Abstract:
A photovoltaic cell having a substrate with at least one curved surface reduces the number of processing steps necessary to manufacture a completed cell. Such a photovoltaic cell can have semiconductor material on the outer surface of a curved substrate or on the inner surface of a curved substrate.

Description:
CLAIM OF PRIORITY 
       [0001]    This is a divisional application of U.S. application Ser. No. 10/704,139 filed on Nov. 10, 2003, which is incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to energy collection, and more particularly to photovoltaic energy cells. 
       BACKGROUND 
       [0003]    Photovoltaic devices have been developed based on crystalline silicon, which requires a relatively thick film such as on the order of about 100 microns and also must be of very high quality in either a single-crystal form or very close to a single crystal in order to function effectively. The most common process for making silicon photovoltaic cells is by the single-crystal process where a flat single-crystal silicon wafer is used to form the device. In addition, crystalline silicon can be made by casting of an ingot but its solidification is not as easily controlled as with single-crystal cylinders such that the resultant product is a polycrystalline structure. Direct manufacturing of crystalline silicon ribbons has also been performed with good quality as well as eliminating the necessity of cutting wafers to make photovoltaic devices. Another approach referred to as melt spinning involves pouring molten silicon onto a spinning disk so as to spread outwardly into a narrow mold with the desired shape and thickness. High rotational speeds with melt spinning increase the rate of formation but at the deterioration of crystal quality. More recent photovoltaic development has involved thin films that have a thickness less than 10 microns so as to be an order of magnitude thinner than thick film semiconductors. Thin film semiconductors can include amorphous silicon, copper indium diselenide, gallium arsenide, copper sulfide and cadmium telluride. These semiconductors have primarily been formed on glass sheet substrates. The glass sheet substrates have been limited in size in order to maintain the planarity of the resultant photovoltaic cell. Furthermore, formation of the photovoltaic cells involves an extensive number of processing steps to ensure adequate formation and functionality of the final cells. Additionally, after fully formed the glass sheet photovoltaic cells are not insignificant in weight, requiring sturdy mounting assemblies. 
       SUMMARY 
       [0004]    In one aspect a photovoltaic cell includes a substrate having a curved surface and a first semiconductor material on the surface. The curved surface can be concave or convex. The substrate can have a polygonal cross-section and can be formed from glass, low iron glass, low expansion glass, borosilicate glass, other types of glass or other materials suitable for use as substrates for photovoltaic cells. 
         [0005]    A photovoltaic cell can include a bottom layer between the curved surface and the first semiconductor material. The bottom layer can include a conductive material. The conductive material can be a transparent conductive layer and can be a transparent conductive oxide. In one aspect the conductive material can be a tin oxide. In another aspect a photovoltaic cell can include a second semiconductor material between the first semiconductor material and the top layer. The second semiconductor material can be a binary semiconductor such as a Group II-VI semiconductor. The first semiconductor material can be CdS and the second semiconductor material can be CdTe. 
         [0006]    In still another aspect a photovoltaic cell can include a buffer layer in contact with the bottom layer and between the bottom layer and the first semiconductor material. A photovoltaic cell can include a top layer covering at least a portion of the first semiconductor material and the top layer can include a metal or an alloy. 
         [0007]    A photovoltaic cell can have an electrical conductor electrically connected to the bottom layer and an electrical conductor connected to the top layer. 
         [0008]    In one embodiment a photovoltaic cell can have a substrate with an annular cross section that includes a first end, a second end opposite the first end, an inner surface connecting the first end and the second end, and an outer surface opposite the inner surface. 
         [0009]    In another aspect a photovoltaic cell can have a substrate in the form of a glass tube and semiconductor material can be on a portion of the inner surface of the substrate. The photovoltaic cell can include a first electrical connection connected to a top layer and a second electrical connection connected to the bottom layer. The first end can form a seal around the first electrical connection and can form a seal around the second electrical connection such that the inner surface, the first end and the second end form a chamber. The chamber can contain a gas or gas mixture having a pressure less than atmospheric pressure and the chamber can contain an inert gas such as helium, argon, nitrogen, or a combination thereof. 
         [0010]    In another embodiment a photovoltaic cell can have the first semiconductor material on a portion of the outer surface of the substrate. 
         [0011]    In another aspect, a method of making a photovoltaic cell includes forming a coating of a semiconductor material on a curved surface of a substrate. The substrate can be extruded prior to coating and can be cut to predetermined dimensions before or after coating. The coating can be formed by depositing a layer of a semiconductor material on a portion of a surface of the substrate. Forming the coating can include generating a substantially uniform thickness layer on a portion of the surface of the substrate. Forming a coating on the surface can also include depositing a chemical vapor on the surface. The surface can be a curved inner surface of the substrate or a curved outer surface of the substrate. The method can include directing a deposition element adjacent to an inner surface of the curved substrate and depositing a chemical vapor on the surface. 
         [0012]    In another aspect, a method of generating electricity includes exposing a photovoltaic cell having a curved surface to a light source. The method can include collecting charge generated by exposing the photovoltaic cell to the light source and may include transporting the charge to an electrical demand source. The electrical demand source can include a charge storage device. 
         [0013]    A system for converting light into electrical energy can include a plurality of photovoltaic cells, with at least one of the photovoltaic cells having a curved surface, and an electrical connection between at least two of the photovoltaic cells. The system can include a storage device for storing electrical energy electrically connected to the photovoltaic cells. In addition, the system can includes a mounting apparatus for securing the photovoltaic cells to a light exposure surface. The mounting apparatus can include electrical connections for each of the photovoltaic cells integral to the apparatus. The light exposure surface can include a roof. The system can also include a protective overlayer surrounding the curved photovoltaic cells. Each photovoltaic cell of the system can include a substrate that has an annular cross section and includes a first end, a second end opposite the first end, an inner surface connecting the first end and the second end, and an outer surface opposite the inner surface. The system can also include a bottom semiconductor layer and a top semiconductor layer on a surface of the substrate. There can be a first electrical connection connected to the top semiconductor layer and a second electrical connection connected to the bottom layer. Each cell can have a first end that forms a seal around the first electrical connection and a second end that forms a seal around the second electrical connection. 
         [0014]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a perspective view of a curved photovoltaic cell coated on an inner surface. 
           [0016]      FIG. 2  is a perspective view of a curved photovoltaic cell coated on an outer surface. 
           [0017]      FIG. 3  is a cross-section of a curved photovoltaic cell coated on an inner surface. 
           [0018]      FIG. 4  is a cross-section of a curved photovoltaic cell coated on an outer surface. 
           [0019]      FIG. 5  is a top view of a system of curved photovoltaic cells. 
           [0020]      FIG. 6  is an end perspective view of a system of curved photovoltaic cells. 
           [0021]      FIG. 7  is a schematic of an example of a system for deposition of semiconductor material on a glass substrate as the substrate is being formed. 
           [0022]      FIG. 8  is a schematic of a system for deposition of semiconductor material on a glass substrate as the substrate is being formed. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Referring to  FIG. 1 , a photovoltaic cell  10  has layers of semiconductor material  20  on a curved inner surface  30  of the cell  10 . The semiconductor material  20  can coat the portion of the inner surface  30  of a curved substrate  15  of the photovoltaic cell  10  in multiple layers. The photovoltaic cell  10  has a first end  40  and a second end  50  that can be sealed around electrical conducting elements  60  and  70 . The electrical conducting elements  60  and  70  are in electrical contact with a bottom  80  and a top layer  90  of the semiconductor material  20  respectively. Sealed ends  40  and  50  in combination with inner surface  30  form a sealed chamber  100  that contains the semiconductor material  20 . The sealed chamber  30  can be evacuated and filled with an inert gas such as argon, nitrogen or helium or a combination of inert gases. 
         [0024]    Referring to  FIG. 2 , a curved photovoltaic cell  200  has a curved surface  210  with layers of semiconductor material  220  deposited on at least a portion of the outer surface  220  of the substrate  15 . Electrical conducting elements  230  and  240  can be attached to the top layer  90  and the bottom layer  80  of the semiconductor material. A protective tube  270  can encase the photovoltaic cell  200  to protect the semiconductor material  220 . The protective tube can include separators  275  that keep the photovoltaic cell  200  from resting on the semiconductor material  220 . The separators  275  can be of any appropriate design, for example, the separators can be bars that connect to an uncoated portion of the substrate. 
         [0025]    Referring to  FIG. 3  and  FIG. 4 , cross-sections  300  and  400  of curved photovoltaic cells  10  and  200  have multiple layers of semiconductor material  20  and  220  deposited thereon. The semiconductor material  20  can include multiple layers. In an example of a common photovoltaic cell, the multiple layers can include: a tin oxide layer  80 , a silicon dioxide layer  310 , a doped tin oxide layer  324 , a cadmium sulfide layer  326 , a cadmium telluride layer  328 , a zinc telluride layer  330 , a nickel layer  332 , an aluminum layer  334 , and another nickel layer  336 . This example illustrates that the bottom layer  80  can be a conductive material such as a transparent conductive material including a transparent conductive oxide. One intermediate layer can be a buffer layer  310  that is composed of, for example, silicon dioxide. Other intermediate layers can be, for example, binary semiconductors such as a group II-VI semiconductor. An example of this would be a layer of CdS followed by a layer of CdTe. A top layer can cap off the intermediate layers and can be made of metal such as nickel or aluminum. 
         [0026]    Referring to  FIG. 5 , a top view of a photovoltaic system  500  is composed of multiple curved photovoltaic cells  510  bundled together. Each photovoltaic cell can be connected in series to an adjacent cell via electrical conducting elements  530  or  540  and electrical connector  535  which connect alternating bottom  550  and top layers  560  of the photovoltaic cells  510  to form a circuit for the photovoltaic cells. End electrical conductors  545  and  547  can be connected to an electrical storage device, or to an electrical demand source. The mounting assembly  570  can hold each of the individual cells  510  and can protect them from the elements. The mounting assembly can consist of multiple parts including mounting elements for mounting the cells to a light exposure surface such as a roof, cell holding elements  580  for securing the cells to the mounting assembly and protection elements  590  for protecting the cells from environmental conditions. The cell holding elements can be integral to the individual slots or can be a function of the formation of the slots themselves. For example, a cell holding element could be one or multiple straps or brackets that can be placed over the cells and connected to the mounting assembly to hold the cells in place. Alternatively, the individual cell slots could be arranged such that the ends of the cells slide into recessed portions that hold the cells in place by preventing the cell ends from sliding out of the slot. Such a recessed portion could be a quick connect/disconnect slot for easy installation and change out of an individual solar cell. The mounting assembly could include wiring for each slot and could provide electrical connections to facilitate collection of the electricity generated by the cells. The wiring could be provided to avoid interruption of current flow during change out of individual cells. The mounting assembly can be made from lightweight durable materials. Such materials could include various rigid plastics and resins or non-conductive lightweight metals, wood or other similar materials. 
         [0027]    Referring to  FIG. 6 , a perspective view of a system of multiple curved photovoltaic cells  600  has a mounting assembly  610 . A plurality of curved photovoltaic cells  600  can be fitted into individual spacings  620  in the mounting assembly  610 . The mounting assembly  610  can be a constructed from lightweight materials such as polymers, plastics, non-conducting metals, composites, wood or other similar materials. The curved photovoltaic cells  600  can be electrically connected in series or in parallel with alternating connections from the top layer of one cell to the bottom layer of an adjacent cell. Specifically, connection  630  is connected to the bottom layer of the individual photovoltaic cell  615 , while connection  635  at the other end of the photovoltaic cell  615  is connected to the top layer connection  630  is connected to the adjacent photovoltaic cell  625  via connector  650 . Connection  640  at the opposite end of connector  650  is connected to the top layer of cell  625 . Connection  645  at the opposite end of cell  625  is connected to the bottom layer and begins the cycle again by connecting to top layer of the next adjacent cell. At the each end of the array are conducting wires  660  and  670 , which connect to the demand or storage device. 
         [0028]    The curved photovoltaic cells can be of various polygonal shapes in cross section and can be cut to a specific length during the formation process. For example, the photovoltaic cells, can have a cross section that is circular, or a half circle, or triangular with one side curved, or n-sided with at least one side and possibly multiple sides being curved with semiconductor material deposited in layers on at least one curved surface. They can be formed from a variety of materials including glass, low iron glass and low expansion glass as defined by the industry, and borosilicate glass. Photovoltaic cells can be formed on annular or solid materials. The semiconductor layers can be deposited on them using a variety of techniques including chemical vapor deposition and vapor transport deposition. They can be encased in a protective coating or enclosure to prevent damage to the semiconductor surface. 
         [0029]    A process for making a photovoltaic device is performed by establishing a contained environment or chamber heated in a steady state during the processing to a starting temperature in a range above about 550° C., and preferably in the range of about 800-1000° C. for the temperature of the glass extruder/distributor during initial formation of the glass substrate from the melted glass. The environment can be kept under vacuum or an inert atmosphere to prevent exposure and possible weakening of the hot substrate due to water vapor exposure. For example, glass fully formed and cooled in the absence of water vapor will have a more desirable and higher modulus of rupture. Referring to  FIGS. 1-4 , the substrate  15  can be directly extruded from a local source of hot substrate, or can be pre-formed. The substrate  15  can be cut to the desired processing dimensions following the extrusion step. For example, the substrate  15  can be cut into any length required for specialized application, or can be cut into standard lengths such as 2 foot or 4 foot lengths for off the shelf devices. Alternatively, the substrate  15  can be kept in 10-20 foot lengths for processing and later cutting. The substrate  15  can be pre-formed or extruded into a solid curved or annular curved substrate, where either the solid curved or the annular curved substrate has a polygonal cross-section with at least one curved surface. The substrate  15  when formed with a circular cross-section can have a diameter greater or smaller than about 0.75 inches. 
         [0030]    After formation and sizing, the substrate  15  is ready for deposition of the bottom conductive layer  80 . Deposition of the bottom layer  80  on the inner surface  30  of the substrate  15  involves forming a substantially uniform layer of a conductive material on the surface of the substrate. This layer can be a transparent conductive material including a transparent conductive oxide. An example of a typical conductive oxide is tin oxide. The deposition on the inner surface  30  can be accomplished by passing the annular substrate  15  around a vapor deposition element at a fixed rate or alternatively inserting a vapor deposition element into the annular substrate  15  at a fixed rate. The rate can be determined based upon the desired thickness of the deposition layer and would be a function of the vapor supply rate and the velocity of the deposition element with respect to the substrate  15 . The substrate  15  could be stationary or moving while the deposition is taking place and could be part of a continuous manufacturing system where the substrate  15  is kept in the contained environment and conveyed to different stations for different treatment. 
         [0031]    Alternatively, deposition of the layers can be performed as the glass substrate is being formed and sized.  FIG. 7  provides an example of an apparatus  700  for accomplishing this. A hot melted glass supply  710  in a melted glass reservoir  720  has an orifice  715  for formation of a glass substrate  705  from the melted glass. The glass substrate  705  can have any polygonal cross-section or may be in the form of a ribbon or a half-tube. Extending through the melted glass reservoir top  730  and through the orifice plug  735  is an annular depositor  740  which deposits a first deposition layer on the substrate. Annular depositor  740  extends through the melted glass reservoir  720 , out the top  730  of the reservoir and connects to an insulated heated flexible deposition gas supply line  765  that provides enough flexibility and length for the depositor to be raised and lowered both to deposit gas and to open the orifice plug  735 . The supply line  765  is connected to an external source of the deposition gas or gases  770 . The deposition layer can be deposited on a portion of the substrate surface or can be deposited across the entire substrate surface, by regulating the extent of the annulus through which gas may pass. 
         [0032]    A second depositor  745  extends from within depositor  740  beyond the first deposition end  742  to a second deposition end  747  to deposit a second deposition layer on a surface of the substrate. The outer wall of the second depositor is spaced away from the inner wall of the first depositor creating the annular space through which the first deposition gas flows. The second deposition gas similarly travels through the annular space between the inner wall of the second depositor and the outer wall of a third depositor  750 . This deposition gas also comes from an external supply  780  via heated, insulated flexile supply line  785 . Similarly, a third depositor  750  extends from within the second depositor  745  to deposit a third deposition gas. For the purpose of this example there are only three separate deposition gas streams, and thus three depositors though more or less of each can be used depending on the number of layers to be deposited. The third deposition gas supply  790  connects via a heated, insulated flexible line  795  to the third depositor  750 . Since this depositor is the last one in this example, the flow is not annular and thus the diameter can be smaller for the same volume of flow. When supplying gases, the external gas supplies and individual depositors can supply gas mixtures, pure gases, or multiple gases that mix at the deposition end of the individual depositors. This can be accomplished using different supply line and deposition line configurations than are shown in this example. The deposition ends of the depositors can have varying shapes and attachments to facilitate deposition of a homogenous layer or layers on the substrate including various spray mechanisms and air mixers. 
         [0033]    Referring to  FIG. 8 , a hot melted glass supply  810  in a melting reservoir  815  has an orifice  820  for formation of glass substrate  825  that can be sealed by plug  827 . The substrate  825  can be formed around the outer surface  830  of an annular depositor  840  which deposits a first deposition layer on the inner surface  850  of the forming substrate  825 . A second annular depositor  835  is shown depositing a second deposition layer onto the inner surface  850  from an annular position within depositor  840 . A third annular depositor  860  is shown depositing a third deposition layer onto the inner surface  850  from an annular position within depositor  835 . Additional annular depositors are possible though not shown. The annular depositors are spaced apart form each other and supported within the ultimate structure using, for example, spacers  865  to ensure adequate flow volume of deposition gas through each annulus. By applying the layers to the glass as it is forming, the deposition can occur at the optimum temperature and the glass is at it&#39;s cleanest when it is initially forming. The annular depositors can be configured to deposit on the whole inner surface, or a portion of the inner surface. Additionally, other configurations using, for example, fins or half-annular blocks can be used to prevent or facilitate gaseous mixing prior to deposition. 
         [0034]    The bottom conductive layer  80  can be deposited on an inner surface  30  of the substrate  15  using a method of chemical vapor deposition in which the deposition element is moved within the annular region of the substrate  15  at a constant rate in order to form a uniform layer on the inner surface  30 . The deposition element can be designed to coat a portion of or the entire inner perimeter of an annular substrate  15 . Similarly, a solid substrate  15  can be coated with the bottom layer  80  using a method of chemical vapor deposition along the curved surface of the substrate  15 . The perimeter, or a portion thereof, can be coated by rotating the substrate  15  as it moves past the deposition element. 
         [0035]    The bottom layer  80  can be a film of tin oxide applied by atmospheric pressure chemical vapor deposition approximately 0.04 microns thick to improve the optical quality. A buffer layer can be applied that includes a silicon dioxide film  310  and is applied by atmospheric pressure chemical vapor deposition to a thickness of 0.02 microns over the tin oxide film to provide a barrier. Next, another tin oxide film  324  that is 0.3 microns thick and fluorine doped is applied over the silicon dioxide film. This second film of tin oxide functions as a reflective film in architectural usage with the fluorine doping increasing the reflectivity and as an electrode for the photovoltaic device as is hereinafter more fully described. 
         [0036]    After the bottom layers have been applied, the substrate  15  can be transported from the chemical vapor deposition zone, to a vapor transport deposition zone. Additional conductive layers can be added at this point. The system includes a suitable heater for heating the substrate  15  to a temperature in the range of about 450 to 640° C. in preparation for semiconductor deposition. The substrate  15  is next transported through a series of deposition stations. The number of stations depends on the semiconductor material to be deposited but can include three deposition zones for depositing three separate semiconductor material layers. More specifically, the first deposition station can deposit a cadmium sulfide layer  326  that can be 0.05 microns thick and acts as an N-type semiconductor. The second deposition station can deposit a cadmium telluride layer  328  that is 1.6 microns thick and acts as an I-type semiconductor. Thereafter, the third deposition station can deposit another semiconductor layer  330  which can be 0.1 microns thick and can be zinc telluride that acts as a P-type semiconductor. The first and second semiconductor layers  326  and  328  have an interface for providing one junction of the N-I type, while the second and third semiconductor layers  328  and  330  have an interface for providing another junction of the I-P type such that the resultant photovoltaic cell is of the N-I-P type. These interfaces normally are not abrupt on an atomic scale, but rather extend over a number of atomic layers in a transition region. This system is not limited to the specific semiconductor materials identified above, and will function using a variety of such materials known to those skilled in the art. 
         [0037]    After deposition of the semiconductor layers, the substrate  15  can undergo a rapid cooling process to strengthen the glass. This process can include rapid blowing of nitrogen or another inert gas inside and outside of and normal to the substrate to cool it, providing compressive stress that strengthens the glass. 
         [0038]    After the rapid cooling step, a sputtering station receives the substrate  15  and deposits a nickel layer  332  over the semiconductor layers. This nickel sputtering is preferably performed by direct current magnetron sputtering and need only be about 100 angstroms thick to provide a stable contact for a subsequent deposition. Thereafter, the substrate  15  is transferred to a sputtering station that deposits an aluminum layer  334  that is 0.3 microns thick over the nickel layer  332  to act as an electrode on the opposite side of the semiconductor layers as the tin oxide film  80 , which acts as the other electrode. The aluminum layer  334  is deposited by in-line multiple cathode, direct current magnetron sputtering. Thereafter the substrate  15  is received by another sputtering station that applies another nickel layer  336  over the electrode aluminum layer to prevent oxidation of the aluminum layer  334 . 
         [0039]    After the sputtering is complete, electronic conducting elements  60  and  70 , for example, wire leads, are attached to the two electrode layers  80  and  334  one at each end of the substrate  15 . For the annular substrate  15  with semiconductor material on the inner surface  30  of the substrate, the annulus is evacuated using a vacuum. The ends of the substrate  15  are melted to form a seal round each of the electronic conducting elements  60  and  70  and an inert gas is inserted into the evacuated annulus. The electronic conducting elements  60  and  70  can be used to connect one cell to another in series or in parallel as part of a photovoltaic system, or can connect individually to a storage device for storing the electricity, or can connect directly to an electrical demand source. The electronic conducting elements may come from alternate ends of the each individual cell or both may come from one sealed end of the cell. The conducting elements may be arranged such that they form a standardized end connection for easy change out of individual cells. The mounting assembly can be configured to receive the specific connection types and can serve to provide electrical connections between the individual cells, including continued service when individual cells are malfunctioning or have failed. The mounting assembly may then serve to distribute the generated electricity to a storage device or a demand source. 
         [0040]    When the semiconducting layers are placed on the outer surface  220  of the curved substrate  15 , the electronic conducting elements  60  and  70  can be attached to the appropriate electrode layers and then the entire cell can be encased in a transparent protective tube or can be covered with a transparent protective layer. The transparent protective layer or tube can also serve to help form a standardized connection for the cell. As such, a photovoltaic system or array can include both cells with the semiconductor material on the inner curved surface and on the outer curved surface or the substrate 
         [0041]    As shown in  FIGS. 5 and 6 , multiple cells can be brought together and connected in electrical series to form a photovoltaic array capable of generating low cost electrical power. The individual cells are connected to each other electrically using the electrical conductors  530  and  540  and electrical connector  535 , and can be held in a mounting assembly for direct exposure to a light source including the sun. The mounting assembly can be any assembly capable of holding the curved photovoltaic cells and exposing them to a light source including the sun, and can incorporate lightweight materials such as polymers, resins, non-conductive metals and composites into the design. The mounting assembly can provide for a modular system of use in which the photovoltaic cells have a standardized electrical connection that connects to the mounting assembly that distributes the generated electricity. Multiple mounting assemblies can be configured to attach to attach to each other. 
         [0042]    The entire contained environment can be heated using electrical resistance heaters, with the temperature controllable at each zone. When operated as a continuous manufacturing process, the substrate  15  can be transported using substrate holders designed specifically for the placement of the semiconductor layers (inner or outer surface). Such transport can be accomplished using a roll conveyor type mechanism or any other conveyancing means suitable for the processing environment. 
         [0043]    In another embodiment, a low reflective coating could be added to the outer surface of the substrate to increase efficiency by allowing more of the incident sunlight to penetrate. Examples of such coatings include a variety of vacuum deposited thin films commonly used in the photography industry to reduce reflection. Other examples include a thin film of MgF 2 , or a thin film sol gel application of silicon powder to make a coating at 1.23 index of refraction 
         [0044]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, other semiconductor materials can be used, and different mounting means can be used. Accordingly, other embodiments are within the scope of the following claims.