PVF), and TEFZEL is ethylene tetrafluoroethylene (ETFE) fluoropolymer. In modules employing thin film devices, such as thin film CIGS solar cells, it is important that the back protective sheet to be a moisture barrier. Standard polymeric back sheets employing TEDLAR do not have the moisture barrier characteristics. The water vapor transmission rate (WVTR) for TEDLAR is 9-57 g/m 2 /day at 39.5° C. and 80% relative humidity. However, when a 18 to 50 um thick aluminum (Al) is laminated into the structure of such polymeric sheets, water vapor transmission rates of 10 −3  g/m 2 /day or lower can be achieved. The front and back protective sheets may be flexible materials that have a water vapor transmission rate of less than 10 −3  g/m 2 /day, preferably less than 5×10 −4  g/m 2 /day. In one embodiment, for external connections, electrical leads of the solar cell strings are extended out of the solar module through one or more connection holes formed through the back protective sheet. Each electrical lead is at least partially coated by a protective shield layer which effectively seals the connection hole against moisture and electrically insulates the portions of the back sheet metallic layer which may be exposed when the connection hole is formed. 
         [0022]    An embodiment of the present invention will now be described in connection to  FIGS. 3 and 4 .  FIG. 3  shows a cross sectional view of a back protective sheet  302  including connection holes  306  extending from a first surface  308 A to a second surface  308 B. As will be described below the back protective sheet may be a component of an exemplary solar module shown in  FIG. 4 . The first surface  308 A faces the outside and the second surface  308 B faces the inside of a module when the module is manufactured. The back protective sheet  302  may have a composite structure including a metallic layer  310 A such as an aluminum layer. The metallic layer  310 A may be interposed between polymeric layers  310 B such as TEDLAR layers. When the connection holes  306  are formed, edge portions  311  of the metallic layer  310 A are exposed, as explained above. The shielded leads  312  are placed in the direction of the arrows ‘A’ and fitted into the connection holes  306  so as to form a moisture seal and to electrically insulate the edge portions  311  within the connection holes  306 . Each shielded lead  312  includes an electrical lead  314  or a conductive wire, e.g., copper ribbon, and a shield layer  316  or shield film surrounding or coating it. The shield layer  316  may have dielectric strength in the range of 500-10000 V/mil, typically 1000-6000, and preferably greater than 4000 V/mil. A volume resistivity for the shield layer  316  may be in the range of 10 14 -10 19  ohm.cm, typically 10 15 -10 19  ohm.cm, and preferably equal to or greater than 10 18  ohm.cm. A water vapor transmission rate of the shield layer  316  may be in the range of 0.1-22 g/m 2 /day, typically, 1-15 g/m 2 /day, and preferably less than 5 g/m 2 /day. 
         [0023]    The electrical leads  314  connect solar cells to various external electrical terminals outside the exemplary module  300  shown in  FIG. 4 . The length of the shield layer  316  surrounding the electrical lead  314  may be at least equal to the depth of the connection holes  306  so that the edge portions  311  of the metallic layer  310 A within the connection holes  306  are electrically insulated and the connection holes  306  are sealed against moisture. The thickness of the shield layer  316  may be 4 mil (0.1 mm) to 40 mil (1 mm) for a 3 mm wide hole in the protective back sheet. 
         [0024]    As shown in  FIG. 4 , in side view, the solar module  300  includes a front protective sheet  304 , which is light transparent, and the back protective sheet  302  described above. The shielded leads  314  are placed into the connection holes  306  formed through the back protective sheet to electrically insulate the edge portions  311  and moisture-seal the connection holes  306 . The module  300  further comprises an edge seal  305  extending between the front protective sheet  304  and the back protective sheet  302  and sealing the edges of them. The back protective sheet  302 , the front protective sheet  304  and the edge seal  305  form a protective shell  318  which protects an exemplary solar cell string  320  or strings contained therein from moisture and other corrosive elements. The solar cells may be Group IBIIIAVIA (CIGS) compound solar cells fabricated on thin stainless steel or aluminum alloy foils, amorphous silicon thin film solar cells, CIS thin film solar cells or CdTe solar cells. The solar cells in the solar cell string  320  may be connected in series. Although the module  300  includes a string of interconnected solar cells in this example, the module  300  may include a single solar cell as well. A support material  321  or encapsulant material, such as ethylene vinyl acetate (EVA) and/or thermoplastic polyurethane (TPU), fills the space between the solar cell strings  320  and the protective shell  318 . The support material  321  may also include thermoplastic olefins, pressure sensitive silicone or acrylic adhesives. The electrical leads  314  are connected to the solar cell strings  320  using methods which are well known in the solar cell manufacturing technologies. A junction box  322  including terminals  323  may be attached to the first surface  308 A of the back protective sheet  302 , may be sealed by a moisture barrier. A housing  324  of the junction box  322  encloses the connection holes  306 . The shielded leads  312  including the electrical leads  314  coated by the protective shield layer  316  are connected to the terminals  323  in the junction box  324 . In one embodiment, the back protective sheet may have a single connection hole, and the shielded leads are formed as a wiring member (not shown) routed through the single connection hole. 
         [0025]    The protective shield layers  316 , which may partially or fully covers the electrical leads and may be made of a high dielectric strength and moisture resistant materials, are placed through the connection holes in a tightly fitting manner so as to minimize any moisture leakage inside the module. Exemplary materials for the protective shield layer  316  may be the following materials: polyethylene terephthalate (PET), which is available under the commercial names Mylar®, Melinex®, heat shrink Mylar; polyimide (Kapton®); polyolefins (EPS 300); and polyethylene napthalate (PEN). 
         [0026]    The dimensions of the connection holes  306  formed through the back protective sheet may for example be about 0.45-2.3 mm wide and 5.2-7 mm long. The electrical lead wires are usually about 5 mm wide and about 0.25 to 0.30 mm thick. In a prior art module with no shield layers around the electrical leads, the lead wires are surrounded with an encapsulant material EVA or TPU with WVTR of 35 and 25 g/m 2 /day, respectively. However, PET, PEN and polyimide layers used as shield layers usually have WVTR of 3-15 g/m 2 /day, 0.9-6 g/m 2 /day and 0.4-21 g/m 2 /day, respectively, which are much lower than EVA and TPU encapsulants. Such shield layers minimize moisture transport through the connection holes  306 . If the thickness of the shield layer  316  is adjusted right with respect to the connection hole size, there will be less of an encapsulant-containing area inside the connection holes. With the thickness adjustment, the shield layer  316  may protect the solar module from moisture better than when there is none. For example, the thickness of the shield layer may be 4 mil (0.1 mm) to 40 mil (1 mm) for a 0.45-2.3 mm wide connection hole in the back protective sheet  302  ( FIGS. 3 and 4 ). A shield layer with 40 mil thickness can protect the panel from moisture better than a thinner shield layer. Of course, the same shielded lead may be inserted into a smaller connection hole in a tight fitting manner by compressing the shield layer. The gain in moisture barrier performance is significant since the shield layer materials have better WVTR than the encapsulants. 
         [0027]    The shield layer containing the electrical lead may also prevent a high voltage shorting between the conductive metallic layer in the back sheet and the solar cells. For example, PET films usually have dielectric strength greater than 4000 V/mil. A heat shrink PET tubing, which is available from Advanced Polymers Inc., (Salem, N.H.), has a dielectric strength greater than 4000 V/mil and volume resistivity on the order of 10 18  ohm-cm. PEN films, such as TEONEX from Dupont, have a dielectric strength greater than 5000 V/mil and a volume resistivity of about 10 18  ohm-cm. Various polyimide formulations, which are available from Dupont, can provide dielectric strengths greater than 4000 V/mil, such as BCL-Y (4500 V/mil), FPC and HPP-ST (7,700 V/mil for 1 mil thickness). Other polyimide, polyolefin, PET, and PEN films under various commercial names may also be used as the shield layer around the electrical leads. The electrical insulation, which is applied to the above defined exposed portions of the metallic layer by the protective shield, is especially important when the solar module is used in high voltage systems, such as a system that connects modules in series to build the voltage up to the 600-1500V range. It should be noted that the typical voltage of a module is in the range of 14-60 V and the above mentioned shorting issue is not significant at these low voltages when a single module is operated by itself. 
         [0028]    At such high voltages, if a high dielectric strength shield layer is not used to coat the electrical leads, there may be electrical arcing between the unprotected electrical leads, and the metallic layer through the exposed edges of the metallic layer, even though in the prior art the holes are usually filled with an encapsulant or a potting material as explained in the back ground section. Such encapsulant or potting materials do not have the 1000V electrical insulation rating to stop such electrical arcing. One prior art solution to this arcing problem is making the connection holes sufficiently large to minimize or avoid electrical arcing between the unprotected lead and the exposed metallic layer. In this approach, the holes must be large so that the unprotected lead is radially at least 15-20 mm away from the lead portion in the connection hole. However, such large holes may allow moisture to enter the module and cause malfunction. 
         [0029]      FIGS. 5A and 5B  show the structure of shielded leads.  FIG. 5A  is a cross sectional view of an embodiment of a shielded lead taken along the width or diameter of the shielded lead. In  FIG. 5A , the electrical lead is placed into a tube shield  316 A. An inner surface  325 A and an outer surface  325 B of the shield tube  316 A may include adhesives to attach the electrical lead  314  to the inner surface  325 A and the outer surface  325 B to the connection hole  306 . As shown in  FIG. 5B  in another embodiment a sheet shield  316 B may be wrapped around the electrical lead  314  to form another shielded lead. An inner surface  326 A and an outer surface  326 B of the sheet shield  316 B may also include adhesives to wrap it around the electrical lead and to attach the connection hole  306 . Although  FIGS. 5A and 5B  show shield layers having round shape, they may have other possible shapes. Specifically, the shields layers may have flat shape and conformally surround the electrical leads  314 . 
         [0030]    Although aspects and advantages of the present inventions are described herein with respect to certain preferred embodiments, modifications of the preferred embodiments will be apparent to those skilled in the art.