Abstract:
A solar conversion apparatus and method includes two or more conversion cells and a reflector assembly. Each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. The reflector assembly comprises at least two integrated reflective sections. One of the at least two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures further comprises a Fresnel microstructure.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/165,129, filed Mar. 31, 2009, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    This invention generally relates to photovoltaic converters and, more particularly, to concentrated solar photovoltaic converters. 
       BACKGROUND 
       [0003]    Optical concentrators are widely used in solar photovoltaic converters for two important reasons. First, they allow for reduced system cost because less photovoltaic conversion material is required which is by far the most expensive component in a photovoltaic system. Typically, concentrated photovoltaic systems have a photovoltaic cell that has less than 1% of the area of a photovoltaic cell used in an equivalent conversion apparatus. Second, it is well known that photovoltaic cells illuminated by higher flux densities achieve higher solar-to-electricity conversion efficiencies. 
         [0004]    A typical prior art concentrating photovoltaic system  10  is illustrated in  FIG. 1 . The concentrating photovoltaic system  10  includes a condensing Fresnel lens  2  and a photovoltaic cell  4  located at the focal point  5  of the condensing Fresnel lens  2 . Both the condensing Fresnel lens  2  and the photovoltaic cell  4  share a common optical axis  1 . In operation, solar radiation  22  is incident on the condensing Fresnel lens  2  which causes the solar radiation  22  to be condensed and brought to a focus at a focal point  5  on the photovoltaic cell  4 . 
         [0005]    A Fresnel optical element can be of two types: one that operates in transmission which is called a Fresnel lens; and one that operates in reflection which is called a Fresnel mirror or Fresnel reflector. Both Fresnel lenses and Fresnel reflectors are commonly employed in solar concentrators and both include a Fresnel microstructure with a series of rather shallow grooves that are generally sawtooth in cross-section. The longer surface of each groove that performs the optical work is called the slope surface and the shorter surface that connects the slope surfaces together is called the draft or riser surface. The angle of the slope surface generally changes slightly from groove to groove being more shallow near the center of the Fresnel and steeper at the edges. At the same time the depth of the draft or riser surface are smaller near the center of the Fresnel microstructure and greater at the edge. 
         [0006]    There are two major problems with this prior art concentrating photovoltaic system  10 . First, the focal point  5  is not a point because of chromatic aberration. Instead, the focal point can be several centimeters in diameter depending on the geometry of the optical configuration and the range of wavelengths passed by the Fresnel lens  2 . An ideal condensing Fresnel lens would transmit and bring to a focus all optical energy within the wavelengths of the sun that contain significant amounts of energy. Typically, the wavelengths of the sun range from about 350 nm to about 1900 nm. The dispersive nature of the material comprising the condensing Fresnel lens  2  causes the refractive index of the material to vary significantly over this range, which in turn causes the optical power of the condensing Fresnel lens  2  to vary as a function of wavelength, which in turn causes the diameter of the focal spot  5  to also vary with wavelength. To compensate for this, additional condensing optics can be installed atop the photovoltaic cell  4  or the photovoltaic cell  4  can be made substantially larger to ensure that it captures all of the energy of the worst-case focal spot  5 . Both of these solutions, however drive up system cost and complexity, and reduce efficiency. 
         [0007]    A second problem with this prior art concentrating photovoltaic system  10  is that only one solar photovoltaic cell  4  is used for each condensing Fresnel lens  2 . Utilizing tandem photovoltaic cells having a variety of stacked photovoltaic junction bandgaps can significantly improve photovoltaic conversion efficiency. These tandem photovoltaic cells are formed by growing two or three photovoltaic cells atop one another in a semiconductor foundry. 
         [0008]    An example of a typical triple junction cell  6  is illustrated in  FIG. 2 . In this triple junction photovoltaic cell  6 , the uppermost junction  7  converts the shortest wavelengths to electricity, the middle junction  8  converts a middle band of solar wavelengths to electricity, and the lowest junction  9  converts the longest wavelengths to electricity. This configuration does offer a significant improvement in conversion efficiency, as photovoltaic cell efficiencies on the order or 40% have been reported. Unfortunately, this triple junction photovoltaic cell  6  requires a large number of layers, only some of which are shown in  FIG. 2 . The addition of each layer dramatically increases device complexity, decreases fabrication yield, and drives up the overall cost of the device. Furthermore, the amount of generated current produced by a tandem photovoltaic cell is limited to the amount of photocurrent produced by the internal junction that is creating the least amount of photocurrent. This governing action can severely limit the amount of electricity produced by a multi junction photovoltaic cell. 
       SUMMARY 
       [0009]    A solar conversion apparatus includes two or more conversion cells and a reflector assembly. Each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. The reflector assembly comprises at least two integrated reflective sections. One of the at least two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures comprises a Fresnel microstructure. 
         [0010]    A method of making a solar conversion apparatus includes providing two or more solar conversion cells where each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. At least one of the two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures comprises a Fresnel microstructure. 
         [0011]    This technology provides a number of advantages including providing a more efficient, better performing, and economical solar conversion apparatus. This technology is able to avoid prior problems with large focal spot sizes and the use of a large and expensive, multi junction photovoltaic cell by utilizing a lower reflector assembly comprising one or more Fresnel reflectors arranged in a cascade configuration. Each of these Fresnel reflectors is reflective to a selected band of wavelengths and is transmissive to other wavelengths that are in turn reflected by lower Fresnel reflectors. Additionally, each Fresnel reflector includes a microstructure that reflects and brings to a focus onto a photovoltaic cell a selected band of wavelengths that the photovoltaic cell is most responsive to. The resulting solar conversion apparatus has a high concentration ratio, is lossless over the range of wavelengths emitted by the sun that have significant energy content, and effectively directs the concentrated solar energy to the appropriate single or multi junction photovoltaic cell. Furthermore, since the semiconductor junctions are not fabricated into the tandem PV-cell but instead are separated into separate PV-cells, a junction producing less photocurrent than the other junctions will not restrict the output of the other junctions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a side-view of a prior art concentrating photovoltaic (CPV) conversion apparatus; 
           [0013]      FIG. 2  is a cross-sectional view of a prior art, multi junction photovoltaic cell; 
           [0014]      FIG. 3  is a side-view of an exemplary solar conversion apparatus with three photovoltaic cells; 
           [0015]      FIG. 4  is a cross-sectional view of a condensing lens in the solar conversion apparatus illustrated in  FIG. 3 ; 
           [0016]      FIG. 5  is a side-view of a reflector assembly in the solar conversion apparatus illustrated in  FIG. 3 ; 
           [0017]      FIGS. 6A-6C  are graphs showing exemplary spectral reflectivities of three reflective layers in a reflector assembly in the solar conversion apparatus illustrated in  FIG. 3 ; 
           [0018]      FIG. 7  is a table of photovoltaic cell types with their bandgaps and operating wavelength bands; 
           [0019]      FIG. 8A  is a spectral response curve for a GaInAs photovoltaic cell; 
           [0020]      FIG. 8B  is a spectral response curve for a GaInP photovoltaic cell; 
           [0021]      FIG. 8C  is a spectral response curve for a GaAs photovoltaic cell; 
           [0022]      FIG. 8D  is a spectral response curve for a Germanium photovoltaic cell; 
           [0023]      FIG. 8E  is a spectral response curve for a Silicon photovoltaic cell; 
           [0024]      FIG. 8F  is a spectral response curve for a GaInP/GaAs double-junction photovoltaic cell; 
           [0025]      FIG. 8G  is a spectral response curve for a GaAs/Ge double-junction photovoltaic cell; 
           [0026]      FIG. 9  is a graph showing an air-mass 1.5 insolation in 20 nm wavelength bands which is the spectral irradiance incident on a typical solar collector pointing directly at the sun; 
           [0027]      FIG. 10  is a graph showing the maximum theoretical efficiency of a photovoltaic converter as a function of the number of junctions, in a system utilizing a solar concentrator that concentrates the solar illumination by 50×; 
           [0028]      FIGS. 11A-11F  are cross-sectional views of a process for manufacturing a reflector assembly for the solar conversion apparatus shown in  FIG. 3 ; 
           [0029]      FIG. 12  is a side-view of another exemplary solar conversion apparatus; 
           [0030]      FIG. 13  is a side-view of a reflector assembly used in the solar conversion apparatus illustrated in  FIG. 12  and without a condenser lens; 
           [0031]      FIG. 14  is a side-view of a multi-cell solar conversion apparatus utilizing condenser lenses and having a lateral cell configuration in accordance with another embodiment of the present invention; 
           [0032]      FIG. 15  is a plan-view of a four-cell solar concentrator assembly utilizing condenser lenses and having a lateral cell configuration in accordance with another embodiment of the present invention; 
           [0033]      FIG. 16  is a side-view of yet another exemplary solar conversion apparatus without a condenser lens; 
           [0034]      FIG. 17  is a side-view of a reflector assembly used in the solar conversion apparatus shown in  FIG. 16 ; and 
           [0035]      FIG. 18  is a perspective view of a solar tracking system used with a solar conversion apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    An exemplary solar conversion apparatus  20  is illustrated in  FIG. 3 . The solar conversion apparatus  20  includes a condensing lens  30 , a reflector assembly  32 , a rear bulkhead assembly  34 , and photovoltaic cells  36 A- 36 C, although the apparatus could comprise other numbers and types of systems, devices, components, cells and other elements in other configurations. The present invention provides a number of advantages including providing a more efficient, better performing, and economical solar conversion apparatus. 
         [0037]    Referring more specifically to  FIGS. 3-4 , the condensing lens  30  is a plano-convex lens that is substantially transmissive to all wavelengths of light that the photovoltaic cells  36 A through  36 C are responsive to. In this example, this range is from about 350 nm to about 1900 nm which is the typical range of wavelengths of the sun. The plano side  31  is generally oriented in a direction towards the sun and can have a subwavelength microstructure  68  to reduce unwanted Fresnel reflection and thereby improve light transmittance, although this microstructure is optional and the side  31  also can have other types of surfaces and treatments, such as an antireflective (A/R) coating or no treatment at all. The subwavelength microstructure on the plano side  31  has the additional benefit of having self-cleaning properties owing to the so-called Lotus Effect. 
         [0038]    Referring more specifically to  FIG. 4 , the condensing lens  30  is a Fresnel lens comprising two individual pieces, although the lens can have other constructions with other numbers of pieces, such as a monolithic unitary construction. If the condensing lens  30  has a monolithic unitary construction, it can be made from glass or a polymer material, such as acrylic, polycarbonate, or from silicone. In this example, the condensing lens  30  has a substrate  60  onto which is installed a layer of a Fresnel microstructure  68 . The substrate  60  is made of glass which has excellent transmissivity, stability, ability to withstand decades of intense solar, especially ultraviolet (UV) radiation, and can also withstand environmental factors, such as extreme temperatures and hail, although the substrate can be made of other types of materials. By way of example only, the substrate  60  can also be made from a film material, such as PET, PEN, PC, or acrylic by way of example. The Fresnel microstructure  68  is made of silicone which is also highly transmissive to the range of solar wavelengths that the photovoltaic cells  36 A- 36 C are responsive to, although the microstructure can be made other types of materials. By way of example only, the Fresnel microstructure  68  can be a UV curable resin installed in a roll-to-roll process. 
         [0039]    The Fresnel microstructure  68  has a series of triangular grooves having slope surfaces  66  and draft surfaces  64 . The slope surfaces  66  which perform the work of optically bending the incident solar energy  22  are designed so the focal length of the condensing lens  30  is approximately twice the cavity depth D (shown in  FIG. 3 ) for the short wavelength band of solar radiation that photovoltaic cell  36 A is responsive to (the focal length of the condensing lens  30  varies with wavelength because of the dispersion of the material comprising the Fresnel microstructure  68 ). In this example, the condensing lens  30  and the reflector assembly  32  comprise circularly symmetric optical elements, such as 
         [0040]    Fresnel surfaces, whose optical axis is substantially collinear with the optical axis  1  of the concentrator. 
         [0041]    Referring to  FIG. 5 , a magnified view of a small section  37  of the reflector assembly  32  shown in  FIG. 3  is illustrated. The reflector assembly  32  comprises layers  201 - 206 , although the assembly can have other types and numbers of layers. In this example, the lowermost layer  201  is a substrate layer substantially planar on each side and is made of a substantially rigid material such as glass, although it can be made in other manners, such as thin and flexible, and made of other materials, such as a sheet of polymer film. The next layer  202  has a Fresnel microstructure  50  and an adhesive-encapsulant  51  that are separated by a reflective layer  48 C, although this layer could have other types and numbers of parts and layers. The next layer  203  also is substantially planar on each side and is made of a substantially rigid material such as glass, although it can be made in other manners, such as thin and flexible, and made of other materials, such as a sheet of polymer film. The next layer  204  has a Fresnel microstructure  53  and an adhesive-encapsulant  54  that are separated by a reflective filtering layer  48 B, although this layer could have other types and numbers of parts and layers. The Fresnel microstructure  53  generally has a different optical prescription than Fresnel microstructure  50 . The next layer  205  also is substantially planar on each side and is made of a substantially rigid material such as glass, although it can be made in other manners, such as thin and flexible, and made of other materials, such as a sheet of polymer film. The material of layer  205  also is substantially planar on each side and is made of a substantially rigid material such as glass, although it may be different than the material used in layers  201  and  203 . Finally, the uppermost surface  206  of layer  205  has a reflective filtering layer  49  deposited onto it, although other types and numbers of layers could be deposited. 
         [0042]    The layers  48 C,  48 B, and  49  will now be described with reference to  FIGS. 6A-6C . The graphs shown in  FIGS. 6A-6C  plot the reflectivity of each of the layers  48 C,  48 B, and  49  over the range of wavelengths that are being concentrated. Each of the layers  48 C,  48 B, and  49  reflects a limited wavelength band, such that the light that is reflected is concentrated and focused onto one of the photovoltaic cells  36 A- 36 C that is most sensitive to that band of wavelengths. The reflective filtering layer  49  has the reflectance illustrated in  FIG. 6A , and reflects wavelengths less than 600 nm and transmits all others. Light reflected from the reflective filtering layer  49  can be concentrated onto a photovoltaic cell having spectral responsivity between 350 nm and 600 nm, such as InGaP. 
         [0043]    In this example, light of a wavelength greater than 600 nm is transmitted through the reflective filtering layer  49  and is incident on the reflective filtering layer  48 B that has the spectral reflectance as shown in  FIG. 6B , although other wavelength ranges could be used. The reflective filtering layer  48 B is transmissive to wavelengths greater than about 900 nm and is reflective to wavelengths between about 600 nm and about 900 nm. The reflective filtering layer  48 B also is transmissive to light at wavelengths less than about 600 nm, although the reflective filtering layer  48 B also could be reflective or even partially reflective as there is essentially no light reaching the reflective filtering layer  48 B in these wavelengths as they are all being reflected by the reflective filtering layer  49 . Additionally, other wavelength ranges could be used. Light reflected from the reflective filtering layer  48 B in the band from about 600 nm to about 900 nm would be concentrated onto a photovoltaic cell having spectral responsivity between about 600 nm and about 900 nm, such as GaAs, although other wavelength ranges could be used. 
         [0044]    Light of a wavelength greater than about 900 nm is transmitted through the reflective filtering layer  49  and the reflective filtering layer  48 B and is incident on the reflective layer  48 C that has the spectral reflectance as shown in  FIG. 6C  although other wavelength ranges could be used. The reflective layer  48 C is reflective to wavelengths greater than about 900 nm and is transmissive to light at wavelengths less than about 900 nm, although the reflective layer  48 C also could be reflective or even partially reflective as there is essentially no light reaching the reflective layer  48 C in these wavelengths as they are all being reflected by the reflective filtering layer  49  and the reflective filtering layer  48 B. Additionally, other wavelength ranges could be used. Light reflected from the reflective layer  48 C reflecting light in the band from about 900 nm to about 1800 nm could be concentrated onto a photovoltaic cell having spectral responsivity between about 900 nm and about 1800 nm, such as Germanium, although other wavelength ranges could be used. The reflectance wavelength bands shown in  FIGS. 6A-6C  are for illustration purposes only. The bands wavelengths may vary in accordance with the spectral characteristics of the photovoltaic cells used in the solar conversion apparatus  20 . By way of example, a table of materials for photovoltaic cells with their respective bandgaps and operating wavelength bands which can be used is illustrated in  FIG. 7 . 
         [0045]    Referring back to  FIG. 5 , the upper surface  206  of layer  205  is a substantially planar surface with no optical power and the reflective filtering layer  49  constitutes a flat mirror that is reflective to the band of wavelengths as described above in connection to  FIG. 6A . If a line  148  is drawn perpendicular to the upper surface  206  of layer  205  at any arbitrary location on the plain of the reflective filtering layer  49 , then an incoming white light ray  24  makes an angle of incidence Φ 1  with respect to the perpendicular line  148 . This light ray  24  follows the law of reflection and reflects from the reflective filtering layer  49  at an angle Φ 2 =Φ 1  into light ray  26 A for the wavelengths that the reflective filtering layer  49  are reflective to. 
         [0046]    The two reflective layers  48 B and  48 C are internal to the reflector assembly  32 . The reflective filtering layer  48 B is installed onto the Fresnel microstructure  53  resulting in a Fresnel mirror that is reflective only to the band of wavelengths as described above in connection to  FIG. 6B . The reflective filtering layer  48 B will cause incident light rays  24  having wavelengths that are transmitted through reflective filtering layer  49  to come to a focus on a photovoltaic cell  36 B whose location on the optical axis  1  is determined by the focal length of the condensing lens  30  and the focal length of the reflecting microstructure  53  in layer  204 . 
         [0047]    The reflective layer  48 C is installed onto the Fresnel microstructure  50  resulting in a Fresnel mirror that is reflective only to the band of wavelengths as described above in connection to  FIG. 6C . The reflective layer  48 C will cause incident light rays  24  having wavelengths that are transmitted through reflective filtering layer  49  and reflective filtering layer  48 B to come to a focus on a photovoltaic cell  36 C whose location on the optical axis  1  is determined by the focal length of the condensing lens  30  and the focal length of the reflecting microstructure  50  in layer  202 . Accordingly, as illustrated and described herein, the reflector assembly  32  with the Fresnel microstructures  50  and  53  results in a solar conversion apparatus  20  with considerable performance and economic advantage over other solar conversion apparatuses using other types of reflective optics. 
         [0048]    Referring back to  FIG. 3 , the solar conversion apparatus  20  includes the photovoltaic cells  36 A- 36 C, although the apparatus could include other numbers and types of solar conversion cells. In this example, the photovoltaic cell  36 A is responsive to short-wavelength solar light, such as in the range of from 350 nm to 650 nm, the photovoltaic cell  36 B is responsive to an intermediate band of wavelengths such as in the range of from 650 nm to 900 nm, and the photovoltaic cell  36 C is most responsive to long-wavelength solar energy, such as in the range from 900 nm to 1800 nm. All of the photovoltaic cells  36 A- 36 C are located substantially on the optical axis  1  with the photovoltaic cell  36 A located at or near the location of an condensing lens  30 , although the photovoltaic cells could have other orientations, such as off axis. 
         [0049]    The photovoltaic cells  36 A- 36 C can be made from a wide variety photovoltaic cell materials and alloys. By way of example only, graphs in  FIG. 8A-8E  show the responsivity of a few of single junction photovoltaic cells while  FIGS. 8F and 8G  show the responsivity of a couple of double junction photovoltaic cells. Additionally, the table illustrated in  FIG. 7  provides an additional exemplary listing of materials that can comprise a single junction photovoltaic cell, their bandgap energies, and usable wavelength ranges. The photovoltaic cells  36 A- 36 C are selected to, in sum, cover the usable wavelength ranges for solar energy from about 350 nm to about 1800 nm as illustrated in FIG.  9 , although as explained in greater detail below other numbers of photovoltaic cells can be used. 
         [0050]    The solar apparatus conversion system  20  economically and efficiently separates the solar energy into three discrete wavelength groupings and directs each group of concentrated solar energy onto the particular photovoltaic cells  36 A- 36 C that is optimal for the wavelengths that are directed to it. As illustrated in  FIG. 10 , as the number of discrete bands the solar conversion apparatus  20  separates the solar energy into increases, the conversion efficiency increases. By way of example only, a solar conversion apparatus which separates the solar energy into four bands for capture by four correspondingly selected photovoltaic cells could achieve about 60% efficiency while a solar conversion apparatus which separates the solar energy into ten bands could achieve nearly 70% conversion efficiency at a 50× concentration ratio. 
         [0051]    Referring back to  FIG. 3 , the solar conversion apparatus  20  also comprises the rear bulkhead surface  34 . The reflector assembly  32  is located on the rear bulkhead assembly  34 . A mechanical mounting assembly retains the position of these photovoltaic cells  36 A,  36 B, and  36 C along an optical axis  1  between the condensing lens  30  and the reflector assembly  32 , although other manners for securing the position of the photovoltaic cells, condensing lens, and reflector assembly and in other configurations can be used. 
         [0052]    The operation of the solar conversion apparatus  20  will now be described with reference to  FIGS. 3-5 . The solar conversion apparatus  20  is exposed to solar radiation  22  that is being concentrated. This solar radiation  22  is polychromatic and for purposes of this discussion comprises three individual rays of solar radiation  22  having wavelength groups λ A , λ B , and λ C , that represent typical wavelength ranges that photovoltaic cells  36 A,  36 B, and  36 C, respectively, are responsive to and are reflected by reflective layers  49 ,  48 B, and  48 C respectively. Wavelength groupings λ A , λ B , and λ C  are generally non-overlapping yet together substantially span the solar radiation spectra as shown in  FIG. 9 . In this example, wavelength group λ A  includes wavelengths between about 300 nm and about 600 nm, λ B  includes wavelengths between about 600 nm and about 900 nm, and λ C  includes wavelengths between about 900 nm and about 1800 nm. 
         [0053]    The condensing lens  30  causes any of the incident solar radiation  22  to converge. These converging rays, such as converging white light ray  24  (which contains all wavelengths of groupings λ A , λ B , and λ C ), are incident on the reflective filtering layer  49  of the reflector assembly  32 . Due to the reflectance characteristics of the reflective filtering layer  49 , light rays  26 A of wavelength group λ A  are reflected in accordance with the Law of Reflection, and all other wavelength groups (λ B  and λ C ) are transmitted into the reflector assembly  32  in accordance with Snells Law. The prescription of the condensing lens  30  is such that the light rays  26 A of wavelength group λ A  are brought to a focus on the photovoltaic cell  36 A. If the location of the photovoltaic cell  36 A is such that it is coplanar with the condensing lens  30 , then the focal length of the condensing lens  30  must be approximately twice the distance between condensing lens  30  and the reflective filtering layer  49 , which is 2×D. The photovoltaic cell  36 A is selected to be highly responsive to wavelength group λ A  of incident rays  26 A and converts the incident solar energy of these rays into electricity with very high efficiency. 
         [0054]    After passing through reflective filtering layer  49  and refracting into the reflector assembly  32 , wavelength group λ B  propagates through layer  205  and into layer  204  where it becomes incident on reflective filtering layer  48 B. Reflective filtering layer  48 B is installed onto the Fresnel microstructure  53  and therefore cooperatively forms a Fresnel mirror. Additionally, reflective filtering layer  48 B in accordance with its spectral reflectance profile shown in  FIG. 6B  is reflective to wavelength group λ B . Therefore, as shown in the close-up view in  FIG. 5 , the light rays  26 B of wavelength group λ B  reflect off the slope surfaces of the Fresnel microstructure  53  within layer  204 . Furthermore, the Fresnel mirror within layer  204  has optical power to wavelength group groupings λ B  such that the output angle θ 2  is not equal to the input angle θ 1  at the upper surface  206  of the reflector assembly  32 . This means that the focal position of the exiting light rays  26 B having wavelength group λ B  will not be at the location of the photovoltaic cell  36 A, but instead are brought to a focus on photovoltaic cell  36 B with little or no interference with the operation of photovoltaic cell  36 A. The photovoltaic cell  36 B is selected to be highly responsive to wavelength group λ B  of incident rays  26 B and converts the incident solar energy of these rays into electricity with very high efficiency. 
         [0055]    After passing through reflective filtering layer  49  and refracting into the reflector assembly  32 , wavelength group λ C  propagates through layers  205 ,  204 ,  203  and into layer  202  whereupon it becomes incident on reflective layer  48 C. Reflective filtering layer  48 B is not reflective to wavelength group λ C  and these rays pass through reflective filtering layer  48 B substantially undeviated in direction. Additionally, the reflective layer  48 C is installed onto the Fresnel microstructure  50  and therefore cooperatively forms a Fresnel mirror. The reflective layer  48 C in accordance with its spectral reflectance profile shown in  FIG. 6C  is reflective to wavelength group λ C . Therefore, as shown in the close-up view in  FIG. 5 , the light rays  26 C of wavelength group λ C  reflect off the slope surfaces of the Fresnel microstructure  50  within layer  202 . Furthermore, the Fresnel mirror within layer  202  has optical power to light rays  26 C comprising wavelength group λ C  such that the output angle θ 2  is not equal to the input angle θ 1  at the surface  206  of the reflector assembly  32 . Furthermore, light rays  26 C exit the reflector assembly  32  at a more aggressive converging rate (i.e., faster f/#) than the other two light ray groupings λ A  and λ B . This means that the focal position of the exiting rays  26 C having wavelength group λ C  will not be at the location of the photovoltaic cell  36 A or the photovoltaic cell  36 B, but instead are brought to a focus on photovoltaic cell  36 C with little or no interference with the operation of photovoltaic cell  36 A or photovoltaic cell  36 B. The photovoltaic cell  36 C is selected to be highly responsive to wavelength group λ C  of light ray  26 C and converts the solar energy of these rays into electricity with very high efficiency. 
         [0056]    Accordingly, as illustrated and described herein, the solar conversion apparatus  20  offers a considerable performance and economic advantage over prior art single junction solar concentrators and triple junction tandem photovoltaic cells. Additionally, although the solar conversion apparatus  20  is illustrated with three photovoltaic cells  36 A- 36 C, the solar conversion apparatus can have additional photovoltaic cells with improved conversion efficiency as illustrated in  FIG. 10 . 
         [0057]    An exemplary method for constructing the condensing Fresnel lens  30  will now be described with reference to  FIG. 4 . A sheet, plate, or film of material that is substantially flat on both its upper and lower sides is provided that serves as the substrate  60  for the condensing Fresnel lens  30 . This substrate is made from glass and ranges from about 0.1 mm to about 10 mm thick, although other types of materials, such as a polymer, and other thicknesses can be used. Since the condensing Fresnel lens  30  needs to be self-supporting and able to withstand a variety of environmental stresses, the substrate  60  is generally made from glass that is between about 2 mm and about 5 mm thick. The input side  62  of the substrate  60  is treated with an A/R coating to reduce unwanted Fresnel reflections at the input surface  62 , although other manners for reducing reflections can be used, such as a subwavelength microstructure formed on the input side  62  of the substrate  60 . 
         [0058]    A Fresnel microstructure  68  is installed on the lower side of the condensing Fresnel lens  30 . The microstructure  68  comprises a polymer material, such as a UV-cured resin, although other types of materials can be used, such as silicone which has transmittance over the entire 350 nm to 18900 nm solar insolation range and it is relatively immune to UV damage from the solar UV light. The prescription of the slope surfaces  66  of the Fresnel microstructure is formed so that it results in a focal length of the condensing Fresnel  30  of 2D for the shorter wavelength band (i.e., λ A ). Longer wavelengths will generally see a longer focal length because the refractive index of the material comprising the microstructure  68  is lower at the longer wavelengths because of the materials dispersion. 
         [0059]    An exemplary method for constructing and assembling the reflector assembly  32  will now be described with reference to  FIG. 11A-11F . In this example, the Fresnel microstructure  50  is formed on a substrate layer  201  resulting in the object shown in  FIG. 11A . The microstructure  50  is a UV-cured resin, although the microstructure can be made of other types of materials, such as a silicone material. Additionally, the substrate layer  201  is glass, although the layer can be made of other types of materials, such as a polymer. The microstructure  50  is installed onto the layer  201  in a cell-cast or other type of casting process, although other methods can be used. For example, layer  201  and microstructure  50  can be formed as a unitary object using a molding process, such as injection molding, compression molding, or injection-compression molding. 
         [0060]    Next, a specularly-reflecting reflective coating layer  48 C is applied to the slope surfaces of the microstructure  50 , resulting in the lower reflecting Fresnel  61 C shown in  FIG. 11B . The reflective coating layer  48 C can be applied to the draft surfaces of the microstructure  48 C, but this is of little consequence because the draft surfaces are substantially unused in the system, and it is preferred that the draft surfaces are left uncoated. The reflecting layer  48 C is metallic, such as gold, silver, or aluminum by way of example only, or an interference stack of thin films that reflects the desired band of wavelengths, in this example for solar conversion apparatus  20  the wavelengths band λ C . 
         [0061]    In addition to the lower reflecting Fresnel  61 C, a reflective filtering Fresnel  61 B also is prepared in a process similar or identical to the process described above for the lower reflecting Fresnel  61 C. After both the lower reflecting Fresnel  61 C and the middle reflective filtering Fresnel  61 B are available, they must be bonded together. As shown in  FIG. 11C , a layer of encapsulant adhesive  51  in a liquid form and that can act as an adhesive when hardened is applied atop the coated microstructure  50  and reflecting layer  48 C. The rear surface of the reflective filtering Fresnel  61 B is then brought into contact with the encapsulant adhesive  51 , and gently compressed to squeeze out any excess encapsulant adhesive  51 . 
         [0062]    The encapsulant adhesive  51  is allowed to cure, dry, or otherwise harden resulting in the assembly depicted in  FIG. 11D . The encapsulant adhesive  51  is an adhesive and is the same material that is used to form the Fresnel microstructure  50  so that the properties of the material are the same on both sides of the reflecting layer  48 C, although other types of adhesives and materials can be used. This will ensure that light rays that are transmitted through the reflecting layer  48 C do not bend or otherwise refract as they cross the interface between the encapsulant adhesive layer  51  and the microstructure  50  because it ensures that the refractive indices of the two materials are the same for all transmitted wavelengths. 
         [0063]    Next, the reflector assembly portion  61 A comprising a substrate layer  205  and reflective filtering layer  49  are prepared. Both the upper and lower sides of the substrate upper layer  205  are planar and the substrate upper layer  205  is made from polymer, although other types of materials can be used, such as glass. The reflective filtering layer  49  is an interference stack of thin films that reflects the desired band of wavelengths (i.e., λ A ). After both reflecting Fresnel assembly portions  61 C and  61 B and reflector assembly portion  61 A are available, they must be bonded together. As shown in  FIG. 11E , a layer of transparent material  54  in a liquid form and that can act as an adhesive when hardened is applied atop the coated microstructure  53  and reflective filtering layer  48 B. The rear surface of the upper reflector assembly portion  61 A is then brought into contact with the transparent encapsulant adhesive  54  and gently compressed to squeeze out any excess encapsulant adhesive  54 , although other manners for joining the portions with other adhesives can be used. 
         [0064]    The transparent encapsulant adhesive  54  is then allowed to cure, dry, or otherwise harden, resulting in the reflector assembly  32  depicted in  FIG. 11F . The transparent encapsulant adhesive  54  is an adhesive and is the same material that is used to form the Fresnel microstructure  53  so that the properties of the material are the same on both sides of the reflective filtering layer  48 B, although other types of adhesives and materials can be used. It is important that the optical properties of the materials be the same on both sides of the reflective filtering layer  48 B because if they are not the same, for example if they have different refractive indices or dispersion, then the difference in refractive index will cause refraction to occur as light rays (i.e., of wavelength band  4 ) pass through the reflective filtering layer  48 B. That is the microstructure  53  will then have optical power and act as a lens and the light rays will not pass through the interface at the reflective and filtering layer  48 B unchanged in direction. This will compromise the optical concentration performance of the solar conversion apparatus  20 . 
         [0065]    In other examples, the solar conversion apparatus assembly process can be streamlined if, instead of having two optically active devices (the condensing lens  30  and the reflector assembly  32 ), there were only one. This can be accomplished by dispensing with the condensing lens  30  and by installing an additional reflecting Fresnel mirror within the reflector assembly. 
         [0066]    Referring to  FIGS. 12-13 , another exemplary solar conversion apparatus  70  with this streamlined configuration is illustrated. The solar conversion apparatus  70  illustrated in  FIGS. 12-13  is the same in structure and operation as the solar conversion apparatus  20  shown in  FIGS. 3-5  except as described and illustrated herein. In the solar conversion apparatus  70 , the condensing lens  30  is replaced with a flat plate  71  that is substantially transparent to all wavelengths that the photovoltaic cells  36 A,  36 B, and  36 C are responsive to. Solar radiation  22  passes through the flat plate  71  substantially unchanged in direction, and travel all the way through the concentrator  70  to the reflector assembly  72 . 
         [0067]    Referring more specifically to  FIG. 13 , an enlarged view of a small section  77  of the reflector assembly  72  is shown. The reflector assembly  72  is the same in structure and operation as the reflector assembly  32 , except as illustrated and described herein. In the reflector assembly  72 , the reflective filtering layer  49  has been eliminated from reflector assembly  72  and optionally replaced with an A/R treatment at the upper surface  79  of the reflector assembly  72 . Additionally, instead of the reflector assembly  72  having two internal Fresnel mirrors as shown with the reflector assembly  32 , there are three Fresnel mirrors  78 A,  78 B, and  78 C. The three Fresnel mirrors  78 A- 78 C each have different optical power and are coated with different reflecting filters to reflect specific bands of wavelengths as previously described in the corresponding embodiment of the solar conversion apparatus  20 . 
         [0068]    In operation Fresnel mirror  78 A reflects and focuses its band of wavelengths (e.g., λ A ) onto photovoltaic cell  36 A and transmits all others (e.g., λ B  and λ C ) substantially undeviated. Fresnel mirror  78 B reflects and focuses its band of wavelengths (e.g., λ B ). onto photovoltaic cell  36 B and transmits all others (e.g., λ C ) substantially undeviated. Fresnel mirror  78 C reflects and focuses all remaining wavelengths (e.g., λ C ) onto photovoltaic cell  36 C. With this configuration for the solar conversion apparatus  70 , the size of the reflector assembly  72  must be increased to fill the entire rear bulkhead surface  34  due to the absence of a condensing lens  30 . 
         [0069]    One problem that is common to the embodiments described thus far has to do with the placement of the photovoltaic cells  36 A- 36 C on the optical axis  1 . This gives rise to the shadow-loss problem wherein a portion of the light that would be incident on an upper photovoltaic cell, such as photovoltaic cell  36 A is blocked by a lower photovoltaic cell such as photovoltaic cell  36 B. In other words photovoltaic cell  36 A is partly shadowed by lower photovoltaic cell  36 B. Accordingly, to overcome the shadow losses it is necessary to install the photovoltaic cells in an off-axis location outside the cone of converging rays. 
         [0070]    The side-view of one such off-axis solar conversion apparatus  80  is shown in  FIG. 14  and a plan view of this apparatus  80  is shown in  FIG. 15 . The solar conversion apparatus  80  is the same as the solar conversion apparatus  20 , except as described and illustrated herein. This solar conversion apparatus  80  comprises a condensing lens  82 , an internal bulkhead  94  having apertures  96 , four reflector assemblies  86 ,  87 ,  88 , and  89  mounted onto a rear bulkhead  34 , and four different types of photovoltaic cells,  90 A,  90 B,  90 C, and  90 D. 
         [0071]    The reflector assemblies  86 - 89  each comprise four reflective filtering Fresnels installed as described earlier that split and reflect the incident converging solar energy  84  into four groups of light  92 A,  92 B,  92 C, and  92 D (each containing only a limited band of wavelengths) and focus the four groups of light  92 A- 92 D onto the four different photovoltaic cells  90 A- 90 D, respectively that are most responsive to the wavelengths of light incident directed onto them. 
         [0072]    The four different types of photovoltaic cells  90 A,  90 B,  90 C, and  90 D are mounted on the internal bulkhead  94 . Additionally, the four photovoltaic cells  90 A,  90 B,  90 C, and  90 D are located away from the optical axis  1  and between the converging rays  84  so that there are no shadowing effects that reduce system efficiency. The four different types of photovoltaic cells  90 A,  90 B,  90 C, and  90 D and are selected to be responsive to four different wavelength bands of light spread across the solar energy spectrum from about 350 nm to about 1800 nm, although other numbers of photovoltaic cells responsive to other bands of wavelengths can be used. As illustrated in  FIG. 10 , increasing the number of bands and photovoltaic cells in the solar conversion apparatus  80  will increase performance and efficiency. Accordingly, the solar conversion apparatus  80  can be constructed with other numbers of photovoltaic cells and reflector assemblies, such as six of each to increase performance and efficiency, but with greater manufacturing complexity. 
         [0073]    With this solar conversion apparatus  80 , the photovoltaic cells  90 A,  90 B,  90 C, and  90 D are located where the corners of several concentrators meet so one photovoltaic cell can collect light of its wavelength band from four different concentrators. As a result, the number of photovoltaic cells in solar conversion apparatus  80  has been reduced by 75%. This is particularly evident in the plan view shown in  FIG. 15 , where by way of example photovoltaic cell  90 C (on the middle-left) receives four groups of light  92 C that it is responsive to from four different concentrators (having reflector assemblies  87 ,  89 ,  88 , and  86 ). 
         [0074]    In operation, the solar conversion apparatus  80  accepts solar radiation  22  that is incident on the condensing lens  82  which condenses the solar radiation into converging cones of light  84 . The converging cones of light pass through apertures  96  in the internal bulkhead  94  and critically illuminates the reflector assemblies  86 ,  87 ,  88 , and  89 . The reflector assemblies  86 ,  87 ,  88 , and  89  each comprise a different Fresnel microstructure which is used to reflect light towards the corresponding one of the photovoltaic cells  90 A,  90 B,  90 C, and  90 D responsive to the reflected band of wavelengths of the solar energy. 
         [0075]    Referring to  FIG. 16 , another embodiment of a solar conversion apparatus  170  is illustrated. The solar conversion apparatus  170  is the same as solar conversion apparatus  80 , except as illustrated and described herein. The solar conversion apparatus  170  also uses four photovoltaic cells arranged in the same lateral configuration as taught with solar conversion apparatus  80 , however solar conversion apparatus  170  does not use an upper condensing Fresnel lens. Instead, solar radiation  22  is directly incident on the reflector assemblies  172  and  173  that separates the incident solar radiation into four distinct groups  174 A and  174 B (groups  174 C and  174 D are not shown) by their wavelengths, and focus these groups  174 A and  174 B onto their respective photovoltaic cells  176 A and  176 B. The photovoltaic cells  176 A and  176 B are matched to the bands of wavelengths (i.e., the photovoltaic cells have high responsivity to the wavelengths contained in the incident light) of the light groups  174 A and  174 B, respectively, that are focused onto them so the light is converted to electricity by the photovoltaic cells  176 A and  176 B with high efficiency. The four photovoltaic cells can be located at each of the four corners of the solar concentrator  170  in the same manner as illustrated with solar conversion apparatus  80 , although other configurations could be used, such as along one or more sides. If several concentrators  170  are arranged in an array, placing the photovoltaic cells at the corners of the concentrators allow for the photovoltaic cells to be shared amongst the concentrators, thereby allowing for a reduction in the total number of photovoltaic cells as previously illustrated and described with reference to  FIGS. 14-15 . 
         [0076]    Eliminating the upper condensing Fresnel lens from the solar conversion apparatus offers several advantages, including: 1) the cost of the condensing lens is eliminated; 2) the Fresnel reflection losses at the input and output surfaces are eliminated thereby increasing efficiency, and 3) the molds for the reflecting mirrors of the microstructure of the reflector assembly  173  are circularly symmetric and easier to tool and fabricate, thereby reducing the costs associated with the reflector assembly as compared to the solar conversion apparatus  80  shown in  FIGS. 14 and 15 . 
         [0077]    A magnified view of a small section  177  of reflector assembly  173  is shown in  FIG. 17 . As seen in the magnified view of the small section  177 , the reflector assembly is made up of four Fresnel mirrors comprising microstructures  191 ,  190 ,  194 , and  195  in layers  187 ,  185 ,  183 , and  181 , respectively. Layers  188 ,  186 ,  184 ,  182 , and  180  are substrate layers made of glass, although other types of materials that support and add rigidity to the microstructure layers, such as a polymer can be used. As in other embodiments, the slope surfaces of the microstructures  191 ,  190 ,  194 , and  195  are coated with reflectors, such as an interference stack, such that the slopes are reflective to the band of wavelengths that their corresponding photovoltaic cell is most responsive to and that their respective slope surfaces are directing the light onto. 
         [0078]    The encapsulating adhesive layers  192  and  193  are used to secure the layers in the same manner as described with earlier examples. The microstructures  194  and  195  in layers  183  and  181  go from side-to-side in this view, and are represented by dashed lines. The encapsulating adhesive, while also present in layers  181  and  183 , are not explicitly shown from this view. 
         [0079]    While four Fresnel mirrors and four types of photovoltaic cells are described as being used in solar conversion apparatus  170 , a lower number, such as one, two, or three, can be used, or a higher number, such as six, can be used. Additionally, the photovoltaic cells can be single junction cells or multi junction type photovoltaic cells. 
         [0080]    The operation of the solar conversion apparatus  170  is the same as the operation of the solar conversion apparatus  80 , except that with the solar conversion apparatus  170  there is no condensing lens that accepts and condenses the solar radiation into converging cones of light. Instead, the solar radiation passes directly through to the microstructures  191 ,  190 ,  194 , and  195  in layers  187 ,  185 ,  183 , and  181  and is correspondingly reflected in bands to the laterally arranged photovoltaic cells with the appropriate responsivity to the reflected band of wavelengths. 
         [0081]    Referring to  FIG. 18 , each of the solar conversion apparatuses also can be mounted on to a heliostat  119  to keep each of the solar conversion apparatuses pointing at the sun, although other manners for managing the positioning of the solar conversion apparatuses can be used. This particular example illustrates the solar conversion apparatus  80  mounted on the heliostat  119  with the general location of the internal bulkhead  94 , the reflector assemblies  87 , the condensing Fresnels  82 , and the various photovoltaic cells  90 A,  90 C, and  90 D also illustrated. The heliostat  119  comprises a base  122  which includes a motor (not shown) for rotating a post  120  connected between the base  122  and the solar conversion apparatus  80 . The heliostat  119  also includes a second motor (not shown) that is attached to the post  120  and the array  124 , and allows for tip-tilt pointing of the solar conversion apparatus  80 . The rotational and tip-tilt angular control of the heliostat meets all the angular positioning requirements of the array  124  of concentrators  80 . 
         [0082]    Accordingly, as illustrated and described herein this technology provides a number of advantages, including providing a more efficient, better performing, and economical solar conversion apparatus. This technology is able to avoid prior problems with large focal spot sizes and the use of a large and expensive, multi junction photovoltaic cell by utilizing a lower reflector assembly comprising one or more Fresnel reflectors arranged in a cascade configuration. Each of these Fresnel reflectors is reflective to a selected band of wavelengths and is transmissive to other wavelengths that are in turn reflected by lower Fresnel reflectors. Additionally, each Fresnel reflector includes a microstructure that reflects and brings to a focus onto a photovoltaic cell a selected band of wavelengths that the photovoltaic cell is most responsive to. The resulting solar conversion apparatus has a high concentration ratio, is lossless over the range of wavelengths emitted by the sun that have significant energy content, and effectively directs the concentrated solar energy to the appropriate single or multi junction photovoltaic cell. 
         [0083]    Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Further, the recited order of elements, steps or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be explicitly specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.