Thermally Stimulated Focused Photon Sources

The present invention relates to a means to efficiently produce, collect, and direct photons from an emitter, such as thermally stimulated superemitters which are capable of producing selected and/or broad wavelength spectral outputs for a variety of applications. These applications include lighting, cooking, laser pumps, heating fluids, and producing electric power when selected wavelengths are collected by photovoltaic materials. This invention uses an optical means to collect and select photons and then direct them to at least one target. A method of preparing such devices is also provided by the present invention. In addition, methods of raising the temperature of combustion to increase radiation output by increased photon output, which may provide higher efficiency electrical production, are also described.

A variety of superemitting materials have been previously described in U.S. Pat. Nos. 4,776,895 and 4,793,799, both issued to Goldstein. Selective emission arises from such superemitting material due to the decay of inner electron shell transitions in a solid phase. Some superemitting materials often have an element present in a mixed oxidation or mixed valence state, forming a nonstoichiometric oxide. Some of the most effective superemitting materials in this class of compounds having mixed oxidation or mixed valance states include rare earth/alkaline earth oxide systems, rare earth/transition metal oxide systems, actinide oxides and the like, and various other mixed metal oxide systems.

Superemitters, when heated to a threshold temperature, emit visible or infrared radiation in a wavelength band that is related to the inner electron shell vacancy of the particular superemitter material. Radiation emitted from such superemitters is often in the form of a narrow band and can, therefore, be absorbed efficiently by a photovoltaic device, such as a silicon cell, InGaAs, and the like, to convert the radiation to an output voltage and current. The thermally-stimulated superemitters produce radiation in relatively concentrated, narrow spectral bands when compared to "blackbody" or "graybody" emitters, which typically produce a broad band thermal emission. As a result of the concentrated, narrow spectral band photon emission from such superemitters, thermophotovoltaic (TPV) power systems that are used in conjunction with such superemitters have greater conversion efficiency, i.e., conversion of photon radiation to electricity, than TPV power systems used in conjunction with blackbody emitters operated at the same heat flux. This is due to the typically broad band emissions from such device. However, blackbody emitters can be made to produce a selective spectral band photon emission by using one or more band-pass filters interposed between the emitter and the TPV power system.

Several thermophotovoltaic devices that are used to collect, select, and direct radiation are shown in FIGS. 1-4. Ideally, superemitters emit radiation having a wavelength near, i.e., slightly higher than, the photovoltaic material's band gap. For example, silicon has a band gap at about 1,100 nm, InGaAsP can be tuned to have a band gap covering a wider range, e.g., from 800 nm to about 1700 nm, and ytterbia has a band gap with a peak at 975 nm, i.e., just above the silicon band gap energy. An ytterbia-based mixed oxide emission spectrum is compared with that of holmium oxide in FIG. 5. Multilayered multiband cells made of InGaAsP and other multilayered group III-V compounds can be used to match the ytterbia and holmium based peaks shown.

Although superemitting fiber matrix burners have been developed, they have generally not been found to be effective in very high efficiency/very high fiber temperature device (as described in copending patent applications Ser. Nos. 07/860,777, now U.S. Pat. No. 5,356,487, and 07/695,783, respectively filed on Mar. 27, 1992 and May 6, 1991) when the gas velocity hitting (impinging) the fibers exceeds a certain amount which destroys the fiber.

Superemitter ceramic burners, which emit radiation in a narrow spectrum when heated above their threshold temperature offer the potential for such high-efficiency energy production for a short period of time when the gas pressure and velocity hitting the fibers are very high, i.e., sufficient to cause damage to the fibers over time. However, if the principle of opposing torches is used to provide the thermal energy needed to effect superemitter photon emission, the fibers will see very low gas velocity and will, therefore, provide a high degree of power for long periods of time without damage. Another method to produce a long lived emitter is shown in FIG. 3, wherein the superemitter is coated onto the surface of a transparent light pipe or waveguide.

It is desirable that these superemitter ceramic burners have highly active emissive surfaces. It is also desirable that these superemitters be inexpensive and easy to produce, strong and durable, and have high-temperature and high-energy density capabilities. The intensity of the light emitted from a superemitter increases dramatically with temperature.

The amount of radiant energy emitted and then collected by a photovoltaic cell used in conjunction with such superemitter will also increase dramatically with temperature if most or nearly all the photons can be collected and converted or recycled. Therefore, optical collection systems are desired to produce electric power efficiently, i.e., convert the photon emission from such superemitters to electrical energy. The efficiency of converting photon emission to electric energy increases if the thermal energy of the exhaust gas is recycled by means of a recuperator system, which transfers the thermal energy in the photons and in the exhaust gases to the incoming reactants. The recuperator may increase the temperature of the reactants above the auto-ignition point. To provide for these important energy conservation features, a fuel injection system has been invented that allows the combustion inlet temperature to reach well over auto-ignition.

There is, therefore, a need for an improved photon collection technology for a wide variety of applications such as photon sources for pumping lasers, and providing photons of specific wave bands for such purposes as photolithography, photochemical reactors, etc. and TPV energy generation.

The invention comprises various novel photon generating devices that produce and concentrate photons onto one or more targets and methods for constructing these devices. A number of examples of novel devices are presented to produce, collect, and concentrate electromagnetic radiation with a predetermined spectral characteristic depending on the specific application. These devices may also be useful for the collection of near blackbody radiation. These photon sources may be used with various targets including photovoltaic cells and the like.

Processes for preparing emissive photon generators having thermal input provided from combustion are also described. One such process comprises placing a base fiber layer solution (consisting of bonding gel, which supports fibers and porosity agents) on one side of a burner skeleton; applying a vacuum to the other side of the burner skeleton to draw the base layer fibers onto the surface of the burner skeleton and to draw the liquid component through the burner skeleton; removing the fibercoated skeleton from the base fiber layer solution, and drying the base fiber layer. An outer fiber layer may also be added to the base fiber layer by placing another fiber layer's solution in contact with the side of a burner skeleton containing the base fiber layer; applying a vacuum to the other side of the burner skeleton, to draw the liquid component through the base fiber matrix and the burner skeleton, to form a fiber matrix; removing the burner with the fibers coated onto the skeleton; drying the fiber layer(s); and firing the coated burner to sublime out porosity agents and bond the ceramic together to produce a cylindrical porous ceramic burner that focuses photons towards its center line. Optical devices such as light pipes, waveguides, or both are placed into the cylindrical cavity. When the device is fired by combustion, the fiber matrix emits and the desired photons pass into the light pipe and are directed to the target or targets.

In another embodiment of the invention, a band pass filter is placed onto the end or around the light pipe to reflect photons that are not with the selected band(s) back to the emitter, thus conserving energy and allowing the selected photons to be delivered to the target such as one or more photovoltaic cells, as shown in FIG. 1.

In another embodiment of the invention, the photon generator comprises a thin coating of emissive material on the light pipe device which is designed to focus the emitted light onto a target placed at the end of the light pipe or waveguide, as shown in FIG. 2.

In another embodiment of the invention, the burner contains separated gas (fuel injection) and oxidizer inlets to provide a torch type burner. This fuel injection system can be used with a number of different designs, including the cylindrical centrally-focused burner shown in FIGS. 1 and 2. One specific novel design involves the use of a cone-shaped object comprising a waveguide material which does not emit significantly when heated. The large end of the cone may be heated by a torch or other means. A conical section is hollowed out and an emitter is coated onto the surface as shown in FIGS. 3 and 4. An infinite variety of similarly designed concept are feasible. If two or more of these devices are placed back to back, photon energy is redirected to the target, exhaust gas energy is transferred into the fuel and oxidizer by means of a recuperator, a very efficient photon generating system can be built, e.g., the thermophotovoltaic devices shown in FIGS. 1-4. Other embodiments are shown in FIGS. 6, 7, and 8.

Other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims, and accompanying drawings where:

FIG. 1a is a perspective end view of a central firing cylindrical burner with light pipe;

FIG. 1b is a cross-sectional view the burner of FIG. 1a across a center of the tube;

FIG. 2 is a cutaway view of one half of a fuel injection burner as used to heat a section of a light pipe coated with an emitter;

FIG. 3a shows a thermophotovoltaic device comprising two opposing torch-type burners for heating a superemitter in the form of fibers suspended from a central area of a cone. The photons are focused through conical waveguides onto one or more targets such as photovoltaic cells;

FIG. 3b shows a thermophotovoltaic device comprising a number of torches impending directly on a superemitter coated onto a waveguide surface, or a superemitter fiber pulled from the waveguide surface by use of focused laser heating;

FIG. 4 shows the same thermophotovoltaic device as illustrated in FIGS. 3a and 3b except that a fluidized bed emitter is in the optical cavity section;

FIG. 5 illustrates the spectral emittance of two different ceramic superemitters when heated above the thermally stimulated quantum emission temperature using the same amount of gas (1500 BTU);

FIG. 6 illustrates two opposing oil fired burners inside a tube which is coated on an inside surface with fibers or a thin layer of superemissive material;

FIG. 7 illustrates a tubular burner with jets of flame impinging, at high velocity, onto a surface of the emitter. The photons generated are focused onto a target by means of a waveguide or other optical device(s);

FIG. 8 shows optical quality ceramic fibers woven in the shape of a U-turn with the open ends directed to the target; and

FIG. 9 illustrates the use of continuous optical fibers, which may comprise superemitter materials or may be coated with a superemitter materials. The use of optical fibers as the enclosure and emitter offers the unique function of almost total photon capture into one or more targets.

The present invention relates to novel photon generators, collectors, and directors and methods of preparing such devices. FIGS. 1a and 1b illustrate the general structural features of a simple porous ceramic burner 100 formed according to principles of this invention with an optical collector 160 (such as a light pipe) and target such as a photovoltaic (PV) cell 170. The porous ceramic burner comprises a fiber layer 132, a burner skeleton, and a fuel oxidizer system.

An intermediate or base layer may be required to bond an emitter material to the optical device. The base layer is preferably any low cost material that can bond the emitter to a light pipe effectively (preferably with thermal-stimulated superemitter materials). The base layer may comprise a high temperature material such as a high temperature fiber or coating, for example, pure or doped oxide(s) of uranium, thorium, ytterbium, aluminum, gallium, yttrium, erbium, holmium, zirconium, chromium or other high-temperature oxides. A preferred base fiber layer is one made from aluminum oxide, which is inexpensive and which lasts longer under oxidative conditions than do other inexpensive materials such as carbides, silicon oxide, or aluminosilicates.

The intermediate layer functions to bond the outer material to an inner portion of the optical device. An yttria intermediate layer may be used when an optical director, i.e., a light pipe, is formed from aluminum oxide, since it is difficult to maintain a bond between ytterbia and alumina after thousands of cycles. If an optical director, i.e., light pipe, other than aluminum oxide, such as YAG, is used the intermediate layer may be omitted. When the intermediate layer is used, it preferably comprises any material which is oxidation resistant and which bonds well to both the light pipe and superemitter.

The same principles apply to fiber systems. The outer fiber layer is preferably formed from a hightemperature superemissive material. The superemitter comprises a material which has an inner electron shell vacancy such that, upon heating, causes one inner electron below to jump into the vacancy as described in U.S. Pat. Nos. 4,906,178, 4,793,799 and 4,776,895, i.e., perhaps by means of a photon-electron interaction. These patents are herein incorporated by reference. Materials suitable for use as the superemitter include a narrow band or selected emitter such as neodymium, yttrium, ytterbium, holmium, erbium, thulium, cerium-thorium, thorium-holmium, aluminum-ytterbium-yttrium mixed oxides or mixtures thereof and other materials that emit radiation by an inner electron shell transition. The use of certain materials increases the useful life, reduces corrosion, and changes the emissivity characteristics of the resultant burner to those desired for a variety of uses such as photovoltaic devices, cooking food, heating water, pumping lasers, reacting materials photochemically, etc.

One method of fabricating such burners from fiber has been described in patent application Ser. No. 07/517,699 and in U.S. Pat. Nos. 4,758,003, 4,776,895, 4,793,799, and 4,906,178.

The fibers shown in FIGS. 3a and 3b and similar devices may be made by one of several methods, e.g., using lasers to heat the material and then pulling it from the surface of the melt on the waveguide or light pipe to form a type of optically coupled whisker. Such laser-fiber methods are commercially used to produce (monofilament) optical alumina fibers. A similar type of process can be used to produce continuous small optical quality fibers that are connected to a target waveguide or light pipe. Thus, all the photons generated inside the optical fiber may be combined and directed to one or two targets. The fibers may be coated with emissive materials by one or more of the methods listed below or in a copending patent application Ser. No. 07/695,983. Alternately, the fibers in the emitter shown in FIG. 3a may be made of small fibers of emissive compositions such as described in the application and in the other cited patents, e.g., U.S. Pat. No. 4,776,895.

One method of coating a superemitter material onto structures such as fibers and waveguides is to coat one or more intermediate layers on the fiber or waveguide, such as a layer of yttria and another layer of the emitting material, such as an ytterbia containing material. This method improves the bonding of the outer layer to the coated intermediate layer for some superemitters. The process of coating the underlying structure, i.e., fiber or waveguide, with one or more intermediate layers enhances bonding of the outer superemitter and involves the use of soluble ceramic precursors or mixtures of solubles and insoluble colloidal particles such as alkoxide, nitrates, colloidal hydroxide and alkoxide and/or nitrates to coat the light pipe by spray, dip or similar process. One such coating process is the subject of a co-pending application Ser. No. 07/695,783 filed May 6,1991. The coating of the intermediate layer is followed by a drying process and then a denitration process (or similar process to solidify the soluble material), such as by exposure to ammonia to form the hydroxide or by hydrolysis with H.sub.2 O and catalysis for the alkoxide case as described by J. Brinker and W. Scherer in "Sol-Gel Science", Academic Press, NY, N.Y. (1990), and the References in that book identified as 13-21 and 25-37 in Chapter 14, pages 839-880. In another method, colloidal hydroxide may be bonded directly to the oxide. Alkoxide coating methods have been extensively described in the literature, such as those for producing light pipe antireflective coatings.

One novel application of the present invention is a thermophotovoltaic (TPV) device having a fiber matrix, thin coating, or both added to the interior of a transparent solid optical tube 600, as shown in FIG. 6. The optical tube 600 comprises an outer body 640 formed from a thickness of an optically transparent material 630. The optical tube has an inside surface coated with fibers 610 of superemissive material. Two opposing oil fired torches provide fuel and oxidizer 620 from each end of the tube to effect thermal stimulation of the superemitter material.

FIG. 7 illustrates a TPV device 700 similar to that described above and illustrated in FIG. 6 comprising an optically transparent tube 710 having an inside surface coated with fibers of superemissive material. In this embodiment, fuel 720 is provided from a single end of the tube into the tube via a single fuel injection line 715 and mixed with oxidant within a centrally located burner 730 to provide thermal energy to effect stimulation of the superemitter material.

An oxidant such as oxygen or air may be used at temperatures well above the ignition point, making recuperation and fuel injection efficient and practical. The recuperator (330), shown in FIGS. 3a and 3b may optionally contain a catalytic surface which may be used to further reduce NOx. Also, the air may be replaced with or enriched with oxygen to further increase the temperature of combustion and the energy density (watts/m.sup.2) output of this design. The recuperator provides a method to significantly increase efficiency by recycling the energy in the exhaust products.

A central-focus firing cylindrical burner with fuel injection 100 is depicted in FIGS. 1a and 1b. Hot oxidant enters an outer distribution chamber 110 under pressure and flows through a porous structural member 120. As shown in FIG. 1b, the porous structural member 120 comprises fuel injection tubes 130 disposed therein. A fiber matrix 132 is disposed along an inside surface portion of the porous structural member. Fuel enters the fiber matrix 132 through fuel injection tubes 130 and is combusted upon mixing with the hot oxidant. Resulting exhaust products 140 move through an exhaust chamber 140 disposed between the porous structural member and a filter 150. One or more targets 170, i.e., photovoltaic cells, is protected from the combustion products by the filter 150. Narrow band radiation emitted from the superemitter fiber matrix 132 passes through the filter 150 and focuses onto a light pipe 160, which in turn directs the photons to the photovoltaic cells 170. A fluid (optionally) flows inside the optical tube (not shown) to cool the tube. Water may flow on the outside of the PV cells to keep them cool (not shown) or fuel and/or oxidizer may be used to further conserve energy.

FIG. 2 is a cutaway view of one half of a fuel injection burner 200 as used to heat a section of a light pipe 250 coated with an emitter (not shown). The burner 200 comprises a number of fuel injectors 220 disposed within a cylindrical body portion 210 of the burner. The injectors are oriented with their ends directed toward an axis running along the length of the burner so that flames 230 projecting from each fuel injector are directed to a central portion of the burner. Oxidant air enters the burner 200 via tubes 240 disposed within a porous structure.

FIG. 3a illustrates thermophotovoltaic device 300 comprising dual opposing torch-type burners 350 for heating a superemitter in the form of fibers 332 suspended from a central area of a cone. A fuel supply 310 and oxidant air supply 320 is routed to each burner 350 and combusted to form a flame 333A impending directly on the fibers. Photons 370 emitted from the fibers 332 focused through conical waveguides to end portions 365 and onto one or more targets 360, such as photovoltaic cells.

FIG. 3b illustrates a thermophotovoltaic device similar to that disclosed above and illustrated in FIG. 3a, comprising a number of torches 350B projecting flames 333B impending directly on a superemitter coated onto a waveguide surface, or a superemitter fiber pulled from the waveguide surface by use of focused laser heating.

Advantages of the optical light pipe systems constructed according to principles of this invention, including those where optical fibers have been pulled or grown from the melt, are that they: (1) are relatively inexpensive to construct; (2) are strong; (3) are durable; (4) are light weight; (5) achieve high radiation power densities of desired spectral wavelength(s); and (7) provide high energy density and efficiency.

Other preferred embodiments of the invention are capable of delivering high photon fluxes to one or more target(s) as shown in FIGS. 2-4 and 6 and may be constructed of bundles of optical fibers. The optical fibers may be attached to a lens or waveguide or other means to collect photons emitted from the superemitter materials. Selected wavelength bands of photons may be used to match the various PV cells, e.g., silicon photovoltaic cells having a band gap of about 1,100 nm., or multi-layer cells can be used in conjunction with multiband emitters. The useful photon fluxes from these multiband emitters are many times those produced by conventional photon sources and may be easily delivered to a variety of targets, such as photovoltaic cells, chemical reactors, heating devices, and optical collection means for lighting or growing plants and pumping other devices such as lasers. Further concentration of photons in an optical fiber system is possible using waveguides and other commercial devices. High electric power conversion efficiencies of from about 10 to 80 percent can be achieved from photon radiation emitted in the selected wavelength bands.

The estimates of efficiency of the TPV systems and other similar systems are based on the novel design described in FIGS. 1-4. The principle that electromagnetic emissions increase dramatically with temperature has been well established in physics by Stefan, e.g., the equation I.sub.(t) =.sigma..epsilon.T.sup.N, where .sigma. is a constant, .epsilon., the causivity, is a number between 0 and 1, and N equals 4 for a near blackbody. Using this principle, an estimate of power potential using data from the TPV measurements in the laboratory has shown N=7 to 14, depending upon the chemical composition of the emitter and the temperature. Recovering some of the exhaust gas energy raises the pre-combustion gas temperature. Similar results were obtained by the American Gas Association (see A.G.A. Research's Bulletin No. 91), i.e., the intensity of light emission increases with the 10th power of temperature for 99% thoria and 1% ceria emitters. Nelson estimates that emission for ytterbia increases with the 8th power of temperature. It is estimated that superemitters (assuming e is constant) go up as N increases from the 7th to the 14th power. Using the recuperation concept or oxygen to raise the temperature, very high photon fluxes can be obtained.

The recuperator burner and emitter may be designed and constructed as a unit. One method of producing a photon generator consisting of a single recuperator burner emitter system includes the following steps: 1) impregnating an organic foam with soluble precursor salts; 2) weaving fibers such as fiber optics or rayon impregnated with superemitter precursor material as described in several earlier patents, e.g., U.S. Pat. No. 4,776,895; 3) drying the fibers; 4) treating the fibers with ammonia to reduce nitrates to hydroxide; and 5) heating the fibers to convert hydroxide to oxide.

In another variation of the invention, a burner recuperator foam structure may be fabricated by weaving optical fibers into a green foam as shown in FIG. 8 as 810, then heating the system to burn out the organics, forming a ceramic foam with fibers or preferred type, orientation, and density. The fibers may be coated with the same various formulas as previously described.

In another embodiment of the invention, rayon may be woven into the organic foam before impregnation. However, this method may use more superemitter material. Generally, methods which reduce the cost of superemitters are preferred. However, sometimes improved product and lower process cost are important, depending on the volume and value added.

The idea of inverting the burner system and collecting the photons in a light pipe or wave guide has two main benefits, it increases the ratio of emissive surface area to collector area, thereby reducing the number of photovoltaics required because a higher photon flux can be obtained, and it results in a more compact design (higher energy density) which will cost less for certain type of hybrid electric vehicles because the energy package will occupy less space and be more durable.

FIG. 8 shows a foam burner system 800 in which fibers 805 are woven into foam 810 so that open ends 820 of the fiber 805 are directed toward a target 830. The fuel and oxidizer entering means into the foam may comprise tubes 850 for the oxidizer 851 and fuel 852 which may be premixed in a very small chamber (not shown) at very high speed to prevent combustion before exiting the chamber to produce a very hot flame (not shown). The flames may be designed so that they oppose each other in the area of the fiber to prevent damage to the fibers. The fibers 805 may be coated with a superemissive material 806. The emitted photons (not shown) are either emitted towards the target or at some angle. The photons that get trapped in the fiber 805 will pass into the foam and out the other end of the U-turn and be directed to the target, thus providing increased photon flux at the target.

Another embodiment of the invention is a fluidized bed combuster 400, as shown in FIG. 4. Fuel and oxidizer enter the combuster through a bottom opening 405 and the resultant exhaust products of the combustion exit the combuster via top opening 410. Photons 415 are emitted from beads 420 disposed within the combuster that are coated with or otherwise comprise superemitter material. The photons are directed to one or more targets 430 by a waveguide 425 and are converted to electricity by PV cells located at one or more targets 430.

FIGS. 3a and 3b illustrate other embodiments of the invention using superemissive fibers made by the process described in U.S. Pat. No. 4,776,895 or by the laser process described above.

A holmium oxide emitter has demonstrated an ability to produce more than twice the absolute radiant intensity that of ytterbia, per Btu of heat, as shown in FIG. 5. Advanced photovoltaic materials such as InGa AlAS, InGaAsP, and similar group III-V compounds (InGaAs) used with this emitter are expected to produce a TPV generator twice as efficient as one with silicon based PV cells.

The recuperation temperature of a normal air/fuel gas premix system is limited by its combustion temperature. For natural gas in air, this combustion temperature is somewhere below 800.degree. C. In order to increase the emissive fiber temperature using recuperation, it is desirable to have the combustion air as hot as possible without preignition of the mixture. FIGS. 2 and 3 depict a possible configuration where fuel and oxidizer are injected through tubes into the premix chamber as in a torch. The recuperated air can then be heated to 1000.degree. C. without fear of preignition if the premix chamber is small and the velocity of the reactants is high enough.

FIG. 9 illustrates an TPV device 900 comprising an optical waveguide cavity 910, in which the primary emitter comprises one or more optical fiber emitters 940, which may comprise a superemitter or may be coated with such material (not shown). The photons (not shown) are emitted from the emissive material when heated above a certain temperature and are either trapped in the optical fiber 940 or the outer waveguide 910, which alternately may comprise a similar form made from the optical fibers (not shown), and thus in either case the photons will be directed to the target 960 by waveguide lens 950, or optical fiber(s). The fuel and air are mixed in the premix chamber,(not Shown) within the burner tube 930 and the flame is produced as the reactants exit the holes 920, and the impinge on the superemitter material-optical fiber system 940. The photons produced are then focused onto the target.

Although the above exemplary embodiments of a ceramic burner comprising the use superemissive materials disposed on light pipes-and waveguides, and method for making the same, has been specifically described and illustrated, variations will be apparent to those skilled in the art. It is, therefore, to be understood that the present invention is not intended to be limited to the particular embodiments described above. The scope of the invention is defined in part by the following claims.