Abstract:
Embodiments of an apparatus that can convert energy across a broad spectrum of wavelengths. These embodiments utilize concentrating optics in combination with one or more of an integrated filter, a cooling mechanism, and a high-efficiency low current cell architecture to form efficient and cost-effective TPV devices. During operation, these components reduce the ratio of cell area to emitter area by concentrating the energy the emitter emits, thereby reducing the total cost of materials and promoting efficiency through integrating the filter and cooling mechanism into the device design.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/536,863, entitled “APPARATUS FOR CONVERTING THERMAL ENERGY TO ELECTRICAL ENERGY,” the contents of which is incorporated by reference it its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The subject matter of this disclosure relates to energy conversion and, in particular, to embodiments of an apparatus that convert thermal energy to electrical signals. 
         [0004]    2. Description of Related Art 
         [0005]    Thermophotovoltaic (TPV) devices convert thermal energy to electric power in accordance with the principles of operation common to solar cells. In particular, an emitter (or “radiator”) emits energy in response to thermal energy that heats the emitter. The thermal energy can arise from a direct source (e.g., through combustion, solar, atomic decay, etc.) or from an indirect source (e.g., industrial waste heat processes). In each case, thermal energy impinges on photoelectric conversion elements, e.g., TPV cells, which convert the energy into electric signals. 
         [0006]    Elements of TPV devices include an absorber/emitter material, an energy filtering media, TPV cells for energy conversion, and a cooling mechanism. To successfully commercialize TPV devices (and systems incorporating TPV devices), proposed designs utilize cost-effective TPV cells that can convert as much of the energy the emitter radiates into electrical signals. Energy emitted at less than the TPV cell semiconductor bandgap cannot be converted to electrical energy. This unused energy is often parasitically absorbed by the TPV device as heat, which decreases efficiency of the TPV cell and, ultimately, reduce cost-effective operation. 
       SUMMARY 
       [0007]    The present disclosure describes embodiments of an apparatus that can convert energy across a broad spectrum of wavelengths. These embodiments utilize concentrating optics in combination with one or more of an integrated filter, a cooling mechanism, and a high-efficiency low current cell architecture to form efficient and cost-effective TPV devices. These components reduce the ratio of cell area to emitter area by concentrating the energy the emitter emit, thereby reducing the total cost of materials and promoting efficiency through integrating the filter and cooling mechanism into the device design. 
         [0008]    During operation, embodiments of the proposed apparatus concentrate a large area of energy onto a small TPV cell area via unique construction that enables cost-effective application of the concentrating thermophotovoltaic cells. These embodiments can deploy in high-temperature environments (e.g., a fire, a boiler, an oven, a turbine, a generator, etc.). To harness energy, one or more embodiments can utilize an outer housing made of an absorber/emitter material (AEM), which can be contoured to various shapes, sizes, and form factors (e.g., an elongated, rod design) to fit the environment and/or contour and configuration of the thermal energy source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is now made briefly to the accompanying drawings in which: 
           [0010]      FIG. 1  depicts a schematic view of an exemplary embodiment of an apparatus that uses thermal energy to generate electrical signals; 
           [0011]      FIG. 2  depicts a perspective view of an exemplary embodiment of an apparatus that uses thermal energy to generate electrical signals; 
           [0012]      FIG. 3  depicts a front, cross-section view of the apparatus of  FIG. 2 ; 
           [0013]      FIG. 4  depicts a side, cross-section view of the apparatus of  FIG. 2 ; 
           [0014]      FIG. 5  depicts a perspective view of an exemplary embodiment of an apparatus that uses thermal energy to generate electrical signals; 
           [0015]      FIG. 6  depicts an example of an array of cells that can generate electrical signals in response to electromagnetic radiation; 
           [0016]      FIG. 7  depicts a front, cross-section view of a cooling element for use in an apparatus, e.g., the apparatus of  FIGS. 1 ,  2 ,  3 ,  4 , and  5 ; 
           [0017]      FIG. 8  depicts a front, cross-section view of a cooling element for use in an apparatus, e.g., the apparatus of  FIGS. 1 ,  2 ,  3 ,  4 , and  5 ; 
           [0018]      FIG. 9  depicts a schematic diagram of an example of a cell for use in an apparatus, e.g., the apparatus of  FIGS. 1 ,  2 ,  3 ,  4 , and  5 ; 
           [0019]      FIG. 10  depicts a perspective view of an example of a concentration feature for use in an apparatus, e.g., the apparatus of  FIGS. 1 ,  2 ,  3 ,  4 , and  5 ; and 
           [0020]      FIG. 11  depicts a perspective view of an example of a concentration feature for use in an apparatus, e.g., the apparatus of  FIGS. 1 ,  2 ,  3 ,  4 , and  5 . 
       
    
    
       [0021]    Where applicable like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated. 
       DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a schematic view of an exemplary embodiment of an apparatus  100  that converts radiation (e.g., thermal energy) to electrical signals (e.g., current). The apparatus  100  includes an emitter element  102 , an energy conversion element  104 , a concentrator element  106 , and a cooling element  108 . The apparatus  100  also includes a filter element  110 , which can reside between the emitter element  102  and the concentrator element  106 . At a relatively high level, during operation, the emitter element  102  generates radiation in response to heat and/or other energy. The radiation impinges on the concentrator element  106 , which directs the radiation onto the energy conversion element  104 . In one example, the energy conversion element  104  converts the radiation into electrical signals. The cooling element  108  circulates a heat transfer medium (also “cooling fluid”) proximate the energy conversion element  104 . The heat transfer medium dissipates thermal energy that the energy conversion element  104  does not convert to electrical signals. 
         [0023]    Embodiments of the apparatus  100  can harness waste energy, e.g., in the form of heat that results from combustion of fuel. These embodiments find use in various applications, e.g., boilers, furnaces, ovens, and other applications where elevated temperatures prevail and/or where thermal energy is abundant. For example, the apparatus  100  can survive fluids (e.g., gas) at temperatures in excess of 500° C., which may exhaust from a combustion chamber that burns fuel (e.g., wood, coal, natural gas, etc). These heated fluids heat the emitter element  102 , which in turn generates electromagnetic radiation that the energy conversion device  104  converts to electrical signals. 
         [0024]    Designs for the apparatus  100  utilize components that comport with the environment and/or application in which the apparatus  100  is found. These designs leverage features and aspects of one or more of the elements (e.g., emitter element  102 , energy conversion element  104 , condenser element  106 , and cooling element  108 ) to convert the thermal energy in the heated fluid to electrical signals. As set forth more below, this disclosure describes features of the apparatus  100  in terms of form factors that may coincide with one or more particular applications. For the combustion applications discussed above, the form factor may conform to the shape of the combustion chamber and/or to the size (e.g., diameter) of pipes, tubes, and conduits that receive exhaust gases from the combustion chamber. However, while indicative of one or more embodiments of the apparatus  100 , these form factors can vary in respect to any number of features to attain an output (e.g., electrical signals) that satisfies potential design criteria and/or output thresholds. Such design criteria may require, for example, electrical signals that exhibit certain current levels to re-charge a battery and/or to operate a specified device (e.g., a radio). 
         [0025]    The emitter element  102  withstands conditions (e.g., temperature, pressure, humidity, etc.) consistent with the designated application. The emitter element  102  also maximizes the spectrum of electromagnetic radiation generated in response to the thermal energy the emitter element  102  absorbs. Examples of the emitter element  102  act as grey-body and/or black-body emitters, comprising one or more materials that absorb thermal energy and emit electromagnetic radiation. These materials allow implementation of the emitters at temperatures of at least 500° C. or more and, in one particular implementation, at around 1200° C. Emitters of this type can generate electromagnetic radiation having wavelengths, for example, of 900 nm and greater. In one example, the emitter element  102  comprises one or more types of metals (e.g., tungsten, steel, tantalum, etc.), ceramics (e.g., aluminum oxide, silicon carbide, etc.), and composites, as well as compositions, derivations, and combinations thereof. The emitter element  102  may further utilize coatings, paints, and like surface treatments (e.g. erbium oxide, yttrium oxide, silicon carbide) that may improve absorption of heat and/or optimize properties of the emitter element  102  for radiating and dispersing the electromagnetic radiation. In one example, the selection of materials and construction for the emitter element  102  tune the electromagnetic radiation that emanates from the emitter element  102 . Such construction may incorporate materials (e.g., tungsten photonic crystals (PhC)) throughout the emitter element  102  to tailor the electromagnetic radiation, e.g., with a cut-off wavelength of 2.3 μm. 
         [0026]    Examples of the energy conversion device  104  include integrated circuit and semiconductor devices with structure that generate electricity from electromagnetic radiation. Such devices include thermophotovoltaic cells and/or combination of discrete photovoltaic elements (e.g, diodes) and photovoltaic materials to generate electricity from electromagnetic radiation. These types of devices can comprise a substrate and various layers of materials, the combination of which may cause the device to operate for the purposes herein. This layered structure operates (e.g., to generate electrical signals) in response to electromagnetic radiation over a wide spectrum of wavelengths that are consistent with black-body and grey-body emitter, as discussed above and further below. 
         [0027]    Construction of the apparatus  100  positions the concentrator element  106  to receive electromagnetic energy that radiates from the emitter element  102 . The construction also directs the electromagnetic energy towards, and in one example, onto the energy conversion device  104 . Effective configurations for the concentrator element  106  gather and/or collect the electromagnetic radiation from a large area (e.g., proximate the emitter element  102 ) and direct the radiation onto a smaller area (e.g., onto the surface of the energy conversion device  102 ). The ratio of the large area to the smaller area can define a concentration factor for the concentrator element  106 . In exemplary devices for use as the concentrator element  106 , the concentration factor can be about 2 or greater and, in one example, in a range from about 2 to about 5. 
         [0028]    The concentrator element  106  exhibits properties that concentrate the electromagnetic radiation from the emitter element  102 , e.g., onto the surface of the energy conversion device  104 . These properties can include optical properties and reflective properties, both of which can change the direction and/or focus of the electromagnetic radiation that passes through the concentrator element  106 . For optical properties, the concentrator element  106  can form optical components (e.g., lenses) that comprise materials that can transmit electromagnetic radiation of wavelengths contemplated herein. These optical components can have various shapes (e.g., concave, convex, oblate, spheroid, etc.) to match operation of apparatus  100  and/or the constituent components. In one example, the optical components comprise magnesium fluoride, fused quartz, fused silica, and like materials, derivations, combinations, and compositions thereof. For reflective properties, the concentrator element  106  can embody structures with an input opening and an output opening that has an output area that is smaller than an input area of the input opening. The structures can further incorporate wall members with reflective materials and/or coatings to direct the electromagnetic radiation from the input opening to the output opening and, ultimately, onto the surface of the energy conversion device  104 . For example, the concentrator element  106  may comprise metals of various types, which form reflective surfaces. 
         [0029]    Examples of the filter element  110  form a barrier that allows electromagnetic radiation of a first wavelength (and/or a first range of wavelengths) to pass and that does not allow other electromagnetic radiation of a second wavelength (and/or a second range of wavelengths) to reach the energy conversion device  104 . This barrier helps minimize the operating temperature of the apparatus  100  and, in particular, the operating temperature of the energy conversion device  104 . Construction of the barrier can be tuned to pass only electromagnetic radiation of wavelengths that will stimulate the energy conversion device  104  to generate electrical signals. In one example, the barrier can reflect other electromagnetic radiation away from the energy conversion device  104 . The reflected electromagnetic radiation may be of wavelengths that the energy conversion device  104  can not convert to electrical signals. Exemplary devices for the filter element  110  can embody a plasma filter or like components, that reflects electromagnetic radiation in a direction, e.g., back toward the emitter element  102 . Suitable materials for use in the filter element  110  comprise transparent material (e.g. sapphire, fused silica, and magnesium fluoride) with properties that transmit electromagnetic radiation of wavelengths emitted by the emitter element of  102 . Such materials may be arranged in one or more layers that reflect radiation of certain wavelengths and allows radiation of certain wavelengths to pass to the energy conversion device  104 . For example, the plasma filter may reflect electromagnetic radiation with wavelengths of 1.8 microns to about 10 microns, of 2.3 microns or greater, and/or at least about 1.8 microns or greater. 
         [0030]    The cooling element  108  can reduce and maintain the operating temperature of the energy conversion device  104  at levels that permit operation, e.g., at or below about 100° C. In one embodiment, the cooling element  108  is in thermal contact with the energy conversion device  100  to promote the most direct path for heat to dissipate. Examples of the heat transfer medium include water, ethylene glycol, as well as like refrigerants and materials. These materials can circulate through the cooling element  108 . For active circulation, the apparatus  100  may couple with a flow generator (e.g., a pump) or other device that can pressurize the material to cause the exchange of material through the cooling element  108 . In some applications, the cooling element  108  may employ passive circulation, e.g., when the cooling element  108  incorporates a heat pipe with multiple chambers and integrated construction that can help to rapidly dissipate heat away from the energy conversion device  104 . 
         [0031]      FIGS. 2 ,  3 , and  4  depict another exemplary embodiment of an apparatus  200 . As shown in  FIG. 2 , the emitter element  202  includes an emitter body  212  with a first end  214 , a second end  216 , and a longitudinal axis  218 . One or more support elements  220  extend from the emitter body  212  to engage, in one example, an inner surface  222  of a pipe  224 . At the first end  214 , the apparatus  200  includes an end cap  226  (also “nose cone  226 ”), which has a shape to enhance thermal distribution (e.g., to make temperature uniform across the emitter body  212 ) between the outer surface of the end cap  226  and a flow F of working fluid (e.g., gases) that flows through the pipe  224 . The apparatus  200  also has one or more external connections (e.g., a cooling connection  228  and a power connection  230 ) that secure to the second end  216 . 
         [0032]    The apparatus  200  may be constructed as a monolithic device, wherein the emitter body  212  and the end cap  226  are formed as a single unitary structure. Such construction may leave the second end  216  open to allow for assembly and installation of components therein. A cover can be placed over the open second end  216  to secure and seal the apparatus  200 . In other examples, the apparatus  200  may be assembled from various pieces that fasten together using adhesives, welds, fasteners (e.g., screws and bolts), and like techniques. 
         [0033]    The support elements  220  can form fins having an aerodynamic shape (e.g., an airfoil). The fins can form integrally with the emitter body  212  or, in other constructions, the fins can fasten to the outer surface of the emitter body  212  using known fastening techniques. In one embodiment, the fins are sized and configured to fit within the pipe  224 . The fit can be loose, i.e., wherein the fins limit movement of the apparatus  200  but the fins do not offer resistance against the inner surface  222  to allow the apparatus  200  to slide through the pipe  224 . In other embodiments, the fins can engage the inner surface  222 , e.g., via a surface that causes friction and/or exerts a force (e.g., a spring force) against the inner surface  222 . Examples of the fins limit conduction, e.g., via embodiments wherein the shape is minimized to limit conduction from the emitter body  212  to the pipe  224 . 
         [0034]    The emitter body  212  is amenable to various form factors (e.g., shapes and sizes) as might be dictated by the application (e.g., the size and shape of the pipe  224 ). Examples of the form factor include the elongated cylindrical structure that is shown in  FIG. 2 . In other examples, the form factor for the emitter body  212  can embody rectangular and cubic features, as well as other multi-sided and non-circular (e.g., ellipsoid) shapes. 
         [0035]    External connections  228 ,  230  allow ingress and egress of cooling fluid (e.g., via cooling connection  228 ) and electrical signals (e.g., via power connection  230 ). Examples of the cooling connection  228  can include tubes and conduits that mate with one or more corresponding fittings on the apparatus  200 . Examples of the fittings include threaded connectors as well as threaded features (e.g., bores) to receive connectors therein. In the same respect, the power connection  230  can comprise electrical cables (e.g., coaxial cables, multi-wire cables, copper cables, etc.) with one or more electrical connections that couple with an electrical connection on the apparatus  200 . For purposes of example, in one implementation the cables can couple with a load (e.g., a motor) and/or a storage unit (e.g., a battery) that the apparatus  200  is to supply with electrical signals. Collectively, the external connections  228 ,  230  can form a single cable and/or conduit, which can function as one or more of the external connections  228 ,  230 . 
         [0036]    As best shown in  FIG. 3 , which is a cross-section of the apparatus  200  taken at line A-A of  FIG. 2 , the emitter body  212  has an outer surface  232  and an inner surface  234  that forms an interior volume  236 . The energy conversion device  204  resides in the interior volume  236  in the form of one or more cells  238  disposed circumferentially about the longitudinal axis  218 . The concentrator element  206  has a plurality of concentrator features  240  positioned radially outward of the cells  238  and, in one aspect, radially inwardly of the emitter body  212 . The concentrator features  240  have an optical axis  242  that aligns with an axis  244  (and/or centerline  244 ) of the cells  238 . This alignment ensures the concentrator features  240  directs electromagnetic energy onto the entire operating surface of the corresponding cell  238 . In one embodiment, the filter element  210  includes a material ring  246  disposed radially outwardly of the concentrator element  206 , e.g., between the concentrator element  206  and the emitter body  212 . 
         [0037]    In one embodiment, the emitter body  212  forms a housing to surround, protect, and maintain the components disposed therein. The housing is resistant to high temperatures (e.g., in excess of 500° C.). As set forth above, the housing absorbs heat and/or thermal energy on the outer surface  232 , transmits the energy towards the inner surface  234 , and disperses the energy as electromagnetic radiation that radiates into the interior volume  236 . In other embodiments, construction of the apparatus  200  forms a hermetically-sealed chamber and/or maintains the interior volume  236  at a desired pressure. In one example, the hermetically-sealed chamber is evacuated, e.g., to form a vacuum. The vacuum helps limit interference, often by air or moisture, of electromagnetic radiation in the desired geometric direction. This configuration may improve output, e.g., by reducing energy loss due to conduction or convection. The hermetically-sealed chamber can also can retain various fluids including liquids and gases (e.g., nitrogen, hydrogen, and helium) that afford a desirable environment for energy conversion to occur. Construction of the emitter body  212  can further entail the use of multiple pieces and/or laminated layers of material. For example, this disclosure contemplates structures for the emitter body  212  in which a first inner surface  234  is part of a first material layer and the outer surface  232  is part of a second material layer disposed on the first materials layer. 
         [0038]      FIG. 4  illustrates a cross-section of the apparatus  200  taken at line B-B of  FIG. 2 . In the example of  FIG. 4 , the cooling element  210  has a tubular structure  248  that couples with the cooling connection  228 . The tubular structure  248  forms a cooling volume  250  through which a cooling fluid  252  is disposed. In one embodiment, the energy conversion device  204  comprises a first array  254  of the cells  238  that extends along the tubular structure  248 . The concentrator element  206  forms a second array  256  of the concentrator features  240 , one each corresponding to the number of the cells  238  in the first array  254 . 
         [0039]    Examples of the tubular structure  248  may contain a fixed amount of the cooling fluid  252 . This fixed amount may not circulate out of the apparatus  200 . However, in other examples, the cooling fluid  252  can circulate through the tubular structure  248 , e.g., under pressure from a cooling fluid supply that is external to the apparatus  200 . 
         [0040]    Examples of the material ring  246  can form a cylinder that extends at least partially along the longitudinal axis  218 . The cylinder permits electromagnetic energy to pass to the concentrator features  240 , but may limit transfer of thermal energy, e.g., via conduction and convention between the emitter body  212  and the concentrator features  240  (and other elements and components of the apparatus  200 ). In one embodiment, the material ring  246  secures directly, by coating or other means, to the inner surface  234 . In other examples, the material ring  246  can be supported at various positions by supports that couple the material ring  246  with the cooling element  210  and/or provide mechanical fastening with the emitter body  212 . As set forth in more detail below, the material ring  246  can be disposed onto the surface of the cells  238  using known deposition techniques. 
         [0041]    While various processes are contemplated, in one example, the material ring  246  can be constructed using various techniques, including via epitaxial lift-off, and incorporated into the apparatus  200 . The material ring  246  can be positioned between the emitter body  212  and the concentrator feature  240  and/or disposed on the cells  238 . For purposes of positions on the cells  238 , the material ring  248  can be grown and/or deposited thereon directly. In one example, bonding material (e.g., a low absorbing yet highly thermally conductive interface material) might be used to secure the material ring  246  to the concentrator features  240  and the cells  238 . In one embodiment, the filter component comprises a layer of InPAs nominally doped to 5E 19 . The InPAs layer can be grown by MOCVD on an InP substrate. 
         [0042]    The material ring  246  may help concentrate the electromagnetic radiation onto the cells  238 . For example, the material ring  246  may incorporate geometric shapes (e.g. concave lenses, convex lenses, linear lenses, etc.) that reflect and/or transmit electromagnetic radiation to the concentrator features  240 . Such enhancements may reduce the size and shapes of certain components (e.g., the concentrator element  206 ), thus resulting in lower costs and reducing absorption of electromagnetic radiation that may increase thermal loading of the concentrator component  206 . It may be desirable to maintain the components (e.g., the concentrator component  112 ) inside the housing at lower temperatures (e.g., at or below 100° C.). This feature can be accomplished by limiting the absorption of radiation by the filter component and, where applicable, by any bonding material that secures the filter component to portions of the apparatus  100  such as to the intermediary member  110 . 
         [0043]      FIG. 5  illustrates a perspective view of another exemplary embodiment of an apparatus  300 , which contemplates configurations of the proposed device in the form of a sheet, a panel, or other substantially flat arrangement. In the example of  FIG. 5 , the apparatus  300  includes a first panel  358  that can emit electromagnetic radiation, a second panel  360  that can convert the electromagnetic radiation into electrical signals, and a third panel  362  that can concentrate the electromagnetic radiation. The apparatus  300  can also include a fourth panel  364  that can dissipate heat, e.g., from the second panel  360 . A fifth panel  366  can act as a barrier to filter electromagnetic radiation of certain wavelengths, as set forth herein. 
         [0044]    The panels of the apparatus  300  may be affixed together into a single device, e.g., using clamps, frames, and similar fixtures. The types of fixtures may secure to the periphery of the panels, as desired. In other constructions, the individuals panels may be flexible, e.g., if constructed using materials that exhibit physical properties consistent with resilient and/or pliable materials. Embodiments of the apparatus that are constructed in this manner may conform to surfaces that are in and/or susceptible to thermal energy on the order disclosed above. These embodiments may comprise an adhesive or other bonding agent and/or material layer that can withstand the temperatures. Such materials may simplify implementation by providing a simple way to position and secure (e.g., adhere) the apparatus  200  into position. 
         [0045]      FIG. 6  illustrates one construction of an array  400  that can position, secure, and couple cells (e.g., cells  238  of  FIGS. 2 ,  3 , and  4 ). The array  400  includes a lead frame  402  with one or more cell areas  404  to receive cells  238  therein. The array  400  can also include interconnects  406  that conduct electrical signals from the cells  238  to a terminal end  408 . The terminal end  408  may include a connector  410  or similar connective feature, which can allow the array  400  to couple with other arrays  400  that are found in an apparatus (e.g., apparatus  100 ,  200  of  FIGS. 1 ,  2 ,  3 , and  4 ) and/or with conductive wiring that connects to the apparatus and with a load. In one embodiment, the lead frame  402  may incorporate one or more supplemental concentrator features (e.g., concentrator features  240  ( FIG. 4 )) to ensure all radiation is directed to the surface of the cell  238 . In other embodiments, the lead frame  402  may likewise incorporate supplemental cooling features that can help to dissipate heat. 
         [0046]    Examples of the array  400  can be constructed using traditional semiconductor high-volume assembly technologies. The lead frame  402  can comprise various materials (e.g., metals, ceramics, etc.), which can receive solder, adhesive, and like bonding agents to mechanically secure and interconnect the cells  238  with the lead frame  402  and to one another, e.g., in series. Construction of the array  400  may incorporate one or more carriers comprising, for example, ceramics (e.g., ALN-DBC) onto which the cells  238  mount. The cell areas  402  can be sized to receive the carriers, which may be larger in size as compared to the cells  238 . Exemplary construction may require the cells  238  to be soldered to the carriers, which mount to the lead frame  402 . Interconnecting leads (e.g., wirebonds) can be added that extend from a first end that couples with interconnects on the cells  238  to a second end that couples with corresponding pads and interconnects on the lead frame  402 . In this configuration, the interconnecting leads conduct electrical signals from the cells  238  to the interconnect  406 , which can then conduct the electrical signals to the terminal end  408 . 
         [0047]    At the terminal end  408 , embodiments of the array  400  may be outfit with terminals and/or other connective elements that electrically connect the leads (and/or the cells  116 ) to the exterior of the apparatus. This configuration will permit the electrical signals to couple, e.g., with a load, a plug, an extension cord, and the like. The leads may be electrically isolated from the components of the apparatus including the heat transfer mechanism  108 . In one embodiment, the connector  410  may further comprise interface circuitry that can accept a plug or can otherwise permit a peripheral device to be coupled with the array  400 . 
         [0048]      FIGS. 7 and 8  illustrate details to describe the tubular structure in examples of a cooling element  500  ( FIG. 7 ) and a cooling element  600  ( FIG. 8 ). In the example of  FIG. 7 , the cooling element  500  includes an outer tube  502  with an inner surface  504  and an outer surface  506  that forms one or more mounting surfaces  508 , e.g., for the array  500 . The cooling element  500  also includes an inner tube  510  that resides inside of the outer tube  502 . The inner tube  510  has an outer surface  512  that is spaced apart from the inner surface  504  to form a gap  514 . Examples of the outer tube  502  can form a hexagon, although this disclosure contemplates other forms with surfaces (e.g., outer surface  506 ) that can receive and position the array  500  to receive electromagnetic radiation. 
         [0049]      FIG. 8  shows a construction in which the cooling element  600  with a central support feature  602  having a core  604  and one or more extension members  606  that extend therefrom. The cooling element  600  also includes outer ring member  608 , disposed radially outwardly from the core  604 . The outer ring member  608  includes one or more mounting positions  610  to receive an array, e.g., the array  400  of  FIG. 6 . This configuration creates one or more channels  612  through which cooling fluid can reside and/or transit the cooling element  600  to dissipate heat from the array  300 . 
         [0050]      FIG. 9  depicts an example of a cell  700  that can generate electrical signals in response to stimulation by electromagnetic radiation. The cell  700  has a layered structure  702  with a substrate  704  and one or more junction layers (e.g., a first junction  706  and a second junction layer  708 ). The layered structure  702  further includes one or more auxiliary layers (e.g., a first auxiliary layer  710  and a second auxiliary layer  712 ) as well as one or more filter layers (e.g., a first filter layer  714 ). For examples of the cell  700  that include the filter layer(s), exemplary construction of the apparatus  100 ,  200  may require the cell  70  is positioned to allow the filter layer(s) to receive the electromagnetic radiation before the remainder of the cell  700 . 
         [0051]    It may also be desirable to maintain a low level of current in the cells  700 , while still enabling the cell  70  to convert radiation energy to electrical energy at a high conversion efficiency (e.g., of about 30% or more and, in one example, between about 15% and about 50%). The level of current can be reduced by interconnecting PN junctions on the cell  700  and, more particularly, by connection of the PN junctions in series. In one embodiment, this feature can be achieved by dividing the area of the cell  700  into smaller cell areas and using metallic interconnections to connect adjacent, smaller cells. In another example, as shown in  FIG. 8 , the cell  700  uses a vertically-stacked multi junction approach where three independent PN junctions are connected in series, e.g., by tunneling diodes. This approach enables the cell  700  to operate at a higher voltage but lower current while maintaining sufficient conversion efficiency when placed under concentration and affords a much simpler method of manufacturing of the cells  700 . Moreover, using this approach may enable lower-cost electricity generation and allow for greater flexibility in designing cells (e.g., thermophotvoltaic cells) that are optimized to the apparatus as set forth herein. 
         [0052]    In one example, the first junction  706  and the second junction  708  can generate electrical signals in response to, respectively, a first wavelength range and a second wavelength range. The first wavelength range and the second wavelength range can include one or more wavelengths not found in the other. For example, the first wavelength range may include wavelengths from about 900 μm to about 1.6 μm and the second wavelength range may include wavelength from about 1.6 μm to about 2 μm. The extend of the ranges can also be clarified in terms of bandgap, e.g., where the ranges cover 0.52 eV, 0.64 eV, and 0.74 eV. 
         [0053]      FIGS. 10 and 11  depict examples of a concentrator feature  800  and a concentrator element  900 . The example of  FIG. 9  shows the concentrator feature  800  in the form of an optical element  802  with a curvilinear surface  804 . The concentrator feature  800  can include a first area  806  and a second area  808  that is smaller than the first area  806 . Examples of the concentrator feature  800  can include optics, lens, and like optical facts that can form and direct electromagnetic radiation, e.g., from the first area  806  to the second area  808 . In the present example, the curvilinear surface  804  forms a convex curve, which focuses the electromagnetic radiation from a larger area to a much smaller area. This disclosure contemplates other shapes for the curvilinear surface  804  to concentrate electromagnetic radiation, e.g., onto thermophotovoltaic cells. 
         [0054]    In the example of  FIG. 10 , the concentrator feature  900  includes a concentrator body  902  with a top  904  and a bottom  906 . The concentrator body  902  is constructed of one or more wall members  908  which couple together to form an interior passage  910  extending from a first opening  912  (at the top  904 ) to a second opening  914  (at the bottom  906 ). The concentrator body  902  forms a first area  916  and a second area  918  that is smaller than the first area  916 . 
         [0055]    The concentrator feature  900  can be made, e.g., of metal, that is formed in the various shapes, and generally reflective and or coated with a reflective coating material at least on the surfaces of the wall members  908  that bound the interior passage  910 . The shape of the concentrator feature  900  is selected to direct electromagnetic radiation through the interior passage  910 , with the dimensional difference between the first opening  912  and the second opening  914  in the present example useful to collect large amounts of electromagnetic radiation at the top  904  and concentrate the electromagnetic radiation at the bottom  906 . 
         [0056]    In view of the foregoing, embodiments of the apparatus discussed herein can convert heat and thermal energy to electrical signals, e.g., for re-charging a battery. These embodiments arrange cells (e.g., thermophotovoltaic cells) in combination with concentrator features to provide the cells with electromagnetic radiation in sufficient amounts to generate electrical signals. One or more embodiments can be combined to form a plurality of the apparatus, which can effectively increase output of electrical signals. These systems may be found in various operating environments and used in various applications. These operating environments include one embodiment comprising a singular apparatus in an outdoor fire application, one embodiment comprising a plurality of apparatus aligned linearly within a home or commercial boiler application, one embodiment comprising a circular array of apparatus aligned at an appropriate angle to take advantage of a singular burner as in a cookstove environment, and one embodiment comprising a linear array of apparatus within an industrial heat application such as a smelting furnace, reflow oven. 
         [0057]    As used herein, an element or function recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or functions, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the claimed invention should not be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
         [0058]    This written description uses examples to disclose embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.