Abstract:
A sun tracking solar concentrator includes a one or two-sided linear Fresnel lens imprinted on a rollable sheet that is curved to form as a cylindrical arc surface. A one sided lens has a first zero line or a center point that transmits sunlight through without any refraction. A two sided lens also has a second zero line that is perpendicular to the first zero line. The Fresnel lens may be spooled onto rollers at its two straight ends. The first zero line or the center point may be positioned along the cylindrical arc by rotating one or both of the rollers. This mechanism aimed at providing horizontal tracking of the sun as it moves from East to West. Vertical tracking is accomplished by a tiltable mount coupled to the two rollers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The field of the present invention relates to solar concentrators, in particular to those that use flexible Fresnel lenses and track the movement of the sun. 
         [0003]    2. Background 
         [0004]    Most of the US landmass has solar potential varying between 3-8 kWh/m2 per day. If one could only convert 30% of the solar energy incident upon a typical 6 m×6 m two-car garage area in a state like Colorado, one could power the whole house, based on the average US daily consumption rate of 32 kWh and send an additional 16 kWh back into the grid for others every day. This potential remains to be realized. To date, much progress has been made, but the amount of electricity generated from solar technologies remains very low. 
         [0005]    The two main technologies for harvesting solar power are (1) photovoltaics and (2) solar thermal. Photovoltaic devices convert solar energy directly to electricity. Solar thermal devices concentrate and convert the solar energy to heat which is then converted to electricity. 
         [0006]    Photovoltaic devices can be in the form of flat panels that are exposed to sunlight or concentrated photovoltaics (CPV) systems that employ sunlight concentrated onto photovoltaic surfaces. Concentrating solar energy leads to increased efficiency in photovoltaics (from 15% to 38.5%) and reduces costs since much less photovoltaic device area is required. Concentration ratios can range from 2 to 800 times, i.e. 2-800 suns. Likewise, in solar thermal systems, the sun&#39;s rays must be highly concentrated (100-1000 suns) for efficient electric power generation. Both systems use optical techniques to focus incident sunlight into a small beam. Higher concentration generally means more efficient power generation. Moreover, in a high concentration design, tracking is critical to keep the sunlight focused onto the small solar cell or a hot spot. 
         [0007]    The three main categories of existing technologies for solar concentrators are parabolic troughs, dish reflectors, and Fresnel lenses and reflectors. Parabolic troughs concentrate incoming light along one dimension leading to a line of concentrated light. Parabolic dishes, on the other hand, concentrate along two dimensions. Fresnel lenses and reflectors can be linear resulting in one dimensional concentration or radial leading to two dimensional concentration. Concentration along two dimensions is required for use with CPV solar cells to make them cost effective. This makes radial Fresnel lenses and dish reflectors the main candidates. Dishes are made of metals which make them expensive and heavy and thus unsuitable for distributed applications. Radial lenses are inexpensive and are the concentrator of choice for home-owner solar technology. There are radial Fresnel lenses on weather-tough acrylic currently available on the market. Even though these lenses are light and inexpensive, external moving parts are needed to orient them to track the sun which raises the cost of the end product significantly. The trackers are also large and bulky and thus not suitable for many applications, especially distributed applications, such as those set up in remote locations or camps, mounted on rooftops, or installed in backyards. 
         [0008]    Both photovoltaic systems and solar thermal systems would benefit greatly from sun tracking solar concentrators built with common inexpensive and lightweight materials. The present invention is aimed at addressing this need. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed toward a sun tracking solar concentrator which includes a single or double layer linear Fresnel lens that can be used with a solar thermal or photovoltaic energy conversion system. In case of a single layer linear Fresnel lens, the rays of the sun are concentrated along one dimension onto a narrow line straddling the focal line of the lens. In case of the double layer Fresnel lens, the solar rays are concentrated along two dimensions onto a spot centered at the focal point of the lens. 
         [0010]    The surface of the Fresnel lens is curved to form a cylindrical arc surface. During use, the axis of the cylinder is preferably positioned such that it lies substantially in the plane perpendicular to the East-West axis. The East-West axis may be defined as the line that connects the two points on the horizon, the first point being where the sun rises and the second being the point where the sun sets on the Spring and Autumn equinoxes. 
         [0011]    The linear Fresnel lens surface has a first zero line substantially parallel to the cylinder axis where the solar rays incident upon it pass through with little or no refraction perpendicular to the cylinder axis. The solar rays incident at other locations are refracted perpendicular to the cylinder axis by the chain of prisms of the Fresnel lens towards the focal area. 
         [0012]    The linear Fresnel lens surface may also have a second zero line substantially perpendicular to the cylinder axis where the solar rays incident upon it pass through with little or no refraction parallel to the cylinder axis. The solar rays incident at other locations are refracted parallel to the cylinder axis by the chain of prisms of the Fresnel lens towards the focal area. 
         [0013]    Based on the described geometry, tracking the sun may be accomplished by accommodating two angles: First is the azimuth angle as the sun travels from East to West during the day. The second is the elevation angle as the sun rises, traverses its daily path overhead, and sets. The azimuth and elevation tracking mechanisms maintain the solar concentrator in its preferred orientation, which is achieved when the Fresnel lens is positioned such that a plane tangent to its surface and passing though the first zero line is substantially perpendicular to the incident solar rays. 
         [0014]    The sun tracking solar concentrator may exhibit local invariance of the angle of incidence as long as the preferred orientation of the solar tracker is maintained. This local invariance renders the angle of incidence of solar rays at any given point on the solar concentrator lens substantially constant despite the movement of the sun across the sky as long as the preferred orientation is maintained. The substantially constant angle of incidence allows for the optimization of the cylindrical Fresnel lens design prism by prism resulting in enhanced optical efficiency. 
         [0015]    Accordingly, an improved sun tracking Fresnel lens solar concentrator is disclosed. Advantages of the improvements will appear from the drawings and the description of the embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings, wherein like reference numerals refer to similar components: 
           [0017]      FIG. 1A  schematically illustrates the Fresnel lens concept as known in the prior art; 
           [0018]      FIG. 1B  schematically illustrates a linear Fresnel lens as known in the prior art; 
           [0019]      FIG. 1C  schematically illustrates a radial Fresnel lens as known in the prior art; 
           [0020]      FIG. 1D  schematically illustrates a section of the Fresnel lens prism chain with nomenclature conventions as known in the prior art; 
           [0021]      FIG. 1F  schematically illustrates a linear Fresnel lens with support structure to hold or suspend the lens as a cylindrical arc surface as known in the prior art; 
           [0022]      FIG. 1G  schematically illustrates the underside of the linear Fresnel lens shown in  FIG. 1F ; 
           [0023]      FIG. 1H  schematically illustrates the rays refracted as they pass through the linear Fresnel lens shown in  FIG. 1F ; 
           [0024]      FIG. 2  schematically illustrates the movement of the sun across the sky on different days of the year as known in the prior art; 
           [0025]      FIG. 3  schematically illustrates a cylindrically shaped linear Fresnel lens that concentrates sunlight along one dimension and utilizes an external pan and tilt mechanism to track the sun as known in the prior art; 
           [0026]      FIGS. 4A and 4B  schematically illustrate a first sun tracking solar concentrator that concentrates sunlight along one dimension; 
           [0027]      FIGS. 5A and 5B  schematically illustrate a second sun tracking solar concentrator that concentrates sunlight along two dimensions; 
           [0028]      FIG. 6  schematically illustrates a first modified sun tracking solar concentrator; 
           [0029]      FIG. 7  schematically illustrates a second modified sun tracking solar concentrator; 
           [0030]      FIG. 8A  schematically illustrates a third embodiment of a sun tracking solar concentrator; 
           [0031]      FIG. 8B  schematically illustrates the folded view of the solar concentrator shown in  FIG. 8A ; 
           [0032]      FIGS. 9A and 9B  schematically illustrate a rolling mechanism for a sun tracking solar concentrator; 
           [0033]      FIGS. 10A ,  10 B, and  10 C schematically illustrate the local invariance of the angle of incidence for a sun tracking solar concentrator; and 
           [0034]      FIGS. 11A and 11B  schematically illustrates a bundle of solar rays that pass through a single prism of a Fresnel lens. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Turning in detail to the drawings,  FIG. 1A  illustrates in a series of steps (a)-(d) how a Fresnel lens  10  can be constructed by collapsing a continuous surface plano convex lens  20  into an equivalent power Fresnel lens  10 . This concept is well known to those skilled in the art. As shown, the Fresnel lens consists of a chain of prisms. Commonly, the prisms can be arranged linearly in rows or radially in concentric circles. A linear arrangement shown in  FIG. 1B  is commonly called a linear Fresnel lens  12  and provides one dimensional concentration of incident sunlight  90  onto an often narrow rectangular area  14  around the focal line of the lens. A radial arrangement shown in  FIG. 1C  is commonly called a radial Fresnel lens  16  and provides two dimensional concentration of incident sunlight  90  onto a small rectangular area  18  around the focal point of the lens. 
         [0036]      FIG. 1D  shows a section of the Fresnel lens prism chain with nomenclature conventions. The prism has two facets, called the slope facet  30  and the draft facet  32 . The distance between the peaks of the prisms is facet spacing  34 . The angle between the line parallel  38  to the plano surface  40  of the Fresnel lens and the slope facet  30  is the slope angle  36 . The angle between the line  42  perpendicular to the plano surface  40  of the Fresnel lens and the draft facet  32  is the draft angle  44 . 
         [0037]      FIG. 1F  illustrates a linear Fresnel lens  100  with support structure  106  to maintain a cylindrical arc shape. The support structure  106  may consist of ribs, wires or struts that maintain the cylindrical shape. The cylindrical outer surface  110  of the lens  100  pointed towards the sun is smooth. The chain of prisms of the Fresnel lens  100  are situated on the inside surface  120 . Alternately, the chain of prisms of the Fresnel lens  100  may be situated on the outside surface  110  with the inside surface  120  being smooth. The material of the lens can be a flexible transparent polymer. If the material is thin and has a tendency to bow or sag, it can be held under slight tension.  FIG. 1G  illustrates a small cutout of the Fresnel lens with the outer surface  110 ′ from another perspective, showing the linear arrangement of prisms  130  and the internal surface cutout area  120 ′. The lens concentrates incident sunlight  90  in one direction around a focal line  140 . As shown in  FIG. 1H , the rays of the sun incident on the first zero line  150  go through the cylindrical Fresnel lens with little or no refraction. The remaining rays incident on the outer surface  110  are refracted towards the focal line  140 . Typically, the ratio of the area of the linear Fresnel lens  100  surface to the area of the focal line  140  area is between 10 and 100. There are numerous examples of this type of solar concentrator in the art. Many of these solar concentrators are made of polymethyl methacrylate (PMMA) which is a transparent thermoplastic sometimes called acrylic glass. 
         [0038]      FIG. 2  illustrates the movement of the sun across the celestial sphere  210  for four days during the course of a year for a location in the Northern hemisphere near latitude  35 . The three solar paths shown are the solar path on Summer solstice  220 , the solar path on Winter solstice  222 , and the solar path on the Spring and Autumn equinoxes  224 . Two angles define the position of the sun. These angles are the elevation angle  230  and the azimuth angle  240 . Tracking the sun is important for solar concentrators. The solar concentrators often have a preferred orientation with respect to the sun. This preferred orientation is the one which substantially maximizes the amount of solar energy that can be concentrated and later converted to other forms of energy. Thus, many solar concentrators are mounted onto solar trackers that cause the solar concentrators to assume their preferred orientations with respect to the incident solar rays  90 . 
         [0039]      FIG. 3  illustrates the cylindrical linear Fresnel lens  100  with support structure  106  (shown earlier in  FIG. 1F ) mounted onto a solar tracking pan tilt mechanism  310 . The pan tilt mechanism  310  is used to maintain the preferred orientation of the Fresnel lens  100  with respect to the incident solar rays  90 . The preferred orientation is when the cylindrical Fresnel lens  100  is positioned such that a plane  320  tangent to the surface  110  of the cylindrical Fresnel lens and passing though the first zero line  150  of the cylindrical Fresnel lens  100  is substantially perpendicular to the rays  90  of the sun. This serves two purposes: First, the solar rays  90  are focused substantially at the same focal line  140  regardless of what the azimuth angle  240  of the sun or the elevation angle  230  of the sun is. Second, the solar energy concentrated along the focal line  140  is substantially maximized. The pan adjustment  340  is substantially coupled to the sun&#39;s azimuth angle  240 ; whereas, the tilt adjustment  372  is substantially coupled to the sun&#39;s elevation angle  230 . There are numerous examples of this type of solar tracking in the art. 
         [0040]      FIG. 4A  illustrates a sun tracking solar concentrator  400  which includes a thin cylindrical linear Fresnel lens  410  spooled onto rollers  430 . The rollers  430  rotate to position the first zero line  450  of the lens along the cylindrical surface of the lens. The support structure  416  which maintains the cylindrical surface of the lens. The support structure  416  may also couple the rollers  430  so that the rollers  430  can pivot around a common axis. This pivoting allows for a tilt adjustment of the midline  450 . Alternately, the support structure  416  may be mounted onto a tilt mechanism  470 .  FIG. 4B  shows the rollers  430  and the Fresnel lens  410  in greater detail. 
         [0041]    As with the Fresnel lens  100  of  FIG. 3 , the preferred orientation is when the cylindrical Fresnel lens  410  is positioned such that a plane  420  tangent to the surface of the cylindrical Fresnel lens  410  and passing though the first zero line  450  of the cylindrical Fresnel lens  410  is substantially perpendicular to the incident solar rays  90 . 
         [0042]    The rollers  430  rotate to position the first zero line  450  so that the cylindrical Fresnel lens assumes the preferred orientation. The tilt adjustment  472  may also be necessary to assume the preferred orientation. The tilt adjustment  472  is substantially coupled to the sun&#39;s elevation angle  230  whereas the positioning of the first zero line  450  by way of turning the rollers  430  is substantially coupled to the sun&#39;s azimuth angle  240 . 
         [0043]    For the thin Fresnel lens to conform to a spool  432 , its material may need to be sufficiently rollable and thin. One suitable material is a plastic sheet made from the resin polyethylene terephthalate (PET). Another generic term for this material is polyester film or plastic sheet. Also, some people refer to it as Mylar®, which is a registered trademark of Dupont Tejjin Films. A Fresnel lens may be imprinted onto a plastic sheet using one of many well-known methods in the art, such as hot-press embossing. 
         [0044]      FIG. 5A  illustrates another sun tracking solar concentrator  500  which includes a thin two sided cylindrical linear Fresnel lens  510  spooled onto rollers  530 .  FIG. 5B  shows the cylindrical linear Fresnel lens  510  flattened and in greater detail. The rollers  530  position the first zero line  550  of the lens along the cylindrical surface of the lens. The chain of prisms on the inside of the cylindrical surface bends the incident rays towards the first zero line  550  whereas the chain of prisms on the outside of the cylindrical surface focuses incident rays towards the second zero line  552 . The result is two dimensional concentration of incident light onto concentration spot  540 . Typically, the ratio of the area of the surface of linear Fresnel lens  510  to the area of the concentration spot  540  is between 100 and 1000. 
         [0045]    The support structure  516  of this sun tracking solar concentrator  500  maintains the cylindrical surface of the lens is mounted onto a tilt mechanism  570 . The tilt adjustment  572  is substantially coupled to the sun&#39;s elevation angle  230 . The positioning of the first zero line  550  by way of turning the rollers  530  is substantially coupled to the sun&#39;s azimuth angle  240 . As with the Fresnel lens  100  of  FIG. 3 , the preferred orientation is when the cylindrical Fresnel lens  510  is positioned such that a plane tangent to the surface of the cylindrical Fresnel lens  510  and passing though the first zero line  550  of the cylindrical Fresnel lens  510  is substantially perpendicular to the incident solar rays  90 . 
         [0046]    Sun tracking solar concentrators may also incorporate Fresnel lens materials which do not permit spooling around rollers.  FIG. 6  and  FIG. 7  illustrate two such modifications. In the sun tracking solar concentrator  600  shown in  FIG. 6 , the Fresnel lens is not spooled onto the rollers  630 . Instead, the two rollers  630  are part of a mechanism used to position the first zero line  650  of the Fresnel lens  610  along the cylindrical arc between lines  660  and  670 . The bottom surface  680  of the support structure  116  can be mounted onto a tilting platform to provide tilt adjustment. In the sun tracking solar concentrator  700  shown in  FIG. 7 , the rollers are eliminated altogether. The Fresnel lens  710  is shaped into a cylinder. The cylinder is rotated around its axis  720 , to position the first zero line  750  of the Fresnel lens along the cylindrical arc between lines  760  and  770 . The bottom surface  780  of the support structure  716  can be mounted onto a tilting platform to provide tilt adjustment. The two sun tracking solar concentrators  600 ,  700  may provide one or two dimensional concentration. One dimensional concentration can be achieved with one sided Fresnel lens and two dimensional concentration can be achieved with a two sided Fresnel lens as shown in the sun tracking solar concentrator  400  and the sun tracking solar concentrator  500 , respectively. The location of concentrated sunlight is not shown to keep the illustrations uncluttered. 
         [0047]      FIG. 8A  illustrates another sun tracking solar concentrator  800 . Four cylindrically mounted strips  810 ,  812 ,  814 ,  816  of a thin two sided linear Fresnel lens are spooled onto rollers  830  and  832  which rotate to position the common first zero line  850  of the lens strips  810 ,  812 ,  814 , and  816  along the cylindrical surface of the composite lens. The chain of prisms on the inward facing side of the cylindrical surface bends the incident rays towards the common first zero line  850  whereas the chain of prisms on the outward facing side of the cylindrical surface focuses incident rays towards the common second zero line  852 . The result is two dimensional concentration of incident solar rays  90  onto concentration spot  840 . The support structure  860  that maintains the cylindrical surface of the lens is mounted onto two legs  866  and  868  that are further coupled to two hydraulic cylinders  876  and  878  which collectively serve as the tilt mechanism. A heat exchange engine  842  is thermally coupled to the concentration spot  840 . The heat engine may be a Stirling engine producing electrical output. 
         [0048]    The support structure  860  is shown with five ribs that maintain the cylindrical shape of the Fresnel lens strips  810 ,  812 ,  814 , and  816 . The support structure  860  may contain further supporting beams or braces to reinforce its strength against external forces, e.g., wind. The support structure  860  may also be equipped by mechanisms that allow it to be stowed when wind speeds exceed safe levels. The support structure  860  may also be built so it can be folded for easy transport or storage as shown in  FIG. 8B . Having a folding support structure  860 ′ is also convenient for reducing assembly complexity, labor, and time. The dimensions of the Fresnel lens strips  810 ,  812 ,  814 ,  816  of the sun tracking solar concentrator  800  determine the solar collection area and hence the energy output from the Stirling engine. It is expected that an area of approximately 10 square meters can be used to generate 1.6 kW of peak electrical power. This size can be achieved with approximately 50 cm wide strips that roll across ribs with arclengths of approximately 5.5 meters. 
         [0049]      FIG. 9A  illustrates one mechanism for rolling the Fresnel lens strip  812  in  FIG. 8A  to position the common first zero line  850  at the desired location. As shown in  FIG. 9B , the Fresnel film strip  812  contains two rows of perforations  920  and  922  along its length positioned at the top and bottom of the strip  812 . The two rows of perforations  920  and  922  are used for transporting and steadying the strip  910 . They are locked onto two rows of sprockets  930  and  932  positioned on two chains  940  and  942 . As the two chains  940  and  942  roll across the two gears  950  and  952 , the film  812  is spooled from one roller  830  to the other roller  832  or vice versa. The rows of sprockets  930  and  932  hold the film strip  812  in slight tension to be suspended over the concentration spot  840 . The rows of sprockets  930  and  932  also prevent the film strip  812  from rubbing against the ribs. A cover may be added to secure the rows of sprockets  930  and  932  in the rows of perforations  920  and  922  and thus prevent the strip  812  from coming loose. 
         [0050]      FIGS. 10A ,  10 B, and  10 C collectively illustrate a common property of the sun tracking solar concentrators  400 ,  500 ,  600 ,  700 ,  800  described above. This common property is referred to herein as “local invariance of the angle of incidence.” This property can be summarized as follows: The angle of incidence  1060  of solar rays  90  at any single point  1011  on the Fresnel lens  1010  remains substantially constant despite the movement of the sun across the sky provided that the preferred orientation of the Fresnel lens  1010  is maintained. As mentioned earlier, the preferred orientation is when the Fresnel lens  1010  is positioned such that a plane tangent to its surface and passing though the first zero line  1050  is substantially perpendicular to the rays  90  of the sun. The substantially constant angle of incidence  1060  at any single point  1011  allows for the optimization of the Fresnel lens  1010  design prism by prism as explained further below. 
         [0051]      FIG. 11A  illustrates a bundle of solar rays  1190  that are incident on a region  1114  of the cylindrical Fresnel lens  1110 . As shown in further detail in  FIG. 11B , the region  1114  is located between surface normal lines  1116  and  1118  of the Fresnel lens  1110 . The bundle of solar rays  1190  incident upon the region  1114  passes through substantially only a single prism  1112 . The facet spacing  1134  of the prism  1112  is small enough that the angles of incidence  1160  of all the rays in the solar ray bundle  1190  are substantially equal to another. The solar ray bundle  1190  is refracted first as it enters the surface of the Fresnel lens  1110  and second as it exits the prism  1112 . The desired angles of refraction  1170  for all rays in the solar ray bundle  1190  are also substantially equal to another. Knowing the angle of incidence  1160  as well as the desired angle of refraction  1170 , both measured with respect to the surface normal  1116  (or  1118 ) of the Fresnel lens  1110  makes it possible to optimize the design parameters of the prism  1112 . These design parameters are facet spacing  1134 , slope angle  1136 , and draft angle  1144 . 
         [0052]    One design process that takes advantage of the property of constant angle of incidence may be described as follows:
       STEP 1. Select material of the Fresnel lens. This will determine the refractive index.   STEP 2. Select the thickness of the Fresnel lens.   STEP 3. Select the Fresnel lens focal length, f number, cylindrical geometry, and dimensions.   STEP 4. Determine the maximum operational curvature of the cylindrical Fresnel lens. This is the angle of the arc which is endowed with Fresnel prisms. Its value is generally between 90 degrees and 180 degrees.   STEP 5. Formulate the initial design for the Fresnel lens. This design will be optimized.   STEP 6. Divide the aperture of the Fresnel lens into segments each of which correspond to a single prism path for the incident solar rays.   STEP 7. Determine the prism inclination and the angle of incidence of solar rays per each segment.   STEP 8. Determine the design parameters of the prism per each segment.       
 
         [0061]    These steps can be iterated as needed. As already mentioned in the text description associated with  FIGS. 11A and 11B , the angles of incidence  1160  and the angles of desired refraction  1170  for all rays in the solar ray bundle  1190  are substantially equal. Any small differences between these angles can be taken into account for further optimizing the design parameters of the prism  1112 . 
         [0062]    Finally, a two layer Fresnel lens for concentrating solar rays  90  along two dimensions may be replaced with a one layer Fresnel lens, by arranging the chain of prisms of the Fresnel lens radially. It has already been mentioned in the text referencing  FIG. 1C  that a radial arrangement—commonly called a radial Fresnel lens  16 —provides two dimensional concentration of incident sunlight  90 . 
         [0063]      FIG. 12A  illustrates the Fresnel lens  1200  with the radial arrangement of the chains of prisms around a center  1250 . The lens  1200  is laid flat.  FIG. 12B  illustrates the cylindrical arc shape in which the Fresnel lens  1200  is to be deployed when used as part of a solar concentrator of this invention. Referring again to the cylindrical arc shape of the Fresnel lens  1200  of  FIG. 12B , solar rays incident upon the center  1250  pass through with little or no refraction. The solar rays incident at other locations are refracted by the chain of prisms of the Fresnel lens  1200  towards the concentration spot  1240 . 
         [0064]    The preferred orientation of the Fresnel lens  1200  is achieved when the Fresnel lens  1200  is positioned such that a plane tangent to its cylindrical arc surface and passing though the center  1250  is substantially perpendicular to solar rays  90 . The center of the Fresnel lens  1200  can be positioned along the cylindrical arc as previously described above for the sun tracking solar concentrators  400 ,  500 ,  600 ,  700 ,  800 . 
         [0065]    Thus, a sun tracking solar concentrator is disclosed. While embodiments of these inventions have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The inventions, therefore, are not to be restricted except in the spirit of the following claims.