Abstract:
A solar power system concurrently generates electricity and a heated transparent fluid while maintaining the solar cells at an optimum temperature and optimizing the heat transfer by matching the refractive index of the secondary sunlight concentrator to the transparent fluid. A solar tracker aligns a primary sunlight concentrator to collect sunlight and directs the sunlight and a system for transferring solar heat to a transparent fluid and into a solar power electrical generating system. The concentrated sunlight transfers solar heat to a transparent fluid via first pass through the transparent fluid. The concentrated sunlight is further concentrated to raise its temperature by passing the concentrated sunlight through a secondary sunlight concentrator, and then passed again through the transparent fluid to transfer heat. The solar energy diminished concentrated sunlight strikes a solar cell array to generate electricity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/584,050 filed Aug. 27, 2009 titled CONCENTRATED PHOTOVOLTAIC AND SOLAR HEATING SYSTEM, and is related to U.S. patent application Ser. No. 12/584,052 titled “LOW NUMERICAL APERTURE (LOW-NA) SOLAR LIGHTING SYSTEM,” filed Aug. 27, 2009 and is related to U.S. patent application Ser. No. 12/584,051 titled “GENERATING ALTERNATING CURRENT FROM CONCENTRATED SUNLIGHT” filed Aug. 27, 2009 all of which are incorporated by reference. These patent applications claim the benefit of priority of U.S. Provisional Application No. 61/094,113 titled “One-axis tracking concentrating photovoltaic and solar hot water hybrid system”, U.S. Provisional Application No. 61/094,115 titled “Alternating current electricity generation from concentrated sunlight”, U.S. Provisional Application No. 61/094,120 titled “Solar lighting system with one-axis tracking”, and U.S. Provisional Application No. 61/094,117 titled “Low Numerical Aperture (Low-NA) Solar Lighting System”, all filed Sep. 4, 2008 and all of which are incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of Invention 
     This invention relates to the field of solar energy and specifically for using concentrated sunlight for the concurrent generation of electricity within the same system as heating of a fluid for heating applications. 
     2. Related Art 
     Typical solar energy systems generate electrical power by either the direct conversion of concentrated or unconcentrated sunlight using solar cells (concentrated photovoltaic, CPV), or by using concentrated solar thermal (CST) energy to generate a pressurized vapor for turning a turbine-generator. 
     Concentrated photovoltaic (CPV) systems have a moderate efficiency of about 40% under a concentration of 500 suns and at an ambient temperature of 25 degrees C. The solar cells are sensitive to temperature, however, so that the efficiency drops to about 35% at about 100 degrees C., which highly concentrated sunlight is capable of achieving as shown in the use of concentrated sunlight to boil water for evaporation systems. In addition, concentrated photovoltaic (CPV) systems need a two-axis solar tracking and are expensive. As such, the return of investment period for localized installations is many years. 
     Concentrated solar thermal (CST) energy systems, on the other hand, can and must operate at a high temperature, and may reach a thermal efficiency of 60-80%. Collector to grid energy conversion losses, however, lowers the overall efficiency to about 15%. In addition, turbine-generator systems have inherent safety issues and are high maintenance, which raises the cost of the delivered power. As such, concentrated solar thermal (CST) energy systems are not suitable for localized installations. 
     SUMMARY OF THE INVENTION 
     Systems and methods provide for the solar generation of electricity and transferring solar heat to a transparent fluid with a solar power generation system having a primary sunlight concentrator and a secondary sunlight concentrator with a refractive index matched to the refractive index of the transparent fluid and using the transparent fluid to maintain the solar cell array at an optimum temperature. 
     A solar tracking system aligns the primary sunlight concentrator towards the sun for concentrating sunlight and directs the concentrated sunlight into a solar power generating unit. The solar tracking system may be a one-axis azimuth tracking system or a two-axis system. 
     A transparent fluid is heated by passing concentrated sunlight through the transparent fluid, then further concentrating the sunlight and passing the further concentrated sunlight through the transparent fluid a second time. The transparent fluid also absorbs some the ultraviolet light that is harmful to some solar cells. The concentrated sunlight then strikes a solar cell array to generate electricity, which generates more heat. The heat is removed from the solar cell by the transparent fluid. In some embodiments the transparent fluid may be pumped through the solar power generating unit. In some embodiments the transparent fluid may be in convection motion through the solar power generating unit. 
     Different embodiments provide for using different concentrator systems. A concentrator system may have a parabolic trough primary concentrator with a compound parabolic secondary sunlight concentrator. A concentrator system may have a Fresnel lens primary concentrator with a compound parabolic secondary sunlight concentrator. 
     The solar power generation unit may be within a single transparent containment system suspended above a second solar cell array for capturing sunlight that misses the solar power generation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Elements in the figures are illustrated for simplicity and clarity and are not drawn to scale. The dimensions of some of the elements may be exaggerated relative to other elements to help improve the understanding of various embodiments of the invention. 
         FIG. 1  shows an embodiment of the concentrated photovoltaic and solar heating system the solar generation of electricity and a heated fluid. 
         FIG. 2  shows the internal components of the solar power generation unit. 
         FIG. 3  shows an embodiment of the at least one secondary sunlight concentrator. 
         FIG. 4  shows an alternate embodiment of the concentrated photovoltaic and solar heating system for the solar generation of electricity and a heated fluid. 
         FIG. 5  shows another embodiment of the concentrated photovoltaic and solar heating system. 
         FIG. 6  shows a flowchart of a method for the solar generation of electricity and a heated fluid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an embodiment  100  of the concentrated photovoltaic and solar heating system. The embodiment  100  may comprise a solar tracker  105 , a primary sunlight concentrator  110 , concentrated sunlight  115 , and a solar power generation unit  120 . 
     The solar tracker  105  supports and orients the concentrated photovoltaic and solar heating system  100  towards the sun. The solar tracker  105  may be one-axis azimuth tracking system, a dual-axis azimuth and elevation tracking system, or in some embodiments, the solar tracker  105  may be a stationary system. 
     The primary sunlight concentrator  110  receives incoming sunlight. In some embodiments, the primary sunlight concentrator  110  may be a parabolic trough. On receiving the sunlight, the primary sunlight concentrator  110  concentrates the sunlight and redirects the concentrated sunlight  115  to the solar power generation unit  120 . 
     The solar power generation unit  120  comprises a transparent tube  125 , an electrical power port  130 , a fluid inlet  135 , a fluid outlet  140  and two solar power generation systems. The transparent tube  125  houses the two solar energy conversion systems while permitting the discharge of electrical energy via the electrical power port  130 , and the ingress and egress of a transparent fluid (described in  FIG. 2 ) via the fluid inlet  135 , and the fluid outlet  140  respectively. 
     The transparent tube  125  may be an optical clear glass tube or acrylic tube. An anti-reflection (AR) coating or AR film may be applied to the outer surface of the transparent tube  125  to reduce the reflection loss. 
       FIG. 2  shows the internal components of the solar power generation unit  120 . The solar power generation unit  120  comprises at least one secondary sunlight concentrator  205 , a solar cell array  210 , a solar cell frame  215 , a cooling gap  220  and a transparent fluid  225 . 
     On reaching the solar power generation unit  120 , the concentrated sunlight  115  passes through the wall of the transparent tube  125  and into the transparent fluid  225  within the transparent tube  125 . 
     Flowing through the interior space of the transparent tube  125  is the transparent fluid  225 , which enters the solar power generation unit  120  via the fluid inlet  135 , and exits via the fluid outlet  140 . In some embodiments, the transparent fluid  225  is in active motion through the transparent tube  125  due to a pump (not shown). In some embodiments, the transparent fluid  225  is in passive motion, i.e. via convection, through the transparent tube  125 . 
     On passing through the transparent fluid  225 , the concentrated sunlight  115  loses some of its heat energy to the transparent fluid  225 . The concentrated sunlight  115  then enters the at least one secondary sunlight concentrator  205 . The at least one secondary sunlight concentrator  205  further concentrates the solar energy of the concentrated sunlight  115  to  300  to  500  suns. On passing through the at least one secondary sunlight concentrator  205 , the concentrated sunlight  115  enters the cooling gap  220 , which is filled with the flowing transparent fluid  225 . Due to the additional energy concentration of the at least one secondary sunlight concentrator  205 , the concentrated sunlight  115  is again hotter than the transparent fluid  225 , and transfers this remaining heat to the transparent fluid  225 . The concentrated sunlight  115  then enters the solar cell array  210  and is converted to electrical energy by the solar cell array  210 , which may be affixed in an optimal location of the transparent tube  125  with respect to the at least one secondary sunlight concentrator  205  by the solar cell frame  215 . The transparent fluid  225  removes the heat generated by the concentrated sunlight striking the solar cell array  210  as well as the heat generated by the electrical energy created by the solar cell array  210 . 
     As shown in  FIG. 2 , the transparent fluid  225  flows below, around and above the at least one secondary sunlight concentrator  205  and is heated by the concentrated sunlight  115  and by the solar cell array  210  before exiting the solar power generation unit  120  via the fluid outlet  140  to a heat exchanger system (not shown). The heat exchanger system uses or removes the heat from the transparent fluid  225  for use in heating or in another application. In some embodiments, a fresh supply of transparent fluid  225  is fed from a source to the solar power generation unit  120  via fluid inlet  135 . In some embodiments, the transparent fluid  225  is re-circulated to the solar power generation unit  120  via fluid inlet  135 . 
     An optimal total energy output of the solar power generation unit  120  may be achieved by matching the at least one secondary sunlight concentrator  205  to the transparent fluid  225  and to the solar cell array  210 . While multiple factors are considered, there is an interdependence of these factors to reduce sunlight energy losses and yet achieve the best heat energy transfer and optimize electrical output. 
       FIG. 3  shows an embodiment  300  of the at least one secondary sunlight concentrator  205 . One factor of optimal total energy output of the solar power generation unit  120  is that the energy level of the concentrated sunlight  115  is dependent on the configuration of the at least one secondary sunlight concentrator  205 . In some embodiments, the at least one secondary sunlight concentrator  205  may be a compound parabolic concentrator (CPC). To achieve optimal energy output in the one-axis tracking system, the collecting angle (q sub in) of the secondary sunlight concentrator along line ‘A’ should be greater than 23.5 degrees. This is due to the angle of incidence onto the normal of the CPC, which varies between +23.5 degrees at the summer solstice and −23.5 degrees at the winter solstice. The collecting angle perpendicular to line ‘A’ should be greater than the maximum exit angle of the primary sunlight concentrator  110 . To achieve a concentration ratio of 8 to 12, the exit angle (q sub out) of the CPC should be about 60 to 70 degrees. The concentration ratio for the primary sunlight concentrator  110  is about 30 to 60. Together, the pair of the primary sunlight concentrator  110  and the secondary sunlight concentrator  205  as described below will concentrate the energy level of the incoming sunlight to about 500 times that of un-concentrated sunlight. 
     To further improve electrical output, the secondary sunlight concentrator  205  may be designed to have a total-internal-reflection (TIR) in the transparent fluid  205 . This may be achieved by having the secondary sunlight concentrator  205  made hollow with a material having a refraction index smaller than the refraction index of the transparent fluid  225  with the transparent fluid  225  inside the secondary sunlight concentrator  205 . For example, the refractive index of transparent Teflon FEP is about 1.34 and the refraction index of Teflon AF 2000 is only about 1.29. Thus, hollow transparent Teflon FEP or Teflon AF 2000 may be used as the secondary sunlight concentrator with mineral oil, which has a refractive index of about 1.46 
     A total-internal-reflection (TIR) may also be achieved by having the secondary sunlight concentrator  205  made solid with a material having a refraction index greater than the refraction index of the transparent fluid  225 . For example, the refractive index of acrylic is about 1.49 and the refractive index of Pyrex glass is about 1.47. Consequently, solid acrylic or Pyrex may be used with water, which has a refractive index of about 1.33. 
     Teflon products, however, are more expensive than other higher refractive index materials. To lower the cost of the secondary sunlight concentrator  205 , transparent Teflon FEP or Teflon AF 2000 may be applied as an internal coating to a hollow secondary sunlight concentrator  205 . In such a case, neither the total-internal-reflection (TIR) nor the reflective index of the secondary sunlight concentrator or the transparent fluid needs to be considered. 
     Another factor is that the amount of heat energy absorbed from the concentrated sunlight  115  and the heat generated by the solar cell array  210  is dependent on the thermal conductivity and heat capacity, i.e. the material composition, of the transparent fluid  225 . The thermal conductivity of water is 0.58 W/mK, while the thermal conductivity of mineral oil is 0.138 W/mK, i.e. about ⅕ of that of water. Conversely, the heat capacity of water is 4.19 kJ/kgK, while the heat capacity of mineral oil is 1.67 kJ/kg K. 
     Another factor is that the electrical output of the solar cell array  210  may be temperature dependent. The temperature of the solar cell array is a function of ambient temperature around it, which in turn depends on the flow rate of the transparent fluid  225 , the solar heat of the concentrated sunlight  115  striking the solar cell array  210  and the heat created by the generated electricity. The temperature of the flowing transparent fluid  225  is proportional to the amount of concentrated sunlight  115  passing through the transparent fluid  225  and is inversely proportional to the flow rate of the transparent fluid  225 , i.e. the solar heat absorbed by the transparent fluid  225  is transferred to the outside. Thus, the flow rate of the transparent fluid  225  may be raised or lowered to adjust the temperature of the solar cell array  210  for optimum electrical generation. 
       FIG. 4  shows an alternate embodiment  400  of the concentrated photovoltaic and solar heating system. The embodiment  400  may comprise the solar tracker  105 , an alternate primary sunlight concentrator  405 , the concentrated sunlight  115 , and the solar power generation unit  120 . 
     The solar tracker  105  supports and orients the concentrated photovoltaic and solar heating system  400  towards the sun. The solar tracker  105  may be one-axis azimuth tracking system, a dual-axis azimuth and elevation tracking system, or in some embodiments, the solar tracker  105  may be a stationary system. 
     The concentrated photovoltaic and solar heating system  400  incorporates a linear Fresnel lens as the alternate primary sunlight concentrator  405 . In addition, the solar power generation unit  120  is oriented towards the sun, rather than receiving the concentrated sunlight  115  as reflected sunlight. 
     As shown in  FIG. 4 , the alternate primary sunlight concentrator  405 , i.e. the linear Fresnel lens, is between the sun and the solar power generation unit  120 . As such, the solar power generation unit  120  is rotated to receive the concentrated sunlight  115  from the primary sunlight concentrator  405 , which is above it in  FIG. 4 , i.e. as though the sun is overhead. In some embodiments, the solar power generation unit  120  and the alternate primary sunlight concentrator  405  may be rotated towards one side, as for higher latitudes or when the sun is low in the sky. This change in configuration may affect the design and assembly of a frame attachment (not shown) to the solar tracker  105 , but the function and operation of the solar tracker  105  and the solar power generation unit  120  are sufficiently the same as to not have distinguishing technical differences. 
       FIG. 5  shows another embodiment  500  of the concentrated photovoltaic and solar heating system. The embodiment  500  may comprise the solar tracker  105 , the alternate primary sunlight concentrator  405 , the concentrated sunlight  115 , the solar power generation unit  120 , a reflective compound parabolic enclosure  505 , and a secondary solar cell array  510  inside the reflective compound parabolic enclosure  505  opposite the solar power generation unit  120  from the alternate primary sunlight concentrator  405 . 
     The solar tracker  105  supports and orients the concentrated photovoltaic and solar heating system  500  towards the sun. The solar tracker  105  may be one-axis azimuth tracking system, a dual-axis azimuth and elevation tracking system, or in some embodiments, the solar tracker  105  may be a stationary system. 
     As with the embodiment  400 , sunlight enters the alternate primary sunlight concentrator  405  and is concentrated and directed towards the solar power generation unit  120 . On a cloudy day, however, a large portion of the incoming sunlight may be scattered by the clouds, so that the alternate primary sunlight concentrator  405  cannot direct the concentrated sunlight  115  towards the solar power generation unit  120 . The reflective compound parabolic enclosure  505  redirects the concentrated sunlight  115  for the generation of electricity by the secondary solar cell array  510 . Consequently, with two solar cell arrays, the concentrated photovoltaic and solar heating system  500  still produces electricity even on a cloudy day, as well as transferring its heat to the transparent fluid  225 . 
       FIG. 6  shows a flowchart of a method for the solar generation of electricity and a heated fluid. 
     At step  601 , sunlight is received and concentrated a first time; 
     At step  610 , the concentrated sunlight is passed though a transparent fluid and transfers solar energy to the transparent fluid; 
     At step  615 , the sunlight is concentrated a second time by passing it through a compound parabolic concentrator; 
     At step  620 , the concentrated sunlight is passed a second time though the transparent fluid and transfers solar energy to the transparent fluid; 
     At step  625 , the concentrated sunlight strikes a solar cell and generates electricity. 
     The embodiments discussed here are illustrative of the present invention. Elements in the figures are illustrated for simplicity and clarity and are not drawn to scale. Some elements may be exaggerated to improve the understanding of various embodiments. The descriptions and illustrations, as well as the various modifications or adaptations of the methods and/or specific structures described are within the spirit and scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is understood that the present invention is in no way limited to only the embodiments illustrated.