Abstract:
The present invention provides systems and methods for conversion of thermal energy to electrical energy. The devices and methods according to the invention allow for efficient conversion of thermal energy to electrical power in a meso-to-micro scale device. In general, the present invention relates to a device for converting thermal energy to electrical energy comprising a working fluid contained within at least one region. The region is exposed to a thermal gradient and thermal energy is transferred through at least a first thermal energy-accepting layer for transferring thermal energy to the working fluid. At least one diaphragm is acted upon by the working fluid, which thermodynamically expands upon thermal energy being transferred thereto to flex the diaphragm. A mechanism is provided for converting the structural flexure of the diaphragm to electrical energy, along with at least one thermal energy transfer mechanism for transferring thermal energy from the working fluid through the diaphragm to cool the working fluid and cause thermodynamic contraction thereof. The thermodynamic cycle of heating and cooling the working fluid causes expansion and contraction thereof in turn causes cyclic flexure of the diaphragm in response to the device being exposed to thermal energy. The cyclic flexure of the diaphragm in conjunction with other structures produces the desired conversion to electrical energy.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to devices and methods for conversion of thermal energy to electrical energy, and more particularly to devices and methods for such conversion using micro-electro-mechanical systems (MEMS). The thermal energy may be created deliberately by combustion or other process, or it may be generated as waste heat, the unintended byproduct of other processes.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many devices and systems produce waste thermal energy, which is typically lost to the environment and results in inefficiencies in operation of such systems. For example, electrical equipment produces waste heat, which can adversely affect operation of the equipment, thereby requiring various approaches to remove the heat from the system, and potentially requiring additional energy. Systems which have been used to remove waste heat in such situations include forced air systems, heat exchanger systems, heat radiating structures or others, which assist in removing waste heat from the vicinity of the heat source.  
           [0003]    Other systems, such as mechanical devices, may also produce waste heat in their operation, due to friction, energy conversion or other operational characteristics. Again, in such systems, waste heat may simply vent to the environment or cooling systems are used to dissipate or remove waste heat from the system.  
           [0004]    In still other systems, such as biological systems, waste heat may be generated due to physiological processes for example.  
           [0005]    In each of these types of systems, as well as a variety of other possible systems and situations, unintended thermal energy may present problems in the desirable operation and function of such systems, or simply may be lost to the environment.  
           [0006]    There have also been developed various systems for converting thermal energy to other energy forms, whether the thermal energy is generated intentionally or unintentionally. As examples, recovery of thermal energy may be possible using heat pumps, including chemical heat and cooling pumps, thermal-acoustical heat and cooling pumps, Stirling cycle systems, or various other systems for capturing and converting thermal energy for re-use. Similarly, various systems have been developed for conversion of thermal energy to other useful forms of energy, including pyro-electrical conversion, thermal-electrical conversion, thermal ionic conversion, gas cycle conversion systems, such as Stirling devices, Brayton, etc., and absorption cycle systems as examples. Devices for conversion of thermal energy to other energy forms have historically been large, and in many cases require moderate-to-high temperature differentials for effective operation. Further, such systems include mechanical components, which are subject to friction and wear, limiting the efficiency of the system. Further, contemporary conventional conversion processes and manufacturing processes limit the device size to dimensions much larger than are compatible with many of the types of systems where thermal energy is generated.  
           [0007]    It would therefore be worthwhile to provide methods and systems which can convert thermal energy into electrical power and which overcome limitations associated with the foregoing systems and approaches. It would also be desirable to provide such capabilities in a meso-to-micro scale device, wherein micro-electro-mechanical systems (MEMS) technology, allows the manufacture of devices having sizes which are greatly reduced from prior art systems.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention addresses the above needs and objectives for providing systems and methods for conversion of thermal energy to electrical energy. The devices and methods according to the invention allow for efficient conversion of heat or other thermal energy to electrical power in a meso-to-micro scale device. In general, the present invention relates to a device for converting thermal energy to electrical energy comprising a working fluid contained within at least one region. The region is exposed to a thermal gradient and thermal energy is transferred through at least a first thermal energy accepting layer for transferring thermal energy to the working fluid. At least one diaphragm is acted upon by the working fluid, which thermodynamically expands upon thermal energy being transferred thereto to flex the diaphragm. A mechanism is provided for converting the structural flexure of the diaphragm to electrical energy, along with at least one thermal energy transfer mechanism for transferring thermal energy from the working fluid through the diaphragm to cool the working fluid and cause thermodynamic contraction thereof. In a thermal gradient, the thermodynamic cycle of heating and cooling the working fluid to cause expansion and contraction thereof in turn causes cyclic flexure of the diaphragm in response to the device being exposed to thermal energy. The cyclic flexure of the diaphragm in conjunction with other structures produces the desired electrical energy, effectively converting the thermal energy.  
           [0009]    In conjunction with the device described above or independently, it is also possible to convert thermal energy to electrical energy by means of a device or mechanism for infra-red (IR) power conversion. The mechanical device described above could also be used with other types of energy conversion system if desired, such as thermionic or thermoelectric devices and/or others. When the IR power conversion or other conversion device is used in conjunction with the thermal energy conversion device described above, the overall efficiency of the conversion system can be further increased. In such an embodiment, an IR power conversion area or other conversion system may be provided in association with the heat transfer mechanism or surface of the device. For such an alternate embodiment, conversion of thermal energy to electrical energy is supplemented, and with respect to an IR power conversion system, conversion can occur even if the device is not positioned in a thermal gradient.  
           [0010]    There is also provided a method of converting thermal energy to electrical energy, and methods of fabricating a device for conversion of thermal energy to electrical energy.  
           [0011]    These and other objects and advantages of the invention will become apparent upon a reading of the description of the invention in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a top elevational view of an embodiment of the invention comprising several individual conversion devices.  
         [0013]    [0013]FIGS. 2 and 3 are enlarged top and side views of one of the conversion devices as shown in FIG. 1.  
         [0014]    [0014]FIG. 4 is a partial cut-away view of a portion of one device as shown in FIG. 2.  
         [0015]    [0015]FIG. 5 is a schematic view of a plurality of devices according to the invention in series.  
         [0016]    [0016]FIG. 6 is a schematic view of a plurality of devices according to the invention connected in parallel.  
         [0017]    [0017]FIG. 7 is a schematic diagram of a system using a device according to the invention for conversion of thermal energy created by an associated device or environment.  
         [0018]    [0018]FIG. 8 is a partial cut-away view of an alternate embodiment of a device according to the invention.  
         [0019]    [0019]FIG. 9 is a partial cut-away view of an alternate embodiment of a device according to the invention, having an infra-red (IR) power conversion system associated therewith. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0020]    Referring now to the drawings, FIG. 1 shows a system  10  for conversion of thermal energy to electrical energy according to an embodiment of the invention. The system  10  may have a square configuration, such as shown in FIG. 1, but the shape of the system or devices according to the invention may be of any desired configuration. In this manner, the system  10  can be configured to be adapted for a particular application or environment, where an alternative shape may be more efficient or convenient for integration and use with a device, medium or environment in which it is used. In this embodiment, the system  10  is also fabricated as a meso-to-micro scale device, wherein it can be used in conjunction with miniaturized equipment or devices or environments where larger scale devices simply could not be implemented. In this regard, the system and methods according to the invention provide the ability to convert thermal energy into electrical energy at an electronic chip-scale, which may be used in a broad variety of applications, such as in association with in-situ sensors or devices, or with miniaturized electrical devices or circuits as merely examples. The conversion system and methods according to the invention thereby may provide auxiliary power for use with portable power sources, which typically dictate the performance characteristics of various electrically powered systems. Additionally, in many devices and environments, waste heat generation by electronics or other systems can create significant problems, particularly as increased power requirements and smaller packaging volumes have exacerbated problems associated with waste thermal energy. The device  10  may include a plurality of conversion modules or devices  20  operating together for providing certain auxiliary power requirements, or for conversion of an amount of thermal energy created or present within a system or by a device. The conversion modules  20  provide conversion of thermal energy to electrical energy, such as for use in heat generating environments where an inherent temperature differential or gradient exists, again being a wide variety of potential systems, equipment or environments. The electrical energy generated may be used as auxiliary power for a system, or may be the primary power for a system such as a self-powered sensor or other electronics. As the system  10  or modules  20  can be fabricated on a meso-to-micro-scale size, it may also be possible to use such a device within the human or other body, wherein waste heat is being generated from physiological processes or the like. Thus, the systems and method according to the present invention are not limited to particular applications, as the variety of applications where a temperature differential exists which can be taken advantage of by means according to the invention are potentially endless.  
         [0021]    A thermal conversion module  20  is shown in FIGS. 2 and 3, according to an embodiment of the invention. As shown in these figures, the conversion module  20  may be formed having a planar geometry and footprint, which may be particularly compatible for use with sensors or other miniaturized electronic components. As an example, the module  20  may have an overall volume less than 10 cubic centimeters (less than 0.6 cubic inches), and at a micro-scale approximately one square centimeter. If desired, the system according to the present invention could also be made at a larger scale for a particular application or environment. Further, as shown in FIG. 1, system  10  may be configured to have a plurality of conversion modules  20  in a planar or curvilinear configuration to increase power output, extend coverage over a surface area, or for use in high heat flux applications as will be described further.  
         [0022]    The conversion module  20  may be formed of a plurality of layers formed in conjunction with one another. Due to the configuration of the conversion modules  20  to be hereinafter discussed, and the possible miniaturization of the devices and systems according to the invention, micro-electro-mechanical system (MEMS) technology may allow desired micro scale structures to be formed in the device  20 . Similarly, microchip fabrication methods and semiconductor processing techniques are suitable for the production of systems and devices according to the invention. For example, microchip fabrication methods used in conjunction with conventional printed circuit board, integrated circuit, metalization and pick and place or flip-chip technologies may be applied.  
         [0023]    In a planar configuration as shown, the conversion module  20  comprises a first thermal energy transfer surface or layer  22 , which may be adapted to physically contact a device or equipment generating thermal energy, or be positioned adjacent a source of thermal energy. A second thermal transfer layer or surface  24  is thereby positioned relative to first layer or surface  22  at a point further from the source of thermal energy. In this manner, the first and second thermal transfer surfaces or layers  22  and  24  operate across a temperature gradient generated in proximity to a source of thermal energy. Electrical power distribution leads (not shown) may be provided in conjunction with conversion module  20  to allow withdrawal of electrical power created by the device. In this configuration, semiconductor-processing techniques allow etching of various structures into the conversion module  20  as will be hereinafter described.  
         [0024]    Turning to FIG. 4, there is shown an embodiment of the conversion module  20  for of thermal energy to electrical energy across a thermal gradient. The conversion device  20  in this embodiment combines the inherent efficiencies of thermodynamic fluid cycles with the scalability of MEMS technology. Low mass microstructures may be created in semiconductor material using wet-etching techniques for example. The first and second surfaces or layers  22  and  24  provide a heat-accepting surface and a heat-dissipating surface respectively; with microstructures formed therebetween to effectively utilize the thermal energy transferred therethrough and convert the same to electrical energy. Between surfaces  22  and  24 , a sealed cavity or chamber  26  is formed, and thermal energy is transferred from surface  22  through the material of device  20  to the chamber  26 . Within chamber  26 , there may be provided a working fluid, such as working gas, which may be selected for its thermodynamic characteristics. Desirably, chamber  26  is hermetically sealed to contain working fluid therein without leakage. The working fluid may also be pressurized within chamber  26  if desired, and surfaces of chamber  26  may be coated to prevent diffusion of the working fluid. A diaphragm member  28  is disposed so as to be acted upon by thermal expansion of the working fluid within chamber  26 . The diaphragm member  28  may have a surface  30  serving to form a surface of chamber  26  so as to be acted upon by thermal expansion of working fluid within chamber  26 , or could be indirectly acted upon by such expansion if desired. Thermal energy is transferred to the working fluid through heat-accepting surface  22 , and is expanded within the chamber  26  to act on the diaphragm member  28  and cause movement thereof. As an example, the working fluid could be hydrogen or helium gas that is hermetically sealed in chamber  26  adjacent the diaphragm member  28 . Any other suitable gas or other fluid may be used. As an example, a single or multi-phase gas, a liquid phase or a combination of gas and liquid phase materials may be used.  
         [0025]    In this embodiment, surface  30  of the diaphragm member is coated with a piezoelectric material layer  32 . An output terminal  34  is schematically shown to be coupled to the piezoelectric coating layer so as to electrically couple the piezoelectric coating for channeling of electrical energy created thereby. Piezoelectric materials produce an electrical charge displacement when strained, which strain could be caused by mechanical deformation of the piezoelectric material, thermal strain imposed upon the piezoelectric material or other stress. Upon straining the piezoelectric material, the charge displacement is associated with an electrical field applied over a distance, and results in an electrical potential difference in the material. The strain-induced electrical displacement results from a reorganization of molecules within the material when subjected to a strain. In this embodiment, movement of the diaphragm member  28  causes mechanical straining of the piezoelectric material layer  32  so as to produce an electrical charge, which can be output to the output terminal  34 .  
         [0026]    Anisotropic etching of the semiconductor material can also form structures in the device to enhance its operation. For example, the diaphragm member  28  may be formed to include structures  38  on surface  36  of diaphragm member  28 . The structures  38  serve to significantly increase the surface area of diaphragm member  28  on this surface to enhance thermal transfer from the surface. In the heat-dissipating layer  24  of device  20 , corresponding shaped recesses  40  may be formed to accept structures  38  associated with the diaphragm member  28  therein.  
         [0027]    The diaphragm member  28  is sealed at its ends with respect to layers  22  and  24  in a cavity formed between these layers, thereby forming in this embodiment, chamber  26  in which the working fluid is hermetically sealed. As should be recognized, upon expansion of the working fluid within chamber  26 , diaphragm member  28  will be moved upwardly such that surface  36  will contact the interior surface of layer  24 . In this manner, the structures  38  formed on the diaphragm member  28  will be received in and contact the recessed areas  40  associated with layer  24 . Above the diaphragm member  28 , the area within the formed cavity may be open to the ambient environment via the slanted through-holes formed in association with recesses  40 . Similarly, the heat-dissipating surface  24  may be wafer bonded to the diaphragm member  28  at two or more of its edges to increase access to the ambient environment from this region. Upon movement of the diaphragm member  28  within the cavity, air (or other fluid) above the diaphragm member can therefore easily move out of the assembly to minimize resistance to diaphragm oscillation. The cavity above the diaphragm member  28  may or may not be open to the ambient environment, but if so, this may also provide self-cooling airflow through and underneath the heat dissipating surface  24 . The particular configuration of the heat dissipation surface  24  to allow or enhance the self-cooling flow through and underneath the surface may be modified according to a particular application, or the material from which the heat-dissipating surface  24  is constructed may have differing thermal transfer characteristics to allow dissipation of thermal energy transferred through the working gas to the diaphragm member  28 . Further, active cooling of heat dissipation surface may be provided if desired.  
         [0028]    In operation, and within a thermal gradient, thermal energy will be transferred through surface  22  to the working fluid within cavity  26 . The working gas expands upon heating up, so as to force movement of the diaphragm upwardly until it is in contact with heat dissipating surface  24 . The movement of the diaphragm member  28  causes mechanical strain of the piezoelectric material layer  32  so as to generate electrical energy in response to the thermal expansion. Upon engaging the heat-dissipating surface  24 , diaphragm member  28  will transfer thermal energy to and through the heat-dissipating surface  24  or to be removed to the ambient atmosphere through the through holes  40  or other passages in layer  24  which are open to the ambient atmosphere. Upon dissipation of thermal energy, the working gas temperature adjacent the diaphragm member  28  will be cooled and will contract to the point that diaphragm member  28  disengages from the heat-dissipating surface  24  to complete a cycle of operation. The oscillation of diaphragm  28  and cyclic flexure of the Piezoelectric material layer  32  in association with the diaphragm  28  produces electrical power which can be rectified to produce a desired output via leads or traces coupled to the Piezoelectric coating  32 . The operating frequency of the cyclic operation of the diaphragm member  28  in association with the gas cycle thermodynamics created by the working gas within cavity  26 , will be dependent upon various factors, and may be tuned for a particular operational frequency. As merely an example, the operating frequency of the device may be dependent upon the heat flux of the thermal gradient, the dimensions of the device, the type of working fluid, whether the working fluid is pressurized as well as other possible factors or other operating characteristics. For a meso-scaled device, wherein the footprint area of the device is on the order of centimeters, and the thickness is on the order of millimeters, the operating frequency may be in the range inclusive of, but not limited to, 100 Hertz to several Kilohertz. As mentioned, based upon a selection of the various factors contributing to the resonant frequency of the device, there is provided the ability to tailor the resonant frequency to interface with the overall system in which it is used, and thereby potentially avoid interference that may be created based upon integration of the device into a system. Further, the resonant frequency may be chosen to be compatible with vibrational frequencies of equipment or the like, to either avoid interference, or to supplement movement of the diaphragm to enhance or increase power output by utilizing environmental vibrations. Depending upon the nature of the vibrational environment, it may even be possible to operate the device  20  without a thermal gradient, wherein only the vibrational environment produces movement of the diaphragm. Alternatively, by providing an ability to tailor the resonant frequency, there is also provided the commensurate ability to cancel out induced vibrations in vibration-sensitive applications, by designing multiple devices  20  to operate out of phase with one another. Such an array of a plurality of devices could therefore be tuned relative to one another to operate at a desired resonance frequency, and to potentially work in conjunction with one another to provide unique capabilities.  
         [0029]    A plurality of devices can also be formed to work in conjunction with one another in a manner to enhance overall operation of the system with respect to a given application or environment. The modular design of the conversion devices  20  allows one or more devices  20  to be used for a particular application or environment, to convert thermal energy to electrical energy. By operating a plurality of devices  20 , it may be possible to increase the conversion capacity in a particular application or environment, to make use of overall heat flux present within a given application or environment. As shown in FIG. 5, a plurality of devices  20  may be operated in series, wherein the devices  20  are stacked upon one another to provide a desired power output or to accommodate applications or environments where higher heat flux is present. In this embodiment, each individual device  20  will operate substantially as described previously, with a temperature gradient provided across each device  20  as thermal energy is successively transferred from a device  20  adjacent a thermal source to the next adjacent device and so forth along a thermal gradient. As thermal energy is converted to electrical energy, the thermal gradient will decrease, but as long as such a gradient is still present, will allow proper operation of the devices accordingly. Although three devices  20  are shown in a series configuration in FIG. 5, any desired number of devices  20  may be used for a particular application or environment.  
         [0030]    Alternatively, as shown in FIG. 6, a plurality of devices  20  may be operated in parallel to provide conversion of thermal energy to electrical energy over any desired surface area. The ability to configure the devices  20  in a parallel arrangement allows the overall system, such as system  10  of FIG. 1, to be tailored to a particular device or apparatus with which it may be used, or other source of thermal energy. Because the possible devices and systems with which the device  20  may be used can vary significantly, the ability to tailor a system comprising a plurality of devices  20  for a particular device or apparatus greatly enhances the flexibility and use of the invention. Similarly, combinations of series or parallel arrangements of a plurality of devices  20  combined into an overall system are contemplated herein. Further, providing a plurality of devices  20  in conjunction with one another may also be used in association with curvilinear surfaces, with the system of devices  20  following a curved or other non-planar surface, as each individual device  20  is very small relative to the non-planar characteristics of surfaces, so as to allow a plurality of units to be configured to follow such surfaces.  
         [0031]    From the foregoing, it should be apparent that the present invention provides devices and methods for conversion of thermal energy to electrical energy to supplement or provide power requirements for varying applications or environments. As merely examples, the electrical power generation capabilities of the devices may provide the ability to not only decrease waste heat generation in many electronic packages and applications, but also provide increased power requirements in smaller packaging volumes. The present invention provides both the ability to effectively manage thermal energy in a variety of environments, but also the ability to effectively convert this thermal energy into useable electrical energy. The generated power may be particularly useable in environments or applications where conventional power sources are cumbersome or otherwise adversely effect a desired configuration for the device or system.  
         [0032]    It should therefore be recognized that the present invention may be particularly adapted for use with electronic devices, self-powered sensors, or a variety of other applications where systems or devices are remote from conventional power sources. In such applications and environments, the present invention can supplement battery power sources associated with systems or equipment, or may provide all power necessary for the particular application. The device according to the invention can also be used in environments where thermal energy is a constant environmental factor, such as in association with the human or other animal body, wherein physiological processes generate heat. Thus, the invention may provide an alternative or supplemental power generation system for implantable devices, self-powered sensors or the like, which are becoming prominent in the medical field. It should also be apparent that for a particular application or system, an artificial source of thermal energy may be provided to cause operation of the device for generating electrical power, and nothing limits operation of the device to only conversion of waste heat. The present invention is also particularly adapted for conversion of thermal energy in low-grade thermal environments, which heretofore have not been addressed, primarily due to limitations on scalability of other thermal energy conversion systems. Certain applications and environments may also allow the diaphragm member associated with the device  20  to be acted upon by other force parameters to generate electrical energy. For example, it may be possible to mechanically stress the diaphragm member  28  in the embodiment of FIG. 4 by means of a source of pressure exerted on the diaphragm member from above, such as fluid pressure from an available source. Electrical power generation could be supplemented in such a fashion.  
         [0033]    Turning now to FIG. 7, a schematic illustration of a system for conversion of thermal energy to electrical energy is shown according to the present invention. Any source of thermal energy  50 , whether waste thermal energy or otherwise, may serve to create a thermal gradient in which at least one device  20  according to the present invention is exposed. In the manner as described previously, at least one device  20  will produce an electrical current which may be directed to a power supply for storage or to operate an electrically powered device  54 . Depending upon the particular source of thermal energy  50 , the characteristics of the system incorporating at least one device  20  may be tailored for efficient and effective conversion of the thermal energy to electrical energy.  
         [0034]    Further, as an alternative to the arrangement shown in FIG. 4, a device  20  may be configured such as shown in FIG. 8. A device  60  according to this embodiment of the invention has similar components referenced by similar reference numerals to that of the embodiment of FIG. 4. In this embodiment, instead of sputtering a coating of Piezoelectric material  32  on at least one surface of the diaphragm member  28 , the Piezoelectric material layer  32  is positioned on the interior surfaces of layer  24 , such that upon exposure to a thermal gradient, the diaphragm member  28  will be moved into engagement with the interior surface of layer  24  in a manner similar to that previously described. In this situation, the piezoelectric material  32  will be strained upon pressure being exerted from the engagement of the diaphragm member  28 , or from the transfer of thermal energy therethrough from the diaphragm member  28  to the layer  24 . As the nature of the piezoelectric material merely requires that it be strained in some manner, this alternative approach may be sufficient to produce the desired electrical current from the piezoelectric material, or other such possible configurations are contemplated herein.  
         [0035]    In a further embodiment as shown in FIG. 9, it is also possible to utilize a complementary thermal energy to electrical energy conversion system with the device  20  or  60  as an example. Alternatively, the complementary conversion system could be used independently or could operate independently in certain environments even if combined with another conversion system as in this embodiment. In FIG. 9, a photonic energy conversion system generally indicated at  70 , such as an IR power conversion system, is employed in conjunction with the conversion device  80 . In this embodiment, the other aspects of device  80  may be consistent with the features of the thermal energy conversion device  20  as described previously, but may also be similar to the embodiment  60  if desired. As shown, the photonic conversion device  70  may be associated with the surface  24  of system  80 , but it should be understood that the IR power conversion system  70  could be positioned in another relationship relative to device  80 . In this configuration, the photonic conversion device  70  does not interfere with the conduction of heat to device  80  from a heat source, and will still be subject to IR radiation from the heat source to convert it to electrical energy. In this embodiment, the photonic conversion device  70  may include one or more layers of photocells, and in this embodiment, layers  72  and  74  are provided. The layers  72  and  74  may include a plurality of photocells thereon, such that IR radiation from a heat source incident upon the layers  72  and  74  will be converted by the photocells to electrical energy. Depending on the nature of the heat source, the frequencies of IR radiation can vary to some degree, and it may therefore be worthwhile to provide photocells, which are tuned to the particular frequencies in an environment. There may also be several frequencies of radiation emitted from a heat source, and different layers could be provided for each dominant frequency encountered, such as with a plurality of layers  72  and  74  (or others), with each tuned to convert radiation in a bandwidth of particular frequencies. The ability to tune the photocells and/or layers to a particular bandwidth of IR energy can tailor the system to capture and convert such energy efficiently. The layers  72  and  74  may be coextensive with the layer  24  or of another configuration as desired, and are desirably spaced from the layer  24  or another portion of the device  80  such that radiant energy from the heat source is transmitted across the space  76  to be incident upon the photocells. Alternatively, the layers  72  and  74  may be disposed to have IR radiant energy directly incident upon the photocells from the heat source. In this embodiment, the photonic conversion system  70  does not rely upon a thermal gradient in the conversion of thermal energy to electrical energy, but merely requires incident radiant energy on the photodetectors. Thus, the device  70  could operate independently of the thermal energy conversion device  20  (or  60 ), or may operate at times where the device  20  or  60  is not in operation.  
         [0036]    Although the present invention has been described with reference to specific embodiments, it is to be understood that the systems and methods of the present invention are not to be limited thereby. The objects and advantages of the present invention, among those made apparent from the preceding description are obtained, and certain changes or modifications may be made without departing from the scope of the invention. The above description is therefore intended to be interpreted as illustrative, and not limiting, and the invention is only to be limited by the appended claims.