Patent Abstract:
The present invention relates to micron-gap thermal photovoltaic (MTPV) technology for the solid-state conversion of heat to electricity. The problem is forming and then maintaining the close spacing between two bodies at a sub-micron gap in order to maintain enhanced performance. While it is possible to obtain the sub-micron gap spacing, the thermal effects on the hot and cold surfaces induce cupping, warping, or deformation of the elements resulting in variations in gap spacing thereby resulting in uncontrollable variances in the power output. A major aspect of the design is to allow for intimate contact of the emitter chips to the shell inside surface, so that there is good heat transfer. The photovoltaic cells are pushed outward against the emitter chips in order to press them against the inner wall. A high temperature thermal interface material improves the heat transfer between the shell inner surface and the emitter chip.

Full Description:
This application claims benefit of U.S. Provisional Application No. 61/308,972 filed on Feb. 28, 2010, and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to micron-gap thermal photovoltaic (MTPV) technology for the solid-state conversion of heat to electricity. More broadly, the invention generates electrical power when it is inserted into a high temperature environment such as an industrial melting furnace. 
     BACKGROUND OF THE INVENTION 
     Thermo photovoltaic devices (TPV) consist of a heated black-body which radiates electromagnetic energy across a gap onto a photovoltaic device which converts radiant power into electrical power. The amount of power out of a given TPV device area is constrained by the temperature of the hot side of the device and generally requires very high temperatures, creating barriers to it practical use. By contrast, micron gap thermal photovoltaic (MTPV) systems allow the transfer of more power between the power emitter and receiver by reducing the size of the gap between them. By employing submicron gap technology, the achievable power density for MTPV devices can be increased by approximately an order of magnitude as compared to conventional TPV. Equivalently, for a given active area and power density, the temperature on the hot-side of an MTPV device can be reduced. This allows for new applications for on chip power, waste heat power generation and converter power. 
     It has been shown that electromagnetic energy transfer between a hot and cold body is a function of the close spacing of the bodies due to evanescent coupling of near fields. Thus, the closer the bodies, approximately one micron and below, the greater the power transfer. For gap spacings of 0.1 microns, increases in the rate of energy transfer of factors of five and higher are observed. 
     The dilemma, however, is forming and then maintaining the close spacing between two bodies at a sub-micron gap in order to maintain enhanced performance. While it is possible to obtain the sub-micron gap spacing, the thermal effects on the hot and cold surfaces induce cupping, warping, or deformation of the elements resulting in variations in gap spacing thereby resulting in uncontrollable variances in the power output. 
     Typically, in order to increase power output, given the lower power density of prior art devices, it has been necessary to increase the temperature. Temperature increases, however, are limited by the material of the device and system components. 
     Micron gap thermal photovoltaic (MTPV) systems are a potentially more efficient way to use photovoltaic cells to convert heat to electricity. Micron gap thermal photovoltaic devices are an improved method of thermal photovoltaics which is the thermal version of “solar cell” technology. Both methods make use of the ability of photons to excite electrons across the bandgap of a semi-conductor and thereby generate useful electric current. The lower the temperature of the heat source, the narrower the bandgap of the semi-conductor must be to provide the best match with the incoming spectrum of photon energy. Only those photons with energy equal to or greater than the bandgap can generate electricity. Lower energy photons can only generate heat and are a loss mechanism for efficiency. A preferred micron gap thermal photovoltaic system would include a source of heat radiated or conducted to an emitter layer which is suspended at a sub-micron gap above the surface of an infrared sensing photovoltaic cell. 
     By using a sub-micron gap between a hot emitting surface and a photovoltaic collector, a more enhanced rate of transfer of photons from solid to solid is observed than is possible with large gaps. Additional transfer mechanisms are involved other than simply Planck&#39;s law of the radiation, although the spectral distribution of the photons is that of a black body. The use of sub-micron gaps, however, implies that a vacuum environment is used to avoid excessive heat conduction across the gap by low energy photons that cannot excite electrons into the conduction hand. To make efficient use of the source of heat, a high fraction of high energy photons must be generated. The structure used to separate the emitting surface from the photovoltaic cell must be both small in diameter and also a very good thermal insulator for the same efficiency considerations. The photovoltaic cell will generally have to be cooled somewhat so that it will function properly. At high temperatures, intrinsic carrier generation swamps the PN junction and it is no longer an effective collector of electrons. 
     Micron gap thermal photovoltaic systems function as though the emitter has an emissivity value greater than one. The definition of a black body is that it has an emissivity value equal to one and this value cannot be exceeded for large gap radiant energy transfer. Equivalent emissivity factors of 5-10 have been experimentally demonstrated using gaps in the region of 0.30 to 0.10 microns. 
     There are at least two ways to take advantage of this phenomenon. In a comparable system, if the temperature of the emitting surfaces is kept the same, the micron gap thermal photovoltaic system can be made proportionately smaller and cheaper while producing the same amount of electricity. Or, if a comparable size system is used, the micron gap thermal photovoltaic system will run at a considerably lower temperature thereby reducing the cost of materials used in manufacturing the system. In a preliminary estimate, it was calculated that by using micron gap technology the operating temperature of a typical system could be reduced from 1,400° C. to 1,000° C. and still produce the same output of electricity. Such a lowering of temperature could make the difference in the practicality of the system due to the wider availability and lower cost of possible materials. 
     U.S. Pat. Nos. 7,390,962, 6,232,546 and 6,084,173 and U.S. patent application Ser. Nos. 12/154,120, 11/500,062, 10/895,762, 12/011,677, 12/152,196 and 12/152,195 are incorporated reference herein. 
     Additional energy transfer mechanisms have been postulated and the ability to build systems using narrow thermally isolated gaps may find use in many types of applications in accordance with the subject invention. 
     SUMMARY OF INVENTION 
     It is therefore an object of this invention to provide a novel micron gap thermal photovoltaic device structure which is also easier to manufacture. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which results in high thermal isolation between the emitter and the photovoltaic substrate. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which can have a large area and is capable of high yield. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which allows for lateral thermal expansion. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which is efficient. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device with a uniform sub-micron gap. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which provides greater energy transfer. 
     It is a further object of this invention to provide such a micron gap thermal photovoltaic device which is constructed without assembling multiple discrete pieces. 
     It is a further object of this invention to provide a method of making a micro gap photovoltaic device. 
     It is a further object of this invention to provide a micron gap device useful as a thermal photovoltaic system and also useful in other applications. 
     The thermo photovoltaic system and apparatus generates electrical power when it is inserted into a high temperature environment, such as an industrial melting furnace. It consists of a heat and corrosion resistant, vacuum-tight shell, and a liquid-cooled mechanical assembly inside that makes contact with the inside walls of the heated shell. 
     The mechanical assembly facilitates and provides a means for achieving sub-micron spacing between large emitter and photovoltaic surfaces. Heat is conducted from the inner surface of the shell to a spectrally controlled radiator surface (hot side). The radiator surface emits the heat in the form of electromagnetic energy, across a sub-micron gap to a photovoltaic (PV) device (cold side). A portion of the heat is converted to electricity by the photovoltaic cell. The rest of the thermal energy is removed from the opposite side of the photovoltaic cell by a liquid cooled, pinned or finned, heat sink. 
     A major aspect of the design is to allow for intimate contact of the emitter chips to the shell inside surface, so that there is good heat transfer. The photovoltaic cells are pushed outward against the emitter chips in order to press them against the inner wall. A high temperature thermal interface material improves the heat transfer between the shell inner surface and the emitter chip. Tiny spacers on the emitter chips always maintain a sub-micron gap between the hot radiating surface and the photovoltaic cells. 
     The mechanical assembly is designed to push the hot and cold chips against the shell inside surface as the shell heats up, expands, and warps. To achieve this, the photovoltaic cells are attached to a deformable body that is able to conform to the shape of the inside surface of the shell. The deformable body is a thin metal foil (membrane). Pressure is imparted to the membrane by means of a pneumatic diaphragm and a liquid metal filled cavity. 
     The liquid metal cavity serves two purposes: 1) to impart pressure to the back side of the membrane, which in turn pushes the photovoltaic chips against the emitter chips, while allowing the membrane to flex and conform to the shape of the shell inside surface; and 2) to carry excess heat away from the photovoltaic to a liquid cooled heat sink. 
     The empty space inside the shell is a nearly perfect vacuum (&lt;10 −3  Torr), so that heat is not conducted by air across the sub-micron gap and between exposed shell inside surfaces and the heat sink. 
     This invention is useful because it generates electrical power from heat that otherwise would be wasted. The electricity can be used to power other devices within the plant, or it can be sold to the utility company. 
     The invention disclosed herein is, of course, susceptible of embodiment in many different forms. Shown in the drawings and described herein below in detail are preferred embodiments of the invention. It is to be understood, however, that the present disclosure is an exemplification of the principles of the invention and does not limit the invention to the illustrated embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
         FIG. 1  illustrates thermo photovoltaic and micron-gap thermo photovoltaic technology in accordance with the present invention; 
         FIG. 2A  illustrates an embodiment of a single-sided MTPV device; 
         FIG. 2B  illustrates an embodiment of a two-sided MTPV device; 
         FIG. 3  illustrates an embodiment  300  the operation of the MTPV device; 
         FIG. 4  illustrates a practical embodiment  400  of a cross sectional view of a front end of a quad MTPV device; 
         FIG. 5  is a cross sectional view  500  of a quad module; 
         FIG. 6  illustrates a complete quad module mounted on the end of its assembly; 
         FIG. 7  illustrates the various parts that are assembled to form a quad module; 
         FIG. 8  illustrates a completely assembled quad module; 
         FIG. 9  illustrates a single quad module within its housing with its top cover removed; 
         FIG. 10  illustrates a quad module sliding into its hot housing through a furnace wall; 
         FIG. 11  shows a module containing four quad modules and coolant connection; 
         FIG. 12  shows an array of quad modules connected to common coolant lines; and 
         FIG. 13  shows required control modules connected to a MTPV panel comprising one or more quad modules. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Turning to  FIG. 1 ,  FIG. 1  illustrates thermo photovoltaic  104  and micron-gap thermo photovoltaic  106  technologies in accordance with the present invention. Both technologies may use heat from the combustion of gas, oil or coal  110 , nuclear energy  120 , waste heat from industrial processes  130  or solar heat  140 . Thermo photovoltaic devices (TPV)  104  consist of a heated black-body  150  which radiates electromagnetic energy across a macro scale gap  190  onto a photovoltaic device  160  which converts radiant power into electrical power. The amount of power out of a given TPV device area is constrained by the temperature of the hot side of the device and generally requires very high temperatures, creating barriers to it practical use. By contrast, micro scale gap  195  thermal photovoltaic (MTPV) devices  106  allow the transfer of more power between the power emitter  150  and receiver  160  by reducing the size of the gap  195  between them. By employing submicron gap technology, the achievable power density for MTPV devices  106  can be increased by approximately an order of magnitude as compared to conventional TPV devices  104 . Equivalently, for a given active area and power density, the temperature on the hot-side of an MTPV device can be reduced. This allows for new applications for on chip power, waste heat power generation and converter power. 
     It has been shown that electromagnetic energy transfer between a hot and cold body is a function of the close spacing of the bodies due to evanescent coupling of near fields. Thus, the closer the bodies  170 , approximately one micron and below, the greater the power transfer. For gap spacing of 0.1 microns  180 , increases in the rate of energy transfer of factors of five and higher are observed. By using a sub-micron gap  195  between a hot emitting surface  150  and a photovoltaic collector  160 , a more enhanced rate of transfer of photons from solid to solid is observed than is possible with large gaps  190 . Additional transfer mechanisms are involved other than simply Planck&#39;s law of the radiation, although the spectral distribution of the photons is that of a black body. The use of sub-micron gaps, however, implies that a vacuum environment is used to avoid excessive heat conduction across the gap by low energy photons that cannot excite electrons into the conduction band. To make efficient use of the source of heat, a high fraction of high energy photons must be generated. The structure used to separate the emitting surface from the photovoltaic cell must be both small in diameter and also a very good thermal insulator for the same efficiency considerations. The photovoltaic cell will generally have to be cooled somewhat so that it will function properly. At high temperatures, intrinsic carrier generation swamps the PN junction and it is no longer an effective collector of electrons. 
     Turning to  FIG. 2A ,  FIG. 2A  illustrates an embodiment  200  of a single-sided MTPV device. The embodiment includes a thermal interface  210  for conducting heat between housing that is exposed to a high temperature and a hot side emitter  215 . The hot side emitter  215  is separated from a cold side photovoltaic cell  225  by a micro-gap that is maintained by spacers  220 . A foil membrane  230  is positioned between the cold side photovoltaic  225  and a chamber  235  containing a liquid metal that is maintained under controlled pressure. This pressurized chamber  235  ensures that the hot side emitter  215  and thermal interface  210  is maintained in close contact with the housing over a wide temperature range. Adjacent to the liquid metal chamber  235  is a heat sink  240  that is cooled by a continuous flow of coolant in a coolant chamber  245 . The coolant chamber  245  is separated from a pneumatic chamber  260  by a coolant chamber seal  250  and a pneumatic chamber flexible seal  255 . The pneumatic chamber  260  is maintained at a controlled pressure to further ensure that close contact is maintained between the heat sink  240 , the liquid metal chamber  235 , the cold side emitter  225 , the hot side emitter  215 , the thermal interface  210  and the housing. A pneumatic chamber fixed seal  265  is positioned between the pneumatic chamber  260  and a coolant water manifold  270 , which is connected to a continuous supply of circulated cooling water for cooling the heat sink  240 . 
     Turning to  FIG. 2B ,  FIG. 2B  illustrates an embodiment  205  of a two-sided MTPV device. The two-sided MTPV device includes the structure described above in relation to  FIG. 2A  and an additional structure that is an inverted image of that shown in  FIG. 2A  attached to the common coolant water manifold  270 . This structure enables the collection of heat from both sides of an MTPV device. 
     Turning to  FIG. 3 ,  FIG. 3  illustrates an embodiment  300  that shows the operation of the MTPV device. The MTPV device  305  is exposed to radiant and convective heat flux  310 , which heats the outer surface and the hot side of the hot side/cold side pair  320 ,  330 . A vacuum is maintained in the interior of the MTPV device  305  and the cold side photovoltaic cell is cooled from the inside by circulating water  340 ,  350 . Output power  360 ,  370  is obtained from the device  305 . 
     Turning to  FIG. 4 ,  FIG. 4  illustrates a practical embodiment  400  of a cross sectional view of a front end of a quad MTPV device. The quad MTPV device is a basic building block for implementing the MTPV technology. The front end includes a thermally conductive graphite interface  410  between a high temperature housing and a hot side emitter  420 . A micro-gap  430  is maintained between the hot side emitter  420  and a cold side photovoltaic cell  440 . A foil membrane  450  is positioned between the cold side emitter  440  and a liquid metal chamber  460 . A surface of a heat sink  470  and the foil membrane  450  enclose the liquid metal chamber  460 . 
     The purpose of the emitters  420  is to absorb heat from the inside of the housing of the quad MTPV device. An emitter chip  420  is typically, but not necessarily, made of silicon and has micro-machined silicon dioxide spacers on the gap side. The smooth side of the emitter  420  is pressed against the inside of the hot housing. A graphite thermal interface material  410  is sandwiched between the emitter  420  and the housing to improve heat transfer. The housing is heated by the radiant and convective energy within a furnace and the heat is conducted through the housing, across a thermal interface material  410  and into the silicon emitter  420 , causing it to become very hot. 
     The photovoltaic cells  440  are designed to convert some of the light emitted from a hot body into electricity. More specifically, the photovoltaic cells  440  have a very flat surface so that when they are pressed against the spacers on the emitting surface  420 , a very small vacuum gap is formed. The spacers are designed so that very little heat flow is conducted from the hot emitter  420  to the relatively cool photovoltaic cell  440 . The photovoltaic cell  440  and emitter  420  are also made of high index materials to obtain a maximum amount of near-field coupled energy enhancement. A percentage of the light passing from the emitters  420  to the photovoltaic cells  440  is converted to electricity. 
     Turning to  FIG. 5 ,  FIG. 5  is a cross sectional view  500  of a quad module. This view is a macroscopic perspective that includes the elements shown in  FIG. 4 . The quad module includes a water distribution housing, also known as a coolant water manifold  510 , a bellow subassembly  560 ,  570 , a heat sink subassembly  470 , a pneumatic subassembly  530 ,  540 ,  550 , a liquid metal compartment  460  (see also  FIG. 4 ), a membrane and photovoltaic subassembly  440 ,  450  (see also  FIG. 4 ), hot side emitter array  410 ,  420  (see also  FIG. 4 ), and a linear actuator pressure regulator (inside the water distribution housing). These elements form the basic quad module building block. One or more quad modules are normally enclosed in an evacuated enclosure or hot housing that is exposed to high temperatures for generating electrical power. 
     The membrane  450 , liquid metal  460 , heat sink  470 , and bellows subassemblies  570  have very coupled functionality. The metal bellows  570  transfer water between the water distribution housing  510  and the heat sink  470 , one set of bellows  570  on the inlet side and the other set on the outlet side. The bellows  570  also act as expansion joints, so that when the housing heats up and expands, the bellows  570  elongate. The bellows  570  are always compressed so that they provide a force that pushes the heat sink and membrane assemblies toward the hot cover, thus pushing the photovoltaic cells  440  against the emitter spacers and pushing the emitter  420  against the hot wall. While the heat sink  470  has internal voids for water to pass through, it also acts as a suspended platform for the photovoltaic cells. Through flexing of the bellows  570 , the platform can move in and out and tilt about two axes. This articulation allows the photovoltaic array  420  to conform, macroscopically, to the orientation of the hot housing. The flexible membrane  450  is there to deal with curvature of the hot housing. 
     The membrane  450  is a second suspension for the chips. The first suspension takes care of rigid body motions due to thermal expansion and tilt offsets due to machining tolerances and differential heating. The membrane  450  is a flexible suspension for the photovoltaic cells  440 , allowing the array of cells to push against the emitters  420  and bend and flex such that the chips conform to the curved shape of the housing. It is important to note that when heat flows normal to a flat plate, there is a temperature drop across the plate which causes thermal bending, or bow. The photovoltaic cells  440  are bonded to membrane  450 . The metal membrane  450  has an insulating layer and a patterned layer of electrical conductors. In this sense, the membrane  450  acts as a printed circuit board, tying the photovoltaic cells  440  together in series and/or parallel and carrying the electricity to the edge of the membrane  450 . 
     The membrane  450  is sealed around the edges to the platform, leaving a small gap between the membrane  450  and the platform. This space is then filled with liquid metal. The liquid metal serves two purposes. First, it provides a thermal path between the photovoltaic cells  440  and the heat sink  470 . Second, because it is a fluid, it allows the membrane  450  to flex. 
     The hot housing is made from a high temperature metal and is securely closed after the quad modules are placed inside. The size of the housing depends on the number and distribution of quad modules. The inside surfaces are polished so that they have a low emissivity. The outside surfaces are intentionally oxidized to a black finish so that they will absorb more radiant heat from the furnace. The housing has pass-through ports for cooling fluid, vacuum pumping, and electrical wires. 
     The pneumatic subassembly  530 ,  540 ,  550  sits between the water distribution housing  510  and the heat sink  470 . In parallel with the bellows  570 , the pneumatic diaphragm  530  pushes the heat sink  470  outward toward the hot housing, thus squeezing the photovoltaic cells  440  and emitters  420  between the membrane  450  and the hot housing. With the proper amount of pneumatic force and pressure in the liquid metal cavity, the membrane  450 , chips, and housing will all take on the same shape and the gap between the emitter  420  and photovoltaic cells  440  will be uniform (but not necessarily flat). 
     The heat flows into the housing, through the thermal interface material  410 , and into the emitter  420 . It is then radiated across a sub-micron vacuum gap to the photovoltaic cell  440 , where some of the energy is converted to electricity and taken away by the metallization on the membrane surface. The rest of the heat passes through the membrane  450 , liquid metal, copper, copper pins, and into the cooling water, which is constantly being replenished. 
     If the photovoltaic cells  440  are all put in series, then bypass diodes can be connected at the ends of each row of cells, such that, if a photovoltaic cell  440  within a row were to fail, the entire row can be bypassed, and the electrical current will be passed to the next row. 
     Turning to  FIG. 6 ,  FIG. 6  illustrates a complete quad module  600  mounted on the end of its assembly. Shown in  FIG. 6  is a hot side emitter array  410 ,  420 , membrane and photovoltaic assembly  440 ,  450 , liquid metal chamber  460 , heat sink  470 , water distribution housing  510 , pneumatic chamber  540 , electrical connections  610  and pneumatic connections  620 ,  630 . 
     The linear actuator consists of a motor and lead screw and is housed inside of the water distribution housing  510 . Its purpose is to control the amount of liquid that is behind the membrane  450 . The actuator drives a piston, which is attached to a rolling diaphragm. The interior of the diaphragm is filled with liquid metal, which can be pumped through channels that lead to the liquid metal/membrane chamber  460 . To increase or decrease the amount of liquid metal behind the membrane  450 , the actuator is driven outward or inward, respectively. The actuator also is used to control the pressure in the liquid metal. Between the linear actuator and the piston is a die spring. Force from the actuator goes through the spring and into the piston, so that the spring is always in compression. This allows for the actuator to modify the liquid metal pressure, even if the piston remains stationary. Compression of the die spring is directly related to the liquid metal pressure. 
     Turning to  FIG. 7 ,  FIG. 7  illustrates the various parts that are assembled to form a quad module  700 . These include a photovoltaic array  710  and heat sink top  715 , heat sink bottom  720 , water housing top cover  735 , servometer bellows  725 , water housing side covers  730 , water housing  740 , bellows connectors  745 , servometer bellows  750 , and bellows tubes  755 . 
     Turning to  FIG. 8 ,  FIG. 8  illustrates a completely assembled quad module  800 . As shown in  FIG. 8 , a quad module includes a photovoltaic array  710  and heat sink top  715 , servometer bellows  725 , water housing side covers  730 , water housing  740 , and electrical and pneumatic connections  770  to external control modules. 
     Turning to  FIG. 9 ,  FIG. 9  illustrates a single quad module  900  within its housing with its top cover removed. Shown are a complete assembled quad module  800  shown in  FIG. 8 , a hot housing  910 , water coolant connections  930 ,  940  and a vacuum port  920 . Not shown is a connection to a pneumatic control module. 
     Turning to  FIG. 10 ,  FIG. 10  illustrates a quad module sliding  1000  into its hot housing through a furnace wall. Shown are a quad module  800 , hot housing  1020 , furnace wall  1030 , a quad module enclosure  910 , water coolant connections  930 ,  940 , and connections to electric power facilities, vacuum control module, and pneumatic control module  1010 . 
     Turning to  FIG. 11 ,  FIG. 11  shows a module containing four quad modules and coolant connection  1100 . It may include up to four double-sided quad modules  800  and coolant connections  1130 ,  1140 . 
     Turning to  FIG. 12 ,  FIG. 12  shows an array of quad modules  1200  connected to common coolant lines. It shows 24 quad modules  800  connected to common coolant lines  1230 ,  1240 . While each quad module contains arrays of photovoltaic cells and emitter chips, a panel may contain an M×N array of quad modules, where M and N are greater than or equal to one. Quad module arrays may be connected together by cooling pipes such that the units are cooled in series or parallel. 
     Turning to  FIG. 13 ,  FIG. 13  shows required control modules connected to a MTPV panel comprising one or more quad modules  1300 . Shown are a MPTV panel  1350 , a cooling control module  1310 , a vacuum control module  1320  and a pneumatic pressure control module  1330 .

Technology Classification (CPC): 7