Abstract:
A small and light cylindrical thermophotovoltaic generator uses gaseous fuels, a counter flow heat exchanger, regenerator and low bandgap photovoltaic cells. In the fuel injection system, with preheated air from a recuperator, fuel combustion begins immediately when the fuel and air first meet. A hot and compact burn results from complete and rapid fuel and air mixing. A venturi necks down the air flow, and a chemically etched jet shim disk creates over 150 small fuel jet streams. The emitter geometric configuration provides good hot gas energy transfer to the IR emitter. Four alternate emitter configurations accomplish the good heat transfer. One emitter is a composite SiC with integrally formed internal fins which extend into the combustion chamber. The photovoltaic converter assembly has good spectral control, good high rate but lightweight heat removal and high current-carrying capability, while maintaining low parasitic IR absorption. A modular photovoltaic converter circuit is complete with series connected low bandgap filtered cells, a heat spreader and high current-carrying mirror-shielded interconnects. An efficient but lightweight and short heat exchanger regenerator is fairly easy to fabricate by inserting an array of angled vanes through slits in a simple cylinder. One regenerator is formed with integrally extruded or machined fins on a high temperature SiC composite.

Description:
BACKGROUND OF THE INVENTION 
     This application claims the benefit of U.S. Provisional Application No. 60/046,588, filed May 15, 1997. 
    
    
     SUMMARY OF THE INVENTION 
     This invention provides a small and light cylindrical thermophotovoltaic (TPV) generator using gaseous fuels, a counter flow heat exchanger and low band gap photovoltaic cells. 
     In the new fuel injection system, with preheated air from a recuperator, fuel combustion begins immediately when the fuel and air first meet. Therefore, for a very hot and compact burn, complete and rapid fuel and air mixing is required. That is accomplished with a venturi to neck down the air flow and a chemically etched jet shim disk which creates over 150 small fuel jet streams. 
     In the new emitter geometric configuration, it is important to have good hot gas energy transfer to the IR (infrared) emitter. Four new alternate emitter configurations accomplish the transfer. 
     In the photovoltaic converter assembly, it is important to have good spectral control, good high rate but light weight heat removal and high current carrying capability, while maintaining low parasitic IR absorption. A modular photovoltaic converter circuit is complete with series connected low band gap filtered cells, a heat spreader and high current carrying mirror shielded interconnects. 
     An efficient but light weight and short heat exchanger which is fairly easy to fabricate is accomplished by inserting an array of angled vanes through slits in a simple cylinder. 
     These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional schematic of the TPV battery charger. 
     FIG. 2 is an enlarged partial bottom view of the etched shim depicting the chemically etched channels. 
     FIG. 3 is a plan view of the fuel injector shim. 
     FIG. 4 is an exploded view of the fuel injector assembly. 
     FIG. 5 is a plan view of the fuel injector feed cup. 
     FIG. 6 is a plan view of the fuel injector feed cap. 
     FIGS. 7-10 are elevational cross-sections of four possible emitter configurations. 
     FIG. 11 is a cross-sectional plan view of a circuit, mirror and PCA configuration. 
     FIG. 12 is an end view of a single circuit. 
     FIG. 13 is an elevation view of a single circuit. 
     FIG. 14 shows fabrication in groups in a sheet of edge strip mirrors. 
     FIG. 15 is a perspective view of a finned heat exchanger. 
     FIGS. 16 and 17 are plan views of the angled fins for inserting in slots in the cylinder shown in FIG.  15 . 
     FIG. 18 is a perspective view of an extruded SiC recuperator finned tube. 
     FIG. 19 is a perspective view of an extruded SiC emitter with inside radial fins. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the TPV battery charger  10  has a cooling fan  11  which blows air over cooling fins  13 . A photovoltaic converter assembly (PCA)  15  is separated from the inner  17  and outer  19  IR emitters by a quartz shield  21 . Insulation  23  supports the outer emitter  19  and the quartz shield  21 . A combustion air fan  25  supplies the combustion air  27  which is preheated by an angled vane heat exchanger  29 . Fuel  31  is supplied to the preheated combustion air  27  by the fuel injector  33  which is shown partially in cross-section. Gaseous fuel enters inlet  35  and is released by the injector  33  into the combustion air. Combustion air enters annular inlet  37  and is blown upward by fan  25 . Venturi  39  speeds the flow of combustion air through the combustion zone  41 . The flames and hot gases in combustion chamber  43  heat the inner and outer IR emitters. The exhaust flows outward at the top of the combustion chamber  43  then downward in annular chamber  45  and transfers heat to heat exchanger  29  to preheat the incoming combustion air  27 . The exhaust gases flow outward through the opening  49  at the bottom of insulation  23  and mix with air flowing out outlet  51 . 
     FIGS. 2 and 3 show details of the jet shim  57  which is positioned in the injector  33 . The purpose of the jet shim is two-fold. First, the shim is used to provide provision spacing (typically 0.003″) between the cap  61  (FIG. 6) and the cup  63  (FIG. 5) of the fuel injector  33 . Accurate control of this gap is necessary to regulate fuel flow into the combustion chamber  43 . Second, since fuel can only flow in the chemically etched channels  59  of the shim  57 , the shim delivers small, discrete jets of fuel to the combustion zone  41  at the neck of the venturi  39  leading to the combustion chamber  43 . Typically, there are more than 150 jets, depending on shim  57  diameter, channel  59  width, and channel pitch. Typically, exit dimensions of each jet are 0.010″ wide by 0.003″ high, depending on channel  59  width and shim  57  thickness, respectively. Air flows up through the venturi neck  39  and perpendicularly intersects the plane of discrete fuel jets, providing excellent fuel to air mixing. 
     FIG. 4 shows an exploded view of the fuel injector  33  assembly. Stainless steel tubing  65  supports and supplies fuel to the fuel injector  33 . Fuel flows axially through eight quarter-inch holes  67  into an annular plenum  69  in cup  63  and then radially outward through controlled channels  59  in shim  57 . An axial screw  68 , shown in FIG. 1, holds the cap  61  (FIG.  6 ), shim  57  and cup  63  (FIG. 5) assembled. The cup base  71  is pressed into the upper end of tube  65 . 
     FIGS. 7 through 10 provide four possible inner  17  and outer  19  IR emitter configurations. A spectrally matched emitter is used with continuous ceramic fiber reinforcement for durability. However, that limitation is not required for the present invention. To insure good heat transfer from the hot gas to the IR emitter, four alternate configurations are shown in FIGS. 7 through 10. In FIG. 7, the hot gases simply flow up axially inside the IR emitter  17 , outward through opening  73  at its top  75 , and axially downward outside the emitter, being confined by a quartz outer cylinder  77 . Alternately in FIG. 8, the hot gases flow radially out through holes  79  in the emitter  81 , again being confined by the outer quartz shield  77 . 
     The problem with the above configurations is that the quartz shield  77  is in direct contact with very hot gases. More desirable configurations are shown in FIGS. 9 and 10. In those configurations, the quartz shield  77  is replaced by a second, larger IR emitter  19  which is heated by both gas contact and by radiation transfer from the inner emitter  17  or  81 . 
     Which of the four configurations is used depends on a trade of material durability against generator power output and efficiency. In each configuration, the gases are confined from escaping upwards by insulated cover  83 . In FIGS. 9 and 10, the quartz shields  77  and  21  are supported by insulation  23 . In FIGS. 7 and 8, insulation  23  supports the quartz shield  21  and the outer IR emitter  19 . In all configurations, the inner IR emitter  17  or  81  is supported at the top of venturi  39 . 
     FIG. 11 shows a photovoltaic converter array (PCA)  15  consisting of an array of circuit boards  87 , which are soldered to copper convoluted fin stock, shown in FIG. 1, and rolled into a cylinder. That forms a light weight array. Mirrors  89  cover connectors at edges of cells  91 . 
     FIGS. 12 and 13 show top and side views of an individual circuit board  87  of the array. Each circuit consists of a copper backing strip  93  with a thin dielectric electric insulating coating  95  on its front side with conducting metal pads  97  on the front of the dielectric. The low bandgap cells  91  with multilayer dielectric filters on their front faces are soldered to the pads  97  and interconnected by leads  98  running at the edges of the circuit. Gold coated edge mirrors  89  are glued over the leads. 
     FIG. 14 shows how the edge strip mirrors  89  are fabricated in groups in a sheet. Since each mirror is 0.060″ thick, a 0.060″ thick aluminum sheet  103  polished on a front face is placed face down in a CNC mill on a vacuum chuck. Several vertical trenches  99  are cut out for the mirror overhang over the cell interconnect buses. Then several horizontal trenches  101  are cut where the mirror needs to bridge over the solder interconnects. Then a slitting saw is used to separate  102  the mirror strips along their length dimension but leaving them still connected to the sheet frame. The sheet is then anodized to form an isolating oxide, and then its front face is coated with a reflecting gold mirror. Finally, the mirror strips are separated by sheering  104  away the frame  100 , and individual mirrors  89  are attached completing the circuit  87  fabrication. 
     FIG. 15 shows a perspective view of a finned heat exchanger  29 . After the hot combustion gases transfer their energy to the radiation emitters  17  and  19 , they continue on a downward path through the heat exchanger  29 . High efficiency TPV burners require recuperation of heat energy from the combustion exhaust steam. Recuperated energy is used to preheat the intake combustion air to increase the combustion flame temperature, and thus increase overall system efficiency. Swirling of the combustion gas products is also known to increase heat transfer to the emitters of the TPV system. The present invention is designed to improve both heat exchanger efficiency and combustion gas swirl in a TPV system. 
     A cylindrical tube  105  forms the inner and outer walls of the heat exchanger. The tube is formed from high temperature materials such as copper-nickel alloys, alumina refractory, silicon carbide or other high temperature ceramics or metal alloys. Alternatively, the tube may be formed in two sections by a combination of two high temperature materials, or by combination of one high temperature material and a lower temperature material, such as aluminum. Slots  106  are cut or milled in the cylindrical tube, and heat exchanger fins  107  are inserted through the slots in the tube wall. Typical fin materials include stainless steel, inconel, copper-nickel alloys, silicon carbide, boron nitride, or other high temperature ceramics or metal alloys with reasonably high thermal conductivity. Fins consist of flat plates, typically 0.010″ or 0.200″ thick and 0.060″ to 2.000″ wide. The length of the fins is determined by design considerations such as fin efficiency and insertion length geometrical constraints on both the hot and cold sides of the heat exchanger. The fins may be either rectangularly shaped, or trapezoidally shaped, as shown in the cylinder in the interior of the tube. The trapezoidal fin shapes will allow higher fin density. Shoulders milled on the fins assist achieving the correct insertion depth. Fins may be loose fitting, press fit, or solder, brazed, or welded to the cylinder. 
     The fins  107  are inserted in a staggered helical pattern as shown in FIG.  15 . The helical pattern establishes a swirl flow of the intake air flowing in the interior (cold side) of the heat exchanger tube  105 . The angle of the fins is typically 45 degrees, and this angle may vary over the length of the cylindrical tube  105  in order to control the swirl. At the fan end of the heat exchanger, the fin angle may be adjusted to match the natural swirl established by the fan, typically about 50 degrees off horizontal and counter-clockwise. That minimizes the pressure drop through the heat exchanger  29 . The swirl established in the intake air side of the heat exchanger  29  is maintained, to some extent, through the venturi  39  and combustion chamber  43 , to give combustion gas swirl as energy is being transferred to the surfaces of emitter  17  and  19 . 
     FIGS. 16 and 17 show plan views of angled trapezoidal  107  and rectangular  109  fins. The use of angled fins  107  to establish a helical air flow pattern on both sides of the heat exchanger  29  improves heat transfer by increasing the path length of the air through the heat exchanger, and thus increasing the heat transfer area on both sides of the exchanger. The relatively short width of the fins  107 ,  109  and the staggered insertion pattern serves to periodically interrupt the boundary layers formed on the fin surfaces, improving heat transfer effectiveness. The exact fin width, stagger pattern, fin density and fin angle will be determined by a tradeoff between pressure drop through the heat exchanger and improved heat transfer efficiency and combustion gas swirl. 
     In TPV recuperators using parallel welded stainless steel plates there is a problem at the hot end. Hot gases exiting the emitter section rapidly corrode the upper stainless steel plate. Thee is a need for a higher temperature material at the hot end of the recuperator. The solution is shown in FIG.  18 . An extruded SiC tube has inside and outside radial fins. 
     The SiC composite regenerator  110  has a thin cylindrical tube  112 , integrally formed internal fins  114  for extending into the combustion air conduit and preheating air, and external fins  116  for removing heat from the hot exhaust gases. The fins are integrally extruded or machined. 
     There is a need to reduce the temperature of hot exhaust gases exiting the emitter section to reduce the thermal stress on the recuperator section. Increasing the heat transfer rate from the hot exhaust gases to the emitter by adding inside radial fins to the emitter meets that need. An inside radial fin emitter is shown in FIG.  19 . 
     The SiC composite emitter  120  has a cylindrical body  122  with internally extending fins  124  for increasing temperature of the radiating body  122 . 
     There is synergy in the fabrication method and material for the recuperator and emitter. There is also a more subtle synergy in that fins may be required on both the recuperator and the emitter for the emitter temperature to hit 1400° C. That fins are required on both parts is not obvious and only follows from careful analysis. 
     Table 1 presents the equations for calculating the temperature difference between a gas and a finned surface given geometry and heat transfer requirements, or the pressure difference through a fin array given geometry and a gas flow rate. These equations were first applied to the photovoltaic cell cooling problem, but they can also be used to analyze heat transfer in a finned recuperator or heat transfer from the hot combustion gases to the emitter. 
     Tables 2 and 3 summarize the ΔT and ΔP results for five different cases. The first column in both tables refers to the photovoltaic cooling fin design used in a first iteration. Although the ΔT of 100° C. that resulted was larger than was desired, the predicted ΔT using these equations matched the measured data and serves to validate the equations. The numbers in the next four columns refer to the present 200 Watt TPV generator design. The predicted numbers are within the required range. 
     The predicted ΔT value of 256° C. in the emitter column of Table 2 is particularly noteworthy. This number results assuming ⅛″ thick ⅜″ high fins on a 3.75″ diameter 4″ tall emitter separated by ⅛″ gaps. The heat to be transferred to the emitter is 2 kW. If, instead of fins, an emitter and a radiator tube are simply separated by ¼″, then the heat transfer coefficient, h, would decrease by 2, the heat transfer area would decrease by 2, and the difference between the emitter surface temperature and the gas temperature would rise by a factor of 4 to over 1000° C. Fins on the emitter are desirable. Without fins, if the radiator tube diameter is increased to fit closer to the emitter tube, the pressure may increase, and any small deviations in concentricity may lead to azimuth non-uniformity. 
     Another reason why fins are desirable is that, without fins, the radiator tube will be much hotter than the emitter tube. The temperature limit for the radiator tube material will then set the temperature limit for the emitter to below 1400° C. With fins on the emitter, the emitter temperature will be closer to the radiator tube temperature limit. 
     The finned SiC recuperator and emitter can be fabricated by extruding parts. It is also possible to machine the parts in graphite and then to convert the graphite to SiC. 
     In the finned emitter, fins may be of constant length. It may be desirable to taper the fin length with shorter fins at the top of the emitter cylinder and longer fins near the bottom to optimize the emitter temperature uniformity. With fins of constant length the emitter would tend to be hotter at its top, because the gas temperature will be hotter there. This tapered fin length could be accomplished by machining an extruded part in the green state before firing or by directly machining a graphite part. 
     Table 1: Fin design equations 
     
       
           ΔP =(32 ηl/d   2 ) F/A   xy   
       
     
     where 
     ΔP is pressure change in Pascal, 
     η is viscosity in Pascal×second, 
     l is fin length in cm, 
     d is fin spacing in cm, 
     F is volumetric flow in cubic cm per second, and 
     A xy  is flow path area in cm 2 . 
     
       
         
           ΔT=Q/h A 
           rz 
         
       
     
     where 
     ΔT is temperature difference between gas and fin surface in ° C. 
     And 
     h=Nu(k/d) is the heat transfer coefficient in Watts/m 2 ° C. 
     Q is the net power transfer rate in Watts, 
     A rz  is the total fin area in square m, 
     k is thermal conductivity of air in Watts/m° C., 
     d is fin spacing in m, 
     and Nu is the Nusselt number (=4 for present geometry). 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Temperature change through cooling fins, heat exchanger, and 
               
               
                 emitter. 
               
             
          
           
               
                   
                 Cooling Fins 
                 HX Up 
                 HX Down 
                 Emit Down 
               
               
                   
                   
               
             
          
           
               
                 Q 
                 1 kW 
                 2 kW 
                 1 kW 
                 1 kW 
                 2 kW 
               
               
                 A rz   
                 0.16 m 2   
                 1.2 m 2   
                 0.036 m 2   
                 0.06 m 2   
                 0.06 m 2   
               
               
                 k 
                 .024 W/m ° C. 
                 .024 
                 .075 
                 0.1 
                 0.1 
               
               
                 d 
                 0.16 cm 
                 0.2 cm 
                 0.3 cm 
                 0.3 cm 
                 0.3 cm 
               
               
                 h 
                 60 W/m 2  ° C. 
                 48 
                 100 
                 130 
                 130 
               
               
                 ΔT 
                 100° C. 
                 35 
                 278 
                 128 
                 256 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Pressure Change Through Cooling fins, heat exchanger, and 
               
               
                 emitter. 
               
             
          
           
               
                   
                 Cooling Fins 
                 HX Up 
                 HX Down 
                 Emit Down 
               
               
                   
                   
               
             
          
           
               
                 F 
                 19 liter/s 
                 75 liter/s 
                 4.5 liter/s 
                 8 liter/s 
                 12 liter/s 
               
               
                 A xy   
                 60 cm 2   
                 184 cm 2   
                 6 cm 2   
                 9 cm 2   
                 10 cm 2   
               
               
                 η 
                 0.2 × 10 −4  Pa s 
                 0.2 × 10 −4   
                 0.4 × 10 −4   
                 0.6 × 10 −4   
                 0.8 × 10 −4   
               
               
                 l 
                 8 cm 
                 10 cm 
                 10 cm 
                 10 cm 
                 10 cm 
               
               
                 d 2   
                 2.5 × 10 −2  cm 2   
                 45 × 10 −2  cm 2   
                 10 −1  cm 2   
                 10 −1  cm 2   
                 10 −1  cm 2   
               
               
                 ΔP 
                 64 Pa 
                 64 Pa 
                 96 Pa 
                 171 Pa 
                 307 Pa 
               
               
                   
               
             
          
         
       
     
     In Table 3, F is volumetric flow in liters per second. 
     In FIGS. 1 and 9 a second, smaller, inner emitter  17  is shown. That second emitter is alternatively and interchangeably referred to herein as a radiator to distinguish it from the outer emitter  19 . The radiator is heated both by heat conduction transfer from the combustion gases and heat radiation. Placing those two emitters or emitter and radiator close together with a narrow gap improves heat transfer but increases undesirable back pressure. By using internal fins on the outer emitter it is heated hotter, and the gap may be increased balancing radiant heat transfer from the inner to outer emitter and the provision of a sufficient gap therebetween to avoid pressure buildup. For example, an ⅛″ gap is desirable for radiant heat transfer but pressure buildup results. Adding internal fins allows a ¼″ gap which reduces pressure, while heating the outer emitter hotter. 
     While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.